摘要:

L e t t e r Might BF and be alternatives to CO? A theoretical quest for BNR2 new ligands in organometallic chemistry F. Matthias Bickelhaupt,*,§,a Udo Radius,b Andreas W. Ehlers,a Roald HoÜmann*,î,c and Evert Jan Baerends*,a a Afdeling T heoretische Chemie, Scheikundig L aboratorium der V rije Universiteit, De Boelelaan 1083, NL -1081 HV Amsterdam, T he Netherlands b Institut Anorganische Chemie, Karlsruhe, Geb. 30.45, f ué r Universitaé t Engesserstraêe, D-76128 Karlsruhe, Germany c Baker L aboratory, Department of Chemistry, Cornell University, Ithaca, NY 14853-1301, USA BF and emerge, in our DFT computations, as ideal alternatives to the CO ligand. BNR2 Carbon monoxide, CO, is ubiquitous in organometallic and coordination chemistry. It plays a key role in many catalytic processes, either as a reacting partner or as a spectator ligand.1 Ligands isoelectronic to CO, e.g.NO` and CN~, N2 , are also quite well-known in metal compounds.2 But, the number of complexes with neutral isoelectronic diatomic molecules terminally ligated to transition metals is somewhat limited, mainly restricted to complexes with ligands of the type CE (E\S, Se, Te, NR, and None of these CH2) N2 .other ligands seems to be as versatile as CO. To –nd potential alternatives for the CO ligand, similar to it, and yet diÜerent, we have undertaken a nonlocal density functional theoretical (DFT) investigation at the BP86/TZ2P level on a series of ìcandidateœ ligands, e.g. SiO, BF and and their coordination in mono- and binuclear –rst- BNH2 , row transition metal complexes (M\Cr, Mn, Fe, Co, Ni) using the ADF program.3,4 Here, we report the preliminary results of our theoretical quest.We begin our study with a careful theoretical investigation of the isoelectronic ligands CO and BF, as well as AB\N2 , their metal bonding capabilities in the model complexes (1, axially substituted), (equa- Fe(CO)4ABax Fe(CO)4ABeq torially substituted) and the homoleptic (2) : Fe(AB)5 It is well-known that the orbital character and energetics of the frontier orbitals, i.e.the 5r HOMO and the 2p LUMOs and in determine the coordination capabilities (3rg 1pg N2),5 of the AB molecules.6 The diatomic HOMO can be viewed as a slightly AwB antibonding lone-pair orbital with an sphybridized lobe along the z axis, which participates in the metalwligand bond through r donation of charge into an empty, mainly hybrid orbital on the fragment, as dz2 Fe(CO)4 shown in 3.The two AwB antibonding p* LUMOs or (1pg 2p) are involved in p backdonation, accepting charge from dxz § Current address : Fachbereich Chemie, Philipps-Universitaé t Marburg, Hans-Meerwein- D-35032 Marburg, Germany. Fax: Straêe, ]49-6421-282189; E-mail: bickel=uni-marburg.de î Fax: ]1-607-255-5707, E-mail: rh34=cornell.edu (4) and hybrid orbitals (5).dyz How exactly does the AB electronic structure change as we go from via CO to BF (see Fig. 1) ? The AOs of the electro- N2 positive atom A rise in energy and become more diÜuse along this series, whereas those of the electronegative atom B decrease in energy and become more compact.This leads to an energy mismatch, poorer overlaps and, therefore, to weaker AwB orbital interactions. As a consequence, the p* LUMOs, i.e. drop slightly in energy and become more 2pp(A)»2pp(B), localized on A (Fig. 1). The ligand donor orbital also becomes more localized on A and moves rather strongly to higher energy (Fig. 1). A more detailed discussion of the subtle interplay of orbital interactions behind these regularities will be given elsewhere.These trends lead us to expect that the AB ligandœs overall metal-binding ability should increase in the order N2\ and that along this series the importance of r don- CO\BF, ation should be enhanced relative to that of p backdonation. These expectations are con–rmed by our further calculations. For instance, the computed bond dissociation Fe(CO)4wAB enthalpies (for 298.15 K) of axially substituted complexes are 18.1, 42.3, 67.9 kcal mol~1 for CO, BF (at the BP86/ N2 , TZ2P level of DFT);4 the corresponding values for the equatorially substituted complexes are very much alike.A similar trend is also found for the homoleptic complexes. Fe(AB)5 Fig. 1 Trend in HOMO and LUMO energies (in eV) of isoelectronic ligands AB.The extent (percentage over all AOs) to which each MO is centered on the more electropositive atom A is given in italics New J. Chem., 1998, Pages 1»3 1M M M A B A B A H 5 4 3 e– flow e– flow e– flow z x y N B H H N B H H 2b2 LUMO x 2b1 LUMO+1 y z 6 7 BF does seem to be a very promising candidate for supplementing CO as a ligand.But it is a reactive molecule (its HOMO-LUMO gap is only 4.6 eV, compared to 7.0 eV for CO and 8.0 eV for and it requires special ways of gener- N2), ation and handling techniques.7 Moreover, the polar BF ligand may remain very reactive even when complexed. How can we overcome the problem of instability of BF, ligated or not? One possibility is to build in steric bulk by substitution of the F by another group.This may be accomplished through with R potentially bulky. We have BNR2 , explored the bonding of such a ligand with R\H and CH3 . Here we discuss the results for the planar, symmetric C2v Its frontier orbital energies suggest that it has even BNH2 . better ligating properties than BF (Fig. 1). The HOMO is 5a1 higher in energy and the LUMO, i.e. the p* orbital lying 2b2 in the molecular plane, is lower in energy.Letœs have a closer look at the and MOs of 2b1 2b2 BNH2 . One way to look at them is from the viewpoint of built BNH2 up from B and The (6) is then the ì free œ AO of NH2. 2b2 2py boron (slightly perturbed by the fragment), whereas the NH2 (7) is the boron AO, destabilized by the of 2b1 2px 2px(N) NH2 . The smaller HOMO-LUMO gap of the free ligand BNH2 (only 2.9 eV) suggests lesser kinetic stability.But this might be alleviated by shielding the frontier orbitals of through BNR2 sterically more demanding substituents R. It is also important to realize that the HOMO-LUMO gap of the free ligand is not automatically an indicator for its inertness after complexation ! The well-known Fischer-type carbenes,8 for example, have an even smaller HOMO-LUMO gap.For the (uncoordinated) archetype C(H)OH we calculate a gap of only 2.2 eV. Yet, these ligands form relatively stable complexes. is isoelectronic with the well-known vinylidene BNH2 ligand which forms stable, isolable complexes.9 We CCH2 , have analysed the frontier orbitals of to see how they CCH2 diÜer from those of The HOMO (83%) and BNH2 .CCH2 3a1 LUMO (80%) are somewhat less localized on the terminal 2b2 atom, in line with the reduced electronegativity diÜerence Table 1 BP86/TZP metal complexwligand, [M]wAB, bond dissociation energies (in kcal mol~1) and, in parentheses, ligand AwB bond lengths (in ”) Ligand AB Compound CO BF BNH2 AB » (1.138) » (1.272) » (1.380) Cr(CO)5wAB 41.8 (1.155) 62.1 (1.281) 72.1 (1.379) Mn(CO)5wAB` 44.2 (1.141) 71.4 (1.259) 94.4 (1.354) Fe(CO)4wABax 48.4 (1.156) 73.8 (1.275) 87.7 (1.378) Co(CO)4wABax ` 37.3 (1.139) 70.6 (1.251) 98.6 (1.346) Ni(CO)3wAB 28.2 (1.151) 45.3 (1.274) 52.7 (1.384) between the two main group atoms.The appearance of the frontier orbitals is, however, very similar to those of CCH2 (see above), and we compute a nearly identical BNH2 HOMO-LUMO gap of 3.0 eV. This suggests similar coordination properties for the two ligands.But still the higher polarity of the ligand makes it potentially more reac- BNH2 tive (more sensitive, e.g. toward nucleophilic attack). There is also some experimental evidence, which indicates that may be a realistic ligand.10 In 1970, Schmid, Petz BNR2 and Noé th10a synthesized the thermolabile compound with and and, very recently, Fe(CO)4BNR2 R\CH3 C2H5 , Braunschweig and Wagner10b reported the –rst X-ray structure of a complex containing the ligand, the binu- BN(CH3)2 clear Mn2(C5H5)2(CO)4BN(CH3)2 .To test the validity of our qualitative considerations, we have carried out an extensive study in which we compare the metal-binding of CO, BF and in mononuclear (axially) BNH2 substituted, hexa- (Cr, Mn`), penta- (Fe, Co`) and tetracoordinate (Ni) as well as in binuclear (Fe, Mn) transition metal carbonyl complexes at the BP86/TZP level.3,4 The trends found for CO and BF in are reproduced in Fe(CO)4AB all the other –rst-row transition metal complexes (see Table 1).Both ligands bind well but the MwBF bond is actually 1.5»2 times stronger.And binds even better than BF. The BNH2 bond dissociation energy, for example, Cr(CO)5wABax increases from 41.8 via 62.1 to 72.1 kcal mol~1 along CO, BF and An analysis of the MwAB bonding mechanism BNH2 . furthermore shows that along this series of ligands r donation becomes increasingly important leading to a build-up of positive charge on the ligand. The balance between r donation and p backdonation is restored in binuclear metal complexes, e.g.(AB\BF, The detailed Mn2Cp2(CO)4wAB BNH2). results will be reported elsewhere. We conclude that the entity should be a superb BNR2 ligand and may well be a good supplement to CO in the design of catalytically active transition metal complexes. BF and other ligands (SiO, BO~) have very interesting properties, too, but they do not contain the potential structural features (i.e. substituents R) that with clever synthetic design might shield their reactive frontier orbitals.The problem of generating a good precursor to remains. BNR2 Acknowledgements would like to thank Dr. N. Goldberg for helpful dis- We cussions. F.M.B., U.R. and A.W.E. thank the Deutsche Forschungsgemeinschaft (DFG) for postdoctoral fellowships.The work at Cornell was supported by Research Grant CHE 94- 08455. We also thank the Cornell Theory Center (CTC) and the Netherlands Organization for Scienti–c Research (NCF/ NWO) for providing grants for supercomputer time. References 1 R. H. Crabtree, T he Organometallic Chemistry of the T ransition Metals, Wiley, New York, 2nd edn., 1994, and references therein, especially Chapters 4, 7.1, 8.1, 12 and 13. 2 H. Werner, Angew. Chem., 1990, 102, 1109; Angew. Chem., Int. Ed. Engl., 1990, 29, 1077, and references therein. 3 (a) C. Fonseca Guerra, O. Visser, J. G. Snijders, G. te Velde and E. J. Baerends, in MET ECC-95, ed. E. Clementi and G. Corongiu, STEF, Cagliari, 1995, pp. 305»395; (b) G. te Velde and E. J. Baerends, J. Comp.Phys., 1992, 99, 84; (c) A. D. Becke, Phys. Rev. 2 New J. Chem., 1998, Pages 1»3A, 1988, 38, 3098; (d) J. P. Perdew, Phys. Rev. B, 1986, 33, 8822; (e) F. M. Bickelhaupt, N. M. M. Nibbering, E. M. van Wezenbeek and E. J. Baerends, J. Phys. Chem., 1992, 96, 4864; ( f ) T. Ziegler and A. Rauk, T heor. Chim. Acta, 1977, 46, 1. 4 The and calculations were done with two Fe(CO)4AB Fe(AB)5 basis sets : TZ2P (reported in the text) and TZP (in Table 1).The other systems (Table 1) were computed with the TZP basis set only. TZ2P is a triple-f basis set of Slater-type orbitals (STOs) augmented with a set of 4p functions on Cr, Mn, Fe, Co and Ni, and a set of 3d and 4f polarization functions on main group atoms (2p and 3d on H). The TZP basis contains only one set of polarization functions per atom.The values reported in the text for the iron complexes are bond dissociation enthalpies, including zero point energies, thermal energy and PV corrections. The comparative values in the table are bond dissociation energies. 5 A leading reference is : T. A. Albright, J. K. Burdett and M.-H. Whangbo, Orbital Interactions in Chemistry, Wiley, New York, 1985, ch 6. 6 For a detailed discussion of the bonding in pentacoordinate complexes see, for example: (a) A. R. Rossi and R. HoÜmann, Inorg. Chem., 1975, 14, 365; (b) C. W. Bauschlicher and P. S. Bagus, J. Phys. Chem., 1984, 81, 5889; (c) H. P. Lué thi, P. E. M. Siegbahn and J. Almloé f, J. Phys. Chem., 1985, 89, 2156; (d) J. Li, G. Schreckenbach and T. Ziegler, J. Am. Chem. Soc., 1995, 117, 486. 7 (a) P. L. Timms, J. Am. Chem. Soc., 1968, 90, 4585; (b) R. W. Kirk, P. L. Timms, D. L. Smith and W. Airey, J. Chem. Soc., Dalton T rans., 1972, 1392. 8 See, for example: T ransition Metal Carbene Complexes, ed. K. H. Doé tz, H. Fischer, P. Hofmann, F. R. Kreissl, U. Schubert and K. Weiss, Verlag Chemie, Weinheim, Germany, 1983, p. 118. 9 For a leading reference to the chemistry of the vinylidene ligand, see : M. I. Bruce, Chem. Rev., 1991, 91, 197. 10 (a) G. Schmid, W. Petz and H. Noé th, Inorg. Chim. Acta, 1970, 4, 423; (b) H. Braunschweig and T. Wagner, Angew. Chem., 1995, 107, 904; Angew Chem., Int. Ed. Engl., 1995, 34, 825 Received 9th July, 1997; Paper 7/08295I New J. Chem., 1998, Pages 1»3 3

ISSN:1144-0546    

DOI:10.1039/a708295i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Opposing steric and electronic contributions in A OsCl2H2(PPr3 i )2 . theoretical study of an unusual structure Feliu Maseras* and Odile Eisenstein* L aboratoire de Structure et Dynamique des et Solides (L SDSMS, CNRS Syste` mes Moleç culaires UMR 5636), 15, CC 014, de Montpellier II, 34095 Montpellier, France§ Ba� t. Universiteç A theoretical study including full geometry optimization is carried out with the integrated molecular orbitals molecular mechanics (IMOMM) method on the complex.The trigonal prism coordination of OsCl2H2(PPr3 i )2 this d4 species is properly reproduced. The origin of this unusual coordination is tracked down to the ML6 balance between an electronic preference for a bicapped tetrahedral coordination and the need to relax the steric repulsions between the proximate Cl and ligands.The geometrical distortion is shown to involve a PPr3 i signi–cant destabilization of the electronic energy, thus showing the importance of steric contributions. Con—it entre facteurs steç riques et e ç lectroniques dans Une interpreç tation theç orique dœune OsCl2H2(PPr3 i )2 . structure e ç tonnante. La geç omeç trie du complexe optimiseç e a lœaide de la meç thode hybride OsCl2H2(PPr3 i )2 , chimie quantique/meç canique moleç culaire (IMOMM) donne un reç sultat en bon accord avec lœexpeç rience.On montre que la structure de type prisme trigonale resulte dœun compromis entre une structure de type tetrae` dre bicappeç preç feç reç e pour des raisons eç lectroniques et la neç cessiteç dœeç loigner les ligands Cl et On montre PPr3 i .eç galement que la structure expeç rimentale est fortement destabiliseç e au point de vue eç lectronique, ce qui rev`` le lœimportance de la contribution steç rique. Steric eÜects have traditionally been the great absentee in theoretical calculations at high computational level for transition metal complexes.1 Nowadays a very promising method for their explicit quanti–cation is emerging in the form of hybrid methods.2h4 These methods are characterized by the combination of diÜerent descriptions for diÜerent parts of the same system (such as quantum mechanics and molecular mechanics).In particular, the integrated molecular orbitals molecular mechanics (IMOMM) scheme3 has already been proven very successful in a number of cases.5h7 This paper provides another example of its performance.The complex studied here is and the main OsCl2H2(PPr3 i )2 ,8,9 novelty of this study resides in the fact that there is a large diÜerence between the geometry predicted by pure electronic eÜects and the experimental X-ray structure. This paper thus complements previous comparisons with crystal structures where only a small distortion, both in geometrical and energy terms, was occurring.6 Computational Details IMOMM calculations are performed with a program built from modi–ed versions of two standard programs: GAUSSIAN 92/DFT10 for the quantum mechanics part and mm3(92)11 for the molecular mechanics part.MO calculations are carried out on the fragment at the OsCl2H2(PH3)2 BECKE3LYP level.12 A quasi-relativistic eÜective core potential replaces the 60-electron core of the Os atom,13 as well as the 10-electron core of the P and Cl atoms.14 The basis set for Os is that associated with the pseudopotential13 with the standard valence double-f LANL2DZ contraction.10 The basis set for P and Cl is also that associated to the corresponding pseudopotentials (LANL1DZ),14 supplemented in this case by a polarization d shell.15 The basis set for the hydrogen atoms is double-f,16 supplemented with a polarization p shell in the case of hydrogen atoms directly attached to the metal atom.17 § E-mail: maseras=lsd.univ-montp2.fr; eisenst=lsd.univ-montp2.fr Molecular mechanics calculations on the full system use the mm3(92) force –eld.11 Van der OsCl2H2(PPr3 i )2 Waals parameters for the osmium atom are taken from the UFF force –eld,18 and torsional contributions involving dihedral angles with the metal atom in a terminal position are set to zero.The mm3(92) default value for the van der Waals radius of chlorine is replaced by 2.47 a value shown ”, previously7 to be more appropriate for inorganic species. All geometrical parameters are optimized in the calculation except the PwH (1.42 bond distances in the ab initio part ”) and the (1.843 bond distances in the molecular PwCsp3 ”) mechanics part. Previous Data on OsCl2H2(PPr3 i )2 The experimental X-ray geometry of is pre- OsCl2H2(PPr3 i )2 sented in Fig. 1.8,19 The arrangement of ligands around the osmium center is so unusual that it has been assigned to a variety of coordination polyhedra by diÜerent researchers.In the beginning it was viewed as a distorted square antiprism of ideal symmetry with two vacant coordination sites.8 Later D4d it has been viewed as a distorted bicapped tetrahedron and as a trigonal prism.9 In our opinion, the labeling as a trigonal prism, with the two triangular faces de–ned by atoms P2wCl4wH7 and P3wCl5wH6, seems to be the simplest one.In what follows, we are going to discuss how this trigonal prism arrangement can be obtained by distortion of the more usual bicapped tetrahedral geometry. The bicapped tetrahedron is one of the most common structures for d4 six-coordinate complexes.20 A previously published theoretical ab initio study on the model system found the bicapped tetrahedron as the sole OsCl2H2(PH3)2 stable isomer.9 The tetrahedron is de–ned by the two hydride and the two chloride ligands, with the phosphine ligands capping each of the two triangular faces de–ned by two hydrides and one chloride.As a result, the two phosphorus, the two chlorine and the osmium atoms all lie in a given plane, while the OswH bonds are in a perpendicular plane. New J. Chem., 1998, Pages 5»9 5R3P Os PR3 H H Cl Cl R3P Os PR3 H H Cl Cl q = 0.0 q q q = 45.0 Bicapped tetrahedron Trigonal prism H Os H PR3 R3P Cl Cl H Os H PR3 R3P Cl Cl Fig. 1 Experimental X-ray structure of the OsCl2H2(PPr3 i )2 complex8 as taken from the Cambridge Structural Database.19 Hydrogen atoms not directly attached to the metal are omitted for clarity The relationship between the ideal bicapped tetrahedron and the trigonal prism geometry observed in the crystal structure is illustrated in Scheme 1.A possible way to convert the bicapped tetrahedron into the trigonal prism is to rotate the ClwOswCl plane with respect to the molecular framework. The bicapped tetrahedron corresponds to the situation where the ClwOswCl plane is parallel to the PwOswP plane Scheme 1 (h\0°) while the trigonal prism corresponds to the case where the ClwOswCl plane is exactly halfway between the two perpendicular PwOswP and HwOswH planes (h\45°).Previously published molecular mechanics calculations on this system hinted at a steric origin for the discrepancy between the ab initio optimized geometry and the X-ray structure. 