摘要:
Anion selective recognition and optical/electrochemical sensing by novel transition-metal receptor systems Paul D. Beer Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR Appropriately designed charged or neutral redox- and/or photo-active transition-metal organometallic and coordination receptor systems can selectively recognise and sense anionic guest species by electrochemical andlor optical methodologies. The topological positioning of the transition-metal Lewis-acid centres and of the ubiquitous hydrogen-bonding functionalities, such as amide (CO-NH) groups, dictates the anion selectivity trend and sensing proficiency the particular receptor displays. Introduction The synthesis of positively charged or neutral electron-deficient abiotic receptor molecules designed to non-covalently bind anionic guest species is an area of ever increasing research activity.' Anions play numerous fundamental roles in biological and chemical processes,2 for example, the majority of enzymes bind anions as either substrates or cofactors, and many anions act as nucleophiles, bases, redox agents and phase-transfer catalysts.In addition, the importance of being able to detect and/ or extract certain environmental anionic pollutants3 such as nitrate, phosphate and radioactive pertechnetate,' produced in the nuclear fuel cycle,4 has only recently been recognised. It is surprising, then, that the construction of specific ligands that have the capability of sensing anions via optical5 and/or electrochemical6 methodologies in polar organic and aqueous media has not been fully exploited.For example, Vance and Czarnik7 have described one of the very few anion fluorescent responsive types, based on acyclic anthracene appended polyammonium receptors. We have initiated a research pro- -V PF6-X gramme aimed at the design and construction of innovative spectral and electrochemical sensory reagents for anions based on novel transition-metal organometallic and coordination receptor systems. This article reviews our current progress in this exciting multidisciplinary field of anion supramolecular chemistry. Acyclic and Macrocyclic Cobaltocenium Receptor Systems In 1989 we reported the synthesis of the first redox-responsive class of anion receptor based on the redox-active, pH independ-ent positively charged cobaltocenium moiety.8 The ester-linked cobaltocenium macrocyclic receptor 1 complexed and electro- chemically detected the bromide guest anion.Unfortunately the general poor solubility of these types of macrocyclic ligands coupled with their arduous syntheses and lability to ester hydrolysis led us to a new synthetic strategy which utilised the ubiquitous amide linkage to construct novel acyclic anion receptors. 0 It 0 1 0 "H&x PFi 7X=OMe \ 9 8X=H Fig. 1 Acyclic, mono-, bis- and tri-podal amide-linked cobaltocenium receptors Chem. Commun., 1996 689 Simple mono- and 1, 1'-bis-substituted cobaltocenium acid chloride or activated ester condensation reactions with alkyl and aryl amines produced a variety of new acyclic mono-, bis- and tri-podal amide-linked cobaltocenium receptors in very good yieldsgJ0 (Fig.1). The addition of tetrabutylammonium salts NBu4X (X = C1, Br, N03, HS04, H2P04) to deuteriated acetonitrile or dimethyl sulfoxide 1H NMR solutions of these receptors resulted in remarkable downfield shifts of the respective receptor's pro- tons. Of particular note were the substantial downfield shifts of the amide protons, A6 = 1.28 ppm for 2 and 1.52 ppm for 3 on addition of 1 equiv. of chloride. These results suggest a significant -CO-NH.-X-hydrogen-bonding interaction is contributing to the overall anion complexation process.The Table 1 Stability constant data for cobaltocenium receptors with various anions Receptor Solvent Anion Kldm3 mol-1 a c1- 100 c1-c1-HzP04- 1200 24 770 c1-c1- 630 30 Br- 25 c1- 35 H2PO4- 320 Errors estimated to be <lo%. u C(W -n(a) H(W Fig. 2 Structure of the bromide complex of 8 Table 2 Electrochemical data Receptor EJVa Anion AEJmVb 3 -0.75" c1-30 3 -0.75~ H2PO4-200 4 -1.10 c1-35 5 -1.08 c1-30 6 -1.03 c1-55 7 -0.80 c1-90 7 -0.80 Br-40 7 -0.80 HzP04-240 8 -0.83 c1-85 8 -0.83 HZPO4-240 a Obtained in acetonitrile solution containing 0.2 mol dm-3 NBu4BF4 as supporting electrolyte. Solutions were ca. 1 x 10-3 mol dm-3 in receptor and potentials were obtained with reference to an Ag-Ag+ electrode.b Cathodic shift in reduction potential produced by presence of anions (up to 10 equiv.) added as their tetrabutylammonium salts. c The-electron reduction process as determined by coulomeb-ic experiments. 