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Russian Chemical Reviews 71 (2) 85 ± 97 (2002) Oxygen compounds of halogens X2O2 (X is a halogen atom) I V Nikitin Contents I. Introduction II. Dioxygen difluoride and related compounds III. Chlorine oxides Cl2O2 IV. Bromine and iodine oxides Abstract. of properties and structure the on studies of results The The results of studies on the structure and properties of the ClOO FOO, the and I r l the O2F2, C , Cl2O2, B , Br2O2 and and I2O2 molecules molecules and the FOO, ClOO and BrOO radicals obtained mainly in the past decade are and BrOO radicals obtained mainly in the past decade are surveyed. The bibliography includes 173 references. surveyed. The bibliography includes 173 references. I. Introduction Compounds of the general formula X2O2 pertain to the class of high-energy compounds.Representatives of this class are ther- mally unstable and are characterised by low binding energies. Some of them are the most potent inorganic oxidants. In my book 1 published in 1986 and devoted to the oxygen compounds of halogens, compounds with the general formula X2O2 were represented by only dioxygen difluoride O2F2. More- over, it was mentioned that this compound has no Cl-, Br- and I-analogues. Such a conclusion was drawn based on the fact that in the mid-80's, the works which admitted the existence of Cl2O2 were few, and virtually none dealt with Br2O2 and I2O2. However, in recent studies on atmospheric chemistry it was reported that halogen oxides X2O2 (and halogen atoms, particularly, those of chlorine and bromine) are involved in depletion of stratospheric ozone.This fact has attracted the attention of researchers to these compounds, and new information appeared on this class of compounds, which allowed the X2O2 oxides to be characterised in more detail. Thus, this review supplements the book.1 It should be noted that, as for other halogen ± oxygen com- pounds, the distinctions between the structures and properties of X2O2 oxides override their similarities. II. Dioxygen difluoride and related compounds 1. Dioxygen difluoride Dioxygen difluoride exists only at low temperatures (m.p.= 119 K, b.p.=216 K). It was obtained for the first time in 1933 by Ruff and Menzel 2 upon action of an electric discharge on a fluorine ± oxygen mixture. The most probable mechanism of O2F2 formation is as follows: O2F, F+O2 I V Nikitin Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region.Fax (7-096) 576 40 09. Tel. (7-096) 522 55 70. E-mail: led@icp.ac.ru Received 7 June 2001 Uspekhi Khimii 71 (2) 99 ± 112 (2002); translated by T Ya Safonova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n02ABEH000684 85 85 88 92 F+O2F O2F2. Inasmuch as the synthesis of O2F2 was described in detail in the book,1 this issue will not be discussed here. Jackson 3 established the structure of the O2F2 molecule in 1962 by using the method of microwave spectroscopy (MW). The dioxygen difluoride molecule was shown to represent spatially skewed chains of atoms F7O7O7F with C2 symmetry.This structure was confirmed later by electron diffraction (ED).4 Table 1 shows the bond lengths (r) and bond and torsion angles in the O2F2 molecule. Being formally close to the hydrogen peroxide structure, the molecular structure of dioxygen difluoride, however, differs sub- stantially in its details. For instance, the O7O bond length in the H2O2 molecule is 1.48 A, which is close to that of a single bond, whereas the bond length in the O2F2 molecule suggests a double- bond nature, being similar to the bond length in the O2 molecule and the Oá2 cation (1.208 and 1.12 A, respectively, according to the data from Ref. 7). Thus, the following resonance structure was proposed for the O2F2 molecule: F7O O+F FO+OF7.Calculations of the structure of O2F2 molecules by the local density functional (LDF) 5 method provided good agreement with the experimental 5 data: the calculated O7O and O7F bond lengths differed from those found experimentally by no more than 0.01 and 0.015 A, respectively (see Table 1). The geometrical parameters of the FOOF molecule calculated using the Becke ± Perdew ± Wang B3PW91 functional 6 fit less the experimental data, and the Hartree ± Fock theory usually applied in calcula- Table 1. Geometrical parameters of the O2F2 molecule. Experimental data Parameter Quantum-chemical calculations LDF5 B3PW91 6 ED4 MW3 1.222 1.523 109.2 88.1 1.216 1.586 109.2 88.1 1.215 a 1.562 a 110.5 a 88.4 a 36 ± 42 1.217(3) 1.575(3) 109.5 87.5 30 r(O7O) /A r(O7F) /A F7O7O /deg.FOO/OOF /deg. Rotation barriers /kcal mol71 aA mean value is given (averaged from nine values calculated using different Gaussians).86 tions of geometrical parameters of molecules gave unsatisfactory results for dioxygen difluoride (see Ref. 5). According to calculations,5 the internal rotation barrier in the O2F2 molecule is *36 ± 42 kcal mol71, which exceeds substan- tially the value of 30 kcal mol71 suggested by Jackson 3, the value of 20.83 kcal mol71 determined by the CNDO/2 method 1 and the energy of the O7F bond dissociation equal to 20 kcal mol71 (see Ref. 8). Vibrational spectra (IR and Raman) of dioxygen difluoride samples in both solid state and solution are described in the book,1 and the IR spectrum of gaseous O2F2 was measured with an IR Fourier spectrometer in a multipass cell at 175 K (Ref.9). The O2F2 molecule is characterised by six normal vibrational modes active in the IR spectrum, viz., O7O stretching (n1), O7F stretching symmetrical (n2) and antisymmetrical (n5), O7O7F bending symmetrical (n3) and antisymmetrical (n6) and also torsion (n4) modes. The most intense absorption bands were obtained at 1210 (n1) and 615 cm71 (n2+n5 combination). The n1 frequency in the spectrum of gaseous O2F2 is 100 cm71 lower as compared with the spectrum of matrix-isolated O2F2. The IR spectral band of gaseous O2F2 observed at 1490 cm71 was attributed to O7O stretches of the O2F radicals present in equilibrium with O2F2.It was noted 9 that the half-life of O2F2 at a temperature below 180 K exceeds 2 h. An experimental value for the enthalpy of formation of dioxygen difluoride DH 298 (O2F2)=4.730.30 kcal mol71 first determined in 1959 10 was never refined. It should be noted that this quantity was calculated by different methods (Table 2) in quite a number of studies (see, e.g., Refs 6, 11 ± 15). In 1996, Chase 11 tabulated the recommended thermodynamic constants for oxygen halides (NIST ± JANAF) based on spectroscopic and thermodynamic data published up to 1994 inclusive, and the value of 4.60.2 kcal mol71 was proposed for the enthalpy of forma- tion of dioxygen difluoride.Later, this value was argued against by Kieninger et al.6 who calculated the thermodynamic character- istics of oxygen fluorides by the density functional method. Using the B3PW91 functional, they have obtained good agreement between experimental and calculated enthalpies of formation of the OF, O2F and OF2 molecules (the largest deviation was observed for the OF molecule and amounted to*0.6 kcal cm71 at 0 K). However, the same calculation method gave overesti- mated values for the enthalpies of formation of O2F2, namely, 7.3 kcal mol71 (at 298 K) and 9.0 kcal mol71 (at 0 K).6 The value for 298 Kexceeded the recommended one 11 by 64%. At the same time, the high value of 9.0 kcal mol71 did not differ dramatically from the values DH 0 (O2F2)= 8.2 kcal mol71 esti- mated 14 by using the B3LYP functional and DH 0 (O2F2)= 8.70.2 kcal mol71 calculated in Ref. 15.Although the differ- ence in experimental and calculated enthalpies of formation of the FOOF molecules cannot be called a dramatic one, however, further studies should be performed in order to refine the experimental value for the enthalpy of formation of FOOF. Table 2. The enthalpy of formation of the FOOF molecule calculated by different methods. T /K Ref. DH8 /kcal mol71 298.15 11 12 136 4.6 a 4.5 5.0 7.3 0 6 14 15 9.0 8.2 8.7 a This value is included in the recommended NIST ±JANAF Thermoche- mical Table. I V Nikitin Dioxygen difluoride is a strong oxidative-fluorinating agent, which reacts over a wide range of low temperatures.Thus, fluorination of ClF3 by O2F2 occurs at 195 K to give chlorine pentafluoride in a quantitative yield.16 All the other methods of ClF5 synthesis require much higher temperatures and pressures.17 Xenon is fluorinated by an excess of O2F2 to give a mixture of XeF2 and XeF4 at 153 ± 195 K, repeated treatment producing pure xenon tetrafluoride.18 Fluorination of actinides by O2F2 can be carried out at ambient and lower temperatures.19 Due to low O7F binding energy, the dioxygen difluoride molecules andO2F radicals behave as more efficient sources of the fluorine atoms as compared with molecular fluorine and rank below krypton difluoride only.20 Synthesis of dioxygenyl salts in the reaction of O2F2 with fluorine-containing Lewis acids is among the most important preparative reactions of dioxygen difluoride.1 Here, dioxygen difluoride is a source of O2F radicals, which react with Lewis acids to give salts, e.g., Oá2 AsF¡6 .O2F+F, O2F2 O2F+AsF5 Oá2 AsF¡6 . The radicalsO2F can be obtained from this salt by thermolysis O2F+AsF5 Oá2 AsF¡6 and used, e.g., for the preparation of electronically excited and vibrationally excited ClF, BrF and IF molecules in their ground electronic states, which are employed in various spectroscopic studies (see Ref. 17 and references cited therein). 2. The O2F radicals The O2F radicals are formed upon a low-temperature impact (photolysis, radiolysis, etc.) on systems containing fluorine and oxygen.Following stabilisation in low-temperature matrices, they can be studied by various spectral techniques. Recently, Bartlett and co-authors 20 reported the possibility of regenerating O2F radicals from dioxygenyl salts at low temper- ature. Solvated O2F radicals were formed from the salts (Oá2 AsF¡6 , Oá2 SbF¡6 ) dissolved in anhydrous HF in the presence of an alkali-metal fluoride by the reaction O2F. Oá2 +F7 A nonlinear O2F radical represents a prolate asymmetrical rotor 21, 22 with the rotational constants A=78.4301, B=10.013935 and C=8.855247 GHz. The ground state of the O2F radical is X2A00; the energy of the 2A0 state exceeds that of the ground state by*1.07 V (8630 cm71).23 The asymmetrical OOF isomer is by 100 kcal mol71 (according to Ref.23) or 85 kcal mol71 (according to Ref. 24) more stable than the sym- metrical OFO isomer. In contrast to OOF, the OFO radical has never been observed experimentally.23, 25 In addition, as com- pared with the OFO+ cation, the FOO+ cation is more energeti- cally favourable. The FOO+ ion represents a charge-transfer complex F(2P)7Oá2 (2Pg) which dissociates readily to F and Oá2(see Ref. 26). Internuclear distances r (O7O) and r (O7F) in the O2F radical are 1.200 and 1.649 A, repectively, and the F7O7O bond angle is 111.2 8, which is somewhat larger 25 than in the FOOF molecule. An increase in the O7F bond length in the O2F radical as compared with the O2F2 molecule agrees well with a decrease in the O7F bond strength when going from O2F2 to O2F.The F7O2F bond strength in dioxygen difluoride is *19.6 kcal mol71 at 0 K.27 In the O2F radical, the fluorine atom is less strongly bound to oxygen: the dissociation energy is 11.70.5 kcal mol71 (0 K).28 Virtually the same value 11.90.2 kcal mol71 (0 K) was obtained in calculations 27 from the third law of thermodynamics when studying the thermo- dynamic properties of the F+O2 reaction mixture. Like peroxy radicals,29 dioxygen monofluoride strongly absorbs in the UV range (200 ± 300 nm). The absorptionOxygen compounds of halogens X2O2 (X is a halogen atom) maximum for the O2F radical was shown 30 to lie at 206.5 nm (the absorption cross-section s= 1.35610717 cm 2), at 215 or 220 nm.27, 28, 31 At 298 K,31 intense absorption of the O2F radical was observed at 215 nm with the absorption cross-section s= 1.15610717 cm 2 (for O2F2 molecules, s=4.2610718 cm 2).31 At 300 K, the absorption maximum was observed at 220 nm, the absorption cross-section being equal to 1.3610717 cm2 (Ref.28) and 1.6610717 cm2 (Ref. 27). The IR spectrum of the O2F radicals in the gas phase has been studied.21, 26, 32, 33 Using an IR Fourier spectrometer (resolution of 0.003 cm71) and a cell with an optical path of 100 m, the vibrational spectrum of the O2F radical was obtained with the most intense absorption bands at 579.32 (n3), 940.22 (n2 + n3), 1142.46 (2n3), 1486.93 (n1), 1495.60 (n2+2n3) and 2948.09 (2n1).21 All vibrational frequencies of gaseous O2F appeared to be lower than those of matrix-isolated O2F.The bending vibrational frequency n2 was in the range of 365 ± 370 cm71. An attempt 9 to resolve the rotational structure of the O7O stretching band has failed; however, this structure was eventually resolved.21 These results show that detachment of a fluorine atom from the O2F2 molecule results in a substantial increase in the O7O stretching mode frequency, the latter approaching a vibrational frequency of 1580 cm71 for the O2 molecule. In spectrokinetic studies 28 of the equilibrium in the gas-phase reaction O2F F+O2 in a temperature range of 295 to 359 K, the enthalpy of formation of the O2F radical was found to be DH 298 (O2F)=6.24 0.50 kcal mol71 (the radicals were generated by pulse irradiation of a gas mixture with electrons of 2 MeV energy).A kinetic study of the reaction of fluorine and oxygen atoms 31 gave a value of 5.490.40 kcal mol71 (298 K). A later study 27 showed similar values for enthalpy (Table 3). Table 3 lists DH 0 (O2F) values calculated using B3LYP 14 and B3PW91 6 functionals and the QCISD(T)/6-31G(d,p) method 34 (QCI is quadratic configuration interaction). Generally, the O2F radical behaves as a mere source of fluorine atoms rather than as a radical. Its relatively low reactivity is explained by the fact that abstraction of the F atom, which represents the active part of the radical, requires an activation energy of *12.6 kcal mol71 (Ref. 35). The O2F radical is inert with respect to other peroxy radicals, whereas the HO2 radical reacts quickly at room temperature with RO2 with the rate constants lying within (4 ± 12)610712 cm3 molecule71 s71 (Ref.29). Table 3. The enthalpy of formation of the O2F radical. T /K Ref. DH exp /kcal mol71 DH theor /kcal mol71 298 28 31 27 346 11 6.240.50 5.490.40 6.132 777 7775.93 6.0 6.10.5 a 0 27 14 34 11 6.571 777 77.22.0 8.93.0 6.50.5 a a This value is included in the recommended NIST ±JANAF Thermoche- mical Table. Solutions containing O2F radicals are stable at temperatures 4223 K. They can be used for oxidative fluorination. For example, Au(III) is oxidised to Au(V) 87 2O2F+AuF¡4 AuF¡6 +2O2. Similarly, the Ag(II) ions are oxidised to Ag(III), Ni(II) is oxidised to Ni(IV) and Pt(IV) is oxidised to Pt(V).Controlled hydrolysis of a solution of a dioxygenyl salt in HF at 195 K represents a rare example of the chemical synthesis of ozone.36 2Oá2 AsF¡6 +3H2O O3+O2+2H3O+AsF¡6 . The same result was obtained with other available and handy dioxygenyl salts, viz., fluoroantimonates Oá2 SbF¡6 and Oá2 Sb2F¡11. It was assumed 36 that the reaction of dioxygenyl Oá2 with a water molecule gives an O2 molecule and an H2O+ cation, the latter decomposing with elimination of H+. Hydroxyl formed upon the H2O+decomposition reacts withOá2 producing protonated ozone OOOH+, which transforms into ozone upon elimination of H+. The recent active studies on the properties of dioxygen difluoride and the O2F radicals were partly associated with the hazards of atmospheric contamination by halogen-containing refrigerants and propellants that produce halogen atoms upon photolysis.The reactions of the fluorine atoms with methane, water and other hydrogen-containing compounds present in the atmosphere of Earth have low activation barriers 37 and result in HF, i.e., stable reservoir molecules rained out of the atmosphere. However, the main mechanism for transformation of the fluorine atoms in the atmosphere is the reaction O2F+M F+O2+M (hereinafter M is any species), which produces the O2F radicals. Moreover, due to a high oxygen concentration and a high rate of fluorine atom reaction with oxygen, the ratio of the equilibrium concentrations [O2F] : [F] in the stratosphere may exceed 10 4 (see Refs 35, 38, 39).The radicalsO2F in turn can take part in catalytic cycles of ozone destruction 36, 38 ± 41 OF+O2 OF+2O2 O2F+O O2F+O3 OF+O3 O2F+O2 _______________________ OF+O3 O2F+O2 ________________________ 2O2 O+O3 3O2 2O3 The efficiencies of the reactions of oxygen, water and methane with F atoms in the atmosphere depends on the altitude: at altitudes of 4 ± 42 km, the reactions with oxygen resulting in O2F2 prevail; below 4 km and above 42 km, the reactions with H2O and CH4, which produce HF, are predominant.42 At altitudes above 28 km, photolysis of O2F proceeds actively, which inhibits accumulation of these radicals.39 The reaction of the F atoms with the oxygen molecules is not the only source of the O2F radicals.The following reaction has been hypothesised: 43 FOOCl F+OClO O2F+Cl, which, however, cannot play any noticeable role due to the lower chlorine dioxide content in the atmosphere as compared with oxygen. Studies of certain reactions of the O2F radicals at 295 K by means of pulse radiolysis coupled with UV absorption spectro- scopy have shown 35 that the reactivity of the O2F radical is relatively low, the upper limits of rate constants of its reactions with O3, CH4 and CO being 3.4610716, 4.1610715 and 5.1610716 cm3 molecule71 s71, respectively.35 Whence it fol- lows that O2F radicals pose no threat to the ozone layer. The reaction (1) products O2F+NO proceeds virtually completely by the channel FNO+O2, O2F+NO probably, via an intermediate complex ON7FO2.The rate constant for the reaction (1) is (1.470.08)610712 cm3 mole- cule71 s71. The reaction88 (2) products O2F+NO2 at 190 K<T<298 K ¡688 377 T k1=(7.5 0.5)610712 exp proceeds at 295 K with the rate constant k2= (1.050.15)610713 cm3 molecule71 s71 (Ref. 40). Kinetic stud- ies of reactions (1) and (2) under continuous flow conditions using microwave electric discharge and mass spectrometry have con- firmed their bimolecular mechanisms.39 At a total pressure of 1 Torr, the following rate constants were obtained (cm3 mole- cule71 s71): and at 260K<T<315 K. ¡2042 456 T k2=(3.8 0.8)610711 exp At room temperature, the upper limits of the rate constants for the reactions of O2F with O3 and CH4 were 3610715 and 2610716 cm3 molecule71 s71, respectively.3. Compounds HOOF and ClOOF The HOOF and ClOOF molecules are of particular interest for atmospheric chemistry. A reaction of OF with OH may result in a compound HOOF, which is kind of hybrid between hydrogen peroxide and dioxygen difluoride. A non-empirical estimate 40 has shown that the HOO/OOF dihedral angle in a skewed HOOF molecule (the C1 group of point symmetry) is *83 8 and the internuclear distance r(O7O)=1.339 A, these values being intermediate between those in H2O2 and O2F2 molecules. The HOOF molecule can be sufficiently stable and decomposes to F and HO2 species. The enthalpy of HOOF formation can be estimated by using the following isodesmic reaction: HOOH +FOH HOOF+HOH and was found to be 0.42 kcal mol71 (at 0 K).Chlorine fluorine peroxide ClOOF can be an intermediate in the reaction of radicals ClO+OF. An estimate 43 gave a value of 85.1 8 for the dihedral angle in the skewed ClOOF molecule. The enthalpy of formation of ClOOF at 300 K was assessed to be 36.82 kcal mol71 (Ref. 43) and 18.5 kcal mol71 (Ref. 13). The presence of a stronger bond F7O in the ClOOF molecule as compared with HOOF, allows one to assume that ClOOF decomposes to Cl and O2F. Dissociation of ClOOF to F and ClO2 is energetically less advantageous.13, 43 The peroxide ClOOF is by 18 kcal mol71 more advantageous with respect to energy than the chlorite FOClO.13 III.Chlorine oxides Cl2O2 In contrast to fluorine which forms a single compound, viz., dioxygen difluoride, chlorine oxides with the composition X2O2 are of greater diversity. The first mention in the literature of a compound with the formula Cl2O2 dates back to 1949. In the 38Cl radiotracer investigations of mechanisms of reactions of chlorine with chlo- rous acid, of the chloride ion with the chlorate ion and of chlorous acid disproportionation, each of these reactions was assumed 44 to involve an intermediate which had an asymmetrical structure of either ClClO2 or ClOClO and decomposed to ClO2. Of all possible structures built of two chlorine atoms and two oxygen atoms which include three-membered and four-membered cycles, only chloryl chloride ClClO2 (Cs symmetry), chlorine peroxide ClOOCl (C2) symmetry) and chlorine chlorite ClOClO (C1 symmetry) can exist.Among these isomers, ClOOCl is the most stable one. Chloryl chloride ranks higher in the energy scale as compared with chlorine peroxide (by 0.9 ± 5 kcal mol71 according to different estimates 45 ± 48) and chlorine chlorite (by 7 ± 10 kcal mol71, see Refs 45 ± 49). An estimate 50 of the Gibbs energy for an equilibrium mixture of all Cl2O2 isomers in the ideal-gas state has shown that at low I V Nikitin temperatures the ClOOCl isomer prevails, whereas at higher temperatures the fraction of ClClO2 increases. 100 0.4 300 21.6 200 8.8 Temperature, K Fraction of ClClO2 in a mixture of Cl2O2 isomers (%) The yield of chlorine chlorite at T4590 Kdid not exceed 1%; however, its content in the mixture increased substantially with further increases in the temperature.1. Chloryl chloride Attempts to obtain chloryl chloride by the reaction of chloryl fluoride ClO2F or chloryl perchlorate ClO2ClO4 with certain chlorine-containing compounds (NOCl, NO2Cl, AlCl3, BCl3, HCl) were undertaken as early as in the 1950's (see, e.g., Ref. 51), but resulted in formation of products of chloryl chloride decomposition, viz., ClO2 and Cl2 in a 2 : 1 ratio. Only in 1992, could the fluorine atom in ClO2F be exchanged for a chlorine atom from AlCl3, BCl3 and HCl to give chloryl chloride in amounts sufficient for spectroscopic measurements; however, an attempt to isolate pure ClClO2 has failed.52 Chloryl chloride was synthesised by passing ClO2F over AlCl3 under a pressure of 0.1 ± 1 mbar, the product being collected at 77 K.52 The reactions of ClO2F with BCl3 and HCl also resulted in ClClO2, while the reaction of ClO2F with NOCl afforded predominantly NO2Cl.In a different preparation of ClClO2, stoichiometric amounts of ClO2F and HCl were mixed in a reactor and then the mixture was expanded into an absorption cell, the temperature in the reactor and the cell being kept at*255 K.53 Chloryl chloride was also formed in the reaction of photochemically generated chlorine atoms with chlorine dioxide and in the recombination of ClO radicals on ice.54 At room temperature, with a ClClO2 partial pressure of 1 mbar and a total gas pressure of 4 mbar, the ClClO2 half-life is 1 min.Under stratospheric conditions, chloryl chloride is more stable, but easily undergoes photolysis. The UV absorption spectrum of ClClO2 in a neon matrix demonstrated two bands devoid of definite structures with max- ima at 236 and 296 nm and half-widths of *40 and *45 nm, respectively.52 The absorption maxima in the gas-phase ClClO2 spectrum were at 231 (s=1.3610717 cm2) and 296 nm (s=1.5610717 cm2). The IR spectra of matrix-isolated and gaseous ClClO2 dem- onstrated six fundamentals n1 ± n6. Table 4 shows the vibrational frequencies for the matrix-isolated 35Cl 35Cl 16O2 (neon matrix) 52 and compares them with the vibrational frequencies calculated by the CCSD/TZ2P method 47 (singles and doubles coupled-cluster).Geometrical parameters of the ClClO2 molecule (Ref. 52) were later refined based on the rotational spectra 53, 54 of chloryl chloride (Table 5). For comparison, Table 5 shows the geomet- rical parameters estimated by the MP2 method using 6-31G* Table 4. Vibrational frequencies in the spectrum of the ClClO2 molecule. Mode Frequ- Experimental Calculations a ency data 52 using the CCSD/TZ2P method 47 1039 524 459 285 1205 1041.20 522.50 440.43 271.37 1216.37 n1 n2 n3 n4 n5 254 251.40 n6 Cl7O stretch, symmetric OClO bend ClClO bend, symmetric Cl7Cl stretch Cl7O stretch, antisymmetric ClClO bend, antisymmetric a Of all the values calculated using the MP2, CCSD and CCSD(T) methods with TZ2P and TZ2Pf basis sets, the CCSD/TZ2P method provided the best agreement with experimental data.Oxygen compounds of halogens X2O2 (X is a halogen atom) Table 5.Geometrical parameters and dipole moment (m) for the ClClO2 molecule. Experimental data Parameter IR 52 Quantum-chemical calculations MW53 MW54 MP2/6-31G* MP2/TZ2Pf (see Ref. 46) (seeRef. 47) a 2.22 1.44 2.269 1.442 103.8 2.1966 2.1978 2.459 1.4367 1.4360 1.469 103.5 104.14 104.06 104.2 115.0 116.0 114.96 115.05 116.7 2.20 3.378 1.570 r (Cl7Cl) /A r (Cl7O) /A Cl7Cl7O /deg. O7Cl7O /deg. m /D a The values shown were calculated by the MP2/TZ2Pf method that provides the best agreement with experimental data.(Ref. 46) and TZ2Pf (Ref. 47) basis sets. The internuclear distance Cl7Cl in a ClClO2 molecule exceeds that in the Cl2 molecule by almost 0.21 A. Chloryl chloride molecules represent asymmetric prolate rotors with the rotational constants A=9450.9218, B= 3588.0617 and C=2787.9468 MHz (for 35Cl35Cl16O2). A non-empirical quantum-chemical calculation 46 of the enthalpy of formation carried out by the MP2 method using 6-31+G(3d, 2p) and 6-31G* basis sets for the following isodes- mic reaction: HOClO2+Cl2 ClClO2+HOCl gave values of DH 0 (ClClO2) equal to 33.8 [MP2/6-31+G(3d, 2p)] and 34.6 kcal mol71 (MP2/6-31G*). The enthalpy of formation of ClClO2 was also calculated 47 using the second-order Mùller ± Plesset perturbation theory and theCCSD and CCSD(T) methods with TZ2P and TZ2Pf basis sets.The CCSD(T)/TZ2P method gave a value of 35.1 kcal mol71, which is most similar to the experimental one. Photolysis of matrix-isolated ClClO2 caused its rearrange- ment into ClOClO and ClOOCl.55 A more detailed study 56 has shown that the effect of light with a threshold wavelength of 610 nm on argon-matrix-isolated ClClO2 results in immediate transformation of the latter into chlorine chlorite ClOClO without formation of any long-lived intermediates. A chlorine atom formed upon ClClO2 dissociation can attack the ClO2 residue either at the chlorine atom giving ClClO2 or at the oxygen atom to give ClOClO. The action of light with a threshold wavelength of 665 nm on ClOClO causes its dissociation to either Cl and OClO (with subsequent regeneration of ClClO2) or two ClO radicals (with subsequent regeneration of ClOOCl).A scheme of photol- ysis of argon matrix-isolated ClClO2 has been described 56 and the wavelengths at which dissociation of Cl2O2 isomers takes place were specified (Scheme 1). Scheme 1 ClClO2 <610 nm ClOClO Cl+Cl O <665 nm 665 ± 488 nm O [ClO+ClO] <488 nm <360 nm [Cl+ClOO] ClOOCl <360 nm Cl2O2 By using narrow-band filters (665, 610 and 360 nm), all three isomers, viz., ClOClO, ClClO2 and ClOOCl, respectively, could be identified. The calculated 56 values of dissociation energies (Edis) 89 for chlorine chlorite and chloryl chloride were substantially lower than the photon energy, which points to the excessive energy of the photofragments.Reaction Edis /kcal mol71 6 *9.5 *19 2 ClO Cl+OClO Cl+OClO ClOClO ClOClO ClClO2 In the photolysis of neon matrix-isolated ClClO2, only chlor- ine and oxygen are formed, and no photoisomerisation of ClClO2 to ClOOCl takes place. A study 57 of photoisomerisation of argon-matrix-isolated chlorine, bromine and iodine chlorites (XOClO, X= Cl, Br, I) obtained by addition of X atoms to OClO has shown that all three chlorites isomerised to the corresponding chloryl halides XClO2. In turn, ClClO2 and BrClO2 were transformed into ClOClO and BrOClO; this process could be repeated many times until XClO2 decomposed to oxygen and the halogen.It was assumed 52 that chloryl chloride takes part in strato- spheric ozone destruction (Scheme 2),52 although one can simu- late 46 a `zero' cycle (Scheme 3), which does not result in ozone loss.46 Scheme 2 Cl2O2+M hn Cl+ClOO Cl+O2+M ClO+ClO+M Cl2O2 ClOO+M 2 (Cl+O3 ClO+O2) _____________________________ 3O2 2O3 Scheme 3 ClOClO+M hn hn O3+M. ClO+ClO+M ClOClO ClClO2 ClClO2 Cl+OClO OClO O+ClO Cl+O3 ClO+O2 O+O2+M For decades, the ClO radicals were the subject of spectro- scopic and kinetic studies (the first works on this subject were carried out as far back as 20 ± 30 years ago); however, little attention was paid to their dimerisation (self-reaction). The possibility of Cl2O2 formation upon recombination of the ClO radicals and further involvement of the Cl2O2 molecules in the presumed catalytic cycle of ozone depletion (see Scheme 2) were the reasons for the appearance of numerous publications devoted to recombination of the ClO radicals and properties of the dimer formed.2. Chlorine peroxide An IR spectroscopic study 58 of recombination of the ClO radicals generated at 220 ± 240 K under continuous flow conditions by the reaction of Cl atoms with ozone has shown that chlorine peroxide ClOOCl, which can take part in the aforementioned catalytic cycle, is the dominant recombination product. Although it was soon shown 55, 59 ± 62 that the IR spectral bands pertain most probably to dichlorine trioxide (chlorine sesquioxide Cl2O3), the main conclusion on the involvement of Cl2O2 in the catalytic cycle remained true.When studying the spectrum of the ClO radical recombination products in the submillimetre range (415 ± 435 GHz) at a temper- ature below 240 K, several Cl2O2 isomers and chlorine dioxide OClO were identified,63 chlorine peroxide ClOOCl being the main product. The ClOOCl molecule has a chain structure with two equivalent chlorine atoms and two equivalent oxygen atoms (C290 Table 6. Geometrical parameters and dipole moment (m) for the chlorine peroxide molecule. Parameter Experimental data MW63 Quantum-chemical calculations MP2/6-31G* MP2/TZ2Pf (see Ref. 46) (see Ref. 47) 1.711 1.407 108.9 83.0 0.84 1.741 1.420 109.0 85.0 0.921 1.7044 1.4259 110.07 81.03 0.72 r (Cl7O) /A r (O7O) /A Cl7O7O /deg. ClOO/OOCl m /D symmetry).Molecules of chlorine peroxide represent slightly prolate asymmetric rotors with the constants A= 13109.4463, B=2409.7892 and C=2139.6786 MHz.63 Earlier,64 the follow- ing constants were obtained: A= 12967, B=2333 and C=2085 MHz. Table 6 shows internuclear distances and bond angles for chlorine peroxide molecules (see also Ref. 65). The calculated values of the internal rotational barrier for ClOOCl are as follows: E /kcal mol71 ClOOCl conformation data from Ref. 66 data from Ref. 67 10.1 5.4 cis trans 8.8 4.9 According to Ref. 63, the dipole moment of a ClOOCl molecule is 0.72 D, although the value of 0.76 D was thought 67 to be the most reliable one for the dipole moment.An IR spectrum of argon-matrix-isolated ClOOCl at 12 K demonstrated absorption bands at 752.6 (O7O, stretch), 649.8 and 647.6 cm71 (Cl7O, stretches); 55 the absorption bands at 750 and 650 cm71 were also present in the spectrum of gaseous chlorine peroxide.60 In a later work,56 the IR spectrum of ClOOCl isolated in an argon matrix was studied. For a molecule with an isotopic composition of 35Cl16O16O35Cl, the frequencies 753.97 (n1), 543.0 (n2), 647.67 (n5) and 418.5 cm71 (n6) were found and the values 310 (n3) and 127 cm71 (n4) were calculated. In the UV absorption spectrum of ClOOCl,68 the absorption maximum was observed at 245 nm and the absorption cross- section (s) was equal to 6.4610718 cm2 (at 350 nm, the absorp- tion cross-section was <1610719 cm2). An adsorption maxi- mum was also found 60 at 245 nm (s=6.5610718 cm2), and measurable cross-section values could be obtained up to 410 nm.The fact that this UVspectrum pertained to chlorine peroxide was confirmed by calculations.48 Later,69 an UV spectrum of ClOOCl obtained by chlorine monoxide photolysis (l=254 nm) was studied; the absorption maximum was found at 244 nm (s=6.4610718 cm2). The measurements were carried out at 195 K in the 200 ¡À 400 nm range. In a number of studies, the enthalpies of ClOOCl formation 1 Keq=exp DS R ¡¦DH1 RT exp and energies of bond dissociation were calculated, which did not coincide completely but did not differ dramatically from one another.The enthalpy of ClO dimerisation found 68 from the temperature dependence of the equilibrium constant for the reaction ClO+ClO Cl2O2, was equal to7170.7 kcal mol71. An experimental value of the enthalpy of ClOOCl formation (DH 298) was equal to 31.30.7 kcal mol71. A non-empirical calculation 46 of the enthalpy of ClOOCl formation carried out within the framework I V Nikitin of the second-order M��ller ¡À Plesset perturbation theory with the 6-31+G(3d) and 6-31G* basis sets gave the values DH 0 (ClOOCl)=35.7 (MP2/6-31 + G(3d) and 32.6 kcal mol71 (MP2/6-31G*), and an estimate 47 obtained by the CCSD method was DH 0 (ClOOCl)= 34.2 kcal mol71. Using an experimental value of the heat of ClO formation and a calculated value of DH 0 (ClOOCl) with a correction made for T=298 K, the energy of ClOOCl dissociation 2ClO, ClOOCl was determined 47 to be D(ClO7OCl)=14.9 kcal mol71.The fact that the enthalpy value of 34.2 kcal mol71 calculated by using the isodesmic reaction ClOOCl+H2O Cl2 O+HOOH deviated from the experimental value may be due to an error introduced by determination of the heat of formation of Cl2O.47 Using another value for the heat of Cl2O formation, the following parameters were obtained:47 DH 0 (ClOOCl)=32.7 kcal mol71 and D(ClO7OCl)=16.4 kcal mol71, which were in better agreement with the experimental data. According to an estimate,58 dissociation of chlorine peroxide ClOOCl to ClO + ClO requires *16.7 kcal mol71, whereas the dissociation to Cl + ClOO requires more than 19 kcal mol71.The value of DH 0 (ClOOCl) was calculated 70 to be 332 kcal mol71. In a kinetic study 71 devoted to reactions of ClOOCl thermal decomposition and ClO dimerisation, the energy of the O7O bond dissociation (18.1 kcal mol71) was calculated and the enthalpy of ClOOCl formation (30.50.7 kcal mol71) was determined. Apparently, without introducing a huge error, one can assume the dissociation energies of Cl7OOCl and ClO7OCl bonds to be 21 and 18 kcal mol71, respectively.69, 72 Thus, the molecule of chlorine peroxide differs sharply from the FOOF molecule in the bond strength: the O7O bond in ClOOCl is much weaker as compared with that in FOOF.Ionisation of ClOOCl makes this bond still weaker: for an ionic dimer ClOOCl+, the values D298(ClO7OCl+)= *11.54.3 kcal mol71 DH and 298(ClOOCl+)= 287.52.9 kcal mol71 were found.73 3. The ClOO and OClO radicals It was assumed 58 that the photolysis of ClOOCl in the polar stratosphere results in the formation of Cl atoms and ClOO radicals. An investigation 74 confirmed this assumption. It was found that it is these products that were formed upon laser pulse photolysis of chlorine peroxide (l=308 nm, 235 K) with a primary quantum yield of 1.030.12. The photodissociation of ClOOCl to Cl atoms and ClOO radicals was also mentioned in Ref. 68; however, the possibility of another dissociation channel resulting in two ClO radicals 56 cannot be ruled out.In contrast to Cl atoms, ClO radicals do not catalyse the ozone destruction; hence, the ratio of the dissociation products [Cl] : [ClO] is impor- tant when assessing the ozone-depletion effect of ClOOCl. Experi- ments 75 on the ClOOCl photolysis (l=248 and 308 nm) have shown that irradiation mainly cleaves the stronger Cl7O bond, the [Cl] : [ClO] ratio being 0.88 : 0.12 (248 nm) and 0.90 : 0.10 (308 nm). Mass-spectrometric studies failed to detect the inter- mediate ClOO that forms in the Cl7O bond scission, because of its fast dissociation. Photolysis of chlorine peroxide at l=308 nm resulted in its dissociation to Cl atoms and O2 molecules, whereas photolysis at 248 nm produced in addition ClO, O and ClOO.76 Abstraction of a chlorine atom from the ClOOCl molecule results in a ClOO radical, which is an isomer of the well-known chlorine dioxide OClO.The possibility of existence of the ClOO radical as an intermediate product in the pulse photolysis of chlorine ¡À oxygen mixtures was reported by Porter and Wright 77 as far back as the early 1950's. The ClOO radical was by 3 ¡À 4 kcal mol71 energetically more favourable as compared with the OClO radical, the former being very reactive, however, in contrast, chlorine dioxide was stable at room temperature andOxygen compounds of halogens X2O2 (X is a halogen atom) could exist as a radical for an indefinitely long period without any sign of dimerisation. Association of OClO with the formation of weakly bound dimers was observed only in the solid state.78, 79 Both ClO2 isomers can be involved in the stratospheric ozone destruction, which explains the attention they have attracted in recent years (photochemistry of chlorine dioxide is the subject of numerous studies, see, e.g., Refs 80 ± 111).Experiments on OClO photolysis most frequently used radiation with wavelengths of 365 ± 368 nm; the absorption of OClO begins at*510 nm. Photolysis of OClO can result in both dissociation of the molecule and its photochemical isomerisation. A model of con- certed isomerisation of the OClO molecule which proceeds via ClOO isomers in the lower excited state 2A0 was proposed. From this state, the radical ClOO can either emit a photon and pass to the ground state 2A00 with the energy 25 ± 30 kcal mol71 lower than the energy of the 2A0 state or dissociate.The ground state 2A00 of ClOO radicals obtained by photoisomerisation of chlorine dioxide and stabilised in an aluminosilicate matrix at 77 K was confirmed by analysis of the ESR spectrum.81 Decomposition of photo-excited OClO molecules results in Cl atoms and O2 molecules. Chlorine atoms can destroy ozone. Decomposition of OClO can proceed by a different pathway, namely, with ClO and O formation. The latter species can regenerate ozone; however, dimerisation of ClO is preferable. Dimerisation of ClO and photoisomerisation of OClO result in the formation of ClOOCl or ClOO which destroy ozone. The direction of OClO reactions, viz., isomerisation or dissociation, and the mechanisms of these processes depend strongly on the photolysis conditions.In low-temperature inert matrices, chlorine dioxide transforms almost completely to the ClOO isomer. The ClOO radicals represent the only photoproduct upon photolysis of OClO in an amorphous ice matrix (360 nm, 80 K).100 Photol- ysis of OClO at 150 K also produced ClOO together with ClClO2. The latter was formed in the reaction of Cl atoms with chlorine dioxide.97, 103 Chloryl chloride was also observed among the photolysis products of thin OClO films either condensed at 100 K (up to 70 monolayers) or adsorbed on ice (0.5 ± 2 mono- layers).105 Photolysis of aqueous and other solutions of chlorine dioxide results in the formation of chlorine atoms.The quantum yield and the mechanism of Cl atom formation were the subjects of a discussion. Thus it was assumed 97 that chlorine dioxide is first isomerised to ClOO, which is decomposed to Cl and O2. On the other hand, it was supposed 89 that nearly 90% of excited OClO molecules dissociate to O and ClO in an aqueous solution at room temperature at l=365 nm and only 10% of the molecules isomerise to ClOO, the latter decomposing to Cl and O2 (the rate constant is *6.7610 9 s71). However, the possibility of chlorine atom formation as the primary photoproduct has not been ruled out hn OClO Cl(2Pu) + O2(1Dg). According to data from Ref. 98, the 10% yield of Cl atoms comprised 9.5% of Cl obtained upon ClOO decomposition and 0.5% of Cl formed upon direct photodissociation. The same conclusion was made by the authors 110, 111 who studied photolysis of an aqueous solution of chlorine dioxide at l=400 nm.They have found that nearly 90% of excited OClO molecules dissociate to ClO and O with subsequent recombination of these fragments. In this process, the quantum yield of the Cl atoms was about 10%, of which 2% were formed upon decomposition of ClOO and 8% were formed upon dissociation. They managed to observe ClOO radicals with a lifetime of*4 ns. Chang and Simon 104 assumed that a `hot' ClOO isomer formed upon phololysis of OClO undergoes relaxation in 9 ps ahen dissociates. Irradiadion of ClOO with light with a wavelength of 365 nm increased the yield of Cl atoms owing to the photolysis of vibrationally excited ClO radicals formed by the dominant photodissociation channel 108 91 ClO+O.OClO A femtosecond spectroscopic study of the photolysis of an aqueous solution of OClO has shown that ClO and O fragments quickly recombine to give vibrationally excited OClO which relaxed to the ground state in 10 ps. Irradiation of OClO with light with l=400 nm produced only Cl and O2 as the photolysis products with a quantum yield of 0.070.03.106, 107 Gas-phase photoisomerisation of OClO molecules seems to be highly unlikely.90, 101 In the UV spectrum, the absorption maximum of ClOO radicals was found at 248 nm [s=(3.40.3)610717 cm2, 298 K] (Ref. 112) and at 246 nm [s=(2.980.23)610717 cm2, 191 K] (Ref.113). An IR spectrum of ClOO radicals generated by either photo- isomerisation of argon-matrix-isolated OClO or the reaction of Cl atoms with oxygen molecules at 17 K demonstrated the following vibrational frequencies: 1442.8 (n1), 408.3 (n2), 192.4 (n3) cm71 and an overtone of 375.6 cm71 (2n3) (Cs point symmetry group).95 IR-Fourier spectra measured for eight neon-matrix-isolated ClOO radicals with different isotope compositions contained all fundamental frequencies, four overtones and five combined frequencies.79 For example, the spectrum of 35Cl 16O16O radicals contained the frequencies 1438.56 (n1), 413.73 (n2), 201.38 cm±1 (n3) (antiharmonicity-corrected frequencies were 1477.8, 432.4 and 214.8 cm71). The O7O stretch frequency for the ClOO radical (1477.8) was close to that for theO2F radical (1487 cm71).The exact values of geometrical parameters for ClOO radicals are unknown. Based on IR spectra measured during the photolysis of a Cl2 + O2 mixture at 4 K, estimates were made 114 for bond lengths and bond angles in a ClOO radical (Table 7). For this radical, the following values were proposed:115 r(O7O)=1.31 A and r(O7Cl)=1.835 A. Noticeably different geometrical parameters of ClOO radicals were calculated by different meth- ods, viz., by the high-level nonempirical CMRCI (contracted multireference configuration interaction) method,116 by using the MP2 theory with different basis sets 117 and by means of the density functional theory (DFT) 118 (see Table 7).According to calculations,116 the dipole moment of ClOO is 1.113 D. Table 7. Bond lengths and bond angle in the ClOO radical. Parameter Quantum-chemical calculations Experimental data DFT118 CMRCI116 MP2117 IR 114 1.174 ± 1.247 1.2154 1.923 ± 2.203 1.9286 114.1 ± 118.6 115.74 1.201 2.139 115.7 1.23 1.83 110 r(O7O) /A r(Cl7O) /A Cl7O7O /deg. In a kinetic study of the reaction of Cl atoms with O2 molecules and the following equilibrium: ClOO +O2 Cl+O2+O2 (181 ± 200 K, oxygen pressure of 15 ± 40 Torr), the rate constant for Cl+O2 association was found to be (8.92.9)610733 cm6 molecule72 s71 (186.55.5 K).115 According to this study,115 the equilibrium constant (Keq) of the reaction Cl + O2 is 18.9 atm71 (185.4 K).A similar value (Keq=20.4 atm71) was found at the same temperature in a different study.112 The enthalpy of formation of ClOO radical was found 112 to be DH 298(ClOO)=23.40.5 kcal mol71; calculations 119 gave the value of 24.22 kcal mol71. In experimental studies,112, 115 cal- culations according to the third law of thermodynamics gave the following values for the dissociation energy D0(Cl7OO): 4.760.49 (Ref. 115) and 4.830.05 kcal mol71 (Ref. 112). The calculated value of D0(Cl7OO) was 4.02 kcal mol71 (see Ref. 119). Quantum-chemical calculations using the density func-92 tional method showed that the dissociation energy of the ClO7O bond is 62.4 kcal mol71. Thus, it is evident that the ClOO radical has a strong O7O bond and a weak Cl7OO bond and in this respect resembles the FOO radical.A rather weak Cl7O bond allows us to consider the ClOO radical as an O2 molecule with a `chaperon', viz., a chlorine atom.120 4. Compound HOOCl Substitution of a hydrogen atom for a chlorine atom in chlorine peroxide gives HOOCl. This compound has not yet been isolated, however, it is of interest for atmospheric chemistry. Reactions Cl+HO2 and ClO+OH can result in the formation of HOOCl. The reaction of Cl with HO2 probably produces an excited HOOCl molecule with the lifetime at 298 K estimated as 1 ns.121 HOOCl molecules can decompose by three channels, viz., to Cl+HO2, HCl + O2 and OH + ClO. A study 122 of the Cl+HO2 reaction under continuous flow conditions at 250 ¡À 420 K and a pressure of 0.9 ¡À 1.5 Torr have shown that in this temperature range its rate constant depends little on the temper- ature and is equal to (4.20.7)610711 cm3 molecule71 s71.At 298 K, the reaction proceeds by roughly 20% by the channel OH+ClO. Cl+HO2 At 243 ¡À 298 K, the reaction of OH and ClO radicals largely proceeds by the channel OH+ClO HO2+Cl, probably, with the formation of an excited HOOCl intermedi- ate.123 The other reaction channel HO+ClO HCl+O2 , is realised to a small degree due to the high energy barrier associated with rearrangement of the intermediate. The probability of stabilisation of excited HOOCl molecules is low under both atmospheric and laboratory conditions.124 How- ever, a non-empirical calculation using the CCSD(T)/TZ2P method has shown 125 that the thermal stability of HOOCl even exceeds that of ClOOCl; hence, when studying atmospheric processes, one must bear in mind the possibility of the existence of HOOCl.In the same work,125 geometrical parameters of the HOOCl molecule were estimated [r(O7O)=1.443 A, r(Cl7O)=1.746 A, r(H7O)=0.968 A, bond angles Cl7O7O and O7O7H were 108.6 8 and 100.6 8, respectively, the dihedral angle ClOO/OOH was 98.1 8] as well as the dipole moment (1.74 D). The enthalpy of HOOCl formation was shown to be 1.51 or 0.21 kcal mol71. The calculated frequencies of HOOCl harmonic vibrations o1¡Ào6 were 3744, 1399, 835, 633, 392 and 361 cm71, respectively. The skewed structure of HOOCl was also confirmed by calculations.126 It was also found 126 that the HOOCl isomer is most stable among the isomers of the general formula HClO2, the energies of branched Y-shaped HClO2 and a chain HOClO (chlorous acid) forms exceeding the energy of the HOOCl form by 51.0 and 8.3 kcal mol71, respectively.The calculated 126 enthalpies of formation of HOOCl, HOClO and HClO2 molecules were 1.6, 11.9 and 56.2 kcal mol71, respectively estimates,13 DH [according 300(HOOCl)= to other 70.1 kcal mol71]. The geometrical parameters of the HOOCl molecule calculated using the MP2/6-31G(d,p) method [r(O7O)=1.408 A, r(O7Cl)=1.751 A] 116 were similar to those found in Ref. 125. 5. Chlorine chlorite and Cl2O�¢2 cation In studies of disproportionation of chlorine-containing com- pounds, it was assumed that chlorine chlorite ClOClO can be an intermediate in these reactions.127 Apparently, chlorine chlorite is the intermediate 128 in the reaction ClO+ClO.O+ClOCl A mass-spectrometric study 73 of ClO dimers has shown that several Cl2O2 isomers exist, of which ClOClO is the least stable. I V Nikitin Table 8. Calculated values of bond lengths and bond and torsion angles for the ClOCl0O0 molecule. Quantum-chemical calculations Parameter MP2/6-31G* (see Ref. 46) CCSD/TZ2P 47 1.717 1.739 1.517 111.6 112.4 76.6 1.713 1.909 1.512 109.7 116.8 67.1 r(Cl7O) /A r(O7Cl 0) /A r(Cl 07O0) /A Cl7O7Cl 0 /deg. O7Cl 07O0 /deg. ClOCl 0/OCl 0O0 /deg. According to different estimates,13, 46, 47 the enthalpy of ClOClO formation is 40.8 ¡À 44.3 kcal mol71.Table 8 shows the results of quantum-chemical calculations 46, 47 of geometrical parameters for the ClOClO isomer. Salts that involve Cl2O�¢2 as a cation have been synthesised.129 A reaction of hexafluoroantimonate O�¢2 SbF¡¦6 with chlorine in anhydrous HF at 195 ¡À 233 K gave Cl2O�¢2 SbF¡¦6 ; the addition of antimony pentafluoride to the same reactants resulted in the formation of Cl2O�¢2 Sb2F¡¦11. Black-violet crystals of both salts precipitated when the were cooled to 193 K. Structural studies showed that the salts contain the Cl2O�¢2 cations shaped as flat trapezia in which the `chlorine part' is spaced at 2.425A from the `oxygen part', which amounts to*75% of the sum of the van der Waals radii of Cl and O.The internuclear distances Cl7Cl and O7O were 1.916 and 1.185 A, respectively. In the Raman spectrum of the Cl2O�¢2 Sb2F¡¦11 salt, an O7O stretch was observed at 1534 cm71 and Cl7Cl stretches were found at 586 and 593 cm71. Despite the apparent weakness of the cation structure, the salts are sufficiently stable: Cl2O�¢2 SbF¡¦6 decomposes at 273 K, and Cl2O�¢2 Sb2F¡¦11 is stable up to 313 K. The dissociation energy of the Cl2O�¢2 cation Cl Cl�¢2 +O2 2O�¢2 is equal to 12.8 kcal mol71. The formation of Cl2O�¢2 cations occurs as a result of chlorine oxidation by a dioxygenyl cation. Cl [Cl2_O2]+. 2 +O�¢2 Cl�¢2 +O2 IV. Bromine and iodine oxides 1.Bromine oxides Currently, bromine oxides remain insufficiently explored com- pounds despite the growing interest in them.130 Probably, this is associated with their low stabilities. Only bromide monoxide Br2O, bromine bromate BrOBrO2 and bromic anhydride O2BrO- BrO2 have been characterised in most detail. Despite being known since the 1930s, BrO2 has probably never been obtained in pure form. Moreover, a substance with this molecular formula is believed to be in fact either a mixture of Br2O3 and Br2O5 (see Ref. 130) or bromine perbromate BrOBrO3 (Ref. 131). The oxide Br2O2 has never been isolated either. However, this was probably observed 132 in the IR spectroscopic studies of the products of atomic ¡À molecular reactions of bromine and oxygen in solid argon.Moreover, it is most probable that this oxide is an intermediate in the recombination of the BrO radicals. A study of disproportionation of the BrO radicals generated by photolysis of either bromine ¡À ozone or bromine ¡À oxygen mixtures BrOO+Br, BrOOBr* BrO +BrO (3a) O O 6�� BrO +BrO BrOOBr* Br2+O2 , (3b) Br Br A has shown 133 that reaction (3) is of the second order. The branching ratio for reactions (3a) and (3) k3a/k3 was equal toOxygen compounds of halogens X2O2 (X is a halogen atom) Table 9. Geometrical parameters of the BrOOBr molecule calculated by different methods. Quantum-chemical calculations Parameter BLYP, SVWN and B3LYP 139 1.264 ± 1.335 1.922 ± 2.082 113.3 ± 115.6 79.2 ± 85.6 r(O7O) /A r(Br7O) /A Br7O7O /deg.BrOO/OOBr /deg. 0.840.03 (at 298 K). According to the data from Ref. 134, the branching ratio k3b/k3 is equal to 0.120.04. The vibrationally excited intermediate BrOOBr* either dissociates to BrOO and Br or passes to a more stable state and then dissociates to Br2 and O2 via a four-centred complex A. Studies of the kinetics and mechanism of the self-reaction of the BrO radicals generated by pulse photolysis of bromine ± ozone mixtures at 220 and 298 K and a pressure of different gases of 75 ± 600 Torr have revealed 135 absorption at 312 nm, which was not reported earlier. The absorption at 220 K was attributed to the Br2O2 dimer. The rate constant for reaction (3) was equal to (2.750.50)610712 cm3 molecule71 s71 at 298 K independent of pressure and changed from (2.000.41)610712 (100 Torr) to (3.100.30)610712 cm3 molecule71 s71 (400 Torr) at 220 K.In the same work, the pressure-independent ratios k3a/k3=0.84 (298 K) and 0.68 (220 K) were found. Estimates 135 have shown that the lifetime of Br2O2 does not exceed 1 min in the stratosphere at 195 K. In a kinetic study,136 the absence of the OBrO radical among the products of the BrO+BrO reaction was reported. Appa- rently, the metastable intermediate BrOBrO the decomposition of which would have given OBrO either was not formed at all or quickly isomerised to BrOOBr. For reaction (3), the following rate constants were found (cm3 molecule71 s71): 136 k3= (2.980.42)610712, k3a=(2.490.42)610712, k3b= (4.690.68)610713 at 298 K and 760 Torr (oxygen as the buffer gas).These values are similar to those obtained in a different study.135 According to data from Ref. 137, in the temperature range of 204 ± 388 K, the rate constant for reaction (3) depends on the temperature in the following way: . 215 50 T k3=(1.700.45)610712 exp At 298 K, k3=(3.510.35)610712 cm3 molecule71 s71. When simulating the atmospheric processes, it is recommended (bearing in mind the results of previous studies) to use the value cm3 molecule71 s71. k3=(1.500.46)610712 exp 230 100 T A detailed study 138 of the kinetics and mechanism of the self- reaction of BrO radicals in the temperature range of 220 ± 298 K has shown that at T<250 K a new reaction channel appears(3c) BrO +BrO +M Br2O2+M.At 222 K, the rate constants k3c and k73c are equal to 8.2610732 cm6 molecule72 s71 and 2.5610718 cm3 mole- cule71 s71, respectively. The equilibrium constant is 1.937610 5 (235 K) and 1.26610 6 atm ±1 (222 K). An enthalpy change in the reaction BrO +BrO Br2O2 was estimated to be7140.02 kcal mol71. The absorption maximum in the UV spectrum of the Br2O2 dimer was found at 305 nm, the cross-section s=1.4610717 cm2 (at 350 nm, the cross-section was 1.28610717 cm2).138 Like Cl2O2, compound Br2O2 can exist in several isomeric forms. Among three possible isomers of Br2O2, bromine peroxide 93 MP2/6-31+G(2d) 141 MP2, CCSD(T), B3LYP140 1.4068 1.8861 111.46 84.75 1.359 ± 1.410 1.861 ± 1.900 109.6 ± 112.6 82.2 ± 84.6 BrOOBr is the most stable.139, 140 A calculation 139 using BLYP, SVWN and B3LYP density functionals have shown that the energies of BrBrO2 and BrOBrO isomers are higher than that of the BrOOBr isomer by 8.4 and 11.3 kcal mol71, respectively.Table 9 shows the results of quantum-chemical calcula- tions 139 ± 141 of geometrical parameters for the bromine peroxide molecule. Depending on the calculation method, the internuclear distance r(O7O) varies from 1.264 to 1.410 A, and r(Br7O) varies from 1.861 to 2.082 A, the bond angle Br7O7O varies from 109.6 to 115.6 8 and the torsion angle BrOO/OOBr varies from 79.2 to 85.6 8. The vibrational spectrum of the BrOOBr molecule was also calculated.139 ± 141 The frequencies of O7O stretches (1103, 1046 and 878 cm71) obtained 139 by three different methods differ substantially from the calculated frequencies 746 cm71 (Ref. 140) and 737 cm71 (Ref.141). The enthalpy of BrOOBr formation was estimated 13 to be 41 kcal mol71 (300 K). Calculated within the framework of different approximations, the enthalpies of BrOOBr, BrBrO2 and BrOBrO formation at 0 K are, correspondingly, 46.1, 52.9 and 54.8 kcal mol71 (Ref. 139) and 441, 576 and 564 kcal mol71 (Ref. 140). It was shown 142 that the reaction of bromine atoms with O2 molecules in an argon matrix at 10 K gives BrOO radicals. The frequency of 1487.0 cm71 in the IR spectrum of the BrOO radicals was assigned to the O7O stretch. In a recent study 143 which analysed the IR spectrum of argon-matrix-isolated BrOO radicals with different isotope compositions at 17 K, the frequencies of 1485.1, 1444.0 and 1402.3 cm71 were attributed to the O7O stretches in Br 16O16O, Br 16O18O and Br 18O18O, respectively. Non-empirical calculations using the QCISD(T) method 144 gave the frequencies of the Br7O stretch (450 cm71) and the Br7O7O bend (261 cm71). Like its chlorine analogue, the BrOO radical is more stable than its symmetrical isomer OBrO by approximately 5 kcal mol71 (Ref.145) or 17 ± 18 kcal mol71 (Ref. 146). By using five different density functionals, the internuclear distances Br7O and O7O and the Br7O7O bond angle were calculated 146 for the BrOO radical in its ground state 2A00 (Table 10).In the other studies,144, 145 higher values were obtained for the Br7O bond length and lower values were found for the O7O bond length. Table 10. Geometrical parameters of the BrOO radical. Parameter Quantum-chemical calculations BHLYP146 BP86 146 QCISD(T) 144 [UMP/2, CCSD(T)] 145 2.302 1.232 2.291 1.214 116.4 2.150 1.242 118 2.178 1.253 1Br7O7O /deg. According to estimates,147 the strength of the Br7O bond is *1 kcal mol71. A later estimate 145 gave practically the same value, viz., 1.2 ± 1.7 kcal mol71. Dissociation of BrOO radicals to BrO and O requires more than 50 kcal mol71 (Ref. 145). The enthalpy of BrOO formation is 35.1 kcal mol71 (300 K).1394 Irradiation of BrOO radicals isolated in a solid argon matrix at 275 nm resulted in reversible photoisomerisation to give OBrO.143 If the matrix also contained oxygen, photolysis of BrOO produced O3 and BrO.Apparently, in this case a reaction of excited BrOO radicals with oxygen molecules proceeds.148 Contamination of the atmosphere of Earth with bromine- containing compounds proceeds both in a natural way and as a result of human activities. Methyl bromide is the main source of stratospheric bromine. Its content in the atmosphere was recently assessed to be*10 ppt (1 ppt=10712).149 The lifetime of CH3Br in the atmosphere is less than two years, it readily photolyses in the lower stratosphere. It was concluded 150 that, of all possible bromine-involving catalytic cycles of ozone depletion, the cycle with the reaction BrO + ClO as the rate-determining step (Scheme 4) is the most efficient.Scheme 4 Br+ClOO Cl+O2+M BrO +O2 BrO +ClO ClOO +M Br+O3 Cl+O3 ClO+O2 _________________________ 3O2 2O3 This cycle is particularly remarkable due to the fact that the loss of two radicals that are inactive with respect to ozone results in appearance of very reactive bromine and chlorine atoms.151 The contribution of such a cycle into the halogen-controlled ozone depletion can reach 20% ¡À 25%.152 Many studies were devoted to the BrO + ClO reaction (e.g., see Refs 153 ¡À 161 and references therein). 