9 The IMOMM calculations presented in this paper aim to provide a conclusive answer to this question.Geometry Optimization Full geometry optimizations are carried out on the model system at the BECKE3LYP computational OsCl2H2(PH3)2 level and on the full system at the OsCl2H2(PPr3 i )2 IMOMM(BECKE3LYP : MM3) level. The most signi–cant parameters of the resulting geometries are summarized in Table 1. The BECKE3LYP calculation on the model system is in qualitative agreement with previous results at the RHF level.9 The optimal geometry for the model system is a bicapped tetrahedron, resulting actually OsCl2H2(PH3)2 in an overall symmetry.The dihedral angle C2v Cl4wOswXwP2 (where X is a dummy atom on the bisector of the P2wOswP3 angle) is 0.0°, that is the two chlorine atoms are contained in the P2wOswP3 plane.This situation is substantially changed in the IMOMM(BECKE3LYP : MM3) calculation on the real system The complex is no longer a bicapped OsCH2(PPr3 i )2 . Table 1 Selected geometrical parameters and degrees) of the trigonal prism X-ray (” crystallographic and IMOMM(BECKE3LYP : MM3)-optimized structures of the complex as well as of the bicapped tetrahedral BECKE3LYP- OsCl2H2(PPr3 i )2 , optimized structure of the model system X corresponds to a dummy OsCl2H2(PH3)2 .atom on the bisector of the PwOswP angle X-ray BECKE3LYP IMOMM(BECKE3LYP : MM3) OswP2 2.289 2.330 2.373 OswP3 2.304 2.330 2.384 OswCl4 2.372 2.405 2.387 OswCl5 2.383 2.404 2.395 OswH6 1.663 1.596 1.595 OswH7 1.663 1.596 1.604 XwOswP2 56.1 56.1 56.6 XwOswP3 56.1 56.1 56.6 XwOswCl4 139.9 135.9 139.2 XwOswCl5 136.5 136.0 137.8 XwOswH6 70.2 59.7 60.0 XwOswH7 79.8 59.8 60.6 P2wOswCl4 91.7 79.8 88.5 P3wOswCl5 91.8 79.8 88.5 P3wOswXwP2 180.0 180.0 180.0 Cl4wOswXwP2 41.9 0.0 35.7 Cl5wOswXwP2 229.2 180.0 220.4 H6wOswXwP2 267.5 269.9 267.5 H7wOswXwP2 95.8 89.9 86.5 6 New J.Chem., 1998, Pages 5»9tetrahedron and the Cl4wOswXwP2 dihedral angle is 35.7°, which is much closer to the experimental value of 41.9°.The overall calculated and experimental geometries are also quite similar (Fig. 1 and 2). Apart from this qualitative success, other more quantitative aspects deserve analysis. There are only minor changes in the metal»ligand bond distances when going from pure QM for the model to IMOMM for the real system, and these do not always give better agreement between calculation and experiment.The OswP distances, already 0.041 and 0.026 too ” long in the BECKE3LYP calculation, become even longer by 0.043 and 0.054 in the IMOMM(BECKE3LYP : MM3) cal- ” culation. This is probably due to a poor estimation of the electronic eÜects of the alkyl substituents in PPr3 i . Experimentally, the OswP3 bond is ca. 0.01 longer than the ” OswP2 one, seemingly indicative of a larger steric interaction for phosphine P3; this diÜerence is well-reproduced by IMOMM.For the OswCl distances, the hybrid method slightly improves on the pure MO calculation on the model system, with the deviation going from 0.032 and 0.021 to ” 0.015 and 0.012 The asymmetry between OswCl4 and ”. OswCl5 is also well-reproduced, with a longer distance for Cl5, the chloride ligand closer to P3.Finally, the OswH bond distances are practically unmodi–ed by the change of the computational method, being always ca. 0.07 shorter than the ” experimental estimation. For parameters concerning the position of hydrogen nuclei, one must recall that X-ray diÜraction does not provide an accurate localization. In the present structure, 8 the hydrides were located through electrostatic potential energy calculations.21 As a matter of fact, the calculated structure provides a con–rmation of their approximate location.It thus appears that IMOMM does not substantially change the metal»ligand distances. The same result has been Fig. 2 IMOMM(BECKE3LYP : MM3)-optimized structure of the complex. Hydrogen atoms not directly attached to OsCl2H2(PPr3 i )2 the metal are omitted for clarity previously found in other systems.6 The situation is quite different for the bond and dihedral angles. The XwOswL angles and XwOswP2wL dihedral angles of the BECKE3LYP calculation on the OsCl2H2(PH3)2 model system show the characteristic pattern of a bicapped tetrahedron.The XwOswL angle is small for and L\PH3 L\H (between 56.1 and 59.8°) and large for L\Cl (135.9 and 136.0°), and the XwOswP2wL dihedral angle values are close to 0.0, 90.0, 180.0 and 270.0°.The pattern of four small and two large XwOswL bond angles is also found in both the X-ray results and the two types of calculations. The major diÜerence is actually between the measured X-ray value and the computed (by both methods) value for the XwOswH angle. While the four computed values are between 59.7 and 60.6°, the reported experimental values are 70.2 and 79.8°.Again, the problematic experimental location of hydrogen atoms invoked above may explain this discrepancy. All the values presented in Table 1, both experimental and computed, for the XwOswP angle are within a range of 0.5° ; while values for the XwOswCl angles are slightly closer to experiment in IMOMM(BECKE3LYP : MM3) than in BECKE3LYP, within a range of 4.0°.More signi–cant diÜerences appear in the P2wOswCl4 and P3wOswCl5 bond angles. The X-ray values (91.7, 91.8°) are larger by ca. 12.0° than the BECKE3LYP values (79.8, 79.8°). This discrepancy is substantially corrected by the IMOMM(BECKE3LYP : MM3) calculation, with values of 88.5 and 88.8°.This is a –rst hint at the importance of the steric repulsion between the phosphine and chloride ligands on the structure of this complex, an aspect that will be analyzed in more detail below. Comparison of the three sets of dihedral angles LwOswXwP2 presented in Table 1 provides the de–nite proof of the qualitative and quantitative improvement of IMOMM(BECKE3LYP : MM3) on BECKE3LYP.Some of these dihedral angles have similar values in the three cases : P3wOswXwP2 is always 180.0° by the de–nition of X, H6wOswXwP2 is always ca. 270.0°, H7wOswXwP2 is always ca. 90.0°. The most impressive change is associated with the ClwOswXwP2 dihedral angles. Cl4wOswXwP2 is 41.9° experimentally, 0.0° in BECKE3LYP, and 35.7° in IMOMM(BECKE3LYP : MM3).Similarly, the values for Cl5wOswXwP2 are 229.2, 180.0 and 220.4°, respectively. Thus, the diÜerences between calculated and experimental values, which amount to 41.9° and 49.2° for MO calculations on the model system, go down to 6.2° and 8.8° with the IMOMM method. IMOMM can thus reduce the diÜerence between computation and experiment for certain geometrical parameters to less than 20% of that found with MO calculations on the model system, while keeping the computational time almost the same.Quanti–cation of Steric EÜects in Terms of Energy Although IMOMM was essentially devised as a method for the calculation of accurate geometries and energetics at a low computational price, it can also be used as a tool for the quanti–cation of steric eÜects in terms of energy.In order to do this, the scheme previously outlined in detail by Barea et al.6 is used. The main idea is to compare the results of the full IMOMM optimization with those of a restricted IMOMM optimization. In the restricted optimization, the positions of the atoms linked to the metal are frozen in the geometry obtained from the pure ab initio optimization of the model system.The restricted IMOMM optimization yields a total energy 11.20 kcal mol~1 higher than the full IMOMM optimization. This energy value can be assigned to the geometry distortion caused by the steric eÜects, and can be further decomposed into ì electronic œ (i.e., quantum mechanics) and ì steric œ (i.e., molecular mechanics) contributions. A steric gain of 18.98 kcal New J.Chem., 1998, Pages 5»9 7Table 2 Decomposition of the molecular mechanics part of the IMOMM energy (kcal mol~1) of complex in the OsCl2H2(PPr3 i )2 complete and restricted geometry optimizations Complete Restricted DiÜerence Compression 1.19 1.46 0.27 Bending 10.24 14.35 4.11 Bend-bend 0.21 0.38 0.17 Stretch-bend 0.09 0.14 0.05 VdW 25.33 37.76 12.43 Torsional [13.4 [11.78 1.96 Torsion-stretch [0.06 [0.12 [0.06 Dipole-dipole 0.36 0.41 0.05 Total 23.62 42.60 18.98 mol~1 is partially compensated by an electronic loss of 7.78 kcal mol~1. The fact that the restricted optimization has a more stable electronic energy is to be expected, since the model system corresponds to the optimal geometrical arrangement for the electronic eÜects.The magnitude of energy diÜerences involved with the distortion of this system is almost twice as large as that found in previous studies.6 This proves the ability of IMOMM to perform well even in cases where large energy, costly distortions are involved.It also provides a quantitative proof of how steric eÜects can alter the electronic properties of a transition metal complex. Steric eÜects push the complex to a geometry where OsCl2H2(PPr3 i )2 its electronic energy is 7.78 kcal mol~1 higher than in its electronically most stable structure.Such an energy diÜerence means that such a structure could not be easily reached in the absence of steric eÜects. These substantial steric eÜects may also aÜect its reactivity.9 The molecular mechanics contribution to the total energy can be further divided into its diÜerent components, as shown in Table 2.The main contribution (12.43 kcal mol~1) to the total MM diÜerence of 18.98 kcal mol~1 is clearly in the van der Waals interactions (ìvdWœ term). This is not surprising since one expects the MM3 force –eld to put the steric repulsions in this term. Since the van der Waals term is nothing other than a summation of single interatomic interactions, it is instructive to analyze the origin of the diÜerence between the complete and restricted optimized geometries.The main difficulty in this analysis is the large number (2147) of interactions. A –lter is thus used to select the most important of these. Only those diÜering by more than a certain threshold between the two IMOMM calculations are considered.By setting this threshold at 0.05 kcal mol~1, only 80 out of the 2147 interactions remain. These few interactions (4% of the total) already account for 11.95 kcal mol~1 of the total van der Waals interaction energy of 12.43 kcal mol~1 (i.e., 97% of the total energy). These remaining 80 interactions can be easily grouped by ligand, resulting in the numbers collected in Table 3.Several data in Table 3 deserve comment. In the –rst place, the two largest terms correspond to phosphine»chloride repulsions : P3»Cl5 at 3.76 and P2»Cl4 at 2.94 kcal mol~1. These Table 3 Classi–cation by ligand of the interatomic van der Waals interactions (kcal mol~1) that change by more than 0.05 kcal mol~1 between the complete and restricted IMOMM geometry optimizations of complex OsCl2H2(PPr3 i )2 P2 P3 P2 1.36 P3 2.27 1.59 Cl4 2.94 0.00 Cl5 0.05 3.76 H6 [0.15 0.52 H7 0.11 [0.50 steric repulsions con–rm their leading role in the geometry distortion, a role that was already hinted at by the geometrical features discussed above, with the PwOswCl bond angles increasing by 9°.It is, however, also worth noting the importance of the P2-P3 term.The phosphine»phosphine repulsion is computed to be 2.27 kcal mol~1, despite the fact that the P2wOswP3 bond angle only increases from 112.2° to 113.2° when going from restricted to full optimization. The likely explanation is that the relaxation of the repulsion between the two phosphine ligands is not related merely to the P2wOswP3 bond angle, but also to the arrangement of the alkyl substituents, which are compressed by the presence of the chloride substituents in the restricted optimization.This hypothesis is actually con–rmed by the importance of the intraligand P2-P2 (1.36 kcal mol~1) and P3-P3 (1.59 kcal mol~1) terms, representing the rearrangement of the steric repulsions within each ligand. The other terms are smaller in magnitude. The P3 ligand seems to move away from H7 towards H6, since it diminishes its repulsion with H7 ([0.50 kcal mol~1) almost by the same amount that it increases its repulsion with H6 (]0.52 kcal mol~1).The same happens with P2, although to a lesser extent ([0.15 vs. 0.11 kcal mol~1) and in the opposite direction (from H6 towards H7). The interactions between the chloride and the hydride ligands are required to be zero by the computational method, since there is no MM contribution between atoms within the model system.A last comment to be made from Table 3 is the lack of symmetry of the ligand»ligand repulsions. In fact, ligand P3 seems to have noticeably larger repulsions than P2. Although this behavior may not be easily foreseen from the X-ray structure, it is by no means in contradiction with it.The complex is not symmetrical (OswP3 is longer than OswP2 by 0.015 ”) as can be readily seen from Fig. 1 and Table 1, a result wellreproduced in the IMOMM calculation. This asymmetry in the OswP distances can therefore be tracked down to the ligand»ligand repulsions just discussed. Conclusions Theoretical calculations with the integrated molecular orbitals molecular mechanics (IMOMM) method reproduce properly the uncommon trigonal prism X-ray structure of the complex and allow an understanding of its OsCl2H2(PPr3 i )2 origin. Pure molecular orbital reasonings predict a bicapped tetrahedron structure that is actually obtained in BECKE3LYP calculations on the model system The small bond angle between the adjacent OsCl2H2(PH3)2 .phosphine and chloride ligands creates a strong steric repulsion that is relaxed by moving the chloride ligands away from their electronically most favored arrangement within the PwOswP plane.This steric repulsion is, however, insufficient to fully overcome the strong trans eÜect of the hydride ligands, and as a result the chloride ligands stay between the PwOswP and the HwOswH planes, giving rise to the trigonal prism structure experimentally observed.IMOMM is thus shown to be able to reproduce in a very satisfactory way the geometry resulting from this balance between large electronic and steric eÜects, and to allow an analysis of the origin of the steric eÜects. Acknowledgements M. thanks the CNRS for a position of Research F. Associate. References 1 (a) A.Veillard, Chem. Rev., 1991, 91, 743. (b) N. Koga and K. Morokuma, Chem. Rev., 1991, 91, 823. 2 (a) A. Warshel and M. Karplus, J. Am Chem. Soc., 1972, 94, 5612. (b) M. J. Field, P. A. Bash and M. Karplus, J. Comput. Chem., 8 New J. Chem., 1998, Pages 5»91990, 11, 700. (c) J. Sauer, Chem. Rev., 1989, 89, 199. (d) U. C. Singh and P. A. Kollman, J. Comput. Chem., 1986, 7, 718.(e) R. V. Stanton, D. S. Hartsough and K. M. Merz, Jr., J. Comput. Chem., 1995, 16, 113. ( f ) J. Gao, Acc. Chem. Res., 1996, 29, 298. (g) M. Noland, E. L. Coitin8 o and D. G. Truhlar, J. Phys. Chem. A, 1997, 101, 1194. (h) M. Strnad, M. T. C. Martins-Costa, C. Millot, I. Tun8 oç n, M. F. Ruiz-Loç pez, J. L. Rivail, J. Chem. Phys., 1997, 106, 3643. 3 F. Maseras and K. Morokuma, J.Comput. Chem., 1995, 16, 1170. 4 (a) S. Humbel, S. Sieber and K. Morokuma, J. Chem. Phys., 1996, 105, 1959. (b) M. Svensson, S. Humbel, R. Froese, T. Matsubara, S. Sieber and K. Morokuma, J. Phys. Chem., 1996, 100, 19357. (c) E. L. Coitin8 o, D. G. Truhlar and K. Morokuma, Chem. Phys. L ett., 1996, 259, 159. 5 (a) T. Matsubara, F. Maseras, N. Koga and K. Morokuma, J. Phys.Chem., 1996, 100, 2573. (b) G. Ujaque, F. Maseras and A. Lledoç s, T heor. Chim. Acta, 1996, 94, 67. (c) M. Svensson, S. Humbel and K. Morokuma, J. Chem. Phys., 1996, 105, 3654. (d) T. Matsubara, S. Sieber and K. Morokuma, Int. J. Quantum Chem., 1996, 60, 1101. (e) R. D. J. Froese and K. Morokuma, Chem. Phys. L ett., 1996, 263, 393. ( f ) Y. Wakatsuki, N. Koga, H. Werner and K.Morokuma, J. Am. Chem. Soc., 1997, 119, 360. (g) M. Ogasawara. F. Maseras, N. Gallego-Planas, K. Kawamura, K. Ito, K. Toyota, W. E. Streib, S. Komiya, O. Eisenstein and K. G. Caulton, Organometallics, 1997, 16, 1979. 6 G. Barea, F. Maseras, Y. Jean and A. Lledoç s, Inorg. Chem., 1996, 35, 6401. 7 G. Ujaque, F. Maseras and O. Eisenstein, T heor. Chem. Acc., 1997, 96, 146. 8 M. Aracama, M.A. Esteruelas, F. J. Lahoz, J. A. Lopez, U. Meyer, L. A. Oro and H. Werner, Inorg. Chem., 1991, 30, 288. 9 D. G. Gusev, R. Kuhlman, J. R. Rambo, H. Berke, O. Eisenstein and K. G. Caulton, J. Am. Chem. Soc., 1995, 117, 281. 10 M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. W. Wong, J. B. Foresman, M. A. Robb, M. Head- Gordon, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart and J. A. Pople, Gaussian 92/DFT , Gaussian, Pittsburgh, PA, 1993. 11 N. L. Allinger, mm3(92), QCPE, Bloomington, IN, 1992. 12 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648. (b) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785. (c) P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys. Chem., 1994, 98, 11623. 13 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 99. 14 W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985, 82, 284. 15 M. M. Francl, W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S. Gordon, D. J. DeFrees and J. A. Pople, J. Chem. Phys., 1982, 77, 3654. 16 W. J. Hehre, R. Ditch–eld and J. A. Pople, J. Chem. Phys., 1972, 56, 2257. 17 P. C. Hariharan and J. A. Pople, T heor. Chim. Acta, 1973, 28, 213. 18 A. K. Rappeç , C. J. Casewit, K. S. Colwell, W. A. Goddard, III and W. M. SkiÜ, J. Am. Chem. Soc., 1992, 114, 10024. 19 F. H. Allen and O. Kennard, Chemical Design Automation News, 1993, 8, 31. 20 (a) R. HoÜmann, J. M. Howell and A. R. Rossi, J. Am. Chem. Soc., 1976, 98, 2484. (b) P. Kubaç có ek and R. HoÜman, J. Am. Chem. Soc., 1981, 103, 4320. 21 G. A. Orpen, J. Chem. Soc., Dalton T rans., 1980, 2509. Received 24th April 1997; Paper 7/06748H New J. Chem., 1998, Pages 5»9 9