690 Chem. Commun., 1996 resulting titration curves suggested 1 : 1 receptor : anion stoi- chiometry in all cases. Stability constants were calculated from the titration curves and a selection of these are shown in Table 1. revealing Br-hydrogen bonding to the amide proton and to aryl and It is noteworthy that with monosubstituted aryl cobaltocenium derivatives 4-6 the strength of chloride ion 1501 100 -50 -4 \-0--50 --100 I I I I I 1 0.5 0.0 -0.5 -1.0 -1.5 -2.0 E I v vs.Ag-Ag+ Fig. 3 Cyclic voltammograms in acetonitrile of 6 in the absence (a) and presence (b) of excess chloride ion -(CHZ),N(M~)(CH~ZN(M~)(CH~)~-15 Fig. 4 (a) Schematic representation of anion recognition by a ditopic bis(coba1tocenium) receptor; (b) examples of ditopic receptors prepared Table 3 Stability constant data for receptors 11-13 and halide anions in CD3CN Receptor C1- Br- I- 11 2500 330 450 12 1300 270 275 13 280 260 100 0 Errors estimated to be <lo%.(b) PF,' Go+ 0 16 PF,' &yAOMe G-OMe 0 17 Fig. 5 (a) Structure of the macrocyclic amide-linked cobaltocenium receptor 16 and (b) acyclic receptor 17 QQ CH3 CH3 19 Fig. 6 Bis(coba1tocenium) calix[4]arene receptors 18and 19 Table 4 Stability constant data for bis- and tetra-cobaltocenium calix[4]ar- ene receptors in (CD&SO Kldm3 mol -1 a Receptor C1-Br-H2PO4--02C(CH2)4C02-18 5035 1680 2800 11510 19 10 -3100 -20 70 -1200 -0 Errors estimated to be <10%.b Titration performed in (CD3)2C0. binding is enhanced when additional favourable amine-halide hydrogen-bonding interactions are sterically accessible as is the case for 5 and 6 but not for 4.10 In addition, the 1,l'-bis- substituted aryl cobaltocenium analogues display an order of magnitude selectivity for the dihydrogenphosphate anion over halide anions. The importance of hydrogen bonding in anion recognition by the cobaltocenium class of receptor is further highlighted with receptors 9 and 10 containing the tertiary amide (CONR2) linkage being unable to form solution halide anion complexes, as evidenced from 1H NMR titration investigations.Hence it is the unique combination of the positively charged cobaltocenium moiety and the appending amide CO-NH unit which can form a favourable hydrogen bond with a coordinated anion guest, which are the essential components for successful anion complexation. This is again highlighted in the single-crystal structure of the bromide complex of 8 (Fig. 2) revealing Br- hydrogen bonding to the amide proton and to aryl and cyclopentadienyl protons of the cobaltocenium receptor. lo Although polyammonium macrocycles have been shown from electrochemical measurements to stabilise hexacyano- ferrate(I1) and hexacyanocobalte(II1) anions," to our knowledge these cobaltocenium ligands represent the first redox-respon- sive class of anion receptor.Cyclic voltammetry was used to investigate the electrochemical anion recognition properties of these receptors and a selection of results are summarised in Table 2. Significant one-wave cathodic shifts of the cobalto- cenium+obaltocene redox couples of amide (CO-NH) contain- ing receptors are observed with all anionic guest species (Fig. 3). The complexed anion effectively stabilises the positive charge of the cobaltocenium unit causing the redox couple to shift to a more negative potential. Very large cathodic perturbations are observed with the H2P04-anion guest, an observation which complements the stability constant data (Table 1) in which the highest K values are obtained with this anion. The tertiary amide containing receptors displayed no electrochemical response to the addition of anions ruling out the possibility of the cathodic shift being caused by simple ion- pairing effects.In an effort to impart selectivity and enhance complex stability for this class of anion receptor we have prepared novel ditopic bis(coba1tocenium) receptor molecules in which two -COCl CH,CN-THF (i) 4 Co+ CT (60%) (ii) excess4v NH,PF,1 20 Scheme 1 Chem. Commun., 1996 691 positively charged metallocene centres, linked via various alkyl, aryl and calix[4]arene spacers, may cooperate in the molecular recognition of mono- or