2. Compounds BrOOCl, BrOClO and HOOBr The class of compounds covered by this review also involves BrOOCl and BrOClO.These can form as intermediates in the BrO+ClO reaction BrOOCl BrO +ClO (4a) Br+Cl+O2 , Br+OClO, BrOClO BrO +ClO (4b) 6�� O O (4c) BrO +ClO BrCl+O2 Cl Br A (Ref. 158) 320 40 T k4=(4.700.50)610712 exp It was found 154, 156 that the rate constant for reaction (4) is (8.21.0)610712 cm3 molecule ¡À1 s71 and does not depend on the temperature in the range of 241 ¡À 408 K. In the same studies, the following branching ratios were found: k4a/k4=0.450.10, k4b/k4=0.550.10 and k4c/k4 < 0.02. The presence of channel (4c) was concluded by Toohey and Anderson, 155 and the for- mation of BrCl in considerable amounts was experimentally observed,157, 158, 160, 161 which suggests that this channel makes a substantial contribution to reaction (4).Using the gas-discharge technique and mass spectrometry and a combination of pulse photolysis and UV absorption spectroscopy, the value k4=(1.290.16)610711 cm3 molecule71 s71 (298 K) and the ratio k4c/k4=0.080.03 were obtained.158, 159 Like many other bimolecular radical ¡À radical reactions, reaction (4) is character- ised by an inverse temperature dependence. Thus the following values were obtained for the rate constant of reaction (4) in the temperature range of 220 ¡À 400 K (cm3 molecule71 s71): and (Ref. 159). 240 60 T k4=(6.11.2)610712 exp I V Nikitin Values of rate constants for reaction (4) similar to the aforementioned values,158, 159 were found in the studies devoted to the kinetics of this reaction and involving mass-spectrometric detection of the products:160 k4=(1.130.15)610711 cm3 mole- cule71 s71, k4b/k4= 0.430.10 and k4c/k4=0.120.04 (298 K).Thus, the BrO + ClO reaction proceeds with formation of two intermediates, viz., BrOClO which gives OClO [channel (4b)] and BrOOCl which either decomposes to Br, Cl and O2 [channel (4a)] or rearranges to the four-centred state A with subsequent elimi- nation of BrCl [channel (4c)]. A calculation 140 has shown that, of the mixed oxides XX0O2 (X=Br andX0=Cl), the peroxide BrOOCl is the most stable; the enthalpy of its formation was found to be 392 kcal mol71 (0 K) 140 or 38.9 kcal mol71 (0 K).139 In an earlier work,155 the geometrical parameters of the BrOOCl molecule were estimated: r(Br7O)=1.8 A, r(Cl7O)=1.7 A, r(O7O)=1.2 A and the BrOO/OOBr dihedral angle was*70 8.Acalculation 140 using the MP2 perturbation theory gave the following values: r(Br7O)=1.859 A, r(Cl7O)=1.710 A, r(O7O)= 1.412 A, bond angles Br7O7O and Cl7O7O were both equal to 109 8 and the dihedral angle was 84.5 8.The values close to those mentioned above were also obtained in calculations using the B3LYP density functional. It was assumed 162 that the reaction of the BrO radical with hydroxyl involves the formation of a short-lived vibrationally excited complex HOOBr. The main and probably the only channel of this reaction is [HOOBr] BrO +OH Br+HO2 . The rate constant for this reaction was estimated to be (7.54.2)610711 cm3 molecule71 s71 at 300 K and 1 Torr.162 The enthalpy of HOOBr formation was determined to be 5 kcal mol71 (298 K) 162 which is similar to the value of 4.2 kcal mol71 (300 K) 13 and to the calculated 139 value of 8.6 kcal mol71 (0 K).The formation of the HOBrO intermediate in the BrO + OH reaction cannot be ruled out either; however, isomerisation of HOOBr to HOBrO is hindered due to the high energy barrier.163 3. Iodine oxides An assumption of the existence of I2O2 and its hydrate H2I2O3 as intermediates in the reaction of the iodate ions with the iodide ions in an acidic medium was put forward back in 1930 (see Ref. 164). In a study devoted to the mechanism of this reaction,164 the following scheme of I2O2 formation (in the IIO2 form) was proposed: H+ + IO¡¦ HIO3, 3 IIO2+H2O, HIO3+H+ + I7I2 + IO¡¦ I7+IIO2 2 .By analogy with the recombination of ClO and BrO radicals, one can assume that the reaction IO +IO also proceeds with the formation of I2O2 dimers as the intermediates, although the evidence of their existence is still insufficient. When studying the recombination of IO radicals generated upon photolysis of CH3I in the presence of ozone at 303 K at an atmospheric pressure (N2 and O2 as the diluent gases), the formation of the I4O9 aerosol was observed, which deposited on reactor walls as a yellow-white film.165 It was assumed 165, 166 that, at first, the bimolecular self-reaction of the IO radicals proceeds IO+IO I2O2. The I2O2 formed reacts with I to give IO2 I2 + IO2, I + I2O2 and the latter transforms into amorphous I4O9. The IO + IO reaction was studied by a combination of pulse photolysis and UV absorption spectroscopy in the temperature range of 250 ¡À 373 K at a pressure of 650 Torr (nitrogen as theOxygen compounds of halogens X2O2 (X is a halogen atom) diluent gas).167 The rate constant for this reaction is independent of the temeprature and has the following form: .k=1.73610712 exp 1020 200 T At 298 K, its value was (5.61.2)610711 cm3 mole- cule71 s71. The rate constant at 300 K was determined 168 to be (6.62)610711 cm3 molecule71 s71. Later,169 the rate constant k=(81.7)610711 cm3 molecule71 s71 was measured at room temperature and a pressure of 60 ± 600 Torr.It was concluded 170 that I2O2 dimers can form in the self- reaction of the IO radicals, although this possibility has never been confirmed in spectroscopic studies. According to certain data,170 the rate constant for this reaction is temperature-independent in the range of 220 ± 320 K and is equal to (9.91.5)610711 cm3 molecule71 s71. A non-empirical calculation using the QCISD(T)/6-311+ G(3df) basis set 171 has shown that all I2O2 isomers are unstable with respect to the elements at 298 K. Below, four possible isomers of I2O2 are shown as well as the calculated bond lengths (in A) in themO O 1.426O O 1.777 2.037 I I 2.785 I I IIO2 (Cs) IOOI (C2) O 2.025 O O 2.007 I 1.822 I 1.809 I O I2.840 OIIO (C2) IOIO (C1) In this family, the IIO2 isomer is the most stable (in contrast to the corresponding chlorine and bromine analogues).The stability of I2O2 isomers decreases in the following sequence: 171 IIO2 > IOIO > IOOI > OIIO. In the IIO2 molecule, the internuclear distance r(I7O)=1.777 A [calculated using the MP2/6-311+ G(3df) method] is the shst distance as compared with the other I2O2 isomers, and the frequency of the I7O stretch is the highest (about 960 cm71). The lifetime of IIO2 with respect to its decomposition to I and OIO at room temperature was estimated to be *4 s. The geometrical parameters of iodine peroxide molecules IOOI were also calculated: 171 r(I7O)=2.037 A, r(O7O)=1.426 A, the bond angle I7O7O and the torsion angle IOO/OOI were equal to 110 and 86 8, respectively.The frequency of the I7O stretch was *510 cm71. Using the follow- ing isodesmic reaction 2HOI + H2O2 IOOI+2H2O the enthalpy of IOOI formation was calculated to be 37.5 kcal mol71 (at 298 K).171 As suggested by the authors of this work,171 OIO may be among the products of the IO + IO reaction. Apparently, IOO radicals are still less stable as compared with ClOO radicals.171, 172 The reaction of IO and ClO radicals at 298 K and the total pressure of 1 Torr was studied by mass spectrometry 173 (5a) I+OClO, IO+ClO (5b) IO+ClO I+Cl+O2, (5c) IO+ClO ICl+O2. For reactions (5), the rate constant k5=(1.10.2)610711 cm3 molecule71 s71 and the branching ratios k5a/k5= 0.550.03, k5b/k5=0.250.02 and k5c/k5=0.200.02 were found.173 The value k5=(1.290.27)610711 cm3 mole- cule71 s71 (298 K) was also obtained.172 95 The channel Cl+OIO IO+ClO was assumed to be highly improbable. The most important source of iodine in the atmosphere is methyl iodide, which is liberated by algae and phytoplankton of the ocean and is rapidly photolysed in the troposphere.Iodine compounds can get to the stratosphere as a result of convection. Nuclear stations can also be a source of atmospheric contami- nation with iodine. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (2) 99 ± 110 (2002) Synthesis and reactions of organic compounds containing bonds of Te to Group 14 elements I D Sadekov Contents I. Introduction II. Synthesis and reactions of compounds R1TeER23 (E=Si, Ge, Sn, Pb) III. Methods of synthesis and reactions of compounds (R3E)2Te (E=Si, Ge, Sn, Pb) IV. Other types of non-cyclic compounds containing Te ±E bonds V. Cyclic compounds containing Te7E bonds VI. Conclusion Abstract. organic of reactions and synthesis the on data The The data on the synthesis and reactions of organic compounds containing bonds of Te to Group 14 elements are compounds containing bonds of Te to Group 14 elements are generalised and systematised. The use of these compounds in the generalised and systematised. The use of these compounds in the synthesis with complexes compounds, organoelement of synthesis of organoelement compounds, complexes with organo- organo- tellurium metal and clusters tellurium-containing ligands, tellurium ligands, tellurium-containing clusters and metal tellur- tellur- ides is reviewed.The bibliography includes 99 references ides is reviewed. The bibliography includes 99 references. I. Introduction The existence of four types of true, i.e., containing at least one Te7C bond, organotellurium compounds with Group 14 ele- ments, viz., (R1Te)4E, (R1Te)3ER2, (R1Te)2ER22 and R1TeER23 (R1=Alk, Ar;R2=H, Alk, Ar; E=Si, Ge, Sn, Pb), is possible in principle. Tellurides R1TeER23 are the most well studied among them. Compounds (TeR)4E have not been described so far, while the other two types are represented by only a few examples.The derivatives (R3E)2Te, the methods of synthesis and reactions of which are similar to those of tellurides R1TeER23 are also consid- ered. The interest in compounds R1TeER23 and (R3E)2Te is deter- mined by the fact that these are currently acquiring an ever increasing importance in the synthesis of organic and inorganic tellurium derivatives. Perfluoroalkyl trimethylstannyl tellurides (RFTeSnMe3) are used for the preparation of telluroketones and previously unknown telluracycles.1 ±3 Compounds R1TeER23 are used as synthetic equivalents of organyl tellurolate anions RTe7 which are easily oxidised in air and are used exclusively in situ.4±9 These types of compounds have been used in the synthesis of aryl methyl tellurides,8 aryl tellurobenzoates 9 and aryl tellurofor- mates 6, 7 in high yields.Triphenylgermyl derivatives and their stannyl analogues, viz., ArTeEPh3 (E=Ge, Sn), are convenient starting compounds for the synthesis of metal tellurolates (ArTe)nM (M is metal),4 whereas BuTeSiMe3 is used for the synthesis of tellurium-containing clusters.5 The reaction of bis- (trialkylsilyl) tellurides with some organoelement compounds, I D Sadekov Institute of Physical and Organic Chemistry, Rostov State University, Prosp. Stachki 194/2, 344090 Rostov-on-Don, Russian Federation. Fax (7-863) 243 46 67. Tel. (7-863) 243 48 94. E-mail: sadek@ipoc.rnd.runnet.ru Received 30 July 2001 Uspekhi Khimii 71 (2) 113 ± 125 (2002); translated by R L Birnova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n02ABEH000692 99 99 103 107 107 108 which enables low-temperature synthesis of practically important metal tellurides MxTey, attracts special attention.10, 11 The use of other compounds with Te7E bonds for the synthesis of organic tellurium derivatives and some organic and organoelement compounds are described in more detail in the sections devoted to reactions of tellurides R1TeER23 .Earlier data on the synthesis and reactions of compounds R1TeER23 and (R3E)2Te are described in the monographs.12 ± 14 Spatially hindered (organoelement)tellurols of the type R3ETeH (R=Me3Si, E=Si, Ge; R=Ph, E=Si) as well as tin and lead tellurolates [(Me3Si)3SiTe]2E (E=Sn, Pb), which also contain Te7E bonds, are used as the starting compounds in low- temperature synthesis of tellurides of these metals and are considered in the reviews.15, 16 3 II.Synthesis and reactions of compoundsR1TeER2 (E=Si, Ge, Sn, Pb) 1. Synthesis of compounds R1TeER23 3ECl.4, 5, 17 ± 19 The most common approach to the synthesis of compounds R1TeER23 (1) with any element E is based on the reaction of alkali-metal tellurolates R1TeM with chlorides R2 Lithium tellurolates are usually prepared by insertion of tellurium into theC7Li bonds of alkyl(aryl)lithium,5,17719 whereas sodium tellurolates can be obtained by reducing diaryl ditellurides with either sodium borohydride in PhH7EtOH 4, 19 or metallic sodium in THF.18 The yields of compounds 1 usually vary from 60% to 80% with the exception of p-methoxyphenyl (triphenylgermyl) telluride (1g) obtained in 23% yield.4 Bis(trimethylsilyl) telluride and dialkyl ditellurides are side products in the synthesis of alkyl trimethylsilyl tellurides 1a,b.5,17 Ph3SiTePh could not be obtained by this method.4 3 R1TeM+R23 ECl 7MCl R1TeER2 1a ± o M=Li, Na.1 E R R2 Ref. Compound 1 175 17 ± 19 194, 19 44 Me Me Me Me Ph 4-MeC6H4 Ph Me Me Bu Ph Ph Ph Ph 4-MeOC6H4 Ph Si Si Si Ge Ge Ge Ge Sn abcdefgh 19100 1 Ref. R2 E R Compound 1 Ph 4-MeC6H4 4-MeOC6H4 Ph 4-BrC6H4 4-MeC6H4 4-MeOC6H4 Ph Ph Ph Ph Ph Ph Ph Sn Sn Sn Pb Pb Pb Pb ijklmno 4, 19 444, 19 444 The silyl derivatives 1a ± c represent pale yellow fluids with an unpleasant odour; their analogues containing heavier Group 14 elements are colourless or pale yellow solids readily soluble in ordinary organic solvents.The reaction of lithium ethanetellurolate with chlorosilanes was used in the synthesis of the silyl derivatives 2a,b containing alkyl groups and a hydrogen atom at the silicon atom.20 EtTeLi +R2ClSiH 7LiCl EtTeSi(H)R2 2a,b R=Me (a), Et (b). Aryl trimethylsilyl tellurides 3a,b are obtained in 30% ±40% yields by treatment of aryl telluromagnesium bromides with chlorotrimethylsilane in THF.21, 22 THF ArTeMgBr+ClSiMe3 7MgBrCl ArTeSiMe3 3a,b Ar=Ph (a), 4-MeC6H4 (b).(Me2Si)2Te and Me3SiO(CH2)4TePh were isolated along with compound 3a in 10% and 45% yields, respectively.21 The latter seems to result from the tetrahydrofuran ring opening under the action of chlorotrimethylsilane and the reaction of Me3SiO(CH2)4Cl formed with PhTeMgBr. Yet another approach to the synthesis of compounds 1, viz., the reaction of the lithium derivatives R3ETeLi with alkyl halides, is represented by a single example. Thus butyl (triethylsilyl) telluride 1p was synthesised 23 in 79% yield by treatment of tetrameric lithium triethylsilanetellurolate (4) with butyl chloride. Compound 4 is prepared from ethyllithium and the corresponding tellurol.23 BuCl Et3SiTeH+EtLi 7LiCl 7C2H6 (Et3SiTeLi)4 4 BuTeSiEt3 .1p Reactions of diorganyl ditellurides 1±3, 24 and diorganyl tel- lurides 6, 7, 25 ± 27 with organoelement hydrides afford tellurides 1 containing Si, Ge and Sn atoms. The synthetic potential of this method is inferior to that based on the reaction of alkali-metal tellurolates with chlorides R23 ECl and is mainly employed for the synthesis of organyl (triorganylstannyl) tellurides 1 (E=Sn). The reaction of diaryl ditellurides with triphenylstannane proceeds under mild conditions (heating of the reactants at 60 ± 70 8C without a solvent) and results in aryl (triphenylstannyl) tellurides 1i, 5a ± c in 50% ±70% yields.24 Ar2Te2+Ph3SnH 7H2 ArTeSnPh3 1i, 5a ± c Ar=Ph (1i), 4-MeOC6H4 (5a), 4-EtOC6H4 (5b), 4-PhOC6H4 (5c).If tributylstannane is used instead of triphenylstannane, no individual compounds similar to compounds 5 could be isolated. However, the mass spectra of oily reaction products contain peaks of molecular ions corresponding to aryl (tributylstannyl) tellur- ides ArTeSnBu3.24 The reaction of bis(perfluoroalkyl) ditellurides with trimethyl- stannane is employed in the synthesis of perfluoroalkyl trimethyl- stannyl tellurides 6a ± e.1±3 Thus trifluoromethyl trimethylstannyl telluride 6a was obtained in *90% yield on warming of a 1 : 3 (CF3)2Te27Me3SnH mixture in diethyl ether from7196 to 0 8C (see Ref. 1). I D Sadekov (RF)2Te2+Me3SnH 7RFH,7Te Compounds 6b ± e were synthesised under nearly identical conditions.2, 3 The reaction of the corresponding ditellurides with Me3SnH (molar ratio 1 : 1) affords perfluorohydrocarbons and Te together with perfluoroalkyl trimethylstannyl tellurides.RFTeSnMe3 6a ± e RF=CF3 (a), C2F5 (b), (CF3)2CF (c), C3F7 (d), C4F9 (e). It is noteworthy that the yields of compounds 6 with a ditelluride :Me3SnH molar ratio of 1 : 2 are much lower due to their reaction with Me3SnH.2 Me3SnTeSnMe3 . RFTeSnMe3+Me3SnH 7RFH Reactions of diorganyl tellurides with organoelement hydrides proceed with the cleavage of one or two C7Te bonds resulting in compounds 1 or bis[triorganylsilyl (germyl, stannyl)] tellurides (R3E)2Te 7. Sometimes, mixtures of both types of compounds are formed.The nature of the reactants is the main factor which determines the direction of these reactions. Thus in the case of dialkyl tellurides, a crucial role is played by the nature of the hydrides R3EH the reactivities of which increase in the following order: Si<Ge<Sn. The reaction of triethylsilane with diethyl telluride (200 8C, 7 h) affords a mixture of compounds 1q and 7a (yields 12% and 55%, respectively). The reaction of triethylgermane with the same telluride (140 8C, 7 h) gives a mixture of compounds 1r and 7b in comparable yields (39% and 59%, respectively), whereas a similar reaction of triethylsilane yields only telluride 7c even at 20 8C.25, 26 R13 EH+R22 Te 7RH 3 + (R13 E)2Te 7a ± c R2TeER1 1q,r R1=R2=Et: E=Si (1q, 7a), Ge (1r, 7b), Sn (7c).In contrast to dialkyl tellurides, the reaction of bis(perfluoro- alkyl) tellurides with trimethylstannane (molar ratio 1 : 1) gives exclusively perfluoroalkyl trimethylstannyl tellurides 6b,d,e in 50%± 65% yields.2, 3 Et2O,7196 8C to 0 8C (RF)2Te+Me3SnH 7RFH RFTeSnMe3 6b,d,e RF=C2F5 (b), C3F7 (d), C4F9 (e). Irrespective of the nature of the hydrides R3EH, their reaction with aryl cyclohexyl tellurides leads to the cleavage of only the Csp37Te bond. Thus tellurides 8a ± f were prepared by heating an equimolar mixture of the reactants in benzene in the presence of catalytic amounts of azoisobutyronitrile (AIBN).6, 7, 27, 28 Com- pounds 8 were not isolated due to their low stabilities and were used in subsequent conversions (see Section II.2) in situ.PhH, AIBN cyclo-C6H11TeAr +R3EH 80 8C ArTeER3+cyclo-C6H12 8a ± f R Ref. Ar E Compound 8 6, 7, 28 6, 7, 28 28 28 27 28 Ph 4-FC6H4 Ph 4-FC6H4 Ph 4-FC6H4 Si Si Ge Ge Sn Sn Me3Si Me3Si Bu Bu Bu Bu abcdef In contrast to tris(trimethylsilyl)silane, triphenylsilane does not split the C7Te bonds in aryl cyclohexyl tellurides.28 It is of note that reaction of alkyl (or cycloalkyl) phenyl tellurides with triphenylstannane was proposed 29 as an efficient preparative procedure for the synthesis of alkanes (or cyclo- alkanes) in high yields long before the publication of the works.6, 7, 27, 28Synthesis and reactions of organic compounds containing bonds of Te to Group 14 elements PhH, 25 8C RH.PhTeR+Ph3SnH 7PhTeSnPh3 In all probability, this reaction follows a radical mechanism.29 Ph3Sn., Ph3SnH R.+PhTeSnPh3 , Ph3Sn.+PhTeR RH+Ph3Sn.. R.+Ph3SnH Germyl (9) and stannyl (1h) derivatives were prepared in nearly quantitative yields by exchange reactions of organyl trimethylsilyl tellurides with the corresponding fluorogermane or chlorostannane.17 MeTeSiMe3+H3GeF 7Me3SiF MeTeGeH3 , 9 PhTeSiMe3+Me3SnCl 7Me3SiCl PhTeSnMe3 . 1h A similar reaction was used in the synthesis of organyl silyl tellurides 10a,b.17 RTeSiMe3+H3SiX 7Me3SiX RTeSiH3 10a,b R=Me (a), Ph (b); X=Br, I. The attempts to synthesise compounds of the type 10 by the reaction of lithium methane(benzene)tellurolate with BrSiH3 were unsuccessful, bis(silyl) telluride (H3Si)2Te being the only reaction product.17 Other reactions resulting in compounds 1 are represented by single examples.Thus ethyl (triethylgermyl) telluride (1r) was synthesised by reaction of diethyl ditelluride with triethylgermyl- lithium (yield 89%) 20 or from the same ditelluride and bis(trie- thylgermyl)mercury (yield 73%).30 Et2Te2+Et3GeLi 7EtTeLi EtTeGeEt3 . 1r Et2Te2+(Et3Ge)2Hg 7Hg Methyl tris(trimethylsilyl)silyl telluride (1s) was synthesised by treatment of lithium tris(trimethylsilyl)silanetellurolate with trimethyltelluronium iodide.31 Photolysis of a hexabutyldistanna- ne ± dimethyl telluride mixture yields methyl tributylstannyl tel- luride (1t).32 (Me3Si)3SiTeLi +Me3Te+I7 7Me2Te,7LiI (Me3Si)3SiTeMe, 1s Me2Te hn Bu3Sn Bu3SnSnBu3 7Me MeTeSnBu3 .1t 2. Reactions of compounds R1TeER23In most cases, reactions of compounds 1 were studied using silyl and stannyl derivatives as examples. Some of these reactions represent valuable procedures for preparative synthesis of various organotellurium compounds including those used in synthetic organic chemistry. The Te7Si bond in organyl triorganylsilyl tellurides is easily cleaved under the action of hydrohalic acids, alcohols and water to give organyl tellurols and halogenosilanes, alkoxysilanes or silox- anes, respectively.17 R1TeH+Me3SiX R1TeSiMe3+HX R1=Me, Ph; X=Br, I, OAlk, OH.Benzenetellurol in situ manifests strong reducing proper- ties.33 ± 35 Several methods for the synthesis of this compound are known, they include methanolysis of phenyl trimethylsilyl tellur- ide 33 ± 35 and reduction of diphenyl ditelluride with hypophos- 101 phorous acid or sodium borohydride.33 The former approach is especially convenient. Phenyltellurol thus generated reduces nitro compounds into amines (in benzene or chloroform),33, 34 alkenes and alkynes into alkanes,34 aldehydes and ketones into alcohols34 and reduction of carbonyl compounds in alcohols in the presence of catalytic amounts of ZnI2 affords ethers.35 These reactions proceed under mild conditions and in high yields. Compounds 1 readily undergo disproportionation.Thus methyl germyl telluride disproportionates into di(germyl) telluride (7d) and dimethyl telluride, whereas disproportionation of methyl silyl telluride yields bis(methyltelluro)silane and silane.17 E=Ge (H3Ge)2Te+Me2Te 7d MeTeEH3 E=Si SiH4+H2Si(TeMe)2 The disproportionation of methyl trimethylstannyl telluride upon long-term storage or irradiation at 488 nm follows the same scheme as in the case of methyl germyl telluride, bis(trimethyl- stannyl) telluride and dimethyl telluride being formed.17 Pyrolysis of perfluoroalkyl trimethylstannyl tellurides 6 pro- ceeds in quite a different manner.1±3 This results in trimethyl- fluorostannane and telluroketones 11, which easily dimerise into the 1,3-ditelluretane derivatives 12 and 13.1±3 D 7Me3SnF R1(R2)C Te 11a ± e Me3SnTeCF(R1)R2 6a ± e Te R2 Te R1 R1 R1 + Te R1 Te R2 R2 R2 12a ± e 13a ± e R1=F: R2 = F (a), CF3 (b), C2F5 (d), C3F7 (e); R1=R2=CF3 (c).Heating of tellurides 6a ± d in chloroform with a large excess of 2,3-dimethylbutadiene yields initially the telluroketones 11a ± d, which then enter into a [4+2]-cycloaddition reaction 1±3 to give 5,6-dihydro-3,4-dimethyl-2H-tellurine derivatives 14a ± d in 70%± 90% yields. Me Me CHCl3, D 7Me3SnF R1(R2)C Te 11a ± d Me3SnTeCF(R1)R2 6a ± d Me Te R1 R2 Me 14a ± d R1=R2 = F (a); R1=F, R2=CF3 (b); R1=R2=CF3 (c); R1=F, R2=C2F5 (d). Phenyl trimethylsilyl telluride cleaves lactones and ethers under mild conditions (dichloromethane, 20 8C) in the presence of catalytic amounts of ZnI2 to give C-tellurination and O-silyla- tion products.18 Thus acids 16 are prepared from lactones 15 and PhTeSiMe3 in>80% yields.Intermediate O-trimethylsilyl esters 17 are hydrolysed upon work-up of reaction mixtures. O CH2Cl2, ZnI2, 20 8C O +PhTeSiMe3 (CH2)n 15 H2O PhTe(CH2)n+1CO2H PhTe(CH2)n+1CO2SiMe3 17 16 n=2±4. Phenyl trimethylsilyl telluride markedly exceeds related sul- fur- and selenium-containing compounds in reactivity. The lac-102 tone ring opening under the action of telluride occurs at 20 8C (PhSSiMe3 reacts at 70 8C,36 whereas PhSeSiMe3, at corresponding metal tellurolates. Thus the reaction of phenyl 80 ± 100 8C).37 Oxirane and other cyclic ethers are cleaved with the same efficiency under the action of phenyl trimethylsilyl telluride to give trimethylsilyloxy-substituted tellurides 18.18 Other examples of such reactions have also been described.18 O R OSiMe3 CH2Cl2, ZnI2, 0±40 8C R +PhTeSiMe3 (CH2)n+1TePh (CH2)n 18 R=Me, Et, n=0;R=H, n=1±4;R=Me, n=2.Organyl trimethylsilyl tellurides RTeSiMe3 are synthetic equivalents of organyl tellurolate anions RTe7 (see Refs 4 ± 9). In some cases, the use of tellurides gives better results in compar- ison with alkali-metal tellurolates. Thus the reaction of MeTeSiMe3 , generated in situ from MeTeLi and Me3SiCl, with arylazo sulfones in MeCN containing catalytic amounts of 18-crown-6 afforded aryl methyl tellurides in 36% ±62% yields.8 MeTeSiMe3+RC6H4N2SO2C6H4Me-4 MeCN, 20 8C, 18-C-6 RC6H4TeMe R=H, 4-Br, 4-COMe, 2-CO2Me.The use of MeTeLi prepared from MeLi and Te in THF does not lead to aryl methyl tellurides in this reaction. Deazosulfona- tion in THF does not take place, but substitution of MeCN for THF yields a mixture of dimethyl telluride and dimethyl ditellur- ide which are inert with respect to arylazosulfones.8 Under mild conditions (THF, 20 8C), phenyl trimethylsilyl telluride reacts with aroyl chlorides to give phenyl telluroben- zoates 19a ± i (yield 62%± 91%).9 The advantages of this proce- dure in comparison with the classical methods based on the use of sodium arenetellurolates lie in higher yields of the esters formed and the simplicity of their isolation from reaction mixtures.38 THF, 20 8C PhTeSiMe3+ArCOCl 7Me3SiCl ArCOTePh 19a ± h Ar=4-ClC6H4 (a), 3-ClC6H4 (b), 4-BrC6H4 (c), 4-NO2C6H4 (d), 4-NCC6H4 (e), Ph (f), 4-MeC6H4 (g), 4-MeOC6H4 (h).THF, 20 8C PhTeSiMe3+1-ClOCC6H4COCl-4 7Me3SiCl PhTeOCC6H4COTePh. 19i Aryl tris(trimethylsilyl)silyl tellurides 8a,b used in situ react with chloroformates exclusively in the presence of catalytic amounts of tetrakis(triphenylphosphine)palladium(0) to give aryl tellurocarbonates 20a ± j 6, 7 (for the use of compounds 20a ± j as precursors of oxyacyl and alkyl radicals see Refs 39, 40). Aryl tellurocarbonates 20e,j with a phenyl radical at the ester oxygen atom can be prepared 6, 7 only from compounds 8a,b. They could not be obtained in the reaction of chloroformates with sodium arenetellurolate.6, 7 PhH, (Ph3P)4Pd, 20 8C 7(Me3Si)3SiCl ROC(O)TeAr.20a ± j ArTeSi(SiMe3)3+ROC(O)Cl 8a,b Ar Ar R R Compo- und 20 Compo- und 20 Me Pri cyclo-C6H11 C8H17 Ph Ph Ph Ph Ph Ph Me Pri cyclo-C6H11 C8H17 Ph 4-FC6H4 4-FC6H4 4-FC6H4 4-FC6H4 4-FC6H4 fghij abcde I D Sadekov Compounds 1 react with some metal halides to give the triphenylgermyl(stannyl) tellurides 1e,i with the palladium(II) chloride ± benzonitrile complex results in elimination of Ph3ECl followed by the formation of red-brown palladium tellurolates 21 which seem to possess polymeric structures.4 CHCl3, 20 8C [(PhTe)2Pd]n 21 PhTeEPh3+Pd(PhCN)2Cl2 1e,i 7Ph3ECl, 7PhCN E= Ge (1e), Sn (1i).7Ph3ECl The reaction of compounds 1 with CuCl in chloroform 4 or acetonitrile 41 gives polymeric copper(I) tellurolates 22. ArTeEPh3+CuCl 1 (ArTeCu)n 22 Ar=Ph, 4-EtOC6H4; E=Ge, Sn, Pb. Metal tellurolates are also formed in those cases where phenyl trimethylsilyl telluride is used as a source of the ligand PhTe. Thus reaction of this compound with dimethylcadmium inCH2Cl2 gave cadmium(II) benzenetellurolate (23).10 20 8C PhTeSiMe3+CdMe2 7Me4Si (PhTe)2Cd. 23 Reaction of organyl trimethylsilyl tellurides, particularly, BuTeSiMe3, with copper(I) salts results in tellurium-containing clusters which were characterised by the X-ray diffraction method.5 a [Cu11(m3-TeBu)7(m4-TeBu)2(m7-Te)(PPh3)5] b [Cu18(m3-Te)6Te6(PPr3)8] BuTeSiMe3+CuX c [Cu23Te13(PEt3)12] d [Cu58Te32(PPh3)16] X=Cl, OAc; (a) PPh3, THF; (b) PPr3, diglyme; (c) PEt3, THF; (d ) PPh3, diglyme. The cluster 24 is the only reaction product of phenyl trime- thylsilyl telluride with a complex cobalt(II) salt.5 THF, 20 8C PhTeSiMe3+[CoCl2(PPh2Pr)2] [Co6(m-Te)8(PPh2Pr)6].24 It has been shown recently 42 that compounds RTeEMe3, similar to (Me3E)2Te (E=Si, Sn), enter into oxidative addition with triethylphosphine platinum complexes Pt(PEt3)n (n=3, 4) to give complexes 25a ± g in 76% ± 100% yields. PEt3 C6H14, 25 8C, 10 min RTe RTeEMe3+Pt(PEt3)3 Pt EMe3 7Et3P 25a ± g PEt3 E=Si: R=Ph (a), 4-PhC6H4 (b), Bu (c), Me3Si (d); E=Ge, R=Ph (e); E=Sn: R=Ph (f), Me3Sn (g).The structures of compounds 25 were confirmed by NMR spectroscopy and the X-ray diffraction analysis using complex 25b 42 as an example. They represent solid red-coloured substan- ces stable under an argon atmosphere but extremely sensitive to oxygen and moisture.42 The reactivities of phenyl trimethylsilyl chalcogenides PhM(SiMe3)3 (M=S, Se, Te) in this reaction increase in the following order: S<Se<Te. Thus phenyl trimethylsilyl sulfide does not react with Pt(PEt3)3 even after 5-h heating at 50 8C, whereas phenyl trimethylsilyl selenide reacts with the Pt(0) com- plex at 25 8C, albeit at a much slower rate than its tellurium analogue.42Synthesis and reactions of organic compounds containing bonds of Te to Group 14 elements PEt3 PEt3 PhH Pt Ph+Ph PhTe PhTe Pt TeSiMe3+ 25a PEt3 PEt3 Complexes 25 are gradually decomposed in benzene even at 25 8C.The following decomposition products of compound 25a have been identified:42 PEt3 Pt SiMe3 PEt3 +PhSiMe3+(Me3Si)2Te. The decomposition reaction is reversible, i.e., the addition of even 1 equiv. of PEt3 inhibits this process.42 III. Methods of synthesis and reactions of compounds (R3E)2Te (E=Si, Ge, Sn, Pb) Compounds containing two bonds of Te to Group 14 elements include both symmetrical (R3E)2Te (E is a Group 14 element) (7) and non-symmetrical derivatives of the type R13 ETeER23 (26). The methods of their syntheses and reactions of both types of compounds are very similar and will therefore be considered in the same section.1. Methods of synthesis Two reactions affording tellurides 7 are described in Section II. The first of them is the reaction of hydrides R3EH with diorganyl tellurides. Depending on the nature of substrates, this reaction gives compounds 1 and 7 or their mixtures.25, 26 The second reaction is disproportionation of tellurides 1.17 The addition of Me3SiCl to an ethereal solution of PhTeMgBr leads to the formation of bis(trimethylsilyl) telluride,43 apparently also as a result of disproportionation of PhTeSiMe3 under the reaction conditions. It should be noted that the same reactants in THF (see Section II.1) give PhTeSiMe3. The general method for the synthesis of both symmetrical (7) and non-symmetrical (26) tellurides is based on the reaction of organolithium derivatives R13 E1TeLi with chlorides R23 E2Cl.44 ± 48 However, this method is only seldom employed, e.g., in the synthesis of phenyl derivatives containing various combinations of Ge, Sn and Pb atoms.This reaction was also used in the synthesis of bis(triphenylsilyl) telluride.49 Ph3E1TeLi +Ph3E2Cl 7LiCl Ph3E1 Te E2Ph3 . 7e ± g, 26a ± c Compound Ref. E2 E1 7e 7f 7g 26a 26b 26c 46 44, 45 47 46 45 46 Ge Sn Pb Ge Pb Ge Ge Sn Pb Sn Sn Pb Non-symmetrical derivatives 26d ± f are also obtained by reactions of triethyltin or -germanium hydrides with tellurides 1q,r (yields 60%± 90%) 26 or 7a,b.50 D 7C2H6 Et3E1TeE2Et3 26d,e EtTeE1Et3+Et3E2H 1q,r D 7Et3E1H Et3E1TeE2Et3 26e,f (Et3E1)2Te+Et3E2H 7a,b E1=Si (1q, 7a), Ge (1r, 7b); E1=Si, E2=Sn (26d); E1=Ge, E2=Sn (26e); E1=Si, E2=Ge (26f).In these reactions, the reactivities of the hydrides decrease in the following order: Sn>Ge>Si. Thus the reaction of Et3GeH with (Et3Si)2Te yields (Et3Ge)2Te (yield 79%) and Et3SiH (yield 50%) at 235 8C for 6 h, while the reaction of Et3SnH with 103 (Et3Ge)2Te results in (Et3Sn)2Te (yield 76%) and Et3GeH (yield 57%) at 70 8C for 1 h.50 Reactions of the mercury derivatives (Et3E1)2Hg with the hydrides R3E2TeH also yield non-symmetrical tellurides 26,51 however, this method has been studied relatively little. Reaction of bis(triethylgermyl)mercury with Et3SiTeH (molar ratio 1 : 1) yields compound 27 with the Si7Te7Hg7Ge fragment, which can easily eliminate the mercury atom to yield the non-sym- metrical telluride 26f under mild conditions (778 8C, 2 ± 3 min).20 8C Et3SiTeH+(Et3Ge)2Hg 7Hg 7Et3GeH Et3SiTeHgGeEt3 27 Et3SiTeGeEt3 . 26f It is of note that in contrast to compound 27, bis(triethylsi- lyltelluro)mercury (28) formed with a tellurol : mercury molar ratio of 2 : 1 is thermally stable and is not decomposed even on heating to 130 8C for 7 h.51 Et3SiTeH+(Et3Ge)2Hg 7Et3GeH Et3SiTeHgTeSiEt3 . 28 The synthesis of symmetrical tellurides 7 has been studied in much more detail than that of the non-symmetrical analogues 26. Reaction of sodium 52 or lithium 21, 53 ± 57 tellurides with the halides R3EX is the most general approach to their synthesis. The yields of tellurides 7 vary from very low (5%) 57 to quantita- tive.55 M2Te+R3EX 7MX (R3E)2Te 7h ± n M=Na, Li.Ref. Compound 7 E R3 53, 55 21, 52 ± 54, 56 56 57 52 52 52 H3 Me3 ButMe2 (CF3)3 Me3 Me3 Me3 hijklmn Si Si Si Ge Ge Sn Pb Reaction of 1,10-bis(chlorodimethylstannyl)ferrocene 58 with lithium telluride was used to obtain 1,3-distanna-2-tellura[3]fer- rocenophane 29 (yield 28%).59 Me2 Sn SnMe2Cl THF Fe Fe +Li2Te Te 7LiCl SnMe2Cl Sn Me2 29 Sodium hydrotelluride was employed in the synthesis of bis(triphenylstannyl) telluride (7f).19, 60 Some digermyl tellurides were prepared using hydrogen telluride as the tellurium-containing reagent.61 Thus warming of a mixture of carbodiimides 30 with an excess of H2Te from 7196 8C to room temperature results in bis(germyl) tellurides 31 in 60% ±90% yields.61 MenH37nGe 7H2NCN N C N GeMenH37n+H2Te 30 (MenH37nGe)2Te 31 n=1±3.Silicon analogues of carbodiimides 30 do not enter into such reactions.61 The symmetrical tellurides 7a ± c were obtained by heating hydrides R3EH with tellurium powder.26, 50, 62, 63 As in the pre- vious cases, the reactivities of the hydrides increase in the follow- ing order: Si<Ge<Sn. Whereas the reaction of tellurium with104 Et3SnH proceeds at 130 8C26, 62 and that with Et3GeH, at 190 ± 210 8C,26, 62 the reaction of Te with Et3SiH resulting in bis(triethylsilyl) telluride 7a in 71% yield can be realised only at 280 8C.50, 63 Te+Et3EH D 7H2 (Et3E)2Te 7a ± c E=Si (a), Ge (b), Sn (c). The reaction of 1,10-bis(dimethylstannyl)ferrocene 58 with tellurium powder was used for the synthesis of telluride 29 (yield 90%);59 the same compound was obtained in 79% yield by reaction of 1,1,2,2-tetramethyl-1,2-distanna[2]ferrocenophane with tellurium.59 SnMe2H Fe SnMe2H Te, PhMe, D 29 SnMe2 Fe SnMe2 Depending on the nature of the hydride and the chalcogen, reactions of sulfur and selenium with hydrides R3EH yield either organoelement hydrochalcogenides R3EMH [E=Si,50, 63 Ge;26 M=S,63 Se (see Refs 26, 50 and 63)] or their mixtures with compounds (R3E)2M.The stabilities of hydridesR3EMHincrease in the following order: Te<Se<S.Thus heating of sulfur with triphenylsilane at 200 8Cgives triphenylsilanethiol Ph3SiSH as the only product (yield 70%),63 whereas reaction of selenium with triethylsilane (260 8C) results in a mixture of Et3SiSeH (yield 62%) and (Et3Si)2Se (yield 32%).50, 63 The latter represents a symmetrisation product.63 (Et3Si)2Se+H2Se. Et3SiSeH Whereas triethylsilanetellurol Et3SiTeH, which is the least stable of all chalcogenols, is symmetrised completely, the degree of conversion of more stable triethylsilaneselenol into bis(triethyl- silyl) selenide and H2Se is only 40%.63 It should be noted that reaction of selenium with germanes R3GeH (R=Pri, cyclo-C6H11) yields predominantly germane selenols R3GeSeH.Tributylphosphine telluride Bu3P=Te, a synthetic equivalent of metallic tellurium, was used instead of the latter in reactions with triorganylstannyl hydrides.64 Its solubility in organic sol- vents makes it possible to synthesise bis(triorganylstannyl) tellur- ides, e.g., bis(tributylstannyl) telluride 7o, under mild conditions in the presence of catalytic amounts (2 mol.% ± 10 mol.%) of Cp2 TiH (Cp*=Me5C5). In the absence of a catalyst, the yield of the target product does not exceed 2%.64 C6D6, 25 8C, Cp2 TiH Te +Bu3SnH Bu3P 7H2,7Bu3P (Bu3Sn)2Te. 7o Tellurides 7 (E=Ge, Sn) were also synthesised by exchange reactions of compounds 7 containing more light weight Group 14 elements with the halides R3EX.Thus bis[tris(pentafluorophe- nyl)stannyl] telluride 7p was obtained (in 57% and 51% yields, respectively) by reaction of bis(triethylgermyl) (7b) or bis(triethyl- silyl) tellurides (7a) with (C6F5)3SnBr.65 This reaction seems to involve stepwise exchange of Et3E groups (E=Si, Ge) for (C6F5)3Sn groups and proceeds through four-membered transi- tion states. I D Sadekov PhMe, D 7Et3EBr (Et3E)2Te+(C6F5)3SnBr 7a,b [(C6F5)3Sn]2Te 7p E=Si (a), Ge (b). With an excess of germyl bromide, di(silyl) telluride (7q) is quantitatively converted into di(germyl) telluride (7d), apparently, by the same mechanism.55 0 8C, 18 h 7H3SiBr (H3Ge)2Te. 7d (H3Si)2Te+H3GeBr 7q Other methods of synthesis of tellurides 7 were studied in separate reactions, although some of them can be used for the synthesis of a larger number of compounds. Thus bis(triisopro- pylgermyl) telluride (7r) was obtained in 78% yield by reaction of bis(triisopropylgermyl)mercury with tellurium powder.30 THF, 20 8C, 30 h (Pri3Ge)2Te.(Pri3Ge)2Hg+Te 7Hg 7r Bis(trifluorosilyl) telluride (7s) was prepared by reaction of trifluorosilyl radicals generated by decomposition of hexafluoro- disilane with tellurium vapour in a low-temperature matrix.66, 67 F3Si.+Te (F3Si)2Te. 7s (25%) The synthesis of silyl derivatives of the type 7, which contain sterically hindered silane residues, requires special strategies. Thus bis(triphenylsilyl) telluride (7t) was obtained by protonation of lithium triphenylsilanetellurolate (32) in THF or by oxidation of this salt with atmospheric oxygen.49 CF3CO2H, THF (Ph3Si)2Te. 7t Ph3SiTeLi .3THF 32 It should be noted that oxidation of tris(trimethylsilyl)silane- tellurol (Me3Si)3SiTeH or its lithium salt results in the ditelluride [(Me3Si)3Si]2Te2.68, 69 The symmetrical telluride 33 is formed together with the corresponding metal tellurides by pyrolysis of tellurolates 34.16 D [(Me3Si)3Si]2Te+MTe 33 [(Me3Si)3SiTe]2M 34 M=Zn, Cd, Sn, Pb, Yb. The symmetrical and non-symmetrical tellurides considered above containing two Te7E bonds have identical substituents on the Group 14 elements. Presumably, telluride 35 69 synthesised from lithium tris(trimethylsilyl)silanetellurolate and trimethyl- chlorosilane is the only compound which has different substitu- ents on these atoms.(Me3Si)3SiTeLi . 2THF+Me3SiCl 7THF,7LiCl (Me3Si)3SiTeSiMe3 . 35 2. Reactions of compounds (R3E)2Te It should be noted that the majority of reactions of tellurides 7 and 26 have been studied for a limited number of compounds only; therefore, it is sometimes difficult to judge the general type of one or another reaction. Most reactions of tellurides 7 (reactions of the non-symmet- rical derivatives 26 have been studied relatively little) occur with elimination of tellurium. The hydride derivatives (H3E)2Te are photosensitive and are decomposed on rough glass surfaces even at room temperature,53 while bis[tris(trifluoromethyl)germyl] tel- luride (7k) eliminates tellurium only at 115 8C.57 Tellurides 7Synthesis and reactions of organic compounds containing bonds of Te to Group 14 elements containing alkyl and trifluoromethyl radicals are rather easily hydrolysed.57 115 8C (CF3)3GeGe(CF3)3+Te [(CF3)3Ge]2Te H2O [(CF3)3Ge]2O+CF3GeH+Te 7k Symmetrical (7e ± g) and non-symmetrical phenyl derivatives (26a ± c) are thermally and hydrolytically stable compounds. Their stabilities with respect to moisture and oxygen increase in the following order: (Ph3Ge)2Te<(Ph3Pb)2Te and (Ph3Ge)2Te< Ph3GeTeSnPh3<Ph3GeTePbPh3.47 The non-symmetrical ali- phatic derivatives R13 E1TeE2R23 easily disproportionate to give the symmetrical tellurides (R13 E1)2Te and (R23 E2)2Te.70 The Te7E bonds in tellurides 7 are easily cleaved under the action of various oxidants.Thus the silyl derivatives 7h,i react with oxygen to give elementary tellurium and the corresponding siloxane.53 (R3Si)2O+Te (R3Si)2Te+O2 7h,i R = H (h), Me (i). The oxidation of tellurides 7 by halogens proceeds under mild conditions and gives halides R3EX (yields 50%± 85%) and Te; some amount of R2SnX2 is formed in the case of stannyl derivatives.56, 71 PhH, 0 8C R3EX+Te (R3E)2Te+X2 7a ± c,i R=Et: E=Si (a), Ge (b), Sn (c); R=Me, E=Si (i); X=Cl, Br, I. The reactions of the tellurides 7 with Ag(I) 43 and Hg(II) 51 salts also afford halides R3EX and, in addition, metal tellurides. Note- worthy, the reactions of these salts with diorganyl tellurides R2Te give complexes with the compositions 1 : 1 or 1 : 2.72 THF, 20 8C Et3ECl+HgTe (Et3E)2Te+HgCl2 7a,b E=Si (a), Ge (b).20 8C Me3SiI+Ag2Te. (Me3Si)2Te+AgI 7i It has been shown for the first time 63 that the cleavage of E7Te bonds in compounds 7 is effected not only by free halogens but also by organic dihalides containing rather active halogen atoms. Thus boiling of ethyl derivatives 7a ± c with 1,2-dibromo- ethane leads to the formation of Et3EBr, Te and ethylene.63 The reaction with bis(triethylstannyl) telluride (7c) proceeds most smoothly. 130 ± 150 8C CH2 Et3EBr+CH2 7Te (Et3E)2Te+BrCH2CH2Br 7a ± c E=Si (a), Ge (b), Sn (c). It is this reaction in particular that underlies the synthesis of alkenes from vicinal dibromides using bis(triphenylstannyl) tel- luride (7f) and CsF as a dehalogenating reagent.73 CsF, MeCN, 20 8C, 3 ± 4 h 7Ph3SnBr,7Te (Ph3Sn)2Te+R1CH(Br)CH(Br)R2 7f R1CH CHR2 R1=H:R2=Ph, n-C8H17 , PhSO2CH2; R1=R2=Ph; R17R2=CH2OCH2OCH2 .This reaction proceeds under mild conditions; alkenes are most frequently formed in virtually quantitative yields. Thus the with of reaction meso-1,2-dibromo-1,2-diphenylethane (Ph3Sn)2Te results in trans-stilbene in 94% yield.73 105 Upon substitution of KF.H2O for CsF, telluride 7f effects dehalogenation of a-chloro(bromo) ketones under mild condi- tions (MeCN, 20 8C).73 With a molar reagent ratio 7f:KF.2 H2O: a-halogeno ketone of 1 : 3 : 1, the ketones are formed in 60%± 90% yields. The feasible mechanism of this reaction includes the intermediate formation of triphenylstanna- netellurol.74 R1R2C(X)C(O)R3 KF.2H2O [Ph3SnTeH] (Ph3Sn)2Te 7Ph3SnX,7Te 7fR1R2CHCOR3 R1=R2=H:R3=Me, Et, Ph; R1=Me, R3=Ph: R2=H, Me; R17R3=(CH2)4, R2=H.Cleavage of the Te7E bonds in tellurides 7a ± c proceeds smoothly under the action of symmetrical and non-symmetrical acyl peroxides.75 This reaction gives high yields of elementary tellurium and triorganylsilyl (germyl, stannyl) esters of the corre- sponding acids. Et3EOCOR1+Et3EOCOR2 (Et3E)2Te+R1CO3COR2 7Te 7a ± c E=Si (a), Ge (b), Sn (c); R1=Ph, R2=Ph, Me. 7Te Reaction of bis(triethylgermyl) telluride (7b) with dicyclo- hexyl percarbonate yields CO2 and cyclohexyloxytriethylgermane together with Te, apparently as a result of decomposition of the initially formed triethylgermyl cyclohexanecarboxylate.75 (Et3Ge)2Te+(cyclo-C6H11OCO2)2 7b [cyclo-C6H11OCO2GeEt3] 7CO2 cyclo-C6H11OGeEt3 .Like diaryl tellurides Ar2Te,72 tellurides 7a,c eliminate Te upon heating with sulfur or selenium to give sulfides and selenides, respectively.50, 63 As in the reaction with diaryl tellurides,72 sulfur surpasses selenium in activity. Thus reaction of (Et3Sn)2Te with sulfur proceeds at 20 8C (90 h), whereas reaction of the same telluride with selenium requires heating at 150 8C for 30 h.50, 63 D (Et3E)2M 7Te (Et3E)2Te+M 7a,c E=Si, M=S; E=Sn:M =S, Se. Formally, reduction of diorganyl chalcogen oxides by bis(trialkylsilyl) tellurides is related to the same class of reac- tions.56 This can be regarded as a preparative method for the synthesis of chalcogenides from chalcogen oxides because of their high yields (80% ± 100%) and easy separation from other prod- ucts, viz., hexaalkyldisiloxanes and elementary tellurium.It should be noted that bis(trimethylsilyl) sulfide and selenide are also rather efficient reagents.56 THF R22 M+(R13 Si)2O (R13 Si)2Te+R22 MO 7Te R1=Me: R2=Ph,M=Te; R2=Me,M=S; R13 =ButMe2: R2=Ph,M=Se; R2=Me,M=S. The use of bis(triphenylstannyl) telluride (7f) for monodesul- furisation of organic trisulfides 76 and synthesis of aryl triphenyl- stannyl sulfides 77 is based on easy elimination of the Te atom from compounds 7. Various types of organic trisulfides (aryl-, benzyl-, alkyl-) are readily converted into the corresponding disulfides under the action of (Ph3Sn)2Te.The rate of the desulfurisation reaction increases with an increase in the solvent polarity.106 MeCN, 20 8C 7Te R S S S R+(Ph3Sn)2Te 7f R S S R +(Ph3Sn)2S R=FC6H4, MeC6H4, Bn, PhCHMe, Me(CH2)2 . Other bis(triphenylstannyl) chalcogenides (Ph3Sn)2M (M=S, Se) can monosulfurise organic trisulfides; the reactivities of chalcogenides (Ph3Sn)2M decrease in the following order: Te>Se>S. Bis(triphenylstannyl) selenide causes desulfurisa- tion of only diaryl and dibenzyl trisulfides, whereas desulfurisa- tion by bis(triphenylstannyl) sulfide proceeds with very low yields and only upon long-term boiling of the reaction mixture.76 Reaction of telluride 7f with diaryl disulfides under mild conditions (MeCN, 20 8C) results in aryl triphenylstannyl sulfides 36 in high, sometimes quantitative, yields.77 Dialkyl and dibenzyl disulfides do not enter into such reactions.Compounds 36 can be synthesised from diaryl disulfides and bis(triphenylstannyl) sele- nide.77 However, the latter is much less efficient than tellurides. 4-Methylphenyl triphenylstannyl sulfide is formed in a quantita- tive yield upon treatment of the corresponding disulfide with telluride 7f and in only 28% yield when a selenium derivative is used, other conditions (solvent, temperature, reaction time) being the same.77 MeCN, 20 8C, 3 h 7Te Ar2S2+(Ph3Sn)2Te 7f ArSSnPh3 36 Ar=4-ClC6H4, 4-FC6H4, Ph, 4-MeC6H4, 2-C10H7.If organyl trimethylsilyl tellurides RTeSiMe3 represent syn- thetic equivalents of organyltellurolate anions RTe7 (see Section II.2), compounds 7 and particularly bis(triphenylstannyl) telluride 7f act as synthetic equivalents of the telluride anion Te27 (see Refs 73, 78). Bis(triphenylstannyl) telluride 7f used in combination with CsF (4 equiv.) is a mild tellurinating reagent.73, 78 Reactions of this reagent with organic bromides and iodides in MeCN at 20 8C afford symmetrical diorganyl tellurides in high yields.73, 78 Aryl iodides and alkyl chlorides are inert with respect to compound 7f, which makes it possible to synthesise diorganyl tellurides contain- ing chlorine atoms.It is believed 73 that the synthesis of diorganyl tellurides proceeds in two steps and includes an attack of the fluoride ion on the tin atom. F7 Ph3Sn Te R X 7Ph3SnF, 7X7 Ph3Sn 7f R RTeR R X Te 7Ph3SnF,7X7 Ph3Sn F7 X=Br, I; R=Pri, Bn, Cl(CH2)6 , Me(CH2)9 , PhCOCH2, CH2CO2Et, CH2CO2Me, CH2CO2Prn, CH2CO2But, (CH2)3CO2Et. Reactions of bis(trialkylsilyl) tellurides with certain organo- element compounds occur with the cleavage of both Te7Si bonds and lead to the formation of tellurides of the corresponding metals rather than elementary Te; these reactions are of special interest. Thus reactions of bis(trimethylsilyl) telluride (7i) with tris(dime- thylamino)antimony or -bismuth occur under very mild condi- tions (hexane, 730 8C) and yield polycrystalline antimony and bismuth tellurides 11 which manifest semiconducting properties.M2Te3 7Me3SiNMe2 (Me3Si)2Te+M(NMe2)3 7i M=Sb, Bi. I D Sadekov Metal dialkyls can be used instead of dimethylamino deriva- tives. This method was used in the synthesis of Cd(II) telluride.10 (PriMe2Si)2Te+Me2Cd CdTe. 7PriMe3Si Reaction of bis(trimethylsilyl) telluride (7i) with copper(I) chloride in the presence of phosphines results in the formation of cluster copper complexes 79 which differ from those obtained using butyl trimethylsilyl telluride (see Section II.2). (Me3Si)2Te+CuCl 7i a [Cu16Te9(PEt3)8] b [Cu16Te9(PPhEt2)8]+[Cu28Te17(PPhEt2)12] c [Cu4Te4(PPri3)4]+[Cu23Te13(PPri3)10]+[Cu29Te16(PPri3)12] d [Cu26Te16(PBut3)10] (a) PEt3, Et2O; (b) PPhEt2, Et2O; (c) PPri3, Et2O; (d ) PBut3, THF.Reactions of tellurides 7 with other than Sb, Bi and Cd organoelement derivatives occur with the cleavage of two (less frequently, one) Te7E bonds; in this case, tellurium becomes a constituent of one of the reaction products. Bis(tributylstannyl) telluride undergoes transmetallation with trimethylaluminium.80, 81 Bis(dimethylaluminium) telluride (Me2Al)2Te used in situ is an efficient tellurinating reagent. Treatment of aldehydes, ketones 80 and N-methylformanilide 81 with this reagent results in telluroaldehydes (ketones) 80 and N-methyltelluroformanilide,81 respectively. PhMe, 80 8C (Bu3Sn)2Te+Me3Al 7Bu3SnMe R1CR2 R1R2CO (Me2Al)2Te 7(Me2Al)2O Te R1=H: R2=Prn, But, Ph, N(Me)Ph; R1R2CO�adamantanone, bicyclo[3.3.1]nonan-7-one.Tellurides (H3E)2Te react with PF2Br with the cleavage of one and two Te7E bonds to give a mixture of compounds 37 and 38.82 These compounds were not isolated; their formation was deduced from the 1H and 19F NMR spectra. (H3E)2Te+PF2Br 7H3EBr H3ETePF2 + (F2P)2Te 38 37 E=Si, Ge. At the same time, reaction of bis(trimethylsilyl) telluride 7i with ButPCl2 occurs with the cleavage of both Te7Si bonds resulting in a mixture of cyclic compounds 39 and 40.83 Te Te ButP PBut+ 7Me3SiCl PBut ButP (Me3Si)2Te+ButPCl2 7i PBut 39 40 Reactions of bis(trialkylsilyl) tellurides with acyl chlorides and bisacyl tellurides have been considered in the recently published review.84 The Te7E bond in tellurides 7 is easily cleaved by protic acids (HCl, anhydrous CF3CO2H).63 Thus the first silanetellurol, viz., triethylsilanetellurol, was obtained by treating bis(triethylsilyl) telluride (7a) with trifluoroacetic acid (molar ratio 1 : 1) at 20 8C.In the presence of an excess of the acid, the Te7Si bond of tellurol is cleaved to give triethylsilyl trifluoroacetate and elementary Te, presumably as a result of oxidation of the intermediate H2Te.63Synthesis and reactions of organic compounds containing bonds of Te to Group 14 elements CF3CO2H Et3SiTeH 7CF3CO2SiEt3 7CF3CO2SiEt3 (Et3Si)2Te+CF3CO2H 7a O2 Te. H2Te 7H2O MenH37nGeI+H2Te Reaction of germyl tellurides 31 with gaseous HI at 20 8C results in the cleavage of both Te7Ge bonds 55, 61 to give germyl iodides and H2Te.(MenH37nGe)2Te+HI 31 n=0±3. Silane- and germane-tellurols 41 are formed by an exchange reaction between the corresponding tellurides and H2Te.85 The equilibrium constants are equal to 0.90 (E=Si) and 0.95 (E=Ge). (H3E)2Te+H2Te 2H3ETeH 41 E=Si, Ge. The signals for the Te7H protons in the 1H NMR spectra of tellurols 41 are observed at very high fields. Thus dTeH of silanetellurol is equal to 77.46 ppm, while that of germanetel- lurol is77.44 ppm.85 The Te7E bonds are not affected upon complexation of tellurides 7 with some Lewis acids. The stable complexes 42a ± c (1 : 1) were prepared from bis[tris(pentafluorophenyl)germyl]cad- mium (43) and tellurides 7a ± c.86 A complex with the composition 1 : 2 was also isolated in the case of the stannyl derivative.[(C6F5)3Ge]2Cd . Te(EEt3)2 42a ± c 7a ± c [(C6F5)3Ge]2Cd+(Et3E)2Te 43 E=Si (a), Ge (b), Sn (c). The complexes 42a ± c are rapidly oxidised in air. It is note- worthy that tellurides 7a ± c form predominantly 1 : 1 complexes, while diorganyl sulfides R2S react with compound 43 to give 1 : 2 complexes. This is attributed to the decrease in the donor activity of the Te atoms of tellurides 7a ± c as a result of dp ± pp coupling of Te with the E atoms and steric hindrances.86 Tellurides 7 react with chromium, molybdenum and tungsten carbonyls (CO5)M.THF (M=Cr, Mo, W) resulting in the displacement of the tetrahydrofuran molecule and the formation of complexes 44 which are sensitive to oxygen and atmospheric moisture.52, 87 The yields of the latter are relatively low (17% ± 45%).The attempts to obtain complexes 44 by irradiation of solutions containing metal hexacarbonyls and tellurides failed due to decomposition of tellurides upon illumination. THF (Me3E)2Te+M(CO)5 .THF 7THF (Me3E)2Te .M(CO)5 44 E=Ge, Sn, Pb;M=Cr, Mo, W. Unlike bis(trimethylstannyl) selenide, which forms binuclear complexes 45 in high yields upon reaction with manganese and rhenium halogen carbonyls, bis(trimethylstannyl) tellurides (7m) react with these carbonyls to give complexes 45a,b in very low yields (e.g., the yield of compound 45a is 11%, whereas the formation of complex 45b followed only from spectroscopic data).88 Treatment of complex 45a with HCl in ether yields tellurol 46 (dTeH712.80 ppm).SnMe3 Te (CO)4M M(CO)4 7Me3SnX, 7CO (Me3Sn)2Te+(CO)5MX 7m Te 45a,b SnMe3 107 HCl 45a 7Me3SnCl (CO)4MnTeH 46 M=Mn (45a), Re (45b); X=Br, Cl. IV. Other types of non-cyclic compounds containing Te ±E bonds 2 Apart from R1TeER23 and (R3E)2Te derivatives, several non- cyclic compounds containing Te7E bonds, e.g., di(organyl- telluro)silanes (germanes, stannanes) (R1Te)2ER2 47 (see Refs 17, 19, 20) and tris(ethyltelluro)silane (48), have been described.20 Synthesis of both types of compounds was performed by reactions of the corresponding chloro derivatives of Group 14 elements with lithium tellurolates in THF19 or ether.20 2 R22 ECl2+R1TeLi 7LiCl (R1Te)2ER2 47a ± c 47a: E=Sn, R1=Ph, R2=Me; 47b: E=Sn, R1=Ph, R2=But; 47c: E=Si, R1=Et: R22 =Et, H. Et2O HSiCl3+EtTeLi 7LiCl HSi(TeEt)3 .48 Dimethylbis(methyltelluro)germane (47d) was prepared by reaction of dimethylchlorogermane with methyl trimethylsilyl telluride.17 Me2GeCl2+MeTeSiMe3 7Me3SiCl Me2Ge(TeMe)2 . 47d Reactions of compounds 47a ± d and 48 have not been studied yet. V. Cyclic compounds containing Te7E bonds Aseries of three-, four-, five- and six-membered cyclic compounds with rings built exclusively of Group 14 elements (Si, Ge, Sn) and tellurium have been described. The methods for the synthesis of rings containing 4 to 6 atoms are similar to those for non-cyclic tellurides (R3E)2Te, whereas reactions resulting in three-mem- bered cycles have no analogues.The reaction of compounds R2E=ER2 50a ± d with tellurium powder is a common procedure for the synthesis of three- membered cyclic compounds 49a ± d containing two Group 14 elements and one tellurium atom. Synthesis of telluradisilirane (49a) 89 and telluradigermirane (49b) 90 is performed in benzene at 20 and 80 8C, respectively; telluradistannirane (49c) 91 is prepared by boiling the reactants in toluene for 2 h. The yields are 66%, 80% and 71%, respectively. Te R2E R2E ER2+Te 50a ± d ER2 49a ± d E=Si, R=2,4,6-Me3C6H2 (a); E=Ge, R=2,6-Et2C6H3 (b); E=Sn, R =2,4,6-Pri3C6H2 (c); E=Ge, R=2,4,6-Me3C6H2 (d).The synthesis of the Te,Ge-cycle 49d was also carried out by boiling hexamesitylcyclotrigermane (51) with tellurium powder in toluene.90 Presumably, this reaction proceeds via the intermediate digermene 50 (E=Ge, R=2,4,6-Me3C6H2). 