ISSN:1144-0546    

DOI:10.1039/a706748h

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Conformational —exibility within the chelate rings of [Pt(en)(CBDCA-O,Oº) ] , an analogue of the antitumour drug carboplatin : X-ray crystallographic and solid-state NMR studies Zijian Guo,a Abraha Habtemariam,a Peter J. Sadler,*a Rex Palmerb and Brian S. Potterb a Department of Chemistry, University of Edinburgh, Kingœs Buildings, W est Mains Road, Edinburgh, UK EH9 3JJ b Department of Crystallography, Birkbeck College, University of L ondon, Malet Street, L ondon, UK W C1E 7HX The X-ray crystal structure of [Pt(en)(CBDCA-O,O@)] 1, an analogue of the anticancer drug carboplatin, shows that platinum has an approximate square-planar coordination.The crystals are orthorhombic with space group Pnma. The Pt-CBDCA chelate ring adopts a —attened-boat conformation, similar to that found for carboplatin, and the Pt-ethylenediamine chelate ring exhibits both d and k conformations with equal populations.Ethylenediamine chelate ring inversion was observed by 13C CP/MAS NMR spectroscopy. The exchange rate and the activation free energy (*Gt) at 317 K were determined to be ca. 415 s~1 and 62 kJ mol~1, respectively. The CBDCA ligand appears to have a direct eÜect on the dynamics of the en ring.Conformational —exibility of the CBDCA ring is also discussed. Such dynamic processes within chelated platinum complexes could play a role in the biological recognition of anticancer complexes. Several side eÜects of cisplatin, such as nephro- and neurotoxicity, have led to the search for the second generation of platinum drugs that circumvent these problems. Such eÜorts have led to the discovery of carboplatin.1 The replacement of the chloride ligand by chelated cyclobutane dicarboxylate maintains the high antitumour activity but greatly lowers the toxicity.There are a large number of clinical trials in progress on analogues that contain chelated diamine ligands and/or chelated dicarboxylates.2 These may recognize biological targets such as DNA by a diÜerent mechanism from cisplatin.In particular, chelated complexes may not hydrolyse before reacting with DNA bases,3 and initial second-sphere interactions may therefore direct the complex to selective regions of DNA. Outer-sphere recognition probably also plays an important role in dictating the target site of cisplatin.4 It is notable that Natile and coworkers have elegantly illustrated that stereospeci–c substitution in diamino chelate rings can have a major in—uence on nucleotide recognition at the trans positions.5 Conformational —exibility within the coordination sphere of platinum complexes is therefore likely to be an important feature in drug design.In this work we have studied dynamic processes within the bis-chelated complex [Pt(en)(CBDCA)] 1 by X-ray crystallography and solid-state NMR spectroscopy, and compared its behaviour with that of carboplatin,6,7 and some related PtII and PdII complexes. Experimental Cyclobutane-1,1@-dicarboxylic acid was pur- (H2CBDCA) chased from Sigma, and from K2PtCl4, LiOH ÆH2O AgNO3 Johnson Matthey, ethylenediamine (en) and other chemicals from Aldrich.and [Pt(en)Cl2], [Pt(en)I2], [Pd(en)2][PdCl4] (carboplatin) were prepared according [Pt(CBDCA)(NH3)2] to literature methods.8,9 [Pt(en)(CBDCA)] 1 was prepared as follows. A suspension of (127.8 mg, 0.39 mmol) and 2 mol equiv of [Pt(en)Cl2] in (2 mL) was stirred for 24 h. The AgCl was AgNO3 H2O –ltered oÜ. To the –ltrate, (56.5 mg, 0.39 mmol) H2CBDCA * Fax ](44) 131 650 6452.E-mail: P.J.Sadler=ed.ac.uk and 2 mol equiv of were added (the pH of the LiOH ÆH2O solution was ca. 6.4). Crystals of 1 were obtained by slow evaporation of the above solution and used for the X-ray diffraction and solid-state NMR studies. Elemental analysis of 1: Calcd (%) for C 24.19 ; H 3.55 ; N 7.05. C8H14N2O4Pt: Found: C 24.28 ; H 3.44 ; N 7.07. X-Ray crystallography The X-ray crystallographic analysis showed that crystals of complex 1 were orthorhombic with space group Pnma.Crystal data and re–nement data are listed in Table 1. Intensity data were measured at room temperature on a Nonius CAD4 diÜractometer in the x/2h scanning mode, Table 1 Crystal data and structure re–nement for [Pt(en)(CBDCAO, O@)] Empirical formula C8H14N2O4Pt Formula weight 397.30 Temperature 291^2 K Wavelength 1.54178 ” Crystal system Orthorhombic Space group Pnma Unit cell dimensions a\8.660 (3) ” a\90° b\9.614 (3) ” b\90° c\12.713 (5) ” c\90° Volume 1058.4 (6) ”3 Z 4 Density (calculated) 2.493 g cm~3 Absorption coef–cient 24.825 mm~1 F(000) 744 Crystal size 0.2]0.3]0.2 mm h range for data collection 5.77° to 71.87° Index ranges 0OhO10, 0Ok\11, [15OlO15 Re—ections collected 2661 Independent re—ections 1103 [R(int)\0.1194] Re–nement method Full-matrix least-squares on F2 Data/restraints/parameters 1054/4/78 Goodness-of-–t on F2 1.130 Final R indices [I[2r(I)] R1\0.0614, W r\0.1691 R indices (all data) R1\0.0771, W r2\0.2225 Extinction coef–cient 0.002 (1) Largest diÜ.peak and hole 4.451 and [4.808 e ”~3 New J. Chem., 1998, Pages 11»14 11employing graphite monochromated CuKa radiation.Two asymmetric units of data were recorded to 72° h. The crystals were weakly diÜracting and suÜered from statistical disorder, factors which aÜected the quality of the X-ray data measurements. The structure was solved using the heavy atom method, employing the program SHELXS-86.10 In addition to the usual Lorentz and polarization corrections, absorption corrections were calculated using the DIFABS method.11 The absorption corrections ranged from 0.678 to 1.832 with an average value of 0.964.The structure was re–ned by fullmatrix least-squares on oF2 o with SHELXL-93.12 Anisotropic displacement parameters were used for the non-H atoms and isotropic ones for the H atoms in the geometrically –xed, riding mode. Re–nement of the structure in the related space group was tried and proved to be unsatisfactory.Pn21a, Solid-state NMR 13C solid-state NMR spectra were recorded on a Bruker MSL-300 NMR spectrometer at 75 MHz, using cross polarization, proton decoupling and magic-angle spinning (CP/ MAS). The sample was inserted into a 7 mm diameter sample rotor. Spinning speeds of 4.5 to 5.0 kHz were employed, and a 1 ms contact time was used.Acquisition times were 30»40 ms and the recycle time between scans was 3 to 8 s. Typically the 90° pulse length for 1H was 4.5 ms and usually 2000 to 8000 scans were acquired. The chemical shifts are externally referenced to liquid TMS (0.0 ppm). Solution NMR Solution 13CM1HN NMR spectra were acquired on a JEOL GSX-270 NMR spectrometer at 67.5 MHz, using 16 K data points, relaxation delay of 2 s, and 4000 to 8000 transients. Tubes of 5 and 10 mm diameter were used.The chemical shifts are referenced internally to dioxane (67.3 ppm). Results and Discussion Crystal structure of [Pt(en)(CBDCA)] 1 The X-ray crystal structure of 1 is shown in Fig. 1(a) and (b). The atomic coordinates are listed in Table 2 and selected bond distances and angles of the molecule are shown in Table Fig. 1 View of [Pt(en)(CBDCA-O,O@)] 1, showing the numbering scheme with the H atoms as spheres of arbitrary diameter. (a) A view perpendicular to the least-squares plane; (b) view perpendicular to the CBDCA plane Table 2 Atomic coordinates (]104) for [Pt(en)(CBDCA-O,O@)] x y z Pt 1149(1) 2500 435(1) O3 1943(12) 4002(9) [531(5) O5 3316(13) 4710(13) [1854(8) N1 168(9) 3908(9) 1411(5) C2A* [1047(32) 3124(34) 2050(24) C2B* [291(25) 1806(19) 2365(15) C4 3051(13) 3810(13) [1188(7) C6 4027(14) 2500 [1089(10) C7 5463(15) 2500 [1814(11) C8 6487(25) 2500 [825(14) C9 5068(21) 2500 [104(14) * Occupancy 0.5. 3. As expected, the platinum atom is square-planar and coordinated to two bidentate ligands.The crystal contains the monomeric molecule, which lies across the crystallographic mirror perpendicular to the b axis. Platinum and the en ring atoms C6, C7, C8, C9 lie exactly in the mirror plane. The en ring is thus constrained to be exactly planar. Atoms O3, C4, O5 and N1 and their mirror-related counterparts complete the molecule, which thus exhibits mirror symmetry. Atom (Cs) C2 is disordered in two sites, C2A and C2B, with a distance of 0.77 apart, giving rise to two conformers, related by re—ec- ” tion across the mirror plane.The crystallographic space group is thus satis–ed statistically by averaging the two conformers. The N1wC2A and C2AwC2B bond distances for the disordered en ring were constrained to the values shown in Table 3. The Pt-CBDCA chelate ring also possesses exact symmetry Cs in a —attened boat conformation.There are no unusual features in the bond lengths and angles of the molecule. The PtwN bond distance of 2.023(7) is very similar to ” values reported for other Pt-ethylenediamine compounds such as dichloroethylenediamineplatinum(II) (2.034 ”),13 chloro(ethylenediamine)M([)-2,3,5,6-tetrahydro-6-phenylimidazo[ 2,1-b]thiazoleNplatinum(II) chloride (2.024 chloro- ”),14 uracil-ethylenediamineplatinum(II) chloride (2.03 trans- ”),15 S,S-[N,N@-bis(2-hydroxyethyl)ethylenediamine(oxalato-O,O@)- platinum(II)] (2.025 ”). The PtwO distance [2.017(8) is also consistent with ”] those found in Pt-bicarboxylate complexes such as carbo- Table 3 Selected bond length and angles (°) for [Pt(en)(CBDCA- (”) O,O@)] C6wC7 1.548(9) C4wC6 1.522(14) C7wC8 1.538(10) C4wO5 1.232(14) C8wC9 1.533(10) PtwN1 2.023(7) C6wC9 1.543(9) N1wC2A 1.48(3) PtwO3 2.017(8) C2AwC2B 1.54(3) O3wC4 1.285(13) C9wC6wC7 90.8(12) C4@wC6wC4 111.7(11) C8wC7wC6 88.7(13) O5wC4wO3 119.0(11) C9wC8wC7 91.5(13) O5wC4wC6 122.3(10) C8wC9wC6 89.0(13) O3@wPtwN1 174.7(4) C4@wC6wC7 113.4(7) O3wPtwN1 92.2(3) C4wC6wC7 113.4(7) C4@wC6wC9 113.0(7) N1wPtwN1@ 84.0(5) C4wC6wC7 113.0(7) C2AwN1wPt 106.6(12) C2BwC2AwN1 105(2) O3@wPtwO3 91.5(5) N1@wC2BwC2A 107(2) C4wO3wPt 123.2(8) C2B@wN1wPt 108.2(9) O3wC4wC6 118.7(9) Ring (PtwO3wC4wC6wC4@wO3@) and ring (PtwN1wC2Aw C2BwN1@) are completed by the operation of m perpendicular to b. 12 New J. Chem., 1998, Pages 11»14platin [(2.029 and (2.025 [Pt(NH3)2(CBDCA-O,O@)] ”)6 ”)7] trans-S,S-[N,N@-bis(2-hydroxyethyl)ethylenediamine(oxalato- O,O@)platinum(II)] (2.039 cis-1R,2R-cyclohexanediamine- ”),16 N,N@-oxalatoplatinum(II) (2.01, 2.04 Pt(malonato-O,O@)- ”),17 cis-1R, 2R- cyclohexanediamine -N,N@ - malonatoplatinum(II) (2.02 potassium anti-bis(2-methylmalonate)platinum(II) ”),17 dihydrate (2.01, 2.00 and potassium dichloro- ”),18 (oxalato)platinate(II) hydrate (2.04, 2.03 ”).18 The O3wPtwO3@ angle [91.5(5)°] is typical for a sixmembered chelate ring in a PtII-bicarboxylate complex.6,7,17,18 The six-membered Pt-CBDCA ring adopts a —attened boat conformation which is similar to that observed in carboplatin crystals.6,7 The N1wPtwN1@ [84.0(5)°] angle has also a normal value for a –ve-membered ethylenediamine chelate ring.The Pt-en chelate ring exhibits both d and l twisted sites C2A and C2B in the alternative conformations (d and k), and both conformations were equally populated in the crystal, with N(1)wCwCwN(1@) torsion angles of 58.7(8)° (d conformer) and [58.7(8)° (k conformer), similar to those reported for other Pt-ethylenediamine complexes.16 CCDC reference number 440/001.NMR studies of [Pt(en)(CBDCA)] 1 The 13CM1HN NMR solution spectrum of 1 at 298 K contains –ve signals that can be assigned to the ring carbons C8 (16.0 ppm), C7 and C9 (31.7 ppm), C6 (56.9 ppm), the carboxyl carbons C4 and C4@ (182.3 ppm) of CBDCA, and the methylene carbons C2A and C2B (48.7 ppm) of ethylenediamine.Similar 13C chemical shifts for the CBDCA ligand have been reported for carboplatin.6 Despite the inequivalence of the cyclobutane ring carbons in the solid-state structure of 1, only one resonance was seen for C7 and C9, showing that rapid ring inversion occurs as for carboplatin.6 Similarly, the d and k conformations of the Pt-ethylenediamine ring are in rapid exchange, only one set of signals being observed for the methylene carbons.The solid-state 13C CP/MAS NMR spectrum of complex 1 at 297 K is shown in Fig. 2(a). Now two broad signals are observed at 49.6 and 50.3 ppm, which are assignable to the en carbons. On increasing the temperature, the two signals CH2 became closer, began to merge at 317 K, and gave rise to a single sharp signal at 390 K [Fig. 2(b), peak c]. Based on the assumption that the two carbon signals are well separated at 297 K Hz), and two-site exchange with equal (*t1@2\186 populations,19 the exchange rate constant was determined to be 415 s~1.At the coalescence temperature of ca. 317 K, the activation energy *Gt was calculated to be 62 kJ mol~1. The energy barriers for this conformational change in an unsubstituted –ve-membered chelate ring, with a metal» nitrogen bond length of 2.0 have been estimated by strain- ”, energy minimization calculations to be 20 kJ mol~1 in solution.20 In the solid state, the energy barrier would be expected to be much higher.Because there are very limited solid-state NMR data available for ethylenediamine complexes, we also studied the 13C NMR spectra of two other platinum(II) and palladium(II) ethylenediamine complexes at 297 K. Only one signal was observed for the two ring carbons of both (50.1 [Pt(en)I2] ppm) and (46.5 ppm).We have recently [Pd(en)2][PdCl4] reported solid-state NMR data for the outer-sphere macrochelate complexes [Pt(en)(5@-GMP) (49.3 ppm) and 2] [Pd(en)(5@-GMP) (48.9 ppm), where again the ethyl- 2] enediamine ligand also gave rise to only one signal.21 Therefore it appears that the slower exchange rate in complex 1 is due to the presence of the CBDCA chelate ring.The other –ve major peaks observed in the solid-state 13C NMR spectrum of 1 [Fig 2(a)] can be assigned to C8 (16.8 ppm), C6 (56.0 ppm), C7 (27.5 ppm), C9 (35.9 ppm) and C4 Fig. 2 The 13C CP/MAS spectra of [Pt(en)(CBDCA-O,O@)] 1 at (a) 297 K and (b) 390 K. Peaks h and j are assigned to the groups of CH2 ethylenediamine and coalesce to give peak c ; other assignments are : a, C8; f and g, C7 and C9; d, C6; e, C4 and C4@.The * denotes minor peaks which may due to a diÜerent conformation of the CBDCA ring (see text) and C4@ (181.3 ppm) of the CBDCA ligand. A series of minor signals were also observed [denoted by * in Fig. 2(a)]. These signals decreased in intensity or disappeared at 390 K [Fig. 2(b)]. From their chemical shifts and temperature dependences, they can tentatively be assigned to a minor alternative conformation of the CBDCA ligand. Further support for the presence of conformational —exibility within the CBDCA ligand of 1 comes from the observation that the cyclobutane ring was not perfectly located in the X-ray diÜraction electron density map. Similarly, in crystalline carboplatin the CBDCA ring also exhibits substantial motion at room temperature, which has been related to the dynamic puckering between two conformations.7 The potential barrier was estimated to be below 6 kJ mol~1.We recorded the solid-state 13C CP/MAS NMR spectrum of crystalline carboplatin at 298 K, but only –ve signals were observed at 16.4, 28.1, 35.3, 56.5 and 182.0 ppm, which can be assigned to the –ve magnetically non-equivalent types of carbon atoms in the CBDCA ligand (four cyclobutane resonances, one peak for the two carboxylates).The shifts are very similar to those of 1 [Fig. 2(a)]. Since no minor signals were observed in the spectrum, it can be concluded that the energy barrier for the dynamic puckering is lower than for 1. Taken together, it seems clear that two chelate rings in bis(chelated)-PtII complexes exhibit interactive conformational dynamics.Such eÜects may be important to molecular recognition processes involving platinum drugs and may be relevant to the design of anticancer drugs. Acknowledgements thank the Association for International Cancer Research, We BBSRC for their support for this work, Dr.P. J. Barrie (University College London) for recording the solid-state NMR spectra and for helpful discussions, and University of London Intercollegiate Research Service for the provision of NMR facilities. We thank the EC support of COST group D1/0002/92. New J. Chem., 1998, Pages 11»14 13References 1 Platinum and Other Metal Coordination Compounds in Cancer Chemotherapy 2, ed.H. M. Pinedo and J. H. Schornagel, Plenum, New York, 1996. 2 J. Reedijk, J. Chem. Soc., Chem. Commun., 1996, 801. 3 U. Frey, J. D. Ranford and P. J. Sadler, Inorg. Chem., 1993, 32, 1333. 4 S. K. C. Elmroth and S. J. Lippard, Inorg. Chem., 1995, 34, 5234. 5 D. Kiser, F. P. Intini, Y. H. Xu, G. Natile and L. G. Marzilli, Inorg. Chem., 1994, 33, 4149. 6 S. Neidle, I. M. Ismail and P. J.Sadler, J. Inorg. Biochem., 1980, 13, 205. 7 B. Beagley, D. W. J. Cruickshank, C. A. McAuliÜe, R. G. Pritchard and A. M. Zaki, J. Mol. Struct., 1985, 130, 97. 8 G. L. Johnson, Inorg. Synth., 1966, 8, 242. 9 R. C. Harrison, C. A. McAuliÜe and A. M. Zaki, Inorg. Chim. Acta, 1980, 46, 205. 10 G. M. Sheldrick, SHEL XS-86, University of Goé ttingen, Germany, 1986. 11 N. Walker and D. Stuart, Acta Crystallogr., Sect. A, 1983, 39, 158. 12 G. M. Sheldrick, SHEL XL -93, University of Goé ttingen, Germany, 1993. 13 L. T. Ellis and T. W. Hambley, Acta Crystallogr., Sect. C, 1994, 50, 1888. 14 G. M. Arvanitis, M. E. Berardini, G. N. Parkinson and B. S. Schneider, Acta Crystallogr., Sect. C, 1993, 49, 1246. 15 R. Faggiani, B. Lippert and C. J. L. Lock, Inorg. Chem. 1980, 19, 295. 16 Q. Xu, A. R. Khokhar and J. L. Bear, Inorg. Chim. Acta, 1990, 178, 107. 17 M. A. Bruck, R. Bau, M. Boji, K. Inagaki and Y. Kidani, Inorg. Chim. Acta, 1984, 92, 279. 18 S. O. Dunham, R. D. Larsen and E. H. Abbott, Inorg. Chem., 1991, 30, 4328. 19 H. Friebolin, Basic One- and T wo-Dimensional NMR Spectroscopy, VCH, Weinheim, 1991, pp. 267»274. 20 C. J. Hawkins and J. A. Palmer, Coord. Chem. Rev., 1982, 44, 1. 21 K. J. Barnham, C. J. Bauer, M. I. Djuran, M. A. Mazid, T. Rau and P. J. Sadler, Inorg. Chem., 1995, 34, 2826. Received 12th May 1997; Paper 7/06739I 14 New J. Chem., 1998, Pages 11»14