di-anionic guest substrates12 (Fig. 4).Proton NMR halide anion coordination studies revealed the alkyl-linked derivatives 11-13 form 1 :1 stoichiometric com- plexes in acetonitrile solution. Stability constant evaluations (Table 3) suggest that as the length of the alkyl chain increased the selectivity preference for chloride and the general stability of the halide complex decreased. This latter observation provides experimental evidence for the existence of an anionic chelate effect. Receptors 14 and 15 containing larger aryl and alkyl amino spacers form complexes of 2 : 1 halide anion :re-ceptor stoichiometry. All the bis(cobaltocenium) systems were found to display electrochemical recognition of varied anion guests with H2P04-again, as found with the monosubstituted cobaltocenium derivatives, producing the largest magnitude of cathodic shift (AE = 250 mV).of 21 e Co+ PF--u22 e+C=OpFG I NH \dNH /C=OI c=o 23 Fig. 7 Cobaltocenium porphyrin receptors 21-23 The macrocyclic amide linked cobaltocenium receptor 16 has recently been prepared and its crystal structure elucidated13 (Fig. 5). Chloride anion stability constant determinations with 16 and the related acyclic analogue 17 revealed the presence of an ‘anion macrocyclic effect’. The stability constant value for the chloride complex of 16 in dimethyl sulfoxide (K = 250 dm3 mol-1) is at least an order of magnitude greater than the chloride complex of 17 (K = 20 dm3 mol-1). Polycobaltocenium Calix[4]arene Receptors The calixarenes14 are attractive host molecules on which to construct additional recognition sites for target guests.Although the calix[4]arene host structural framework has been modified at the lower rim for the recognition of metal cations,15 the design and synthesis of calix[4]arene anion receptors is still relatively rare.16 The bis-cobaltocenium calix[4]arene deriva- tive12,1718 (Fig. 6) was prepared and shown to form extremely stable 1 : 1 anion complexes in polar dimethyl sulfoxide 0 R = (CH*)*OCHs M= C‘, /‘Pt CI’ CI ‘Z”< CI ’ 0 /X(OC)3Re\ and 5,5’-bipyanalogues &&lNH2FP Q 24 Q 25 @$$/IB,,OH OH ,:jI 26 Fig. 9 New classes of charged and neutral anion receptors n Fig. 8 Depicting the basic requirements for an anion receptor Fig.10 Polyazaferrocene macrocycles 692 Chem. Commun., 1996 solutions and with the adipate anion in acetone (Table 4). Interestingly this receptor displays the uncommon anion selectivity preference of chloride over dihydrogen phos- phate.12 Modifying the substituents on the calix[4]arene lower rim has a dramatic effect on the anion coordination properties of this type of receptor. For example, derivative 19 (Fig. 6) containing tosyl groups para to the upper-rim amide substituted cobalto- cenium moieties exhibits the H2P04- >> C1- selectivity trend18 (Table 4), the reverse of 18.Presumably the bulky tosyl groups alter the topology of the upper-rim anion recognition site in favour of phosphate complexation.Recently the novel tetrakis(coba1tocenium) calix[4]arene 20 has been synthesisedlg (Scheme 1). Table 4 shows this receptor exhibits a remarkable selectivity preference for H2PO4- over C1-. Electrochemical investigations disclose significant anion- induced cathodic shifts are observed with each calixE41arene receptor, including 18 sensing adipate. Halide and Nitrate Selective Cobaltocenium Porphyrin Receptors A series of new cobaltocenium porphyrin receptors have been prepared and shown to spectrally and electrochemically sense anions20.21 (Fig. 7). Notable anion selectivity differences are displayed by the various atropisomers which highlights the importance of the relative positions of the cobaltocenium amide moieties in the anion recognition process.For example, the cis-a,a,a,a-atropisomer 21 exhibits the selectivity trend C1- > Br->> NO3-in which all four cobaltocenium moieties cooperatively form a cavity complementary to the spherical halide anion guest.20 In contrast the &,a,-22 and a,a,PP-23 atropisomers display the rare selectivity sequence N03-> Br-> C1- indicating a complementary trigonal host cavity exists for nitrate.21 New Classes of Anion Receptor containing Charged and Neutral Transition-metal Lewis-acidic Recognition Sites Taking into account the crucial importance of hydrogen bonding to the anion recognition process22 for cobaltocenium receptors we reasoned that in principle any Lewis-acidic centre in close proximity to one or more amide (CO-NH) groups may lead to the successful molecular recognition of an anionic guest ~pecies23.