49d. [(2,4,6-Me3C6H2)2Ge]3+Te 51 Compounds 49a ± d represent high-melting crystalline sub- stances which are resistant against oxygen and atmospheric moisture. The germyl derivatives 49b,d manifest thermochromic108 properties: they are colourless at7196 8C but become pale yellow (20 8C) and orange (140 8C) colour.90 The structures of the cyclic compounds 49a ± c were studied by the X-ray diffraction method. Reaction of compound 49c with an excess of tellurium powder in toluene affords cyclo-1,3-ditellura-2,4-distannane 52 in a quan- titative yield.91 The same cycle is obtained in 73% yield by boiling a mixture of the corresponding distannene 50c with tellurium powder in toluene (molar ratio 1 : 3).91 Te Te PhMe, D +Te SnR2 R2Sn R2Sn SnR2 49c Te 52 PhMe, D 52 R2Sn SnR2+Te 50c R=2,4,6-Pri3C6H2.The tert-butyl analogue of compound 52, viz., cyclo-1,3- ditellura-2,4-distannane 53, was synthesised by reaction of tetra- meric di(tert-butyl)tin with tellurium as well as by reaction of But2SnCl2 with sodium telluride.92 Te (But2Sn)4+Te SnBut2 But2Sn But2SnCl2+Na2Te Te 53 The reaction similar to that used in the synthesis of tellurides (R3E)2Te (see Section III.1) was used to prepare cyclo-1,3-ditel- lura-2,4-digermane 55.Heating of the hydride 54 with tellurium powder gives a cyclic compound 55 in a nearly quantitative yield.93 It is noteworthy that reaction of the hydride 54 with selenium yields a mixture of four- and five-membered cyclic products.93 Te 220 8C GeBut (But2GeH)2+Te 2 But2Ge 54 Te 55 The synthesis of the five-membered derivativesR8E4Te (56a,b, E=Ge, Sn) was carried t using structurally similar derivatives of Group 14 elements. Octaphenyltelluratetragermacyclopentane (56a) was obtained by reaction of 1,4-diiodooctaphenyltetrager- mane with sodium hydrotelluride in a benzene ± ethanol mix- ture.94 Reaction of 1,4-diiodocta(tert-butyl)tetrastannane with H2Te in the presence of triethylamine leads to octa(tert-butyl)tel- luratetrastannacyclopentane (56b).95 Since the reaction of tri- ethylamine with H2Te yields triethylammonium hydrotelluride Er3NH+HTe7, it may be assumed that the HTe7 anion is a tellurium-containing substrate of this reaction.Sodium telluride is not suitable for the synthesis of the heterocycle 56b, since it cleaves the Sn7Sn bonds in the starting compound. Compound 53 and the five-membered tert-butyl derivative of 1,3-ditellura-2,4,5- tristannacyclopentane 57 are by-products in the synthesis of telluratetrastannacyclopentane 56b.95 ER2 R2E PhH, EtOH I(R2E)4I+NaHTe ER2 R2E Te 56a,b E=Ge, R=Ph (a); E=Sn, R=But (b). SnBut2 But2Sn PhMe, Et3N 53+56b+ Te I(But2Sn)4I+H2Te 2 Te SnBut 57 Compound 56b is rather stable upon storage, whereas its germanium analogue 56a is rapidly oxidised in air to give I D Sadekov Ph2Ge-Ph2Ge-O-GePh2-GePh2.94 According to X-ray diffraction analysis data,95 the ring in compound 56b is practically planar.2,4,5-Hexamethyl-1,3-ditellura-2,4,5-tristannacyclopentane, a representative of yet another type of five-membered cyclic compound containing Te and Sn atoms, was prepared by reaction of tellurium powder with dimethylstannane which in turn was synthesised by reduction of dichlorodimethylstannane with lith- ium aluminium hydride in an Et2O±DMF mixture (30 : 1, by volume).96 It was characterised by mass spectrometry as well as by 1H (see Ref. 96) and 119Sn NMR spectroscopy (see Ref. 97). Reduction of dichlorodimethylstannane or Me2SnO with lithium aluminium hydride in Et2O and subsequent reaction of dimethylstannane with tellurium powder results in 2,2,4,4,6,6- hexamethyl-1,3,5-tritellura-2,4,6-tristannacyclohexane (58) in 58% yield.98 SnMe2 Te Te Et2O Me2SnH2+Te 7H2 SnMe2 Me2Sn Te 58 Compound 59 was also synthesised in 62% yield by reaction of dimethyldichlorostannane with sodium hydrotelluride synthes- ised from Te and NaBH4 in water 99 and was characterised by the X-ray diffraction method 99 and mass spectrometry.98 VI.Conclusion The data presented suggest that compounds containing bonds of Te with Group 14 elements belong to a synthetically promising class of organoelement compounds. They can be used for the synthesis of organotellurium derivatives of various types, e.g., organoelement tellurols, alkyl aryl tellurides, aryl telluroben- zoates and aryl telluroformates, various telluroketones and het- erocycles.Apromising, although still insufficiently studied area of application of these compounds is the synthesis of various metal tellurolates and tellurium-containing clusters and low-temper- ature synthesis of metal tellurides.These are only a very few examples which illustrate possible applications of compounds containing Te7E bonds in organic synthesis, particularly, in the synthesis of organic sulfur derivatives and some other organoele- ment compounds. The expansion of the range of well-studied reactions, primar- ily those resulting in tellurolates and metal tellurides, a search for novel procedures for the synthesis and characterisation of various compounds, e.g., R13 ETeR2 and (R3E)2Te, elaboration of syn- thetic procedures for and studies of reactions of other types of compounds containing bonds of Te with Group 14 elements, e.g., (RTe)4E and (R1Te)nER24¡n (n=2, 3), present indisputable inter- est for both organotellurium chemistry and organic chemistry in general.This review has been written with the financial support of the Russian Foundation for Basic Research (Project Nos 99-03- 33132a and 00-15-97320) and Grant No. 2000-5-117 of the RF Ministry for Education. References 1. P Boese, A Haas, C Limberg J. Chem. Soc., Dalton Trans. 2547 (1993) 2. J Beck, A Haas,W Herrendorf, H Heuduk J.Chem. Soc., Dalton Trans. 4463 (1996) 3. M Baum, J Beck, A Haas, W Herrendorf, C Monse J. Chem. Soc., Dalton Trans. 11 (2000) 4. 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Russian Chemical Reviews 71 (2) 111 ± 139 (2002) Metal complex catalysis in organic electrosynthesis Yu H Budnikova Contents I. Introduction II. General principles of metal complex electrocatalysis in organic synthesis and methods of investigation of reaction mechanisms III. Homocoupling of organic halides IV. Cross-coupling of organic halides V. Formation of compounds containing the metal ± carbon bond VI. Addition of organic halides to unsaturated groups VII. Electrocatalytic reduction of carbon dioxide VIII. Synthesis of carboxylic acids IX. Carbonylation of organic substrates X. Electrochemical synthesis of organophosphorus compounds from chlorophosphines or white phosphorus XI. Other examples of electrochemical syntheses catalysed by metal complexes XII.Characteristic features and mechanisms of reactions catalysed by nickel and palladium complexes XIII. Conclusion Abstract. metal generated electrochemically of use the on Data Data on the use of electrochemically generated metal complex catalysts in organic synthesis are analysed, systematised complex catalysts in organic synthesis are analysed, systematised and generalised. The considerable potential of organic electro- and generalised. The considerable potential of organic electro- chemistry is shown. Some electrochemical methods provide chemistry is shown. Some electrochemical methods provide advantages in the synthesis of compounds where conventional advantages in the synthesis of compounds where conventional chemical methods fail, are experimentally complicated or have chemical methods fail, are experimentally complicated or have environmental limitations.Particular attention is given to the environmental limitations. Particular attention is given to the mechanisms of electrocatalytic reactions. The bibliography mechanisms of electrocatalytic reactions. The bibliography includes 333 references includes 333 references. I. Introduction The development of environmentally safe and resource-saving chemical and technological processes is one of the strategically important lines of research in science and engineering. Homoge- neous catalysis is finding increasing use in organic synthesis because it offers advantages over heterogeneous catalysis, partic- ularly, from the viewpoint of selectivity and efficiency.Actually, complexes of transition metals, such as Ni, Pd or Co in low oxidation states, can react with many functional groups thus catalysing the formation of C7C, C7P, C7Si and other bonds. Electrochemical methods enable one to synthesise many organo- metallic compounds, study the reactivities of coordination com- pounds and elucidate possible reaction mechanisms. In recent years, the use of electrochemically generated cata- lysts in reactions of organic compounds has assumed great importance both in organic synthesis and in thorough investiga- tions into electron transfer, bond cleavage, substitution, addition Yu G Budnikova A E Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, ul.Akad. Arbuzova 8, 420088 Kazan, Russian Federation. Fax (7-843) 275 22 53. Tel. (7-843) 275 23 92. E-mail: yulia@iopc.knc.ru Received 4 October 2001 Uspekhi Khimii 71 (2) 126 ± 158 (2002); translated by T N Safonova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n02ABEH000697 111 112 113 115 117 118 125 126 127 128 129 130 135 and other reactions. The results of these investigations find application in many fields of physical, organic and analytical chemistry. The major breakthrough in the use of electrochemical reactions in industry, particularly, in low-tonnage chemistry, occurred in the past few years. From an industrial standpoint, electrochemical reactions have a number of advantages.Thus the conversion, the rate and, within certain limits, the selectivity of the process can be easily controlled by means of such parameters as the current density and the potential.1 ±3 Methods of indirect reduction and oxidation of organic compounds where the formation and regeneration of reactants occurs at an electrode find increasing use in modern organic synthesis. These methods enable one to perform various trans- formations of organic compounds under conditions of efficient and environmentally safe processes.4 Presently, the electric energy cost is only an insignificant part of the cost price of the product and it is expected that it will grow more slowly than the price of chemical reagents.5 Electrochemical processes are environmen- tally non-polluting and the control over these processes can be easily automated.Electrochemical reactions can provide higher selectivity as compared to conventional chemical transformations with simultaneous reduction of energy consumption and attain- ment of higher current densities. Electrochemical metal complex catalysis attracts interest because it enables one to carry out reactions under mild conditions (moderate temperature, ambient pressure) and in a virtually closed system with the use of a minimum amount of the catalyst (mediator), which is cyclically regenerated. It should be noted that mediator processes are not only of practical importance but also offer a unique possibility of studying reaction mechanisms of reduction and oxidation of various substrates by electrochemical methods.For the simple cases, the mathematical approach was elaborated. This approach is based on approximating formulae, which provide a way of determining the rate constants of individual steps of the overall process of formation, consumption and regeneration of the mediator and enable one to obtain information on the energy characteristics of these steps and on the influence of various external factors. For this purpose, the112 dependencies of the current (its catalytic increase as a measure of the rate of the process) and the potential on the conditions of electrolysis are used. The ever-increasing number of studies are aimed at replacing traditional catalysts based on toxic heavy metals with environ- mentally safer and cheaper compounds.The present review surveys the principal results, which were obtained in organic synthesis with the use of electrochemical methods under the conditions of metal complex catalysis in the last 15 years. Some electrochemical reactions related to metal complexes, such as reversible haptotropic shifts, ligand exchange, modelling of molecular switches and modelling of the active sites of metalloenzymes as well as the architecture of electrochemically functionalised fullerenes are beyond the scope of the present review. II. General principles of metal complex electrocatalysis in organic synthesis and methods of investigation of reaction mechanisms 1.General principles Reactions involving transition metal complexes can be divided into two classes. One class includes reactions, such as isomer- isation, dimerisation or oligomerisation of unsaturated com- pounds, in which no redox processes occur. In these reactions, the function of the catalyst is to control the kinetics and selectivity of thermodynamically possible processes. In this case, electro- chemical methods employing transition metal catalysis are used as an alternative to conventional procedures for generation of some low-valent species that are difficult to access.6 However, the above-mentioned reactions are not, in essence, electrochemical processes because they require no electricity consumption and, hence, they are not considered in the present review.The reactions of the second class involve both catalysts (transition metal complexes) and a stoichiometric number of electrons (either from a reducing agent or from an electrochemical source). This class can be subdivided into two groups according to the type of action of the catalyst. In the first group of reactions, the transition metal complex can serve as a carrier of electrons from their source to the reagent. Generally, these processes proceed by the so-called outer-sphere mechanism without the formation of an intermediate complex of the substrate (A) with the catalyst (the redox P/Q pair) and the mediator is regenerated electrochemically.7± 12 Q, P+e P+B, Q+A B C, where P is a mediator, Q is a catalyst, A is a substrate, B is a reduction product and C is the target product.The function of the redox catalyst (mediator P) is to transfer an electron from the electrode to the substrate after which the initial product of reduction of the substrate B is transformed into the final product C in the course of the chemical reaction. In the case of the rapid irreversible conversion of B into C, the substrate can be reduced under the conditions of homogeneous electron transfer in the direction opposite to the standard potential drop (E P=Q>EA=B),7, 8 which substantially extends the possibilities of redox catalysis. The apparent benefit of using redox catalysts is achieved only when these catalysts are stable and can be rapidly regenerated at an electrode in high current yield at potentials which are more positive than the potentials of direct electro- reduction of the substrate, or at potentials which are less positive than the potentials of direct electrooxidation of the substrate.In addition, the catalysts must rapidly react with the substrate. Hence, it is advantageous to use such metal complex catalysis in two cases: (1) if the employment of the homogeneous electron transfer leads to acceleration of the process as a whole, i.e., if the Yu H Budnikova kinetic barrier of the electrochemical process is lowered and the potential approaches the thermodynamical value owing to trans- fer of the electron to the bulk of the solution and a decrease in the overpotential of the electrochemical reaction; (2) in processes involving substrates whose homogeneous redox reactions proceed more rapidly than heterogeneous reactions for one or another reason.The second group of faradaic and catalytic reactions includes those proceeding by the inner-sphere mechanism, i.e., processes involving the formation of various intermediate coordination compounds (PA or QA) in the course of the reaction. This approach was described for the first time 15 years ago and was considered in the reviews.13 ± 19 The inner-sphere electron transfer has some advantages over the outer-sphere mechanism for the following reasons. 1. Redox processes are independent of the difference between the potentials of the mediator and the substrate and, hence, a large reduction in the overpotential becomes possible.2. The inner-sphere transfer has a lower activation barrier. 3. Only one electron can be transferred in the outer-sphere process, whereas two or more electrons can be transferred through the inner-sphere mechanism. This is of particular importance in biocatalysis and in activation of small molecules. In the last decade, attempts have been made to provide theoretical justification for these processes and to extend the range of catalytic reactions. However, many details of the reaction mechanisms remain unclear. The mechanism of action of metal complex catalysts can be represented by the following scheme Q P + e PA P+A QA Q+A or e e P+C PB PA QB QA P+C.Since transformations of the substrate in the coordination sphere of the metal complex play a decisive role in the catalytic cycle, this type of catalysis is termed coordination electrocataly- sis.13 The electrochemical step of the process and the chemical reaction of the substrate share a common catalyst ± substrate complex. The function of the catalyst is to bind and activate the substrate in the coordination sphere of the metal complex with the resulting increase in the reactivity of the substrate in the electron exchange with the electrode. Unlike redox catalysis, coordination electrocatalysis requires that either the mediator P or the catalyst Qbe coordinatively unsaturated or their ligands be readily leaving groups, which is necessary for the formation of the intermediate PA and QA complexes.Different approaches to investigation of mediator processes were first described approximately two deca- des ago and were covered in the reviews.7 ±19 Abundant data on complex formation and activation of substrates that are kinetically inert in redox reactions are presently available,14, 20 which facilitates the choice of an appropriate metal complex. For the reactions considered below, electrochemical methods do not necessarily have advantages over conventional chemical methods. However, it should be noted that electrochemical methods make it possible to generate unusual low-valent inter- mediates, which cannot be prepared with the use of chemical reducing agents. In addition, many reactions require reduction of intermediates of the catalytic cycle, which can be readily accom- plished in the electrochemical process by controlling the potential of the working electrode.In recent years, electrochemical methods were substantially simplified and became cheaper. Many electrochemical reactions are carried out in the direct current mode with the use of constructionally simple current sources. Undivided cells find increasing use because divided cells are less efficient in polar aprotic solvents and require a large amount of a supporting electrolyte. Processes with the use of sacrificial anodes haveMetal complex catalysis in organic electrosynthesis considerable promise both for direct and catalytic reactions. In the latter case, the conductivity is enhanced and metal ions generated at an anode can influence the reactivities of chemical inter- mediates.21, 22 Data on the electrochemical and chemical behaviour of the simplest transition metal complexes were surveyed in the reviews and papers.22 ± 33 2.Methods of investigation of the mechanisms of reactions catalysed by transition metals Reactions catalysed by transition metals proceed through catalytic cycles involving metal complexes in different oxidation states. Most of the metal complexes are either oxidised or reduced, or both. Consequently, they can be readily detected and charac- terised by electrochemical methods (voltammetry, amperometry, chronoamperometry, etc.). These methods also allow one to estimate the reactivities of metal complexes.Presently, stationary and transient voltammetry are the most readily avail- able methods. Stationary voltammetry is used for characterising a metal complex which is already present in solution and is characterised by the redox potential and its concentration is determined by the reduction or oxidation current. The reactivity of the complex with respect to the substrate can be examined by monitoring the change in its redox current as a function of time using, for example, the method of rotating disc electrode. If a new electroactive metal complex is formed in the course of the reaction, it can also be detected from its redox peak and its yield in the reaction is determined by the redox current.By this means, both the reagents and the products of the chemical step and its kinetics can be characterised. The stationary state method enables one to obtain kinetic data for chemical reactions whose half-lives t1/2 are larger than several tens of seconds. Transient voltammetry is used for proving the existence of the equilibrium between different complexes by estimating the relative change in the current at the redox peaks of different species as the scan rate is changed. The redox process of one species at the electrode surface leads to a shift of the equilibrium in the diffusion layer. The concentration of the species, which is measured from their redox current, reflects not their true concentration but the dynamic concentration [the CE (chemical ± electrochemical) mechanism].The scan rate fixes the time scale. The equilibrium is more difficult to shift as the time is decreased. In the limit, at very high scan rates, the dynamical equilibrium is frozen though it can be realised within longer periods of time (low scan rates). This method allows one to determine the equilibrium concentrations of different complexes and then their equilibrium constants. In these cases, the thermodynamics and kinetics of the equilibrium can be characterised using staircase transient voltammetry or chronoam- perometry (the pulse duration y) of redox waves of various species. In this case, electrochemical methods are similar to Table 1. Products of electroreductive coupling catalysed by Ni and Co compounds.Solvent Metal ± ligand Substrate EtBr n-C6H13Br n-C5H11Br MeOC2H4Br MeO2CC5H10Br Cl(CH2)4Br CH2=CHCH2Cl PhCHCl2 DMF DMF or NMP DMF or NMP DMF or NMP DMF or NMP DMF or NMP H2O DMF DMF DMF Ni ± phen Ni ± bipy Ni ± bipy Ni ± bipy Ni ± bipy Ni ± bipy Co ± bipy Ni ± salen Co ± salen Ni ± salen X(CH2)nCl (X=Br, I; n=2± 6) Note: phen is 1,10-phenanthroline; bipy is 2,20-bipyridyl; NMP is N-methyl-2-pyrrolidone; salen is the bis(salicylidene)ethylenediamine dianion. 113 NMR spectroscopy because progressive freezing of the equili- brium as the scan rate is increased (voltammetry) or as the pulse duration y is decreased (chronoamperometry) is analogous to coalescence in NMR spectroscopy.In the equilibrium reactions, the major species observed in solution are not necessarily reactive because they can be involved in the endergonic equilibrium (which is established with energy absorption). Since conventional meth- ods provide a way of controlling the reaction, efficient reactive species can be identified only in detailed kinetic studies. Cyclic voltammetry can also be used for generation of stable or short-lived organometallic species through successive electron transfers and chemical reactions in the course of redox processes involving metal complexes. A knowledge of the absolute number of electrons involved in an electrochemical process enables one to determine oxidation states of a cascade of complexes, which can be generated from a particularly stable complex.The kinetics of reactions of electrogenerated species with a particular substrate can be controlled by increasing the scan rate in cyclic voltammetry (or decreasing the pulse duration in chronoamperometry), i.e., by reducing the time scale of the chemical reaction. Analysis of the changes in the voltammogram as a function of the scan rate (or the time) makes it possible to obtain the kinetic data for chemical reactions whose half-lives t1/2 range from 0.1 s to 10 ns. Hence, intermediate species can be generated and/or identified and their reactivities can be characterised. However, although electrochemical methods provide a way of studying the kinetics and thermodynamics of reactions, they give no information on the structures of intermediates.Electrochem- ical methods in combination with NMR spectroscopy allow the elucidation of the characteristic features of the reaction mecha- nisms and provide detailed structural data. Other spectral meth- ods can also be combined with electrochemical methods to obtain kinetic and structural parameters of the complex reaction system. The efficiency of this approach was demonstrated in many studies on elementary stages, such as oxidative addition, ligand exchange and reductive elimination. III. Homocoupling of organic halides 1. Homocoupling of organic monohalides Reductive dimerisation of organic halides is the first known reaction involving both faradaic reduction and catalysis by transition metal complexes.In 1976, Jennings et al.34 reported electrodimerisation of alkyl bromides in DMF by electrolysis between two metal electrodes (Ni, Al or Cu) in the presence of iron or nickel acetylacetonates and triphenylphosphine. Later on, other analogous reactions were carried out, the electrochemical properties of catalyst precursors were studied and the reaction mechanisms were investigated.35 ± 44 The results published within the last decade are summarised in Table 1. Although dimerisation products can be obtained in high yields, these reactions are of Ref. Yield (%) Product 85 89 60 65 38 35 36 36 36 36 36 37 40 41, 42 43, 44 C4H10 C12H26 C10H22 (MeOC2H4)2 MeO2CC10H20CO2Me Cl(CH2)8Cl (CH2=CHCH2)2 PhCH(Cl)CH(Cl)Ph+PhCH=CHPh PhCH2CH2Ph+PhCH=CHPh Cl(CH2)2nCl 80 ± 90114 limited synthetic usefulness because most of them can be carried out using either chemical or electrochemical methods in the absence of catalysts.45 The synthesis of biaryls from aryl halides attracts great interest because these compounds are of practical use, whereas they are difficult to prepare by chemical methods.First electro- syntheses were carried out in 1980s simultaneously by several research groups 46 ± 49 with the use of NiCl2PPh3 (1% ± 20%) as the catalyst precursor in the presence of an excess of PPh3 . Later on, nickel complexes with 2,20-bipyridyl and bidentate phosphines, viz., 1,2-bis(diphenylphosphino)ethane (dppe) and 1,2-bis(dial- kylphosphino)benzene, were prepared.50, 51 The Ni(II) complex with bipy, which can be reduced to the Ni(0) complex, proved to be an efficient catalyst of dimerisation of aryl compounds.52, 53 In combination with sacrificial anodes, this process can be performed in an undivided cell under very simple experimental conditions in the presence of an excess of bipyridyl.NiBr2(bipy) (7%) Ar Ar+2X7 2 ArX+2 e7 DMF, Mg anode Ar=Ph, 3-MeC6H4 , 4-MeCOC6H4 , 2-naphthyl; X=I, Br, Cl. Interestingly, biaryls were obtained in only slightly lower yields when the reactions were carried out in alcohols, such as methanol or ethanol, instead of DMF with the use of a sacrificial iron or aluminium anode.54, 55Anickel complex with either bipy 55 or di(2-bipyridyl)amine can serve as a catalyst. The results obtained in the cited studies evidence the possibility of formation of organonickel intermediates, which are stable in protic solvents and give rise to biaryls.Efficient arylation of activated alkenes was carried out under the same conditions.55 The electrosynthesis of biaryls from bromo- or iodoarenes 56 can be carried out in the presence of PdCl2(PPh3)2 as the catalyst precursor. Aryl triflate can also serve as the substrate,57, 58 but the latter process requires higher temperature. These homocoupling reactions catalysed by Ni or Pd complexes can also be performed using conventional chemical methods in the presence of zinc as a reducing agent to obtain biaryls in comparable yields.58 PdCl2(PPh3)2 (7%) 2ArX+2 e7 DMF, 20 8C Ar Ar+2X7 (50% ± 98%) Ar=Ph, 4-ButC6H4 , 4-MeCOC6H4 , 4-Me2NC6H4; X=I, Br. PdCl2(PPh3)2 (10%) 2ArOTf +2 e7 DMF, 90 8C Ar Ar +2TfO7 (50% ± 70%) Ar=Ph, 4-NCC6H4 , 4-CF3C6H4 , 4-ClC6H4; Tf=O2SCF3 .Catalytic reduction of cyclohexanecarboxylic acid chloride (A) in the presence of the electrochemically generated Ni(I)(salen) complex yielded the tetramer, viz.,1,2-dicyclohexylethene-1,2-diol dicyclohexanoate (B).59 The first step involves electron transfer from the Ni(I)(salen) complex to acyl chloride to form the acyl radical C. The radical anion E is generated from diketone D either by electroreduction or by the reaction with the Ni(I)(salen) complex. Subsequent reactions of the intermediates E and F with the initial acid chloride give rise to the reaction product B.Ni(II)(salen)+e7 Ni(I)(salen) +Ni(II)(salen)+Cl7 RCOCl+Ni(I)(salen) RC A O C RC CR RC 2 O O D O C RC CR RC CR + e7 O O D O7 O E Yu H Budnikova E+Ni(II)(salen) RC CR+Ni(I)(salen)O O O D R C O e7 E+RCOCl C C R 7Cl7 O R O O R C O R C O RCOCl R C C R C C R 7Cl7 O C R R O7 B O F R=cyclo-C6H11 . However, the function of Ni(I)(salen) is conceivably more complex than the mere role of an outer-sphere electron carrier to form acyl radicals. Heterogeneous reduction of acyl chlorides RCOCl (R=Alk), which also proceeds through acyl radicals, yields aldehydes rather than the tetramers. 2. Homocoupling of organic dihalides Electroreductive coupling of organic dihalides is one of the approaches to the synthesis of polymers.Polyynes were prepared from diiodoacetylene in the presence of the Ni complex with dppe.60 NiI2(dppe) (5%) +2n I7. C C n IC CI+2n e7 n DMF Electroreductive coupling of aromatic and heteroaromatic dihalides was examined.61 The results of studies on reductive electropolymerisation of aryl dihalides are summarised in Table 2.51, 62 ± 72 Depending on the experimental conditions, polymers either deposit from the bulk of the solution in the course of electrolysis or precipitate as films on the electrode surface. This method is also suitable for preparing copolymers from a mixture of two aryl dihalides. Recently, the mechanism of polymerisation catalysed by the Ni complex with bipy was investigated.73 Homogeneous catalytic reduction of a,o-alkanediyl dihalides under the action of electrochemically generated Ni(I)(salen) com- plexes was studied.Thus, 1,8-dichlorooctane was prepared from 1-bromo-4-chloro- and 1-chloro-4-iodobutane in a yield of higher than 88%. It was demonstrated that the reaction followed the radical pathway.73, 74 Electrochemical reductive coupling of some gem-dihalides, in particular, of dimethyl dibromomalonate in the presence of the copper complex with 1,10-phenanthroline and a sacrificial copper anode gave rise to cyclopropanes.75 Br CO2Me CuBr2(phen), e7 C + Cu anode Br CO2Me CO2Me +2Br7. CO2Me Studies by cyclic voltammetry and chronoamperometry showed that reduction of dimethyl dibromomalonate with the Cu(I) complex is a catalytic process, which is kinetically controlled by the rate of concerted electron transfer and cleavage of the carbon7bromine bond.In the presence of styrene, cyclopropane is formed through the addition of R2C.Br (R=MeO2C) at the double bond.75 The radical intermediate is subsequently reduced to the corresponding anion, no transformations into carbene or alkylcopper intermediates being observed. In the reactions with other gem-dihalides, the products are formed by the chain mechanism involving addition.Metal complex catalysis in organic electrosynthesis Table 2. Reductive electropolymerisation of aryl dihalides with the use of a nickel catalyst. Substrate Cl Cl Br Br Br Br C(O)NH Br NHC(O) Br NHC(O) C(O)NH Br Br NR(R=Alk, aminoalkyldisiloxane) Br N Br Br Br O Br Br + NR Br Br + (R=Alk, aminoalkyldisiloxane) aDMA is N,N-dimethylacetamide. IV.Cross-coupling of organic halides 1. Cross-coupling of aryl halides Asymmetrical biaryls, particularly, those containing electron- donating and electron-withdrawing groups in the aromatic nucleus, find use in nonlinear optics. Attempts were made to prepare these compounds, which are difficult to access, from mixtures of two different aryl halides employing electrochemical methods in combination with catalysis by Ni or Pd complexes. The chief drawback of this approach is the low selectivity of the process due to competitive cross- and homocoupling.It was demonstrated 76 that the reactions involving two aryl halides, which possess similar reactivities in the oxidative addition step, afforded three possible products (one asymmetrical and two symmetrical) in a ratio corresponding to statistical distribution (2 : 1 : 1). If the reactivities of the starting aryl halides differ substantially, an asymmetrical product can be obtained in sat- isfactory yield only upon slow addition of the more reactive aryl halide in the course of electrolysis.76 Ref. Solvent Metal ± ligand 51 DMSO Ni ± dppe 62 DMAa Ni ± dppe MeCN 63 Ni ± PPh3 64 Ni ± bipy DMAa Br 64 DMAa Ni ± bipy Br 65 ± 67 DMAa Ni ± bipy 68, 69 Ni ± bipy+ MeCN PPh3 70 MeCN Ni ± bipy 71, 72 DMAa Ni ± bipy 115 NiBr2(bipy) (4% ± 18%), e7 Cl X+R2 R1 NMP, Mg anode R2 R1 (15% ± 17%) R1=MeO, Me2N, MeS; R2=F, CN, CF3; X=Br, Cl.Intramolecular cross-coupling catalysed by the Ni complex with 1,2-bis(diisopropylphosphino)benzene was reported.51 Ph O N e7, NiCl2L N DMSO, 65 8C Cl (30%) Cl PPri2 . L= PPri2 In the electrochemical syntheses of asymmetrical biaryls, high selectivity is achieved in two-step processes. The reaction involv- ing a stoichiometric amount of the PdCl2(PPh3)2 complex is an example.56, 77, 78 The Pd(0) complex generated at an electrode first reacted with a molecule of one aryl halide followed by electro- reduction of the resulting s-arylpalladium complex in the pres- ence of another aryl halide to give asymmetrical biaryls in good yields.Ar1X 2 e7, PPh3 Pd(0)(PPh3)2 PdCl2(PPh3)2 Ar2X, 2e7 Ar1Ar2+Pd(0)(PPh3)2 . Ar1PdX(PPh3)2 Yet another two-step synthesis of biaryls was carried out with the use of the catalytic Ni ± bipy system. Electroreduction of aryl halides in the presence of Zn2+ ions and an excess of bipy with the use of a sacrificial zinc anode afforded not biaryls but arylzinc compounds stable in the reaction medium.79 The addition of another aryl halide and a catalytic amount of PdCl2(PPh3)2 to this reaction mixture gave rise to asymmetrical biaryls in high yields (Table 3).80 Ar2X e7, Ni(BF4)2(bipy)3 (7%) Ar1Ar2 Ar1ZnX Ar1X DMF, Zn anode PdCl2(PPh3)2 (0.5% ± 2%) This method is also applicable to coupling reactions of aryl halides with allyl chlorides.79 Functional substituents in aryl halides do not interfere with the reaction.Table 3. Two-step cross-coupling of aryl halides catalysed first by Ni complexes and then by Pd complexes.80 Yield (%) Ar1X Ar2X 83 83 90 84 84 84 83 55 4-BrC6H4OMe 4-BrC6H4CN 4-BrC6H4CN 4-BrC6H4NO2 4-BrC6H4OMe 4-BrC6H4NO2 4-BrC6H4CN 2-BrC6H4CN 4-ClC6H4CF3 4-ClC6H4CF3 4-BrC6H4OMe 4-BrC6H4OMe 4-ClC6H4CO2Me 4-BrC6H4NMe2 4-BrC6H4NMe2 2-BrC6H4OMe116 2. Cross-coupling of aryl halides with activated alkyl halides The syntheses of pharmaceuticals, agrochemicals and perfumes make use of arylacetic or arylpropionic acids and benzyl ketones as important intermediates.These compounds can be prepared by electrochemical methods. It was shown that esters of a-halocar- boxylic acids can react with arylnickel compounds, which are generated electrochemically from aryl halide in a divided cell, to give esters of arylacetic or arylpropionic acids.81 The chief draw- back of this two-step procedure is the use of a large excess of PPh3 , which is necessary for preventing homocoupling. X2CH(R)CO2Et PPh3 ArNi(0)X1(PPh3) ArX1+2e7+NiCl2 ArCH(R)CO2Et R=H, Me; X1, X2=Cl, Br. Recently, a more efficient procedure, which combines the use of a sacrificial anode with catalysis by nickel complexes, was developed. To reduce undesirable homocoupling to a minimum, the most reactive substrate, viz., activated alkyl halide, was slowly added to the reaction mixture at 60 ± 80 8C.This procedure was applied to cross-coupling of aryl halides with a-chloro esters (Table 4),82, 83 a-chloro ketones (Table 5) 83 ± 85 and allyl and vinyl derivatives (Table 6).84 Table 4. Electroreductive cross-coupling of aryl halides with a-chloro esters catalysed by the NiBr2(bipy) complex.82, 83 e7, NiBr2(bipy) (5% ±10%) ArCH(R)CO2Me ArX+ClCH(R)CO2Me DMF, Zn or Al anode Yield (%) ArX R 75 60 67 65 59 66 70 51 HHHMe Me Me Me Me 4-FC6H4Br 4-NCC6H4Br 4-Me2NC6H4Br 4-FC6H4Br 3-CF3C6H4Br 4-CF3C6H4Br 4-CNC6H4Br 4-MeOC6H4Br 4-MeOC6H4I Me 85O 2-Bromo-6-methoxynaphthalene 55 Me This procedure was also successfully used in the asymmetric synthesis of 2-arylpropionic acids.86 The resulting compounds possess higher biological activities than those of S-enantiomers [for example, in the case of 2-(3-phenoxyphenyl)propionic acid (phenoprofen) or 2-(6-methoxy-2-naphthyl)propionic acid (1a, R=H) (naproxen)]. However, conventional procedures for the synthesis generally afford racemates. The electrochemical syn- Table 5.Electroreductive cross-coupling of aryl halides with a-chloro ketones catalysed by the NiBr2(bipy) complex.83 ± 85 e7, NiBr2(bipy) (5% ± 10%) ArCH(R1)COR2 ArX+ClCH(R1)COR2 DMF, Zn or Al anode Yield (%) R2 ArX R1 Me 4-FC6H4Br H Me 56 4-NCC6H4Br H Me 80 4-Me2NC6H4Br H Me 52 4-FC6H4Br H Me 65 3-CF3C6H4Br H Ph 52 4-CF3C6H4Br H Ph 63 4-CNC6H4Br Me Me 70 4-MeOC6H4Br Me Me 70 4-MeOC6H4I Me Me 53 2-Bromo-6-methoxynaphthalene Me 45 Yu H Budnikova Table 6.Electroreductive cross-coupling of aryl halides with vinyl or allyl derivatives catalysed by the NiBr2(bipy) complex.84 e7, NiBr2(bipy) (5% ± 10%) ArR ArX+RX DMF, Zn or Al anode ArX RX (the ratio between Z and E isomers) Yield of ArR (%) (the ratio between Z and E isomers) 66 (20 : 80) 66 (20 : 80) 44 (25 : 75) 56 38 52 63 a 56 b BrCH=CHMe (50 : 50) BrCH=CHMe (50 : 50) BrCH=CHMe (50 : 50) AcOCH2CH=CH2 AcOCH2CH=CH2 AcOCH2CH=CH2 AcOCH2CH=CHMe MeCH(Cl)CH=CH2 3-CF3C6H4Br 4-NCC6H4Br 4-MeOC6H4Br 3-CF3C6H4Br 4-CF3C6H4Br 4-MeOC6H4Br 4-MeOC6H4Br 4-MeOC6H4I aA mixture of ArCH=CHEt and ArCH(Me)CH=CH2 was obtained in a ratio of 86 : 12.bA mixture of ArCH=CHEt and ArCH(Me)CH=CH2 was obtained in a ratio of 96 : 4. thesis allows asymmetric arylation of derivatives of a-chloropro- pionic acid using chiral auxiliary residues bound to the carboxy group. This reaction was used in the synthesis of (S )-fluorobipro- fen starting from compound 1b. Compounds 1a,b were prepared in good yields and with a diastereomeric excess (de) using one equivalent of a-chloropropionylimide 2.86 Me Br C(O)R RC(O)CHMe a Cl 2 +MeO MeO 1a Me a 2+ Br C(O)R 1b F F Me N N R= ; (a) e7, NiBr2(bipy) (5% ± 10%), DMF, Al anode. Ph Me de ee Yield (%) Compound Major configuration 93 92 62 58 1a 1b SS 85 82 Recently, it has been demonstrated that cross-coupling of some aryl halides with hetaryl halides proceeded in high yields under the action of the Ni(II) complex with bipy.87 ± 89 e7, NiBr2(bipy) X+ Mg or Zn anode N R R Y N R=H, 4-F, 4-MeO, 4-Me2N, 4-CN, 4-MeO2C, 2-MeO; X, Y =Cl, Br.Products of cross-coupling of aryl halides are listed in Table 7. In the course of electrolysis, the nickel(II) complex was regener- ated and its content in the electrolyte is at most 10 mol% with respect to Ar1X. Organic halides were taken in equivalent ratios. In all cases, the electrosynthesis afforded asymmetrical biaryl as the major product. If aryl halides contain electron-withdrawing substituents in the aromatic nucleus, their cross-coupling reac- tions proceed in higher yields with the use of a zinc anode.Apparently, this is attributable to higher reactivity of magnesiumMetal complex catalysis in organic electrosynthesis Table 7. Products of cross-coupling of organic halides under the action of the electrochemically generated Ni(0)(bipy) complexes. Q=2.0 ± 2.5 e per ArX molecule. Ar1X Ar2X or RX Yield (%) 4-MeOC6H4Br 4-Me2NC6H4Br PhBr 4-FC6H4Br 4-MeOC(O)C6H4Cl 4-MeOC(O)C6H4Cl 2-MeOC6H4Br 4-CNC6H4Br Br S Br S Br O Br S Br S Br S PhBr aZ: E=25 : 75. and its compounds with respect to electron-withdrawing func- tional groups resulting in transformations of the latter as side reactions. The mechanism of cross coupling was studied in detail and discussed in Ref. 89.V. Formation of compounds containing the metal7carbon bond Cathodic alkylation of catalysts upon reduction of alkyl halides is characteristic of various chelate complexes of transition metals, which form rather stable compounds with the alkyl7metal s-bond (such as cobalamine, phthalocyanine, salen and dithio- carbamate complexes of Co, Fe, Ru, Rh, Ni and Cr).90, 91 The electrochemistry of vitamin B12 and reactions of its reduced form (B12s) with organic halides have been studied extensively.92 ± 99 Schrauzer and coworkers 95 ± 97 examined alkylation of vita- min B12s , cobaloximes and other chelate Co(I) complexes [includ- ing Co(I)(salen)] with alkyl halides. Generally, these reactions proceed through the SN2 mechanism including the supernucleo- philic derivative of vitamin B12 , viz., B12s , in which cobalt exists in the Co(I) state.Alkylcobalt intermediates were demonstrated 97 to decompose either photolytically or pyrolytically to form alkyl radicals. Rusling and coworkers 98 studied electrocatalytic reduc- tion of 1,2-dibromoethane and 1,2-dibromobutane under the action of vitamin B12s and found that these vicinal dibromides were transformed into alkenes without the intermediate formation of the alkylcobalt(III) complex. Later on, the catalytic reactions involving B12s were compared with those involving Co(I)(salen) 99 and the rate of reduction of 1,2-dibromocyclohexane was found to Anode Ref. 2-ClC5H4N 77 Mg 87 2-ClC5H4N 67 Mg 87 2-ClC5H4N 70 Mg 87, 89 2-ClC5H4N 66 Zn 87 2-ClC5H4N 33 Mg 87 2-ClC5H4N 76 Zn 87 2-ClC5H4N 58 Mg 87 2-BrC5H4N 60 Zn 87 2-BrC5H4N 78 Mg 88 2-BrC5H4N 50 Mg 88 2-BrC5H4N 50 Zn 88 2-BrC5H4N 60 Mg 87 and simulation of the mechanism was performed.Preparative O Me Cl 88 Al 59 O Me MeCHCN 88 Al 41 Cl 88 Al MeCH=C(Me)Br 80 a 89 Mg 65 Me3SiCl 117 depend on the formal potential of the Co(II) ± Co(I) pair. In addition, it was demonstrated 100 that reduction of benzyl bro- mide under the action of Co(I)(salen) gave rise to either benzyl radicals or carbanions depending on the potential at which the intermediate benzylcobalt(III) complex is subjected to electrolysis. Costa and coworkers 101 ± 103 examined electrochemical reduc- tion of salen complexes of organocobalt(III) and catalytic reac- tions of organic halides with the electrochemically generated Co(I)(salen) species.Methyl- and ethylcobalt(III)(salen) were electrochemically reduced to Co(I)(salen) and the alkyl radical.101 It was shown that Co(I)(salen) slowly reacted with tert-butyl bromide or chloride to give 2-methylpropene and molecular hydrogen. Apparently, the initially formed unstable intermediate decomposed into Co(II)(salen) and the highly reactive tert-butyl radical due to steric hindrances induced by the tert-butyl group, no organocobalt(III) derivatives being generated.103 Peters and coworkers 104 studied these reactions in most detail. Thus, the products of the reaction of Co(II)(salen) with iodoethane were identified and characterised, the reaction scheme was proposed, its kinetic and thermodynamical parameters were measured, the UV spectrum of the EtCo(salen) intermediate was recorded, the electrochemical reduction of this intermediate was characterised electrolysis showed that one electron was consumed per iodo- ethane molecule.The compositions of the reaction products depend primarily on the solvent. Thus the dimer, viz., n-butane, and ethane were obtained as the major products in acetonitrile and DMF, respectively (in all cases, ethylene was present in small amounts). The reaction scheme includes the following steps: (1) [Co(I)(salen)]7, Co(II)(salen)+e7 (2) EtCo(III)(salen)+I7, [Co(I)(salen)]7+EtI (3) [EtCo(II)(salen)]7, EtCo(III)(salen)+e7 (4) [Co(I)(salen)]7+Et., [EtCo(II)(salen)]7 (5) 2Et.C4H10 , (6) 2Et. C2H6+C2H4 , (7) Et.+SolvH C2H6+Solv.. SolvH is the solvent. It was suggested that the solvents have different effects on the relative rates of the last three steps [reactions (5) ± (7)]. Dimerisa- tion of ethyl radicals dominated over disproportionation (in acetonitrile, the yield of n-butane was 7.2 times higher than that of ethylene; in DMF, this ratio ranged from 3 to 4). In all cases, the yield of ethane was higher than that of ethylene because hydrogen atoms are readily detached from the solvent under the action of ethyl radicals, the detachment from DMF proceeding more read- ily than that from MeCN [reaction (7)]. To confirm the proposed mechanism, EtCo(III)(salen), which was prepared by the reaction of electrochemically generated Co(I)(salen) with EtI [reaction (2)], was reduced at a controlled potential.In this process, one electron was demonstrated to be consumed per molecule of the s-complex with the resulting formation of the green Co(I)(salen) complex and the ethyl radical [reactions (3) ± (4)].104 Upon reduction of 1,8-diiodooctane under the same condi- tions,105 two s-complexes, viz., 8-iodooctylcobalt(III)salen and m-(1,8-n-octyl)bis[cobalt(III)(salen)], were detected (both by elec- trochemical methods and UV spectroscopy) as intermediates, their relative amounts being dependent on the substrate : catalyst ratio. The mechanism proposed in the study 99 was supported by the results of the investigation 106 on catalytic reduction of alkyl halides with the Co(II)(salen) complex at microelectrodes, the reaction scheme being supplemented by two more steps.[RCo(II)(salen)]7+RI R7R+[Co(II)I(salen)]7, Co(II)(salen)+I7. [Co(II)I(salen)]7118 Electrochemical alkylation of mercury metal with butyl bro- mide in the presence of bis(dimethylglyoximato)(pyridino)Co(II) [Co(DH)2Py2] was examined.107 It was suggested that Co(II) was initially reduced to Co(I). Then the Co(I) complex reacted with BuBr to give BuCo(III)(DH)2Py under the action of which the alkyl group was transferred to mercury metal. Cathodic alkylation of the catalyst is typical of various chelate complexes of transition metals, which form rather stable com- pounds containing the alkyl7metal s-bond, for example, of cobalamine, phthalocyanine, salen and dithiocarbamate com- plexes of Co, Fe, Ru, Rh, Ni and Cr.90 These compounds contain metal atoms, which can exist in three successive oxidation states (for example, +3, +2 and +1 for Co), and possess the external p-system of the ligand favourable for stabilisation of the anionic complex.However, the ability of metal to exist in three oxidation states (n+1), n and (n71) and the presence of a chelating ligand are not necessarily sufficient for alkylation of metal complex anions. For example, attempts to subject the Ir dimethylglyox- imate complex to either chemical or electrochemical alkylation failed 108 in contrast to the analogous Co and Rh complexes.The potential of formation of the nucleophilic anion interacting with alkyl halides plays a large role in this process. If this potential is more negative than the potential of reduction of the alkylation product, the latter is not formed because of its irreversible reduction accompanied by the cleavage of the metal7alkyl bond. The potential of formation of the metal complex anion and the potential of reduction of the alkylation product depend on the nature of the metal atom, the chelating ligand, the solvent and the s-bonded organic group.109 For example, reduction of most metalloporphyrins, which contain an organic group s-bonded to the metal atom, occurs at more negative potentials than reduction of metalloporphyrins with coordinated ligands.However, some ironporphyrins containing the Fe7Alk bond are reduced more readily than the corresponding complexes with ionic ligands.110 Studies of the electrochemical properties of a series of Co2+ complexes with substituted phthalocyanines in DMF allowed the conclusion that alkylation of electrochemically generated mono- anionic complexes in the presence of n-butyl bromide proceeded at the metal atom.91 Electrochemical reduction of n-butyl bromide was studied in the presence of the cobalt phthalocyanine complex PcCo(II) (Pc is substituted phthalocyanine) and the catalytic cycle was pro- posed.111 This catalytic cycle was called `chemical catalysis of electrochemical reduction of alkyl halides'.111 [PcCo(III)L]0 e7 [PcCo(II)L]7 7L7 [PcCo(II)]0 e7 R7 [PcCo(I)]7 [PcCo(II)R]7[PcCo(III)R]0 RBr Br7 Br7 e7 [PcCo(I)]27 e7 RBr The above scheme is common to many chelate compounds of transition metals and rare-earth elements. At potentials more cathodic than the potential of reduction of the Co7Alk bond, electrolysis resulted in regeneration of the highly reactive [PcCo(I)]7 anion, which was rapidly alkylated with n-butyl bromide present in the solution and then reduced thus resuming the catalytic cycle.91 Metalloenzymes are natural catalysts in which metal ions exhibit different coordination modes.Vitamin-B12-dependent enzymes containing the cobalt atom as the catalytic site catalyse Yu H Budnikova different isomerisation reactions accompanied by skeleton rear- rangements.112 ± 123 New models were proposed 115 ± 123 for enzyme catalysis on the basis of a hydrophobic analogue of vitamin B12 in which the peripheral amide groups of natural vitamin B12 are replaced by ester groups and this complex was demonstrated to be an efficient catalyst of electrochemical rearrangements of the carbon skeleton of 1-bromopropane derivatives containing two electron-withdrawing groups (COMe,CO2Et or CN) at position 2.Among other catalytic processes, the electrochemical trans- formation of nitrate anions into ammonia was carried out in water under the action of the Co(I) complex with 2,3-dimethyl-1,4,8,11- tetraazacyclotetradeca-1,3-diene, which was prepared from the corresponding Co(II) complex.124 Some theoretical and applied problems of the use of cobalt complexes with ligands, which can stabilise metals in low-valent states, in electrocatalytic dehalogenation, homocoupling and cross-coupling reactions and in functionalisation of white phos- phorus were covered in the studies.125, 126 It was shown that electrochemical reduction of organic halides proceeded under the action of either the Co+(bipy) or Co7(bipy)¡2 complexes.Intermediates of the catalytic cycle, viz., cobalt s-complexes, were isolated and characterised. It was demonstrated that arylation (alkylation) of white phosphorus occurred under the simultaneous action of electrochemically generated nucleophilic [Co(I)(bipy) or RCoX2(bipy)] and electrophilic (Mn+, RX) reagents. Functional- isation of P4 can follow two pathways, viz., P4 can react with ArCoX2(bipy) or it can be reduced with the Co(I)(bipy) complex to the d-P3-cyclotriphosphorus complex of cobalt, which, in turn, reacts with organic halides to form compounds containing the P7C bond.The Ni s-complexes are difficult to access and little studied. However, these compounds can be readily synthesised by electro- chemical methods under mild conditions. It is known 89 that these complexes are intermediates of electrocatalytic cycles of cross- coupling, functionalisation of alkenes and reactions of aldehydes proceeding in the presence of Ni(0) complexes. In some cases, Ni s-complexes can be isolated in pure form and they can be considered as model compounds for gaining insight into the mechanisms of organometallic synthesis.89 Recently,89 several s-complexes containing Ni in different oxidation states [for example, RNiX(bipy), R2NiX(bipy) and R2Ni(bipy)] and exhibit- ing different reactivities were found to exist.VI. Addition of organic halides to unsaturated groups 1. Addition at double and triple carbon7carbon bonds First studies of electroreductive addition of organic halides at double or triple carbon7carbon bonds go back to the 1980s. Two main procedures using various catalysts were developed. In both processes, the key step involves the radical addition to unsaturated hydrocarbons. The first approach, which was considered primar- ily by Scheffold et al.,127, 128 is based on the use of vitamin B12 or related cobalt complexes as mediators of indirect electroreduction (in some cases, in combination with photolysis).The second approach, which was proposed by Gosden and Pletcher,129 is based on the use of nickel complexes with macrocyclic tetraden- tate ligands, which can stabilise Ni(I) species. In recent years, various organonickel and organocobalt com- pounds were studied and their redox properties and reactivities were examined depending on the nature of the ligand coordinated to the metal atom. The reactions described earlier were inves- tigated once again with the aim of optimising the conditions and improving their efficiency. In particular, the reactions using the Ni(cyclam) complexes (cyclam is 1,4,8,11-tetraazacyclotetrade- cane) were examined and carbanion analogues were generated using the Ni(0) complexes in the presence of Michael acceptors.Metal complex catalysis in organic electrosynthesis a.Reactions with electron-rich double and triple carbon7carbon bonds Most of reactions with electron-rich double and triple carbon7 carbon bonds occur as free-radical cyclisation processes, which are used primarily for the preparation of five-membered rings with the desired stereochemistry.130 Br Cat, e7 + X X X Major product X=C, O, N. Electrochemical processes involving cobalt complexes were studied in detail.127, 131 In recent years, the results of new studies on the application of cobalt complexes other than vitamin B12 and nickel complexes with tetradentate macrocyclic ligands were reported.In these reactions, complexes of both types generate radical species. Direct cyclisation of bromoacetals through cobaloxime(I) complexes was described as early as 1985.132 The reactions were carried out in a divided cell in the presence of a base (40% aqueous NaOH) and (chloropyridine)cobaloxime(III) (50%) as a catalyst precursor. Later on, it was found 133, 134 that the amount of the catalyst can be reduced to5%(the number of the catalytic cycles is *50) and the presence of a base is not needed if the reaction is performed in a divided cell with a zinc anode. This method was applied 133 to the synthesis of condensed bicyclic derivatives from various ethylene and acetylene com- pounds in the presence of (chloropyridine)cobaloxime(III) or Co[C2(DO)(DOH)pn] (see Table 8).Either saturated or unsatu- rated cyclic products can be obtained depending on the amount of the catalyst used, the cathodic potential and the presence of the hydrogen donor, for example, RSH (see Table 8).136, 138 Studies of some model reactions showed that the electrochemical method is more selective and efficient than conventional reduction with the use of Zn.135 Nickel compounds, for example, the Ni(cyclam) com- plexes,129 can also be used for generating radicals in electro- reductive processes, including free-radical cyclisation reactions. The Ni complexes with cyclam and its analogues and also with Ni(CR) and Ni(tet a) serve as efficient catalysts in DMF in the presence ofNH4ClO4 as a proton source.136 ± 138, 141 Electrochemi- cally generated Ni(I) species rapidly react with organic halides to form alkynyl, alkenyl or aryl radicals whose intramolecular addition at double or triple bonds gives rise to cyclopentane derivatives (see Table 8).136 ± 138 These reactions can also be carried out in an undivided cell with a magnesium anode, as exemplified 139 by the reactions of aryl halides bound to an unsaturated hydrocarbon chain (see Table 8).Nickel complexes with cyclam and other tetradentate donor ligands are catalysts of choice.139, 142, 143 The Ni(salen) complex is a good alternative to the Ni(cyclam) complex for generating Ni(I)-containing intermediates. Thus cyclisation of 