ISSN:1144-0546    

DOI:10.1039/a706739i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Transition-metal derivatives of the functionalized cyclopentadienyl ligand. XVI. Synthesis of the bridged complexes [ (l-g5- (M = Cr, Mo, W). X-Ray crystal structure of C5H4PPh2)M(CO)2 ] 2 the dihydride derivative [ (l-g5-C5H4PPh2)W(CO)2H] 2 Brigitte Brumas-Soula, Dahan and Reneç Poilblanc* Franc” oise L aboratoire de chimie de coordination du CNRS,§ 205 route de Narbonne, 31077 T oulouse cedex, France Four synthetical methods, implying basically the oxidation of the anionic species [(g5-C5H4PPh2)M(CO)3]~ [M\Cr, (2a~), Mo (2b~), W (2c~)] to produce the new homobimetallic derivatives [(l,g5- (MwM), [M\Cr (1a), Mo (1b), W (1c)] of the heterodifunctional C5H4PPh2)M(CO)2]2 diphenylphosphinocyclopentadienyl bridging ligand, have been investigated.The –rst approach proceeds in two steps : thus the electrochemical oxidation of the complexes 2a~and 2b~ leads to the metal»metal bonded dimetallic complexes [(g5- (MwM), [M\Cr (5a), Mo (5b)] ; the irradiation of these C5H4PPh2)M(CO)3]2 complexes 5a and 5b with a high-pressure Hg lamp aÜords the corresponding decarbonylated bridged complexes 1a and 1b.The second method, using silver tetra—uoroborate as the oxidant of the anions 2a~ to 2c~, leads to the formation of tetrametallic cyclic complexes of silver and Group 6 transition metals [(l-g5- [M\Cr (6a), Mo (6b), W (6c)] but the splitting of these compounds into bimetallic C5H4PPh2[M(CO)3Ag]2 , complexes 1a»c and metallic silver appears neither easy nor selective.As a third procedure, the hydrido complexes (g5- [M\Cr (3a), Mo (3b), W (3c)] are irradiated with a high-pressure Hg C5H4PPh2)M(CO)3H lamp.This procedure is useful to prepare 1b but is non-selective in the two other cases, aÜording mainly bimetallic dihydrido-bridged complexes [(l-g5- [M\Mo (7b), W (7c)] and 1a or 1b, as C5H4PPh2)M(CO)2H]2 a result of the expected competition between the dehydrogenation and the decarbonylation processes. The X-ray molecular structure of 7c points out the transoïé d disposition of the hydrido ligands, which could well be a factor of its inertness in a spontaneous dehydrogenation process towards 1c.Finally, the most efficient method requires the preliminary preparation of the iodo complexes (g5- [M\Cr (4a), Mo (4b), W (4c)], C5H4PPh2)M(CO)3I which are reacted in toluene with their anionic parents 2~.This last method is particularly useful for preparing 1a and 1c. Complexes des meç taux de transition avec les ligands cyclopentadienyles fonctionnaliseç s. XVI. Synthe` se des complexes dimeç talliques (M = Cr, Mo, W). Structure moleç culaire du deriveç [ (l-g5-C5H4PPh2)M(CO)2 ] 2 dihydrure dœacceç der a` la seç rie des complexes homobimeç talliques ponteç s [ (l-g5-C5H4PPh2)W(CO)2H] 2 .A–n [(l-g5- (MwM), [M\Cr (1a), Mo (1b), W (1c)] du ligand heç teç rodifonctionnel pontant C5H4PPh2)M(CO2)]2 dipheç nylphosphinocyclopentadienyle, quatre possibiliteç s de preç parations ontç eteç eç tudieç es. Elles impliquent fondamentalement lœoxydation des anions [(g5- [M\Cr (2a~), Mo (2b~), W (2c~)]. La C5H4PPh2)M(CO)3]~ premie` re approche met dœabord en jeu lœoxydation eç lectrochimique de 2a~ et 2b~ conduisant aux complexes a` liaison meç tal»meç tal [(g5- (MwM), [M\Cr (5a), Mo (5b)] dont lœirradiation sous UV C5H4PPh2)M(CO3)]2 conduit aux deç rives rechercheç s 1a et 1b.La deuxie` me, utilisant comme oxydant le teç tra—uoborate dœargent, conduit a` la formation de complexes teç trameç talliques cycliques [(l-g5- [M\Cr (6a), C5H4PPh2)M(CO)3Ag]2 Mo (6b), W (6c)].La troisie` me voie met en jeux lœirradiation UV des hydrures (g5-C5H4PPh2)M(CO)3H, [M\Cr (3a), Mo (3b), W (3c)]. Celle-ci se traduit par la deshydrogeç nation de 3b en 1b, mais la deç carbonylation de 3c, conduit au complexe dihydrure ponteç [(l-g5- 7c dont la structure a eç teç C5H4PPh2)W(CO)2H]2 , deç termineç e. Finalement, la quatrie` me meç thode exige la preç paration preç alable des iodures (g5- [M\Cr (4a), Mo (4b), W (4c)] qui reç agissent aiseç ment avec les anions correspondants C5H4PPh2)M(CO)3I 2a~»2c~.We are currently involved in the synthesis of various dinuclear compounds in which two functionalized cyclopentadienyls, acting as eight-electron ligands, bridge the two metallic centers. This bridging forces the metallic atoms to remain in close proximity and has been shown, specially in the case of rhodium(I) complexes,1 to promote reactions in which the two metal centers cooperate.2 In extending these studies to metal» metal bonded dimers of the Group 6 transition metals, we anticipated that bridging might stabilize new dimetallic § UPR 8241 lieç e par convention a` lœUniversiteç Paul Sabatier et a` lìInstitut National Polytechnique de Toulouse.species and modify the course of reactions in which homolytic or heterolytic splittings of the metal»metal bond play a prominent part.3 In the present paper, we shall describe and compare synthetic processes aÜording the dinuclear complexes of the general formula [(l-g5- [M\Cr C5H4PPh2)M(CO)2]2 (1a), Mo (1b), W (1c)]. Using the monometallic anionic complexes [(g5- (2a~), Mo (2b~), W (2c~)] as C5H4PPh2)M(CO)3]~[M\Cr starting materials, four synthetic attempts implying basically oxidative processes were investigated, namely: (i) electro- and photo-chemical transformations of the anions 2a~, 2b~ and 2c~; (ii) chemical oxidation of the anions 2a~, 2b~ and 2c~ by New J.Chem., 1998, Pages 15»23 151a–1c silver tetra—uoroborate; (iii) photochemical dehydrogenation of the hydrides (g5- [M\Cr (3a), Mo C5H4PPh2)M(CO)3H (3b), W (3c)] ; and (iv) metal»metal bond formation by reaction of the lithium salts [Li][2] with the corresponding iodo complexes (g5- [M\Cr (4a), Mo (4b), W C5H4PPh2)M(CO)3I (4c)].Interestingly, the –rst process includes the primary formation of metal»metal bonded complexes [(g5- [M\Cr (5a), Mo (5b)], the second one C5H4PPh2)M(CO)3]2 aÜords novel tetrametallic MwAg cyclic complexes [(l-g5- [M\Cr (6a), Mo (6b), W (6c)] and C5H4PPh2)M(CO)3Ag]2 the third one also leads to novel dimetallic dihydrides [(l-g5- [M\Mo (7b), W (7c)].The X-ray C5H4PPh2)M(CO)2H]2 molecular structure of the dihydride 7c is also presented and discussed in comparison with the already known parent compounds [(g5- (WxW ), related to the C5H5)W(CO)2(l-H)]2 series of non-bridged complexes.4 Preparations of the molybdenum complex 1b and its X-ray crystal structure, together with that of one of the tetrametallic cyclic complexes [(l-g5- have C5H4PPh2)Mo(CO)3Ag]2 , already been reported in two preliminary communications5,6 Results The complex [(l-g5- 1b, is an inter- C5H4PPh2)Mo(CO)2]2 , esting example of a dinuclear complex having a metal»metal bond supported by ì—exibleœ bridging ligands.As described hereafter chromium and tungsten analogs have now been prepared. Beside elemental analysis data, the three compounds 1a»1c have been identi–ed by their mass spectra (DCI/NH3), which are in good agreement with the expected isotopic pattern calculated for dimetallic complexes.In addition, they were also easily characterized by spectroscopic methods as shown below. As shown in Table 1, the most evident analogies appear in the IR spectra in the CwO stretching frequency region, where the observation of four bands agrees with the expected C2v symmetry. Moreover, comparison of these spectra with that of the centrosymmetric metal»metal bonded complex [(g5- [1850 (s), 1831(vs) cm~1] suggests C5H5)Mo(CO)2PPh3]2 7 the assignment of the two higher frequency bands of the C2v bridged complexes 1a»1c, to the in-phase stretching modes.The observed spectra of 1a»1c are also comparable with that exhibited by the [l- com- (Ph2P)CH2][M(g5-C5H5)(CO)2]2 plexes, which nevertheless, interestingly show a low frequency shift [1919(s), 1882(vs), 1848(m), 1828(s) for M\Mo8a and 1911(s), 1875(vs), 1837(m), 1815(s) for M\W8b] in the same solvent.In the 1H NMR spectra, the four protons of each cyclopentadienyl ligand are non-equivalent, con–rming the absence Table 1 Spectral characteristics of the bridged complexes 1a-1c IR/cm~1 a 31PM1HN NMR/ppm [(l-g5-C5H4PPh2)Cr(CO)2]2 1941 vs 88.5b 1a 1890 vs 1864 w 1844 s [(l-g5-C5H4PPh2)Mo(CO)2]2 1943 vs 68.2c 1b 1888 vs 1868 w 1846 s [(l-g5-C5H4PPh2)W(CO)2]2 1938 vs 39.9 (JPvW\140 Hz)b 1c 1892 vs 35.0 (JPvW\164 Hz)c 1863 w 1838 s a In toluene solution.b 81.015 MHz, c 32.40MHz, [2H6]acetone [2H6]benzene. Scheme 1 of a symmetry plane in the fragment as shown in C5H4wP Scheme 1. From the values of the coupling constants, it is JHvP also suggested that the two signals at higher –eld in the three complexes 1a»1c correspond to the protons in the a positions to the phosphorus atoms.Noticeably, the four cyclopentadienyl signals exhibit an important solvent eÜect (see Experimental). This last phenomena is most clearly observed with 1a and 1b. A last argument in favor of the equivalency of the two [(g5- fragments in 1a»1c is pro- C5H4PPh2)M(CO)2] vided by the 31PM1HN NMR spectra, which exhibit only one signal (Table 1).In all three cases, the chemical shifts appear as characteristic of the coordination of the phosphorus atom to the metal atom. In compound 1c, each 31P nucleus is coupled with one 183W leading to a low intensity doublet that brackets the uncoupled singlet ; the isotopic –gure corresponds to the isotopic ratio 183W/W (14.28%). Preparation of the diphenylcyclopentadienyl monometallic starting materials Fig. 1 sums up the various synthetic pathways used to prepare the three dimetallic complexes 1a»1c via four methods, each starting from the anionic complexes [(g5- 2a~»2c~. These anions were prepared C5H4PPh2)M(CO)3]~, from the lithium diphenylphosphinocyclopentadienyde using tricarbonyl metal derivatives since the reactions with the hexacarbonyl complexes produce largely the monomeric pentacarbonyl anion [(g1- (in which the PPh2C5H4)M(CO)5]~ phosphorus atom is bonded to the metal).By using the heptatrienyl tricarbonyl complexes (g6- (M\Cr, Mo) in re—ux with THF, the anions C7H8)M(CO)3 2a~ and 2b~ were obtained in good yield (99 and 91%, respectively, with respect to (g6- Neverthe- C7H8)M(CO)3).9 less, the synthesis of (g6- itself is slow (it needs C7H8)Cr(CO)3 about 20 h of re—ux) ; in addition the substitution reaction of (g6- with needs a three-day C7H8)Cr(CO)3 Li(C5H4PPh2) period of re—ux to reach completeness.Moreover, the product [Li][2a] was formed along with a pyrophoric compound whose separation is awkward and decreases its yield (60%).These constraints prompted us to use the complex as the starting material. In our hands, (CH3CH2CN)3Cr(CO)3 the preparation of this complex following the published procedure10 also gave a modest yield (60%), but its reaction in toluene with a suspension of very conveniently Li(C5H4PPh2) aÜords [Li][2a], which precipitated in high purity and good yield [99% with respect to (CH3CH2CN)3Cr(CO)3].The preparation of 2c~, starting from the cycloheptatrienyl complex (g6- has also been tested. In addition C7H8)W(CO)3 , to the slowness of the preparation of this starting material, its reaction with in THF re—ux is accompanied by Li(C5H4PPh2) decomposition. Therefore a second method of preparation was preferred using as the starting (CH3CN)3W(CO)3 material.11 As in the case of the chromium compound, the use of suspensions in toluene of the reactants (CH3CN)3W(CO)3 and aÜords easily a precipitate of the product Li(C5H4PPh2) [Li][2c] in high purity and good yield with respect to (97%).(CH3CN)3W(CO)3 The lithium salts [Li][2a], [Li][2b] and [Li][2c] were identi–ed in 31PM1HN NMR by signals at chemical shifts (2a~, 16 New J.Chem., 1998, Pages 15»23Fig. 1 The four synthetic reaction paths aÜording the bridged homobimetallic derivatives [(l-g5- [M\Cr (1a), Mo (1b), C5H4PPh2)M(CO)2]2 W (1c)] d[17.0(s) ; 2b~, d[18.2 (s) ; 2c~, d[17.2 (s]d, 2JPvW\41 Hz) in consistent with a non-coordinated phos- [2H6]benzene, phorus arm and in the infrared, surprisingly, by four CwO stretching bands.The occurrence of four bands instead of the three normally expected for tricarbonyl compounds has been attributed to the formation of a species in which an interaction occurs between the CO groups and a lithium cation.12 The new hydrido complexes (g5-C5H4PPh2)M(CO)3H [M\Cr (3a), Mo (3b), W (3c)] were readily prepared by adding one equivalent of glacial acetic acid to toluene solutions of the lithium salts [Li][2].The solutions (from orange for chromium to yellow for the tungsten derivatives) instantaneously turned red (from bright red for chromium to orange red for the tungsten derivatives) together with the formation of a light precipitate of lithium acetate. The hydrido complexes were identi–ed in 31PM1HN NMR spectra by singlets at chemical shifts consistent with a dangling phosphino group (3a, d[19.8 ; 3b, d[19.5 ; 3c, d[19.8), in 1H NMR spectra by their hydride signals at high –eld [3a, d[5.39(s) ; 3b, d[5.26(s) ; 3c, d[7.28 (s]d, Hz)] and in infra- 1JHvW\36 red by two bands that compare quite well with those of their parent compounds (g5- The formation of C5H5)M(CO)3H.byproducts was also observed and will be discussed further.The three complexes 3a»3c have been obtained as crystalline solids by concentrating and cooling their solutions in toluene (or THF) to [18 °C. The transformation of the preceding anionic compounds 2a~»2c~ into the iodo derivatives (g5-C5H4PPh2)M(CO)3I, (4a»4c) has been easily performed, by simply adding iodine to a toluene solution of 2a~»2c~. The three new complexes 4a»4c were easily identi–ed, namely in 31PM1HN NMR spectra, which show singlets at d [14.0 and [12.8 in [2H6]acetone, respectively, for 4a and 4b and at [16.1 (s]d, Hz) JPvW\71 in for 4c, fully consistent with a pendant phos- [2H6]benzene phorus atom.Electro- and photo-chemical processes leading to the bridged dinuclear complexes 1a and 1b: method A The transformation of the mononuclear anionic complexes 2a~»2c~, into the dinuclear neutral complexes 1a»1c can be regarded as the succession of two processes, the oxidative coupling of the anions, then the substitution of a carbonyl ligand bonded to one of the metal centers by the phosphino group bonded to the other metal center.We have already reported our observations in the case of the molybdenum complexes. 6 We mention that in this case, the primary formation of a dimetallic neutral species [(g5-C5H4PPh2)Mo(CO)3]2 (MwM), 5b (Fig. 1), resulted directly from the electrochemical oxidation of the mononuclear anionic complexes 2b~. We have attempted to apply the same processes to the chromium compounds. An oxidative electrolysis of 2a~ was performed at 0 mV on a platinum-gauze electrode in acetone with 0.1 M Noticeably, as in the case of the molyb- Et4NBF4 .denum compounds, a phenomenum of saturation of the electrode occurred, limiting the number of electrons exchanged per mole of 2a~ during the exhaustive electrolysis. Therefore, for purposes of electron counting and electrosynthesis, an electrolytic cell without a frit was used. After the electrolysis, the acetone solution showed complicated infrared and New J.Chem., 1998, Pages 15»23 1731PM1HNNMR spectra, from which the bridged complex 1a was easily identi–ed. Additionally, a peak at d[19.0 suggests the presence of a complex bearing a dangling phosphine arm. To this signal observed in the 31PM1HN NMR spectra, one can associate, in respect of the concomitant intensity variations during various essays, two bands in the infrared spectra at 1978 and 1908 cm~1.These data were –nally assigned to the metal»metal bonded complex 5a resulting probably from the dimerization of the electrogenerated radical M(g5- Attempts to transform 5a into 1a C5H4PPh2)Cr(CO)3N.13 failed, leading to the precipitation of an unidenti–ed green powder. We have also tried to use the considered two-phase electrophotochemical process to get the tungsten compound 1c.In this case the electrolysis, performed at 600 mV, also on a platinium-gauze electrode in acetone with 0.1 M Et4NBF4 , stopped after the consumption of only 0.5 Faraday mol~1. The absence in the infrared spectra of any band in the 2000» 1800 cm~1 region where 5b was characterized and the presence in the 31PM1HN NMR spectra of peaks without the characteristic satellites due to the coupling with 183W show that the expected products, 5c and 1c, do not form.Considering this result, we thought it useless to perform further photochemical experiments. An attempt to oxidize the anion by silver tetra—uoroborate : method B A further possibility to get the dimeric complexes 1a»1c is chemically to oxidize the anionic complexes 2a~»2c~.Considering the value of the oxidation potential of 2b~ (Ep\100 mV), it is advisable to use ferrocenium as an oxidizing reagent ; unfortunately we were unable to get a clean reaction. For this reason, we extended the investigation using silver cation as the oxidant. As already reported in the case of the molybdenum compounds, 5 when 1 equiv of silver tetra—uoroborate crystals was added to a toluene solution of one of anions 2a~»2c~, a grey precipitate of lithium tetra—uoroborate appeared, which was easily removed. In each case one can obtain from the resulting solution, in moderate yield, yellow crystals of the crown-like complexes [(l-g5- [M\Cr (6a), Mo C5H4PPh2)M(CO)3Ag]2 (6b), W (6c)].These complexes have been fully characterized by elemental analysis, MS, IR and NMR spectroscopies and their crystal structures have been solved.14 The scheme shown in Fig. 1 is based on the results of this study. As far as the synthesis of the complexes 1a»1c is concerned, it was eÜectively possible to observe their formation by heating at re—ux the toluene solutions of their respective silver derivatives 6a»6c.But these reactions are non-selective and unidenti–ed products form through a decomposition process. Synthesis of the hydrido complexes (g5-C5H4PPh2)M(CO)3H [M= Cr (3a), Mo (3b), W (3c) ] and [ (l-g5- [M= Mo (7b), W (7c) ] : some C5H4PPh2)M(CO)2H] 2 observations on the photochemical dehydrogenation processes : method C The hydrido complexes (g5- [M\Cr C5H4PPh2)M(CO)3H (3a), Mo (3b), W (3c)] were expected to be the direct precursors of the dimeric compounds 1a»1c and lead to efficient pathways for their dehydrogenation and decarbonylation reactions.The preparation of these hydrido compounds was attempted from the lithium salt [Li][2] through reaction with acetic acid. In the case of the chromium complex, the reaction in toluene at room temperature aÜords mainly the monometallic hydrido complex 3a, but the formation of small quantities of 1a suggests also a spontaneous multistep transformation of 3a into 1a.In the case of the molybdenum complex the reaction of [Li][2b] with acetic acid aÜords similarly the hydrido complex (g5- 3b, but C5H4PPh2)Mo(CO)3H, in addition gives noticeable amounts of a compound 7b that was characterized in 1H NMR spectra by a high-–eld signal at d[5.29 and in 31PM1HN NMR spectra by a singlet at d 59.37, suggesting the coordination of the phosphine arm.In the tungsten case, analogous mixtures were obtained, from which the hydrido complex (g5- 3c, was easily C5H4PPh2)W(CO)3H, identi–ed by its IR and 1H NMR spectra. Moreover, by slow evaporation of an acetone solution of 3c, crystals of the compound 7c were obtained, suggesting the occurrence of a substitution process, even in absence of irradiation : *, ~CO (g5-C5H4PPh2)W(CO)3H 3c »»»’ 12 [(l-g5-C5H4PPh2)W(CO)2H]2 7c The crystals of 7c were suitable for X-ray analysis which con–rmed the dimetallic phosphino-bridged molecular structure [(l-g5- (see below).C5H4PPh2)W(CO)2H]2 To support the discussion of the photochemical transformation of the preceding compound, it is worth summarizing some aspects of the photoreactivity of the parent complexes (g5- (M\Cr, Mo, W).Fig. 2 recalls part of C5H5)M(CO)3H the published observations in this –eld.4b,c The various pathways postulated for the photolysis of these complexes diÜer essentially, depending on the experimental conditions (in CO matrice, in gas phase, in n-pentane solution, .. .), in the nature of the dihydrogen elimination processes. They are postulated to occur either (i) from monometallic or (ii) from bimetallic species. Therefore, the CO dissociation»association equilibrium (iii) can be regarded as dispatching the system into one or the other process. Interestingly, the hydrido-bridged complexes [(g5- 8 can lose reversibly C5H5)M(CO)2(l-H)]2 H2 upon UV irradiation, forming the dimers [(g5- (M\Mo and W).These dimers are known C5H5)M(CO)2]2 to add CO in a dark reaction, forming the saturated [(g5- complexes. C5H5)M(CO)3]2 Concerning the presently studied hydrido complexes 3a»3c, the presence of a pendant ligand led us to anticipate PPh2 photoprocesses notably diÜerent from those described in Fig. 2. Because of the spontaneous decarbonylation of 3b into 7b (or of 3c into 7c), it was not expected to be easy to get signi–- cant data for the separate pathways. Therefore we restricted Fig. 2 Photoreactivity in the parent series of non-bridged cyclopentadienyl hydrido tricarbonyl complexes (g5-C5H5)M(CO)3H (M\Mo and W) as summarized from references 4 18 New J.Chem., 1998, Pages 15»23C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) H(1) W P Cp P¢ C(3¢) W¢ O(1) O(2) ourself to preliminary experiments. Thus the solutions containing both 3b and 7b (or 3c and 7c) have been irradiated with a UV high-pressure lamp. The mixture of molybdenum complexes transformed in toto into complex 1b, aÜording no additional observations.In contrast, in the case of the tungsten complexes, monitoring by infrared spectroscopy of the progress of the photoreaction showed the characteristic CwO stretching bands of the complexes 1c and 5c [this compound being identi–ed by its bands at 1963(s) and 1906(s) cm~1, similar to those of 5b at 1959 and 1913 cm~1]. The appearance of 5c shows that the irradiation of 3c induces a dehydrogenation process, which ends with the formation of a metal»metal bonded complex:15 hl, ~H2 (g5-C5H4PPh2)W(CO)3H 3c »»»’ 12 [(g5-C5H4PPh2)W(CO)3]2 (WwW ) 5c Finally in the case of the chromium compounds, the irradiation of 3a (in fact of a mixture containing in addition small amounts of 1a) aÜorded a light green powder.This green product, which apparently from its ponderal analysis, does not contain the diphenylphosphinocyclopentadienyl ligand, was not considered for further studies.To sum up, as regards the efficiency, the preparation of compounds 1a»1c by the irradiation of the hydrido complexes (method C) appears limited to the previously described case of the molybdenum compound 1b, which was obtained with high yield (90%).6 The X-ray molecular structure of 7c [ (l,g5-C5H4PPh2)W(CO)2H] 2 , The noticeable stability of the dihydrido complex 7c with respect to the dehydrogenation process led us to try to obtain a better understanding of its structure.Therefore an X-ray crystallographic investigation of a suitable crystal of 7c was carried out. The molecular geometry and atomic numbering scheme of 7c are shown in Fig. 3 and selected bond lengths and angles are given in Table 2. In a perspective view the most prominent features are the dinuclear nature of the complex and the headto- tail disposition of the bridging ligands. The geometry of Fig. 3 Perspective view of the dihydrido complex [(l-g5- 7c C5H4PPh2)W(CO)2H]2, Table 2 Selected bond lengths and angles (deg) with e.s.d.s in (”) parentheses for the complex [(l-g5- (7c) C5H4PPh2)W(CO)2H]2 W… … …W@ 4.5408(6) WwP 2.413(2) WwC(1) 1.923(10) WwCp 2.004(8) WwC(2) 1.942(10) WwH(1) 1.65(5) WwC(3) 2.294(7) C(3)wC(4) 1.443(10) WwC(4) 2.323(7) C(4)wC(5) 1.421(12) WwC(5) 2.358(9) C(5)wC(6) 1.372(13) WwC(6) 2.367(8) C(6)wC(7) 1.400(11) WwC(7) 2.312(8) C(7)wC(3) 1.360(10) C(1)wO(1) 1.145(13) C(2)wO(2) 1.195(12) PwWwCp 125.4(2) CpwWwC(2) 125.6(4) PwWwC(1) 103.2(3) CpwWwH(1) 119(2) PwWwC(2) 79.4(3) C(1)wWwC(2) 77.7(4) PwWwH(1) 66(2) C(1)wWwH(1) 61(2) CpwWwC(1) 127.5(4) C(2)wWwH(1) 116(2) @ denotes the 1[x, [y, 1[z symmetry operation.Cp is the centroid of the C(3)C(4)C(5)C(6)C(7) cyclopentadienyl ring. each (g5- moiety conforms to the C5H4w)W(CO)2(Ph2Pw)H ìfour-legged piano stoolœ description with the four ì legs œ including one phosphine, two carbonyl ligands and the hydride ligand.The two carbonyl groups are in cisoïé d positions. Those two moieties are related by centrosymmetry and, in particular, the two MwH bonds occupy transoïé d positions with respect to the molecular center of symmetry. The metal» metal distance, 4.5408(6) is long enough to prevent any ”, metal»metal interaction.There have been numerous studies of the cyclopentadienyl complexes that adopt a ìfour-legged piano stoolœ geometry but, among the hydrido-substituted complexes of the type (g5- whose structures are similar to the frag- C5R5)M(CO)2LH, ment (g5- of7c, only the structure C5H4w)W(CO)2(Ph2Pw)H of the chromium derivative (g5-C5H5)Cr(CO)2(PPh2Cy)H (Cy\cyclohexyl)16 and of the molybdenum derivatives (g5- and (g5- C5Me5)Mo(CO)2(CNBut)H 17 C5Me5)Mo(CO)3H 18 have been reported.In the former molybdenum compound a MowH distance of 1.63(4) was observed while the neutron ” diÜraction study of the latter aÜords 1.789(7) Our determi- ”. nation of a WwH distance of 1.65(5) is coherent with the ” former observation as the covalent radius of tungsten is only 0.01 longer than that of molybdenum; nevertheless the ” lower precision of our result precludes any comparison with the latter neutron diÜraction determination.To our knowledge, 7c is the –rst reported structure of a tungsten derivative of the type (g5- and this C5R5)M(CO)2LH, limits therefore the comparisons that can be made. The average WwCO distance of 1.93 in 7c appears, as expected, ” some 0.05 shorter than the values generally observed in ” [(g5- complexes (R\R@\Me; R\H, C5R4R@)W(CO)3] R@\various groups).19 Concerning the CwO distances, it would be unwise, in view of their e.s.d.s, to try a precise comparison ; we will just consider that they lie in the normal range.The angles between the ì legsœ and the (Cp)wW axis and also between them indicate no signi–cant deviations worth mention.Considering the dehydrogenation processes in 7c, it is worth noticing the transoïé d geometry of the hydrido ligands. Such a mutual position could well be a considerable endergonic hindrance to a reductive elimination process of dihydrogen and it was surprising that both complexes 3c and 7c disappear when they are irradiated.Nevertheless, the existence of a facile pathway to the cisoïé d isomer, based on the cis» trans rearrangement observed20 in the related monometallic hydride (g5- is not excluded C5H5)W(CO)2(PMe3)H, and could provide a reasonable explanation of the observations reported above in the description of ììmethod Cœœ. New J. Chem., 1998, Pages 15»23 19Formation of metalñmetal bonds by reaction of the lithium salts [Li] [2] with the corresponding iodo complexes (g5- [M= Cr (4a), Mo (4b), W (4c) ] : C5H4PPh2)M(CO)3I method D Such syntheses of some dinuclear metal»metal bonded complexes of molybdenum and tungsten, bridged by one or two heterodifunctional ligands have already been C5H4PPh2 , reported.21 Following this method, T HF was used as solvent.Thus, by heating at re—ux solutions of 2b~ with one equivalent of (g5- (M\Mo, W) for 16 h, the mono- C5H5)M(CO)3I bridged (M\Mo, (CO)3Mo(l-g5-C5H4PPh2)M(g5-C5H5) W) were obtained but in low yield. Following this process using 2b~ with one equivalent of 4b, the dibridged complex 1b was obtained, also in a low yield (32%). Overall this method appears to be non-selective and all products need to be puri- –ed by column chromatography.We have used the above principle to synthesize the three complexes 1a»1c, in toluene solution. First, iodo complexes 4a»4c were obtained with a good yield from solutions of anionic complexes 2a~»2c~ with one equivalent of iodine. 