~~(Fig. 8).The anion is complexed via the combina- tion of favourable Lewis acid-anion electrostatic and amide (CO-NH)-anion hydrogen-bonding interactions. Substantial evidence in support of this simple concept came from the syntheses of a variety of new classes of anion receptor R= incorporating positively charged and neutral organometallic and coordination transition-metal Lewis-acidic binding sites in combination with amide groups (Fig. 9). Proton NMR anion titration experiments provided solution evidence of halide, HS04-and H2P04- anion complexation. With the neutral acyclic 24, tripodal 25 and calix[4]arene 26 ferrocene receptors significant anion guest induced cathodic perturbations of the respective ferrocenyl oxidation wave were 0bserved.2~Of interest to the future design of amperometric chemical sensors were the novel results of electrochemical competition experi- ments, in agreement with stability constant determinations, which demonstrated 24-26 were capable of detecting the H2P04- anion in the presence of tenfold excess amounts of HS04-and C1- i0ns.2~ We have recently prepared a series of water-soluble polyaza ferrocene macrocyclic ligands25 (Fig.10) which are able to electrochemically recognise phosphate anions (HP042-, ATP) in an aqueous environment at pH values of 67. Fluorescence Emission Spectral and Electrochemical Sensing of Anions by Acyclic, Macrocyclic and CalixC41arene Ruthenium(I1) Bipyridyl Receptor Molecules We have recently incorporated the Lewis-acidic redox- and photo-active ruthenium(I1) bipyridyl moiety, in combination with amide (CO-NH) groups, into acyclic, macrocyclic and lower-rim calix[4]arene structural frameworks to produce a new class of anion receptor with the dual capability of sensing anionic guest species via electrochemical and optical metho- d0logies26,~~ (Fig.11). Single-crystal X-ray structures of 27.C1-(Fig. 12) and 31.H2P04- (Fig. 13) display again the importance of hydrogen bonding to the overall anion com- plexation process. In the former complex (Fig. 12) six hydrogen bonds (two amide and four C-H groups) stabilise the C1- anion and three hydrogen bonds (two amide and one calix[4]arene hydroxy) effect H2P04- complexation with 31 (Fig.13). Stability constant determinations in dimethyl sulfoxide demon- strated these receptors form strong, and in the case of the macrocyclic 30 and calix[4]arene containing receptor 31, highly selective complexes with H2P04- (Table 5). Substantial anion- induced cathodic perturbations of the respective ligand-centred amide substituted bipyridyl reduction redox couple were detected in electrochemical anion recognition experiments (Table 6) with 31, in agreement with stability-constant values, able to sense H2PO4- in the presence of tenfold excess amounts of HS04- and C1-. Fluorescence emission spectroscopic measurements were also undertaken to probe anion bindi11g.~~-28 All receptors R= -(CH~)~OMC28 Fig.11Acyclic, macrocyclic and calix[4]arene ruthenium(I1)-bipyridyl receptors Chem. Commun., 1996 693 exhibited significant blue shifts in the respective MLCTA,,, emission band on addition of C1- and H2PO4-, not observed with [Ru(bpy)#+, with 31 displaying the largest perturbation of 16 nm (Fig. 14). These shifts were accompanied by large increases in emission intensity (higher quantum yields) (Fig. 14)which may be a consequence of the bound anion rigidifying the receptor, inhibiting vibrational and rotational relaxation modes of non-radiative decay. Upper-rim substituted calix[4]arenes functionalised with two and four ruthenium(I1) bipyridyl amide groups have also been very recently prepared by our gro~p269~9 (Fig.15) and shown to 0 Fig. 12 Structural view of the chloride complex of 27, with thermal ellipsoids at 50% probability *, 2.48-% ‘’P Fig. 13 Structure of the dihydrogen phosphate complex of 31; in the solid state the anion hydrogen bonds to another anion molecule Table 5 Stability constant data for acyclic, macrocyclic and calix[4]arene ruthenium(i1) bipyridyl receptors in (CD3)*S0 K/dm3 mol- * a Receptor C1-H2P04-27 500 8000 28 480 7700 29 90 5600 30 420 8000 31 1600 28000 Errors estimated to be S5%. 