1-halo-6-phenylpent-5-ynes proceeded much more efficiently than direct electroreduction of the substrate in the presence of 10% of a catalyst (see Table.8).140 Electroreductive cyclisation of N-alkenyl-2-bromoanilines with the use of the Pd complex with PPh3 as the catalyst 144 and electrocatalytic dehalo- genation of a-haloacetic acids under the action of the reduced form of vitamin B12 were reported.145 b. Addition to Michael acceptors Radicals or carbanions and their equivalents, viz., organometallic compounds, can be added to Michael acceptors. Free-radical reactions in the presence of nickel or cobalt complexes as catalysts have been studied previously.127 ± 129 119 The synthesis of ethyl 8-oxodec-2-enoate, which was isolated from honey queen bees, provides a good example of the potential of the electrochemical approach to the preparation of natural products. This process involves two major electrochemical steps, viz., the formation of the C7C bond catalysed by vitamin B12 and initiated by visible light and the Michael addition of intermediate 3 at the triple bond.128 O e7, B12, hn Ac Br+ DMF O Ac (95%) O HC CCO2Et, e7, B12 , hn Br DMF 3 O CO2Et (85%) This approach was applied to the synthesis of the pheromone of California red scale,146 prostaglandin PGF2a ,147 the phero- mone of house mice 128 and jasmonates.148 Alkylation and acyla- tion of activated alkenes was used for the preparation of amino acid derivatives (Table 9).149 O O e7, B12 (4%), hn RX+ +X7.OMe OMe R DMF Y Y In some cases, protonation of an intermediate radical formed through the addition of alkyl halide to the Michael acceptor proceeds diastereoselectively. This situation is observed upon the addition of ButBr to diethyl mesaconate catalysed by vitamin B12 .150 However, standard reduction with zinc in the presence of vitamin B12 proved to be more efficient because the stereoselec- tivity in the latter case was determined by the addition of amines. CO2Et e7, B12 (25%) ButBr+ (40%) Me EtO2C CO2Et But But CO2Et+ Me Me EtO2C EtO2C 84 : 16 The Ni(cyclam) complexes and related compounds can also be used for alkylation of Michael acceptors. For Ni(tet a) in acetoni- trile, the number of catalytic cycles is small.151 However, this catalyst (2%) in DMF containing NH4ClO4 as the proton source shows efficiency in alkylation of unsaturated ethers, ketones or nitriles.152 In the case of terminal alkenes, the reaction products are obtained in good yields.R4 R1 e7, Ni(tet a)2+ R1X+R2CH CR3R4 DMF, NH4ClO4 R3 R2 Yield (%) R1X R2 R3 R4 CO2CH2Ph HH HH (CH2)4CO Me HHHH 53 CO2H 32 HCO2Me CO2Me CO2Me CN COMe CO2Me BuBr BuBr Ph(CH2)3Br Ph(CH2)3Br Ph(CH2)3Br Ph(CH2)3Br Ph(CH2)3Br Ph(CH2)3Br Ph(CH2)2CH(Me)Br H 14 13 53 57 72 37 60 HMe HHHH120 Table 8. Reductive cyclisation catalysed by Co and Ni complexes. SubstrateBr n X OCH2C CR n=1, 2; X = O,NCO2Et; R=H, Et, C5H11 , SiMe3 R1 Br R2 O BunO R1=H, Me, Ph; R2=H,Me O R1 Br R2 R3 R1=H, R2=R3=Ph; R17R2=(CH2)n , R3=H R1 Br R2 R3 O R1=H: R2=R3=Ph; R17R2=(CH2)n, R3=H R1 Br R2 O RN3 R1, R2=H, Me; R3=H, Ts, All, Bn Br O RN R=H, Ts, All, Bn R1 Br R2 E E R1, R2=H, Me; E=CO2Me XY X=I, Br; Y=O, NH PhC C(CH2)3CH2X X=Br, I a Hereinafter, cyclam is 1,4,8,11-tetraazacyclotetradecane. Catalyst [short name] Amount (mol %) Cl N O N 7O H H Co MeOH, 5 55±608C, X Zn anode N O O7 [(chloropyridine)- cobaloxime(III)] N Py I N O N 5 H [CoC2(DO)(DOH)pn] CoN O7 N I 40 CoC2(DO)(DOH)pn N N 20 NiN N N N 20 NiN N Me Me N 20 N N Ni [Ni(CR)] NH 20 Ni(CR) H H N N Ni [Ni(tet a)] 20 N N H H 20 Ni(tet a) 10 Ni(cyclam) a N N 10 Ni [Ni(salen)] O O Reaction product Reaction conditions n BunO MeCN C12H25SH MeCN BunO DMF O DMF O R1 CH R2 DMF or MeCN DMF or MeCN NR R1 CH R2 DMF E E DMF YY DMF, Mg anode Ph H DMA Yu H Budnikova Yield (%) Ref.CR 133 54 ± 80 O CHR1R2 135 42 ± 96 O CR1R2 135 68 ± 97 O 136 53 ± 75 R1 R2 R3 136 85 ± 86 R1 R2 R3 137 11 ± 67 O RN3 137 7 ± 33 O 138 47 ± 81 136, 138 60 ± 90 139 60 ± 90 140 84Metal complex catalysis in organic electrosynthesis Table 9. Alkylation or acylation of activated alkenes catalysed by vitamin B12 .149 Reaction product RX Y Yield (%) Me(CH2)4CHCO2Me NHAc 70 Me(CH2)3Br NHAc 75 H Me(CH2)5CO2Me Me(CH2)3Br AcCH2CHCO2Me 67 NHAc Ac2O NHAc 70 H Ac(CH2)2CO2Me Ac2O MeO2C(CH2)4CHCO2Me 72 MeO2C(CH2)3Br NHAc NHAc The nucleophilic addition can be performed in the presence of Ni(II) complexes, which are transformed into Ni(0) complexes upon cathodic reduction.The Ni(0)(bipy) complexes rapidly react with alkyl, alkenyl and aryl halides. However, in this case the organonickel intermediate generated through oxidative addition does not give the corresponding addition product in the presence of activated alkenes. In the presence of aNi complex with a weaker bound ligand (for example, with pyridine, which prevents acti- vated alkene from being coordinated to the metal atom), the regioselective 1,4-addition of aryl groups to activated alkenes was carried out under very mild conditions (60 8C, an undivided cell with a sacrificial aluminium or iron anode).Arylation products were obtained in 20%± 63% yields,153 which are comparable with those obtained in reactions using arylcopper or cuprates. The reactions performed in methanol or ethanol instead of DMF gave products in only slightly lower yields [the nickel complex with di(2-bipyridyl)amine as the catalyst].55 Ar R3 e7, NiBr2 (5% ± 10%) ArBr+R1CH CR2R3 R1 R2 DMF±Py (9 : 1), Al or Fe anode Yield (%) R3 Ar R2 R1 H H CO2Et 63 H H CO2 Et 48 HHHMe HH HCO2Et CO2Et HMe H Ph 4-NCC6H4 2-Naphthyl 4-MeOC6H4 4-MeCOC6H4 2-Naphthyl 2-Naphthyl 2-Naphthyl 2-Naphthyl 50 46 31 27 20 61 20 CO2Et CO2Et CO2Et CO2Et CO2Et CN H (CH2)4CO Intramolecular nucleophilic addition can follow either the radical or carbanion pathway.51, 137, 138, 154 Some examples are given below.51, 137, 138 Br [Ni] PPh3 + O DMSO O O NMe (13%) Me N NMe (40%) OMe Br OMe Ni(tet a) (20%) O DMF O Me N Me N (23%) O Br CO2Me Bicyclic ketones were synthesised in the presence of 5% of vitamin B12.127 Some other Co complexes as well as Ni complexes catalysed cyclisation of bromocyclohexenones 4a,b and 5a,b to form bicyclic ketones 6a,b and 7a,b, respectively,154 but the numbers of catalytic cycles in these cases were rather small.These reactions afforded open-chain compounds 8a,b and 9a,b as by-products. The Ni(cyclam)(ClO4)2 , Ni(CR)((ClO4)2 and Co(bpp)OAc [bpp= C5H5NC(O)N7(CH2)3N7C(O)C5H5N] complexes proved to be most efficient as catalysts.154 O (CH2)nBr 4a,b n = 4 (a), 5 (b). Catalyst Ni(cyclam)(ClO4)2 Ni(CR)(ClO4)2 Co(bpp)AcO O (CH2)nBr 5a,b n = 4 (a), 5 (b). Catalyst Ni(cyclam)(ClO4)2 Ni(CR)(ClO4)2 Co(bpp)OAc Intramolecular arylation of unsaturated amides gives rise to lactams. These reactions can follow either the radical or carbanion pathway. The carbanion mechanism is realised in the presence of the nickel triphenylphosphine complex to give a mixture of g- and d-lactams in a ratio of 3 : 1.51 The reactions through the radical mechanism [in the presence of the Ni(tet a) complex] proceed selectively to give d-lactam, although in rather low yield.137 The electroreductive addition of (Z)- or (E )-alkenyl halides to electron-deficient alkenes in the presence of a sacrificial iron anode proceeds in high yields with complete retention of the stereochemistry of the alkenyl fragment.These reactions can be used for functionalisation of isomerically pure (Z)- or (E )- alkenes.155 Ni(tet a) DMFO 6a,b Substrate 4a 4b 4a 4b 4a 4bO 7a,b Substrate 5a 5b 5a 5b 5a 5b 121 O CO2Me (86%) O (CH2)nH (CH2)n72 + 8a,b Yields of products (%) 8b 6b 8a 6a 7 718 7 714 7 721 70 6 7 7 33 60 1 7 7 42 52 5 7 7 28 O + (CH2)n72 (CH2)nH 9a,b Yields of products (%) 9b 7b 9a 7a 30 42 47 4 7 7 65 4 7 7 16 11 7 7 407 721 7 719 7 713122 C5H11 C5H11 NiBr2 .3H2O (10%), e7 + 60 ± 80 8C A A X X=Cl, Br, I; A =CO2Et, COMe, CN.The conditions were optimised using the model reaction of methyl vinyl ketone (A=COMe) with (Z)- or (E)-1-halohept-1- enes. It was found that high yields (up to 84%) can be achieved in the presence of 1,2-dibromoethane and an iron anode. The process involves preelectrolysis, viz., oxidation of an iron anode and reduction of 1,2-dibromoethane, followed by electrolysis of a mixture of alkenyl halide and methyl vinyl ketone. The role of iron ions is not understood. Conceivably, they function as Lewis acids, activate electron-deficient alkenes and facilitate the forma- tion of the C7C bond.The reaction mechanism is not entirely known. Presumably, it involves coordination of electron-deficient alkene by electrochemically generated Ni(0) species. Oxidative addition of vinyl halide to the complex A affords the organonickel compound B, which is either protonated by residual water (path a) to form an addition product or transmetalated with iron ions (path b) to release Ni(II). O COMe RX 2 e7 LnNi(0) LnNi(II) Me A LnNi(0) X O Ni(II) Ln Me Ln Me R Ni(II) R O B X Path a H+ B Me R O Path b FeXn71 R OFeXn71 FeXn B Me R 7NiX2Ln Me O Recently, it was found 156 that cobalt bromide, either in the free form or in the complex with 2,20-bipyridyl, catalyses the electrochemical addition of aryl halides to activated alkenes in a DMF± pyridine or acetonitrile ± pyridine mixture.The reactions proceeded only with the use of a sacrificial iron anode, whereas the addition products were not detected in the reactions performed with other anodes (Zn or Al). Me a or b X+ (CH2)2COMe R R O (30% ± 70%) R=4-CO2Et, 4-COMe, 4-CF3, 4-CN, 4-CHO, H, 4-OMe, 2-COMe; X=Br; (a) DMF± PyH, CoBr2 ± bipy (1 : 2), Fe anode, Ni cathode, 70 8C; (b) MeCN± PyH, CoBr2 , Fe anode, Ni cathode, 60 8C. 2. Addition to carbonyl compounds Data on electrochemical allylation of carbonyl compounds and the Reformatsky reaction are presently available. Some carbonyl compounds can be subjected to direct electro- chemical allylation; however, coupling reactions catalysed by transition metals are more efficient, particularly, in the case of ketones.One of procedures involves PdCl2(PPh3)2 as the catalyst in the presence of the zinc salt (ZnCl2). This procedure allows one to transform allyl acetates into the corresponding tertiary alcohols in good yields.157 Yu H Budnikova R1 R1 R3 R2 O OAc + R2R3C PdCl2(PPh3)2 (5%), ZnCl2 , e7 DMF OH Yield (%) R3 R2 R1 H 77 H 84 H 6840 62 H Ph H Ph H 4-MeOC6H4 H 4-ClC6H4 H (CH2)5 Me Of two possible isomers, the more branched isomer A was obtained as the major reaction product. PdCl2(PPh3)2 (5%), ZnCl2 , e7 R OAc +PhCHO DMF Ph Ph R OH OH + B A R R=Me (56%, A: B=91 : 9); Ph (71%, A: B=83 : 17).The reactions with conjugated carbonyl compounds proceed as 1,2-addition. Exploration into the mechanism of this reaction revealed the key role of ZnCl2 ,158 which is most readily reduced to give Zn(0). The latter is capable of reducing the p-allylpalladium intermedi- ate. The reaction products are formed through addition of diallylzinc to carbonyl compounds. Zn(0) Zn2++2e7 Zn(0) Pd(0) OAc Pd(II) R1 R2 R1R2C O Zn 1/2 OH 2 Carbonyl compounds can be allylated with the use of a nickel catalyst.82, 159, 160 R1 R1 R2 R3 X+R2R3C O NiBr2(bipy), e7, Zn anode DMF OH 1 Yield (%) R3 X R R2 85 HH 70 Ph Me Me H Ph Me(CH2)5 Ph But CH2=CH Ph Me Me Me Me Me HH 86 60 43 80 83 Cl Cl Cl Cl Cl OAc OAc (CH2)5 When employing an undivided cell, this reaction proceeds efficiently only in the presence of a sacrificial zinc anode.The reactions of aromatic and aliphatic aldehydes or ketones give rise to the corresponding homoallylic alcohols in good yields. The reactions of branched ketones, such as diisopropyl ketone, afford products in lower yields. In the case of a,b-unsaturated carbonyl compounds, the addition occurs exclusively at the 1,2-position. The Pd ±Zn catalytic system is also efficient in these reactions. It was suggested 160, 161 that the process is accompanied by trans- metalation of the allylnickel complex, which is formed upon the oxidative addition of allyl halide (acetate) to the Ni(0) complex, with anodically generated Zn(II) species.160, 161 The Reformatsky reaction can also be carried out in an electrochemical mode either directly or with the use of a mediator.Metal complex catalysis in organic electrosynthesis Catalysis by nickel compounds provides an efficient way of transforming the corresponding a-chloro derivatives to b-hydroxy esters or nitriles.162 These reactions also involve the oxidative addition of cathodically generated Ni(0)(bipy) to a halogen- containing derivative as the first step.In this case, the nature of a sacrificial anode is also of importance although the formation of an organozinc derivative was not unambiguously confirmed. NiBr2(bipy), e7, Zn anode O ClCH(R1)CO2Me+R2R3C DMF OH R2 R3 CHCO2Me R1 Yield (%) R3 R2 R1 (CH2)5 Me H Prn Ph Et 7786 Ph 7764 80 50 H Et HH Ph Me Me Me CH=CH(CH2)3 The reaction of carbonyl compounds with methyl chlorodi- fluoroacetate,163 which cannot be activated by conventional chemical methods, provides an interesting example demonstrating the possibilities of this procedure.The best results were obtained with the use of a CH2Cl2 ±DMF mixture (9 : 1). The 19F NMR spectroscopic studies demonstrated that the reaction was accom- panied by transmetalation. OH R1 ClF2CCO2Me +R1R2C O NiBr2(bipy), e7, Zn anode CH2Cl2 ±DMF R2 CF2CO2Me R2 R1 Yield (%) Me Ph (CH2)5 H 72 63 70 H 77 Ph Me(CH2)2 H 45 Me O Under analogous conditions, the reactions of methyl dichloro- acetate with acetophenone or cyclohexanone afforded epoxides in high yields.162 NiBr2(bipy), e7, Zn anode Cl2CHCO2Me+R1R2C O DMF or CH2Cl2±DMF O R1 CO2Me R2 (54% ± 86%) R1=Ph, n-C6H13, But; R2=H, Me, Ph; R17R2=(CH2)5 .The reactions of chloroacetonitrile and a-chloropropionitrile with ketones proceeded analogously.162 OH R2 NiBr2(bipy), e7, Zn anode R1CHClCN+R2R3C O DMF CHCN R3 R1 R1=H, Me; R2=Ph, n-C6H13, But; R3=H, Me, Ph; R27R3=(CH2)5 . The intramolecular addition to the carbonyl group has not been adequately investigated. Only the ring expansion reactions of a-bromomethylcycloalkanones, which were carried out in the 123 presence of Co[C2(DO)(DOH)pn]Cl2 as the catalyst at 55 ± 60 8C, were reported.133, 164 It was assumed that the reaction proceeds via the radical pathway in accordance with the usual behaviour of alkylcobalt intermediates. The use of a sacrificial zinc anode in combination with a cobalt catalyst leads to an increase in the number of catalytic cycles and improvement in the yield.133 O CH2Br Co[C2(DO)(DOH)pn]Cl2 (5%), e7 MeOH, Zn anode R n O O + R R n n (19% ± 24%) (54% ± 68%) R=Bun, n-C6H13 , n-C11H23; n=1±4.Much attention has been given to the use of catalytic systems based on cobalt complexes in the organic synthesis, particularly, in the ring expansion reactions suitable for the preparation of natural compounds. The traditional electrolytic scheme of the synthesis was proposed.Under electrochemical conditions, Co(II) in heptamethyl cobyrinate perchlorate {[Cob(II)7C1ester]ClO4}, which is a catalyst simulating vitamin B12 , is readily reduced to Co(I) to give highly nucleophilic intermediates. The latter react with alkyl halides to form alkylated complexes containing the Co7C bond. The electrochemical reactions or photolysis of these complexes afford the final products. Torii and coworkers 164 studied the ring expansion reactions of 2-(bromomethyl)cyclo- penta(hexa)nones with the use of cobaloxime (MeOH, 60 8C) in the galvanostatic mode under irradiation with visible light. Murakami and coworkers 165 used [Cob(II)7C1ester]ClO4 as the catalyst of ring expansion reactions in the potentiostatic mode.O Br B12, e7 CO2Et n O O O Me CO2Et + + CO2Et CO2Et n n n 10 n=1±4. In different electrolytic modes, the ring-expansion product 10 was obtained as the major product (in yields of up to 56%). The course of the reaction was monitored by electronic and ESR spectroscopy, which made it possible to identify intermediates formed at different potentials either in daylight or in the dark. The potentials 471.5 V (with respect to a saturated calomel elec- trode) are optimum for electrolysis. The reaction cycle involves electrochemical reduction of the Co(II) complex to supernucleo- philic Co(I) species, which react with the substrate to give the corresponding alkylated complex. The latter decomposes upon electrolysis (rather than photochemically) to the final product and the cobalt complex is recycled back to the catalytic cycle as the mediator. The ratio of the reaction products is determined by the strain energy of alicycles. Thus the highest yield of the ring- expansion product was achieved in the transformation of five- membered cyclic ketone to six-membered ketone.165 Under particular conditions, electroreductive coupling of organic halides with aldehydes catalysed by the nickel(0) complex with 2,20-bipyridyl proceeds in good yields (Table 10).166 e7, [Ni] RCH(OH)Ar.ArX+RCHO Alcohols can be obtained in high yields with the use of nickel, iron or stainless steel anodes. Aromatic aldehydes bearing the124 Table 10. Coupling of organic halides with carbonyl compounds catalysed by the NiBr2(bipy) complex (1 mmol) (see Ref.166).a ArX 2-BrC6H4Me 2-BrC6H4NH2 2-BrC6H4OMe 4-ClC6H4CO2Me MeC(Br)=CHMe a Reaction conditions: ArX, 15 mmol; R1R2CO, 7.5 mmol; Bu4NBF (1072 mol litre71) as the supporting electrolyte; I=0.1 A, a nickel cathode, 20 8C. b The yields are given with respect to the carbonyl compound; c NiBr2(bipy), 7 mmol; d inox is a stainless steel anode; e the yield is given with respect to ArCl (ArCl, 7.5 mmol, PhCHO, 30 mmol). electron-donating groups in the ring, aliphatic aldehydes and cyclohexanone can be involved in these reactions. 2-Bromoaniline and 2-bromoanisole can serve as arylating agents, whereas other isomers cannot perform this function due, apparently, to stability of the arylnickel intermediate ArNi(I)(bipy).The reactions of aryl halides containing substituents at positions 3 and 4 yield predom- inantly biaryls, whereas the addition at the carbonyl group proceeds to only a small extent. The reaction of benzaldehyde with aryl halide containing the electron-withdrawing methoxy- carbonyl group in the ring proceeded in satisfactory yield upon the replacement of Br by Cl. In this case, homocoupling of ArNi(I)(bipy) with ArX was suppressed but a three- to fourfold excess of the aldehyde was required. The fact that aromatic aldehydes containing electron-withdrawing substituents in the ring, which are favourable for p-bonding, virtually do not enter into the reactions (whereas aliphatic aldehydes can be involved in these reactions) is indicative of the predominant s-character of the bond between the organonickel complex and the carbonyl group of aldehyde. Selective reduction of aryl propargyl ethers to the correspond- ing phenols in the presence of electrochemically generated nickel complexes proceeds under mild conditions and in good yields.This reaction provides the means of removing the allyl and propargyl protective groups.167 The carbonyl group in allyl ether of salicylaldehyde was subjected to electrochemical intramolecular allylation [NiBr2(bipy), Mg anode, 71.2 V with respect to a saturated calomel electrode] using nickel complexes as the catalyst.168 It is believed that this process involves electrochemically generated Ni(0) complexes as active catalytic species capable of reacting with the allyl fragment of the ether to form the p-allyl nickel complexA.R1R2CO PhCHO 2-MeOC6H4CHO O C8H17CHO PhCHO PhCHO PhCHO PhCHO Reaction product CH OH Me CH OH Me MeO OH Me C8H17CH OH Me CH OH NH2 CH OH H3CO CH OH CH C(Me) CHMe OH The intramolecular transfer of the allyl fragment to the carbonyl group affords intermediate alkoxide, viz., nickel(II) phenoxide B. Conceivably, the Ni2+ ions in this complex can be replaced by the Mg2+ ions to give the intermediate C, which is accompanied by liberation of the Ni(II) complex and completion of the catalytic cycle. Hydrolysis of the alkoxide C gives rise to the final product: Mg(II) L=bipy. Anode Mg Zn Ni Fe inox d Ni inox d Ni inox d inox d Ni CO2Me inox d inox d CHOH OH H2O OMg(II) O C LNi(II) ONi(II)L O B CHO ONi(II) L A Yu H Budnikova Yield (%) (see b) 14 (75) c 18 61 63 80 60 71 50 58 42 85 50 e 52 2 e7 LNi(0) CHO OMetal complex catalysis in organic electrosynthesis Later on, it was demonstrated 169 that the propargyl fragment in o-(propargyloxy)arenecarbaldehydes was transferred to the carbonyl group under the action of electrochemically generated nickel complexes to produce homopropargyl alcohols.OH CHOR2 R1 1) e7, Ni(II)(bipy)3 , DMF, Mg anode 2) hydrolysis R1 O R2 OH (33% ± 71%) R1=H, 3-Cl, 3-MeO; R2=H, n-C5H11 .It was believed that the C7O bond in propargyl ether was initially cleaved followed by the addition of the propargyl frag- ment to the carbonyl group. The reactions of ring-unsubstituted propargyl ethers afforded allene isomers as by-products. Electrochemical reductive deprotection of the amino group in allylcarbamates containing several functional substituents in the presence of catalytic amounts of the Ni(II)(bipy)3 complex pro- ceeded selectively to give the corresponding amines in 40% ± 99% yields.170 O e7, Ni(II)(bipy)3, Zn anode RNH2 RHN O DMF R=Ph, 4-MeO2CC6H4 , 4-NCC6H4 , 2-MeCOC6H4 , Bn, cyclo-C6H13 , (MeO)2CHCH2 . The reactions proceed under mild conditions (20 8C) in an undivided cell with a sacrificial zinc anode.The allyl groups in arylcarbamates are selectively removed in 70%± 99% yields. Under these conditions, the ether, ketone, acetal and nitrile groups remain intact. VII. Electrocatalytic reduction of carbon dioxide CO2=CO¡ = 2 The data on electrocatalytic reduction of nitrogen and its oxides, carbon oxides and small molecules containing triple bonds (acetylene, cyanides) were surveyed in the reviews.13, 171 All these substrates are electrochemically inactive and contain very strong bonds, which are rather difficult to cleave. Hence, activation of these molecules by inserting them into the coordination sphere of metal followed by (or preceded by) the electrochemical electron transfer is the most promising way for subjecting poorly reactive molecules to various chemical transformations.Besides, electro- catalytic procedures involving metal complexes attract consider- able attention because they allow the use of vast biosphere reserves of N2, CO2 and O2 as cheap and accessible starting materials in various chemical processes. Direct reduction of CO2 at a metal cathode, for example, at a mercury cathode, requires a high overpotential (E 72.21 V with respect to a saturated calomel electrode, DMF). Other metal cathodes, for example, copper or gold cathodes, can reduce CO2 in aqueous solutions at somewhat lower cathodic potentials (from 71.3 to 1.7 V). However, even these values are too high for practical applications because water is reduced at these potentials with hydrogen elimination.In principle, it is possible to use non-aqueous solvents, but these processes require larger energy consumption and are often accompanied by side reactions. The use of various metal complexes as catalysts has the most promise for selective electroreduction of CO2. Generally, these processes are carried out using complexes with nitrogen-contain- ing macrocyclic ligands, viz., porphyrins and their analogues, which are well known as biocatalysts.172 Compared to natural catalysts, artificially synthesised metal complexes with nitrogen- containing macrocyclic ligands (for example, with phthalocya- nines) possess even more extended p-conjugated chains, whereas their catalytic activity is highly competitive with that of natural analogues.Metal complexes with phthalocyanines,173 ± 185 Co(II) 125 complexes with porphyrins,90 Ni(II) complexes with cyclam,186 ± 191 bis- and polypyridyl complexes of Rh(II),192 Re(I),193 Cu(II),194 Co(II), Fe(II) and Ni(II),195 ± 199 iron and cobalt complexes with 4,5-dihydroxybenzo-1,3-disulfonate and 2-hydroxy-1-nitrosonaphthalene-3,6-disulfonate 200, 201 ligands proved to be catalysts of CO2 reduction. Most of the above- mentioned complexes act as efficient catalysts of CO2 reduction only in non-aqueous media because CO2 reduction in water is preceded by proton reduction. Only the cobalt complex with phthalocyanine (PcCo)173, 174, 176 ± 181, 201, 202 and the nickel com- plex with cyclam 185, 195, 203, 204 proved to be efficient electrocata- lysts of CO2 reduction in water or aqueous acetonitrile, the latter being active only at a mercury cathode at which the overpotential of hydrogen elimination is higher than that at a platinum cathode.It was suggested 186, 187 that higher selectivity of electroreduction of CO2 compared to that of H2O observed in these processes is associated with the larger size of the macrocyclic ligand and the presence of the NH groups in Ni(cyclam). A considerable advantage of metal complexes as catalysts is the possibility of changing their reactivities by modifying the macrocyclic ligand with electron-donating or electron-withdraw- ing substituents. Thus the nickel(II) complex with a cyclam analogue containing two methyl groups in the macrocycle (3,10- dimethyl-1,3,5,8,10,12-hexaazacyclodecane) exhibits much higher catalytic activity than the nickel(II) complex with unsubstituted cyclam.205 It should be noted that the insertion of s-acceptor substituents (for example, of fluorine) into the nitrogen-contain- ing ring also leads to improvement in the catalytic activity and selectivity of CO formation (compared to hydrogen elimination).In these cases, the Ni(II) complex with 3,3,10,10-tetrafluorocy- clam proved to be most efficient. The further increase in the number of the fluorine atoms in the macrocycle leads to deterio- ration of activity.206 Metalloporphyrins can also exhibit catalytic activity in reduc- tion of CO2 to CO in aqueous and non-aqueous media.207 However, porphyrins are rapidly consumed in the course of electrolysis and the catalysis ceases after a rather small number of catalytic cycles.Catalysts are poisoned due to carboxylation and hydrogenation of porphyrins (see Ref. 208).{ Mechanisms for formation of different products of CO2 reduction catalysed by metallophthalocyanines have been pro- posed.173, 174 CO2+PcM [PcM_CO2]. The formation of carbon monoxide 2H++2e7+[PcM_CO2] H2O+[PcM_CO], CO+PcM. [PcM_CO] The formation of formic acid [PcM_OOCH], H++e7+[PcM_CO2] [PcM_HOOCH], H++e7+[PcM_OOCH] HCOOH+PcM. [PcM_HOOCH] The formation of methane H2O+[PcM_C], 2H++2e7+[PcM_CO] [PcM_CH4], 4H++4e7+[PcM_C] [PcM_CH4] CH4 +PcM. { Undesirable cathodic processes can be prevented performing gas-phase reactions of CO2 with electroactive films at solid electrolytes.209 In many cases, heterogeneous catalytic reduction of CO2 was carried out with the use of an active catalyst, for example, metalloporphyrin or metallophtha- locyanine, deposited on a solid electrolyte (graphite) and coated with a conducting film (Nafion), which prevents it from passing into solu- tion.173, 174, 176 ± 181, 210 However, these systems are beyond the scope of the present review.126 It should be noted that the formation of the metal7CO2 complex was postulated as the key step in virtually all studies devoted to catalytic binding of CO2 by transition metal com- plexes.211 In most cases, this suggestion was supported only by indirect evidence obtained in IR and NMR spectroscopic studies because, first, the lifetime of these intermediates is short and, second, the presence of the solvent molecules, which are also capable of being coordinated, hinders the detailed analysis of the structure of the complex.The reaction mechanism was investigated also with the use of semiempirical quantum-chemical calculations. For example, an attempt was made to relate the catalytic activity of phthalocya- nines to the energy of the frontier orbitals of the complex.211 It appeared that the catalytic activity of phthalocyanine complexes correlates with the ability of the metal atom to coordinate an additional ligand, which, in turn, depends on the difference between the energy of the d orbitals of the metal atom and the energy of the frontier orbitals of the coordinated ligand.Electrocatalytic reduction of CO2 was examined in aqueous solutions at a graphite electrode, which was coated with PcCo bound to poly-4-vinylpyridine as a donor axial ligand.179, 183, 184 It was demonstrated that the presence of the latter leads to a substantial improvement in the catalytic activity of PcCo due, apparently, to a rise in the electron density on the metal atom. In this catalytic system, the selectivity of CO2 reduction is much higher than that in the case of pure PcCo in spite of the fact that competitive water reduction persists. This is attributable to an increase in the local concentration of CO2 in the near-membrane layer through hydrophobic interactions of CO2 with polyvinyl- pyridine.177 The reaction scheme was proposed.183, 184 H+ e7 [Co(I)Pc27]7 Co(I)Pc 27H+ Co(II)Pc27 e7 [Co(I)Pc37H+]7 CO CO2 7 H 7 O O7 7 O O C C Co(II)Pc 37H+ OCCo(II)Pc27 Co(II)Pc27 H2O H+ The catalytic process starts only at the second reduction wave of PcCo because the first reduction step is followed by rapid addition of H+ to the peripheral nitrogen atoms of the phthalo- cyanine ring and the active species capable of reducing both H+ and CO2 to form H2O and CO, respectively, are generated only at the second wave.However, no direct evidence for the formation of the PcCo ±CO2 adduct was obtained. VIII. Synthesis of carboxylic acids 1. Carboxylation of organic halides In the early 1980s, it was demonstrated that electroreduction of aryl halides catalysed by nickel complexes with PPh3 (see Ref.212) or dppe 213 in the presence of CO2 affords predom- inantly arenecarboxylates rather than biaryls. Based on the results of electroanalytical investigation of the [Ni] ± dppe system, the catalytic cycle was proposed.214, 215 The Pd complex with PPh3 also catalyses electrocarboxylation of aryl iodides and bromides.216 PdCl2(PPh3)2 (7%), PPh3 RC6H4X+CO2+2e7 RC6H4CO¡2 +X7 DMF (50% ± 90%) R=H, 4-But, 4-MeO; X=I, Br. In this reaction, CO2 was assumed to react as an electrophile with [ArPd(0)(PPh3)2]7 generated upon reduction of the aryl- Yu H Budnikova palladium(II) s-complex.217 Aryl chlorides react with Pd(0) too slowly to ensure efficient carboxylation.Aryl triflate and aryl bromide possess higher reactivities and, hence, arylcarboxylic acids can be synthesised starting from the corresponding aryl triflate.57, 58 PdCl2(PPh3)2 (10%) ArOTf+CO2+2e7 ArCO¡2 +TfO7 DMF, 90 8C (52% ± 95%) Ar=4-RC6H4 (R=Cl, CF3, CO2Et, F, Me), 1- or 2-naphthyl. Nonsteroid anti-inflammatory drugs, viz., a-arylpropionic acids, were prepared from the corresponding benzyl chlorides and CO2 in the presence of the Ni ± dppe or Ni ± dppp system [dppp is 1,3-(diphenylphosphino)propane] and cyclooctadiene (COD).218 There is no need to use a catalyst in these reactions, but it suppresses homocoupling, which becomes the main reaction at high concentrations of benzyl chloride and lower CO2 pres- sure.219 ArCH(Me)CO2H ArCH(Me)Cl +CO2 NiCl2(dppp) (10%), COD (10%), e7 THF, HMPA HMPA is hexamethylphosphoramide.Yield (%) Name Product 89 PhCH(Me)CO2HMe 81 naproxen CO2H MeO Pri Me 80 ibuprofen CO2H Me CO2H 76 phenoprofen PhOThere is also no need to use a catalyst in electrocarboxylation of aryl halides or various benzyl compounds in an undivided cell in the presence of a sacrificial aluminium 220 or magnesium 21, 221 anode. Organic halides containing functional groups, which make it impossible to apply direct electrochemical reduction, are sub- jected to electrocarboxylation using a catalyst simultaneously with a sacrificial anode. Interest in transition metal complexes with tetraazamacrocy- clic ligands as applied to electrocatalysis is associated with the fact that these complexes are similar to natural photosynthetic sys- tems.222 The mechanism of CO2 reduction catalysed by tetraaza- macrocyclic complexes of Ni(II) and Co(II) was examined by cyclic voltammetry and preparative electrolysis.Reduction affords CO, which then reacts with a Ni(I) complex to form Ni(0) carbonyl compounds, which eventually poison the cata- lyst.222 Electrocatalytic reduction of CO2 under the action of trinu- clear nickel cluster radicals was investigated by electrochemical methods and spectroelectrochemistry.223 2. Carboxylation of alkenes and alkynes It is possible to subject unsaturated compounds to direct electro- chemical carboxylation.18, 224 However, this procedure is of lim- ited usefulness because only arylated or activated alkenes can be used as substrates. For alkynes, only a few examples of the successful application of this reaction have been reported.225 ± 229 Indirect electrochemical reductive coupling of alkynes with CO2 gives rise to substituted acrylic acids through hydroxycar- boxylation.60, 225 ± 231 Electrolysis was carried out at 20 ± 80 8C in an undivided cell in DMF containing a catalytic amount of a nickel complex in the presence of a sacrificial magnesium anode atMetal complex catalysis in organic electrosynthesis ambient or slightly elevated (5 atm) CO2 pressure.Alkynes containing electron-donating groups are more readily subjected to carboxylation at elevated temperatures, whereas carboxylation of alkynes bearing electron-withdrawing groups proceeds more readily at room temperature. R2 R1 R2 R1 a, b + R1C CR2+CO2 H H CO2H HO2C B A (a) Ni(bipy)3(BF4)2 (10%), e7, DMF, Mg anode; (b) hydrolysis.Ratio A: B Yield (%) R2 R1 71 : 29 38 : 62 H 65 90:10 Prn HMe CO2Et 93 55 72 75 a n-C6H13 Prn Ph Ph Ph a The ratio between the di- and monocarboxylation products was 74 : 26. The reactions proceed predominantly as the cis-addition with good regioselectivity (in the case of terminal alkynes,CO2 reacts at position 2). Monocarboxylation takes place only in the absence of electron-withdrawing groups. This procedure has advantages over the chemical method devised by Hoberg et al.,232, 233 which, for the most part, required stoichiometric amounts of air-sensitive Ni(0)(COD)2 ; instead, readily accessible Ni(II) complexes are used in the electrochemical procedure.In these reactions, the advantages of a sacrificial anode are evident, a magnesium anode being most suitable. Oxidation of the latter gives rise to Mg2+ ions, which can be involved in the catalytic cycle to decompose nickelacycle A thus liberating Ni for further transformations. R R Mg2+ H CO¡ 2 2 2 e7 [NiL2]2+ L, Mg2+, H+ R R Ni(0)L2 LNi O RC CR OA CR CO2 LNi CR L CO2 The formation of the nickelacycle A, which was established by cyclic voltammetry, confirms the proposed mechanism. In the absence of Mg2+ ions (the reactions were carried out in a divided cell in the presence of ammonium ions), the nickelacycle was not decomposed and the reaction was terminated once the starting nickel compound has been completely consumed. After the addition of MgBr2 to a solution of the nickelacycle prepared electrochemically, Ni(II) was regenerated.234 Diacetylenes are used for the construction of polycyclic compounds.Thus bicyclic pyrones were prepared by electro- chemical carboxylation in the presence of CO2 and a stoichiomet- ric amount of a Ni(0) complex.235 This procedure was also applied to the synthesis of linear and cyclic monocarboxylic acids from nonconjugated diacetylenes.236, 237 The composition of the reac- tion products depends on the nature of the ligand.Thus cyclic compounds 11 were prepared with the use of a nickel complex with bipyridyl at normal CO2 pressure, whereas linear adducts 12 were obtained with the use of pentamethyldiethylenetriamine (PMDTA) as the ligand and elevated CO2 pressure (5 atm). 127 +CO2 + 1) 2 e7, NiL2+, Mg anode 2) H2O CO2H CO2H 12 11 Ratio 11 : 12 Yield (%) L 59 : 4.5 9 : 45 35 30 bipy PMDTA The addition of CO2 to diacetylenes containing both the terminal and internal triple bonds proceeds predominantly at position 2. Electrochemical carboxylation of 1,3-diacetylenes in the pres- ence of Ni(PMDTA) as the catalyst proceeded regio- and stereo- selectively to give predominantly (E )-2-alkylidenealk-3-ynoic acids 13 (sometimes, with an impurity of isomeric products 14 and 15).237, 238 R1 1) 2 e7, NiL2+ R1 R2 R2 H+ 2) H2O 13 CO2HR2 R1 R2 H + CO2H + R1 15 14 H CO2H L=PMDTA.Yield (%) R2 R1 15 14 13 58 40 60 37 50 n-C5H11 Ph MeOCH2 Bun Ph n-C5H11 Ph MeOCH2 Ph MeOCH2 7773 11 2501 160 In this process, double bonds proved to be less reactive than triple bonds.239 In nonconjugated as well as in conjugated alkenynes, only the triple bond is carboxylated with regio- and stereoselectivity analogous to those observed for acetylenes.240 According to this procedure, monocarboxylic acids were prepared in good yields starting from 1,2-dienes.241 In the case of alkyl- and cycloalkylallenes, the addition of CO2 proceeds pre- dominantly at the C(2) atom.In the case of arylallenes, the addition occurs predominantly at the C(3) atom.R R R CO2H 1) 2 e7, NiL2+ C +CO2 2) H2O + CO2H H Ar CO2H Ar 1) 2 e7, NiL2+ C + +CO2 2) H2O Ar CO2H R=n-C8H17 , cyclo-C6H11; Ar=Ph, 4-MeOC6H4 ; L=PMDTA; pCO2 =5 atm. Some other examples of homogeneous electrochemical acti- vation of CO2 were also described among which are the synthesis of lactones by telomerisation of butadiene and CO2 ,242 synthesis of bicyclic lactones using nickel or cobalt complexes,243 trans- carboxylation 244 and insertion of CO2 into epoxides catalysed by Ni(cyclam)Br2 .245 IX. Carbonylation of organic substrates Carbonylation of organic substrates is still a complicated problem because the presence of CO leads to deactivation of the catalytic system.The reactions of carbon monoxide with transition metals128 in low oxidation states afford different metal carbonyls, which exhibit low reactivity with respect to the carbon7halogen bonds because of strong coordination with CO. Some metal carbonyls can be activated by electrochemical reduction with generation of active anionic species. For example, aldehydes can be synthesised by electrolysis of a stoichiometric mixture of alkyl halides and iron pentacarbonyl.246, 247 1) e7, DMF or MeCN RX+Fe(CO)5 2) H+ RCHO (30% ± 70%) R=n-C5H11 , Et, Bn; X=I, Br. Carbon dioxide can be used as a CO source. It is well known that complexes of transition metals in low oxidation states catalyse chemical and electrochemical reduction of CO2 to CO.This approach was applied to the generation of the mixed Ni(0)bipy(CO)2 complex by electrochemical reduction of Ni(bipy)2+ in N-methylpyrrolidone or DMF in the presence of CO2 .248, 249 The resulting complex can react with alkyl, benzyl and allyl halides to form symmetrical ketones with regeneration of Ni(bipy)2+. The proposed two-step procedure for the synthesis of ketones involves electroreduction and chemical coupling. It was demonstrated 250 that carbonylation can also be carried out by electrolysis of a solution of organic halide containing a catalytic amount of bipy under an atmosphere of CO in an undivided cell with a stainless steel anode. This process produces symmetrical ketones in good yields.FeCl2 (5%), bipy (5%), RCOR+2X7 2RX+CO+2 e7 DMF or MeCN Yield (%) RX 80 70 62 75 65 62 50 BnCl 2-MeC6H4CH2Br 4-MeC6H4CH2Br 2-ClC6H4CH2Cl MeCH=CHCH2Cl Me(CH2)5Br 4-CF3C6H4Br The anodic reactions afford the Ni(II)(bipy) and Fe(II)(bipy) complexes, which are reduced at a cathode to give intermediates catalysing the synthesis of ketones. In these processes, complexes of nickel coordinated both by bipy and CO serve as reactive species. Iron complexes, apparently, exhibit a synergistic effect. Ketones were successfully prepared by electroreductive cou- pling of organic halides with CO using the Ni(bipy)2+ complex as the catalyst. Either CO entering the solution 250 ± 252 or metal carbonyls 253 serve as a source of carbonyl groups.Electroreduc- tion of CO2 proceeded under the action not only of Ni(0)(bipy) 248, 249 but also of iron porphyrinates in the presence of anodically generated Mg2+ ions.254 Ketones can be prepared not only by electrochemical carbon- ylation of organic halides but also by some other reactions catalysed by nickel complexes. Thus unsymmetrical ketones were obtained by NiBr2(bipy)-catalysed electroreductive coupling of acyl chlorides with alkyl halides.255 Acid chlorides are trans- formed into symmetrical ketones in an undivided cell equipped with a nickel or stainless steel anode. Ni(bipy), MeCN 2 RCOCl+2 e7 stainless steel anode RCOR+2Cl7+CO (45% ± 80%) R=Bn, Ph, 3-FC6H4, 3-MeC6H4, 4-MeC6H4, 4-FC6H4, , 4-BrC6H4, 2,4-F2C6H3, 1-naphthyl.CH2 S It is believed that the first reaction step gives rise to alkynyl- nickel intermediates through oxidative addition of acid chloride to an electrochemically generated unstable low-valent nickel com- Yu H Budnikova plex. Regeneration of divalent Ni yields a bisacylnickel complex. Ketone is derived from the bisacylnickel complex by reductive elimination of Ni bound with one CO molecule.256 In the presence of the Ni(bipy) complex, arylzinc compounds, which are electrochemically generated from aryl halides and zinc salts, react with (CF3CO)2O to give the corresponding aryl trifluoromethyl ketones.79 The reactions involving the electroreductive formation of the carbon7heteroatom bond under the conditions of transition metal catalysis are few in number.For example, mention may be made of electrochemical silylation of allyl acetates in the presence of the Pd ± PPh3 system 257 and the electrochemical synthesis of aryl thioethers from thiophenol and aryl halides.258 Based on electrochemical activation of salts and complexes of some transition metals, catalytic systems were designed for carbonylation of methanol to form dimethyl carbonate.259 The reactions are carried out by passing CO through a methanolic suspension or through a solution of a complex under atmospheric pressure at 20 8C in an ion-exchange membrane cell. Metal complexes catalyse oxidative carbonylation of methanol. 2MeOH+CO+2M(n+1)+ (MeO)2CO+2H++2Mn+.The anodic process leads to regeneration of metal in a higher oxidation state.M(n+1)++e7. Mn+ The redox pairs, which are active in chemical carbonylation promoted by oxygen, viz., Cu(I)/Cu(II), Pd(0)/Pd(II) and Co(II)/ Co(III), were tested as catalysts. However, the electrochemical conditions appeared to be milder than those used in conventional chemical reactions. In some cases, the complexes exhibited higher catalytic activity. In these complexes, bipy, salen, acac and PPh3 were used as ligands. The highest faradaic yields (%) of dimethyl carbonate were achieved with the complexes CuCl(bipy) (84.8), PdCl2(bipy) (64.0), CoCl2 (26.0), RhCl3 (25.0) and AgBF4(bipy) (10.2). The VCl3 , Mn(salen), Mn(acac)2 , ReCl3(bipy), Cr(acac)3 , NiCl2(bipy) and RuCl2(PPh3)2 compounds proved to be either poorly reactive or inactive at all.Only in the presence of the PdCl2(bipy) complex, did more than one redox cycles occur. It is known that many transition metals catalyse oxidation of CO to CO2 in water. It is these transition metals that exhibit the highest activity in oxidative carbonylation of alcohols to form dialkyl carbonates. This reaction mechanism involves the forma- tion of methoxycarbonylmetal intermediates (MCO2Me, where M=Cu, Co or Pd) whose stability and reactivity depends substantially on the structure of the complex and the nature of the ligands.260, 261 For example, CuCl exhibits activity only at 70 8C, whereas it is active even at 20 8C in the presence of bipy. Electroactivation of CuCl, PdCl2 , PtCl2 and AgBF4 occurs only in the presence of bipy.To the contrary, the addition of bipy as well as of acac, salen and PPh3 to CoCl2 , NiCl2 and AuCl3 impairs the efficiency of the process. X. Electrochemical synthesis of organophosphorus compounds from chlorophosphines or white phosphorus The development of synthetic approaches to the preparation of compounds containing P7C bonds based on white phosphorus or phosphorus chlorides is of importance in the preparative chemistry of organophosphorus compounds. Considerable atten- tion is given to the synthesis of tertiary phosphines, which are widely used in organometallic and coordination chemistry. The promising process 262 ± 266 for the preparation of tertiary phosphines is based on electrochemical coupling of mono- or dichlorophosphines with aromatic and heteroaromatic halides catalysed by the nickel complex with bipy in an undivided cell using a sacrificial magnesium or zinc anode (Table 11).Metal complex catalysis in organic electrosynthesis Table 11. Electrochemical coupling of aryl or hetaryl halides and chloro- phosphines catalysed by NiBr2(bipy).262 ± 266 Product ArX Yield Anode (%) a 54 56 52 70 63 66 80 63 65 66 50 45 PhBr 2-BrC6H4Me Ph2PC6H4Me-2 4-BrC6H4Me Ph2PC6H4Me-4 4-MeOC6H4Br Ph2PC6H4Me-4 4-Me2NC6H4Br Ph2PC6H4NMe2-4 4-EtO2CC6H4Br Ph2PC6H4CO2Et-4 3-EtO2CC6H4Br Ph2PC6H4CO2Et-3 2-ClC5H4N Ph2 PC5H4N-2 3-ClC5H4N Ph2 PC5H4N-3 2-ClC4H3S Ph2 PC4H3S-2 2-Cl-6-MeOC5H3N Ph2 P(6-MeO-2-C5H3N) 2-ClC4H3N2 Ph2PC4H3N2-2 3-NCC6H4Br Ph2PC6H4CN-3 5-Cl-1,3-Me2C3HN2 Ph3P 87MgMg Mg Mg Mg Zn Zn Mg Mg Mg Mg Mg Zn Ph2P(5-C3HN2-1,3-Me2) 25 Mg a The preparative yield is given with respect to chlorophosphine.e7, NiBr2(bipy) Ar37nPhnP ArX+PhnPCl37n Mg or Zn anode R Ar= [R=Me, MeO, Me2N, CN, CO2Et, MeC(O)]; N N Me , , , NMe ; n=1, 2. N S N The electrochemical synthesis of tertiary phosphines can be carried out using aryl halides containing either electron-donating or electron-withdrawing substituents in the ring as well as hetaryl halides as substrates. In reactions of aryl halides bearing electron- donating substituents in the ring, a Mg anode must be used, whereas reactions of aryl halides containing electron-withdrawing substituents in the ring must be carried out with the use of a Zn anode.The general scheme of cyclic regeneration of the catalyst was proposed (this scheme is considered below).262 ± 266 The problem of the selective opening of the P4 tetrahedron of white phosphorus and its direct functionalisation assumes increas- ing importance in connection with a search for new environ- mentally safe procedures for the synthesis of organophosphorus compounds. Under the action of electrochemically generated catalysts, viz., the Ni(0)(bipy) complexes, and organic halides, white phosphorus can be converted into compounds containing the P7C bonds (phosphines and phosphine oxides),267 ± 269 Ni(II) being cathodically reduced to Ni(0).Ni(0)L2+L Ni(II)L3+2e7 L=bipy. Mn+ M07n e7 M=Mn, Zn, Al. P R P4+RX Ni(0)L2 7X7 The key step involves the reaction of the organonickel com- pound RNi(II)BrLm with white phosphorus according to the following scheme: P4 P R+Ni(II)Lm . RNi(II)BrLm Ni(0)Lm+RBr The opening of the P4 tetrahedron and the cleavage of the remaining P7P bonds under the action of Ni(0) complexes give rise to binuclear complex A, which rapidly reacts with organic halides to produce mononuclear complex B with the Ph3P3 ligand. 129 N P PhHal P Ni Ni Br2 P N A n+ Ph N P Ph P Ni P N Ph B The formation of the P7Ph bond through this path competes with the reaction of the PhNiBr s-complex with P4.This proce- dure made it possible to perform functionalisation of white phosphorus under mild conditions to obtain compounds with the P7C bonds. For example, the electrochemical method was applied to the preparation of Ph3P and Ph3PO in 70% and 59% yields, respec- tively (DMF, Mg or Zn anode) and to the synthesis of (n-C6H13)3PO in 47% yield (MeCN, Al anode).265 ± 269 XI. Other examples of electrochemical syntheses catalysed by metal complexes Nitro compounds were selectively reduced to amino derivatives using the mediator, viz., the Cp2Ti+ complex, in CH2Cl2 in the presence of dilute sulfuric acid.270 Indirect electrolysis of various nitrobenzene derivatives containing the ester, carbonate or amide group in the ortho position was carried out in dichloromethane.6Cp2Ti++6H2O, 6Cp2TiOH++6e7+6H+ 6Cp2Ti++ArNO2+4H2O 6Cp2 TiOH++ArNH2 . In the second step, the catalyst is regenerated thus resuming the catalytic cycle. The rearrangements of anilines containing the ester or carbonate group in the ortho position into N-acylated o-aminophenols proceed in situ. Upon refluxing in acetic acid, N-acylated o-phenylenediamines are converted into 2-substituted benzoimidazoles. NHC(O)R NH2 NO2 Indirect electrolysis OH OC(O)R OC(O)R NH2 NO2 Indirect electrolysis 7H2O NHC(O)R NHC(O)RHN R N In recent years, the principles of indirect anaerobic electro- chemical regeneration of galactose oxidase (GOase) were devel- oped.271 This enzyme oxidises the primary alcohol group of D-galactose to the corresponding aldehyde.The synthetic use of GOase is of interest because it is highly sensitive to the nature of the substrate and can perform selective oxidation of primary hydroxy groups in polyols. Thus xylitol is oxidised to L-xylose. Substituted ferrocenes exhibiting high catalytic activity were used as mediators. The iron(II) and ruthenium(II) complexes with the phenanthroline ligands appeared to be less efficient. For these processes, phosphate buffers with pH 10.8 are the media of choice. Electrochemical regeneration of GOase allows the pre-130 vention of the undesirable formation of the enzyme inhibitor (hydrogen peroxide), which is generated under aerobic conditions in reactions involving GOase.Mred Sred GOaseox e7 Pox Mox GOasered Anode + R R FeII FeII Anode 7e7 S is a substrate, P is a product,M is a mediator (ferrocene derivative). XII. Characteristic features and mechanisms of reactions catalysed by nickel and palladium complexes 1. Kinetic regularities The application of electrochemical methods to generation of complexes of Ni(0) and other metals in low oxidation states makes it possible to use catalytic amounts of complexes generated at an electrode. In addition, electrochemical methods allow one to obtain quantitative estimations of the rates of catalytic reactions and in some cases even to separate the overall process into individual steps and to evaluate the reactivities of intermediates and stability of different forms of metal complexes.Based on the external morphological traits, three qualitatively different catalytic waves can be distinguished 24, 33, 272 ± 274 in the Ni(II)Ln ± Ni(0)Ln system (L=PPh3 or bipy) (Fig. 1). These are an increase in the current of reduction of the starting Ni(II)Ln complex (see Fig. 1 a), the wave corresponding to reduction of the complex involving the substrate as the ligand (see Fig. 1 b) and the wave of catalytic reduction of the organometallic compound, which is a product of the reaction of the reduced form of the starting complex with the substrate (see Fig. 1 c). In all these cases, the observed pattern can be (with some reservations) considered as a classical pattern and, hence, the increase in the current (the ratio of the catalytic to diffusion current, ic/id) can be used for calculat- ing the rate constant of catalyst regeneration (keff), which describes the overall process.33 c b a I 2 0 2 0 2 0 2 2 1 2 1 1 7E Figure 1.Morphology of catalytic waves in the Ni(II)Ln±Ni(0)Ln sys- tem;24 (a) an increase in the current of reduction of the starting complex; (b) the wave corresponding to reduction of the complex with the substrate, (c) the wave of catalytic reduction of the organometallic compounds; (1) the Ni(II)L complex, (2) and (2 0) the Ni(II)L complex in the presence of increasing amounts of organic halides. Yu H Budnikova k1 Ni(0)(bipy)+bipy, Ni(0)(bipy)2 k71 k2 products, Ni(0)(bipy)+RHal k1k2 keff= [RHal] [bipy] k71 The lower the donor number of the solvent, the higher the efficiency of the catalytic process.33 The nickel complex with bipy proved to be the most active and versatile catalyst, which exhibits a pronounced catalytic effect in reactions with various substrates containing the C7Hal, P7Cl or Si7Cl bonds.24, 33 2.Relationship between electrochemical redox properties of nickel complexes and their reactivities in dehalogenation The redox potentials of the complexes were used as the starting characteristics for predicting the reactivities and the reaction mechanisms of organometallic compounds.24, 26, 32, 33, 275 For a series of Ni(II) and Ni(0) complexes, the electrochemical gap G=Eox7Ered , the electrochemical electronegativity w=(Eox+Ered)/2 and the degree of charge transfer DN=Dw/SG were estimated experimentally.24, 26, 32, 33 The G, w and DN param- eters can be used for evaluating the reactivities of the complexes in oxidative addition to organic halides.RNi(II)LnHal . Ni(0)Ln+RHal These parameters are also suitable for estimating interactions between the complex and various ligands and substrates and for obtaining information on the complex-forming properties and donor-acceptor complex ± substrate interactions. This is of partic- ular importance in the case of organophosphorus substrates exhibiting both s-donor and p-acceptor properties in interactions with transition metals. The largest electrochemical gap character- ising the degree of `hardness' of the complex was found for Ni(II)Br2 .24, 25, 32, 33 For phosphine and phosphite complexes of Ni(II), the G values are much smaller.On the contrary, the electrochemical gap increases and, consequently, the donating ability and polarisability decrease in the series of the Ni(0) complexes with bipy (0.60 V), phen (0.72 V) and Ph3P (1.04 V). Based on these characteristics, the degrees of charge transfer between the complexes and substrates were calculated on the assumption that the Ni(II) or Ni(0) complex and the substrate act as an electron donor and an electron acceptor, respec- tively.24, 25, 32, 33 In the case of Ni(II) complexes, the degrees of charge transfer are small, whereas these values are an order of magnitude higher in the case of Ni(0) complexes. In the latter case, oxidative addition reactions are assumed to proceed. The sign of the degree of charge transfer indicates that the Ni(0) complexes with all substrates exhibit pronounced donor properties (DN440).The reactivities of nickel complexes in catalytic processes were accounted for based on the data from voltammetric measure- ments, results of quantum-chemical calculations and estimations of the G, w and DN values.24, 25, 32, 33 The reactivities of Ni(0) complexes in reduction reactions of organic halides are deter- mined by a number of factors among which are the electro- chemical gap G, the degree of charge transfer DN, the strength of the Ni7P bond, the electronic and steric effects of the ligands and the occurrence of the subsequent redox processes.In the series of the Ni(0)L, Ni(0)L7., L7. mediators (L=bipy or phen) possess- ing reducing properties, the energy of stabilisation of the tran- sition state decreases on going from the inner-sphere to outer- sphere reduction processes, which is accompanied by the corre- sponding decrease in the reaction rate, as compared to the inner- sphere mediator characterised by the same driving force for the process.30 In the studies,26, 276 ± 278 the mechanism of two-electron reduc- tion of the complexes was proposed with consideration for competitive homogeneous redox reactions, the evidence for theMetal complex catalysis in organic electrosynthesis individual steps was cited, and the rate-limiting processes were revealed.In reduction of the nickel(II) complex with bipy, the transfer of the second electron was the rate-limiting step, DE8= E 1 ¡ E2 &760 to 770 mV (E1 and E2 are the standard potentials of transfer of the first and second electrons, respec- tively), whereas competitive redox reactions did not proceed. In reduction of the nickel(II) complex with phen, the transfer of the first electron was the rate-limiting step and DE8&730 to 760 mV; competitive redox reactions were also not observed. Reduction of the Ni(II) complexes with PPh3 and phosphites (PriO)3P, PhP(OBu)2 and (PhO)3P as the ligands involves the transfer of the first electron as the rate-limiting step and is accompanied by chemical coproportionation (PPh3 , DE8&90 mV) and disproportionation (phosphites, DE8<0) reactions.Redox disproportionation and coproportionation reactions are typical of phosphine and phosphite complexes of Ni(II) characterised by higher polarisabilities (`softness') due to small electrochemical gaps. Competitive processes lead to a decrease in catalytic activity of these complexes