4a»4c were added to the corresponding anionic species 2a~»2c~ and the resulting toluene solutions were re—uxed during 16 h for the chromium and molybdenum complexes and 24 h for the tungsten complex.This method appears quite selective, the three complexes 1a, 1b and 1c are obtained as crystalline powders without puri–cation, with excellent yields of 95, 95 and 99%, respectively. Experimental General methods and materials All reactions of water- or air-sensitive compounds were conducted under a dry argon atmosphere using Schlenck techniques.Solvents were puri–ed as follows : toluene, diethyl ether and tetrahydrofuran were distilled under nitrogen over Na/benzophenone. Dichloromethane was distilled over CaH2. Starting materials (g6- (M\Cr, Mo),9 C7H8)M(CO)3 and as well (CH3CH2CN)3Cr(CO)3 10 (CH3CN)3W(CO)3,11 as the cyclopentadienide were prepared as Li(C5H4PPh2),22 described in the literature.All irradiations were performed by a 500-watt, highpressure, water-cooled, mercury lamp with a Hanau power supply. Infrared spectra were obtained on a 1725 X Perkin Elmer FT-IR spectrometer. 1H, 13C and 31P NMR spectra were recorded using 80-, 200-, or 250-MHz Bruker instruments. Mass spectra were recorded on a Nermag R10 instrument. Elemental analyses (%) were performed by the Microanalytical Laboratory of the Coordination Chemistry Laboratory (Toulouse, France).Syntheses Only the selected methods of preparation of compounds 1 are described hereafter in detail. 1a. Method D: To an [ (l-g5-C5H4PPh2)Cr(CO)2 ] 2 , orange solution of 1.44 mmol of 2a~ in 20 mL of toluene was added a red solution of 1.44 mmol of 4a in 20 mL of toluene. The resulting violet solution was re—uxed for 16 h, then cooled to room temperature. After concentration and –ltration, dark metallic crystals were obtained, giving 0.977 g of 1a (95% yield). 1H NMR (200 MHz, d 7.90 and 7.75 (2m, 8H, C6D6) : ortho), 7.13 (m, 12H, meta and para), 4.64 (s, 2H, 4.38 C5H4), (s, 2H, 3.81 (s, 2H, 3.34 (s, 2H, 1H NMR C5H4), C5H4), C5H4) ; (200 MHz, d 8.05 and 7.70 (2m, 8H, ortho), [2H6]acetone) : 7.65 (m, 12H, meta and para), 5.15 (s, 2H, 4.48 (s, 2H, C5H4), 3.58 (m, 2H, 3.41 (s, 2H, 31P M1HN C5H4), C5H4), C5H4) ; NMR (32.40 MHz, d 83.6 (s) ; 31P M1HN NMR (81.015 C6D6) : MHz, d 88.5 (s) ; IR 1940 (vs), [2H6]acetone) : (CH2Cl2 , mCO) : 1889 (vs), 1864 (w), 1842 (s) cm~1; IR (toluene, 1941 (vs), mCO) : 1890 (vs), 1864 (w), 1844 (s) cm~1; MS m/z 714 (DCI/NH3), [MH`] showing an isotopic pattern characteristic for a dichromium compound.Anal. calcd for C, Cr2C38H28O4P2: 63.87 ; H, 3.95. Found: C, 63.45 ; H, 3.69. [Li] [2a] . First method: To a Li[ (g5-C5H4PPh2)Cr(CO)3 ] , yellow solution of 1.20 g (4.68 mmol) of in 130 Li(C5H4PPh2) mL of THF was added 1.07 g of (g6- (4.68 C7H8)Cr(CO)3 mmol). The resulting red solution was re—uxed for 3 days, giving an orange solution that was cooled to room temperature. The solvent was removed in vacuo to give a brown yellow residue, which was washed with pentane to give 1.82 g of [Li][2a] as a beige solid (99% yield).Second method: To a heterogeneous yellow solution of 0.128 g (0.5 mmol) of in 15 mL of toluene was added a green solu- Li(C5H4PPh2) tion of 0.150 g (0.5 mmol) of in 15 mL (CH3CH2CN)3Cr(CO)3 of toluene.The heterogeneous mixture was heated and when the re—ux temperature was obtained the solution became homogeneous. A white precipitate rapidly appeared. The new mixture was cooled to room temperature and the suspension was –ltered and washed with pentane. [Li][2a] was obtained as 0.160 g of a dried white product (82% yield). 1H NMR (200 MHz, d 7.84 (2m, 4H, ortho), 7.66 to 7.08 (m, 6H, meta C6D6) : and para), 5.02 (s, 4H, 31P M1HN NMR (32.40 MHz, C5H4) ; d [17.0 (s) ; IR (THF, 1903 (vs), 1811 (vs), 1788 C6D6) : mCO) : (s), 1725 (s) cm~1. Anal. calcd for C, 61.24 ; LiCrC20H14O3P: H, 3.57. Found: C, 60.82 ; H, 4.28. 3a. To a dark orange solution of (g5-C5H4PPh2)Cr(CO)3H, 0.12 g (0.31 mmol) of [Li][2a] in 30 mL of toluene was added 18 lL of glacial acetic acid (0.31 mmol, one equivalent).The solution, which had turned bright red, was stirred for 10 min. After –ltration and elimination of the solution CH3COOLi, was concentrated and cooled to [18 °C. A red solid precipitated and the suspension was –ltered. 3a was obtained as 0.11 g of a dried red powder (92% yield). 1H NMR (200 MHz, d 7.70 and 7.45 (2m, 4H, ortho), 7.20 (m, 6H, meta and C6D6) : para), 4.98 (s, 2H, 4.74 (s, 2H, [5.39 (s, C5H4), C5H4), hydride) ; 31P M1HN NMR (32.40 MHz, d [19.8 (s) ; IR C6D6) : (toluene, 2012 (vs), 1927 (vs) cm~1. Anal. calcd for mCO) : C, 62.18 ; H, 3.91. Found: C, 62.34 ; H, 3.79. CrC20H15O3P: 4a. To an orange solution of [ (g5-C5H4PPh2)Cr(CO)3I ] , 1.78 mmol of 2a~ in 40 mL of toluene was added 0.451 g of I2 (1.78 mmol).The resulting red solution was stirred for 10 min. After –ltration, the solution was concentrated and cooled to [18 °C. An orange solid precipitated, and the suspension was –ltered. 4a was obtained as 0.738 g of a dried orange powder (81% yield). 1H NMR (200 MHz, d 7.98 to 7.30 (m, C6D6) : 10H, ortho, meta and para), 5.82 (s, 2H, 5.38 (s, 2H, C5H4), 31P M1HN NMR (32.40 MHz, d [16.1 (s) ; 31P C5H4) ; C6D6) : M1HN NMR (81.015 MHz, d [14.0 (s) ; IR [2H6]acetone) : (toluene, 2029 (vs), 1975 (vs) with a shoulder at 1955 (s) mCO) : cm~1. Anal.calcd for C, 46.90 ; H, 2.76. CrC20H14O3PI: Found: C, 46.77 ; H, 2.81. 1b. Method A: The hydride [ (l-g5-C5H4PPh2)Mo(CO)2 ] 2 , 3b (in fact a mixture also containing 7b can be used) was dissolved in 30 mL of toluene and the solution was irradiated with a low-pressure Hg lamp for 5 h.The red solution was then concentrated and cooled to [18 °C. A red solid precipitated and the suspension was –ltered. The product was washed with pentane and dried under vacuum. 1b was obtained as 0.17 g of a dried red powder (99% yield). Method B: After linear voltametry of Li[(g5-C5H4PPh2)Mo(CO)3], [Li][2b], on a platinum-gauze electrode in acetone with 0.1 M electrolyte, electrolysis at 100 mV was performed. Et4NBF4 The bright orange solution turned progressively dark red when one electron/mol of 2b~ was exchanged.The supporting electrolyte was eliminated through two cycles of evaporation 20 New J. Chem., 1998, Pages 15»23to dryness and redissolution in diÜerent solvents, from acetone to THF in which is not soluble, and then from THF Et4NBF4 to toluene.The 31P M1HN NMR (32,40 MHz, spectrum C6D6) shows the presence of [(g5- 5b, at d C5H4PPh2)Mo(CO)3]2 , [17.0 (s), of 1b at d 68.1 (s) and of unidenti–ed species at d 62.5 (s) ; in IR (THF, the bands at 1959(s), 1913 (s) cm~1 mCO) were assigned to 5b (main product) by comparison with [(g5- which are at 1960(s), 1914 (s) cm~1 C5H5)Mo(CO)3]2 for mCO in The red solution was irradiated with a high-pressure CCl4.Hg vapor lamp for 3 h. It was then concentrated, –ltered, and cooled to [18 °C. The precipitate was washed with pentane and dried under vacuum. A dried red powder of 1b was obtained with a 40 to 50% yield for electrolysis and irradiation. Method D: To an orange solution of 1 mmol of 2b~ in 50 mL of toluene was added a red solution of 1 mmol of 4b in 50 mL of toluene.The dark solution was re—uxed for 16 h and then cooled to room temperature. After concentration and –ltration, 0.761 g of 1b were obtained (95% yield). 1H NMR (200 MHz, d 7.90 and 7.75 (2m, 8H, ortho), 7.13 (m, C6D6) : 12H, meta and para), 4.89 (s, 2H, 4.42 (s, 2H, C5H4), C5H4), 3.73 (s, 2H, 3.15 (s, 2H, 31P M1HN NMR (32.40 C5H4), C5H4) ; MHz, d 68.2 (s) ; 13C M1HN NMR (50.323 MHz, C6D6) : C6D6) : d 244.5 (d, Hz, 2CO), 232.3 (s, 2CO), 140.4 (d, 1JCP\27 Hz, 4C, ipso), 134.3 and 133.4 (2s, 4C, para), 135.2 1JCP\41 and 132.2 (2d, Hz, 8C, ortho), 131.6 and 130.7 (2s, 1JCP\10 8C, meta), 94.1 and 88.2 (2s, of 91.8 and 88.6 (2d, 4C3 C5H4P), Hz, of 53.6 (d, Hz, of 1JCP\11 4C2 C5H4P), 1JCP\41 2C1 IR (THF, 1941 (vs), 1896 (vs), 1865 (w), 1843 (s) C5H4P); mCO) : cm~1; IR (toluene, 1943 (vs), 1898 (vs), 1868 (w), 1846 (s) mCO) : cm~1; MS m/z 802 [MH`] showing an isotopic (DCI/NH3), pattern characteristic for a dimolybdenum compound.Anal. calcd for C, 56.87 ; H, 3.52. Found: C, 57.0 ; Mo2C38H28O4P2 : H, 3.56.[Li] [2b] . To a yellow solu- Li[ (g5-C5H4PPh2)Mo(CO)3 ] , tion of 2.40 g (9.37 mmol) of in 275 mL of THF Li(C5H4PPh2) was added 2.55 g of (g6- (9.37 mmol). The red C7H8)Mo(CO)3 solution was re—uxed for 3 h and the resulting yellow solution was cooled to room temperature. The solvent was removed in vacuo to give a brown yellow residue, which was washed with pentane to give 3.72 g of [Li][2b] as a beige solid (91% yield). 1H NMR (200 MHz, d 7.70 and 7.46 (2m, 4H, ortho), C6D6) : 7.14 (m, 6H, meta and para), 5.50 (br s, 4H, 31P M1HN C5H4) ; NMR (32.40 MHz, d [18.2 (s) ; IR (THF, 1909 C6D6) : mCO) : (vs), 1813 (vs), 1790 (s), 1724 (s) cm~1; IR (KBr, 1980 (s), mCO) : 1901 (vs), 1789 (vs), 1763 (s) cm~1. Anal. calcd for C, 55.1 ; H, 3.2. Found: C, 52.2 ; H, 3.9.LiMoC20H14O3P: 3b and (g5-C5H4PPh2)Mo(CO)3H, [ (l-g5- 7b. An orange solution of 0.21 g C5H4PPh2)Mo(CO)2H] 2 , (0.48 mmol) of [Li][2b] in 30 mL of toluene was treated with 28 lL of glacial acetic acid (0.48 mmol, one equivalent). The bright orange solution was stirred for 10 min. After –ltration and elimination of the solution was concentrated CH3COOLi, and cooled to [18 °C. An orange solid precipitated and the suspension was –ltered.A dried orange powder (0.18 g) was obtained. Spectroscopic analyses showed that the monometallic hydride compound 3b and the dimetallic dihydride compound 7b were present in an approximate ratio of 6 : 1. 1H NMR (200 MHz, d 7.70 and 7.45 (2m, 32H, ortho), C6D6) : 7.20 (m, 48H, meta and para), 4.98 (s, 24H, 3b) and C5H4 of 4.74 (s, 8H, 7b), [5.26 (s, 6H, hydride of 3b) and C5H4 of [5.29 (s, 2H, hydrides of 7b) ; 31P M1HN NMR (32.40 MHz, d [19.5 (s, 3b) and 59.4 (s, 7b) ; IR (toluene, 2024 C6D6) : mCO) : (s), 1937 (s) cm~1.Anal. calcd for 6 and 1 MoC20H15O3P C, 53.4 ; H, 3.4. Found: C, 54.1 ; H, 3.9. Mo2C36H30O4P2 : 4b. To an orange solution of 1 (g5-C5H4PPh2)Mo(CO)3I, mmol of 2b~ in 30 mL of THF was added 0.254 g of (1 I2 mmol).The red solution was stirred for 10 min. After –ltration, the solution was concentrated and cooled to [18 °C. An orange solid precipitated and the suspension was –ltered. 4b was obtained as 0.500 mg of a dried orange product (90% yield). 1H NMR (200 MHz, d 7.57 to 7.45 (m, [2H6]acetone) : 10H, ortho, meta and para), 6.24 (m, 2H, 5.81 (m, 2H, C5H4), 31P M1HN NMR (81.015 MHz, d [12.8 C5H4) ; [2H6]acetone) : (s) ; IR (THF, 2039 (vs), 1964 (vs) cm~1.Anal. calcd for mCO) : C, 43.19 ; H, 2.54. Found: C, 43.01 ; H, 2.65. MoC20H14O3PI: 1c. Method D: To a solu- [ (l-g5-C5H4PPh2)W(CO)2 ] 2 , tion of 0.120 g of [Li][2c] (0.23 mmol) in 15 mL of toluene was added a red solution of 0.147 g of 4c (0.23 mmol) in 15 mL of toluene. The solution was re—uxed for 16 h.and then cooled to room temperature. After concentration and –ltration, a red powder was obtained, giving 0.224 g of 1c (99% yield). Red crystals of 1c were obtained by slow diÜusion of diethyl ether in a saturated dichloromethane solution. 1H NMR (200 MHz, d 7.92 and 7.41 (2m, 8H, ortho), 7.23 C6D6) : to 7.09 (m, 12H, meta and para), 4.64 (s, 2H, 4.38 (s, C5H4), 2H, 3.81 (s, 2H, 3.34 (s, 2H, 1H NMR C5H4), C5H4), C5H4) ; (200 MHz, d 8.08 and 7.81 (2m, 8H, ortho), [2H6]acetone) : 7.67 to 7.30 (m, 12H, meta and para), 5.45 (s, 2H, 5.00 C5H4), (s, 2H, 4.19 (s, 2H, 3.89 (s, 2H, 31P M1HN C5H4), C5H4), C5H4) ; NMR (32.40 MHz, d 35.0 (s]d, Hz); IR C6D6) : JPvW\328 1940 (vs), 1889 (vs), 1864 (w), 1842 (s) cm~1; IR (CH2Cl2 , mCO) : (toluene, 1938 (vs), 1892 (vs), 1863 (w), 1838 (s) cm~1; IR mCO) : (THF, 1936 (vs), 1890 (vs), 1862 (w), 1835 (s) cm~1; MS mCO) : m/z 978 [MH`] showing an isotopic pattern (DCI/NH3), characteristic for a ditungsten compound.Anal. calcd for C, 46.65 ; H, 2.89. Found: C, 46.20 ; H, 3.00. W2C38H28O4P2: [Li] [2c] . To a yellow solu- Li[ (g5-C5H4PPh2)W(CO)3 ] , tion of 0.835 g (3.26 mmol) of in 50 mL of Li(C5H4PPh2) toluene was added a solution of 1.275 g of (CH3CN)3W(CO)3 (3.26 mmol) in 50 mL of toluene.The heterogeneous mixture was heated and when the re—ux temperature was obtained, the solution became homogeneous. After 15 min a beige precipitate appeared. The new mixture was cooled to room temperature and the suspension was –ltered. [Li][2c] was obtained as 1.655 g of a dried white powder (97% yield). 1H NMR (200 MHz, d 7.56 to 7.48 (m, 4H, ortho), [2H6]acetone) : 7.45 to 7.38 (m, 6H, meta and para), 5.25 (s, Hz, 2H, JHvP\2.3 5.04 (s, Hz, 2H, 31P M1HN NMR C5H4), JHvP\1.9 C5H4) ; (32.40 MHz, d [17.2 (s]d, Hz); IR C6D6) : 2JPvW\41 (THF, 1903 (vs), 1808 (vs), 1786 (s), 1724 (s) cm~1. Anal. mCO) : calcd for C, 45.84 ; H, 2.69. Found: C, 46.12 ; LiWC20H14O3P: H, 3.27. 3c. To a yellow solution of 0.290 (g5-C5H4PPh2)W(CO)3H, g (0.55 mmol) of [Li][2c] in 30 mL of THF was added 30 lL of glacial acetic acid (0.55 mmol). The resulting red solution was stirred for 10 min. After –ltration and elimination of the solution was concentrated and cooled to CH3COOLi, [18 °C. An orange solid precipitated, and the suspension was –ltered. 3c was obtained as 0.275 g of a dried orange powder (96% yield). 1H NMR (200 MHz, d 7.57 to [2H6]acetone) : 7.37 (m, 10H, 6.03 (s, 2H, 5.75 (s, 2H, C6H5), C5H4), C5H4), [7.23 (s]d, hydride, Hz); 1H NMR (200 MHz, JHvW\36 d 7.45 to 7.25 (m, 10H, 5.59 (s, 2H, CDCl3) : C6H5), C5H4), 5.40 (s, 2H, [7.28 (s]d, hydride, Hz); 31P C5H4), 1JHvW\36 M1HN NMR (32.40 MHz, d[19.8 (s]d, C6D6) : 2JPvW\63 Hz); IR (THF, 2019 (vs), 1926 (vs) cm~1. Anal.calcd for mCO) : C, 62.18 ; H, 3.91. Found: C, 62.34 ; H, 3.79. WC20H15O3P: 4c. To a yellow solution of 0.315 (g5-C5H4PPh2)W(CO)3I, g (0.60 mmol) of [Li][2c] in 30 mL of THF was added 0.152 g of (0.60 mmol). The resulting red solution was stirred for 10 I2 min. After –ltration, the solution was concentrated and cooled to [18 °C. An orange solid precipitated and the suspension was –ltered. 4c was obtained as 0.314 g of a dried orange powder (81% yield). 1H NMR (200 MHz, d 7.40 and C6D6) : New J. Chem., 1998, Pages 15»23 21Table 3 Crystal Data for [(l-g5- 7c C5H4PPh2)W(CO)2H]2 Chemical formula C38H30O4P2W2 FW 980.3 Crystal system Orthorhombic Space group Pbca (no.61) a/” 12.042(1) b/” 16.535(2) c/” 16.871(2) U/”3 3359(1) F(000) 1872 Z 4 qcalc/g cm~3 1.938 Radiation MoKa (j\0.71073 ”) l(MoKa)/mm~1 6.70 T&»T' 0.922»1.000 2h range/deg 3»46 Scan mode x[2h No.of data collected 2328 (all unique) No. of observed data 1193 [Fo2[3r(Fo2)] No. of variable params 128 S 1.093 w [r2(Fo)]0.0011Fo2]~1 (*/r)max 0.008 R\&[(oFo o[ oFc o)/&oFo o 0.030 Rw\[&w(oFo o[oFc o)2/&woFo o2]1@2 0.034 (*/q)max, min/e ”~3 0.55, [0.53 7.31 (2m, 4H, ortho), 7.17 to 7.12 (m, 6H, meta and para), 5.07 (quint., 2H, 4.87 (t, 2H, 31P M1HN NMR (32.40 C5H4), C5H4) ; MHz, d [16.1 (s]d, Hz); IR (THF, C6D6) : 2JPvW\71 mCO) : 2033 (vs), 1951 (vs) cm~1.Anal. calcd for C, WC20H14O3PI: 37.30 ; H, 2.19. Found: C, 37.52 ; H, 2.24. Formation of the complex [ (l-g5-C5H4PPh2)W(CO)2H] 2 , 7c. In the 1H NMR tube of a 3c solution in [2H6]acetone, orange crystals of 7c were obtained by slow concentration MS m/z 980 [MH`] showing an [2H6]acetone.(DCI/NH3), isotopic pattern characteristic for a ditungsten compound. Anal. calcd for C, 46.56 ; H, 3.08. Found: C, W2C38H30O4P2 : 46.42 ; H, 3.01. Table 4 Atomic coordinates for 7c Atom x/a y/b z/c W 0.51845(2) 0.12256(2) 0.44077(2) H(1) 0.4614(62) 0.1285(41) 0.3522(21) P 0.3671(2) 0.0294(1) 0.4198(1) C(1) 0.4576(7) 0.2245(6) 0.4072(6) O(1) 0.4262(7) 0.2885(5) 0.3940(5) C(2) 0.4224(8) 0.1510(6) 0.5289(6) O(2) 0.3677(6) 0.1747(4) 0.5832(4) C(3) 0.6529(6) 0.0465(4) 0.5018(4) C(4) 0.6857(6) 0.1301(4) 0.5089(5) C(5) 0.7071(7) 0.1596(7) 0.4313(5) C(6) 0.6919(7) 0.0962(5) 0.3800(5) C(7) 0.6581(5) 0.0283(5) 0.4233(4) C(8) 0.2335(4) 0.0803(3) 0.4119(3) C(9) 0.1480(4) 0.0638(3) 0.4654(3) C(10) 0.0452(4) 0.1015(3) 0.4567(3) C(11) 0.0277(4) 0.1557(3) 0.3946(3) C(12) 0.1131(4) 0.1722(3) 0.3411(3) C(13) 0.2160(4) 0.1345(3) 0.3498(3) C(14) 0.3631(4) [0.0301(4) 0.3289(3) C(15) 0.4514(4) [0.0297(4) 0.2754(3) C(16) 0.4450(4) [0.0748(4) 0.2057(3) C(17) 0.3502(4) [0.1204(4) 0.1895(3) C(18) 0.2618(4) [0.1209(4) 0.2430(3) C(19) 0.2683(4) [0.0757(4) 0.3127(3) X-Ray crystallography for 7c DiÜraction data were collected on an Enraf-Nonius CAD-4 diÜractometer at room temperature using MoKa graphitemonochromated radiation (k\0.71073 Accurate unit cell ”).parameters were obtained from least-squares re–nement of 25 re—ections in the 11»15° h range. The intensities were corrected for Lorentz and polarization eÜects and for a slight ([6.3%) linear decay.23 Empirical absorption corrections24 were made from w scans.The structure was solved by Patterson techniques.25a Relevant crystallographic data for 7c are listed in Table 3. Full-matrix least-squares re–nement25b minimizing was performed with non-hydrogen &w(oFc o[oFc o)2 atoms anisotropic but those of phenyl rings re–ned as isotropic rigid groups.The hydride ligand H(1) was located on a diÜerence-Fourier map. Its position agreed with that obtained by seeking sites of potential energy minima.26 It was re–ned isotropically. All other H atoms were introduced in calculated positions. Final atomic coordinates are given in Table 4. CCDC reference number 440/002. Conclusions The possibility to obtain in very good yields a series of bridged metal»metal bonded complexes now opens opportunities to study comparatively the structures and physical properties in that series. Chemical properties, in particular the electrochemical behavior, are currently extended from the case of the molydenum complex5 to the chromium and tungsten ones.Acknowledgements authors would like to express their gratitude to Suzy The Richelme and Dr.D. de Montauzon for helpful contributions and comments on the mass spectra and electrochemical determinations, respectively and to Dr. Andreç Maisonnat for helpful discussions. References 1 (a) A. Iretskii, M. C. Jennings and R. Poilblanc, Inorg. Chem., 1996, 35, 1266. (b) J. B. Tommasino, D. de Montauzon, X. He, A. Maisonnat, R. Poilblanc, J.N. Verpeaux and C. Amatore, Organometallics, 1992, 11, 4150. 2 (a) M. J. Hostetler, M. D. Butts and R. G. Bergman, J. Am. Chem. Soc., 1993, 115, 2743. (b) D. Baudry, A. Dormond and A. Ha–d, New J. Chem., 1993, 17, 465. (c) M. Visseaux, A. Dormond, M. M. Kubichi, C. Moïé se, D. Baudry and M. Ephritikine, J. Organomet. Chem., 1992, 433, 95. (d) M. Ogasa, M. D. Rausch and R. D.Rogers, J. Organomet. Chem., 1991, 403, 279. (e) X. He, A. Maisonnat, F. Dahan and R. Poilblanc, Organometallics, 1991, 10, 2443. ( f ) A. Dormond, P. Hepiegne, A. Ha–d and C. Moïé se, J. Organomet. Chem., 1990, 398, C1. (g) M. El Amane, R. Mathieu and R. Poilblanc, New J. Chem., 1988, 12, 661. (h) G. K. Anderson and M. Lee, Organometallics, 1988, 7, 2285. (i) K. W. Lee, W. T. Pennington, A.W. Cordis and T. L. Brown, J. Am. Chem. Soc., 1985, 107, 631. ( j) F. Faraone, S. Lo Schiaoo, G. Bruno and G. Bombieri, J. Chem. Soc., Chem. Commun., 1984, (k) G. Bruno, S. Lo Schiavo, P. Priraino and F. Faraone, Organometallics, 1985, 4, 1098. (l) P. Kalk, J.-M. Frances, P. M. P–ster, T. G. Southern and A. Thorez, J. Chem. Soc., Chem. Commun., 1983, 510. (m) M. El Amane, R.Mathieu and R. Poilblanc, Organometallics, 1983, 2, 1618. 3 (a) J. R. Knorr and T. L. Brown, J. Am. Chem. Soc., 1993, 115, 4087. (b) S. C. TenhaeÜ, K. J. Covert, M. P. Castellani, J. Grunkemeier, C. Kunz, T. J. R. Weakley, T. Koenig and D. R. Tyler, Organometallics, 1993, 12, 5000. (c) S. L. Scott, J. H. Espenson and W.-J. Chen, Organometallics, 1993, 12, 4077. (d) Q. Yao, A.Bakac and J. H. Espenson, Organometallics, 1993, 12, 2010. (e) W. C. Watkins, T. Jaeger, C. E. Kidd, S. Fortier, M. C. Baird, G. Kiss, G. C. Roper and C. D. HoÜ, J. Am. Chem Soc., 1992, 114, 907. ( f ) S. Fortier, M. C. Baird, K. F. Preston, J. R. Morton, T. Ziegler, T. J. Jaeger, W. C. Watkins, J. H. MacNeil, K. A. Watson, K. Hensel, Y. Le Page, J.-P. Charland and A. J. Williams, J.Am. Chem. Soc., 1991, 113, 542. (g) S. J. McLain, J. Am. Chem. Soc., 1988, 110, 643. (h) R. H. Hooker, K. A. Mahmoud and A. J. Rest, J. Organomet. 22 New J. Chem., 1998, Pages 15»23Chem., 1983, 254, C25. (i) R. J. Kazlauskas and M. S. Wrighton, J. Am. Chem. Soc., 1982, 104, 6005. ( j) D. S. Ginley, C. R. Bock and M. S. Wrighton, Inorg. Chim. Acta, 1977, 23, 85. (k) R.M. Laine and P. C. Ford, Inorg. Chem., 1977, 16, 388. (l) M. S. Wrighton and D. S. Ginley, J. Am. Chem. Soc., 1975, 97, 4246. (m) J. L. Hughey IV, C. R. Bock and T. J. Meyer, J. Am. Chem. Soc., 1974, 97, 4440. (n) R. D. Adams, D. E. Collins and F. A. Cotton, J. Am. Chem. Soc., 1974, 96, 749. 4 (a) H. G. Alt, T. Frister, E. E. Trapl and H. E. Engelhardt, J. Organomet. Chem., 1989, 362, 125.(b) K. A. Mahmoud, A. J. Rest and H. G. Alt, J. Chem. Soc., Dalton T rans., 1984, 187. (c) H. G. Alt, K. A. Mahmoud and A. J. Rest, Angew. Chem., Int. Ed. Engl., 1983, 22, 544. 5 B. Brumas, F. Dahan, D. de Montauzon and R. Poilblanc, J. Organomet. Chem., 1993, 453, C13. 6 B. Brumas, D. de Caro, F. Dahan, D. de Montauzon and R. Poilblanc, Organometallics, 1993, 12, 1503. 7 M.A. Bennett, L. Pratt and G. Wilkinson, J. Chem. Soc., 1961, 2037. 8 (a) A. Alvarez, E. Garcia, V. Riera, M. A. Ruiz, C. Bois, Y. Jeannin, V. Riera and M. A. Ruiz, Angew. Chem., Int. Ed. Engl., 1993, 32, 1156. (b) V. Riera, M. A. Ruiz and F. Villafane, Organometallics, 1992, 11, 2854. 9 (a) R. B. King and A. Fronzaglia, Inorg. Chem., 1966, 5, 1837. (b) E. W. Abel, M. A. Bennett, R. Burton and G.Wilkinson, J. Chem. Soc., 1958, 919, 4559. (c) E. W. Abel, M. A. Bennett and G. Wilkinson, Proc. Chem. Soc., 1958. 152. 10 G. J. Kubas, L. S. van der Sluys, R. A. Doyle and R. J. Angelici, Inorganic Syntheses, ed. R. J. Angelici, Wiley, vol. 28, p. 32. 11 (a) T. J. McNeese, J. Chem. Educ., 1991, 68, 678. (b) R. B. King, Organomet. Synth., 1965, 1, 109. 12 M. Y. Darensbourg, Prog. Inorg. Chem., 1985, 33, 221. 13 (a) Following the suggestion of Baird et al., brace brackets will be used to distinguish the 17-electron species from the closed-shell compounds of similar formulae. (b) T. A. Huber, D. H. Macerney and M. C. Baird, Organometallics, 1993, 12, 4715. 14 B. Brumas-Soula, G. Commenges, F. Dahan and R. Poilblanc unpublished results. 15 In addition, a species characterized by two low frequency stretching bands at 1753 and 1728 cm~1, whose concentration increased with the duration of the experiment, was attributed to the formation of CO-bridged dimetallic species. 16 W. C. Watkins, K. Hensel, S. Fortier, D. H. Macartney, M. C. Baird and S. J. MacLain, Organometallics, 1992, 11, 2418. 17 H. G. Alt, H. E. Engelhardt, T. Frister and R. D. Rogers, J. Organomet. Chem., 1989, 366, 297. 18 L. Brammer, D. Zhao, R. M. Bullock and R. K. McMullan, Inorg. Chem., 1993, 32, 4819. 19 (a) H. G. Alt, J. S. Han and R. D. Rogers, J. Organomet. Chem., 1993, 454, 165. (b) T. J. R. Weakley, A. Avey Jr. and D. R. Tyler, Acta Crystallogr., Sect. C, 1992, 48, 154. (c) A. L. Rheingold and J. R. Harper, Acta Crystallogr., Sect. C, 1991, 47, 184. (d) T. E. Bitterwolf and A. L. Rheingold, Organometallics, 1991, 10, 3856. (e) A. Avey, S. C. TendeÜ, T. J. R. Weakley and D. R. Tyler, Organometallics, 1991, 10, 3607. ( f ) I. R. Lyatifov, T. K. Casanov, T. K. Kurbanov, A. N. Shnulin and V. K. Shilœnikov, Koord. Khim., 1990, 16, 1343 (Chem. Abstr., 1991, 114(15), 336k). (g) W. Abriel and J. Heck, J. Organomet. Chem., 1986, 302, 363. 20 B. Wrackmeyer, H. G. Alt and H. E. Maisel, J. Organomet. Chem., 1990, 399, 125. 21 T. J. Duckworth, M. J. Mays, G. Conole and M. McPartlin, J. Organomet. Chem., 1992, 439 327. 22 F. Mathey and J.-P. Lampin, J. Organomet. Chem., 1975, 31, 2685. 23 C. K. Fair, MolEn Molecular Structure Procedures, Enraf-Nonius, Delft, Holland 1990 24 A. C. T. North, D. C. Phillips and F. S. Mathews, Acta Crystallogr., Sect. A, 1968, 21, 351. 25 (a) G. M. Sheldrick, SHEL XS86 Program for Crystal Structure Solution, University of Goé ttingen, Goé ttingen, 1986. (b) G. M. Sheldrick, SHEL X76 Program for Structure Determination, University of Cambridge, Cambridge, 1976. 26 A. G. Orpen, J. Chem. Soc., Dalton T rans., 1980, 2509. Received 29th January 1997; Paper 7/06746A New J. Chem., 1998, Pages 15»23 23