694 Chem. Commun., 1996 selectively sense via fluorescence emission and electrochemical investigations the H2PO4- anion. Remarkable Chloride Selective Macrocyclic Ruthenium(r1)- Bipyridyl-Metallocene Receptors The majority of the various anion receptors already discussed in this article exhibit pronounced selectivity for the dihydrogen phosphate anion in preference to halide anions. Indeed it is Table 6 Cathodic perturbations of the first ligand-centred amide-substituted bipyridyl reduction couple observed on addition of various anionsa AE(HzP04-) AE(HS04-) AE(C1-) AE(Br-) Receptor /mV /mV /mV /mV 27 h h 40 30 29 175 - 70 10 30 150 20 65 - 31 175 15 70 60 a Obtained in acetonitrile solution containing 0.1 mol dm-3 NBu4PF6 as supporting electrolyte.Cathodic shifts of reduction potential produced by presence of anions (up to 10 equiv.) added as their tetrabutylammonium salts.Precipitation of complex prevented a AE value from being determined. 4.5r 4.0 3.5 A 8 3.0 v .$2.5 C 2.0 1.5 1.o 5.0 0.0 500 550 600 650 700 htnm Fig. 14 The effect of addition of stoichiometric amounts of H2P04-on the fluorescence emission spectrum of 31 in dimethyl sulfoxide; 0 (a),3 (b), 6 (c), 9 (4,12 (e) or 15 equiv. (t) of H2PO4-’co ,cb ‘NH NH II II Tbs T~s Fig. 15 Upper-rim bis- and tetra-ruthenium(I1) bipyridyl calix[4]arenes noteworthy that this selectivity trend is also displayed by uranyl amide based receptors.30 The chloride anion is crucial for a large number of biological processes.31 For example, the relatively common hereditary disease cystic fibrosis is known to result from a genetically caused misregulation of chloride anion channels.32 Thus there is a real need for selective detection as (PF;),*4HZ0 " 32 7"."-N\ 35 Fig.16 Macrocyclic bis[ruthenium(r~)-bipyridyl] 32 and ruthenium(I1)- bipyridyl-metallocene receptors 33 and 34 Table 7 Stability constant data for receptors 32-35 and chloride anion in (CD3)ZSO Proton of receptor Receptor monitored Kldm3 mol-' a 4.05 x 104 3.95 x 104 9.00 x 103 9.95 x 103 1.00 x 104 9.88 x 103 1.86 x 104 1.23 x 104 2.07 X 102 1.52 X 102 1.57 x 103 1.57 x 103 a Errors estimated to be 610%. b EQNMR analysis of titration curve of particular receptor proton. established methods for chloride determination based on titrimetric analysis lack selectivity and are not suitable for biological appli~ations.~3 We have prepared the novel macrocyclic bis[ruthenium(II) bipyridyl] and ruthenium(I1)-bipyridyl-metallocene receptors 32-34 (Fig.16) and 1H NMR titration studies suggested each receptor formed an extremely stable 1 : 1 stoichiometric com- plex with chloride in (CD3)2S0 s0lutions.3~ In fact the magnitudes of the stability constants (Table 7) are amongst the largest known for any anion-abiotic amide receptor complex and are ca. two orders of magnitude greater than the stability constant obtained for the acyclic receptor 35 (Table 7). Analogous IH and 31P NMR titration experiments with H2P04- gave no evidence of binding this anion by these macrocyclic receptors in (CD&SO solution.This contrasts with acyclic receptor 35 which forms a stronger complex with H2P04- than C1-(Table 7). This remarkable C1- over H2P04- selectivity preference exhibited by the macrocyclic systems may be attributed to their inherently rigid structures. Molecular model- ling calculations (MM2) and C.P.K. models suggest the minimised structure of 32 has all the amide and 3,3'-bipyridyl protons lying in a coplanar arrangement which creates a host cavity of similar dimensions to the chloride anion (Y = 1.81 A)capable of forming eight hydrogen bonds with this spherical anionic guest species. The larger size and tetrahedral shape of H2PO4-is non-complementary to the macrocyclic receptor's host cavity and consequently complex formation with this anion is not favoured.Fluorescence emission spectroscopic measurements corro- borated the NMR findings. Although excess amounts of H2PO4-had no effects on the emission spectra of the macrocyclic receptors the addition of chloride produced substantial blue shifts (Ah,,, = 6 nm) with significant intensity increases (Fig. 17) demonstrating 32-34 are first-generation prototype chloride selective sensory reagents. Conclusions We have seen in this article a variety of transition-metal organometallic and coordination receptor systems which are capable of recognising and sensing anionic guest species both by electrochemical and spectroscopic means. The respective topological positioning of redox-/photo-active transition-metal Lewis-acidic centres and hydrogen-bonding functionalities such as amide (CO-NH) groups in particular, dictates not only the receptor-anion complex thermodynamic stability and sensing proficiency, but also crucially the distinct anion 45 1 40 ? ?2.30 .