in dehalogenation because the concentration of the active form of the catalyst in the reaction zone falls; the higher the reaction rate the more rapid the decrease. Actually, it was demonstrated that the Ni complex with bipy possesses the highest catalytic activity of all the complexes under study and reduction of this complex is not accompanied by competitive redox reactions.Undoubtedly, this improves the efficiency of the Ni complex with phen; however, the efficiency is lower than that of the complex with bipy and, consequently, other factors must be taken into account. In the series of phosphine and phosphite complexes of nickel, the catalytic activity is determined by the ratio between the rates of the steps giving rise to the final product and those of the competitive reactions.26 3. Mechanisms of reactions catalysed by nickel and palladium complexes Under conditions of homogeneous catalysis by transition metal complexes, the synthesis of organic compounds involves at least one redox process in which one of the reagents is activated.279, 280 In many processes catalysed by metal complexes, oxidative addition plays a key role in the transformation of the organic substrate into a highly reactive intermediate.280 The mechanisms of these reactions were described in many papers and monographs (see, for example, Refs 280 ± 284).Functionalisation of organic halides (RX) in the course of oxidative addition to a zero-valent metal complex [M(d 10)] affords organometallic derivative A. R M(d 10) + RX M(d 10) X A Two main mechanisms were proposed for this process. The first mechanism involves both the three-centre addition 285 and SN2 substitution.286 The second mechanism describes a multistep process proceeding through the formation of paramagnetic inter- mediates.287 Thus the observed selectivity in the reactions of polyhalides 288 indicates that their mechanism is analogous to the well-known nucleophilic aromatic substitution.289, 290 However, the mechanism of the second type must not be ruled out taking into account the formation of paramagnetic nickel(I) complexes and products derived from radical intermediates in reactions of some aryl halides with Ni(0) complexes.Tsou and Kochi 291 proposed the radical-chain mechanism for the stoichiometric reactions of aryl halides with zero-valent nickel complexes. This mechanism assumes intermediate paramagnetic nickel complexes to account for the formation of diarylnickel(III) species in the catalytic cycle. The authors demonstrated that oxidative addition of aryl halides to (triphenylphosphine)nickel(0) 131 afforded trans-arylnickel(II) halide A.In addition, the reaction gave rise to paramagnetic nickel(I) halide B as a by-product. Ni(0)L4+ArX ArNi(II)XL2+Ni(I)XL3 B A L=PEt3 , PPh3; X=Cl, Br, I. In this case, the ratio between the products A and B depends on the nature of the halide ion (increases in the series I<Br<Cl), the nucleophilicity of the substituents and the polarity of the solvent. The general scheme of oxidative addition was reported. k1 Ni(0)L3+L Ni(0)L4 k2 [Ni(I)L3ArX.7] Ni(0)L3+ArX ArNi(II)XL2+L [Ni(I)L3ArX.7] diffusion Ni(I)L3+X7+Ar. However, the proposed mechanism does not give a precise idea of the sequence of the reaction steps yielding the diaryl- nickel(III) complex from ArNi(II)XLn under the conditions of the catalytic cycle, i.e., in the presence of reduced metal.Amatore and coworkers,78, 215, 292 ± 295 Bontempelli and co- workers 296 ± 299 and research teams headed by Perichon 300 ± 302 and Kargin 166, 303, 304 used electroanalytical methods in combina- tion with the preparative electrosynthesis, which allowed them to reveal the key steps of many electrocatalytic reactions. One of the first studied processes is catalytic coupling of allyl halides, which proceeds with a very high rate according to the following scheme:298 E1, 2 e7, 2L [Ni(II)L2Solv4]2+ 74Solv C C C X C L Ni(I) C 73L [Ni(0)L4] A X C B C B E2, e7, 3L X7+ +A 7 C C 3L C3H5X E2<E1 L=PPh3. In this case, the catalyst, viz., the highly reactive nickel(0) triphenylphosphine complex A, is generated in situ by electro- chemical reduction of the corresponding Ni(II) complex, which is stable and can be prepared by anodic oxidation of nickel in the presence of PPh3 .48, 297 ± 299 The Ni(0)(PPh3)4 complex (A) is involved in oxidative addition with allyl halide resulting in functionalisation of the C7Hal bond to form the allylnickel p-complex B.The adduct B is reduced at a potential E2 , which is more negative than the potential of reduction of the complex (E1) but is more positive than the potential of reduction of allyl halide (*71.8 V with respect to a saturated calomel electrode). The reduction is accompanied by the cleavage of the Ni7C bond. The subsequent transformation of the allyl fragment affords a coupling product, viz., hexa-1,5-diene.The zero-valent nickel complex is regenerated and the new catalytic cycle starts. The proposed mechanism was experimentally confirmed. An analogous electrocatalytic cycle was suggested for the synthesis of hexa-1,5-diene from allyl bromide in the presence of nickel complexes with 2,20-bipyridyl 298 as well as from allyl132 chloride under the action of the electrochemically generated cobalt complex with 2,20-bipyridyl.305 However, the numbers of the catalytic cycles in these processes were rather low due to rapid decomposition of the catalyst. Shiavon et al.48 made an attempt to generalise the notions of the reaction mechanisms of organic halides under the conditions of homogenous catalysis by Ni(0) complexes with phosphine ligands.In this case, the efficient electrochemical process occurs through a lowering of the activation energy of RX reduction and cyclic regeneration of the catalyst. At less negative potentials (jERXj>jENi(II)Lj), a lowering of the activation energy and RX reduction lead to a change in the reaction pathway compared to that observed in non-catalytic conditions because heterogeneous reduction of RX proceeds at substantial overpotentials and high cathodic potentials. As a consequence, the radical generated upon the transfer of the first electron is reduced to the carbanion and the corresponding hydrocarbon RH and does not give coupling products. In all cases, reduction can proceed via a series of simple steps involving electrochemical reduction of the nickel(II) complex to Ni(0), activation of organic halide through its oxidative addition to the Ni(0) complex and reductive elimination with the cleavage of the metal7carbon bond (either thermal decomposition or further cathodic reduction) as the key steps.48 a RX 2 e7, 2L [Ni(II)L2(MeCN)4]2+74MeCN [Ni(0)L4] 72L [Ni(II)L2RX] A [Ni(II)L2R2] RX e7, L [Ni(I)L2RX]7 [Ni(0)L3R]7 7X7 7X7 R R+[Ni(0)L4] b A R R and other products [Ni(I)L3X]+R c A alkene+[Ni(II)L2HX] B alkane+alkene+H2+Ni(I)L3X L=PPh3; X=I, Br, Cl; R =Alk, Ar.It should be noted that intermediates of the catalytic cycles were neither detected nor isolated.48 The path a is realised if the s-organonickel complexAis rather stable, as in the case of PhBr and BnCl at low temperature.For the catalytic process involving these halides to occur, the complexes A must be reduced. Later on, it was shown 87, 301, 302, 306 that a Ni(0) complex serves as a reducing agent and the catalytic process takes place at ENi(II)/Ni(0) . The path b is typical of neopentyl bromide and benzyl iodide at 25 8C, the coupling product being formed via dimerisation of the radicals generated upon the homolytic cleav- age of the nickel7carbon bond in the complex A. The reaction with butyl bromide follows the path c. In the latter case, thermal decomposition of an organonickel intermediate occurs through b-elimination involving the nickel hydride complex B as an intermediate.For homocoupling to proceed, the reaction must take the path a, which is realised at more negative working potentials, the composition of the products being determined by the ratio between the paths a and c. Oxidative addition of organic halide to the Ni(0) complex is the rate-limiting step. An increase in catalytic current allows the comparison of the reaction rates for various halogen-containing derivatives. The reactivity was found to decrease in the series BunI>ButCH2I>BnCl>PhBr in parallel with the change in dipole moment. Electroanalytical methods provide information on the reac- tion mechanisms of coupling of aryl halides. In the general case, a zero-valent nickel or palladium complex is formed at a cathode if PPh3 , dppe or bipy serve as ligands.The first step of the catalytic cycle involves oxidative addition of aryl halide to a Ni(0) or Pd(0) complex giving rise to a s-arylnickel or s-arylpalladium inter- Yu H Budnikova mediate. Coordinatively unsaturated complexes of the Ni(0)(bipy),272 Ni(0)(dppe) 292 or Pd(0)(PPh3)2 type 295 proved to be much more reactive than the corresponding coordinatively saturated complexes. The reactivities of the Ni(0) or Pd(0) species depend also on the nature and the concentration of halide ions bound to the starting complex or added to the reaction medium.218, 238 The formation of biphenyl from bromobenzene catalysed by Ni(dppe) was examined in detail 292, 293 and the catalytic cycle for this process was proposed.In this catalytic cycle, the s-arylnickel intermediate A is initially reduced to the corresponding Ni(I) complex B followed by the transformation into the diarylnickel(III) complex C. e7 ArX ArNi(II)X A X7 e7 e7 Ni(I) Ni(0) Ni(II) ArNi(I) B ArX X7 e7 Ni(I)X Ar2Ni(III)X C Ar Ar The complex C is subjected to reductive elimination to form the corresponding product and the Ni(0) complex from which Ni(0) is generated.292, 293 Hence, homocoupling proceeds through intermediate paramagnetic [Ni(I) and Ni(III)] and diamagnetic [Ni(0) and Ni(II)] species. The threshold potential (72 V with respect to a saturated calomel electrode) was demonstrated to exist, which is necessary for the formation of biphenyl. At a less cathodic potential, the reaction affords the phenylnickel deriva- tive PhNi(II)(dppe)Br, which needs to be reduced for the cyclic process to occur.It should be noted that the scheme proposed based on voltammograms 292, 293 lacked experimental support because none of the postulated intermediates was isolated or identified. Equations describing the kinetic characteristics of the assumed sequence of the reactions were derived and the rate constants of different steps were calculated. It is believed that reductive elimination is the rate-limiting step. The formation of biphenyl was completely suppressed upon introduction of CO2 into the reaction system giving rise to a new catalytic cycle to form carboxylation products. ArX ArCO¡2Ni(0)L2 e7 ArNi(II)XL2 Ar CO2Ni(I)L2 e7X7 Ar Ni(III)L2 ArNi(I)L2 O C O CO2 The reaction mechanism was proposed based on the results of an investigation of the reaction products, examination of the voltammetric behaviour of the starting complexes in the presence of the substrate and the behaviour of intermediates and kinetic calculations of individual steps of the process.Since the addition of CO2 to ArNi(I)L2 (k=1±2 mol71 litre s71) proceeds approx- imately 100 ± 200 times faster than the addition of ArX, the formation of biaryls in the presence of stoichiometric amounts of CO2 is completely eliminated.215, 294, 307 By analogy with the reactions catalysed by Ni(dppe), two hypothetical catalytic cycles involving the formation of a s-aryl- nickel intermediate were proposed for the reactions in the presence of the Ni(bipy) complex.In the catalytic cycle A, the Ni(I)XMetal complex catalysis in organic electrosynthesis complex (16), which is derived from the Ni(III) complex (17) in the reductive elimination step, undergoes disproportionation into Ni(0) and Ni(II) followed by reduction of Ni(II).305 An alternative mechanism assumes the formation of biaryl in the course of metathesis of s-arylnickel(II) complex 18 accompanied by regen- eration of Ni(II) (the catalytic cycle B).164, 230 Ni(II) 2 e7 72 e7 Ni(0) ArX ArNi(II)X 18 X7 ArNi(I) e7 ArNi(II)X Ar2Ni(II) Ar2Ni(III)X 17 The reaction mechanism of coupling of organic halides under A B the action of the electrochemically generated Ni(0)(bipy) com- Ar Ar Ar Ar Ni(0) Ni(I)X 16 Ni(0) ArX ArX 72 e7 Ni(II) Ni(0) 2 e7 The reactivity of the nickel(0) complex with bipy with respect to carbon monoxide and organic halides under conditions of competitive reactions was examined by electroanalytical methods, which enabled the authors to devise a new procedure for the synthesis of ketones.251, 252 Reduction of Ni(bipy)2+ in the pres- ence of an excess of CO rapidly afforded the inactive Ni(0)(CO)2(bipy) complex.It was noted that RX remained intact when electrolysis was carried out with bubbling ofCOthrough the reaction mixture, i.e., when the solution was saturated with CO. Organic halides were efficiently converted into ketones when the concentration of CO was limited and the possibility existed of generating a mixture of Ni(0)(CO)2(bipy) and Ni(0)(bipy).Based on the kinetic data, two alternative pathways for the formation of acylnickel complex 19 were proposed. Ni(0)(CO)2(bipy) Ni(0)(CO)(bipy) Ni(0)bipy k3 RX k1 RX k2 RNi(II)(CO)X(bipy) RNi(II)X(bipy) CO k4 RCONi(II)X(bipy) 19 The first pathway involves oxidative addition of RX to Ni(0)(CO)(bipy) (the rate constant k1), whereas the second path- way includes oxidative addition of RX to Ni(0)(bipy) (the rate constant k2) followed by addition of CO from the mixed com- plexes Ni(0)(CO)2(bipy) or Ni(0)(CO)(bipy) or fromCOpresent in the solution (the rate constant k4). The reactions of highly reactive organic halides (for which k2>k3 ), such as aryl iodides, follow the second pathway.In contrast, benzyl chlorides, aryl bromides and alkyl halides are moderately reactive with respect to Ni(0)(bipy) (k2 55 k3) and their reactions follow the first path- way.The preparative synthesis of ketones was carried out in the presence of catalytic amounts of Ni(bipy)2+ (5% with respect to RX).250 Under these conditions, the constants k2 and k3 have similar values and the two pathways for formation of the acylnickel complex 19 compete with each other.228 The Pd ± PPh3 system is characterised by two-electron reduc- tion of the s-arylpalladium intermediate 77 analogous to reduc- tion of the corresponding intermediate in the Ni ± PPh3 ± ArX system studied earlier.48, 231 In the presence of the Pd ± PPh3 system, the biaryl is formed, apparently, through reductive elimination from diarylpalladium(II) followed by regeneration of Pd(0). 133 ArPd(II)X ArX 2 e7 X7 ArPd(0)7 Pd(0) X7 ArX Ar Ar Ar2Pd(II) Many key steps of the mechanisms of reactions catalysed by Ni(0) complexes were suggested without sufficient substantiation.In some cases,301 the catalytic waves in voltammograms were incorrectly interpreted and the calculated rates of particular steps were in error. plexes was investigated.303 o-Bromotoluene was chosen as a model aryl halide. Its reaction with Ni(0)(bipy) afforded the stable s-complex 2-MeC6H4Ni(II)Br(bipy) (20) through oxidative addi- tion. Ni(0)(bipy)+2 Br7, NiBr2(bipy)+2 e7 Ni(0)(bipy)+2-MeC6H4Br 2-MeC6H4Ni(II)Br(bipy).20 Reduction of the complex 20 gave rise to a new complex, viz., (2-MeC6H4)2Ni(0)Br(bipy) (21), which was isolated and charac- terised. The stability of arylnickel s-complexes containing the methyl group in the ortho position of the aromatic nucleus is attributed to the steric factors, which hinder the rotation about the Ar7Ni s-bond and the axial attack of the reagents on the nickel atom.53, 308 The complex 21 was reversibly reduced to radical- anionic intermediate 22. e7 2-MeC6H4Ni(I)(bipy)+Br7 (2-MeC6H4)2Ni(0)Br(bipy) 2-MeC6H4Ni(II)Br(bipy) 20 2-MeC6H4Ni(II)Br(bipy) 20 21 e7 21 [(2-MeC6H4)2Ni(0)Br(bipy)]¡ 22 As the outer-sphere electron carrier, the complex 22 can reduce 2-bromotoluene to toluene, which was confirmed by chromatography.Slow decomposition of [(2-MeC6H4)2. .NiBr(0)(bipy)]¡ (22) in solution to form a biphenyl derivative was a competitive reaction. This reductive elimination was induced by electron transfer. (2-MeC6H4)2+Ni(0)(bipy)+Br7. [(2-MeC6H4)2Ni(0)Br(bipy)]¡ 22 The mechanism of Ni(0)(bipy)-catalysed cross-coupling of aryl and hetaryl halides was investigated taking into account the notion of the mechanism of homocoupling reported earlier.303 It was demonstrated that a Ni(I) complex serves as the active form in cross-coupling. 2-MeC6H4(Ar)Ni(II)X(bipy) 2-MeC6H4Ni(I)(bipy)+ArX 2-MeC6H4Ar+Ni(II)X(bipy). Aparticular complex can either serve as an efficient catalyst of homocoupling of organic halides or be poorly active or even inactive in cross-coupling of two different RX.In the general case, the efficiency of the catalyst depends on the following factors. 1. The possibility of rapid regeneration of the catalyst, i.e., the number of catalytic cycles. 2. The reaction rate of reductive addition of the arylnickel intermediate to aryl halide must be higher than the rates of its competitive decomposition, protonation, etc.134 3. The reaction rate of reductive elimination of the Ar1Ar2NiX complex must be rather high for regeneration of the catalyst (conceivably, promoted by the electron transfer) to be successful. The electrochemically synthesised stable s-complex 2-MeC6H4Ni(II)Br(bipy) (20) proved to be a convenient model for studying the mechanism of formation of tertiary phos- phines.304 ± 306 Thus it was demonstrated that the cross-coupling product containing the P7C bond was generated in the substitu- tion reaction (37n) ArNi(II)Br(bipy)+PhnPCl37n Ar37nPhnP+(37n) NiBr(II)Cl(bipy) n=0±3.The competitive pathway through nickel phosphides is not realised. Phosphines are generated primarily according to the following sequence of reactions bipy ArNi(I)(bipy)+X7, ArNi(II)X+e7 ArPh2PNi(II)Cl(bipy), ArNi(I)(bipy)+Ph2PCle7 [ArPh2PNi(I)Cl(bipy)]7 ArPh2PNi(II)Cl(bipy) 23ArPh2P+Ni(0)(bipy)+Cl7. Reductive elimination of the (triarylphosphine)nickel com- plex ArPh2PNi(II)Cl(bipy) (23) affords the target compounds. Ni(0) or e7 Ph2PAr Ph2PCl Ph2PCl ArNi(I) X7 A Ni(II)ClX B ArNi(II)X ArPh2PNi(II)Cl 2 e7 e7 Cl7 Ni(0) Cl7, X7 Ni(0) ArX Ph2PAr ArX X=Cl, Br.In the initial steps of electrolysis, products are formed through the reactions of the catalytic cycle A, whereas the reactions of the catalytic cycle B become dominant as the catalyst is consumed. The use of the complex 20 as a model compound made it possible to follow the pathway of formation of secondary alcohols in electroreductive coupling reactions of organic halides with aldehydes catalysed by the nickel complex with 2,20-bipyridyl.166 The yield of the product was found to be proportional to the concentration of NiBr2(bipy) in solution. It was suggested that nickel ions were involved in the corresponding nickel alkoxide thus being removed from the catalytic cycle.In addition, it was shown that alcohol was not generated at a potential of Ni(II)/Ni(0) reduction (71.2 V with respect to a saturated calomel electrode). Cross-coupling proceeded only at more cathodic potentials cor- responding to further reduction of the s-complex 2-MeC6H4Ni(II)Br (71.35 V). PhCHO e7 2-MeC6H4Ni(I)(bipy) 2-MeC6H4Ni(II)Br(bipy) 7Br7 PhCHC6H4Me-2 +bipy. ONi 4. Mechanisms and kinetics of reactions in the presence of palladium catalytic systems Palladium serves as the most versatile catalyst because of its high efficiency in reactions of aryl halides, vinyl halides (triflates) and Yu H Budnikova allyl derivatives with nucleophiles (cross-coupling, Stille coupling, Suzuki coupling, Heck reaction, Tsuji ± Trost reaction) to form the C7C, C7H, C7N, C7O, C7P, C7S and C7C(O)7C bonds.309 In the presence of an electron source, the reactions of these substrates with electrophiles can give rise to the C7C, C7H and C7CO2 bonds.310, 311 The efficiency of palladium is associated with the fact that zero-valent palladium is capable of activating C7X bonds (X=I,312, 313 Br,313 Cl,313 O314) by oxidative addition yielding organopalladium(II) complexes, which are able to react with nucleophiles.315 ± 318 The nucleophilic reaction affords a new organopalladium(II) complex from which the final product is generated in one or several steps.309 2 The recent data published in the literature (see the review 308 and references cited therein) allowed revision of the commonly accepted mechanisms of palladium-catalysed reactions.It appeared that the reactivity of the low-coordinate 14-electron Pd(0)L2 complex, which serves as an active species initiating the catalytic cycle, depends not only on the nature of the ligand at the Pd atom but also on the precursor of the Pd(0) complex. With the aim of revealing the reasons for this specificity, the structures and reactivities of different Pd(0) complexes prepared by oxidative addition were studied. It was found that the oxidative addition to aryl halides afforded various arylpalladium(II) complexes (neu- tral, anionic or cationic).309 For example, the oxidative addition of aryl triflates to Pd(0)(PPh3)4 gave rise to the cationic ArPd(II)(PPh3)á complexes, whereas the neutral ArPd(II)Cl(PPh3)2 complexes were obtained in the presence of chloride ions. These complexes can be related by the dynamical equilibrium.The structures of arylpalladium(II) complexes depend substantially on the structure of the efficient Pd(0) com- plex involved in the oxidative addition and on the nature of the anions bound to Pd(0). Earlier, it was assumed 295, 309, 312, 313 that two-electron reduction of PdCl2(PPh3)2 afforded the Pd(0)(PPh3)2 complex. However, the 31P NMR spectrum of this complex after its exhaustive electrolysis has three signals 295 belonging to three different anionic complexes (24 ± 26), which are at dynamical equilibrium. 27 Cl L L Pd(0) Pd(0) L L Cl 24 27 7 Cl L L 2Cl7 2 2 Pd(0) Cl Pd(0) L L Cl 26 252 e7 PdCl2L2 L=PPh3 .The dimer 24 is the most reactive species. Hence, the Pd(0)(PPh3)2 complex does not exist in solution. When generated in the presence of halide ions, this complex gives anionic Pd(0) complexes. Oxidative addition proceeds more rapidly in the presence of cations (Li+ or Zn2+).309 The reactions of the cations with the chloride anions bound to palladium(0) produce ionic pairs and improve the reactivity of the complex. L L + 7 7 Cl ZnCl Pd(0) Cl Li+ Pd(0) L LIn the reactions with poorly reactive aryl bromides, the oxidative addition of aryl bromide to zero-valent palladium complex 27 is conceivably the rate-limiting step.315 ± 317 For more reactive aryl halides, transmetalation [the nucleophilic attack on trans-ArPdXL2 (28)] was considered as the rate-limiting step.318, 319 Reductive elimination from cis-ArPdNuL2 (29) gen-Metal complex catalysis in organic electrosynthesis erated upon the rearrangement of the trans-ArPdNuL2 complexes (30) can also be the rate-limiting step.318, 320, 321 Pd(0)L4 ArX ArNu Pd(0)L2 27 L L Ar Pd X Ar Pd L L trans-28 Nu cis-29 Nu7 L Ar Pd Nu X7 L trans-30 The results of the study 322 provided convincing evidence that reduction of PdCl2(PPh3)2 yielded the anionic palladium(0) com- plex Pd(0)(PPh3)2Cl7 and that the oxidative addition afforded the anionic five-coordinate arylpalladium(II) complex ArPdXCl(PPh3)¡2 , which was slowly transformed into trans- ArPdX(PPh3)2 .322 Under these conditions, the Cl7 anion was the best nucleophile and simultaneously the best leaving group.Hence, this rapid equilibrium is the degenerate SN1 nucleophilic substitution. Based on the above-mentioned results, the new catalytic cycle was suggested. PdCl2L2 Reduction 7 L ArNu ArX Pd(0)Cl L 7 7 L L Cl Nu Ar Pd Ar Pd X X L33 L32 Solv L Solv Solv Ar Pd Solv+Cl7 Nu7 X L trans-31 L=PPh3. The nucleophile attacks the intermediate five-coordinate neutral arylpalladium(II) complex (31) to give the anionic five- coordinate complex [ArPd(II)XNuL2]7 (32) in which the aryl group and the nucleophile are arranged in such a way as to provide rapid reductive elimination. Hence, the trans-ArPdXL2 complex (28) may be ignored as an intermediate of PdCl2(PPh3)2-catalysed cross-coupling.In the absence of nucleophiles, the formation of trans-PhPdI(PPh3)2 was demonstrated to be accelerated by cati- ons 322 because the equilibrium 31 33 was shifted toward the complex 31. In the presence of a rather strong nucleophile, the reaction follows the above-described scheme. Evidently, its mech- anism can be changed in the catalytic cycle depending on the reaction conditions. The reaction of alkenes with aryl halides catalysed by palla- dium complexes was discovered by Heck in 1968 (see the review 323). In the presence of chiral ligands, enantioselective reactions can be carried out.323, 324 Ar [Pd] +ArX+NEt3 R +Et3NH+X7 R [Pd]=Pd(OAc)2+nL.135 The Heck reaction is performed in the presence of a base and, generally, in DMF. Of the catalytic systems, which were empiri- cally chosen for the Heck reaction, mixtures of Pd(OAc)2 and phosphines exhibit the highest efficiency. The catalytic cycle proposed for the Heck reaction in the study 309 differs from that postulated earlier 315 ± 317 by the reduction of Pd(II) to Pd(0) under the action of phosphine 325 ± 327 and by the presence of the anionic Pd(0)(PPh3)2(OAc)7 complexes (34) generated in situ. The latter undergo oxidative addition to aryl halides yielding the short-lived five-coordinate arylpalladium (II) complex (35). Then the ArPd(II)(OAc)(PPh3)2 complex (36) is subjected to the nucleo- philic attack by alkene.Reductive elimination of complex 37 affords the reaction product and hydride complex 38 from which the catalyst 34 is regenerated under the action of triethylamine. Pd(OAc)2+2 PPh3 Pd(II)(OAc)2(PPh3)2 PPh3 ArX NEt3 [Pd(0)(PPh3)2(OAc)]7 34 [HNEt3]+ [ArPd(II)X(OAc)(PPh3)2]7 35 HPd(II)(OAc)(PPh3)2 38 X7 Ar ArPd(II)(OAc)(PPh3)2 36 Ar Pd(II)(OAc)(PPh3)2 R R H 37 R The results of the cited studies provide evidence that the anionic complexes generated from precursors of zero-valent palladium complexes play a key role and account for the empirical data concerning the characteristic features of catalytic palladium systems.328 ± 333 XIII. Conclusion To summarise, many interesting synthetic applications combining electroreduction with transition metal complexes were reported in the last 15 years.Electrochemical equipment and procedures for electrosynthesis become increasingly simple due to which many electrochemical methods compete with conventional chemical methods. Presently, some reactions, which have been previously described as catalytic processes, for example, carboxylation of organic halides or homocoupling of alkyl halides, can be carried out by direct electroreductive coupling in the presence of a sacrificial anode. However, electrochemical transition metal cat- alysis is often more selective. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (2) 141 ± 158 (2002) Sorption mechanism and prediction of sorbents' behaviour in physicochemical systems I V Melikhov, D G Berdonosova, G I Sigeikin Contents I. Introduction II. General pattern of sorption III. Sorption gas dynamics and hydrodynamics IV. Adsorption kinetics V. Sorbate migration into the bulk of a body VI. Equilibrium sorption VII. Accompanying phenomena VIII. Sorbent degradation IX. The search for an optimum sorbent X. Conclusion Abstract. semiquantitative and quantitative of possibility The The possibility of quantitative and semiquantitative prediction of sorbents' behaviour in various physicochemical prediction of sorbents' behaviour in various physicochemical systems is discussed. The attention is primarily devoted to the systems is discussed.The attention is primarily devoted to the investigation of elementary processes at different surface areas of investigation of elementary processes at different surface areas of a sorbent grain. It is noted that the achieved level of the a sorbent grain. It is noted that the achieved level of the description of sorption processes based on semiquantitative description of sorption processes based on semiquantitative estimations provides sufficient information to predict sorption estimations provides sufficient information to predict sorption kinetics and equilibrium sorption of possibility The posteriori kinetics and equilibrium a posteriori. The possibility of sorption by agglomerates grain and grain sorbent separate a both by both a separate sorbent grain and grain agglomerates is is analysed.on processes sorption of peculiarities The analysed. The peculiarities of sorption processes on biosorbents biosorbents are for sorbent optimum an for search the in Steps considered. are considered. Steps in the search for an optimum sorbent for a given 242 includes bibliography The described. are system given system are described. The bibliography includes 242 refer- refer- ences. I. Introduction In the present review, sorption implies the transfer of a substance onto the surface or into the bulk of solids or liquids from the environment without formation of a separate phase of the substance. Such a viewpoint corresponds to a common sorption definition (see Ref. 1, for instance), though it is much broader than the interpretation of the process as the uptake of a substance from the medium by a specially prepared sorbent only.In real systems, any solid or liquid phase of a substance is a sorbent for the other substances being present in the environment. The character of such a sorption differs from that on a specially prepared sorbent in terms of quantity only. I V Melikhov, D G Berdonosova Department of Chemistry,M V Lomo- nosov Moscow State University, Leninskiye Gory, 119992 Moscow, Russian Federation. Fax (7-095) 939 02 83. Tel. (7-095) 939 34 49. E-mail: melikhov@radio.chem.msu.ru (I V Melikhov) Tel. (7-095) 939 32 07. E-mail: berd@radio.chem.msu.ru (D G Berdonosova) G I Sigeikin Interdepartmental Centre of Analytical Research at the Presidium of the Russian Academy of Sciences, ul.Dm. Ul'yanova 5, 117333 Moscow, Russian Federation. Fax (7-095) 135 88 25. Tel. (7-095) 135 20 58 Received 2 July 2001 Uspekhi Khimii 71 (2) 159 ± 179 (2002); translated by A F Nasonov #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n02ABEH000688 141 142 143 146 148 150 152 154 155 155 The sorption accompanies any processes occurring in hetero- geneous systems, and so researchers from almost all laboratories over the world deal with it to a greater or lesser extent. Informa- tion about sorption is mainly an incidental result of laboratories' work, although a vast number of studies specially cover the study of sorption and sorbents every year.The scope of sorption phenomena under investigation has significantly been extended 2± 11 during the last decade. Methods for the detailed and express analysis of sorbents' and sorbates' state have gained ground, efficient methods for the mathematical simulation of sorption processes have become available. Yet the main thing is that the possibility of observing the behaviour of separate adsorbed atoms and molecules in situ has arisen. The first observations of adsorbed atoms were made as early as 1958.12 However, such observations became widespread as late as the present time. As a result, a transition to a new stage of sorption investigation is possible, which can be called the stage of visual- isation of sorbed molecules. A change in direction of many sorption investigations can be considered as the first indication of the advent of the new stage.If previously one aimed to create a macrokinetic model of sorption first and then make conclusions about molecules' motion, at present, one aims to obtain experimental data about behaviour of separate molecules first and, based on this, interpret macro- kinetic sorption characteristics.13 The second indication is the introduction of a new research methodology avoiding application of a priori conceptions of the character of elementary processes. This methodology includes successive alternate implementation of laboratory, computing and, further, pilot experiments, based on the data obtained from observations in situ on connection of molecular motion with sorption macrokinetics.14 The fact that there is a tendency to predict the behaviour of sorbing bodies quantitatively by solution of kinetic equations describing the state change of these bodies and the medium during sorption can be considered as the third indication.The prediction can be incorrect if the kinetic equations employed ignore a number of necessary factors. However, methods for composing and solving kinetic equations of sorption are presently being improved rapidly. This review considers only new studies giving an idea of the recent achievements in the investigation of elementary processes142 with an emphasis on the data obtained by visualisation of molecular motion.The matter of the synthesis and usage of sorbents to solve applied problems is not concerned here. II. General pattern of sorption Consider a system which consists of sorbing bodies and a liquid or gaseous environment containing a sorbate. The `bodies' can be represented by single crystals; microcrystals' agglomerates accreted so that the medium can penetrate into pores remaining between them; aggregates of particles of the amorphous sub- stance; globules of polymeric molecules; pieces of tissue from sorbing fibres, etc. The bodies can form dense layers, localise in certain parts of the system or move freely in its bulk. The sorbate molecules (ions) migrate from the bulk of the medium to the surface of each body and transfer into its adsorption layer during sorption. The adsorbed molecules migrate in this layer, diffuse into the bulk of the body or desorb.Molecules in the adsorption layer interact with each other and with the sorbent atoms located in the near-surface monoatomic layer of the bodies (Fig. 1). If the interaction of sorbate molecules with each other is sufficiently strong, they assemble into two- dimensional clusters that can grow over the surface until they contact neighbouring clusters. The two-dimensional clusters do not form on weak interaction of absorbed molecules with each other, while the influence of surface atomic relief on adsorption increases. Migrating over the surface, molecules stay on an adsorption centre for some time, then jump to an adjacent centre.Molecule's dwell times on vertices, edges or steps of single crystal faces can differ by factor of tens in this process. This difference is the case for molecules' dwell times on adsorption centres located at different crystal faces with different atomic relief. Therefore, faces with an unequal number of steps have different sorbing abilities. Each face is characterised by its own local adsorption factor and sorptive capacity.15 The difference in sorbing properties of different faces is levelled if a body represents an agglomerate of microcrystals with pores between them. In that case, the sorbate first gets into pores between the microcrystals and subsequently adsorbs on their surfaces and diffuses into the bulk.Microcrystals have usually various habits and differ in composition, defectiveness, faces' relief and mutual arrangement. Hence, agglomerate proper- ties can vary significantly.16 Considerable distinctions in proper- ties are observed for aggregates of amorphous particles, globules of polymeric molecules, etc.17, 18 Taking into consideration the discussion above, one should operate with a distribution function of sorbing bodies (sorbent particles) over properties (state parameters) to describe sorption 2 3 1 7 5 4 6 Figure 1. Scheme of the arrangement of adsorbed molecules on the surface of the sorbent crystal. (1) Single molecule, (2) two-dimensional cluster on a face, (3) cluster at the vertex, (4) molecule at the adsorption site on the step end, (5) molecule that passed to the near-surface monolayer, (6) molecule at the edge, (7) single molecule at the vertex. I V Melikhov, D G Berdonosova, G I Sigeikin q pN , jOfxig; tU a qx1 qxi qxp where xi (i=1, 2, ..., p) is one of the state parameters of the particles at the moment t, p is the number of the state parameters considered, N is the number of particles in a volume unit of the system, which have values of state parameters not exceeding {xi}.The state of each body is being changed during sorption: they undergo Ostwald curing and structural ordering,19 ¡¾ 21 the bodies dissolve partly and sorb extraneous agents.21, 22 If bodies are in a stirred medium, they are partly destroyed upon collisions with each other and with walls of the system (vessel) and decompose at high temperature.23 Bearing this in mind, one should define the state of each body with external parameters, among which are the spatial coordinates of its centre of gravity {Xi}, the velocity of movement relative to the walls (~uk) and the mass mk, and with internal parameters, the habit, characteristics of surface relief, and the phase and elemental composition.Each sorbing body can be presented as an aggregate of several species of structural elements. The structural element implies an aggregate of similar sorbing centres, and the sorbing centre is a group of atoms in the bulk or on the surface of a body, which can hold one molecule of the sorbate. Hence, fxig a Xi;~uk;mk; fnjg; fyjg, where nj is the number of adsorbing centres in the same state, which is determined by the centre's location in the body, the composition and configuration of molecules forming it, yj is the fraction of centres of a given sort occupied by the sorbate.A conservation condition for the number of sorbent particles and all species of adsorbing centres can be written in the form 24 ¡¾ 26 (taking into account the continuous change of their states) G (1) q qxi ij ¢§ ¢§qj qt a q qxj ODijjU aQOxi;jU, ja1 ia1 XpXp where G (2) Dt?0 i a lim hDxii , Dt D 2 Dt?0 ij a 1 lim hDxiDxji . Dt In these equations, Gi is the rate of the change of parameter xi for particles with approximately the same properties, Dij is the fluctuation factor of the rate Gi upon the influence of random changes of the parameter xj ; Q(xi , j) is the frequency of particles' dissociation upon collisions with each other, thermal decomposi- tion or phase transitions in the particles' bulk; Dxi is the increment of xi over the interval Dt; averaging is performed for an ensemble of particles.Equation (1) reflects the concept that every particle can with a certain probability move from one state to another, which is consistent with the conservation laws. The probability of some transitions is negligibly small, but they must be taken into account within the framework of a general pattern of sorption. Another conservation condition has the form (3) ¢§qOecU a divaeO~uLc ¢§ DgradcUa a dMdt , qt where M is the number of moles of a substance extracted by the sorbent from a volume unit of the medium, qjdxi , M a O a? ¢§?Sorption mechanism and prediction of sorbents' behaviour in physicochemical systems Vkjdxi , e a 1 ¢§ O a? ¢§? e, c and ~uL stand correspondingly for the local porosity, the concentration of the sorbate, and the velocity of medium move- ment in the interval where particles have distribution j(xi , t), D is the molecular diffusion constant for the sorbate in the medium, q is the sorbate quantity in a particle with parameters {xi}, Vk is the volume of the particle, integration is performed over all parame- ters xi .Variable q is usually represented as a sum of sorbate quantities in the near-surface layer of a body q qs= s j j Xq qv= vj , j and in its bulk X where index j belongs to the adsorption centres of the jth species.If a body is a single crystal, then the layer of adsorbed molecules (sometimes along with one or two additional monolayers of crystal atoms) is regarded as a near-surface one.27 If a body is composed of accreted microcrystals, then one includes both those which have at least one facet contacting the medium and the pores between them in the near-surface layer. For the near-surface layer which is homogeneous along a normal to the surface, we have s (4) dq dt a j X ojaSc ¢§ Olj a vjUqs j , v ljqs j , dq dt a j X where oj , nj are correspondingly the probabilities of the sorbate intake from the medium into the near-surface monolayer through sorption centres of the jth species and its leaving from these centres for the medium in a time unit, a is the normalising factor (effective width of the layer, from which sorbate molecules can get into the near-surface layer as a result of unit translation), S is the surface area, lj is the probability of a sorbate molecule transfer from the adsorption centre of the near-surface layer into the bulk of the body in a time unit.Terms in the second member of Eqn (4) can differ in the order of magnitude, which will reveal itself in the stepwise character of sorption.28 If a body is homogeneous, a transition of a sorbate from the near-surface layer into the bulk can approximately be described as a convective transfer with the effective diffusion constant Dv upon ordered motion of sorbate particles relative gravity centre of the body with velocity~uv. The velocity~uv rides on the shift of the near- surface boundary due to dissolution and swell (compression) of the body upon sorption.If a body is highly porous,~uv is associated with the flow in pores caused by external pressure or phase transitions in the body.29 The change with time of the sorbate concentration cv in each region of the body in which the position is fixed relative to the body's surface is described by the equation (5) ¢§qcv qt a divO~uvcv ¢§ DvgradcvU aWOtU, where W(t) is the rate of a sorbate transition into a motionless state in the bulk of the body.In terms of Dv,~uv, cv quantities, we have 143 v (6) r?0 ljqs j a lim qc vcv ¢§ Dv qr S j u dS, a X where r stands for the distance from the near-surface layer to the point where the sorbate concentration is equal to cv, uv is the normal component for the velocity of a surface shift to the centre of the body over the area dS. Quantities included in Eqns (1) ¡¾ (6) are functions of medium properties {yi}, from which the following are usually defined: {yi}=T,ci ,~uL,Hi, rL, mL , where ci is the concentration of the system components in the medium,~uL is the medium velocity, Hi is the field strength for an external field, rL and mL are the medium density and viscosity, respectively. Equations (1) ¡¾ (6) are applied in a simplified form.For example, one assumes that Dij=0 (considering sorption as a purely determinate process) and~uv=0. If one takes into account the fluctuations of bodies' properties, then one assumes that Dij=const, and changes in state parameters {xi} are independent of each other.30 ¡¾ 32 This makes the solution easier, but excludes `cross effects' reflecting the mutual influence of the body's state parameters and the medium's ones 33, 34 from the consideration. The cross effects are indirectly taken into account by the intro- duction of functions Gi (xi , yi ) and Dij (xi , yi ) reflecting all the elementary processes. III. Sorption gas dynamics and hydrodynamics Velocities~uk and~uL determine the bodies' positions in the system.The sorption systems are heterogeneous as a rule, therefore, the position of a body is its most important state parameter. 1. Motion of an individual body in the flow of the medium Any component of the instantaneous velocity of a body's move- ment can be represented as a sum of the velocities of the ordered motion Gi=uki and the random fluctuations characterised by coefficient Dij=Dki, i.e. (7) dZi dt a uki a DkixkiOtU, where Zi is the coordinate of the gravity centre of the body, xki (t) is a random function of time, Dki a DB a ukipki , uki is a component of the velocity~uk, averaged over time, DB is the diffusion constant when uki?0, and pki is the characteristic scale for fluctuations of the body's position.Velocity uki is formed from the velocities of the body sedimen- tation in the field of terrestrial gravity, the approach of the bodies under the action of molecular interaction forces, the bodies' shifts in electric and magnetic fields, the thermo- and diffusiophoresis, the drift caused by the flow of medium and the chemoreactive movement. These kinds of motion are investigated with regard the sorbing bodies of simple form with little internal porosity.35 ¡¾ 38 The movement of the bodies of complex forms has not been adequately studied. Quantity DB is determined by the Brownian motion. Func- tions xki (t) and pki (xi, yi) have different shapes. Theoretical approaches to their determination have been developed for the movement of particles in homogeneous suspensions.38 ¡¾ 41 The experiment has repeatedly been confirming the propriety of using correlation (7).As a result of the observations of trajectories of particles with dimensions of 1 ¡¾ 5 mm in a liquid medium, it was established in recent time that the motion of the particles is really the sum of the ordered sedimentation in the field of terrestrial gravity and the random fluctuations characterised by coefficient DB invariable with time.42, 43144 2. Liquid flow near the surface of a body The motion of a liquid near the surface of a solid determines in many respects the mass transfer to the surface of the solid.35, 42 ¡¾ 46 The motion of a liquid whose molecules' interaction with each other and with molecules of the body is described by the Lennard ¡¾ Jones potential, has been calculated by the molecular dynamics method 47, 48 12 6 i i sr 0 ¢§ vOr 0U a 4e si r 0 ¢§ r 0 0 0i where r 0 is the distance between the gravity centres of the molecules, ei, si are correspondingly the energy parameter and the dimension parameters of the potential of the interaction of molecules of the liquid with each other (i=1) and with molecules of the solid (i=2), r is the boundary of the potential action area.It was assumed in calculations that the body's surface is molecular smooth, and the molecules of the liquid move chaoti- cally with a resulting shift along the direction of the stationary flow. The calculations showed that in a broad interval of flow velocities, when e2/e1>0.6 and s1=s2, the liquid moves along the surface so as the liquid's molecules in the layer of width s practically do not move along the flow, and so this layer is motionless.The molecules which are more remote from the body surface move in accordance with the velocity gradient of the shear flow. Provided that e2/e140.6 and s2<s1 , a jump in the flow velocity takes place at a distance s from the surface, in other words, the liquid `slides' over the body's surface. This result means that numerous considerations of the near-surface motion of the liquid that do not take into account the slide motion and the existance of a `motionless' layer need refinement. When motion velocity increases, `vortexs' are produced near the surface, which, multiplying, form wave structures disintegrat- ing into chaotic vortexs in velocity flows l.49 The most important peculiarity of the liquid's motion relative to a body is the anisotropy of motion velocity that leads to an unequal accessi- bility of different surface areas to the sorbate.35, 50, 51 Figure 2 illustrates the results of the calculations of velocities of medium motion near a body of size d at different flow velocities~uL.52 In this case, there are several critical values of the Reynolds numbers Re=1:6d j~uL j , mL a y/x1 2 2 1 3 1 0 b 2 2 2 1 1 0 3 2 1 Figure 2.Results of calculations for solution streams flowing around a single grain of the sorbent (a) or a group of grains fixed on the support (b) by numerical solution of the Navier ¡¾ Stokes equation for Re=10.52 (1) Frontal vortex, (2) grain, (3) rear vortex; y and x are the spatial coordinates, x1 is the size of the body.12 s i i (8) sr 0 a 0i 0i 6 , 3 6 5 4 x/x1 I V Melikhov, D G Berdonosova, G I Sigeikin changing the character of the velocity distribution of medium motion near them. If Re<0.1, the flux flows around a body without producing vortexes; a stationary `rear' vortex is produced near each body in the interval of 0.1<Re<100; provided that Re>100, vortexes break periodically away from the body. [An analogous vortex forming occurs for a body being in an ascending current (the Karman track) and for other motion regimes.53] Vortexes magnify anisotropy of the sorbate concentration field around the body and, therefore, increase unequal accessi- bility of its surface areas.If a body is fixed, its surface is unequally accessible at any Re values; if not, the difference in the area accessibility is substantial upon the laminar medium motion. In the latter case, the body orients in the flow in such a way that the medium makes a minimum resistance to the motion. The body retains such orientation (if we neglect re-orientation shifts caused by the rotary diffusion).54 Thereby, the body retains unequal accessibility of its surface. It chaotically migrates and rotates in a turbulent flow so that its surface becomes equally accessible on average. The concept of accessibility of different surface areas of bodies is fundamental to describe the external-diffusion regime of sorp- tion.The consideration of this regime requires the determination of velocities of bodies' motion and medium flows around the bodies with high accuracy. 3. Chemoreactive movement Chemoreactive movement is added to the common forms of the body's motion if the surface of a body is sorptively heterogeneous, in other words, if there are active sites distributed irregularly over the surface.55 ¡¾ 60 Such a movement was discovered, for example, for sorbing water vapour on Al2O3 grains. The bodies' movement was investigated in a release reactor of which a scheme is shown in Fig. 3. Sorbent grains are introduced into section 1 for sorbent preparation, then they are blown in pre-reactor 2.Aportion of the grains gets over channel 3 into adsorber 4, where they fall freely down in a sorbate gas.Flying through the adsorber, the grains deflect from the vertical trajectory if their velocity has a horizontal component. The bodies were of size d=305 microns and represented agglomerates of microcrystals. They were one by one introduced into the reactor filled with a mixture of water vapour and air. The agglomerates fell freely down reaching the bottom in several seconds. After that, distribution functions of agglomerates were determined with respect to motion velocities in the horizontal direction over the falling time. The measurements demonstrated that the distribution of the bodies over the velocities was the same as in the case of a dry sorbent in dry air if the bodies were brought to an equilibrium with water vapour before introduction into the reactor.If the bodies contained a non-equilibrium amount of water and fell in a humid medium, the velocity of their motion was considerably higher (Fig. 4). There were areas with a high rate of adsorption of water on the bodies' surfaces, which was caused by the capillary condensation started to proceed there. These sites adsorbed water vapour at the same rate as the other sites did, but desorbed it to a lesser degree, since a portion of the adsorbed water 2 1 3 4 L Figure 3. Scheme of the release reactor.55Sorption mechanism and prediction of sorbents' behaviour in physicochemical systems N 1 0.8 2 0.40 8 16 u /mm s71 Figure 4.Distribution of Al2O3 grains according to the movement velocities in water vapour (from Refs 55 and 56). N is the fraction of grains that move in the horizontal direction at a rate lower than u. Partial vapour pressure, kPa: (1) 2.0, (2) 3.0. was removed as a condensate. As a result, the momentum flux of recoil from desorbed water molecules over such sites was less than the momentum flux over the other areas, and this caused the bodies' movement. 4. Motion in an ensemble of sorbing bodies The motion of bodies in an ensemble of them is affected by the superposition of fields of velocities in the environment around the bodies and by direct contacts of different bodies.Interaction in the vortex medium furthers the approach and coordinated movement of bodies. For instance, if a homogeneous suspension of glass balls of diameter *50 mm in a water ± glycerol medium (mL= 561072 Pa s, e=0.5 ± 0.9) is poured into an empty vessel, the coordinated motion of aggregates of hundreds of balls will take place in different directions against a background of slow sed- imentation after the cessation of the motion caused by pour- ing.61, 62 The bodies can stick together, undergo a plastic deformation and even be destroyed if the velocity of their approach is great.63 ± 65 The dimensions and shapes of the aggregates produced in ensembles of moving bodies depend on the flow velocity, the medium composition, the sizes, shapes, and relief of the bodies' surfaces.Any suspension flow is characterised by its own distri- bution function of aggregates over dimension and shape param- eters. This function shifts to smaller dimensions of the aggregates on increasing the flow velocity and decreasing the concentration of particles in the flow. In contrast, the function shifts to large aggregates on increasing the concentration of the particles, and the aggregates can change their shapes from fractal ones to colloid crystals depending on the movement regime. All these results follow from the observations of moving suspensions with an optical or atomic force microscope in situ, and also from the observations of `preserved' suspensions with an electronic micro- scope (see Refs 66 ± 69, for example).The force of resistance to the flow changes upon aggregation of particles and, therefore, so does the regime of their motion. One can make conclusions about the scale of such changes from data on effective viscosities mef of sorbent ± medium suspensions. Thus, the effective viscosity of a suspension of polystyrene spherical particles with sizes of d=70 nm in an aqueous solution of gelatin (1 mass %) with particles' concentration q0=2.761020 m73 and shearing strain ps=0.1 Pa in the shearing strain of the suspension is 500 times greater than in the absence of the particles.67 The dependence mef (ps) shows that the suspension is a single `loose' aggregate which moves as a whole at the indicated concentration q0 and ps<0.3 Pa.This aggregate dissociates into smaller frag- ments upon increasing ps , and most particles move at ps>30 Pa almost independently from each other. The value of mef keeps at the level of 108 Pa s in the shearing flow of SiO2 particles in an ester (T=300 K, d=14 nm, q0=1.761020 m73) at ps<10 Pa, 145 while at ps?10 Pa it undergoes a jump until*0.1 Pa s, at which it stays in the interval of ps=10 ± 100 Pa (see Refs 67 and 68). This jump indicates a mechanically stimulated rearrangement of the suspension, resembling a first-order phase transition. The accessibility of the surface of bodies to a sorbate on such a rearrangement changes with a jump too. The possibility of aggregation of bodies in an ensemble depends on the composition of the adsorption layer.The sorption can both accelerate and suppress the aggregation.67, 70 This dependence becomes of particular significance in the presence of a sorbate whose molecules can adsorb on two close particles at once, producing `molecular bridges' between them.70 Such sor- bates are employed to control the aggregation. These facts highlight the change in the velocity of a stationary flow of a suspension due to a change in its effective viscosity caused by the adsorption-induced acceleration or the deceleration of aggregation. It seems that such `adsorption-dependent destabi- lisation' of a flow is more intensive for concentrated suspensions of small particles. Dissipative structures reflecting cooperative interactions of bodies arise in moving suspensions.71 ± 73 For example, if a mixture of glass balls of diameter of*0.27 mmand sucrose crystals of size *0.80 mm is discharged to an empty vessel with the form of a rectangular parallelepiped with dimensions of 562006300 mm along one of the long edges, a texture is formed of inclined layers of the glass balls alternating with layers of the sucrose crystals.The widths of the layers and their angles of inclination relative to the bottom of the vessel will depend on the form and subtle peculiarities of the surface relief of sucrose crystals.73 Separation of the initial mixture of particles with the formation of a spatially dissipative structure occurs at a comparatively low input of mechanical energy in the system.The grounds of such separation could be explained after a considerable specification of conditions of particles' contacts in the flow. The examples above indicate that the variety of hydrodynamic phenomena accompanying sorption increases nonlinearly with an increasing number of bodies in the system. Each of these phenom- ena can influence or depend on sorption under certain conditions as it takes place for the adsorption destabilisation of a suspension flow. The phenomena in question can be described by the motion equation of solids in the flow and by Navier ± Stokes equation by detailed accounting of the forces affecting the sorbing bodies. 