ISSN:1144-0546    

DOI:10.1039/a706746a

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Unusual properties of ruthenium(II) diphenylcyanamide complexes: chemistry and application as sensitizers of nanocrystalline TiO2 Stefan Ruile,* Oliver Kohle, Henrik Pettersson and Michael Graé tzel Institut de Chimie Physique, Ecole Polytechnique de L ausanne, CH-1015 L ausanne, Feç deç rale Switzerland In this study, a series of ruthenium(II) polypyridyl complexes of the general formula (dcbpy Na4[RuII(dcbpy)2X2] is 2,2@-bipyridine-4,4@-dicarboxylate and X is a substituted phenylcyanamide anion) are investigated.When introduced into the ruthenium co-ordination sphere, the phenylcyanamide ligand showed linkage isomerism and was co-ordinated via the nitrile- or amide nitrogen, as shown by 1H NMR, 13C NMR spectroscopy and use of a 13C-labeled ligand. The title compounds exhibited unusually high –rst values between 5 and 6, determined pKa via spectrophotometric titrations.The –rst protonation occurred at the phenylcyanamide ligand, rendering the energy of the lowest energy 1MLCT absorption strongly pH-dependent. In photovoltaic devices, overall efficiencies were in the range of 3»4%, due to photon-to-current conversion efficiencies of 50»60% at 520 nm. The long-term stability of a representative complex under irradiation was investigated in solution and in a solar cell device.The potential of dye-sensitized titanium dioxide electrodes in solar-to-electrical energy conversion schemes is considerable. Demonstrated solar energy conversion efficiencies of 10% are promising under the aspect of an industrial application of the photoelectrochemical cell.1,2 Ruthenium(II) 2,2@-bipyridine-4,4@-dicarboxylic acid complexes are the most frequently used sensitizers (dcbpyH2) for due to their favorable light absorption, electro- TiO2 , chemical and photoelectrochemical properties, and their photostability in the device.3h8 In particular, cis- 1, showed an unmatched conversion Ru(dcbpyH2)2(NCS)2 , efficiency in solar cell devices.2 The main reasons for this lie in an apparently perfectly positioned p* energy level, a relatively high-lying Ru energy level, a favorable geometry and size (t2g) of the molecule, and in the presence of the two thiocyanate ligands. Most of the efficient ruthenium polypyridine sensitizers contain this ligand, as it seems to support quantitative electron injection.7 Four carboxylic acid groups enhance the absorption coefficients of the MLCT transitions.The limitation of ruthenium polypyridyl sensitizers is, however, their small absorption coefficients at longer wavelengths (above 700 nm). In complexes of the type absorption proper- Ru(dcbpy)2X2 , ties are mainly controlled by the donor strength of the ligand X. A strong electron donor destabilizes the ruthenium ground state energy level level), giving rise to MLCT transitions (t2g shifted to lower energies.9,10 Hence a promising approach to a more efficient sensitizer is to replace the thiocyanate ligands in 1 by stronger electron donors.Substituted phenylcyanamide anions are candidates for this purpose.8 Their donor strength may in addition be –ne-tuned by a judicious choice of the substituents at the phenyl ring.11 The long-term stability of the dye is a key requirement when an industrial application as a sensitizer for in a TiO2 solar cell device is considered.For a long time, it was questioned whether ruthenium polypyridine complexes could ful–ll the strenuous requirement of millions of turnovers without decomposition. Such a long-term stability in a solar cell device could only recently be shown for 1.12,13 However, under irradiation in solution, 1 decomposed with loss of sulfur.13,14 In the present study, the synthesis, chemistry and photoelectrochemistry of a series of ruthenium polypyridyl complexes containing two phenylcyanamide ligands are reported.The stability of the new complexes under irradiation is studied in solution and in a photovoltaic cell.Experimental Materials All solvents and chemicals used were at least reagent grade and were purchased from Fluka AG or Aldrich (Switzerland). Sephadex LH-20 was obtained from Pharmacia. Electrodes were coated with nanocrystalline prepared via a sol-gel TiO2 procedure.15 The molten salts (3-hexyl-1,2-dimethyl- HM2I imidazolium iodide) and HMI (1-hexyl-3-methylimidazolium iodide) were synthesized in a similar manner as previously reported.16 The phenylcyanamide ligands were prepared from substituted anilines via the thiourea route.17 A sample of 13Clabeled thallium 4-chlorophenylcyanamide was available from a previous study.8 2, was a gift of Sol- cis-Ru(dcbpyH2)2Cl2 , aronix SA (Aubonne, Switzerland). Elemental analyses were carried out by the analytical service of Ciba-Geigy (Basel, Switzerland). 3. 2 (90 mg) was dissolved Na4 [Ru(dcbpy)2(2-Clpcyd)2 ] , in a mixture of 0.5 M aqueous NaOH (20 mL) and dimethylformamide (45 mL). Subsequently 2-chlorophenylcyanamide (435 mg) was added and the reaction mixture was re—uxed for 4 h. After cooling to room temperature, the solvent was evaporated and the product was puri–ed on a short Sephadex LH-20 column.The product band was collected and evaporated to dryness. Then the solid was redissolved in methanol (10 mL), and diethyl ether was slowly added until the product precipitated. The solid was –ltered oÜ, washed with small amounts of a 5 : 2 diethyl ether»methanol mixture and dried in vacuo. Yield : 96 mg (62%). Anal. calcd.(%) for C, 38.14 ; H, 3.71 ; N, 9.36. C38Cl2H20N8Na4O8Ru … 12H2O: Found: C, 37.74 ; H, 3.32 ; N, 9.16. 4. 4 was prepared in the Na4 [Ru(dcbpy)2(4-Clpcyd)2 ] , same way as 3, starting from 200 mg of 2 and 900 mg of 4- chlorophenylcyanamide. Yield : 171 mg (54%). Anal. calcd. (%) for C, 41.58 ; H, 3.03 ; N, C38Cl2H20N8Na4O8Ru … 6.5H2O: 10.21. Found: C, 41.55 ; H, 3.32 ; N, 10.26.New J. Chem., 1998, Pages 25»31 255. The preparation Na4 [Ru(dcbpy)2(2,4,5-Cl3pcyd)2 ] , from 80 mg of 2 and 470 mg of 2,4,5-trichlorophenylcyanamide was identical to the one of 3. Yield : 80 mg (56%). Anal. calcd. (%) for C, C38Cl6H16N8Na4O8Ru … 7H2O: 36.68 ; H, 2.43 ; N, 9.00. Found: C, 36.95 ; H, 2.85 ; N, 8.91. 6. 6 was prepared like Na4 [Ru(dcbpy)2(2,4,6-Cl3pcyd)2 ] , 3, with 80 mg of 2 and 500 mg of 2,4,6-trichlorophenylcyanamide as starting materials.Yield : 64 mg (49%). Anal. calcd (%) for C, 40.16 ; C38Cl6H16N8Na4O8Ru …H2O: H, 1.60 ; N, 9.86. Found: C, 40.17 ; H, 1.55 ; N, 9.52. 7. The synthesis was Na4 [Ru(dcbpy)2(3,5-Cl2pcyd)2 ] , identical to the one of 3, starting from 82 mg of 2 and 480 mg of 3,5-dichlorophenylcyanamide. Yield : 78 mg (55%).Anal. calcd (%) for C, 38.24 ; H, C38Cl4H18N8Na4O8Ru … 8H2O: 2.87 ; N, 9.39. Found: C, 38.15 ; H, 2.47 ; N, 9.27. Methods Spectroscopic studies. 1H and 13C NMR spectra were measured on a Bruker AC-P 200 spectrometer. Chemical shifts are given in ppm, relative to tetramethylsilane. The pulse repetition time in 13C NMR measurements was 3 s. UV/VIS spectra were obtained on a Hewlett Packard 8452A diode array spectrophotometer.Emission spectra were measured on Perkin- Elmer LS 50 B luminescence spectrometer. Fourier transform infrared spectra were recorded from KBr pellets on a Perkin- Elmer Paragon 1000 FTIR spectrophotometer. Spectrophotometric titrations were performed in 10~5 M aqueous dye solutions. Their ionic strength was kept constant at a value of one with a phosphate»citrate buÜer.After adjusting the pH value with dilute HCl, solutions were stirred for at least one minute until the equilibrium was attained. Electrochemistry. Cyclic voltammetry was performed in argon-purged DMSO, dried over a molecular sieve, in the presence of 0.1 M tetrabutylammonium tri—uoromethanesulfonate as the supporting electrolyte. In the three-electrode setup, a glassy carbon working electrode (surface 0.07 cm2), a glassy carbon counter electrode (separated from the working electrode compartment by a bridge containing the same electrolyte as the test solution), and a silver wire as a quasireference electrode were used.The quasi-reference electrode was calibrated with a Ag/AgCl/KCl saturated reference electrode. Reported potentials refer to the latter.Scan rates between 100 and 300 mV s~1 were used. Photoelectrochemical measurements. electrodes were TiO2 coated for 15 h under exclusion of light in 5]10~4 M ethanolic dye solutions at room temperature. If there was no deep coloration of the electrode, soaking was continued for 2 h at 70 °C. Unless otherwise stated, the dye solutions contained 50 mM of 3a,7b-dihydroxy-5b-cholanic acid as a coadsorbate. As the electrolyte, a mixture containing 0.6 M 40 mM of 40 mM of LiI and 200 mM of 4-tert- HM2I, I2 , butylpyridine in acetonitrile was used.The setup and procedure for the measurement of action spectra and current» voltage characteristics are described elsewhere.2 Stability. The long term performance of 3 was studied with semi-transparent cells of a surface area of 1 cm2, sealed with an epoxy resin.The cells were prepared following the general procedure for the fabrication of dye-sensitized, nanocrystalline solar cells.2,18 Three sets of cells were prepared. In the –rst, the working electrodes were sensitized with a 3]10~4 M ethanolic solution of 3. The second set was coated in the same way, except that the solutions contained in addition 50 mM 3a,7b-dihydroxy-5b-cholanic acid.For the third set, sensitizer 1 was used as a dye. Each set consisted of –ve cells. HMI containing 300 mM of 4-tert-butylpyridine and 10 mM of I2 was used as an electrolyte. This experimental setup is useful to fabricate regenerative photoelectrochemical cells for lowpower applications.12 During stability tests, cells were illuminated with a light source of 5000 lx (AM 1 is B120 000 lx) under a polycarbonate UV –lter.The cell temperature was kept constant at 50 °C. Current»voltage characteristics were measured using a 60 W incandescent lamp at a light intensity of 50 lx. Results and discussion Synthesis Starting from the dichloro compound 2, Ru(dcbpyH2)2Cl2 , two reaction pathways to the desired diphenylcyanamide complexes of the general formula Na4[Ru(dcbpy)2(pcyd)2] were tested : one involved the direct reaction of 2 with an excess of protonated phenylcyanamide ligand in a DMF» NaOH mixture; in the other, the thallium(I) salt of phenylcyanamide was used in 2.5 molar excess.Dimerization of phenylcyanamide was not a problem under the applied reaction conditions, as the ligand was found to be introduced in the complex exclusively in its monomeric, anionic form.Hence, the –rst route was preferred, as it avoids the very toxic thallium(I) compounds. The phenylcyanamide complexes were subsequently passed over a short Sephadex LH-20 column and isolated as the tetrasodium salts by precipitation with diethyl ether from methanol.This procedure gave rise to products showing satisfactory elemental analyses. The described procedure failed for more basic phenylcyanamide ligands, such as 4-(N-diethylamino)phenylcyanamide or 2,4,6-trimethylphenylcyanamide. In these cases, resulting complexes showed strongly blue-shifted absorption maxima at around 470 nm, suggesting incorporation of the ligands in the protonated form.NMR spectra and bonding mode of phenylcyanamide 1H NMR spectra of complexes 3 and 4 are presented in Fig. 1. All products were free of uncoordinated phenylcyanamide ligand, which would appear up–eld shifted. Complete 1H NMR data can be found in Table 1. The 2,2-bipyridine-4,4-dicarboxylate signals show a pattern con–rming the cis geometry of the complexes, where the pyridine units of each ligand are inequivalent, but each is equivalent to one pyridine ring of the other bipyridine.19 These spectra thus contain signals of six distinguishable hydrogens.Their positions are only weakly aÜected by the nature of the phenylcyanamide ligand. The occurrence of only six bipyridine signals con–rms the presence of one compound, in which Fig. 1 Aromatic region of the 1H NMR spectra of complexes 3 and 4, measured in [2H4]methanol 26 New J.Chem., 1998, Pages 25»31Table 1 1H NMR data of complexes 3[7 in [2H4]methanol 3 4 5 6 7 3 9.10 9.10 9.13 9.04 9.13 d (2) s (2) d (2) d (2) d (2) 1.2 1.1 1.4 1.1 5 8.26 8.24 8.30 8.23 8.28 dd (2) dd (2) dd (2) dd (2) dd (2) 5.8, 1.5 5.8, 1.4 5.7, 1.6 5.7, 1.6 5.8, 1.5 6 9.72 9.66 9.67 9.73 9.62 d (2) d (2) d (2) d (2) d (2) 5.8 5.8 5.9 5.9 5.7 3@ 8.95 8.95 8.97 8.89 8.97 d (2) s (2) d (2) d (2) d (2) 1.3 1.3 1.3 1.3 5@ 7.65 7.65 7.67 7.60 7.67 dd (2) dd (2) dd (2) dd (2) dd (2) 5.7, 1.6 5.9, 1.5 5.9, 1.6 5.8, 1.6 5.8, 1.6 6@ 7.89 7.88 7.89 7.77 7.87 d (2) d (2) d (2) d (2) d (2) 5.8 5.8 5.8 5.9 5.8 pcyd 3*: 7.05 3*, 5*: 6.87 3*: 7.19 3*, 5*: 7.00 4*: 6.52 dd (2) m (4) s (2) s (4) t (2) 7.8, 1.5 1.7 pcyd 4*: 6.54 2*, 6*: 6.36 6*: 6.41 2*, 6*: 6.25 m (2) m (4) s (2) d (4) 1.7 pycd 5*: 6.80 m (2) pcyd 6*: 6.41 dd (2) 8.0, 1.5 the two bipyridine units have an identical chemical environment, which implies that the two phenylcyanamide ligands are co-ordinated in the same mode.This is the case for the complexes 3, 5, and 6, which have in common a chloro substituent in the 2 position of the phenylcyanamide ligand. Due to the asymmetric CwN stretching vibrations in the infrared spectra at about 2170 cm~1 , it is concluded that both phenylcyanamide ligands are co-ordinated via the nitrile nitrogen.20 The proton NMR spectra of the 4-chlorophenylcyanamide complex 4 and of the 3,5-dichlorophenylcyanamide derivative 7 show a slightly diÜerent pattern in both spectra, additional signals occur.The position of the 2,2-bipyridine-4,4-dicarboxylate 6H proton signal, which is the most down–eld shifted, gives further hints on the nature of the second compound. The NMR spectra show the occurrence of two new doublets of identical intensity. In complexes of the type the two bipyridine units are in a diÜer- Na4[Ru(dcbpy)2XY], ent chemical environment, hence each of them shows its own set of signals.The presented NMR spectra of 4 and 7 thus indicate the presence of two diÜerently co-ordinated phenylcyanamide ligands (signal integration reveals the incorporation of two bipyridine and two phenylcyanamide units), tentatively assigned to amide- and nitrile-bound, respectively. Structures of the two isomeric complexes are depicted in Fig. 2. Linkage isomerism in this system is known for the Fig. 2 Structures of the diphenylcyanamide complexes Only the nitrogen atoms of the dcbpy ligands [Ru(dcbpy)2(pcyd)2]4~. are shown. Left : major isomer. Middle: mixed isomer. Right: free phenylcyanamide ligand (deprotonated form) dithiocyanate complex 1, whose samples always contain a few percent of the isomer in which one thiocyanate ligand is Sbound. 7 In order to con–rm linkage isomerism of the phenylcyanamide ligand, a sample of complex 4, with 13Clabeling at the cyanamide carbon, was prepared from 2 and 13C-labeled thallium 4-chlorophenylcyanamide. The reaction time was, however, limited to 90 min, as, at least in other complexes such as the previously studied Na[Ru(bmipy)(dcbpy)(4- Clpcyd)] [bmipy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine], prolonged re—uxing favored the formation of nitrile-coordinated phenylcyanamide.8 The proton NMR spectrum of the 13C-labeled compound revealed an isomer content of around 10%.The 13C NMR spectra of the labeled and the unlabeled complex are compared in Fig. 3. They show some marked diÜerences due to the 13C-labeling. The weak signals appearing at 129.6 ppm are assigned to phenyl signals of the phenylcyanamide ligand, as they appear in the spectral region between 126 and 135 ppm.21 Low intensity signals at around 123.5 ppm might be due to 5 and 5@ carbon atoms on the bipyridine ligand.22 In unlabeled samples, the cyanamide carbon signal is not visible due to its lower spectral sensitivity, compared with the phenyl carbon atoms.The isotope-labeled compound shows, apart from the phenyl carbon signals, three additional peaks. The main signal at 130.0 ppm is due to N-bound phenylcyanamide in the double nitrile-bound isomer, and two smaller peaks at 130.6 ppm and 125.4 ppm show the nitrile- and amide-co-ordinated ligands in the mixed isomer, respectively. The signal at 130.6 ppm is typical for carbodiimides and the one at 125.4 ppm appears in a spectral region usually assigned to organonitriles.8,23 Absorption properties Absorption spectra of the diphenylcyanamide complexes 3»7 are dominated by strong ligand-centered absorptions at 310 nm and two 1MLCT maxima.The position of the lower energy 1MLCT band maximum varies between 514 nm (5) and 530 nm (6) and yields absorption coefficients of around 12000 M~1 cm~1.Emission maxima occur between 760 and 785 nm in methanol. Spectroscopic data can be found in Table 2. The investigated complexes show only weak solvatochromism, but spectral changes on pH variation were dramatic. These eÜects were scrutinized in spectrophotometric titrations of the di-4-chlorophenylcyanamide complex 4 in water.In Fig. 4, absorption spectra at various pH values are displayed. Lowering the pH of the test solution was accompanied by an impressive color change from red-violet to bright orange. The fact that only the MLCT maxima were aÜected, but not the bipyridine-centered transitions, reveals that the variations must have their origin in a phenylcyanamide protonation. The spectral changes directly re—ect the lower donor strength of Fig. 3 13C NMR spectra of a 13C-labeled sample (upper curve) and an unlabeled sample (lower curve) of complex 4 in [2H4]methanol New J. Chem., 1998, Pages 25»31 27Optical density Wavelength / nm Optical density Wavelength / nm Optical density Wavelength / nm Table 2 UV/VIS, emission, IR, and electrochemical data Redox potentialsb Eox/mV Ered/mV UV/VIS dataa Emissiona IRc Complex j/nm (e/103 M~1 cm~1) k/nm m/cm~1 3 524 384 310 302 785 450 [1540 2166 (12.0) (14.5) (51.4) (45.6) 4 524 378 310 298 780 405 [1560 2180 (11.6) (15.0) (54.9) (52.1) 5 516 376 310 298 775 560 [1525 2181 (12.0) (14.4) (57.0) (55.6) 6 530 384 310 302 770 460 [1560 2174 (11.8) (14.5) (58.7) (54.0) 7 514 376 310 300 775 540 [1510 2179 (12.2) (14.3) (57.9) (52.6) a In methanol. b V ersus Ag/AgCl/NaCl satd, measured in DMSO; all waves were quasi-reversible. c KBr pellet.protonated phenylcyanamide, compared with the deprotonated form.24 The origin of these changes is in the protonation/ deprotonation equilibra given in equations (1) and (2) : `H` [Ru(dcbpy)2(4-Clpcyd)2]4~ A8B ~H` [Ru(dcbpy)2(4-Clpcyd)(4-ClpcydH)]3~ (1) `H` [Ru(dcbpy)2(4-Clpcyd)(4-ClpcydH)]3~ A8B ~H` [Ru(dcbpy)2(4-ClpcydH)2]2~ (2) There are no examples of investigations of the acid-base properties of phenylcyanamide ruthenium(II) complexes in the literature.However, there are some studies on the cyanide ion. In the complex (M\Ru, Fe) the cyanide M(bpy)2(CN)2 ligand showed appreciable proton affinity (yet values pKa were below one), and both the mono- and the di-protonated species could be identi–ed.25 Their presence was veri–ed by the occurrence of two sets of isosbestic points.No clear isosbestic points would have indicated that the mixture contained more than two compounds at the same time. The same analytical method was applied for the phenylcyanamide complex 4. In Fig. 5, spectra at pH values between 8.10 and 6.22 are shown.Lowering of the pH in that range yielded a blue-shifted absorption maximum of both 1MLCT Fig. 4 Absorption spectra of complex 4 at various pH values in water. Insert : dependence of the 1MLCT absorption maximum on solution pH maxima, accompanied by a loss in absorption coefficient. A set of isosbestic points at 496, 440, 364, 324, 308 and 296 nm con–rmed the presence of only two species, which are the deprotonated and the monoprotonated form, as given in equation (1).The subsequent lowering of the pH to 5.11 resulted in a Fig. 5 1MLCT absorption bands of 4 at pH values of 8.10, 7.13, 6.86, 6.65, 6.45 and 6.22. The curve with the highest absorption was measured at pH 8.10 Fig. 6 1MLCT absorption bands of 4 at pH values of 5.83, 5.67, 5.51, 5.30 and 5.11.The curve with the lowest energy absorption maximum was measured at pH 5.83 28 New J. Chem., 1998, Pages 25»31further shift of the 1MLCT maxima to higher energies. The spectra given in Fig. 6, measured at pH values between 5.11 and 5.83, contained a second series of isosbestic points at 476, 420, 352, 322, 308 and 293 nm. The protonation process is given in equation (2).Curves measured at pH values between 5.92 and 6.11 did not coincide with any isosbestic point, revealing that, at such pH values, species in all three protonation states are present in the solution. The insert in Fig. 4 shows the pH dependence of the 1MLCT absorption maximum. Further lowering of the solution pH gives a redshift of the lowest energy 1MLCT maximum, due to protonation at the carboxylate group.The latter is also manifested by a shift of the bipyridine-centered p»p* absorption to 316 nm. In order to estimate the value of 4, relative absorption pKa changes at the 1MLCT maxima of the fully deprotonated compound (518 nm) and the di-protonated species (maximum at 460 nm) were calculated. A plot of the absorbance change vs.solution pH yielded complex values of 6.4 and 5.4. pKa The phenylcyanamide complexes can also be prepared in their protonated form by precipitation from aqueous solution by the addition of dilute HCl. Samples prepared in this way contained, as expected from the experiments described above, the phenylcyanamide ligands in their protonated form. Infrared spectra of 7, precipitated as the tetrasodium salt, showed the typical very strong and broad band at 2179 cm~1 for the asymmetric CwN stretching vibration.However, after dissolving the same complex in water and precipitating it with dilute HCl, a completely diÜerent infrared spectrum was obtained. The CwN band was split into two new bands of low intensity at 2170 and 2262 cm~1. Furthermore, a weak absorption at 3077 cm~1 occurred, which was assigned to an NwH stretching vibration.As a consequence of the pH-dependent behavior, the phenylcyanamide complexes were exclusively used in their fully deprotonated form when operating in solar cell devices to ensure an optimal light harvesting at longer wavelengths. Furthermore, it should be noted that the phenylcyanamide ligand in its anionic form is expected to be more strongly bound to the ruthenium center.Electrochemistry and photoelectrochemistry Phenylcyanamide complexes 3»7 displayed very similar electrochemical behavior. At sweep rates between 100 and 500 mV s~1 , all oxidation and –rst reduction waves were quasireversible, with a peak separation between 100 and 130 mV. The oxidation potentials were measured to be between 405 and 560 mV vs.Ag/AgCl/NaCl satd (which has a potential of 192 mV vs. NHE). The –rst reduction occurred between [1510 and [1555 mV vs. Ag/AgCl/NaCl satd. Electrochemical data are listed in Table 2. The complexes were tested as sensitizers for nanocrystalline Current»voltage characteristics and overall cell effi- TiO2 . ciencies were obtained with a thin-layer sandwich-type cell under illumination with simulated AM 1.5 solar light.The incident monochromatic photon-to-current conversion efficiency is derived from equation (3) :2 IPCE(%)\ (1.25]103 Æ photocurrent density(lA cm~2) wavelength (nm) Æ photon —ux (W cm~2) Æ 100 (3) Reported IPCE values (Table 3) are not corrected for light losses due to re—ection or absorption by the supporting electrode.All presented values are the average of three measured electrodes. IPCE values at wavelengths above 550 nm depend strongly on the –lm thickness2 of which was 9 lm in TiO2 , this work. For all phenylcyanamide complexes, short-circuit currents did increase in a non-linear fashion with light intensity, indicating surface aggregation. Hence, 50 mM 3a,7b-dihydroxy- 5b-cholanic acid was added to the ethanolic solutions of the dyes before soaking the electrodes.Cholanic acids have been shown to improve both the photocurrent and voltage of copper chlorophyllin-sensitized cells.26 For the TiO2 phenylcyanamide complexes, the same eÜect was observed. Device efficiencies reported in Table 3 were over three percent with all complexes except 7. Complex 3 showed the highest open-circuit voltages and the best –ll factors, hence yielding the highest cell efficiencies.Spectral characteristics of the investigated dyes are similar. Maximum injection efficiencies were at 47»61%, whereas at 700 nm, values of around 20% were measured. The redox potentials are obviously in a range to allow for efficient charge injection and dye regeneration, and hence are not responsible for reduced cell efficiencies.The results underline the importance of a quantitative light-to electrical energy conversion efficiency at shorter wavelengths between 400 and 600 nm in order to obtain high device efficiencies. 7 The main reason for the reduced IPCE values of the diphenylcyanamide complexes, compared with the dithiocyanate complex 1, is probably surface aggregation that was not completely suppressed, even by the presence of additives.The aggregated dye on the surface might cause a mass transport problem by blocking the semiconductor pores. This is supported by the observation that cell efficiencies are higher under low light intensities. Another reason may be that the complexes catalyze the dark current.Stability The stability of complex 4 in solution was studied under irradiation in quartz cells or NMR tubes with a 420 nm cut-oÜ UV –lter, while being kept at room temperature in a water bath. Combining absorption and NMR data reveals that in 0.25 M NaOH solution, a fast photocatalytic conversion of 4 to the complex occurs, while in meth- [Ru(dcbpy)2(OH)2]4~ anolic solution, multiple products are formed, manifested by NMR spectra and a slight blue-shift of the 1MLCT absorption maximum to 506 nm.Table 3 Photoelectrochemical data under one sun illumination IPCE(max) IPCE (700 nm) g/ Uoc Isc FF Complex % % %a (mV)b /mA cm~2 c /mA cm~2 d 3 55 22 3.7 660 8.7 0.65 4 61 16 3.0 625 8.7 0.55 5 52 23 3.1 640 7.8 0.63 6 50 23 3.1 660 7.4 0.65 7 47 17 2.6 620 6.7 0.63 a Solar cell efficiency.b Open-circuit voltage. c Short-circuit current. d Fill factor. New J. Chem., 1998, Pages 25»31 29Photovoltage / mV Photocurrent / mA cm–2 Irradiation time / days The long term stability of 3 in the photovoltaic device under irradiation was studied with semi-transparent cells, sealed with an epoxy glue. Three sets of cells were investigated. The –rst one only contained 3 and the second one contained 3 with a cholanic acid derivative as co-adsorbate.As the photostability of cholanic acids is known to be limited, the two sets of cells containing 3 were prepared, in order to avoid a misinterpretation of decomposition eÜects of the additive. The third set was sensitized with 1 and was used as a reference. Resulting average open-circuit photovoltages and shortcircuit currents are displayed in Fig. 7. Initial photovoltages directly re—ect conduction band position engineering by TiO2 protons present in the sensitizer solutions. The highest opencircuit voltage of 475 mV was reached with the proton-free 3, whereas the use of the tetraprotonated 1 lowered the potential by about 30 mV. The addition of the cholanic acid derivative diminished the potential by 80 mV.However, the positive eÜect of the additive was visible in the short-circuit currents, which were nearly doubled in comparison with the cells containing 3 without additive. The highest currents of over 10 lA cm~2 were obtained with 1. Fill factors were at around 0.70 mA cm~2 for all cells. The standard deviations of these values were extremely low, con–rming the high reproducibility of the system.These current»voltage characteristics are in good agreement with the data obtained for 3 in open cells. The efficiency of the cells sensitized with 1 was estimated to 7.4%.27 Efficiencies of 4.7 and 2.9% were obtained for cells containing 3 plus additive and without additive, respectively. In the –rst twelve days after fabrication, the cell performances improved.The eÜect was much more pronounced for the cells containing the phenylcyanamide complex. The new efficiencies were estimated at 5.1 and 4.0%, respectively. In the following, only the performance of cells containing 1 remained constant. For the cells sensitized with 3, both current and voltage began to decrease, regardless of the presence of an additive.The degradation process was accompanied by dramatic losses in –ll factor. For some of the cells, it was already at 0.4 mA cm~2 after 40 days of irradiation. After two months, the average photovoltage was at around 300 mV and currents were at 2 lA cm~2. The standard deviation of these values was over 50%. It should be noted that the positive eÜect of the additive was no longer visible.Cells containing 3 without additive were superior in all parameters after two months of irradiation. In contrast, cells sensitized with 1 Fig. 7 Open-circuit voltage (upper curves) and short-circuit current (lower curves) monitored with time after exposure to 5000 lx at 50 °C and polycarbonate UV –lter (average of –ve cells). Measurements were taken at 50 lx displayed an outstanding stability both in the current»voltage characteristics and the –ll factor (see Fig. 7). Comparison of the dithiocyanate complex 1 and the diphenylcyanamide dye 3 in the same system reveals dramatic diÜerences for the long term stability of the cells. With the results presented, a slow decomposition of 3 in the device appears likely. However, care must be taken with such an interpretation, as there are other possible explanations for the deteriorating performance.When a electrode is TiO2 immersed in a solution of 3 without additive, surface aggregation takes place. This leads to the presence of small amounts of 3 not directly attached to the semiconductor, in the cell. As shown above, species in solution are not stable under irradiation.Their decomposition products might increase the dark current of the cell. The addition of cholanic acid efficiently controls the surface coverage with dye, but decomposition products of the additive might have the same detrimental eÜects on the dark current. The dark current in—uences the open-circuit voltage of the solar cell, and in more extreme cases, the –ll factor of its maximum power point.Another source of instability might be the increased crystal water content of the phenylcyanamide dyes. In fact, the evolution of the short-circuit current of the cells containing 3 is very similar to that of cells of 1 containing some additional water. With the experiments performed, it was not possible to determine the source of instability. Nevertheless, the system used was optimized in electrolyte and redox mediator concentration for an application of 1, hence further eÜorts would have to be taken in order to optimize it for the diphenylcyanamide sensitizer.The existence of a stable system sensitized with 3 cannot yet be excluded. Conclusions The chemistry and photoelectrochemistry of ruthenium polypyridyl complexes containing two phenylcyanamide ligands was investigated.The compounds are structurally very similar to the most efficient known sensitizer, cis- 1. Due to the higher donor strength of Ru(dcbpyH2)2(NCS)2 , the phenylcyanamide ligands, compared with thiocyanate, light absorption of the new complexes at longer wavelengths was improved. The use of two phenylcyanamide ligands caused a redshift of 14 to 30 nm of the 1MLCT maximum.In the preparation of some diphenylcyanamide complexes, linkage isomers occurred, containing one nitrile- and one amide-bound phenylcyanamide ligand. This was veri–ed with 1H and 13C NMR spectroscopy. Despite better light harvesting at lower energies, solar cell efficiencies of the new complexes were limited to values between 3 and 4%. This is due to reduced charge injection efficiencies at 540 nm (which are between 47 and 61%) and to relatively poor –ll factors of the cells.In the standard solar cell system, with a given semiconductor, electrolyte, and redox mediator, the diphenylcyanamide complexes appear not to be competitive with although the positions of their Ru(dcbpyH2)2(NCS)2 , ground and excited state energy levels should not prevent them from quantitative charge injection.Surface aggregation, which was not suppressed completely by additives, and reduced dye regeneration due to mass transport problems, might be responsible for the reduced IPCE values, compared with the dithiocyanate complex. The stability of phenylcyanamide complexes under the in—uence of light was investigated. Irradiation of dye solutions in 0.25 M NaOH yielded an activation of the phenylcyanamide ligand and its fast replacement by hydroxide.The introduction of the strong electron donor hydroxide caused a strong red-shift of the MLCT bands in the UV/VIS spectra. In 30 New J. Chem., 1998, Pages 25»31methanol, the MLCT bands of the investigated complexes were blue-shifted under the in—uence of light.NMR spectra revealed phenylcyanamide complex decomposition, the formation of multiple new complexes, and the appearance of free cyanide. Solar cell devices containing 3 degraded in long-term experiments. It appears likely that complex decomposition is responsible for the loss in performance, but an unambiguous conclusion is not possible, as there might be other reasons for cell degradation.Acknowledgements research was –nancially supported by the Institut fué r This Angewandte Photovoltaik, (INAP), Gelsenkirchen, Germany. Furthermore, it is a pleasure to acknowledge experimental help of Dr. Tobias Meyer (Solaronix SA), Dr. Marie Jirousek, and Stefanie Maier. References 1 B. OœRegan and M. Graé tzel, Nature, 1991, 353, 737. 2 M. K. Nazeeruddin, A.Kay, I. Rodicio, R. Humphry-Baker, E. Mué ller, P. Liska, N. Vlachopoulos and M. Graé tzel, J. Am. Chem. Soc., 1993, 115, 6382. 3 J. Desilvestro, M. Graé tzel, L. Kavan, J. Moser and J. Augustynski, J. Am. Chem. Soc., 1985, 107 2988. 4 N. Vlachopoulos, P. Liska, J. Augustynski and M. Graé tzel, J. Am.Chem. Soc., 1988 110, 1216. 5 R. Amadelli, R. Argazzi, C. A. Bignozzi and F.Scandola, J. Am. Chem. Soc., 1990, 112, 7099. 6 R. Argazzi, C. A. Bignozzi, T. A. Heimer, F. N. Castellano and G. J. Meyer, Inorg. Chem., 1994, 33, 5741. 7 O. Kohle, S. Ruile and M. Graé tzel, Inorg. Chem., 1996, 35, 4779. 8 S. Ruile, O. Kohle, P. Peç chy and M. Graé tzel, Inorg. Chim. Acta, 1997, in the press. 9 A. B. P. Lever, Inorg. Chem., 1990, 29, 1271. 10 P. A. Anderson, G. F. Strouse, J. A. Treadway, F. R. Keene and T. J. Meyer, Inorg. Chem., 1994, 33, 3863. 11 R. J. Crutchley, K. McCaw, F. L. Lee and E. J. Gabe, Inorg. Chem., 1990, 29, 2576. 12 N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonho� te, H. Petterson, A. Azam and M. Graé tzel, J. Electrochem. Soc., 1996, 143, 2999. 13 O. Kohle, M. Graé tzel, T. Meyer and A. Meyer, Adv. Mater., 1997, 9, 904. 14 O. Kohle, PhD Thesis, No 1532, EPF Lausanne, 1996. 15 C. J. Barbeç , F. Arendse, P. Compte, M. Jirousek, F. Lenzmann, V. Shklover and M. Graé tzel, unpublished work. 16 P. Bonho� te, A. P. Dias, N. Papageorgiou, K. Kalyanasundaram and M. Graé tzel, Inorg. Chem., 1996, 35, 1168. 17 R. J. Crutchley and M. L. Naklicki, Inorg. Chem., 1989, 28, 1955. 18 M. Graé tzel and P. Liska, US Patent 5 084 365, 1992. 19 J. D. Birchall, J. R. Wood and T. D. Odonoghue, Inorg. Chim. Acta, 1979 37, L461. 20 A. R. Rezvani and R. J. Crutchley, Inorg. Chem., 1994, 33, 170. 21 M. L. Naklicki and R. J. Crutchley, Inorg. Chem., 1989, 28, 4226. 22 R. H. Herber, G. Nan, J. A. Potenza, H. J. Schugar and A Bino, Inorg. Chem., 1989, 28, 938. 23 S. Ruile, PhD Thesis No 1629, EPF Lausanne, 1997. 24 H. E. Toma and H. E. Malin, Inorg. Chem., 1973, 12, 1039. 25 A. A. Schilt, J. Am. Chem. Soc., 1963, 85, 904. 26 A. Kay and M. Graé tzel, J. Phys. Chem., 1993, 97, 6272. 27 White light conversion efficiencies were estimated as follows : 120 000 lxB1000 W/m2F50 lxB0.42 W/m2; WinB0.42]10~4 W/cm2; W/cm2; WoutBÜ…UOC … ISC\0.70]445]10]10~6 g\Wout/WinB7.4%. Received 8th April 1997; Paper 7/06740B New J. Chem., 1998, Pages 25»3