-? 25 -c .-al2 20 -8 15-E 10 5 7 i I I I I I I 550 575 600 625 650 675 700 h/ nrn Fig.17 Fluorescence emission spectra of 34 in acetonitrile with addition of chloride; 0 (a),0.2 (h),0.4 (c), 0.6 (4,0.8 (e), 1.0 (R and 1.2 equiv. (8)of c1-Chem. Commun., 1996 695 selectivity trend the receptor displays. For example, this is highlighted by receptors 19, 23 and 32 exhibiting respective selectivities towards HzP04-, N03-and C1-.Having establi- shed the basic requisite types of building blocks needed for the potential sensing of anions it remains the imagination of the coordination chemist to further design and synthesise more specific and complementary shaped receptors for target anionic substrates of particular environmental and medical concern. The fabrication of these types of systems into membranes, electron- ically conducting polymeric supports and optical fibres will no doubt produce novel prototype molecular sensory devices of the future. Acknowledgements I thank my postgraduate and postdoctoral coworkers for their undaunting motivation and determination to succeed in this multidisciplinary research field; their names appear in the references.Special thanks go to Dr Michael G. B. Drew for his many structure determinations and molecular modelling ex- pertise. I gratefully acknowledge the financial support of EPSRC, The Royal Society, Kodak Limited, MediSense and Serpentix. Paul Beer was born in Totnes, Devon. In 1979 he obtained a first class honours degree in chemistry from King’s College London, and remained there to undertake research in the field of organophosphorus chemistry under the supervision-of Dr C. D. Hall. In 1982 he received a PhD and a Royal Society postdoctoral fellowship enabled him to conduct research in supramolecular chemistry with Professor J.-M. Lehn at the Universitk Louis Pasteur, Strasbourg, France. After a demonstratorship at the University of Exeter in 1983, in 1984 he took up a New Blood Lectureship at the University of Birmingham.In 1990 he moved to a lectureship at the Inorganic Chemistry Laboratory, University of Oxford, where he is also a tutorial fellow at Wadham College. He was awarded in 1987 the RSC Meldola medal, in 1993 the UNESCO Javed Husain prize and 1994 the RSC Corday-Morgan medal. 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Hazlewood, D. Hesek, J. Hodacova and S. E. Stokes, J. Chem. SOC., Chem. Commun., 1993, 229. 18 P. D. Beer and D. Hesek, unpublished work. 19 P. D. Beer, D. Hesek and R. J. Mortimer, unpublished work. 20 P. D. Beer, M. G. B. Drew, D. Hesek and R. Jagessar, J. Chem. Soc., Chem. Commun., 1995, 1187. 21 P. D. Beer, D. Hesek and R. Jagessar, unpublished work. 22 For examples of organic based ligands that complex anions exclusively through hydrogen bonding, see R.A. Pascal, J. Spergal and D. van Engen, Tetrahedron Lett., 1986, 27, 4099; S. Voliyavaeettil, J. F. J. Engersen, W. Verboom and D. N. Reinhoudt, Angew. Chem., Int. Ed. Engl., 1993, 32, 900; C. Raposo, M. Crego, M. L. Mussons, M. C. Caballero and J. R. MorBn, J. Am. Chem. Soc., 1993, 155, 369; S. Nishizawa, P. Biihlmann, M. Iwao and Y. Umbezawa, Tetrahedron Lett., 1995, 36, 6483. 23 P. D. Beer, C. A. P. Dickson, N. C. Fletcher, A. J. Goulden, A. Grieve, J. Hodacova and T. Wear, J. Chem. Soc., Chem. Commun., 1993, 828. 24 P. D. 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Reinhoudt, J. Am. Chem. SOC.,1994,116,4341. 31 K. L. Kirk, Biochemistry of the Elemental Halides, Plenum, New York, 1991. 32 W. Kaim and B. Schwederski, Bioinorganic Chemistry: Inorganic Elements in the Chemistry of Life, Wiley, New York, 1991, p. 283. 33 Official and Standardized Methods of Analysis, ed. C. A. Watson. The Royal Society of Chemistry, Cambridge, 3rd edn., 1994. 34 P. D. Beer and F. Szemes, J. Chem. SOC., Chem. Commun., 1995, 2245. Received, 27th October I995; 51071025 696 Chem. Commun., 1996
ISSN:1359-7345
DOI:10.1039/CC9960000689
出版商:RSC
年代:1996
数据来源: RSC