5. Prediction possibilities Acomplete a priori prediction of the hydrodynamic behaviour of a system of sorbing bodies in the medium is still impossible in the general case (although it can be made with an acceptable accuracy under some sorption conditions 74, 75).To make the complete prediction, one needs to have methods to calculate the parameters of Eqns (5) ± (7) based on data on molecular motion as well as methods to solve the equations specified by accounting for the variety of hydrodynamic behaviour of an ensemble of bodies in the medium. However, no universal methods for such a calcula- tion exist so far. The methods of molecular dynamics allow prediction of properties of flows of bodies based on data on potentials of interatomic interactions. However, specification of the kind of interatomic potentials is necessary to apply these methods widely to describe flows of particles and the medium.Observations of adatoms of gold in the contact area of two crystals in a tunelling microscope has shown that atoms' behaviour cannot be explained by using the Lennard ± Jones potential [see Eqn (8) or other known potentials].76, 77 Therefore, the contemporary conceptions of the potentials of interatomic interactions are approximate. Nevertheless, this does not exclude successful semi-empirical prediction of motion of particles and a medium in narrow intervals of sorption conditions, for instance, upon sedimentation in reactors of different shapes 78, 79 or in shearing flows of concentrated suspensions.67 However, such calculations require expenses which are commensurable to those for experimental146 work.Hence, one has to restrict oneself to the estimations obtained using various models, e.g. the cell model,80, 81 if the geometry of a system is complex. IV. Adsorption kinetics It is convenient to present the velocity of sorption in the form (9) OojaSc ¢§ vjqsjUjdxi , dMdt a j O a? X¢§? where . (10) vj a kT h wj exp ¢§Ejbj kT Here k and h are the Boltzmann and Planck's constants, respec- tively, wj is the frequency factor, Ej is the detachment energy of a sorbate molecule from an adsorption centre, bj is the ratio of an excess of the activation energy of desorption to the detachment energy. Correlations (9) and (10) demonstrate that it is necessary to take adsorption centres of all sorts into account, although adsorption centres do not always differ by values of nj as sharply as, for example, for the sorption of antimony by the faces (001) of a GaAs crystal.(Three forms of the antimony are discovered in this case with Ejbj values differing by almost 1 eV.82) Nevertheless, three species of adsorbed molecules were also found upon an exploration 83 of the sorption of a 5,10,15,20-tetrakis[3,5-di-tert- butylphenyl]porphyrin complex of copper onto the surface of a gold crystal at 420 K from a gaseous environment by scanning tunelling microscopy in situ. The central porphyrin part of the molecule was not observable in the microscope against a back- ground of the substrate while the tert-butyl groups manifested themselves as bright spots.The conformations of molecules of all species could be judged from the location and the intensity of the spots. The kinetics of mutual conversions of the forms could be deduced from the change in the number of the images. It turned out that one form localised on the steps, the second one on the breaks of rectangular terraces, the third on the terraces. The tert- butylphenyl groups of the second from localised at an angle of 65 8 to the plane of the porphyrin part of the molecule, and those of the third form at an angle of 45 8. Heating the substrate till 520 Kover 12 min led to the disappearance of the second form from the substrate without any observable desorption of the third form. In this case, the energy of gold atoms on the breaks of the terraces and far from them differed by a few millielectronvolts only.84 However, such a difference causes a noticeably distinct behaviour of the second and the third forms of the mentioned molecules.Therefore, an estimation of the role of the centres of all sorts is necessary in any case. Yet it is hard to measure the values of oj=oj (~uL ,D) for the centres of all sorts, and an a priori calculation of these quantities is only possible after a specification of the form of interaction potential. Specification can be achieved by a search for quantities which are independent of the form of the potential. Such invariants will restrict the possibility of changes in the potential. One of these invariants can be connected with the diffusion constant, D, of the sorbate in the medium and, hence, with oj.85, 86 Indeed, D along with self-diffusion constant Ds characterises thermal motion of molecules D=g(yi)Ds , where g (yi) is a function that does not depend onDs explicitly.The coefficient Ds calculated by the method of molecular dynamics for 1.66104 atoms at different forms of a potential of molecules' interactions can be presented in the following form for a dense monoatomic liquid:85, 86 Ds a BexpOS0U , g0r40 I V Melikhov, D G Berdonosova, G I Sigeikin where B is a coefficient that does not depend on the form of a potential, S0 is the difference between the entropy of a liquid and a gas of the same composition, and g0 is the value of radial distribution function of liquid's atoms at the point of the first maximum of the distribution for which the interatomic distance is equal to r0.This correlation indicates the possibility of searching a corre- lation of coefficient D and, therefore, frequency oj with the potential of the interaction of medium molecules through the invariants resembling coefficient B. Functions qsj=qsj (yi ,t) and wj=wj (yi ,t) reflect the kinetics of all the elementary surface processes leading to sorbate migration between centres of different sorts, chemical transformations of adsorbed molecules and a sorption change of the state of the surface. Rates of elementary processes usually differ significantly, and so functions qsj change with time stepwise.Some steps can be slow in this case. For instance, a three-step process proceeds during adsorption of oct-1-ene by crystals of aluminosilicate zeolite H-ZSM-5 at 290 K over 102 ¡¾ 26105 s. A transfer of the oct-1-ene into the adlayer occurs at the first step, the adsorbed oct-1-ene reacts with hydroxy groups of the zeolite, producing carbenium ions at the second step, and at the third step, the carbenium ions undergo a rearrangement CH3C+H(CH2)5CH3 CH2=CH(CH2)5CH3 CH3(CH2)2C+H(CH2)3CH3 . qsj q0sj These steps were identified by observing 13C NMR spectra in situ (in magnetic fields up to 18 T with the sample spinning at a 2 ¡¾ 5 kHz frequency at a magic angle, using cross-polarisation). It follows from the intensities of lines in the NMR spectrum at 14.3 ppm which correspond to the terminal groups 13CH3 and are given in the study 87 that the third step is determined by the relation a o 0:31 a 0:69 exp ¢§¢§pAAAAAAA qtA, where q0sj and qsj stand for the amount of cabenium ions in the adlayer at the beginning of the third step and at moment t, correspondingly, oq=4.561075 s71 stands for the character- istic frequency of rearrangements.The three-step nature of the process in question can be explained by the fact that centres of three sorts exist on the surface of the zeolite that differ in composition and mutual location of atoms. The centres of the first sort have a much larger value of oj than the other centres, thus, sorption mainly starts with transition of molecules onto these centres.At the second step, the molecules move to the centres of the second sort, which have hydroxy groups reacting with the oct-1-ene. The carbenium ions rearrange in the slow third step, remaining on the centres of the first sort or moving to the centres of the third sort. The surface processes are so varied that one has to consider their pecularities at a qualitative level. This has been done below using the sorption by single crystals as an illustration and the level of details of description achieved to date is also presented. 1. Accommodation of sorbate molecules Different sorbing centres have different probabilities of accom- modation, so that j (11) oj=aj SS xj , where aj is the local factor of accommodation at centres of the jth sort, Sj is the area of the body's surface containing these centres, S is the total area of the body's surface, xj is the probability that a sorbate molecule contacts the surface in a unit time as a result of transition of molecules from a gas or a liquid.A sorbate molecule is promoted to an excited state upon collision with the surface.88 The promotion proceeds in a period t0Sorption mechanism and prediction of sorbents' behaviour in physicochemical systems in the order of 10 fs. It is accompanied by deformation of electron density and activation of vibrational motion of both the molecule and the body in the contact point and sometimes by a config- uration change of the molecule.89, 90 Then the relaxation of the electronic excitement of the molecule proceeds in 10 ¡¾ 100 fs, and the relaxation of the vibrations in ts=1 ¡¾ 10 ps.Vibrations of atoms of a body can accidentally synchronise giving a molecule sufficient momentum to break it away from the surface. Molecules dwell at the surface of a body for different time t owing to the accidental character of the synchronisation process and aj a 1 ¢§ jjOtUdt, Ots 0 where jjOtU is the density of the distribution of molecules occupy- ing centres of the jth sort over t. Function jjOtU implicitly characterises the dependence of t on molecule's velocity and orientation at the beginning of the con- tact. An elastic reflection of the molecules from the surface is observed and aj?0, if all the molecules have the velocity and the orientation at which function jjOtU is localised in the region of t&t0<ts.If most molecules have t&ts , then almost complete accommodation takes place and aj?1. Coefficient aj can be determined for the adsorption of a rarefied gas on a homogeneous surface with high accuracy based on the data on a molecular beam reflected from a body. When considering sorption from a liquid medium, one has to content oneself with rough values of aj. It is of some use to determine the function jjOtU characterising the first step of sorption by methods of femtochemistry.91 2. Migration on the surface The sorbate molecules fixed on the surface jump from one adsorbing centre to another.The frequency of the long distance jumps which are accompanied by re-orientation of the molecules increases with increasing temperature. It is established by obser- vation with the tunelling microscope that the jumps of acetylene molecules on faces (111) of a lead crystal occur with a change of orientation at T>70 K, and all six orientations are realised.92 Superficial mobility of molecules depends on their configu- ration.93, 94 Molecules can move from one centre to another due to conformational changes. Increasing the number and the strength of bonds of a molecule with an adsorbing centre weakens the tendency to such changes. This results, for instance, in sorption of a dendritic DNA on a graphite crystal, whereDNA molecules can bind to the crystal through numerous guanidine bridges.95 In the case of adsorption of 1,4-bis[b-(2-pyridyl)vinyl]benzene on face (100) of a silicon crystal, the sorbate molecules that are sufficiently strongly bound to silicon atoms migrate with low velocity so that their motion cannot be detected even on a sufficiently long observation in a tunnelling microscope.Molecules of this sorbate exist as four conformational species on the surface, however, motion with an observable velocity is not a specific feature for any of them. The migration proceeds much faster if it occurs at the face (100) of silicon where every near-surface silicon atom is blocked by a hydrogen atom.96 Cooperative effects reveal themselves upon increasing the fraction yj of the surface occupied by the sorbate.The mobility gets smaller with the rise of yj if the attraction dominates in lateral interaction of sorbate molecules. The mobility increases first and then diminishes if the repulsion dominates.97 Thus, the detailed description of molecules' motion over the surface requires that possible changes in their configuration upon jumping from one centre to another and their collective motions be taken into account. Migration is quantitatively characterised by quantity Dj (12) Dj=kT h ¢§OEj ¢§ EiUbji , kT i wji exp X 147 where wji is the frequency function for the transition of a molecule from the jth sorbing centre to the ith one, Ej and Ei are the energies of interaction of the molecule with the crystal on the jth and the ith sorbing centres, respectively, and bji is the correction function reflecting cooperative effects of activation.It is assumed while calculating Dj that the shifts to the nearest vacant sorbing centre dominate among jumps of each molecule, and Ej=njEj 2 and Ei=niEi 2, where nj and ni are the number of the crystal's atoms in the nearest neighbourhood of the molecules on the jth and the ith sorbing centres, correspondingly, Ej 2 and Ei 2 are the average energies of the pairwise interaction of the molecules with the nearest neighbours.98 It is also taken that bji=1 and wji=const.99 The calculation of Ej and wji is attempted based on `the first principles', but it is a success for the simple sorbates only.100 It was possible to calculate these quantities for the migration ofH2 on the face (001) of a nickel crystal with a 10% discrepancy with the experimental data.However, one failed to explain the absence of an observable isotope effect in the super- ficial diffusion of hydrogen at low temperatures.101 If the energy of adsorption is sufficiently high, the near- surface layers of a crystal get disordered, molecules come to the surface from these layers, which leads to acceleration of superficial migration.102 One can get an idea of the scale of such acceleration based on the data on superficial migration of atoms during the adsorption of hydrogen and benzene on the crystals of iron and nickel. For instance, the hydrogenation of the adsorbed benzene is accompanied by a 104¡¾ 105-fold increase in the intensity of the superficial migration of iron and nickel atoms.3. Formation of superficial clusters Adsorbed molecules can join into clusters that grow up to the size of a whole face for large yj.103 ¡¾ 105 The rate of the formation of the clusters increases quickly if the concentration of the sorbate in the medium achieves a critical value, which is substantially different in the case of the weak and strong bonds of the sorbate molecules with the crystal and with each other. For example, the clusters can be observed in the sorption of hexadecyltrimethylammonium bromide by the surface of clinop- tilolite crystals only at the concentration for which yj?0.25. In the case of a lesser extent of the adlayer filling, the sorbate exists on the surface as separate molecules, which are observed as oblong knobs of 0.4 nm height in the atomic force microscope.106 The heights of some knobs double at yj?0.25, which can be inter- preted as the formation of clusters.The probability of their formation and growing up depends on the extent of conformity between the structures of the sorbate molecules and the near-surface monolayer of the body. In the case of the adsorption of simple molecules, this probability is higher for the sorbates whose crystal lattices have planes with an arrange- ment of atoms close to that in the near-surface monolayer of the body. The formation of clusters is facilitated in the case of the adsorption of complex molecules at certain configuration of the sorbate molecules.107, 108 This is confirmed by the observation of the adsorption of oligosaccharide polymers on the face (100) of a graphite crystal from an aqueous solution in atomic force micro- scopy in situ.109, 110 The molecule of the polymer, representing a polyvinylamide backbone with numerous dextran and hexanoyl branches, adsorbs so that its backbone is stretched along the surface.The hexanoyl chains contact the carbon atoms of graph- ite, and the dextran ones are directed outwards. The orientation of molecules on the surface of a body depends on the ratio between the numbers of the hexanoyl chains and the dextran ones (qm). The majority of molecules orients along one of two crystallographic directions of the crystal lattice at qm=1/5.In the case of a sufficiently high concentration of oligosaccharides in solution, a number of molecules successively attach to each molecule fixed on a free surface area, producing a two-dimen- sional cluster. The clusters spread out over the surface until they merge into a continuous monolayer of 0.7 ¡¾ 1.2 nm width. The148 orientation of molecules is arbitrary, and no two-dimensional clusters form at qm=1. The two-dimensional clusters, enlarging, can turn into three- dimensional formations. For example, the clusters turn with time into epitaxial incrustations 111 upon adsorption of cadmium arachidonate Cd[CH3(CH2)12CO2]2 from the vapour on the face (001) of a crystal of KCl. Clusters can change their compositions and structures,112, 113 and successive monolayers of an adsorbed substance can be deposited over the produced ones 114 at certain values of yj.Such a phenomenon was in situ observed in tunnelling and atomic force microscopies. The conclusions from the observations were con- firmed by calculations using the methods of molecular dynam- ics.115, 116 In particular, it was established that multi-layer crystals arise, which can have different structures varying in the mutual orientation of molecules, upon the adsorption of H2O on the face (100) of a crystal of MgO at yj?1. One of the orientations dominates at T<200 K, and the other one at higher temper- atures.116 Structural transitions occur in the monolayer of lead on the face (100) of a copper crystal: the first one at yj?0.38, the second, at yj?0.5, and the third, at yj?0.6.In this case, each of the structures undergoes `two-dimensional' fusion 117 on heating. The structural rearrangement of monolayers is accompanied by significant acceleration of adsorption over the adsorption of dodecanethiol and butanethiol on the face (111) of a gold crystal at 298 K.118 4. Two-dimensional solid solutions Adsorbed atoms can be incorporated into near-surface interstices or occupy vacancies in the near-surface monolayer of crystal atoms, i.e. form two-dimensional interstitial solid solutions or substitutional ones.27 This could be observed directly in the case of the sorption of InAs from its vapour on the faces (100) of a crystal of InP by atomic force microscopy.119 The sorption of vanadium oxide on the face (001) of a crystal of TiO2 was detected by emergence of an additional component in the band V2P3/2 of the photoelectronic spectrum at 623 K.(This band is caused by the transition of a vanadium atom into the lattice of TiO2.120) It was possible not only to detect solid solutions, but also to determine their structures by means of X-ray diffraction analysis while studying the sorption of bismuth from its vapour on the face (001) of a copper crystal.121 It turned out that the bismuth atoms substitute the copper ones at small yj chaotically, and each bismuth atom transferred into the near-surface monolayer is shifted towards the vapour with respect to the copper atoms by 0.06 nm.The sorbed atoms of bismuth form a two-dimensional structure of the type C(262) at yj?0.35, their radius is equal to 0.163 nm, which is considerably smaller than that of a bismuth atom in the bulk of its crystal (0.182 nm). Thus, the structure of two-dimensional solutions can change with increasing yj . The mobility of molecules in the near-surface monolayer of the crystal is far lower than that in the adlayer.27, 122 Hence, the sorbate transfers to the adlayer at the first stage rather rapidly and forms a two-dimensional solid solution at the second stage slowly. The third stage arises after large periods of sorption, which consists of the transfer of the sorbate to the bulk of the sorbent. 5. Change in the properties of a sorbing body All properties {xi} of the body change during sorption, and the scale of these changes is nonlinearly connected with the body's state and can be great in a narrow interval of sorption conditions.For example, a change in velocity of the body's motion under the influence of sorption becomes detectable if the body can freely move in the sorbed gas for a long time and the sorption is localised on a portion of the body's surface.60 The change in temperature is considerable if the medium is characterised by the low thermal conductivity and the great heat of sorption. Sorptive changes in electroconductivity, magnetic, thermophysical and optical prop- erties can turn out to be substantial for porous bodies, which is employed in sensor devices.123 I V Melikhov, D G Berdonosova, G I Sigeikin There can exist both positive and negative feedback between k is positive since increasing the sorption and the properties of a sorbing body.For instance, the feedback of the quantity of an adsorbed substance q with the velocity of chemoreactive movement~u ~uk leads to growth in oj, and growth in oj facilitates the acceleration of the movement.55 The feedback between q and T is usually negative since increasing the temperature leads to growth in the rate of desorption dominating over increasing aj.124 The character of the feedback between q and parameters {xi} is of great significance in the development of sensors.123, 124 6. The possibilities of computational experiment The direct observations of the motion of adsorbed molecules encouraged the application of the methods of molecular dynamics and Monte Carlo for the simulation of elementary processes of sorption under the quantum-chemical interpretation of intera- tomic interactions or the usage of atom-atomic potentials of the type (8).Simultaneously, the computational methods of chemical kinetics based on differential equations of the type (4) become more widely applied in investigating the sorption processes. As a result, separate elementary processes of sorption could approx- imately be described for some of the simplest molecules, based on the `first principles'. It has become clear at the same time that some elementary processes can a priori be excluded from the consid- eration only in the case if we limit ourselves to a narrow set of conditions when modelling sorption.A complete kinetic model of sorption which is applicable for a broad set of conditions must take into account all the details of elementary superficial proc- esses. However, the complexity of the model does not allow making computational experiments in corpore. V. Sorbate migration into the bulk of a body 1. Diffusion into the bulk of crystals Sorbate migration into the bulk of a body is usually characterised by coefficient Dv reflecting all elementary processes leading to the translation movement of sorbate molecules (ions).125 Function W(t) can be excluded from Eqn (5) while the sorption at the crystals is described. Coefficient Dv can be considered as a tensor taking into account the anisotropy of crystals' properties in this case.To solve Eqn (5), one should reflect the polyhedral form of crystals and the possibility of their formation with different faces in edge conditions. One also has to take into consideration the fact that the sorbent crystals have `biographical' structural defects affecting Dv. Besides, the crystals acquire new defects during sorption that are distributed irregularly in their bulk. For exam- ple, the defects are localised at the periphery of each crystal for the crystals of K2SO4 or NaCl being in an intensely agitated aqueous solution. In this case, the sorbate accumulates significantly faster at the defect areas than at the other ones.63, 64 The values in the order of Dv of 10725 m2 s71 are a characteristic of the perfect crystals of BaSO4 with an ion-covalent bond of atoms at 300 K; for the sorption of SrSO4, in particular, by the crystals of BaSO4 with high defectiveness of growing, Dv=10716 m2 s71 (see Refs 126 and 127). The molecules of sodium ethylenediaminetetraacetate and trinitrophenol diffuse into interlayer spaces of the crystals of hydrotalcite Mg3[Al(OH)8][CO3]0.5 , which have a lamellar struc- ture with the ion-covalent bond of atoms within the layers and the van der Waals one between the layers, with the diffusion constant Dv&10716 m2 s71 at 300 K.128 Diffusion rate is especially high for sorption by the organic analogues of zeolites.The crystals of organic zeolites consist of groups of covalently bound atoms producing ring-shaped for- mations (blocks).129 Closing each other up, the blocks produce channels whose width can reach several nanometres. Atoms of the crystal occupy not less than half volume of a channel, the rest space is occupied by molecular pores. Sorbate molecules can migrate along the channels with high values of Dv, which, for example, takes place in the crystals of C72H98N12O40Zn3 with aSorption mechanism and prediction of sorbents' behaviour in physicochemical systems trigonal lattice (the diameter of a channel *1 nm, the channels orient along axis c).130 The channels accounting for 47% of the crystals' volume are filled with water molecules in an aqueous medium.Migration, for instance, of Na+ ions over the channels proceeds at 300 K with the diffusion constant Dv& 10714 m2 s71, which was estimated from the data on the rate of ion exchange between the crystals and the solution. One can judge elementary acts of sorbate migration in molecular crystals by the results of calculations made by the molecular dynamics method.131 ¡¾ 133 The calculations showed a sharp change in the frequency of jumps of sorbate molecules over the channel and the number of molecules in the channel at small changes of the potential of interactions with atoms of the channel's `walls'. The motions of sorbate molecules lead to significant fluctuations of the number of the molecules localised per length unit of the channel.The scale of such fluctuations also depends on the potential of interaction of molecules with atoms of the channel's walls. Thus, if a carbon nanotube with a diameter of 0.81 nm and a length of l=1.34 nm is placed in a medium containing 103 molecules of H2O, these molecules get into the nanotube and move inside it. If the interaction potential is of the type (8) and ei=0.549 kJ mol71 and si=0.328 nm, the mole- cules of H2O, having passed through the channel, get out of the tube with the frequency oe=261010 s71 (see Ref. 133), which corresponds to the coefficient Dv of the order l 2oe=3.661078 m2 s71. Frequency oe proves to be lower by several times if ei=0.272 kJ mol71 and si=0.341 nm. In the channel of the nanotube, H2O molecules form a monomolecular chain of 2 ¡¾ 7 molecules arranged with higher order than in the medium.The molecules of the chain approach each other to a distance at which the hydrogen bond forms much more fre- quently, existing in such a state for te=5.6 ps (te=1.0 ps in water) on average. In addition, the H2O molecules possess not only the axial mobility but also the radial one (less frequently). In this case, the average number of molecules in a chain varies from 4 to 6 when ei=0.549 kJ mol71 and si=0.328 nm, and from 1 to 2 for ei=0.272 kJ mol71 and si=0.341 nm. This example dem- onstrates the possibility of a sufficiently detailed description of the movement of a sorbate in channels when data on interaction potential of atoms are available.2. Migration into polymeric grains Coefficient Dv of the polymeric sorbents has as a rule a complex dependence on their structure, values of yj, T, composition of the medium and preliminary swelling of the grains, and this depend- ence is difficult to predict. For example, Dv=3.4610710 m2 s71 at pH=1.0 and Dv=1.65610710 m2 s71 at pH=2.0 for the sorption of CdCl2 by grains of formaldehyde resin from an aqueous solution at 300 K.134 The value of Dv grows with increasing yj first, and then diminishes during the sorption of LiCl by a resin containing sulfo groups.135 Sorbents based on dendrimers and hydrogels have been obtained in recent years. Their coefficient Dv is almost the same as in the medium surrounding a body.136 Two models are usually employed to describe sorption by polymeric grains.137, 138 The first model implies that all grains are quasi-homogeneous and isotropic, i.e., characterised by a unique value of Dv when~uv=0 and W(t)=0; the second one assumes a grain to be a population of small polymeric globules jointed by rare molecular bridges.In this model, a grain is characterised by the constants of the sorbate diffusion over the interglobule bulk (the channels) and from the interglobule space into the bulk of a globule. In this case, function W(t) is determined as a total diffusion flux through surfaces of all the globules being in a volume unit, into their bulk. Both models reproduce basic features of the process only approximately. The calculations 30 have shown that if sorbing bodies are homogeneous and have a Gaussian size distribution, then the time t0.9 for which they capture 90% of an equilibrium quantity of the sorbate, is 149 (13) t0:9 a O0:182 a 0:255s0 ¢§ 0:190s20 Ul¢§2 , Dv where s0 is the ratio of the root-mean-square deviation of distribution of particles' sizes to their average size l¡¾.The deviation varies in the interval from 0 to 0.45. For the lognormal distribution over sizes, we have (14) t0:9 a O0:1943 a 0:057s0Ul¢§2 . Dv Correlations (13) and (14) characterise reversible sorption (Fig. 5). It follows from these correlations that it takes sufficiently great time to achieve the sorption equilibrium. One has to restrict oneself to the adsorption if it is required to perform the sorption sufficiently rapidly, i.e. within the time tk 55 t0.9.At small tk, the adsorption behaves as a process with a slight deviation from the reversibility, which is related to the transfer of the sorbate into the bulk of grains. Such a deviation is often revealed in practice.139, 140 M/M? 1 0.9 23 4 0.6 0.30 1.5 0.5 t Figure 5. Results of the numerical experiment on sorption by a sorbent with different size distributions of grains.30 Normal distribution at s0=0 (1), 0.1 (2), 0.2 (3), 0.3 (4). t=p2Dvt/l¡¾2 is dimensionless time,M?is the equilibrium amount of the sorbate in grains. 3. Sorption by the agglomerates of microparticles If a body consists of accreted microparticles, coefficient Dv depends on their shape, size, and the character of their pack- ing.141, 142 The relief of the surface of the particles also affects the value ofDv, which was demonstrated with the use of the molecular dynamics method, employing the diffusion ofN2 and isobutane in a cylindrical space between microparticles of 0.4 ¡¾ 1.4 nm size at 200 ¡¾ 800 K as an illustration.143 In addition, the value of Dv can be reduced by infilling the space by the sorbate. All this suggests that the description of sorption by the agglomerates with the use of coefficient Dv provides little infor- mation and shows a necessity to work Eqn (5) out, taking into account the structures of the agglomerates. Bodies consisting of multiple accrete macroparticles retaining free spaces (macropores) between them are widespread among the agglomerates.In this case, each of the macroparticles is a combination of microparticles divided by the micropores pro- vided that (15) Vk44V¢§k1 44V¢§k2 , k1 and V¢§k2 are the average volumes of macro- and micro- where V¢§ particles, respectively. If the state of every particle in an agglomerate is characterised by its size li and the volume (Vpi) of pores surrounding it, then one can write condition (15) in a continuum approximation150 (16) m1j(l1 ,Vp1)dl1dVp1 , cv=e1cv1+ O? ?O00 m1= m2j(l2 ,Vp2)dl2dVp2 dV, e2cv2+ O? O? OV 0 0 k1 where e1 is the total volume of macropores in a volume unit of a body, cv1 and cv2 are the bulk concentrations of the sorbate in macro- and micropores, m1 and m2 are the quantities of the sorbate in a macroparticle in state {l1, Vp1} and in a microparticle in state {l2, Vp2}, e2 is the porosity of the microparticles, j(l1, Vp1) and j(l2, Vp2) are the distribution functions of the particles over their states.Concentrations cvi (i=1, 2) change during sorption according to a transfer equation, which is analogous with relation (5) qcvi=div [ei (Di grad cvi7~uicv1)]7 bi l 2 i Ji j(li ,Vpi)dli dVpi , (17) qt O? O?0 0 where Di is the diffusion constant of the sorbate diffusion into the bulk of pores,~ui is the velocity of ordered motion of a liquid or a gas filling the pores, bi is the shape factor for the particles, Ji is the sorbate flux from the pores through the surface of a particle.In relations (16) and (17), (18a) (J2+J2s)dt , m2=b2l 22 Ot 0 v2 (18b) , l J1=(o 1 cv17vl1 ca1)a(17e2)+D2e2 l1 qc qr (18c) l J2=(o 1 cv27vl1 ca2)a, 1 1 1 1 where J2s is the intensity of the sorbate transfer into the bulk of microparticles, oi and vl are the specific frequencies of the adsorption and desorption, respectively, ca1 and ca2 are the concentrations of the sorbate in the adsorption layer of micro- particles localised on the surface of a macroparticle and in its bulk, Oqcv2=qrUl is the normal component of the gradient of sorbate concentration in pores near the surface of the macroparticle, and r is the distance from the centre of the macroparticle to the point where the sorbate concentration is equal to cv2, a is the effective width of the adsorption layer.Equations (17) ¡¾ (18a ¡¾ c) contain only those parameters which can be measured by experiment. Thus, j(li, Vpi), ei and bi can be determined by sectioning the bodies and measuring the sizes of particles and pores on the cuts with the use of optical and electronic microscopy. Frequencies ol and vl can be measured upon adsorption by the microparticles which are not assembled into agglomerates. The intensity of sorbate transfer J2s can be determined by the use of single crystals with the same molecular relief that the microparticles have. In addition, quantity J2s can be measured directly observing the motion of adsorbed molecules in atomic force or tunnelling microscopy. The motion of individual molecules and atoms could repeatedly be observed in the tunnel- ling microscopy.144, 145 To tell the truth, their trajectories were determined by the electric field of the microscope to a great extent, but there are all reasons to consider that the influence of the field on the surface movement of molecules can be taken into account.It is not difficult to determine the coefficient D1, while to determine D2 is more complicated. This can be made by the conductometric method in the case where the size of sorbate molecules is not more than 0.5 nm. The current strength fluctuates at limits that are proportional to D2 if two conductive micro- particles of size 5 ¡¾ 10 nmare approached so as to retain a space of 1 ¡¾ 10 nm width between them, and the space is filled up with a sorbate solution containing NaCl, and the current is passed through the electric circuit formed.146, 147 The volume of the solution determining the electrical resistance of the circuit is very I V Melikhov, D G Berdonosova, G I Sigeikin low at the given size of microparticles.Thus, if even one molecule of the sorbate gets into the space, this results in a jump of the electrical conductivity and, therefore, the fluctuation of the current. It appears that a way to conductometric determination of the rate of sorbate accumulation in the bulk and on the walls of an individual micropore was suggested by Bezrukov et al.146, 147 Therefore, a way to the direct measurements of ol1, vl1 and coefficient D2 for molecules of an arbitary size is also open. It was shown 148 that the value of D2 amounts to (1 ¡¾ 2)610711 m2 s71 at 300 K for the diffusion of polyethylene glycol of a mass of 200 ¡¾ 103 a.u.over a channel filled with an aqueous solution of NaCl in the lipid ¡¾ peptide membrane with a channel radius of 1 nm. This value is lower than that in a free solution by an order of magnitude. This fact confirms the necessity to describe the sorption with Eqns (16) ¡¾ (18a ¡¾ b) taking into account possible differences in the values of D, D1, and D2. The use of the functions contained in the indicated equations is complicated by the fact that some of them are unknown even for the most common sorbents. Simplifying assumptions are made for these functions, hence, the description of the sorption kinetics becomes semi-quantitative even if it agrees with the experimental data. The following assumptions are the most widespread: ~ui a 0, j(li ,Vpi)=const .j(Vpi)d(l¡¾i), Di=Di (l¡¾i), bi=bi (l¡¾i), l o 1 =vl1 =0, ca2=Bacv2 , where j(Vpi) is the distribution function of pores over the bulk, d(l¡¾i) is the Dirac function, and Ba is the equilibrium coefficient of adsorption. As can be seen, the number of a priori assumptions is large. Nevertheless, one has to employ most of them even in the most consistent descriptions.149, 150 4. Possibilities of prediction Quantum-chemical methods and correlative empirical depend- ences give an opportunity to estimate coefficient Dv a priori with a relatively small error for the bodies whose structure is near- equilibrium.But the acting sorbents are non-equilibrium as a rule. The structures of their particles depend on the `biography' of the sorbent. Therefore, the problem of the estimation becomes complicated, and one has to solve it experimentally. Some peculiarities of the migration into the bulk of a body can be predicted by the molecular dynamics method. However, the predicting possibilities of this method are restricted to nano- particles and small periods of sorption. VI. Equilibrium sorption The equilibrium sorption is usually charaterised by a distribution coefficient K=(Vf cNA)71 yj nj j(xf ,t)dxi , j O a? X¢§? (19) ¢§DF , kT where Vf is the volume of the sorbing phase, and NA is the Avogadro constant. Approaching the equilibrium, function j(xf, t) converges, and parameters yj and nj become independent from {xj}, the number of all the centres except the most stable diminishes, and coefficient K tends to a value Ke=y1N1(cNA)71=K0exp where y1 is the fraction of the most stable filled centres, N1 is the number of such centres in a volume unit of the sorbent phase, K0 isSorption mechanism and prediction of sorbents' behaviour in physicochemical systems the ratio between the coefficients of thermodynamic activity of the sorbent in the sorbent phase and in the medium, DF is the standard change in the free energy of the sorbent during sorption.A dynamical quasi-equilibrium is achieved between the centres sorbing most rapidly and the medium in the system upon completion of the first step if the sorption proceeds in several steps and adsorption centres of only one type are involved in the process at each step.Many examples of adsorptive quasi-equilibrium on the `external' surfaces of bodies or on the surfaces of micro- particles in the bulk of bodies with a successive transition to the equilibrium in the bulk have been described. Equilibrium coefficients Ke are determined for several thou- sands of systems (though often without proof of their equili- brium).K0 and DF are calculated for many systems by the methods of statistical mechanics and molecular dynamics. The matter connected with the differentiation of the partial and total equili- brium and the indentification of centres that are responsible for the quasi-equilibria still remains unclear.Functions DF(yj, yi) and K0(yj, yi) that can be converted into isotherms of sorption are diverse.151 ± 154 The Langmuir, Freund- lich, Frumkin, Totte, Dubinin ± Serpinsky isotherms describing a variety of experimental results are applicable in a narrow range of conditions, as a rule. They are special cases of more complex isotherms. Systems are widespread both with a small entropy contribution in DF and with a great one.155, 156 The great contri- bution of entropy has been found upon investigation of the sorption of bovine serum albumin and ovalbumin on a series of ion-exchangers.155 The sorption process of these proteins is endothermic, and coefficients Ke are great.1. The variety of equilibrium forms Sorptive centres of several sorts can retain commensurable quantities of a sorbate at the quasi-equilibrium. For instance, it has been proved by the methods of scanning electronic and tunnelling microscopy and also by the diffraction of slow electrons that CO molecules are retained on centres of two sorts on the face (111) of a platinum crystal. In this case, the adsorption on both centres is commensurable if yj is high, and the adsorption on one of them dominates if yj is low.157 Four forms of the sorbate, being present in commensurable quantities and manifesting themselves in its magnetic properties, were discovered in the study of the adsorption of Cu(II) salts at TiO2 single crystals.158 Three forms of the sorbate with the adsorption heat of 101.6, 112.4, 178.1 kJ mol71, respectively, were observed upon the adsorption of NH3 on crystals of CdSO4 .0.66H2O.23 The presence of these forms can be inter- preted as a result of sorbate localisation at the centres of different type. The presence of several adsorptive forms can be related to the different orientation of molecules on the surface in the case of the adsorption of complex molecules. The forms whose orientations are favourable to produce the largest number of bonds with the surface dominate over the others. This was demonstrated by Diaz et al.,159 for instance, who compared the sorption ability of the SiO2 surface covered with an alkyl monolayer with respect to eight polyacrylate speci- mens (sorbates) to which different number of dodecyl radicals was grafted.2. The role of complex formation Coefficient Ke increases if centres arise on the surface and in the bulk of a body which act according to the laws of complex formation: the more stable sorptive complexes, the greater values of Ke.160 This was cogently shown for the sorption of actinides by sorbents containing organophosphorus substances 161 and for the sorbent intended to remove bilirubin from the blood plasma.162 This fact makes possible the use of ideas of the chemistry of coordination compounds for the interpretation of the sorption, taking into account the fact that the stability of complexes in the bulk or on the surface of the body differs from that in the environment.151 Sorbents with centres forming complexes can be selective. For example, the selective adsorption of a lanthanum salt at tetra- cycline centres is observed if tetracycline hydrochloride is sorbed at zirconium phosphate, and then the resulting sorbent is intro- duced into a solution of lanthanum, cerium, barium, and thorium salts.163 If functional groups based on ethylenediamine or dieth- ylenetriamine are grafted to a matrix of mesoporous molecular sieve MCM-41, a possibility will arise to extract Co2+ from its aqueous solution selectively due to the formation of amino complexes in the bulk of grains.159 The selectivity of sorption depends on the difference in the stability of sorptive complexes.The mechanisms of the processes leading to different stability are similar to those determining the selectivities of homogeneous reactions. 3. Deformation of the electronic structure over adsorption The electronic structures of sorbate molecules change upon sorption of any type, and the changes can be as substantial as for the formation of chemical compounds.140 ± 145 Thus, the deforma- tion of electronic structure of potassium atoms upon the adsorp- tion of the potassium vapour on the face (110) of a GaP crystal is so strong that lines 2p (P) and 3d (Ga) in X-ray electron spectra shift by 1.6 eV at yj?0.1.164 Changes of electronic structure over adsorption affect all atoms of an adsorbed molecule to a greater or lesser extent. However, they are mainly localised on one or several atoms.As a result, some atoms can have great mobility, remaining within a molecule or breaking away from it. In particular, the change of electronic structure affects several atoms connecting the sorbate to the surface over the adsorption of theH8Si8O12 clusters at the face (100) of a silicon crystal. The evidence for this is the coincidence of a measured reflection IR spectrum and a calculated one (by the method of density functional).165 Calculations have shown that an H2O molecule changes its configuration insignificantly upon its transition on regular adsorption centres localised at the faces (100) of NaCl crystals. But if a molecule of H2O gets near an anion vacancy in the near- surface monolayer of the crystal, its electronic density changes so much that the molecule dissociates.166 It has also been shown by the method of density functional that the electronic structure of a thiophene molecule changes to the configuration Z5 upon the sorption on the (010) surface of a crystal of MoS2.167 A cleavage of the C7S bond and the hydro- genation take place in this case.It was established by the same method that a hydrogen bond is formed between hydrogen atoms of the molecule of H2O and any oxygen atoms of the near-surface monolayer (mostly the oxygen atoms of the vanadyl group) upon the adsorption of H2O molecules on the face (010) of a perfect crystal of V2O5. If the sorbent is non-stoichiometric, the electronic structures of H2O molecules are deformed so that some of adsorbed molecules dissociate.168 It was shown by the X-ray electron spectroscopy and temperature-programmed desorption that the adsorption of H2S on the face (100) of a crystal of FeS2 at 500 K is accompanied by a strong deformation of electron shells of atoms and dissociation of the sorbate molecules.The hydrogen comes further into the bulk of the crystal and sulfur localises near the vacancies of the near-surface monolayer.152 The molecules of water adsorbed at the surface containing silicon and aluminium oxygen tetrahedra, are deformed to a lesser extent so that one of the protons of H2O has an increased mobility without the dissociation of the molecules.Such a conclusion was made as a result of the calculation of configuration and normal vibrations of the molecule of water by the Hartree ± Fock method.169 The changes of the electronic structure of the sorbate are specific for the centres of a different sort. This manifests itself, for instance, in the adsortion of CO on the surface of crystals with the composition Cs2HPW12O48, where the electronic structure of CO changes in different ways depending on whether it is localised on the atoms of cesium or hydrogen. Adsorption bands with maxima at 2165 and 2145 cm71 emerge in the IR spectra of the sorbate.152 The intensity of the former band increases if some of the cesium atoms are replaced with hydrogen ones by changing the conditions of the synthesis of the sorbent.170 The examples considered above indicate the variety of the forms of adsorption-induced deformation of the electronic struc- tures of sorbate molecules.Information of such deformation is accumulated rapidly, though the general regularities of the response of an electronic structure to the adsorption are not defined concretely.154 It is worth noting that the observation of R17NHCONH7 R27NHCONH7R1 molecules adsorbed on the graphite by scanning tunnelling microscopy revealed that the contrast in the images of carbamide groups is the same if the hydrocarbon chain contains an odd number of carbon atoms. If the number of the atoms is even, the contrast of these images is different.171 The contrast of the images of the carbonyl group is directly connected with its electronic structure.Therefore, one can conclude that the electronic structure of the sorbate's molecule becomes unsym- metrical with an even number of carbon atoms in the chain. This observation indicates the possibility of a direct microscopic study of some of the peculiarities of the intramolecular distribution of the electronic densities of adsorbed molecules. 4. The relationship between the distribution coefficient and the properties of a body to be sorbed Experiments have demonstrated that the following condition holds true for some series of sorbates 172 s (20) Ke=K1 áK2 , P m P where K1, K2, andmare the characteristic parameters of a series of sorbates, Ps and P are the solubilities (for the sorption from a liquid) or the pressures of saturated vapour (for the sorption from a gas) of the sorbent and sorbate, respectively. As a rule, m>1 and K2=0 in the investigated series.Never- theless, m=1 and K2>0, for instance, for the sorption of a-naphthylamine by the crystals of naphthalene from solutions in various solvents.173 Series for which m=71 have appeared in the literature, but this can only be explained by the absence of the equilibrium in the system.172 Correlation (20) reflects a general tendency for substances with low solubility or pressure of satu- rated vapour to be sorbed predominantly. The results of quan- tum-chemical calculations do not conflict with such a tendency, though they show that one can expect a monotonic dependence of coefficient Ke on quantities Ps and P in the case of similar properties of sorbed substances only.174, 175 Numerous facts indicate the importance of the similarity of sorbate's and sorbent's properties.For example, substances whose crystal lattice is similar to the sorbent one for at least one crystallographic direction adsorb better owing to favourable conditions created for the formation and growth of two-dimen- sional clusters in this case.176 Inert gases dissolved in water are sorbed by the crystal solvate SO2 . 6H2O at T=268 K. In this case, coefficient Ke is monotoni- cally reduced by increasing the difference between the molecular mass of SO2 and the atomic mass of a gas � Ar, Kr, Xe, and Rn (Fig. 6).177 A number of admixtures are sorbed by the crystals of germanium and silicon from a melt at the melting point.The smaller the difference in sublimation enthalpy between the sorbed substance and the sorbent, thereater Ke for the formation of solid solutions.178 Summarising the results of studies,172 ± 178 one should note the absence of a determining property, which would uniquely charac- terise coefficient Ke, among the investigated properties of the sorbate and sorbent. Ratio Ps/P can be considered as such a property as a first approximation. This is indicated by the prevalence of correlation (20). However, the connection of Ke with this correlation is not always unequivocal. I V Melikhov, D G Berdonosova, G I Sigeikin 7logK3 4 6 3 4 2 2 1 0 1 2 logm0 Figure 6.Coefficients of equilibrium sorption by the SO2 .6H2O crystals at 268 K of radon (1), xenon (2), krypton (3) and argon (4) vs. the sorbate atomic mass;177 m0 is the atomic mass /a.u.; K3 is co-crystallisation coefficient. 5. Ways of prediction The qualitative prediction of function Ke(yi , yj) is possible based on the regularities of equilibrium sorption mentioned above. The semi-quantitative prediction is performed by analogy, using correlation dependences that are similar to Eqn (20). Another way is to employ approximate calculations of functions DF(yi , yj) and K0(yi , yj) with subsequent application of correlations of the type (19). The calculations are performed with the use of statistical thermodynamics and quantum chemistry.179 ± 181 One aims to determine the values DF regarding the resulting interaction of the sorbate and sorbent as a sum of interactions of different types, by considering their influence to be additive.182 For example, the dispersion, dipole, and pairwise electronic interactions are discriminated while describing sorption.183 How- ever, more precise information about the interaction potentials of atoms is required in such a method.Especially great attention is paid to the calculation of function Ke(yi , yj) for porous grains.184 ± 187 Pores of different diameters and configurations have different sorptive capacities usually. In other words, each of them represents a specific sorbing centre.The assumption that pores and adsorbing centres are distributed over the free energy of sorption continuously could be used without a loss of accuracy if there exist many pores.188 ± 190 The calculations of sorption are carried out for many systems. Nevertheless, some of them deviate from the experimental results sharply.191 Other calculations allowed quantitative explanation of the existing experimental results only, but the new data could be explained at the qualitative level only. VII. Accompanying phenomena 1. Cosorption on the surface and in the bulk If a sorbent takes several sorbates up simultaneously, the sorption of each sorbate is rarely additive.15, 192 Sorbate particles usually not simply compete with each other for sorbing centres, but also change the sorptive capacity of these centres with respect to their competitors.193 If these changes are great, one can speak about negative or positive cosorption of substances.Each cosorption type can be divided into a kinetic one, at which competitors speed up or slow down the sorption of each other, and an equilibrium one, at which they affect not only the rate, but also significantly change the value of Ke. The range of the kinetic cosorptive effect can be considerable. For example, the rate of the formation of mixed adsorptive layers of Cu(ClO4)2 and KBr on the faces (100) of a copper crystal in the aqueous medium is so much greater than the rate of the formation of the pure layers of Cu(ClO4)2 that the adsorption proceeds from the kinetic regime to the diffusion one under the influence of KBr.194 A powerful negative kinetic effect has been revealed 90 forSorption mechanism and prediction of sorbents' behaviour in physicochemical systems the cosorption of H2 and Cl2 on the rearranged (261) (001) surface of silicon at 300 ± 1500 K, the initial temperature being 300 K.The molecules of HCl are produced in such an excited state upon the reaction of chlorine and hydrogen adatoms that most of them desorb in picoseconds after the beginning of the contact of the adatoms. The probability of desorption increases to 0.18 in a temperature range from 300 to 1000 K. After that, the probability remains constant, which indicates complex forms of functions wj (T) and bj (T) in formula (10).These functions point out the possibility of cleaning the surface from molecules of one sorbate by the molecules of another one owing to the interaction of the sorbates. The equilibrium cosorption takes place in different systems. Thus, the sorption of strontium from a hydrothermal solution by Montmorillonite increases by 30% if the Sr2+ ions sorb together with Li+ ions.195 An analogous positive cosorption of these elements by soils was described by Chitra et al.196 The adsorption of carbon monoxide on SiO2 from a gaseous phase is negligible under ordinary conditions if the sorbent is not doped with a silver salt. The adsorption is enhanced if the sorbent contains silver. A band at 2169 cm71 has appeared in the IR spectra of the system, which is a characteristic of the Ag7CO bond.197 The adsorption of polyvinylpyrrolidone on the crystals of a-Al2O3 from an aqueous solution at pH 5 ± 7 is enhanced in the presence of polyacrylic acid.198 The adsorbed molecules of the acid form hydrogen bonds with the polyvinylpyrrolidone.This results in a positive coadsorption. Such a bond changes the conformations of sorbate molecules, which was proved for samples with a spin label by ESR. At the same time, the biomass of pine bark sorbs substances from a mixture of aqueous solutions of cadmium, copper, and nickel worse than from an individual solution of each salt.199 A sterilised soil suppresses the sorption of phenanthrene if the soil contains pyrene in a concentration of 561074 mol g71.The pyrene affects the rate of adsorption slightly, but increases the rate of desorption sharply.200 Some sorbents interact with the sorbates, producing micro- crystals. This is the case, for instance, for the sorption of AgNO3 from an aqueous solution by copper hexacyanoferrate Cu2[Fe(CN)6] (see Ref. 201) or for the sorption of SeO2 from a gaseous flow by calcium hydroxide.202 An abnormally strong sorption is observed in the latter case in the interval 770 ± 970 K owing to the formation of CaSeO4. Thus, cosorption is a consequence of chemical reactions between the sorbates in the bulk or on the surface of sorbing bodies where the conditions of reactions of molecules differ from those in the medium surrounding the bodies.Investigating sorp- tion, one can obtain information about the specificity of the interactions of the sorbate molecules in a force field of the sorbent molecules, which is necessary for a directed search for sorbents. 