ISSN:1144-0546    

DOI:10.1039/a706740b

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

N N N bpy–pyr Photoredox pathways for the polymerization of a pyrrole-substituted ruthenium tris(bipyridyl) complex Alain Deronzier,*,a Pierre Jardon,a Agne` s Martre,a Jean-Claude Moutet,a Clara Santato,a Vincenzo Balzani,b Alberto Credi,b Francesco Paoluccib and Sergio Roffiab a L aboratoire dœElectrochimie Organique et de Photochimie (CNRS UMR 5630), Reç dox Joseph Fourier Grenoble 1, BP 53, 38041 Grenoble cedex 9, France Universiteç b Dipartimento di Chimica ììG.Ciamicianœœ, di Bologna, via Selmi 2, 40126 Bologna, Universita` Italy The oxidative quenching by a diazonium salt of the excited state of a pyrrole-substituted complex in Ru(bpy)32` acetonitrile leads to its polymerization, forming a soluble metallopolymer. The same phenomena is observed with dioxygen as the quencher.The data are consistent with the initial oxidation of the pyrrole moieties by RuIII species formed upon electron-transfer quenching of by This process appears competitive [Ru(bpy)3]2`* O2 . with pyrrole oxygenation by singlet oxygen formed by energy transfer, due to the subsequent fast polymerization reaction. Photopolymeç risation par transfert dœeç lectron dœun complexe tris(pyridine) de ruthenium(II) substitueç par un groupe pyrrole.Lœinhibition par un sel de diazonium de lœeç tat exciteç dœun complexe du type substitueç par Ru(bpy)32` un groupe pyrrole conduit dans lœaceç tonitrile a` sa photopolymeç risation, pour former un polyme` re soluble. Le me� me pheç nome` ne est observeç en preç sence dœoxyge` ne moleç culaire, lœensemble des reç sultats eç tant en accord avec un meç canisme dans lequel les groupes pyrrole sont oxydeç es par les espe` ces RuIII, formeç es a` la suite de lœinhibition par transfert dœeç lectron de par Ce processus est compeç titif avec les reç actions dœoxygeç nation [Ru(bpy)3]2`* O2 .du pyrrole par lœoxyge` ne singulet formeç par transfert dœeç nergie, en raison de la grande vitesse de la reç action de polymeç risation conseç cutive au transfert dœeç lectron.The fabrication of redox-active metallopolymers is an important subject in the construction of electrochemical and photochemical devices.1 The applications include electrocatalysis, electroanalysis, molecular display, and energy conversion. Polymers for these purposes are readily deposited as –lms on electrode surfaces by oxidative electropolymerization of functionalized pyrroles.2 In contrast, the photosensitized polymerization of pyrroles has only rarely been applied to the synthesis of polypyrroles.3h6 The photochemical approach is of great interest for depositing polypyrroles onto any type of surface : conducting,5,6 semiconducting,3 or even insulating4h6 materials, and might provide an entry into soluble transitionmetal- complex-based polymers.Photopolymerization of pyrrole is readily eÜected in multicomponent solutions containing a photoactive species and an irreversible electron acceptor, e.g. (bpy is 2,2@-bipyridine) with [Ru(bpy)3]2` or (dpp is 2,9-diphenyl-1,10- [Co(NH3)5Cl]2` 4 [Cu(dpp)2]` phenanthroline) with p-nitrobenzyl bromide.5 Photopolymerization of a pyrrole-functionalized [Ru(bpy)3]2` complex with the reversible oxidative quencher 1,1@-dimethyl- 4,4@-bipyridinium was also proved possible,6 but the mechanism remains to be fully elucidated. This paper reports the oxidative photopolymerization of the pyrrole-substituted ruthenium complex [Ru(bpy)2(bpywith a diazonium salt as an irreversible oxidative pyr)]2` quencher to form a soluble metallopolymer.Since we have recently shown that adsorbed layers of amphiphilic pyrrolesubstituted polypyridyl ruthenium(II) complexes can be readily photopolymerized with dioxygen, as the only quencher in both organic and aqueous electrolytes,7 we report here a test of this unconventional polymerization process in —uid solution with This study is of great inter- [Ru(bpy)2(bpy-pyr)]2`.est in the much debated question of the electron transfer quenching of the excited state of by molecular [Ru(bpy)3]2` oxygen.8 Results and Discussion The electrochemical behavior and oxidative electropolymerization of pyrrole-substituted ruthenium tris(bipyridyl) complexes in acetonitrile electrolyte have already been investigated.9 The cyclic voltammetry curve for [Fig. 1(a), curve a] shows three well- [Ru(bpy)2(bpy-pyr)]2` behaved pairs of peaks in the negative potential area, due to the successive one-electron reductions of the ligands.10 Upon oxidation, the RuII@III couple appears to be weakly reversible and is characterized by an abnormally high anodic peak. Since N-alkylpyrroles are known to be oxidized around 1.3 V,12 this behavior has been attributed to the catalytic twoelectron irreversible oxidation of the pyrrole group by RuIII species, leading to a functionalized polypyrrole.In fact, thin poly[pyrrole ruthenium(II) complex] –lms could be grown on a platinum electrode by repeated scans over the [0.1 to 1.6 V range.9 However, the –lm-forming ability of [Ru(bpy)2(bpywhich contains only one pyrrole group, is poor in pyr)]2`, acetonitrile electrolyte.Exhaustive oxidation at 1.3 V consumed 3.6 electrons per molecule of complex and led mainly New J. Chem., 1998, Pages 33»37 33E / V vs. SCE (a) (b) (c) Fig. 1 Cyclic voltammograms at l \ 0.1 V s~1 in 0.1 CH3CN] M TBAP. (a) 2 mM before (curve a) and after [Ru(bpy)2(bpy-pyr)]2` (curve b) exhaustive oxidation at 1.3 V on a large platinum gauze.(b) 1 mM mM before [Ru(bpy)2(bpy-pyr)]2`]2.3 p-CH3C6H4N2 ` (curve a) and after (curve b) visible irradiation under argon. (c) 1 mM after visible irradiation under air [Ru(bpy)2(bpy-pyr)]2` to a solution of oligomers and/or polymer in a RuIII form.9 The cyclic voltammetry curve of these oxidized species [Fig. 1(a), curve b] presents a fully reversible RuII@III wave and illbehaved waves for the ligand-centered reductions, along with several broad peaks due to protons released upon polymerization (two protons per pyrrole ring2). It must be emphasized that no wave corresponding to the regular polypyrrole electroactivity can be seen on the voltammogram.Obviously the conductivity of the polypyrrole matrix, and hence its electroactivity, have been destroyed by over-oxidation, due to the high anodic potential used for its synthesis.13 The voltammogram of remains the [Ru(bpy)2(bpy-pyr)]2` same upon visible irradiation (k[405 nm) of the solution for hours under an argon atmosphere.We checked that the luminescence of the excited state in deaerated ace- [Ru(bpy)3]2`* tonitrile was not quenched by pyrrole, even when present in large excess.This result was further corroborated by the determination of the luminescence lifetime (D950 ns) and the emission quantum yield (0.064) of which [Ru(bpy)2(bpy-pyr)]2`, are very close to those of It should be noted [Ru(bpy)3]2`.14 that the absorption and emission spectra of the pyrrolesubstituted complex in acetonitrile solution at room temperature are also identical to those exhibited by [Ru(bpy)3]2`. In contrast, we found that was [Ru(bpy)2(bpy-pyr)]2` readily photopolymerized in deareated acetonitrile and in the presence of a slight excess (2.3 equivalents) of pmethylbenzenediazonium salt.This compound is known15 to efficiently quench to form [Ru(bpy)3]2`* [Ru(bpy)3]3` (kq\ 1.8]109 M~1 s~1) with a good quantum yield ('\0.13). The cyclic voltammetry curve recorded after visible irradiation [Fig. 1(b), curve b] shows that the RuII@III wave has become well-reversible, and that the diazonium salt has been almost fully consumed, as demonstrated by the disappearance of its irreversible reduction wave V;15 Fig. 1(b), curve [Ep\[0.23 a]. In addition, the appearance of several new reduction waves can be explained by the presence of protons released during the photoprocess. Comparison between curves b of Fig. 1(a) and (b) shows that the visible irradiation of the pyrlesubstituted complex in the presence of this oxidative quencher gives the same result as its electrooxidation. This means that the complex is photopolymerized in the presence of a diazonium salt to give a soluble polymer, as a consequence of pyrrole oxidation by the photogenerated RuIII species.We have veri–ed that the photopolymerization is incomplete in the presence of less than two equivalents of diazonium salt. The overall process is close to that reported for the photosensitized polymerization of regular pyrrole with as the sensitizer and as the oxida- [Ru(bpy)3]2` Co(NH3)5Cl tive quencher.4 However, here the process is probably intramolecular, since the pyrrole group is covalently linked to the photosensitizer.Highly irreversible oxidative quenching of by the diazonium salt (denoted [RuII(bpy)2(bpy-pyr)]2`* (eqn 1) leads to Fast intra- ArN2 `) [RuIII(bpy)2(bpy-pyr)]3`. molecular electron transfer can then occur between RuIII and the pyrrole moities (eqn 2), giving rise to the polymerization of the complex (eqn 3).Owing to the moderate quenching rate of the excited complex by the diazonium salt and the rather low overall photopolymerization quantum yield (see below), some back reactions and other deactivation pathways are expected to take place. For clarity they are not discussed here. [RuII(bpy)2(bpy-pyr)]2`*]ArN2 `] [RuIII(bpy)2(bpy-pyr)]3`]ArN2~ (1) ArN2~]Ar~]N2~ Ar~]SH]ArH (S\solvent) [RuIII(bpy)2(bpy-pyr)]3`][RuII(bpy)2(bpy-pyr~`)]3` (2) n [RuII(bpy)2(bpy-pyr~`)]3`]poly[RuII(bpy)2(bpy-pyr)]2` (3) The photopolymerization can be followed by 1H NMR experiments conducted in deuterated acetonitrile solutions of complex and diazonium salt.At the end of the photolysis, the aromatic resonances at 5.64 ppm for the H atoms of the pyrrole ring have vanished, as have those at 8.32 and 7.72 ppm of the diazonium (Fig. 2). The quantum yield of the photopolymerization at 436^4 nm was evaluated from the decrease of the pyrrolic 1H NMR signals. An overall quantum yield of 0.08 was obtained when 73% of the initial complex (1 mM in was polymerized in the presence of an excess CD3CN) of p-methylbenzenediazonium salt (4 mM). Thus the polymerization by RuIII species is 64% efficient, taking into account that these species are formed with a quantum yield of 0.13 15 in these experimental conditions.In addition, the FTIR spectrum of the isolated photopolymer (see Experimental) shows a carbonyl band at 1685 cm~1, typical of the formation of some pyrrolidone moeties in the polymer.16 A similar band was found at 1703 cm~1 in the polymer synthesized by oxidative electropolymerization.These observations indicate that the polypyrrole matrix has been over-oxidized during the photopolymerization process, since RuIII is a strong oxidizing agent. In a similar fashion, the RuII@III couple of [Ru(bpy)2(bpybecomes well-reversible after visible irradiation for a pyr)]2` few minutes under air or dioxygen [Fig. 1(c)].This observ- 34 New J. Chem., 1998, Pages 33»37d (b) (a) d (c) (b) (a) Fig. 2 250 MHz 1H NMR spectra of [Ru(bpy)2(bpy-pyr)][PF6]2 ]4 mM in before (a) and after (b) visible p-CH3C6H4N2 ` CD3CN irradiation under argon ation suggests that the complex is also photopolymerized in these experimental conditions. It is noteworthy that no peak due to the reduction of protons released upon pyrrole polymerization can be seen in the voltammogram.We will see in the proposed photopolymerization scheme that released protons are quantitatively trapped by This photo- O2~~. polymerization could also be followed by recording the 1H NMR spectra of in aerated [Ru(bpy)2(bpy-pyr)][PF6]2 solution before and after visible irradiation (Fig. 3). CD3CN After photopolymerization, the aromatic resonances for the H atoms of the pyrrole ring have vanished while new small peaks in the 5.4»6.9 ppm range appear.The spectra now exhibit broad resonances in the aliphatic region. After the photopolymerization process the signal corresponding to the Fig. 3 250 MHz 1H NMR spectra of in [Ru(bpy)2(bpy-pyr)][PF6]2 before (a) and after (b) visible irradiation under air ; spectrum CD3CN (c) was recorded after about 30% of the initial complex was photopolymerized methylene group adjacent to the pyrrole ring is strongly disturbed and is shifted about 0.6 ppm down–eld with respect to the monomer.The resonance for the methylene group adjacent to the bpy moieties broadens but is not shifted, and that for the methyl group in the 4@ position of the ligand is unchanged after polymerization.It should also be noted that the spectrum remains essentially the same in the aromatic region. Owing to the extreme insolubility of most polypyrroles, very little information on their 1H NMR spectra can be found in the literature. However, similar features have been reported for the 1H NMR spectra of poly(3-butylsulfonate pyrrole), which is slightly soluble in It should be noted D2O.17 that the FTIR spectrum of the isolated photopolymer is very similar to that of the electropolymer and of the photopolymer synthesized in the presence of diazonium salt.In particular, it shows a strong carbonyl band at 1709 cm~1, typical of the over-oxidation of the polypyrrole matrix (see above). Additional experiments have been carried out to obtain more information on this photoprocess.First, the quantum yield of the photopolymerization at 436 nm was evaluated from the evolution of the 1H NMR spectra of the complex. A quantum yield of 0.36 for the overall process was obtained when 80% of the initial complex (1 mM in aerated CD3CN) was transformed. It is well-known that the quenching of the excited states of and related complexes by molecular oxygen can [Ru(bpy)3]2` occur via both electron transfer and energy transfer processes. 18 Since the polymerization reaction must involve –rst the oxidation of the pyrrole moieties of the complex,2,9 we outlined a possible photopolymerization mechanism in the presence of oxygen on the basis of the excited state and redox properties of and The known [RuII(bpy)2(bpy-pyr)]2` O2 .energy levels for this system are shown in Fig. 4. On the basis of the energy level diagram, oxidative quenching of by (eqn 4) can take place with for- [RuII(bpy)2(bpy-pyr)]2` O2 mation of the superoxide radical anion and O2~~ Before the back-electron-transfer [RuIII(bpy)2(bpy-pyr)]3`. reaction (eqn 5) takes place, can rear- [RuIII(bpy)2(bpy-pyr)]3` range by intramolecular electron transfer to [RuII(bpy)2(bpy- (eqn 2), as seen above.19 At this stage, other back- pyr~`)]3` electron-transfer reactions can occur, leading to and either ground state oxygen (eqn [RuII(bpy)2(bpy-pyr)]2` 6) or singlet oxygen (eqn 7).Some [RuII(bpy)2(bpy-pyr~`)]3` molecules, however, can escape the back reactions and polymerize (eqn 3).Scavenging of by H` to form a hydro- O2~~ peroxyl radical and free RuIII complex (eqn 8) can of course Fig. 4 Energy level diagram for and [Ru(bpy)2(bpy-pyr)]2` O2 solution) (CH3CN New J. Chem., 1998, Pages 33»37 35help avoid the back electron transfer.8 [RuII(bpy)2(bpy-pyr)]2`*]O2 ] [RuIII(bpy)2(bpy-pyr)]3`]O2~~ (4) [RuIII(bpy)2(bpy-pyr)]3`]O2~~] [RuII(bpy)2(bpy-pyr)]2`]O2 (5) [RuIII(bpy)2(bpy-pyr)]3`][RuII(bpy)2(bpy-pyr~`)]3` (2) [RuII(bpy)2(bpy-pyr~`)]3`]O2~~] [RuII(bpy)2(bpy-pyr)]2`]O2 (6) [RuII(bpy)2(bpy-pyr~`)]3`]O2~~] [RuII(bpy)2(bpy-pyr)]2`]*O2(1*g) (7) n [RuII(bpy)2(bpy-pyr~`)]3`]poly[RuII(bpy)2(bpy-pyr)]2` (3) O2~~]H`]HO2~]12 H2O2]12O2 (8) [RuII(bpy)2(bpy-pyr)]2`*]O2 ] [RuII(bpy)2(bpy-pyr)]2`]*O2(1*g) (9) *O2(1*g)]O2(3&g)]hm(1260 nm) (10) [RuIII(bpy)2(bpy-pyr)]3`]O2~~] [RuII(bpy)2(bpy-pyr)]2`]*O2(1*g) (11) [RuII(bpy)2(bpy-pyr)]2`]*O2(1*g)] photooxygenation products (12) can also be quenched by via [RuII(bpy)2(bpy-pyr)]2`* O2 energy transfer (eqn 9).It is well-known20 that the quantum yield of formation upon excitation of the regular *O2 (1*g) complex in aqueous solution is only 0.5, indicat- [Ru(bpy)3]2` ing that there is some other deactivation path that competes with the energy-transfer mechanism. It has also been established20,21 that for the yield of the [Ru(bpy)3]2` electron-transfer products and is negligi- [Ru(bpy)3]3` O2~~ ble.In for example, it is lower than 10~3.21 It was CH3CN, concluded20 that be quenched by oxygen via [Ru(bpy)3]2`can both energy- and electron-transfer processes, and that the electron-transfer products cannot be easily evidenced because of the fast back electron transfer of the ion pair within the solvent cage.However, it has been very recently demonstrated8 that can be released from the [Ru(bpy)3]3` solvent cage by protonation of upon excitation in acidic O2~~ solutions. Formation of was obtained with a [Ru(bpy)3]2` limiting quantum yield of 0.49 at in–nite acid concentration, showing the efficiency of the photoredox process in these conditions.In a similar fashion, in the case of [RuII(bpy)2(bpythe fast intramolecular electron transfer from the pyr)]2` pyrrole moiety to the oxidized RuIII center (eqn 2) can compete with the back electron transfer (eqn 6 and 7), allowing for the photopolymerization process.However, a photoprocess involving singlet oxygen cannot be ruled out since formation of singlet oxygen in our system is proved by the observation of the sensitized infrared (1260 nm) luminescence of singlet oxygen (eqn 10) upon excitation of We found that the quantum yield of [RuII(bpy)2(bpy-pyr)]2`. singlet oxygen formation for is essen- [Ru(bpy)2(bpy-pyr)]2` tially the same as that found for Besides energy [Ru(bpy)3]3`. transfer (eqn 9) there are in principle two other reaction paths for singlet oxygen formation: (i) back electron transfer from to (eqn 11), which does not O2~~ [RuIII(bpy)2(bpy-pyr)]3` occur for and can probably also be excluded [Ru(bpy)3]2` 22 for and (ii) back electron transfer [RuII(bpy)2(bpy-pyr)]2`, from to the oxidized pyrrole moiety of O2~~ [RuII(bpy)2(bpy- (eqn 7).It can then be anticipated that the pyrrole pyr~`)]3` groups could be photooxygenized by singlet oxygen (eqn 12), rather than undergo oxidative polymerization, since the photosensitized oxygenation of pyrrole and its derivatives is well-established.23 In order to investigate this other possible photoprocess, we have photolyzed in [Ru(bpy)2(bpy-pyr)]2` aerated in the presence of singlet oxygen quenchers. CD3CN We –rst chose 2,5-dimethylfuran, which is known to react with with a high rate constant (ìpreferred valueœ :24 *O2 (1*g) 2.3]10~8 M~1 s~1). In the presence of one molar equivalent of this quencher, the quantum yield for the reaction of pyrrole dropped from 0.36 to 0.10.This result might indicate that, in the absence of singlet oxygen quencher, pyrrole consumption is mainly due to its oxygenation rather than to its oxidative polymerization.However, we found that 2,5-dimethylfuran is readily oxidized by RuIII species. This was easily demonstrated by a CV experiment conducted in a solution of [Ru(bpy)3]3` in acetonitrile electrolyte, which showed that the RuII@III couple became fully irreversible in the presence of one molar equivalent of 2,5-dimethylfuran. Thus, this compound is able to quench photogenerated RuIII species as well as singlet oxygen.Additional quenching experiments have been carried out using the less oxidizable 2,3-dimethyl-2-butene (tetramethylethylene, TME), which is know to react with singlet oxygen with a good rate constant (ìpreferred valueœ :24 3]10~7 M~1 s~1), to give 2,3-dimethyl-3-hydroperoxy-1- butene.25 1H NMR measurements have shown that photolysis of the pyrrole-substituted complex (1 mM in in the CD3CN) presence of TME (3 mM) only resulted in a small decrease of the quantum yield for pyrrole consumption (0.25, against 0.36 without quencher). An important result is that this yield remained the same upon increasing the concentration of TME, up to 10 molar equivalents.The efficiency of the quenching of singlet oxygen by TME in these experimental conditions can be evaluated from 1H NMR spectra, since the hydroperoxide formed presents characteristic resonances at 8.81 ppm (wOOH) and 4.89 ppm The quantum (xCH2). yield for TME photooxygenation was high (0.30) and remained the same regardless of the amount of added TME (3, 6, or 10 molar equivalents).In addition, we have veri–ed in separate experiments that 2,3-dimethyl-3-hydroperoxy-1- butene was also formed upon visible irradiation of a pyrrolefree complex dmbpy\4,4@-dimethyl- M[Ru(bpy)2(dmbpy)]2`, 2,2@-bipyridineN 1 mM in aerated together with 3 mM CD3CN TME; quantum yield : 0.5, even in the presence of 1 mM Nmethylpyrrole.All these results mean that the singlet oxygen formed upon photolysis of is quantitatively [Ru(bpy)2(bpy-pyr)]2` quenched by TME. This proves that pyrrole comsuption is mainly due to its oxidative polymerization with RuIII species formed via electron-transfer quenching of the excited state of the complex with dioxygen (quantum yield 0.25). However, in the absence of a singlet oxygen quencher, some pyrrole photooxygenation is likely to take place with a lower quantum yield. Conclusion The present novel photosensitized polymerization procedure appears to be a useful approach for the elaboration of soluble photoredox polymers. It might also provide a way to synthesize soluble supramolecular polymers. On the other hand, experimental evidence strongly suggests that electron transfer from to molecular oxygen is able to compete [Ru(bpy)3]2`* with energy transfer in organic media, if RuIII species are released from the solvent cage by a fast chemical reaction such as pyrrole oxidation and polymerization.Experimental Materials The bpy-pyr ligand and were syn- [Ru(bpy)2(bpy-pyr)][PF6]2 thesized according to literature procedures.9 The p- 36 New J.Chem., 1998, Pages 33»37methylbenzene diazonium salt was prepared and puri–ed by a standard procedure.26 Acetonitrile (Rathburn HPLC grade S or Merck Uvasol) and deuterated acetonitrile (S.d.S.) were used as received. Tetrabutylammonium perchlorate (TBAP, Fluka puriss) was dried under vacuum at 80 °C for 3 d. Measurements The electrochemistry measurements in elec- CH3CN»TBAP trolyte were done with a PAR 273 electrochemical system.The working electrode consisted of a 5 mm diameter platinum disc sealed in glass. Potentials are reported vs. the saturated calomel reference electrode (SCE). Solutions of photopolymers were evaporated under vacuum. The resulting solids were dissolved in a minimum of and precipitated by the addition of The elec- CH2Cl2 Et2O.tropolymer was synthesized using as the sup- (CH3)4NBF4 porting electrolyte. The solvent was removed under vacuum and the residue extracted with which was then con- CH2Cl2 , centrated. The polymer was precipitated by the addition of IR spectra were recorded in KBr pellets. Et2O. Visible irradiations were done with the light of a 250 W Hg lamp –ltered through UV and IR cutoÜ –lters.For the determination of the quantum yield, the 436^4 nm emission from a Xe lamp was isolated by using a bandpass –lter (Oriel 5645). The intensity of light adsorbed by the sample was measured by a standard actinometry procedure. Quantum yields were determined from the integration of the 1H NMR spectral signals of the sample before and after irradiation. Luminescence lifetimes were measured with an Edinburgh single-photon-counting apparatus lamp, 337 nm).Lumi- (N2 nescence quantum yields were obtained following a literature procedure27 using as a standard. All the photo- [Ru(bpy)3]2` physical experiments were performed on 10~4 M CH3 CN solutions degassed by repeated freeze»pump»thaw cycles. Near-infrared oxygen emission was measured with a homebuilt instrument consisting of an Ar ion laser as the excitation source nm), a monochromator (Edinburgh) with a (kex\488 grating of 1200 lines/nm, 1-mm blaze, and a liquid-nitrogencooled germanium detector and preampli–er (Northcoast, model EO-817L).The sample (air-equilibrated CH3CN solution) was held in a 10 mm]10 mm cuvette, and a beam chopper (Stanford Research, model SR540) was placed between the excitation source and the sample; luminescence was monitored at right angles to the excitation.A muon –lter (Northcoast, model 829B) was used as an electronic signal –lter, after the signal was sent to a lock-in ampli–er (Stanford Research, model SR510). References 1 H. D. Abrun8 a, Coord. Chem. Rev., 1988, 86, 135. 2 A. Deronzier and J.-C. Moutet, Acc. Chem.Res., 1989, 22, 249 and Coord. Chem. Rev., 1996, 147, 339. 3 For the photoelectrochemical polymerization of pyrrole on semiconductors, see for example: (a) R. Nou–, D. Tench and L. F. Warren, J. Electrochem. Soc., 1980, 127, 2310; (b) T. Skotheim, I. Lundstroé m and J. Prejza, J. Electrochem. Soc., 1981, 128, 1625; (c) T. Kobayashi, Y. Taniguchi, H. Yoneyama and H. Tamara, J.Phys. Chem., 1983, 87, 768; (d) M. Okano, K. Itoh, A. Fujishima and K. Honda, Chem. L ett., 1986, 469. 4 H. Segawa, T. Shimidzu and K. Honda, J. Chem. Soc., Chem. Commun., 1989, 132. 5 J. M. Kern, J. P. Sauvage, J. Chem. Soc., Chem. Commun., 1989, 657. 6 P. Subramanian, H.-T. Zhang and J. T. Hupp, Inorg. Chem., 1992, 31, 1540. 7 A. Deronzier, J.-C. Moutet and D. Zsoldos, J.Phys. Chem., 1994, 98, 3086. 8 X. Zhang and M. A. Rodgers, J. Phys. Chem., 1995, 99, 12797. 9 S. Cosnier, A. Deronzier and J.-C. Moutet, J. Electroanal. Chem., 1985, 193, 193. 10 From investigations made in DMF»tetrabutylammonium tetra- —uoroborate at [54 °C with the use of high vacuum techniques,11 this complex shows six one-electron reduction waves at [1.39 V, [1.53 V, [1.77 V, [2.42 V, [2.62 V and [2.94 V, respectively. 11 S. Roffia, M. Marcaccio, C. Paradisi, F. Paolucci, V. Balzani, G. Denti, S. Serroni and S. Campagna, Inorg. Chem., 1993, 32, 3003. 12 A. Diaz, J. Castillo, K. K. Kanazawa, J. A. Logan, M. Salmon and O. Fajardo, J. Electroanal. Chem., 1982, 133, 233. 13 S. Cosnier, A. Deronzier and J.-F. Roland, J. Electroanal. Chem., 1990, 285, 133. 14 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. von Zelewsky, Coord. Chem. Rev., 1988, 84, 85. 15 H. Cano-Yelo and A. Deronzier, J. Chem. Soc., Faraday T rans. 1, 1984, 80, 3011. 16 (a) F. Beck, P. Braun and M. Oberst, Ber. Bunsenges. Phys. Chem., 1987, 91, 967; (b) P. A. Christensen and E. A. Hamnett, Electrochim. Acta, 1991, 36, 1263. 17 E. E. Havinga, W. ten Hoeve, E. W. Meijer and H. Wynberg, Chem. Mater., 1989, 1, 650. 18 See for example: C. T. Ling and N. Sutin, J. Phys. Chem., 1976, 80, 97. 19 We have not necessarily ruled out the formation of by an intermolecular electron transfer [RuII(bpy)2(bpy-pyr~`)]3` between and [RuIII(bpy)2(bpy-pyr)]3` [RuII(bpy)2(bpy-pyr)]2`. 20 Q. G. Mulazzani, M. DœAngelantonio, M. Venturi and M. A. J. Rodgers, J. Phys. Chem., 1994, 98, 1145. 21 Q. G. Mulazzani, M. DœAngelantonio, M. Venturi and M. A. J. Rodgers, J. Phys. Chem., 1991, 95, 9605. 22 Q. G. Mulazzani, M. Ciano, M. DœAngelantonio, M. Venturi and M. A. J. Rodgers, J. Phys. Chem., 1988 110, 2451. 23 H. H. Wasserman and R. W. Murray, Singlet Oxygen, Academic, New York, 1979. 24 F. Wilkinson and J. G. Brummer, J. Phys. Chem., Ref. Data, 1981, 10, 809. 25 A. M. Winner and K. D. Bayes, J. Phys. Chem., 1966, 70, 302. 26 A. Roe, in Organic Reactions, ed. R. Adams, Wiley, New York, 1949, vol. 5, p. 203. 27 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991. Received 15th April 1996; Paper 7/06749F New J. Chem., 1998, Pages 33»37 37