2. Specific features of biosorbents Biosorbents consist of plant biomass or microbial cells immobi- lised on supports.203 ± 210 Formally, the behaviour of biosorbent grains or separate cells does not differ from that of particles of inorganic sorbents. For example, Madrid et al.205 properly predicted the variation of the concentrations of a number of sorbates in a well-stirred reactor containing algae as sorbents. It was found that biosorbents can be characterised by particular diffusion coefficients Dv and by sorption isotherms and that the surface and volume sorption processes can be simulated in the usual way.If the sorbent is a set of identical cells, the membrane, cytoplasm and internal cell organelles should be regarded as its structural units.209, 210 Sorbate accumulation by cell membrane can be treated as adsorption, while migration of the sorbate inside the cell, as bulk sorption. Many biosorbents demonstrate selective sorption by separate structural units.211 For example, redwood bark and moss cells accumulate cadmium, copper and nickel ions from aqueous 153 solutions by membranes almost without involvement of the cytoplasm.212 The sorption capacities of cells depend on their physiological state but, generally, their `preferences' do not change.213 ± 215 For example, the sorption sequence Au3+>Cu2+>Ag+ always holds for the Chlorella vulgaris cells in aqueous solutions.Inside the cells, the sorbed substance undergoes chemical transforma- tions and passes to the colloidal state. In a study of sorption of various compounds by seeds of legume crops, it was found 52 that the coefficient Kdecreases along the series of chromates, molybdates and tungstates: Cr(VI)> W(V)>Mo(VI). However, no general relationship between the nature of the biosorbent and the Dv or K values has been found so far. Elucidation of this relationship is complicated by strong influence of sorbates on one another 204, 208 and by changes in the sorbate state in the cells or in the physiological state of the cells themselves.The cells or destroyed biomass are to be regarded as `black boxes'; thus, prediction of sorption becomes groundless, although for- mally it can provide some results.207 3. Structural memory of sorbents If sorbate molecules are introduced into particles of a sorbent during its synthesis so that the former can escape to the medium, `imprints' of the sorbate molecules can be retained in the sorbent structure and may play a role of specific sorption sites for this particular sorbate. The sorbent can `remember' that the sorbate was present in the system during its synthesis. The `memory' of inorganic sorbents was discovered long ago.216 It has now been proved that organic polymeric sorbents also possess `memory'; this fact is used successfully for selective and complete removal of side products of organic syntheses.217 4.The influence of external fields on sorbents Recent studies showed that the sensitivity of a sorbent with respect to an electric field can be increased to a practically significant level without deterioration of their sorption properties. For example, sorbents based on titanium phosphate have been developed the sorption capacity of which changes dramatically upon application of an electric field; these sorbents absorb a number of metal ions from aqueous solutions by the ion exchange mechanism and, after that, desorption can be accomplished by merely changing the potential sign.218 The application of an electric field accelerates sorbate diffusion inside the bulk of grains.219 The field also influences the surface mobilities of atoms; this has been demon- strated, for example, by direct observation in a tunnelling micro- scope of the behaviour of potassium and sodium atoms on the (100) surface of aluminium.220 Multiphase sorbents sensitive to acoustic fields have been prepared.221, 222 The sorbent grains consist of a heat-sensitive polymeric hydrogel, which can undergo a structural phase transi- tion at some temperature T0 .Microcrystals of a poorly soluble substance with high density are introduced in the polymeric matrix, together with particles of a sorption-active phase. When T<T0, the polymer molecules form a three-dimensional network with large cells, which ensures fast diffusion of the sorbate into the grains.If the acoustic power is relatively low, the properties of the polymer network do not change on direct exposure to an acoustic field but the microcrystals inside the matrix are heated. They absorb most of the field energy and function as `internal heaters' for the grains. If the grain temperature increases to T0 , the polymer molecules are compacted to form denser stacks and the grain permeability decreases abruptly. Once the sound has been switched off and the grain has been cooled to T<T0, the permeability increases again. Thus, the grain responds to acoustic treatment by a triggered transition from one permeability level to another. New magnetite-containing sorbents have also been prepared.Particles of such a sorbent can move on exposure to a magnetic field.223154 5. Phase antagonism and synergism in multiphase sorbents If a sorbent grain contains several phases, they can compete for a sorbate. In this respect, sorbent phases are antagonists. The points where particles of different phases contact with one another can give rise to specific sorption sites with increased Ke values. If this effect predominates over the antagonistic effect, these phases exhibit a synergistic effect.224 ¡¾ 227 For example, synergism was attained in the synthesis of a fibrous sorbent based on cellulose and zinc or copper ferrocyanide.224 This sorbent proved to be especially efficient for extraction of cesium salts from aqueous solutions and was recommended for deactivation of natural and technological waters contaminated by 137Cs.A sorbent based on CuO and g-A12O3 phases with a large internal surface area of microparticles and a high porosity of grains having a narrow size distribution was prepared by the sol ¡¾ gel procedure.227 At 790 K, this sorbent showed a sorptive capacity toward sulfur dioxide of no less than 22.5 mass %, which is much greater than the values observed for other sorbents. 6. The molecular electrical conductivity of adsorbed species If an adsorbed molecule (a two-dimensional cluster) occurs in a flow of electrons coming to the surface from the bulk, then, with some probability, the electrons adhere to a molecule, remain in contact with it for some period, and then pass to the substrate. Thus, the adsorbed molecule influences the electron flow.This influence can be described by attributing to each molecule an individual electrical conductivity. This parameter can be found by observing separate adsorbed molecules through a tunnelling microscope or by measuring the electric current between electro- des bridged by individual adsorbed molecules. For example, by applying a dilute solution of DNA (the length of the molecule is 10.4 nm) onto two platinum nanoelectrodes located at a distance of up to 8 nm using a special application procedure, one can ensure that one end of the molecule is adsorbed to the cathode and the other, to the anode, thus forming a `bridge' across the gap between the nanoelectrodes.228 When the electrodes have been dried in a nitrogen flow and the voltage U=1 ¡¾ 5 V has been applied, the cathode ¡¾DNA¡¾ anode circuit carries the current (21a) I(U)=s(U7Uo), U>Uo , (21b) I(U)=0, U<Uo , where s=5.3610710 O71 is the molecular electrical conductiv- ity at voltages higher than the threshold value Uo=1 V.229 The electrical conductivity s of the DNA molecule is due to tunnelling of electrons through the contact surface between the molecule and the anode and to their one-dimensional diffusion along the molecule.228, 230 However, it is not clear yet what are the contributions of these processes to the DNA conductivity.Relations (21a) and (21b) reflect the threshold character of the molecular electrical conductivity of DNA.Apparently, this is a typical character of the electrical conductivity of molecules of different nature, as follows from the data on the interaction of adsorbed molecules with a flow of tunnelling electrons. If a molecule is located on the surface of the substrate electrode, a second electrode represented by a spike with a `monoatomic' tip is placed at a distance R=1 ¡¾ 5 nm, and the voltage U is applied to the spike, then current would flow from the spike to the substrate.231 ¡¾ 233 The current intensity can be expressed by the relation I(U)=or0e(r0 ,U,R)[UZ1+(U7Uo)Z2], where o is the probability for an electron to pass from the spike to the molecule per unit time, r0 is the electron density of the substrate under the spike before application of the field, e(r0 ,U,R) is the function characterising transition of electrons from the molecule to the substrate, Uo is the voltage correspond- ing to the change in the Z1 and Z2 values from Z1=1 and Z2=0 for U<Uo to Z1=0 and Z2=1 for U>Uo .I V Melikhov, D G Berdonosova, G I Sigeikin The threshold voltages Uo were found for C2H2 , CO, benzene and pyridine molecules adsorbed on the surface of copper crystals.233 ¡¾ 235 The o value usually depends slightly on r0 and in the kT<Ue<je range (e and je are the charge and the work function of an electron), it can be described by the exponent o=exp ¢§U0 , U where U0=10 ¡¾ 100 V is the characteristic voltage of the tunnel- ling field emission.236 The e(r0 ,U,R) function has a discontinuity at U=Uo.For U>Uo, it has several maxima; the {Ui} values corresponding to these maxima comply with the condition eUi?ohi h , where ohi is the frequency of a vibrational transition of the molecule or of the substrate below the molecule. This implies that the tunnelling electrons provide a resonance stimulation of vibrations of atoms in the adsorbed molecule, and the vibrations excited by the electrons are synchronised in such a way as to increase the probability of electron transition onto the substrate. The resonance excitation of intramolecular vibrations was found, for example, in theH2Omolecules adsorbed from vapour at 300K on the surface of titanium oxidised in air.231 ¡¾ 233 In this case, prior to H2O adsorption, the I(U) function exhibited maxima corre- sponding to the stretching vibrations in TiO2 (ohi h&0.1 eV).After adsorption, this function acquired large-scale maxima at e(U27U1)^0.2 eV, e(U37U1)^0.5 eV, e(U47U1)^0.7 eV, e(U57U1)^1.0 eV. The first two of these values are close to the quantum energies of bending (0.2 eV) and stretching (0.46 eV) vibrations of adsorbed water molecules (from IR-spectroscopic data). Hence, the other maxima can be attributed to two-quanta and combined vibration transitions.233 When H2O is replaced by D2O, the positions of the I(U) maxima shift in conformity with the expected isotope effect. This attests unambiguously to the stimulation of intramolecular vibra- tions by the tunnelling electrons.VIII. Sorbent degradation The properties of a sorbent change inevitably during operation as sorbent bodies are gradually dissolved and destroyed upon collisions and chemical reactions with admixtures. The mass of each grain m varies at the rate (22) dm dt a bk0SOc ¢§ PsU ¢§ gkokmf a Gk , where bk0 is the kinetic coefficient of dissolution, gk is the probability of grain fragmentation, ok is the frequency of colli- sions of the grain with other grains or with reinforcing structures, mf is the average mass of the chopped-off fragments, Gk is the rate of mass variation caused by chemical reactions between the sorbate and the reagents present in the system as impurities. Each of the above-mentioned paths of the change in the sorbent properties is characterised by the proper time ti : m¢§ t1=bk0SOc ¢§ PsU , m¢§ t2=gkokmf ,Sorption mechanism and prediction of sorbents' behaviour in physicochemical systems t3= m¡ , Gk where m¡ is the average mass of grains.The ti values can be predicted;237, 238 however, the bk0 coef- ficients are unknown for most substances. They should be determined experimentally. The gk value can be estimated using the theory of contact plastic deformation, the frequency ok can be determined using the collision theory and mf is found from the model of fracture of solids.239, 240 These values can be appreciably different, depending on the reactor shape and the method of stirring of the suspension.If one deals with a block crystal, chopping-off of blocks is the most probable way of destruction. As a result, the mf value is similar to the average mass of blocks. If the particle is an agglomerate, the points of accretion of microcrystals are the first to be destroyed upon collisions, thus, mf becomes equal to the average mass of the separate microcrystal. IX. The search for an optimum sorbent The quest for the optimal sorbent is carried out more and more often according to the principle: `a specific sorbent for each system.' An individual system is characterised by a unique set of Ke(yi), t0.9(yi) and t1(yi) functions. The sorbent of choice is one ensuring the specified values of these functions. The best sorbent for a given system is usually selected by searching through the sorbents optimum for similar systems.241, 242 If none of these materials is appropriate, the following multistep procedure is executed.1. Identification of the substance to form the basis of the desired sorbent At the stage of looking for the substance that would form the basis of the desired sorbent, correlations such as relation (20) are used and the array of materials with rather high K values and low solubility Ps is identified. The K values for each of these materials are determined experimentally and the material with the greatest K is chosen. This material would form the basis of the optimum sorbent. 2. Study of the kinetics of sorption by the disperse phase of the chosen material When investigating the kinetics of sorption by the disperse phase of the material chosen as the basis for the optimum sorbent, the kinetics of sorbate absorption by the material particles moving freely in the medium should be measured over a broad range of conditions {yi}.Using kinetic equations such as Eqns (3) ± (11), the aj (yi), vj (yi), lj (yi), and t0.9 (yi) functions are determined, while relations of type (13) and (14) are used to determine the Dv(yi) function. The mL (yi) and rL (yi) functions are also found. Using experimental data on sorption for long sorption times and high concentrations of the sorbate and relations of the type (19) and (20), the K(yj) and DF (yj) values are determined and the isotherms of sorption yj (yi) by the disperse phase are elucidated.The results of measurements are summarised as Gi (xi ,yi) and Dij (xi ,yi) dependences [see relations (1) and (2)]. This stage results in quantitative descriptions of the adsorption and desorption, the formation of two-dimensional solid solutions and surface clusters, the formation of multiatomic layers and particles of the sorbate phase on the surface of freely moving sorbent particles, and migration of the sorbate inside the bulk of the chosen sorbent. 3. Study of the role of agglomeration Agglomerates of particles differing in porosity are studied to identify the effects of contacts between the particles, capillary condensation and crystallisation in the pores on the K(yj), DF (yj) and yj (yi) functions.By simulating the sorbate interaction with pore walls, the Dv (xi , yi) and Ke (yi ,yj) values for agglomerates can be determined. 155 The results of investigation of the kinetics of sorption by the disperse phase of the material chosen as the basis of the optimum sorbent and study of the role of agglomeration are generalised as kinetic equations of sorption of the type (1) ± (13) devoid of arbitrary assumptions concerning the parameters of elementary steps in the sorption by separate particles and their agglomerates. 4. Determination of the rate of sorbent degradation The rates of dissolution, fragmentation and chemical degradation of the sorbent for the most drastic operation conditions are determined using Eqn (22), and the bk0(yi), gk(yi) and Gk(yi) functions are found.The ways of sorbent modification aimed at increasing its chemical and mechanical stability are developed. 5. Numerical experiment The kinetic equations of sorption supplemented by edge condi- tions for the whole range of {yi} parameters formulated as a closed boundary problem are usually solved by numerical methods. The results are presented as the following functions: Ke(x0i , yi ,tk), t0.9(x0i , yi ,tk), ti (x0i , yi ,tk), where x0i are parameters for the state of sorbent grains before use, tk is the time period during which operation of the sorbent is planned. These functions allow one to answer the question what properties are demanded of the initial sorbent for this sorbent to be regarded as optimal.At the stage of numerical experiment, the {x0i} values are found. 6. The nearest goal of the search The stepwise procedure outlined above has been implemented thus far only for systems that are most important for practical purposes. Most often, in choosing the sorbents, the material that provides the most complete, fast and selective sorption and can be recovered from the medium over the shortest period of time is assumed to be the sorbent of choice. These conditions are met by a sorbent with dendrite-shaped grains consisting of accreted filamentary nanocrystals forming a highly porous texture. The surface of nanocrystals bears func- tional groups capable of forming complexes as stable as possible with the given sorbate.The structure of nanocrystals allows the sorbate molecules to be inserted into interstices with a high K coefficient. Judging by the experimental results presented above, grains of such a sorbent would form highly porous filter layers characterised by low fluid-flow resistance, high velocity ~uL and approximately equal accessibilities of grain surfaces for a sorbate and would ensure favourable conditions for adsorption of sorbate molecules on nanocrystals and diffusion of molecules into their bulk. X. Conclusion Analysis of the published data shows that the research into sorption is approaching a new stage of development which can be termed as the molecule visualisation stage. There are grounds to expect that the gap between macroscopic and molecular description of sorption would be filled.By the beginning of this stage, conditions have been created for experimental investigation of all elementary processes taking place on the surface and in the bulk of sorbent bodies. To date, these opportunities have not been exploited to a degree sufficient for the elaboration of a theory of sorption able to describe the whole set of elementary processes. Therefore, it is still impossible to predict quantitatively the behaviour of sorbents over a broad range of sorption conditions. The current level of detail in the description of sorption makes it possible to compose a particular plan for the accumulation of data sufficient for quantitative prediction of the sorbent behav- iour under various sorption conditions on the basis of a priori semiquantitative estimates (`by analogy').156 This plan should include theoretical analysis of the possible variants of implementation of sorption processes and accumula- tion of missing experimental results.Theoretical analysis implies the solution of evolution equations that describe the variation of the sorbent state during the synthesis and operation, while experimental studies are aimed at `information loading' of the evolution equations. The authors are grateful to V E Bozhevol'nov and S S Ber- donosov for assistance and active discussion of the work. This work was supported by the Russian Foundation for Basic Research (Project No. 00-03-32644). 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Chem. (Engl. Transl.) o�Inorg. Mater. (Engl. Transl.)

ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (2) 159 ± 174 (2002) Polyurethane foams in chemical analysis: sorption of various substances and its analytical applications S G Dmitrienko, Yu A Zolotov Contents I. Introduction II. General characteristics of polyurethane foams III. Sorption of elements and organic compounds on unloaded polyurethane foams IV. Sorption of elements on polyurethane foams loaded with organic reagents V. Classification of sorption processes involving polyurethane foams VI. Analytical application of polyurethane foams VII. Conclusion Abstract. foams polyurethane of properties sorption the on Data Data on the sorption properties of polyurethane foams including their modified forms are surveyed. Examples of appli- including their modified forms are surveyed.Examples of appli- cation and separation the for foams polyurethane of cation of polyurethane foams for the separation and preconcen- preconcen- tration given. are compounds organic and inorganic of tration of inorganic and organic compounds are given. The The mechanisms in described foams polyurethane on sorption of mechanisms of sorption on polyurethane foams described in the the literature of development the in Advances analysed. are literature are analysed. Advances in the development of combined combined and sorbents these of use the with analysis of methods hybrid and hybrid methods of analysis with the use of these sorbents are are discussed. references 259 includes bibliography The discussed. The bibliography includes 259 references. I.Introduction proposed the use of PUF as solid polymer matrices for immobi- lisation of various organic reagents.3, 4 Since the mid-1970s, PUF have been used in analytical chemistry for preconcentration of inorganic and organic compounds from aqueous solutions or air and for the development of combined or hybrid analytical techniques. During the elapsed time, vast amounts of experimen- tal information dealing with sorption by means of unloaded and loaded PUF has been accumulated and data on sorption mecha- nisms have been gained. The following practical applications of PUF have become traditional and most important: separation of mixtures in which elements and compounds are present in com- parable amounts; isolation of components for preconcentration and subsequent determination directly in the sorbent phase using neutron-activation analysis, X-ray fluorescence, photometry, or another method; and isolation of components for preconcentra- tion and subsequent determination in the eluate phase using chromatography, spectroscopy, or some other analytical techni- que.The application of PUF in analytical chemistry is the subject of a monograph 5 (difficult to access for a Russian reader) and several reviews,6± 10 which, however, do not cover all applications of this class of sorbent.In addition, after the publication of the latest of the above-mentioned works (1992), the number of studies dealing with this topic has substantially increased and new trends in the use of PUF in chemical analysis have taken shape.This has stipulated the need for generalisation of the accumulated data at a qualitatively new level taking into account, first of all, the modern views on the mechanisms of sorption onPUFand the properties of these sorbents. Polyurethane foams (PUF), or foamed polyurethanes, represent an extensive class of synthetic materials prepared from polyur- ethanes � heterochain polymers that contain urea, amide, ether, ester, or other groups in addition to urethane groups. Polyur- ethanes were first synthesised by Bayer and co-workers in 1937.1 Later, industrial production of PUF was started on the basis of these polymers and polyesters or their analogues (Germany, 1944) and, subsequently, on the basis of cheaper polyethers (USA, 1957).Currently, PUF are produced in large amounts (millions of tons a year) in many countries all over the world. Polyurethane foams (in the household, these materials are called foam-rubbers) are widely used for domestic purposes: to produce soft furniture, car seats, mats, sponges, toys, and so on. In industry, PUF are employed as damping and heat insulating materials. Some sim- ilarity between the characteristic functional groups present in polyurethane molecules and in macromolecules forming living tissues provides grounds for assuming that PUF-based materials would be promising for use in medicine, all the more so, because polyurethane fibres have long been used in surgical practice as sutures and for the manufacture of vessel prostheses. II.General characteristics of polyurethane foams The possibility of using PUF as sorbents for extraction of metal ions and organic compounds from water and air was first established by Bowen in 1970.2 Two years later, Braun and Farag S G Dmitrienko, Yu A Zolotov Department of Chemistry, M V Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russian Federation. Fax (7-095) 939 46 75. Tel. (7-095) 939 46 08. E-mail: dmitrienko@analyt.chem.msu.ru (S G Dmitrienko) Tel. (7-095) 939 55 64 (Yu A Zolotov) Received 19 November 2001 Uspekhi Khimii 71 (2) 180 ± 197 (2002); translated by Z P Bobkova Polyurethane foams are prepared by polycondensation of isocya- nates with polyols (glycols, triols, polyethers, or polyesters) followed by foaming of the polymer bulk by carbon dioxide.Polyether and polyester foams can be distinguished. In the industrial synthesis, polyethers with terminal OH groups and molecular masses of 400 ± 6000 are used most often as the initial compounds, together with mixtures of 2,4- and 2,6-toluylene diisocyanates in 65 : 35 or 80 : 20 ratio. In the one-step process, diisocyanate, polyether resin, water, catalysts, stabilisers, and emulsifiers are mixed simultaneously. The reaction takes place immediately; foaming starts several seconds after mixing and is over after 1 ± 2 min. Toluylene diisocyanates play an especially important role in the synthesis of PUF. The reaction between #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n02ABEH000703 159 159 161 164 165 166 171160 polyhydric alcohol and isocyanate is a chain propagation reac- tion, resulting in urethane derivatives R17NH7C(O)7O7R2.R17N C O+R27OH The reaction of isocyanate with water yields amine and carbon dioxide, which foams the polymer. Amine can react with excess isocyanate to give substituted urea derivatives R1NH2+CO2, R17N C O+H2O [R17NH7C(O)7OH] R17NH7C(O)7NH7R1. R17N C O+R17NH2 Isocyanate can also react with urea and urethane derivatives giving rise to allophanate and biuret, which induce additional cross-linking of the polymer: NH R1 R1 N C(O) R1 N C O+R1NH C(O) NHR1 NH R1 C(O) OR2 R1 N C(O) R1 N C O+R1NH C(O) OR2 NH R1 C(O) Depending on the number of cross-links, flexible and rigid PUF are distinguished.The materials with low cross-linking density are called flexible PUF, while rigid polymers are charac- terised by high cross-linking density. Flexible PUF are usually synthesised from polyhydric alcohols with not very high molec- ular mass and low degrees of branching, while rigid ones are prepared from highly branched alcohols with low molecular mass. Detailed information on the synthesis of PUF can be found in monograph.1 Foamed polyurethanes contain different functional groups, namely, urethane [7NHC(O)O7], ether (7O7), ester [7C(O)7O7], amide [7C(O)7NH7], urea [7NH7C(O)7 NH7], and terminal toluidine [7C6H3(CH3)NH2] groups and also both aromatic (7C6H47) and aliphatic (7CH27) groups.The diversity of functional groups in the flexible polymer chains of PUF creates favourable conditions for the formation of intermo- lecular (secondary) bonds of various chemical natures and with different energies (ranging from van der Waals to hydrogen bonds), which has a substantial influence on the physicochemical properties of the materials.11, 12 The hydrogen bonds arising upon the interaction of hydrogens of the proton-donating urethane groups with oxygens of the proton-withdrawing ether, ester or carbonyl groups play an especially important role. The type of hydrogen bond depends on many factors, in particular, on the proton-donating and protonithdrawing capacity of groups, their concentrations and distribution along the polymer chain and on the steric conditions of the formation of the H-bonds.Exhaustive information on the types of hydrogen bonding has been obtained by IR spectroscopy.13 Study of IR absorption of polyurethanes of different chemical structures in the region ofNH stretching frequencies showed that most of the NH groups in polyurethanes are involved in hydrogen bonds. The vibration frequencies of NH groups perturbed by hydrogen bonding are different in the spectra ofPUFbased on polyethers and polyesters. This may be due to different types of H-bonding in these systems. From analysis of the IR spectra, it was concluded that the major type of hydrogen bond in polyester-based polymers is that between the hydrogen atom of the urethane group and the carbonyl oxygen of the ester group, while in the polyether foams, this is the bond between the urethane-group hydrogen and oxygen of the polyether chain; the hydrogen bond involving ether oxygen is stronger.13 Due to the strong hydrogen bonds which form in PUF a continuous three-dimensional network, in certain cases, linear polyurethanes, cross-linked only through physical bonding exhibit properties normally typical of chemically cross-linked linear chains.13 Yet another structural feature of the three-dimen- sional network formed in PUF is lability.The ability of the S G Dmitrienko, Yu A Zolotov network to be destroyed and reconstructed on heating, treatment with solvents or mechanical stresses suggests that this network can be likened to thixotropic structures, well-known in colloid chem- istry.The presence of the labile structural network accounts for many specific properties of PUF. For example, the tensile and compression strengths of PUF are believed to be mainly due to the presence of intermolecular interaction. In the case of pronounced deformations (bending, stretching, etc.), the secondary bonds are destroyed, while the primary (chemical) bonds remain intact. Evidently, the ability of polyurethanes to `heal' the defects arising upon deformations can be explained by the relative ease of network destruction and subsequent restoration. Therefore, after multiple compression, PUF is softened and its initial strength is restored after a certain `rest'.11 Polyurethane foams are foamed plastics in which some of the solid has been replaced by gas, usually air, which exists in the polymer as numerous bubbles (cells).Depending on the relation- ship between the rate of the macromolecule growth and the rate of gas evolution at the foaming stage, the cell walls are either broken (ruptured) or not; this gives rise to polymers with open-cellular or closed-cellular structure, respectively. As a rule, flexible PUF have an open-cellular structure and rigid ones are characterised by a closed-cellular structure. If the volume occupied by gas bubbles is less than 76% of the total volume of the specimen, the bubbles can be spherical; if this volume is greater than 76%, the bubbles are shaped most likely as quasi-spherical polyhedra, mainly, pentag- onal dodecahedra.5, 6, 9 The polymer itself forms the walls of the bubbles, which are actually polyurethane membranes.In open- cellular PUF, at least two membranes (faces of the pentagonal dodecahedron) are ruptured. Among the properties of PUF, one should mention the light- ness (the apparent density is 0.015 ± 0.045 g cm73), enhanced stability against thermooxidative destruction (180 ± 220 8C), and chemical and radiation stability. The properties of PUF do not change (not counting the reversible swelling) on contact with hydrochloric (up to 6 M), sulfuric (up to 4 M), nitric (up to 2 M), or glacial acetic acid, solutions of sodium hydroxide or ammonia (up to 2 M), and many organic solvents such as benzene, tetra- chloromethane, chloroform, diethyl ether, other ethers or esters, methyl isobutyl ketone, alcohols.Poluyrethane foams based on esters are less stable in acidic or alkaline media than those based on ethers because the former are more susceptible to hydrolysis. Both types of PUF are soluble in concentrated sulfuric acid or a hot solution of arsenic(III) chloride; they are destroyed on treat- ment with concentrated nitric acid and oxidised by an alkaline solution of KMnO4 .2 Estimation of the purity of PUF of various brands showed that the content of inorganic impurities in them is comparable with or even less than that in other sorbents such as active (activated) carbons, synthetic ion exchange resins or chelating sorbents.5 It was found by neutron activation analysis that the major impurity in PUF is tin, its content being substantially lower in polyester PUF.14 This may be due to the fact that organotin catalysts are used in the synthesis of PUF based on ethers.In addition, small amounts of Na, Mg, Al, Mn, V, Cl, Br and I have been found in PUF. The organic impurities extracted from industrial PUF speci- mens by various methods, namely, on treatment with sol- vents,15, 16 by thermodesorption 17, 18 or by supercritical fluid extraction 19 have been identified by a combination of gas chro- matography and mass-spectrometry techniques. The impurities were mainly toluylene diisocyanates,15 various aliphatic and aromatic amines 16, 18 and fluorocarbons (Freons).17, 19 The com- position of the impurities depends on the solvent used and on the features of the PUF manufacturing process.{ { It is known 1 that amines and Freons are used as catalysts and as additional foaming agents, respectively, in the production of some sorts of PUF.Polyurethane foams in chemical analysis: sorption of various substances and its analytical applications The specific surface area of PUF can be determined from isotherms of low-temperature adsorption of nitrogen 20, 21 or carboxy-14C-stearic acid.22 The specific surface areas of the PUF specimens studied were 0.007 ± 0.035 m2 g71, which is much smaller than the values found in sorbents such as Tenax GC (6.4 ± 25 m2 g71) or XAD-2 (300 ± 360 m2 g71).As applied to PUF, a method for the estimation of the microenvironment hydrophobicity, based on the interfacial parti- tion of pyrene between water and the medium to be tested, has been developed.23 The partition coefficient of pyrene between the sorbent and water, aqueous acetonitrile or aqueous ethanol is used as a measure of hydrophobicity. A comparative estimate of the surface hydrophobicities for six industrially manufactured speci- mens of PUF based on ethers and esters and for other sorbents such as Diasorbs C4, C8, C16, phenyl and carboxyl, XAD-8, cellulose, cellulose triacetate and activated carbon AX-21 showed that the PUF specimens studied are characterised by approx- imately equal surface hydrophobicities and that, regarding this parameter, they should be placed between Diasorb C16 and activated carbon AX-21.Fluorescence spectroscopy has been demonstrated to provide extensive possibilities in investigations of the properties of polyur- ethane foam membranes.24, 25 Using pyrene as the fluorescence probe, the polarity of the pyrene microenvironment (eef) in the matrix of various PUF has been estimated.24 For low degrees of filling of the sorbent phase (0.3 ± 0.9 mmol g71), the eef value ranges from 24 to 39, polyether PUF being less polar than polyester PUF; for high degrees of filling of the sorbent phase (4.2 ± 5.6 mmol g71), the polarity of the pyrene microenvironment decreases, the eef values being 10 ± 19 for polyester PUF and *2 for polyether PUF.The polarity of the pyrene microenvironment in the matrices of protonated PUF proved to be higher than those for the initial PUF. The data on quenching of the fluorescence of pyrene sorbed on PUF by quenchers of different natures, namely, potassium iodide and bromoform, gave rise to the suggestion that solid-phase foamed polyurethane membranes contain polar and hydrophobic microregions characterised by different physical properties.25 Lower values for the calculated Stern ± Volmer constants for pyrene in the PUF matrix compared to the values for aqueous solutions point to a decrease in the mobility of pyrene and quencher molecules, related, first to the their transition to a thicker medium and, second, to the rigid fixing of these molecules at particular sections of the polymer chain.The fact that the pyrene molecule is rigidly fixed in the PUF matrix is also indicated by the absence of the excimer band in the fluorescence spectra of pyrene sorbates, even when it is sorbed from suspended media.25 In a study of the protolytic properties of PUF, it was shown that stepwise protonation of PUF takes place in aqueous solutions of mineral acids (pH<3).25, 26 The protonation constants were calculated; it was suggested that the terminal toluidine groups in the PUF molecules are the first to be protonated. Thus, PUF are foamed plastics with a `membrane-like struc- ture' characterised by a relatively low specific surface area and high chemical stability and purity. III. Sorption of elements and organic compounds on unloaded polyurethane foams 1.Sorption of elements At present, the sorption of *50 elements on unloaded PUF has been studied. Most often, PUF adsorb elements as metal acido complexes,{ more rarely, as heteropolyacids, ion associates or metal chelates. Sorption in the form of metal acido complexes is observed for most elements able to form negatively charged halide, cyanide, and thiocyanate complexes (Table 1). Sorption of chloride and thiocyanate metal complexes is the most wide- { Acido complexes are complexes containing only acid anions as the ligands. Table 1. Sorption of elements by unloaded PUF from solutions in mineral acids. Sorbable forms of elements 6 MF¡ MClq¡n n MBr¡4 4 4 MI¡ M(CN)¡ M(NCS)q¡n n UO2(NO3)2 HnMMo12O40 spread.Data on the sorption of fluoride,27 ± 30 bromide,2, 49, 52 iodide 39, 53 and cyanide 54 complexes are scarce. The sorption of metal acido complexes is carried out both from solutions of hydrohalic acids and from mixtures of alkali metal salts with mineral acids, for example, LiCl ± HCl, KNCS± HCl and KNCS±H2SO4 . The use of these mixtures is especially typical of sorption of thiocyanate complexes. The time required for sorption equilibrium to establish depends on the nature of the sorbed acido complex and varies from several hours (for metal chloride complexes) to 10 ± 30 min (for thiocyanate complexes). It should be noted that the conditions for sorption of metal acido complexes on PUF are the same as conditions for extraction of these complexes with oxygen-containing solvents, in particular, diethyl ether.Polyurethane foams adsorb metal acido complexes as one- and two-charged anions of the composition MF¡6 , MX¡4[X=Cl, Br, I, CN, NCS and M(NCS)2¡ 4 ] with high partition coefficients (n6102 ± n6104 ml g71). The sorption capacity is also high, varying from *0.2 to *2 mmol per gramme of the sorbent, which makes up to 10% of the sorbent weight. As a rule, PUF based on polyethers sorb metal acido complexes better than polyester foams. The metal acido complexes can be quantitatively desorbed from PUF on treatment with acetone or dilute solutions of hydrohalic acids. 161 Ref. Element 27 ± 30 2, 31 ± 34 2, 33, 35 ± 40 2, 20, 33, 35, 36, 41 42 ± 45 46, 47 2, 48 21, 33 49 50, 51 2, 52 49 39, 53 54 33 Nb, Sb, Ta Au Fe Ga Pt Rh Sb Sn Te Tl Ag, Au Te Bi Au Au, Fe, Ga, Hf, In, Mo, Nb, Pa, Sb, Sc, Sn, Ta, W, Zr Cd Co Cu Fe 55 56 ± 67 39, 66 55 ± 57, 60, 63, 64, 66, 68 ± 70 33, 71 33, 72 14, 73, 74 14, 33, 73 75, 76 39, 66, 71, 77 78, 79 60, 80, 81 82 83 55, 57, 73, 84, 85 2, 36, 86, 87 88, 89 90 90, 91, 95 Ga Hf, Zr Hg In Ir, Rh Mo, W, Tc Os, Ru, Pd Pd Pt Ti Zn UAs GeP 91±95 Si162 Sorption on PUF of acido complexes of platinum group metals has some specific features.Thus Rh(III) and Ir(IV) are poorly collected from thiocyanate solutions at room temperature; however, after short-term heating to 90 8C, sorption of these metals becomes quantitative.76 The selectivity of PUF with respect to chloride and, especially, thiocyanate metal complexes is relatively low and depends on sorption conditions, namely, solution acidity, supporting salts and other substances present in the solution.The sorption of metals from fluoride solutions proved to be more selective. Polyurethane foams quantitatively collect Ta(V) and Sb(V) from 0.1 ± 5 M HF as MF¡6 complexes and do not adsorb Co, Zn, Fe(III), Hf, Zr, Sn(IV), Nb, Pa, As(V), Mo, or W.27, 28 Apart from metal acido complexes, PUF quantitatively sorb some elements, for example, As, Ge, P and Si, as heteropolyacids (see Table 1).In this case, too, the conditions of sorption of heteropolyacids on PUF coincide with conditions of their extrac- tion with oxygen-containing solvents. Like crown ethers and acyclic polyethers, PUF collect alkali and alkaline earth metals 59, 96 ± 98 as well as Ag, Pb and Tl(I) 59, 98 in the form of ionic associates with large hydrophobic anion partners such as picrate or anilinonaphthalenesulfonate. The sequence of sorption of metal picrates is in agreement with the series of extractability of these metals with crown ethers, for example, 18-crown-6 or dicyclohexyl-18-crown-6. Polyether foams are better sorbents for these metals than polyester-based PUF. The possibility of sorption of metal chelates on unloaded polyether type PUF has been demonstrated.Thus Hg, Pt(II) and Zn are sorbed as complexes with dithizone,2, 99 Y is sorbed as a complex with 8-hydroxyquinoline,100, 101 and Cd, Fe(II) and Eu, as complexes with 1,10-phenanthroline.102 ± 106 2. Sorption of organic compounds The interest in the use of PUF as sorbents for preconcentration of organic compounds is in many respects due to a study 107 demonstrating that ether-based PUF collect quantitatively poly- chlorinated biphenyls (PCB) from large water samples. In the subsequent years, several dozens of publications have been devoted to sorption on PUF of polycyclic aromatic hydro- carbons (PAH), PCB, phenols and some other organic com- pounds from various water 108 ± 112, 125 ± 127, 137 ± 145, 147, 150 ± 174 and air 113 ± 124, 128 ± 136, 146, 148, 149 samples (Table 2).Table 2. Sorption of organic compounds by unloaded PUF. Sorbable compounds Type of compound Aromatic hydrocarbons Hydroxy-containing aromatic compounds Esters Insecticides Dyes Other compounds monocyclic aromatic compounds polycyclic aromatic compounds polychlorinated biphenyls phenols carboxylic acids alcohols esters of phthalic acid esters of acetic acid organochlorine organophosphorus acaricides azo dyes xanthene acridines sulfophthalein other anionic surfactants cationic surfactants alkylammonium salts bilirubin It was found in relation to aromatic compounds that the partition coefficients of substances between PUF and the aqueous phase are proportional to their partition coefficients for the extraction into diethyl ether.108 Similar results were obtained in a comparison of sorption of 59 organic dyes (triphenylmethane, xanthene, sulfophthalein) by polyurethane foams from water or aqueous ethanol with the extraction of the same compounds into diethyl ether or ethyl acetate.160 Sorption of dyes depends on the nature, the structure, and polarity of the dye.The pattern of the dependence of sorption of ionising organic compounds (phenols and carboxylic acids) on the solution pH attests that these compounds pass to PUF as neutral species.108, 137, 138, 141, 142 The efficiency of sorption of anionic 105, 171 and cationic surfactants 172 and 4-nitrophenylazo derivatives of phe- nols 161, 163, 164 increases if they are collected as ionic associates with large hydrophobic counter-ions.In this case, sorption is substantially dependent on the hydrophobicity of the cationic and anionic components of the associate, the size and charge of the ions they contain, the concentration and the nature of the counter- ion, the composition of the aqueous medium, and the chemical structure of the sorbent.25, 105, 167 ± 169 The capacity of PUF with respect to volatile organic com- pounds sorbed from air has been studied to elucidate the depend- ence of the volume before breakthrough on the key parameters of the process: flow rate of the mobile phase, temperature, sample volume and some other.114, 115, 119 ± 121, 129, 130 The relationship between the size of the PUF block, the volume of the air under analysis and the volume before breakthrough has been elucidated in relation to PCB.It was found that PUF blocks with thicknesses of 4 and 7 cm collect 90% of PCB from 500- and 900-m3 air specimens, respectively.130 It was noted 132 that a 15 cm-thick PUF block completely absorbs hexachlorobenzene, 3,30-dichloro- biphenyl and 2,40,5-trichlorobiphenyl impurities from air speci- mens with volumes of 250, 330, and 2880 m3, respectively. Pankov 119 demonstrated that the relationship between the spe- cific retention volume of a compound on PUF (vg, 293) and the vapour pressure (p0L;293) of the pure target compound is described by correlation equations: log vg, 293=71.195 log p0L;293 71.884 (r2=0.989, PAH) andlog vg, 293=71.059 log p0L;29371.764 (r2=0.950, PCB), where r is the correlation coefficient.S G Dmitrienko, Yu A Zolotov Ref. 2, 108 109 ± 125 107, 124, 126 ± 136 2, 108, 122, 137 ± 142 108 143, 144 145, 146 147 127 ± 129, 133, 148 ± 150 133, 151 ± 157 158, 159 160 ± 166 160, 167 160, 168 160, 169 160, 166 105, 170 ± 172 172, 173 174 175Polyurethane foams in chemical analysis: sorption of various substances and its analytical applications A number of studies have been devoted to the transport of phenols 141 and organic dyes 166 through thin polyurethane mem- branes. Using a polyurethane membrane, ethanol has been isolated from aqueous solutions.144 3. The mechanisms of sorption on unloaded polyurethane foams A critical analysis of the views on the mechanisms of sorption of metal complexes on PUF has been reported in a monograph 5 and in a review.10 The mechanisms discussed as the most probable ones include extraction, anion exchange, cation chelation, ligand exchange and ligand addition mechanisms.The views on the extraction mechanism, proposed by Bowen in his first study 2 and confirmed subsequently by other research- ers are underlain by the fact that the conditions of extraction of metal acido complexes into diethyl ether are similar to the conditions of sorption of these complexes on PUF. The relatively high sorption capacity of PUF calculated on the basis of sorption isotherms of Au(III),2 Fe(III),2, 38 Hg,73 Sb(V) 48 and Te 49 halide complexes and some ions (0.5 ± 1.5 mmol g71) with relatively low specific surface area (0.03 ± 0.08 m2 g71) is regarded as evidence supporting this mechanism.Experimental data concerning the transport of Ga,20, 36 Fe(III) 36, 37, 40 and Au(III) 34 halide com- plexes and uranyl nitrate 36, 86 through polyurethane membranes indicate that the membranes collect compounds not only due to adsorption but also due to absorption processes. A layer-by-layer microscopic study of polyurethane foam membranes after sorp- tion of coloured compounds, for example, iodine showed that they are uniformly distributed throughout the sorbent bulk rather than concentrated on the surface.2 In several studies devoted to sorption of Ga,20, 35 Fe(III),35, 38 Sb(III, V) 48 Sn(II, IV) 21 chloride complexes and Co and Fe(III) 56 thiocyanate complexes, PUF are characterised as solid polymeric analogues of oxygen-containing solvents.Extraction of metal acido complexes by polyurethane foams increases in the presence of alkali metal salts, i.e., a salting-out effect is observed, which is usually typical of liquid extraction. For example, when polyether-based PUF are used, the partition coefficient of Sn(IV) from 3 M HCl does not exceed 140 cm3 g71, whereas for a solution containing 10 M LiCl and 0.12 M HCl, this value is 5600 cm3 g71 (see Ref. 21). A similar increase in the partition coefficient was also observed for PUF based on polyesters; however, the values are much lower in this case, namely, 120 and 730 cm3 g71, respectively.Actually, the extraction mechanism proposed in early studies 20, 35 ± 37, 48 reflected the known ability of polymers to dissolve metal salts and organic compounds, which was later (1982) called solid-phase extraction,176 but did not disclose the factors that made this dissolution possible. Even the first investigations of the sorption properties of PUF showed that these sorbents possess low anion-exchange capacity. It has been suggested 2, 20 that these properties appear due to protonation of the electron-donating nitrogen and oxygen atoms incorporated in PUF macromolecules. Since the basicity con- stants of the groups present in the foamed polyurethane units are rather low, it has been considered that PUF can exhibit anion exchange properties only in highly acidic solutions.In 1982, a cation chelation mechanism was proposed,59 which increases provided a different interpretation of the anion exchange proper- ties in PUF. In a study of sorption of thiocyanate metal complexes and some organic anions (in particular, picrate) in the presence of various salts, the researchers concluded 59 that polyester-based PUF exhibit complexing properties with respect to alkali and alkaline earth metal cations, Ag, Tl(I), Pb, NH4, RNHá3 , and H3O+. When binding cations, neutral macromolecules of foamed polyurethane are converted into macrocations with variable charge density. To maintain neutrality, an anion should be bound (or exchanged) together with the cation.Sorption of metal acido complexes and organic anions by polyether-type polyurethane foams in the sequence 163 Li<Na<Cs<Rb<K<NH4<Ag, Tl(I) 55Ba<Hg<Pb. Since the same sequence was observed for the sorption of these substances by many crown ethers 177 and acyclic ethers,178, 179 it was suggested 59 that the selective binding of cations is ensured by the polyether segments of PUF, which are able to form a helix around the sorbed cation. To confirm the selectivity of cation binding, the sorption properties of PUF of various types were compared with the extraction capacity of macrocyclic ethers containing polymeric units similar to those found in foamed sorbents.59 It was shown that the cation-binding capacity of PUF based on poly(ethylene oxide) is higher than that of PUF based on poly(propylene oxide).The results of a study of sorbates by IR spectroscopy attest in favour of the cation chelation mechanism of sorption of organic compounds on PUF. This study showed the presence of strong interactions between the ether units of PUF and sorbed thiocyanate metal complexes.59 The presence of ester groups, which are conformationally more rigid, in the polymer chain of PUF has an adverse effect on the cation binding. This is confirmed experimentally by the facts that the sorption properties of polyester-based PUF are poorer than those of polyether-based PUF and that the pattern of influence of alkali metal halides on sorption changes.The sorption of anions by polyurethane foams based on esters varies according to Hofmeister lyotropic series: Li<Na<K<Rb<Cs. The cation chelation mechanism has found quite a few advocates. This mechanism is believed to be responsible for the sorption on PUF of Cd,55 Co,59, 61 Fe(III),55 Mo, W, Tc,77 Os,78 Pd,81 Pt,82 Ru,79 Zn,55 Zr and Hf 72 thiocyanate complexes from slightly acidic aqueous solutions and hexachloro platinum and iridium complexes from acetone and ethyl acetate.42 In the ligand exchange or addition mechanism, the polymer segments containing electron-donating nitrogen or oxygen atoms act as ligands able to coordinate metal ions. This `ligand addition' has been considered to interpret the sorption of yttrium hydroxy- quinolinate complexes 101 and iron thiocyanate complexes 39 on PUF.It was proved using IR spectroscopy that the role of the ligand is played by the urethane group (7CO7NH7). Sorption of organic compounds on PUF is explained in most cases in terms of the extraction and cation chelation mechanisms. According to the extraction mechanism, PUF are regarded as solid polymeric extractants which dissolve organic molecules in the polymer films that form the framework of these sorbents.108 A piece of evidence supporting the extraction mechanism is provided by the fact that the efficiency of extraction of organic compounds by polyurethane foams can be increased by introduc- ing inorganic salts in the aqueous solution ±PUF system.108 In the presence of alkali metal salts, sorption efficiency increases in the series: K<Na<Li.Double-charged cations influence the extraction efficiency to a markedly greater extent than single- charged ones, i.e., a salting-out effect typical of extraction is involved. The extraction mechanism has also been used to interpret sorption by polyurethane foams of PAH,125 phe- nols,108, 138, 141, 142 cationic dyes 160, 167, 168 and organophosphorus and organochlorine acaricides.158 Negatively charged forms of organic compounds such as anionic surfactants,172 sulfophthalein dyes 160, 169 and azo dyes 160, 161 are extracted by PUF by the cation chelation mecha- nism. This is indicated by the data on the influence of alkali metal and ammonium salts on sorption.Sorption of these compounds increases in the following order: Li<Na<Cs< Rb<K*NH4. In some publications, the important role of hydrophobic interactions and interactions giving hydrogen bonds in the sorp- tion of organic compounds is noted. A reliable correlation has been established between the main parameter of PAH sorption, the partition coefficient (logD), and the hydrophobicity parame- ter of their molecules [the Hansch parameter defined as the partition coefficient in the 1-octanol ± water system (log Pow)].125 Two-parameter correlation equations relating the partition coef- ficients of phenols to their hydrophobicity parameters and164 Me P NH2 Me OC P = NH medium pKa values have been proposed.142 The logarithms of the partition coefficients of cationic surfactant homologues were found to follow a linear dependence on the number of carbon atoms in the alkyl groups of these compounds.25, 172 Thus, various mechanisms of sorption of metal complexes and organic compounds by polyurethane foams have been proposed; each mechanism has its own adherents and has been confirmed by experimental data.A common drawback of the views in these mechanisms is that sorption is idealised. In most cases, consid- eration actually refers to special cases in which only one process takes place in the system, other processes being entirely sup- pressed. Meanwhile, real systems normally provide favourable conditions for the simultaneous occurrence of several processes.4. Chemisorption processes involving polyurethane foams Experimental data attesting to a previously unknown ability of the terminal toluidine groups of PUF to be involved in oxidation, diazotisation and azo coupling reactions have been reported.180 ± 182 When PUF samples are brought in contact with aqueous solutions of free active chlorine, sodium nitrite and 4-nitrophenyldiazonium tetrafluoroborate, chemisorption proc- esses take place to give intensively coloured products. The composition of the products was determined using diffuse reflec- tion spectroscopy, elemental chemical analysis and other techni- ques. The optimal conditions of heterogeneous chemical reactions with participation of PUF and the conjectural composition of the products are shown in Scheme 1.The introduction of the diazo group into PUF provides activation of these materials toward covalent attachment of func- tional groups to their surface.182 Diazotised PUF enter into azo coupling reactions with azo components such as 8-hydroxyquino- line (Scheme 1), 1- and 2-naphthols, phenol, aniline and other compounds to give intensely coloured products. Heterogeneous chemical reactions involving PUF proceed under relatively mild conditions (aqueous solutions, room temperature, atmospheric pressure), the coloured products being rapidly formed at relatively low concentrations of compounds present in the aqueous phase. Therefore, PUF were recommended as new polymeric reagents for determination of free active chlorine and nitrite ions.180, 181 IV.Sorption of elements on polyurethane foams loaded with organic reagents Polyurethane foams can collect and retain substantial amounts of inorganic and organic compounds. This provides the possibility of Me P 8-Ox, 0.2 M Na2CO3 NaNO2 , 0.5 ± 2 M HCl + N Cl7 N Me P + 4-O2NC6H4N2 BF¡4 , pH 7±9 N N NH2 Me P Cl2 , 0.05 ± 0.2 M H2SO4 NHCl O ; where is the ether group; 8-Ox is 8-hydroxyquinoline. n R O n N C O R O H m S G Dmitrienko, Yu A Zolotov Scheme 1 Me P OH N N N NO2 modifying PUF to improve their sorption properties, while preserving the required hydrodynamic characteristics of the material. This is done using oxygen- and nitrogen-containing extractants or chelating organic reagents.The general information about sorption of elements on PUF loaded with various oxygen- and nitrogen-containing extractants and chelating reagents is presented in Table 3. The major application of loaded PUF is preconcentration with subsequent determination of metal ions and organic compounds in waters. Two ways of immobilising reagents, differing in the method of fixing of compounds in the PUF matrix, have been described, namely, physical and chemical immobilisation. Physical immobi- lisation is used most often because chemical fixing of reagents, which is carried out during the polymer synthesis, is a more laborious procedure. Organic compounds can be introduced into the foaming material owing to the hydrophobic nature of the polymer matrix and its dissolving capacity with respect to various compounds.Immobilisation of liquid extractants is an exception- ally simple procedure; this is attained by keeping PUF pellets or small cubes in an appropriate liquid extractant or in its mixture with some organic solvent. Excess liquid is then removed and the PUF samples are dried between sheets of filter paper. Foams obtained in this way contain 50 mass% to 80 mass% of an organic compound. Immobilisation is performed for liquid extractants, for example, tributyl phosphate,3, 183, 184, 187 trioctyl- amine,196, 197 bis(2-ethylhexyl) hydrogen phosphate,192 and for solutions of these compounds: tributyl phosphate in benzene,185 trioctylphosphine oxide in diethyl ether, bis(2-ethylhexyl) hydro- gen phosphate in nitrobenzene 190, 191, 193 or in dichloroben- zene.194, 195 Immobilisation of solid chelating reagents on PUF is carried out using two procedures.According to one procedure, the reagent is dissolved in a non-volatile organic solvent (plasticiser) and physically immobilised on PUF as described above. The other method consists of keeping the polymer in a solution of the reagent in a highly volatile solvent followed by removal of the solvent. In both procedures, it is required that the reactant solubility be rather high. This requirement is readily fulfilled in the second procedure because there is a wide choice of volatile solvents; however, the range of solvents suitable for the first procedure is substantially narrower.The solvents play a dual role in this case: they should efficiently dissolve the reagents and should also act as plasticisers for the polymeric sorbent. Tributyl phosphate 57, 192, 213, 215 and a-dinonyl phthalate 57, 209, 225 are used most often as plasticisers.Polyurethane foams in chemical analysis: sorption of various substances and its analytical applications Table 3. Sorption of elements by polyurethane foams loaded with organic reagents. Organic reagent Tributyl phosphate Trioctylphosphine oxide Bis(2-ethylhexyl) hydrogen phosphate Trioctylamine Dibenzoylmethane Thenoyltrifluoroacetone Dimethylglyoxime 1-(2-Pyridylazo)-2-naphthol aAM are alkali metals.Immobilisation of various chelating reagents on PUF has been described, for example, dimethylglyoxime,204 ± 207 1-(2-pyrid- ylazo)-2-naphthol,14, 57, 208 ± 210 dithizone,211 ± 219 ammonium pyr- rolidinedithiocarbamate,220, 221, 223 hexamethyleneammonium hexamethylenedithiocarbamate,224 and sodium,222 diethylammo- nium 225 and potassium 228 diethyldithiocarbamates. It was shown using numerous examples that the conditions of formation of compounds with PUF-immobilised reagents do not differ from the conditions of reactions taking place in aqueous solutions. Parameters such as the strength of retention and the uniform- ity of distribution of the organic reagent in a polymer matrix are little studied. It was only found that on contact with large amounts of an aqueous phase (up to 1 litre), not more than 15% of the reagent is desorbed, whereas on treatment with acetone, ethanol and other organic solvents both the reagent and the resulting complex are removed from PUF almost completely.217 It was shown in a study of operational characteristics of PUF loaded with 1-(2-pyridylazo)-2-naphthol,210 that an increase in the reagent content leads to a poorer retention of the reagent on contact with water.The optimal amount of the reagent is 1 mass %. As in the case of unloaded PUF, isolation and preconcentra- tion of compounds on loaded PUF are carried out in either a static or dynamic regime. In the latter case, the flow rate of the analysed solution through the column can vary over a broad range, from 0.1 to 250 ml min71.The scope of the dynamic preconcentration technique can be extended by using plasticised PUF,210, 225 as this allows quantitative collection of the analyte for flow rates exceed- ing 100 ml min71. Nevertheless, the solution flow rates used most often are 0.1 ± 10 ml min71. Physical immobilisation of organic reagents is usually carried out for polyether-type PUF. No results of comparative studies of the properties of different PUF have been found in the literature; only in one study,209 was it noted that the use of polyether-type PUF allows a 4-fold increase in the solution flow rate with respect to that for the polyester foam in the collection of Co, Mg and Fe using the polymer loaded with 1-(2-pyridylazo)-2-naphthol.Organic reagent Ref. Element dithizone diethyldithiocarbamates diphenylcarbazide benzoylhydrazone phenylhydrazine dibenzo-18-crown-6 14, 183, 184 4, 57 185 186 4162, 187 188, 189 190, 191 192 193 ± 195 196 197, 198 199, 200 201 ± 203 204 ± 206 207 14 57, 208 57, 208 ± 210 57, 209 14, 57, 73, 208 14, 73 57, 73, 208 Au Bi Cr Nb Pd UUCe Th UCo UUCo, Fe Ni Pd Au Cd Co Fe, Mn Hg In Zn V. Classification of sorption processes involving polyurethane foams A classification of sorption systems involving PUF has been proposed,25 based on the views of various types of sorbent ± sor- bate intermolecular interactions developed by Kiselev.237 Accord- ing to this classification, three types of sorption systems with participation of PUF can be distinguished, depending on the type of intermolecular interactions between the sorbent and the sorbate (Table 4).The first type includes unloaded PUF in the polymer chains of which the active centres are represented by hydrophobic hydro- carbon chains and aromatic groups and electron-donating nitro- gen and oxygen atoms contained in urethane, amide, ester, ether or terminal toluidine groups. The second type comprises PUF loaded with alkali metal ions and mineral acids. This modification becomes possible due to protonation of nitrogen atoms incorporated in polar groups (first of all, terminal toluidine groups), which takes place in acidic solutions, due to introduction of cations capable of complexation with polyether (polyester) units in the polymer macromolecule.Upon binding alkali metal or ammonium cations or some other ions, polyurethane foams are converted into macrocations with variable charge density, which depends on the nature of the cation involved, salt concentration, and the nature of the PUF unit. Since this binding is less typical of PUF based on esters, it is possible to increase the selectivity of sorption on passing from polyether foams to those based on polyesters. The properties of the initial polymers would differ from the properties of PUF loaded in this way; thus, the latter can be used to recover negatively charged species. Finally, PUF, loaded with various organic reagents upon immobilisation of the reagent without covalent bonding with the sorbent belong to the third type.In the majority of cases, the main role in binding the sorbates on unloaded polyurethane foams is played by hydrophobic interactions and hydrogen bonds. Hydrophobic interactions play Element Ag Bi Cd, Cu Hg Pb Zn As, Bi, Hg, Pb, Sb, Se, Sn Cd, Co, Cr, Cu, Fe, Mn, Ni Hg Sb REE Cr Cu, Pd Mo Pb AMa 165 Ref. 211 212, 213 214 215 ± 217 218, 219 212, 218 220 ± 222 222 ± 224 225, 226 227 228, 229 230 ± 232 233 234 235 236166 Table 4. Classification of sorption systems with PUF according to the main types of sorbent ± sorbate intermolecular interactions. Type of sorption system Unloaded PUF PUF loaded with alkali metal ions and mineral acids PUF loaded non-covalently with organic reagents a crucial role in the collection of PAH, associates containing simultaneously large hydrophobic cations and anions and asso- ciates containing hydrophobic cations.Compounds of the two first-mentioned groups are sorbed quantitatively on unloaded PUF (95% ± 99.9%); this can be used to develop procedures for preconcentration followed by spectroscopic determination of these coloured (luminescing) compounds. The efficiency of sorption of compounds the molecules of which contain polar groups together with hydrophobic fragments is determined by a number of factors, which was demonstrated convincingly in relation to phenols.142 Of the group of compounds under consideration, either molecules incorporating hydrophobic fragments with Hansch parameters (logPow in the octanol ± water system) greater than 3 (e.g., 1-naphthol), or relatively strong acids (e.g., 2,4,6-trinitrophenol) can be of interest from the practical standpoint.The main types of intermolecular interactions involved in the binding of strong acids and alkali metal salts include hydrogen bonding and donor ± acceptor and ion ± dipole interactions. In the case of chemical interactions taking place due to high reactivity of the terminal toluidine groups, chemical individuality of the sorbed molecules is lost upon the formation of surface compounds and, thus, sorption becomes irreversible, unlike the cases described above.Ion ± ion interactions make a crucial contribution to the sorption on PUF loaded with alkali metal ions and mineral acids of compounds such as thiocyanate metal complexes, heteropolya- cids, single- and double-charged anions of sulfophthalein dyes, anionic surfactants and 4-nitrophenylazo derivatives of phenols. Binding of negatively charged species to PUF, which has an electrostatic nature, can be enhanced by hydrophobic interactions of anions containing hydrophobic groups. Usually, the com- pounds listed above are quantitatively sorbed on PUF; most of them, are intensively coloured, which accounts for their high practical significance. In sorption systems of the third type, PUF function as hydro- phobic polymeric matrices that fix organic compounds, the key interactions being due to the formation of chemical bonds between metal ions and functional groups of the reagent.VI. Analytical application of polyurethane foams Polyurethane foams are of substantial interest for analytical chemistry. These sorbents are characterised by high efficiency combined with sorption capacity with respect to many classes of sorbates, chemical and mechanical strength and stability against organic solvents. The advantages of PUF include the presence of a membrane structure, which ensures good hydro- and air-fluid dynamics of the sorbents and allows preconcentration of trace Main types of intermolecular interactions hydrophobic hydrophobic +H-bonding H-bonding +donor ± acceptor ion ± dipole ion ± ion ion ± ion+donor ± acceptor ion ± ion+hydrophobic chemical bond between the metal and the functional group of the organic reagent elements and organic compounds from large samples of water and air.These sorbents are relatively inexpensive and readily avail- able. Sorption on PUF is used for separation, preconcentration and determination of trace elements and organic compounds from water and air. Determination of elements and organic compounds after preconcentration or separation can be carried out by a variety of methods. Most often, preconcentration by sorption is combined with photometry, diffuse reflection spectroscopy, atomic-absorption spectroscopy and chromatographic techni- ques.Determination can be done either directly in the sorbent or in a solution after desorption of the target components using an appropriate eluent. In some cases, determination is carried out after decomposition of the sorbent on treatment with mineral acids, for example, HNO3 184 or its mixture with HCl.44, 54, 192 1. Separation and preconcentration The success of preconcentration and separation by sorption on PUF depends on a rational choice of the sorption system, the composition of the aqueous phase and on the procedure used for separation and preconcentration. The practice of preconcentra- tion by sorption on PUF has some specific features differing from those used with other sorbents. Preconcentration and separation of elements and organic compounds on PUF can be performed in either a static or dynamic regime.Static sorption can be performed in two ways. The traditional procedure is based on shaking (mixing) of the solution to be analysed with a known amount of the sorbent; air bubbles should be necessarily removed from the sorbent. Usually, PUF are used as cubes, cylinders, cylindrical discs or pellets cut out or knocked out of a polymer sheet. According to the second procedure, PUF blocks or discs situated in the solution under analysis are squeezed at some intervals either manually, for example, by the bottom of a glass measuring cylinder, or using a special automatic device.} In the dynamic version of sorption, the solution under analysis is passed through a chromatographic column packed with PUF cubes or cylinders.Various methods for packing the columns have been developed; a vacuum procedure is used most often. In yet another specific method of preconcentration on PUF, called the pulsating column technique, the solution to be analysed is pumped through a sorbent layer located in a glass or plastic medical syringe. Since the sorbent is flexible, the cylindrical polyurethane foam plug can be easily compressed (or released) when the syringe } The automatic device consists of an eccentric shaft operated by an engine, which makes one or several glass or plastic pistons move up and down and thus forces the solution through the PUF sample placed under the piston. S G Dmitrienko, Yu A Zolotov Groups of compounds sorbed polycyclic aromatic hydrocarbons ionic associates containing large hydrophobic cations ionic associates containing large hydrophobic cations and anions ionic surfactants phenols heteropolyacids alkali metal and ammonium salts metal acido complexes heterolyacids sulfophthalein dyes anionic surfactants 4-nitrophenylazo-derivatives of phenols metal complexes with organic reagentsPolyurethane foams in chemical analysis: sorption of various substances and its analytical applications piston moves.The pulsating column technique is very convenient regarding process automation in the sorption ± desorption regime. More detailed information about different procedures of sorption on PUF from aqueous solutions can be found in the monograph 5 and in reviews.9, 10 Various types of samplers used for the collection of organic compounds from air by sorption are described in the same publications.Numerous studies have been devoted to PUF for the separa- tion and preconcentration of metals, mainly, as acido complexes. Some examples are presented below. The separation of the Ta ±Nb (from 1 M HF), Mo± Re, Mo±Tc (from 0.1 M HF+1 M NH4F) pairs on 4.560.9 cm columns packed by polyether-based PUF (0.27 g of the sorbent) has been described.28 It was shown that Ta, Re and Tc are desorbed quantitatively from the column on treatment with 1 M HNO3.28 Separation of Ga and Fe in the presence of large amounts of aluminium (1 : 1000) is attained by sorption from 6 ± 7 M solutions of HCl by polyether-based polyurethane foams.35 Procedures for the separation of binary and ternary mixtures (W ± La, W± Al, La ± Ga, La ±W, Ga ± Al, Al ±W±Ga and Mo± La ± Al)71 under dynamic conditions using columns packed with 1 g of PUF at a flow rate of 1 ± 5 ml min71 have been developed. The separation is based on different sorbability of elements from thiocyanate solutions, which increases in the series Al<Ga<La< Mo<W.The Mo and La ions are desorbed from the column with acetone, while Al, Ga and W, by 1 M HCl, 2 M NH3 and an HCl ± acetone mixture, respectively. The difference between the rates of sorption of elements on PUF underlies the procedure of separation of Te and Se.49 Tellurium is sorbed quantitatively (> 99%) over a period of 2 min from a solution containing 1 M HCl and 5 M NaBr, while quantitative sorption of selenium requires that the contact time be increased to 2 h.6 6 Procedures for separation of Os from Ru,78 Pd from Pt,82 Rh from Ir 76 and Ru from Rh79 based on the difference between the rates of formation of thiocyanate metal complexes have been developed. It was found that palladium can be collected at room temperature from a solution (0.002 ± 0.006 M KNCS and 2 M KCl) in the presence of a large amount of platinum (1 : 20).82 To separate Ru from Os or Rh, a 0.3 ± 0.4 M solution of KNCS containing 2 M NaCl is heated for 5 min at 90 8C and the Ru(NCS)3¡ complex is sorbed by polyether-based poly- urethane foams.The degree of recovery of Ru is 95%, while Os and Rh are sorbed under these conditions to not more than 5%.78, 79 For separation of Rh and Ir (0.2 M KNCS+0.2 M LiCl), the solution is heated for 30 min at 90 8C; as this takes place, rhodium is transferred quantitatively in the Rh(NCS)3¡ complex. The complex is sorbed by PUF to 941% and iridium, to not more than 5%. The procedure allows separation of rhodium in the presence of a large amount of iridium (1 : 5).76 The preconcentration of cobalt and iron as thiocyanate complexes from large (up to 1 litre) samples on a PUF-packed column at a flow rate of 30 ml min71 has been described.56 The recovery of metals present in a concentration of 1 mg litre71 was 96%. An efficient method for preconcentration of Co, Fe and Zn on 562 cmcolumns packed with 1.5 g of PUF (u=5 ml min71) has been proposed.57 It was shown 73 that Zn, Hg and In are sorbed quantitatively on a PUF column from a 0.2 M solution of KNCS at pH 2 ± 3 and at flow rates of 25 ± 50, 10 ± 25 and 20 ± 50 ml min71, respectively.Zinc is desorbed using 1 M HNO3, and mercury and indium, by 1 M KNCS in 80% acetone. Molybdenum is recovered from sea water using sorption of its thiocyanate complexes on PUF with subsequent desorption with acetone.77 A number of studies have been devoted to the use of PUF for the preconcentration of PCB,107, 126 ± 128, 153 PAH,109, 110, 125 phe- nols,139, 140, 165 organophosphorus and chlorinated acari- cides 158, 159 and organophosphorus insecticides from water.It was noted that PUF offer a number of advantages over other polymeric sorbents and carbons for preconcentration of PAH, 167 PCB, and chlorine- and phosphorus-containing insecticides present in water.118, 127, 132, 135, 136 Polyurethane foam sorbents provide highly efficient recovery and allow a very high sampling rate (up to 800 litres min71). 2. Photometry Two variants of using PUF in photometric analysis can be distinguished. The simplest, although not the most popular method is based on the sorption of elements on PUF as coloured compounds, desorption by acetone and spectrophotometric deter- mination in the eluate. A classical example of using this method is determination of Co, Fe, Mo and Cu,66 Ti,83 Co,238 Fe 239 as intensely coloured thiocyanate complexes after desorption from PUF on treatment with acetone.This technique was used to determine iron in glass and ceramic materials.103 The method includes the preparation of Fe(II) phenanthrolinate, its sorption on PUF as an ionic associate with perchlorate ions, desorption by acetone and spectrophotometric determination at 510 nm. The range of iron concentrations determined in this way is 0.05 ± 3 mg ml71. The method for cadmium determination in industrial zinc oxide specimens is based on the sorption of the [Cd(Phen)2]2+ associate with dithizone as the counter-ion.102 A method for platinum determination in glass with a limit of detection of 0.1 mg ml71 has been developed.99 This includes sorption of the complex with dithizone on PUF, desorption by HCl-acidified acetone and spectrophotometric determination at 530 nm. In some cases, a different, more suitable reagent is added to the eluate to increase the sensitivity and selectivity of the spec- trophotometric determination. A method for determination of cobalt in nickel salts and in steels is based on the collection of cobalt on PUF from thiocyanate solutions, desorption with 80% ethanol and the subsequent determination in the eluate using 2-(2-benzothiazolylazo)-2-cresol.240 The method allows determi- nation of cobalt in the presence of large amounts of Ni, Mo and Fe.A photometric method was described for arsenic determina- tion in glass; the method includes preparation of the oxidised form of molybdoarsenic acid, its sorption on PUF, desorption with 0.1 M HNO3 and determination in the eluate as the reduced form after the addition of SnCl2 .89 A reported 84 method for determi- nation of zinc in cadmium salts is based on sorption of zinc thiocyanate complexes on PUF, desorption on treatment with water and subsequent determination using 4-(2-pyridylazo)resor- cinol.The same principle underlies determination of cobalt in natural waters.210 The procedure includes preconcentration of cobalt from a 10-litre sample by sorption under dynamic con- ditions on a column with PUF loaded with 1-(2-pyridylazo)resor- cinol. The second variant is associated with the techniques of thin layer spectrophotometry, which rationally combines the precon- centration of elements on PUF with direct determination of the absorption of light by the solid phase.53, 62, 94, 185, 214, 216, 230 The essence of the method is in the sorption of coloured compounds by a PUF specimen shaped like a thin parallelepiped with dimensions corresponding to those of the cell (26160.1 cm), transfer of the sorbent into a cell filled with an organic solvent and measurement of the optical density. The solvent serves as an immersion liquid reducing light scattering. Methods have been reported for deter- mination of bismuth as an iodide complex;53 cobalt as the thiocyanate;62 phosphate ions as the reduced form of the molyb- dophosphoric heteropolyacid;94 zinc and bismuth,212 divalent mercury chloride and phenylmercury chloride 216 as dithizonates; and chromium as blue peroxochromic acid and a complex with diphenylcarbazide.185, 230 However, these methods are character- ised by relatively narrow ranges of linearity of calibrating plots and relatively low reproducibility of the results.Good prospects for the use of PUF in a flow-injection system were first demonstrated in relation to photometric determination of zinc in specimens of biological origin.85 In this system, zinc is collected in the `on-line' mode as the thiocyanate complex on a168 microcolumn with PUF (0.363 cm, 100 mg of the sorbent), eluted with water and determined with 4-(2-pyridylazo)resorcinol. A flow-injection system has been used 241 for the photometric determination of aluminium with methyl thymol blue after separation of the interfering ions as thiocyanate complexes by sorption on a microcolumn with PUF (0.367 cm, 200 mg of the sorbent).The calibration plot is linear in the 0.25 ¡¾ 2.0 mg ml71 range, the productivity of the method is 17 samples per hour. Determination of 1 mg ml71 of aluminium can be carried out in the presence of large amounts of iron (1 : 170), zinc (1 : 100), copper and cobalt (1 : 50). This method has been used to determine aluminium in silicates and ores. The same procedure is employed to separate Fe, Cu, Zn and Co in a flow-injection method of determination of nickel in brasses, bronzes and silicates.242 A method for determination of carbaryl with a limit of detection of 12 mg ml71 after collection on a PUF microcolumn and desorp- tion by dichloromethane has been proposed; on-line spectropho- tometric detection was carried out at 280 nm.243 3.Diffuse reflection spectroscopy The combination of preconcentration on PUF and subsequent determination of the sorbed substances by diffuse reflection spectroscopy is a promising expedient for analytical chemistry. The possibility of this combination was first discussed by Dmi- trienko and coworkers.63, 64 It was shown 25 that PUF offer a number of advantages over other sorbents regarding their use in sorption ¡¾ photometric and sorption ¡¾ luminescence procedures and in test methods. First of all, this facilitates the final stage of analysis because the sorbate can be easily separated from other components present in the sorption system, which looks like a coloured tight pellet; owing to the uniform distribution of the analyte over the sorbent matrix, PUF pellets can be used without any additional operations.In diffuse reflection spectroscopy, the Kubelka ¡¾ Munk func- tion (F) is related to the analytical signal, diffuse reflection (R), and the sorbate concentration in the following way: F a O1 ¢§ RU2 a 2:3 e c s¢§1, 2R where e=n6104 is the molar extinction coefficient of the sorbate (n=1 ¡¾ 10), c is the sorbate concentration, s is the light scattering coefficient. Methodical aspects of the quantitative measurements of the diffuse reflection of sorbates on PUF are considered in a paper.244 The main factors influencing the accuracy and reprodu- cibility of diffuse reflection measurements, namely, sample layer thickness, sample humidity, the brand of the PUF used and the sorbate concentration, have been studied.For all the compounds studied, the Kubelka ¡¾ Munk function was found to be linearly related to the sorbate concentration not only in the PUF matrix but also in aqueous solution within the limits of the linear sections of sorption isotherms. The limits of detection of intensely col- oured (e ^ n6104) compounds in the PUF matrix reach n61072 mg in a 0.04 ¡¾ 0.07 g PUF pellet or n61074 mg ml71 for a sample volume of 25 ¡¾ 100 ml. Various methods are used to obtain a coloured compound in the sorbent phase. One method is based on the sorption on PUF of the coloured compounds formed in the solution under analysis upon the reaction between a specially added analytical reagent and the target component. Numerous publications have been devoted to determination of metal ions and organic compounds by this method.For example, in a sorption ¡¾ photometric deter- mination of cobalt 63 ¡¾ 65 and iron,63, 64, 69 they are transferred into coloured thiocyanate complexes and sorbed on PUF. The limits of detection are n61073 mg ml71. To increase the selectivity of analysis, sorption is carried out in the presence of various masking compounds.Asimilar method was used to determine silicon as the oxidised and reduced forms of molybdosilicic acid with limits of detection of 361073 and 161072 mg ml71, respectively. A sorption ¡¾ photometric procedure for determination of ionic sur- S G Dmitrienko, Yu A Zolotov factants using PUF is based on sorption of the associates formed by surfactants and coloured hydrophobic counter-ions.Sulfoph- thalein dyes are used as the counter-ions in the determination of cationic surfactants,173 while tris(1,10-phenanthroline)iron(II) was used for anionic surfactants.105, 171 This procedure makes it possible to determine ionic surfactants present at a level of n61072 mg ml71. A sorption and photometric method for deter- mination of phenols and 1-naphthol is underlain by sorption of their coloured azo derivatives.161, 163 ¡¾ 165 Azo derivatives of phe- nols were synthesised by azo coupling with 4-nitrophenyldiazo- nium tetrafluoroborate.To enhance the recovery, sorption was carried out in the presence of cetyltrimethylammonium bromide. This procedure provides determination of phenols with limits of detection of n61073 mg ml71. The second method is related to the use of loaded PUF. In this case, the coloured compound is formed directly in the sorbent phase due to complexation or redox reactions between the target component present in an aqueous solution and an analytical reagent immobilised on PUF. Polyurethane foam loaded with dimethylglyoxime was proposed for sorption ¡¾ photometric deter- mination of nickel,64, 206 and diphenylcarbazide-loaded PUF was proposed for chromium(VI) determination.64, 231, 232 A sorption ¡¾ photometric determination of ascorbic acid is based on reduction of heteropolyacids immobilised on PUF.245 The procedure allows determination of 0.3 to 2.4 mg ml71 of ascorbic acid.The time of analysis is 75 min. When the reduction of heteropolyacid is carried out on exposure to microwave radiation, the duration of the analysis decreases to 6.5 min.246 The third method for preparation of coloured analytical forms in the PUF phase is based on chemisorption processes involving the terminal groups of these polymers. In this case, a higher selectivity is to be expected. This technique was used, for example, to determine nitrite ions 180 and free active chlorine 181 in water. 4. Luminescence A number of studies have been devoted to the use of PUF in luminescence analysis.The key factors influencing the intensity of fluorescence of sorbed molecules such as the thickness of the sample layer, sample humidity, the structure of the polymeric framework of PUF and the sorbate content have been studied in relation to the [Ru(Phen)3]Cl2 complex, pyrene, rhodamine 3B, rhodamine 6G and butylrhodamine sorbed on PUF.244, 247 It was found that the polymer framework of PUF has virtually no influence on the fluorescence emission and fluorescence excitation spectra of the sorbates; the spectra of sorbates on different PUF normally do not differ from one another and in most cases, they are similar to the spectra of these substances in aqueous solutions. The limits of detection of compounds with intense fluorescence in the PUF matrix are n61073 mg (the weight of PUF ^ 0.04 ¡¾ 0.07 g) or n61075 mg ml71 for a sample volume of 25 ¡¾ 100 ml.The range of linearity of calibrating plots coincides, as a rule, with the initial linear sections of sorption isotherms and holds up to a change in the sorbate concentration by one to two orders of magnitude. In most cases, compounds that luminesce on the sorbent surface (for example, metal complexes with organic ligands) are obtained by sorption from the solution in which they have been synthesised. A sorption ¡¾ luminescence method for yttrium deter- mination includes the formation of a complex with 8-hydroxyqui- noline, sorption of the complex on PUF and measurement of the luminescence intensity at 510 ¡¾ 513 nm (lexcit =398 nm).The method was applied for determination of yttrium in lanthanum 100 and scandium 101 oxides. In a highly sensitive sorption ¡¾ lumines- cence method of selenium determination in soil, the target element is sorbed as 4,5-benzodiazoselenol, which is prepared by the reaction with 2,3-diaminonaphthalene.248 In a method for deter- mination of Eu in scandium oxide, the mixed-ligand complex of this element with thenoyltrifluoroacetone and 1,10-phenanthro- line is sorbed by polyurethane foam and then Eu is determined by luminescence directly in the sorbent, the limit of detection beingPolyurethane foams in chemical analysis: sorption of various substances and its analytical applications 161076 %.106 Thallium is sorbed on PUF as the chloride com- plex, then the sorbate is treated with rivanol, a diamino derivative of acridine, and the fluorescence intensity is measured at 497 nm (l excit=367 nm).51 Only gallium is sorbed together with thal- lium.The analysis of these elements is not interfered with by the presence of large amounts (1 : 1000) of Cu, Co, Ni, Cd, Zn, Hg, In and La and alkali and alkaline earth metals; the limit of detection is 0.5 mg litre71. A method for determination of Cr(III) based on the quenching of fluorescence of rhodamine 6G sorbed on PUF has been described.249 The limit of detection equals 8 ng ml71 for a sample volume of 25 ml. A scheme of analysis has been proposed to perform fast screening of water samples for the content of PAH.According to this procedure, PAH are collected on PUF and their total amount is determined by the sorption-luminescence method.125 Individual PAH are determined in the eluate after desorption of the sorbed impurities by acetonitrile using high-performance liquid chroma- tography (HPLC) with a fluorescence detector.25 5. Atomic-absorption and atomic-emission spectroscopy The preconcentration of elements by sorption on PUF is often combined with determination in eluates by atomic-absorption spectroscopy (AAS) and atomic-emission spectroscopy with inductively coupled plasma (ICP AES). The flame AAS method was applied to determine platinum 44 and thallium 50 after pre- concentration of the elements as halide complexes on PUF followed by desorption on treatment with acetone.Combination of group preconcentration of elements by sorp- tion on PUF loaded with ammonium pyrrolidinedithiocarbamate (PUF ± APDTC),220, 221, 223 a mixture of hexamethyleneammo- nium hexamethylenedithiocarbamate with methyltrioctylammo- nium chloride (MTOA) 224 or on PUF loaded with sodium diethyldithiocarbamate 222 followed by determination of elements by non-flame AAS or ICP AES proved successful. A procedure for determination of As, Bi, Sb, Hg, Sn and Se in natural waters 220 includes collection by sorption on columns with PUF± APDTC, elution of the element pyrrolidinedithiocarbamates with metha- nol, generation of hydrides directly in a methanol solution by the reaction with NaBH4 and subsequent determination by ICP AES.The limits of detection of elements are, ng ml71: 0.2 (As), 30 (Bi), 0.03 (Se, Sb and Sn). In determination of Cd, Co, Cu, Fe, Hg, Ni and Pb in high- purity oxalic acid,224 trace elements are concentrated under dynamic conditions on columns with PUF±APDTC or PUF±APDTC±MTOA; the degree of recovery of elements is higher than 97%. The components are desorbed by methyl isobutyl ketone (for AAS determination) or 4 M HNO3 (for ICP AES determination). The limits of detection are 0.1 (Cd), 0.3 (Pb), 1 (Ni), 2 (Co and Cu) ng g71 and 0.1 (Hg) mg g71 for non-flame AAS and 0.05 (Cd), 0.1 (Cu), 0.2 (Ni) and 1 (Pb) mg g71 for ICP AES. The column version of the solid-phase extraction using PUF loaded with APDTC was used 221 to collect As, Bi, Hg, Sb, Se and Sn impurities in waters of various types.The degree of recovery of elements on passing of 150 ml of a sample (pH*4.5) at a flow rate of 2 ml min71 through a column was 97%± 100%. After elution with methanol, the concentrate was analysed by ICP AES; the limits of detection were, ng ml71: 3 (As, Se), 8 (Bi), 0.12 (Hg), 2 (Sb) and 6 (Sn). It should be noted for comparison that the use of non-flame AAS after desorption of the elements with methyl isobutyl ketone decreased the limits of detection of elements down to 0.06 (As and Sb), 0.1 (Bi and Sn), 0.08 (Se) and 0.3 (Hg) ng ml71. A method for lead determination in sea food has been proposed;250 lead is collected on a microcolumn packed with PUF loaded with 2-(2-benzothiazolylazo)-2-cresol, desorbed with 0.1 M HCl and determined by flame AAS.The limit of detection of lead by this method is 1 mg l71. 169 A flow-injection system has been used 251 for atomic-absorp- tion determination of zinc in waters and biological materials after preconcentration by sorption on a microcolumn packed with PUF. 6. X-Ray fluorescence spectroscopy 4 4 4 X-Ray fluorescence spectroscopy has been used to determine Ga,41 Co,58, 60 Fe, Pd, Pt and Zn,60 As,88 Ge,90 PO3¡ (see Refs 92, 93) methyl- and phenylmercury chloride, inorganic mer- cury 226 and U.252 Indirect determination of phosphorus on the basis of molybdenum or antimony after preconcentration on PUF as phosphomolybdate 92 or molybdoantimonylphosphoric acid 93 has fairly low limits of detection (0.1 and 0.02 mg ml71, respec- tively).The latter procedure is more selective: large amounts of silicon do not interfere with the determination of phosphorus. The relative standard deviation does not exceed 0.05. In an X-ray fluorescence determination of arsenic 88 and germanium,90 they are transformed into arsenomolybdate and molybdogermanate, which are sorbed on PUF. The range of concentrations deter- mined in this way is fairly broad: 0.5 ± 3.5 and 0.25 ± 35 mg ml71, respectively. Arsenic can be determined in the presence of large amounts of PO3¡ and SiO2¡ (1 : 10) and an equal amount of germanium. The limits of detection are 40 and 70 ng ml71 and the relative standard deviations do not exceed 0.07 and 0.09 for arsenic and germanium, respectively.Determination of Co, Zn, Pd, Pt, Fe is carried out using sorption of their thiocyanates from acid solutions in the presence of NH4Cl.60 The ranges of linearity of the calibrating plots are rather narrow: 1 ± 5 (Co, Zn, Fe) and 1 ± 10 mg ml71 (Pd, Pt), the corresponding relative standard deviation being not more than 0.05. In sorption ± X-ray fluorescence determination of chloride, methyl- and phenylmercury chloride, PUF loaded with sodium diethyldithiocarbamate is used.226 7. Neutron activation method Direct determination of sorbed elements by neutron activation analysis (NAA) was shown to be, in principle, possible in a number of studies.14, 31, 227 The NAA method with a neutron flux density of 361013 neutron cm72 s71 (irradiation for 10 min) allows determination of Au, In, Hg and Zn in the polyurethane foam matrix after collection of these elements from thiocyanate media.14 It was noted that irradiation of PUF for more than an hour results in polymer destruction; irradition in H2SO4 leads to complete dissolution. Determination of gold in natural waters after sorption from 6 M HCl on ether-based polyurethane foams has been described.31 Neutron-activation determination of anti- mony is preceded by its isolation using PUF loaded with potas- sium diethyldithiocarbamate.227 8.Enzymic methods In enzymic methods of analysis, PUF are used as polymeric supports for immobilised enzymes. A method for the immobilisa- tion of cholinesterase comprising capture of an enzyme into a starch gel followed by impregnation of the gel into the PUF matrix has been proposed.253 An analytical system in which an enzymic minireactor is combined with an electrochemical sensor within one unit has been developed 254 to determine toxic substances of the anticholinesterase action (for example, insecticides) in air and in liquid media.As the air or solution sample under analysis is pumped through the minireactor, the inhibitor is collected on the immobilised enzyme and inhibits it. The combination of precon- centration and analytical reaction in one unit with the sensor increases the method sensitivity and shortens the response time. The limit of detection of organophosphorus insecticides by such a system is 0.1 ± 4.5 ng ml71, that for carbamates is 0.5 ± 20 ng ml71.A method for immobilisation of horse-radish peroxi- dase 255, 256 and alkaline phosphatase 257 in a chitosan film fol- lowed by impregnation into PUF has been developed. This procedure was used to develop highly sensitive test methods for170 determination of mercury 255 and organomercury compounds.256 The oxidation of o-dianisidine in the presence of thiourea by hydrogen peroxide is used as the indicator reaction. This proce- dure provides mercury(II) determination at a level of 1 ng ml71; the lower limits of determination of methyl-, ethyl- and phenyl- mercury are 0.008, 0.01 and 25 mmol litre71. A test procedure for determination of 0.05 ± 10 000 ng ml71 of lead has been described.257 In the development of this proce- dure, the inhibitory action of lead on the activity of PUF- immobilised alkaline phosphatase in the hydrolysis of p-nitro- phenyl phosphate was utilised.It was noted that the key advan- tages of PUF over polystyrene and chromatographic paper used previously for enzyme immobilisation include a long (1 ± 1.5 years) period of maintenance of enzyme activity; the researchers also noted good metrological characteristics of the developed procedures. 9. Chromatographic methods PAH,110 ± 113, 117, 122, 124 The most important applications of PUF in chromatographic methods of analysis include preconcentration and isolation from waters and air of PCB 107, 121, 124, 131, 133, 153 and chlorine- and phosphorus-con- taining insecticides.127, 128, 133, 134, 152 After collection on PUF, organic compounds are determined using gas chromatography (GC) and HPLC in combination with various types of detector: flame ionisation (FID), electron capture (ECD), mass-spectro- metric (GC/MS technique), N/P-selective and UV and fluores- cence detectors.The existing chromatographic methods for the determination of trace amounts of organic compounds consist of several stages: collection of the trace elements on PUF, desorption, evaporation of the solvent and analysis of the concentrate. Various solvents have been proposed for desorption of com- pounds from PUF: acetone,11, 107, 110, 117, 152 hexane,107, 133, 134, 153 methanol,117 an acetone ± hexane mixture,133 diethyl ether and petroleum ether.121 Solvent desorption is carried out in the Sohxlet apparatus for 18 ± 24 h.107, 113, 115, 116, 118, 130, 132 ± 134, 149 In some cases, samples are treated with ultrasound to intensify the process.122 Supercritical fluid extraction (SFE), which makes it possible to shorten the desorption time to 10 ± 20 min,124, 156 and a method based on extraction of impurities with a small amount of an organic solvent after compression of PUF to 10% of its initial volume in a special column can provide alternatives to the prolonged and cumbersome procedure of Sohxlet desorption.123 In has been shown 123 that 4 ml of dichloroethane or 6 ml of a 10% solution of acetone in hexane is sufficient for quantitative elution of eighteen PAH from a PUF sample with an initial volume of 115 cm3.The preconcentration of organic compounds from waters is carried out in a dynamic regime; the flow rate reaches 270 ml min71. Traces of PAH are collected from water samples having large volumes (4 ± 60 litres) using columns filled with one or several cylindrical inserts of PUF (45645 mm).109, 110 A water sample heated to 622 8C is passed through a column at a flow rate of 250 10 ml min71. It was noted that pre-heating decreases the loss of PAH caused by co-precipitation on the suspended particles present in water samples.109 For most of PAH including 3,4-benzopyrene, the degree of recovery amounts to 90%± 98% for the model samples prepared from distilled water and 65%± 85% for real potable or river water samples.109, 110 It was demonstrated in relation to 3,4-benzopyrene that PAH samples sorbed on PUF are stable for 7 days when the storage temperature is 4 8C.109 The PAH are desorbed by successive treatment with acetone and cyclohexane and determined by GC with a FID or by HPLC with a luminescence detector.110, 111 The efficiency of PUF as sorbents for the isolation of PCB (2 ± 20 ng litre71) from natural water with a view to subsequent GC determination has been evaluated.107, 126 ± 128, 135 When large sample volumes (up to 1790 litres) are passed through columns packed with PUF cylinders (3.862.2 cm) at a flow rate of S G Dmitrienko, Yu A Zolotov 250 ml min71 and then elution with acetone or hexane is carried out in a Sohxlet apparatus, 80%± 100% of PCB is recovered.The influence of the flow rate on the degree of recovery of organic compounds under dynamic conditions has been studied. It was noted that an increase in the rate from 10 to 100 ml min71 has no influence on the completeness of PCB recovery; however, an increase in the flow rate to 250 ml min71 decreases the degree of recovery to 30% ± 60%.128 Polyurethane foams are used to collect thirteen organochlor- ine insecticides (DDT, Lindane, Aldrin, etc.) and polychlorinated biphenyls by passing water through a (2062 cm) column packed with PUF at a flow rate of 100 ml min71.127, 128 The collected compounds were washed out from the column by successive treatment with acetone and hexane and determined by chroma- tography.Several types of samplers which allow a flow rate of 800 litres min71 to be reached have been developed for analysis of air.117, 118, 121, 128, 133 ± 135 The samplers are stainless-steel, glass or plastic tubes filled with cylindrical inserts made of PUF, in some cases, in combination with other sorbents or with glass-fibre filters. Prior to the use, sorbents are subjected to prolonged (up to 24 h) purification by means of various organic solvents and water in the Sohxlet apparatus.113, 117, 133, 134 A large potential of portable samplers based on combination of PUF with other sorbents such as Tenax CG121, 135 and XAD-2 118, 121, 128 for preconcentration of PAH, PCB and chlorine-, phosphorus- and nitrogen-containing insecticides in urban air samples has been demonstrated.For a sampling velocity of 0.1 ± 0.3 m3 min71, the recovery of compounds is not less than 75%.121 After desorption with a diethyl ether ± petroleum ether mixture (15 : 85) 111 or 5% diethyl ether in hexane 133 in a Sohxlet apparatus and evaporation of the solvent, the concentrate was analysed by various chromato- graphic methods: HPLC with UV detector and GC with ECD, PID and N/P-selective detectors. One sampler of this type was used to determine organochlorine compounds in the air above the Arctic.134 Air samples with a volume from 300 ± 600 m3 to 2000 ± 4000 m3 were pumped through a sampler for a period from 8 h to 2 ± 7 days at a flow rate of 750 ± 800 litre min71.Solid particles were retained on the glass-fibre filter, while the vapours of organochlorine compounds were sorbed in two cylin- drical PUF inserts. The impurity components were desorbed by 100 ± 300 ml of hexane for 8 h in a Sohxlet apparatus, then the solvent was partially removed and the concentrations of the impurities were determined using GC with an ECD, GC/MS and HPLC with an UV detector. The Arctic air was found to contain various PCB isomers,DDTand other organochlorine compounds whose concentrations are several pg m73. Amethod developed 113 for determination of PAH in air at the ng m73 level includes the following stages: preconcentration by sorption on PUF for 24 ± 48 h at a flow rate of 0.5 m3 min71, successive elution with pentane and a mixture of dichloromethane (30%) with pentane, solvent evaporation from the eluate and analysis of the concentrate by GC/MS or by HPLC with a fluorescence detector.Aprocedure for isolation, preconcentration and identification of volatile mutagens present in air has been described.117 The procedure includes sorption on PUF, elution with methanol or acetone, removal of the solvents, bioassays and analysis of the concentrate by GC/MS or by HPLC with a fluorescence detector. Eighteen PAH were determined using GC/MS in exhaust gases after preconcentration for 20 min on a PUF-packed column and desorption with a small volume of dichloromethane or 10% of acetone in hexane.123 Combination of the desorption of components collected on PUF using supercritical fluid extraction with subsequent determi- nation of these components by the GC/MS technique was reported to be efficient.124 This combination underlies a scheme for determination and identification of PAH, PCB and n-alkanes present in exhaust gases and the cigarette smoke.The scheme implies preconcentration of organic impurities on PUF followedPolyurethane foams in chemical analysis: sorption of various substances and its analytical applications by desorption by the CO2 fluid directly to the separating chroma- tographic column and allows gas-chromatographic analysis to be carried out over a period of less than an hour. 10. Test methods The high efficiency of preconcentration by sorption on PUF of many compounds, the intensive colour of the sorbent observed in some cases, the ready availability and low cost of the sorbent are attractive features stipulating the use of PUF in test methods of analysis � simple and cheap methods for detection and determi- nation of substances.258 A colour change of the sorbent after the reaction indicates the presence of the target component, and the content of the component is found by comparing the intensity of the sorbent colour with the appropriate scale or by measuring the length of the coloured zone or the time it takes for the character- istic colour to appear.The reliability of visual detection can be improved in some cases using portable miniphotometers which record light reflection from solid samples.The possibility of using PUF in test methods of analysis was first mentioned by Braun and Farag.218 Determination was preceded by element preconcentration on chromofoams (the term was introduced by Braun 218), i.e., PUF loaded with photo- metric reagents. When these foams, as small cubes, are brought in contact with the solution under analysis, the white polymer acquires a colour typical of the given photometric reaction. These reactions are highly sensitive, for example, for sorption in the static regime, tenths (Co,218 Cu,214, 218, 233 Cd,214 Pd,233 Pb,218, 219 Cr,230 Mo234) and in some cases, even hundredths (Ag,211 Ni,205 Cu,214 Zn,218 PO4 94) of a microgram of an impurity can be determined in a sample volume not exceeding 1 ± 2 ml.The sensitivity of determination increases by at least an order of magnitude when Co,218 Cu,218 Cd,214 Cr 230 andMo 234 are sorbed in the dynamic regime. Detection of metals at a level of several nanograms in ml is attained by passing relatively large volumes of a solution (*1 litre) through a chromofoam filled column. The use of these materials ensures higher sensitivity of analysis than the use of impregnated paper or than drop reac- tions.205, 214, 218, 219, 230 Chromofoams are also suitable for a semi- quantitative determination, which implies either comparison with standard colour scales after sorption in the static regime or measurement of the length of the coloured zone in the column after letting-through the solution under analysis.Usually, the colour intensity of chromofoams is linearly related to the metal concentration in the aqueous solution. One more way of using these sorbents in test methods of analysis, not associated with the preparation of loaded PUF, is also possible. Pellets of unloaded PUF complete with ampoules containing solutions of reagents have been used as test kits.105, 163, 165, 172, 180, 181 The reproducibility of the results of visual detection based on a colour scale (sr) varies in the range of 0.1 ± 0.5. Test kits based on PUF for determination of Co, Ni, Fe(III), nitrite ions, free active chlorine, total content of phenols, and anionic and cationic surfactants have passed metrological certification, have been registered as a type of test system 259 and are currently produced in industry. The test systems based onPUF are simple to produce, reliable in operation and suitable for determination of toxic substances in waters of various types including potable and mineral waters, extracts from soils or juices. If necessary, the test form can be retained and analysed in a laboratory using diffuse reflection spectroscopy or, after desorp- tion of compounds, HPLC.165 VII.Conclusion The presented data demonstrate that polyurethane foams are of considerable interest for the analytical chemistry of inorganic and organic compounds. Characteristic features of these sorbents include high efficiency in combination with versatility, chemical and mechanical strength, stability against organic solvents, rela- tively low cost and ready availability. The unique sorption 171 properties of synthetic polymeric materials are largely due to their cellular membrane-like structure, which allows the molecules being sorbed to penetrate inside the sorbent, and to the combi- tion of various hydrophilic and hydrophobic active sites and reactive terminal groups.The applications of PUF in analytical chemistry are diverse. These sorbents are used for preconcentration of elements and organic substances from water and air and in spectroscopic, sorption ± spectroscopic and chromatographic methods of analy- sis. The good prospects of using PUF in flow-injection analysis are demonstrated. Other applications of these materials are also noted, including the use as polymer matrices for enzyme immobi- lisation in enzymic methods of analysis.Polyurethane foams are of interest not only regarding prac- tical use but also in purely scientific aspects. In our opinion, the scope of application of PUF in analytical chemistry has not been adequately appreciated because most studies are aimed at the solution of applied problems of substance preconcentration and only few publications deal with investigation of physicochemical processes that proceed during sorption. Too little attention is devoted to elucidation of the relationship between sorption parameters and the properties of the sorbed molecules and evaluation of the contributions of intermolecular sorbent ± sor- bate interactions.Virtually no works are related to the study of physicochemical properties of the polyurethane foams themselves. The study of heterogeneous chemical reactions involving the PUF surface are of considerable interest. This line of research, which is successfully developing in other fields of chemistry, can provide important results that would allow one to approach the problem of target-directed change of the properties of PUF by surface modification. One may hope that PUF would be used more and more extensively in hybrid methods of analysis based on the combina- tion of preconcentration by sorption with the subsequent deter- mination in the sorbent matrix using diffuse reflection spectroscopy, luminescence or X-ray fluorescence spectroscopy and in test methods of analysis.This review was supported by the Russian Foundation for Basic Research (Project No. 01-03-33102). References 1. J H Saunders, K C Frish Polyurethanes (New York; London: Academic Press, 1965) 2. H J M Bowen J. Chem. Soc. A 1082 (1970) 3. T Braun, A B Farag Talanta 19 828 (1972) 4. T Braun, A B Farag Anal. Chim. 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