ISSN:1144-0546    

DOI:10.1039/a706749f

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

A study of the electron-transfer reaction between and Fe(CN)2(bpy)2 in solvent mixtures: the translational component of solvent S2O8 2ó reorganization Manuel Sanchez Matamoros-Fontenla, Pilar Loç pez-Cornejo, Pilar Peç rez, Rafael Prado-Gotor, Reyes de la Vega, Maria Luisa Moyaç and Francisco Saç nchez* Departamento de Facultad de Universidad de Sevilla, C/ Prof. Quïç mica Fïç sica, Quïç mica, Garcïç a s/n, 41012 Sevilla, Spain Gonzaç lez The kinetics of the oxidation of dicyanobis(2,2@-bipyridine) iron(II) by peroxodisulfate [Fe(CN)2(bpy)2] (S2 O82~) has been studied in diÜerent water»cosolvent mixtures.The cosolvents used were methanol, tert-butyl alcohol, ethylene glycol and glycerol. The results are explained assuming an additional component of the reorganization free energy of the solvent in the mixtures, caused by a translation of the solvent molecules, as a consequence of the changes in the composition of the (at least) innermost solvation shell.A quantitative estimation of this component is attempted. Understanding solvent eÜects on chemical reactivity is of prime importance in chemistry. These eÜects, as is well known, are extremely diÜerent, depending on the solvent and the reaction under study; sometimes the solvent simply provides a physical environment for the reaction.At the other extreme, it participates as a reactant. For electron-transfer reactions the solvent plays an essential role that is well understood, since the seminal papers of Marcus1, Hush2 and others.3 In the last few years, we have studied electron-transfer reactions in mixed solvents, constituted of water and an organic cosolvent, as well as aqueous electrolyte solutions.Solvent eÜects in these media are more difficult to explain because in the mixed solvents the reactivity can depend on preferential solvation phenomena. Indeed, there are at least three diÜerent solvent»solvent interactions that can also have kinetic in—uences.However, these kinds of solvents are of interest in relation to many areas of chemistry and biology. In particular, it is possible, by using mixed solvents, to continously change the macroscopic properties of the reaction media. They have, therefore, become a subject of both experimental and theoretical interest 4. This paper was motivated by previous results, on optical5 and thermal6 electron-transfer reactions in mixed solvents, that seemed to point to the possibility of anomalous behaviour in these media: reactivity trends are the opposite of those expected, based on the continuum model of the solvent.To con–rm this –nding and to quantify the magnitude of the deviation from the model, if any, is the objective of this work. This question is of interest in relation to the types of models to be used for solvent mixtures.Thus, although simple continuum models seem to be sufficient for pure solvents,7 the application of these models to mixed solvents has been criticised and still remains an open question.8 In order to deal with the questions raised above, we have studied the kinetics of the oxidation of by Fe(CN)2(bpy)2 in diÜerent water»cosolvent mixtures. This system is S2O82~ appropriate for several reasons : –rst of all, the zero charge of the iron complex implies that the true electron-transfer rate constant, diÜers only by an approximately constant factor, ket , from the observed rate constant, is the equi- KIP , kobs ; KIP librium constant corresponding to the formation of the precursor (encounter) complex from the reactants.On the other hand, the iron complex, after the electron-transfer reaction, becomes a positively charged species. This circumstance will probably cause a signi–cant change in the (preferential) solvation of this reactant, this being the point we want to address here. The iron complex is also useful as a reactant because of a recently published paper9 concerning the electrochemical reduction of This paper gives the value of the Fe(CN)2(bpy)2 `.internal reorganization free energy of this reactant, a datum that, as will be seen later, permits the calculation of the internal reorganization free energy for This calculation is S2O82~. important in order to check the reliability of our calculations (see point iii in the Appendix).As for the other reactant, the peroxodisulfate, there are data on the redox potential of the couple in water. These data and the transfer S2O82~/S2O83~ free energies of the peroxodisulfate ions from water to the aqueous mixtures, also available, permit a reasonable estimation of the free energy for the reaction studied in these mixtures (see below). More importantly, in the oxidations by peroxodisulfate, the inner-shell reorganization energy of this reactant controls the kinetics of these reactions (but not the solvent eÜects).So, the use of this oxidant guarantees the constancy of the preexponential factor in (see Discussion). This ket is especially useful in order to obtain the variations of the activation free energy caused by the solvent. Results Table 1 contains the second-order rate constants, for the kobs , process studied in the diÜerent water»cosolvent mixtures (D\dielectric constant). The standard formal potentials of the couple, collected in Table Fe(CN)2(bpy)2 `/Fe(CN)2(bpy)2 2, were corrected for liquid junction potentials using those of the Fe(g5-Cp) couple, which also appear in the 2 `/Fe(g5-Cp)2 same table.After correction for liquid junction potentials, a correction for ionic strength eÜects (from 0.1 to 0.069 mol dm~3) was also taken into account. Table 3 presents the true rate constants for the electrontransfer process. These constants can be obtained from kobs through: ket\ kobs KIP (1) In this case, as the formation of the encounter (precursor complex) involves a neutral species, can be considered KIP independent of the dielectric constant of the reaction media. New J.Chem., 1998, Pages 39»44 39Table 1 Second-order rate constants (102 mol~1 dm3) for the reaction of with at 298.2 K in diÜerent water» kobs/s~1 Fe(CN)2(bpy)2 S2O82~ cosolvent mixtures D Methanol tert-Butyl alcohol Ethylene glycol Glycerol 76 (0.033) 9.91 (0.007) 24.8 (0.028) 2.28 (0.020) 1.87 74 (0.060) 6.00 (0.013) 19.6 (0.051) 1.66 (0.036) 1.73 70 (0.112) 4.81 (0.025) 15.1 (0.110) 1.03 (0.077) 1.47 66 (0.169) 3.00 (0.040) 11.5 (0.174) 0.75 (0.131) 1.24 64 (0.200) 2.41 (0.047) 9.24 (0.215) 0.49 (0.165) 1.18 60 (0.265) 1.56 (0.062) 7.64 (0.289) 0.36 (0.234) 0.98 * Parenthesis correspond to molar fractions of the cosolvent used.mol~1 dm3 s~1. kobs(water)\30.9]10~2 For this reason, the same value of this parameter was used in all the media mol~1dm3).10 (KIP\1.839) Table 4 contains the calculated redox potentials for The calculation of these redox potentials S2O82~/S2O83~.was performed as follows : the starting point is the value of this potential in water (1.39 V).11 From this value, the redox potential in the mixtures can be calculated if the free energies of transfer of and from water to the mixtures S2O82~ S2O83v are known.These free energies are known, for but S2O82~ 12 not for (an unstable species). So, we have estimated the S2O83v latter using the following approximation: *Gt(S2O33~) *Gt(S2O82~) \ Z2(S2O83~) Z2(S2O82~) \ 9 4 (2) where Z is the charge of the ion being transferred. This permits the calculation of standard redox potentials for this couple in the diÜerent solvents through: (E0)\EH2O 0 ] RT F ln ct(S2O82~) ct(S2O83~) (3) and RT ln ct(i)\*Gt(i) (4) However, we are more interested in the standard formal redox potentials corresponding to the actual conditions of the reaction, the latter being carried out at an ionic strengh of 0.069 mol dm~3.Thus a Debye»Hué ckel correction was applied to give the results appearing in Table 4.The procedure used to calculate the variations of the redox potential of the redox couple could be considered a S2O82~/S2O83~ rather crude approximation. However, hydration free energies of anions are proportional to the square of their charges (see point ii in the Appendix). The validity of eqn 2 depends on the supposition that the radii of and are similar.S2O82~ S2O83v After electron transfer the wOwOw bond will increase its length though there are no data on this length increase. However, for cobalt complexes that, like present a S2O82~, high value of the internal reorganization free energy, this increase after electron transfer is about 6%.13 This is thus about the diÜerence in the radii to be expected in the present case.(The diÜerences in radii of the anions in Fig. A-1 of the Appendix are of this order and as can be seen, eqn 2 holds.) Finally, the average transfer of for water to the mix- S2O82~ tures studied in this work is about 5 kJ mol~1. Six per cent of this value is about 0.3 kJ mol~1, which is of the order of magnitude of the uncertainty in the values of the redox potentials measured directly.From data in Tables 2 and 4 the free energy of reaction, *G°, can be easily calculated. However, for the reason given in the discussion, we are interested in *G°@, the free energy of the process : ket precursor complex »»»’ successor complex (5) rather than in *G°, the free energy of the reaction : kobs reactants »»»’ products (6) The corrections to *G° in order to obtain *G°@ were done as in reference 14: *G°@\*G°]wp[wr (7) with and being the work corresponding to the formation wp wr of the succesor complex from the products and the precursor complex from the reactants, respectively.The work (i\r or wi p) can be calculated with the following equation: wi\ ZiZj e2NA DR(1]jR) (8) where and are the charges on the two reactants or pro- Zi Zj ducts, considered with their corresponding signs, and j the inverse Debye length : j\A 8pNA e2 1000Ds kB T BI1@2 (9) The values of *G°@ calculated in this way are given in Table 5.According to these data the reaction becomes less favourable, from a thermodynamic point of view, when the amount of cosolvent in the mixtures is increased. Table 2 Standard formal redox potentials (E°@/mV vs.NHE)a of the (a) and ferrocinium/ferrocene [Fe(g5-Cp) Fe(CN)2(bpy)2 `/Fe(CN)2(bpy)2 2 `/ Fe(g5-Cp) (b) couples at 298.2 K in diÜerent water»cosolvent mixtures 2] Methanol tert-Butyl alcohol Ethylene glycol Glycerol D (a) (b) (a) (b) (a) (b) (a) (b) 78.5 water 775 514 76 776 506 782 522 758 506 779 507 74 776 502 789 516 768 515 773 506 70 778 495 803 513 779 515 779 502 66 780 491 815 515 788 522 779 498 64 787 492 819 516 792 526 780 490 60 781 486 834 516 804 530 783 493 a Potentials measured in the presence of 0.1 mol dm~3 as supporting electrolyte after correction for liquid junction potentials and ionic NaClO4 strength eÜects from 0.1 to 0.069 mol dm~3. 40 New J. Chem., 1998, Pages 39»44In( ket / s–1) Y GW In( ket / s–1) E T (30) Table 3 Electron-transfer rate constants (102 values for the ket/s~1) reaction of with at 298.2 K in diÜerent water» Fe(CN)2(bpy)2 S2O82~ cosolvent mixtures D Methanol tert-Butyl alcohol Ethylene glycol Glycerol 78.5 water 16.8 76 5.40 13.5 1.24 1.02 74 3.26 10.7 0.90 0.94 70 2.61 8.21 0.56 0.80 66 1.63 6.25 0.40 0.67 64 1.31 5.02 0.27 0.64 60 0.85 4.15 0.20 0.53 Table 4 Standard formal redox potentials (E°@/V vs.NHE) of the at 298.2 K in diÜerent water»cosolvent mix- S2O82~/S2O83~couple tures D Methanol tert-Butyl alcohol Ethylene glycol Glycerol 78.5 water 1.421 76 1.418 1.437 1.405 1.425 74 1.414 1.430 1.386 1.420 70 1.406 1.421 1.368 1.410 66 1.398 1.410 1.352 1.404 64 1.393 1.405 1.341 1.401 60 1.383 1.393 1.338 1.396 Table 5 [*G°@/kJ mol~1 values for the reaction of Fe(CN)2(bpy)2 with at 298.2 K in diÜerent water»cosolvent mixtures S2O82~ D Methanol tert-Butyl alcohol Ethylene glycol Glycerol 78.5 water 65.59 76 65.33 66.61 65.84 65.59 74 65.06 65.31 63.15 65.96 70 64.26 63.20 60.52 64.54 66 63.40 61.23 58.27 64.08 64 62.35 60.38 56.81 63.86 60 62.19 58.00 55.66 63.26 It is worth pointing out that as given in eqn 8, does not wi , include the cavity terms. However, these terms would be similar for reactants and products and, consequently, they would cancel.This term, however, has been taken into account in the calculation of (for this reason a value KIP KIP diÜerent from unity appears). Discussion First, it is interesting to note that, as can be seen in Tables 1 and 3, the addition of a small amount of cosolvent decreases the rate constant by about an order of magnitude.This apparently abnormal situation is clearly shown in Fig. 1 and 2, which are plots of vs. two polarity parameters. The –rst ln ket one, Y , (Grundwald»Winstein)15 is obtained from kinetic measurements and the second one from spectro- ET(30),16 photometric measurements. In both cases the points corresponding to the rate constants in water are outside the correlation.The –gures correspond to water»ethylene glycol mixtures, but similar behaviour is found for methanol»water and glycerol»water mixtures. However, for tert-butyl alcohol» water mixtures the linear correlation includes the water point. This special situation can be understood by considering the molar fraction of cosolvent in the mixtures (Table 1), which are much lower in the case of tert-butyl alcohol.This suggests, at –rst, that the deviation of the water point in the other cases Fig. 1 Plot of the logarithm of vs. the Grundwald»Winstein ket/s~1 polarity parameter, in ethylene glycol»water mixtures at 298.2 K YGW , Fig. 2 Plot of the logarithm of vs. the Reichardt polarity ket/s~1 parameter, in ethylene glycol»water mixtures at 298.2 K ET(30), can be related to the phenomenom of preferential solvation.If this is so, it is clear that the treatments based on simple continuum models of the solvent cannot explain quantitatively the kinetic behaviour. So we need to consider the solvent from a molecular point of view. As a starting point we will consider the expresion of ket given by classical electron-transfer theory :14 ket\jel tn exp([*GE/RT ) (10) Here, and are the electronic transmission coeffi- jet , tn *GE cient, the nuclear frequency factor and the (Gibbs) free energy of activation for the electron-transfer process.The latter is given by: *GE\ (k]*G°@)2 4k (11) The k parameter appearing in this equation is the so-called (free) energy of reorganization for the electron-transfer process.This free energy is considered to consist of a solvent contribution, and a contribution arising from the reorgan- ko , ization of the bonds within the donor and aceptor, The ki. New J. Chem., 1998, Pages 39»44 41latter contribution to the reorganization energy can be safely considered as independent of the reaction media.The values of can be obtained from the data in Table 3 once the *GE value of the preexponential term in the rate constant is known. Assuming adiabatic behaviour we need to (jelB1),11 know This parameter is given by:17 tn. tn\Atin 2 ki]tout 2 ko ko]ki B1@2 (12) and being the characteristic frequencies for the internal tin tout and external (solvent) reorganization. If (as will be kiAko shown to be the case for our process) one can safely assume: tn\tin (13) because is nearly two orders of magnitude greater than tin That is, the preexponential factor in the rate constant can tout .be considered independent of the reaction media. According to this, a value of s~1 seems reasonable. We have tnB1013 used a value of 6.62]1012 s~1 corresponding to the value of at 298 K.However, it is important to indicate that we kB T /h have performed calculations with preexponential factors in the range 108»1014 s~1. In this range, although there are variations in the values obtained for the trends in this param- *GE, eter and the conclusions reached from these trends do not change. Once the values of been calculated, obtaining k is *GEhave a straightforward matter, using the data in Table 5 and eqn 11.These k values appear in Table 6, which also includes the values of calculated from:1 ko ko\NA e2A 1 2rA ] 1 2rB [ 1 dABBc (14) where c\1/n2[1/D is Pekarœs factor, and the acceptor rA rB and donor radii, respectively and and (rA\3.4 ” rB\5.6 ”),18 is the donor»acceptor distance in the precursor dAB complex (assumed to be the sum of the reactant radii).From the values of and k in the table, the value of can be ko ki obtained. This value is about 330 kJ mol~1and, consequently, the above assumption is supported. (kiAko) It is of interest to consider the k values, which are greater in the mixtures than in water. This circumstance is unexpected since Pekarœs factor decreases as the amount of cosolvent in the mixtures increases.Given that is a constant, a decrease ki in k should be expected. According to this, must have ko another component, which is not included in eqn 14. This equation is based on the consideration of the solvent as a continuum and, consequently, does not include contributions arising from preferential solvation, which probably in—uences the kinetics, as suggested by Fig. 1 and 2. In fact, as a consequence of the electron transfer, the neutral iron complex becomes a (positively) charged species and a change in the preferential solvation is expected. This change will produce an extra reorganization of the solvent caused by a translational movement of some solvent molecules, because at the transition state the position of the molecules of the two components of the solvent (and not only the solvent polarization) must be intermediate between the positions corresponding to the (preferential) solvation of the initial and –nal states.It is important to realize that the cause of the extra component in k is not the preferential solvation itself, but the changes in this preferential solvation in the activation process, which implies a movement of solvent molecules in this process.This extra contribution in mixed solvents has been suggested by Curtis et al.19 from thermodynamic measurements and by Hupp and Weydert4 from a study of the spectra of some complexes in mixed solvents. Also, Piotrowiak and Miller20 and others21 have explained results corresponding to optical electrontransfer processes in electrolyte solutions as caused by an extra component of the reorganization free energy due to the translational movement of the ions of the supporting electrolyte.But, to the best of our knowledge, no previous results have been reported showing the in—uence of this component of k on the kinetics of electron-transfer processes in mixed solvents. However, related eÜects in the kinetics of the electron-transfer processes have been pointed out by Nielsen et al.22 in relation to primitive recognition eÜects in electrontransfer reactions (see also ref. 23). The magnitude of this contribution to k, caused by the translational movement of the solvent molecules, can be calculated from: kt , k t\(k)mix[ki[(ko)mix (15) and: ki\(k)H2O[(ko)H2O (16) Notice that in pure water, because preferential solva- kt\0 tion is absent.The values of obtained in this way are given in Table 6. kt Fig. 3 gives the plot of in methanol»water mixtures vs. the kt diÜusion coefficient of the organic component in the mixtures. 24 The increase of when the diÜusion coefficient kt decreases, that is, when the translational movement of the molecules becomes more hindered, supports the idea of a translational origin of in such a way that, as established kt before, the movement of solvent molecules in the activation process, rather than the preferential solvation itself, is at the origin of kt .As to the values obtained (except for tert-butyl alcohol» kt water mixtures), it is worth pointing out that they represent an important fraction of Since increases when decreases, ko .kt ko this extra component can cause changes in reactivity trends, as observed in this work, in relation to the predictions of the continuum model. The importance of will be outstanding in kt the cases where is small. Indeed, the phenomena causing ki kt can produce a breakdown of the linear response of the solvent when substantial diÜerences exist between solute»solvent interactions in the initial and –nal states.25 The observed solvent eÜects can now be explained : after the addition of the –rst portions of methanol, ethylene glycol or glycerol there is a marked decrease in the rate constant.This Table 6 Reorganization energies (k), outer reorganization energies and ìtranslationœ reorganization energies (all in kJ mol~1) for the (ko) (kt), reaction of with at 298.2 K in diÜerent water»cosolvent mixtures Fe(CN)2(bpy)2 S2O82~ Methanol tert-Butyl alcohol Ethylene glycol Glycerol D k ko kt k ko kt k ko kt k ko kt 78.5 water 431 99.6 0 76 442 99.0 10.6 435 98.5 3.4 458 97.6 27 460 97.3 31 74 447 98.5 16.1 436 98.3 3.6 457 96.6 28 460 96.0 33 70 448 98.0 17.6 434 97.2 4.7 457 94.5 31 460 93.3 35 66 451 97.5 21.1 434 96.5 5.1 456 92.5 33 461 90.5 38 64 452 97.3 22.3 434 96.2 5.7 458 91.5 34 461 89.2 39 60 456 96.9 26.7 432 95.3 6.6 459 89.9 35 462 87.0 42 42 New J.Chem., 1998, Pages 39»44lt / kJ mol–1 10–5 D / cm2 s–1 Fig. 3 Plot of the translational reorganization energy, mol~1, kt/kJ vs. the diÜusion coefficient of methanol in methanol»water mixtures at 298.2 K decrease comes from the fact that the free energy of reorganization, k, increases by about 10»30 kJ mol~1 when going from water to the –rst water»cosolvent mixtures (see Table 6).Indeed, the addition of cosolvents makes the process somewhat less favourable from a thermodynamic point of view (see Table 5). Further addition of the cosolvents glycerol and ethylene glycol does not signi–cantly change k. This happens since the extra component compensates for the decrease in ko due to the change in the dielectric properties of the mixtures (in all cases Pekarœs factor decreases with increasing concentration of the organic component).The decrease in the rate constant in these media must be ascribed, consequently, to the fact that the reaction becomes thermodynamically less favourable with increasing cosolvent amount (see Table 5). In methanol»water mixtures, both the kinetic parameter, k, and the thermodynamic one, *G°@, contribute to the decrease of ket .In conclusion, we have shown that a simple continuum dielectric model is unable to explain the ì–ne structureœ of the solvent eÜects in solvent mixtures, due to the preferential solvation phenomena. In relation to electron transfer, this model cannot (obviously) account for an extra component of the solvent reorganization caused by molecular translations, which contribute substantially to the total solvent reorganization energy, as shown here.Experimental Materials The iron complex was prepared Fe(CN)2(bpy)2 … 3H2O according to the literature.28 Its purity was tested by UVvisible spectroscopy and by CHN analysis.EDTA (disodium salt) was obtained from Merck (P.A. grade) and sodium peroxodisulfate from Carlo Erba (P.A. grade). Ferricinium was synthesized according to the method given in the literature.27 All the solutions were prepared with deionized water (conductivity\10~8 S m~1). Kinetics Kinetic runs were performed using a Hitachi 150-20 spectrophotometer at 298.2 K employing a matched 1 cm quartz cell.The temperature was maintained within a range of ^0.1 K by using a Julabo thermostat. All the experiments were carried out under pseudo-–rst-order conditions using an excess of oxidant. The reactant concentrations used were the following : mol dm~3, [Fe(CN)2(bpy)2]\1.18]10~4 mol dm~3 and [EDTA2~]\ [Na2S2O8]\2.25]10~2 5]10~4 mol dm~3. The kinetics were followed at 520 nm.Rate constants were obtained from the slopes of the plots of vs. time, A being the absorbance at time t and ln(A[A=) A= the –nal absorbance. These rate constants were found to be reproducible within about 5%. Electrochemical measurements The apparatus and electrodes used in this study have been previously described.28 The concentration of the iron complex used in these experiments was 2]10~4 mol dm~3.To measure the redox potential of the Fe(g5-Cp)2 `/Fe(g5- couple, a 1.5]10~4 mol dm~3 concentration of the oxi- Cp)2 dized component of this couple was used. In all cases potentials were obtained in the presence of 0.1 mol dm~3 NaClO4 . Acknowledgements authors thank D.G.I.C.Y.T. (PB92-0677), the Consejeriç a The de Educacioç n y Ciencia de la Junta de Andaluciç a and Fundacio ç n Caç mara for their support of this work.Appendix: Checking calculations In the previous paragraphs some assumptions and approximations have been used. Therefore it seemed necessary to check them in order to support our conclusions. The main approximations are : (i) The use of the Eigen»Fuoss treatment for the calculation of the association constants (ii) The (KIP).approximations made in the calculation of the redox potential of the couple. (iii) The assumption that is S2O82~/S2O83~ kin a constant in the water»cosolvent mixtures. We will try to justify the assumptions we have made. (i) In order to calculate we have used the Eigen»Fuoss KIP treatment. Even if these equilibrium constants are in error by a factor of two, this would imply an error in of about 2 *GE kJ mol~1 which is probably smaller than the error in the other approximations.(ii) To estimate the redox potential of the S2O82~/S2O83~ couple, we have used the Debye»Hué ckel formulation. This approach, in the range of concentrations used in our experiments mol dm~3), can be applied safely. Indeed we (ItotalO0.1 have used eqn 2 in order to estimate the transfer free energies of from those of It is important to realize S2O83~ S2O82~.that eqn 2 is not based on any model. It simply supposes that the free energy of transfer of an anion is proportional to the square of its charge. Fig. A-1 gives the experimental free energy of hydration of I~, and vs. the square of SO42~ PO43~ their charges.29 This –gure supports our approximation. (iii) In order to check that the variations of k are not due to the in—uence of the solvent on due to the (possible) disso- ki , ciative character of our electron-transfer process, we will consider Fig.A-2. This –gure gives a plot of k values in methanol»water mixtures vs. the values of this parameter obtained for the same mixtures by an independent procedure.The k values on the x axis correspond to an (optical) electrontransfer reaction within the binuclear complex:5 In this complex, a MMCT band is [(NH3)5RuNCRu(CN)5]~. observed (k\683 nm, eB3000 mol~1 dm3 cm~1 in water). The maximum energy for this band, is related to the k Eop , and *G°@ parameters characterizing the electron transfer through:18 Eop\k]*G°@ (18) New J.Chem., 1998, Pages 39»44 43Square of ion charge Hydration free energy (10–3 D Gh 0 / kJ mol–1) lspectroscopy / kJ mol–1 l / kJ mol–1 Fig. A-1 Plot of the free energy of hydration (kJ mol~1) of some anions vs. the square of their charge As *G°@ can be obtained by performing electrochemical measurements on both ruthenium centers of the type described above, k can be obtained directly from two experimental magnitudes without any additional hypothesis.As can be seen in Fig. A-2, a correlation is observed between the k values obtained in this work by using a kinetic procedure and the values of this parameter obtained from spectroscopy measurements. In the case of the binuclear complex is small, ki (26.84 kJ mol~1)30 and consequently a dissociative character of the electron transfer is ruled out in this case. This correlation shows that the changes in k are due to the in—uence of the solvent only on as we have supposed, because in the ko , case of a binuclear complex it can be safely assumed that the solvent does not in—uence ki .Finally, a word in relation to the high value for this reac- ki tion. As mentioned above, this parameter measures the energy required to change the internal bonds of the reactant in order to reach the con–guration of the transition state.The contribution of the iron complex to has been obtained recently by ki Terrettaz et al.9 They give a value to this parameter of 0.04 eV Fig. A-2 Plot of the reorganization energy obtained in this work, k/kJ mol~1, vs. the reorganization energy obtained from spectroscopy measurements, mol~1, in methanol»water mixtures kspectroscopy/kJ molecule~1\3.86 kJ mol~1.According to this, the main contribution to comes from the other reactant, the ki S2O82~ ions. Taking into account that : kiBkiox]kired 2 (19) we estimated ( ) to be about 660 kJ mol~1, in rea- ki S2O82~ sonable agreement with the value of about 600 kJ mol~1 that can be estimated from the study of Fué rholz and Haim11 on the oxidation of by peroxodisulfate.This Ru(NH3)5 pz2` agreement also gives support to our calculations. Indeed, this result can explain the relatively low rate of reaction observed when the oxidant is in spite of its high redox poten- S2O82~, tial. References 1 R. A. Marcus, Ann Rev. Phys. Chem., 1964, 15, 155 and references therein. 2 N.S. Hush, J. Chem. Phys., 1956, 28, 962. 3 See for example: I. Rips, J. Klafter and J. Jortner, J. Phys. Chem., 1990, 94, 8557. 4 J. T. Hupp and J. Weydert, Inorg. Chem., 1987, 26, 2657. 5 F. Saç nchez-Burgos, M. Galaç n, P. Peç rez-Tejeda and M. Domïç nguez, unpublished data. 6 P. Peç rez-Tejeda, J. Benko, M. Loç pez, M. Galan, P. Loç pez, M. Domïç nguez, M.L. Moyaç and F. Saç nchez, J. Chem. Soc., Faraday T rans., 1996, 92, 1155. 7 J. Aqvist and T. Hannson, J. Phys. Chem., 1996, 100, 9512. 8 A. Chandra and B. Bagchi, J. Chem. Phys., 1991, 94, 8367. 9 S. Terrettaz, A. M. Becka, M. J. Traub, J. C. Fetlinger and C. J. Miller, J. Phys. Chem., 1995, 99, 11216. 10 A. J. Miralles, R. E. Armstrong and A. Haim, J. Am Chem. Soc., 1977, 99, 1416. 11 U. Fué rholz and A. Haim, Inorg. Chem., 1987, 26, 3243. 12 (a) M. J. Blandamer and J. Burgess, Can. J. Chem., 1983, 61, 1361; (b) Y. Marcus, Z. Naturforsch., 1995, 50a, 51; (c) A. Rodrïç guez, C. Carmona, E. Mun8 oz, F. Saç nchez and J. Burgess, T rans. Metal Chem., 1991, 16, 535; (d) F. Saç nchez, A. Rodrïç guez and J. Burgess, J. Chem. Soc., Faraday T rans., 1990, 86, 3731. 13 B. S. Brunschwig, S. Ehrenson and N. Sutin, J. Phys. Chem., 1986, 90, 3657. 14 R. A. Marcus and N. Sutin, Biochim. Biophys. Acta, 1985, 811, 265. 15 M. L. Moyaç , F. Saç nchez and J. Burgess, Int. J. Chem. Kinet., 1993, 25, 891 and references therein. 16 (a) Y. Marcus, Ion Solvation, Wiley, London, 1985, ch. 7; (b) R. J. Sindreu, M. L. Moyaç , F. Saç nchez Burgos and A. Gustavo Gonza ç lez, J. Solution Chem., 1996, 25, 289. 17 M. J. Weaver, Chem. Rev., 1992, 92, 463. 18 (a) Reference 14; (b) J. C. Curtis and T. J. Meyer, Inorg. Chem., 1982, 21, 1562; (c) E. Kremer, G. Cha, M. Morkevivius, M. Seaman and A. Haim, J. Am. Chem. Soc., 1979, 101, 883. 19 J. C. Curtis, R. L. Blackbourn, K. S. Ennix, J. A. Roberts and H. T. Hupp, Inorg. Chem., 1989, 28, 3791. 20 (a) P. Piotrowiak and J. R. Miller, J. Phys. Chem., 1993, 97, 13052; (b) P. Piotrowiak, Inorg. Chim. Acta, 1994, 225, 269. 21 R. L. Blackbourn and J. T. Hupp, J. Phys. Chem., 1990, 94, 1788. 22 (a) R. M. Nielson, J. T. Hupp and D. I. Yoon, J. Am. Chem. Soc., 1995, 117, 9085; (b) R. M. Nielson, L. A. Lyon and J. T. Hupp, Inorg. Chem., 1996, 35, 970. 23 J. Wilson, O. H. K. Ting and J. Lipkowski, J. Electroanal. Chem., 1988, 247, 85. 24 M. Ferrario, M. Haughney, I. R. McDonald and M. L. Klein, J. Chem. Phys., 1990, 93, 5156. 25 P. V. Kumar and M. Maroncelli, J. Phys. Chem., 1990, 99, 3038. 26 A. A. Schildt, J. Am. Chem. Soc., 1960, 82, 3000. 27 H. Schumann, J. Organomet. Chem., 1985, 82, 3000. 28 E. Roldan, M. Domïç nguez and D. Gonzaç lez, Comput. Chem., 1986, 10, 187. 29 (a) Y. Marcus, Ion Solvation, Wiley, London, 1985, ch. 5; (b) Y. Marcus, J. Chem. Soc., Faraday T rans., 1991, 87, 2995. 30 K. Tominaga, D. A. V. Klener, A. E. Johnson, N. E. Levinger and P. Barbara, J. Chem. Phys., 1993, 98, 1228. Received 27th January 1997; Paper 7/06747D 44 New J. Chem., 1998, Pages 39»44

ISSN:1144-0546    

DOI:10.1039/a706747j

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

N N N .O3S Fe N N N N N III .O3S SO3 . SO3 . Sequential addition of pH and solvent e¢­ ects as key factors in H2O2 , the oxidation of 2,4,6-trichlorophenol catalyzed by iron tetrasulfophthalocyanine Anke Hadasch,a Alexander Sorokin,a Alain Rabionb and Bernard Meunier*,a a L aboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 T oulouse 4, France cec dex b Centre de Recherche EL F-Atochem, Chimie Organique et Biochimie, L acq, BP 34, Dec partement 64170 Artix, France The efficiency of the oxidation of 2,4,6-trichlorophenol (TCP) catalyzed by iron tetrasulfophthalocyanine H2O2 (FePcS) is highly dependent on the pH value of the reaction mixture, the local hydrogen peroxide concentration and the organization of FePcS molecules in solution.Among the several forms of FePcS in aqueous solutions (dimer or monomer), monomeric FePcS is proposed to be the catalytically active complex.The key role of the organic co-solvent (acetonitrile, acetone, alcohol, . . .) is to shift the dimer/monomer equilibrium toward monomeric FePcS, the efficient catalyst precursor. A stepwise addition of hydrogen peroxide signi.cantly improves the conversion of TCP and allows a low catalyst loading, below 1% with respect to the pollutant, to be used.Facteurs clec s dans l©«oxydation du 2,4,6-trichlorophec nol catalysec e par la tec trasulfophtalocyanine de fer : ajout sec quentiel d©«eau oxygec nec e, eUets du pH et du solvant. L©«efficacitec de l©«oxydation du 2,4,6-trichlorophec nol (TCP) catalysec e par la tec trasulfophtalocyanine de fer (FePcS) est tre` s dec pendante de la valeur du pH du milieu rec actionnel, de la concentration locale en eau oxygec nec e et de l©«organisation des molec cules de FePcS en solution.Parmi les diUec rentes formes possibles de FePcS en solution aqueuse (dime` re ou monome` re), la forme monomec rique de FePcS est probablement la forme active du catalyseur. Le ro¢� le clec du co-solvant organique (acec tonitrile, acec tone, alcool, .. .) est de dec placer l©«ec quilibre dime` re/monome` re vers la forme monomec rique de FePcS, le prec curseur catalytique efficace. Une addition fractionnec e d©«eau oxygec nec e amec liore de manie` re signi.cative la conversion du TCP et permet d©«utiliser moins de 1% de catalyseur par rapport au polluant.Degradation of chlorinated aromatics is an important environmental research .eld since these compounds are among the most recalcitrant pollutants.1 Recently we found an efficient catalytic oxidation of 2,4,6-trichlorophenol (TCP) catalyzed by iron tetrasulfophthalocyanine (FePcS, see Fig. 1 for the structure).2h4 With hydrogen peroxide (ithe clean peroxide©«), FePcS has been shown to catalyze aromatic ring cleavage of trichlorophenol and pentachlorophenol.2h4 Chloromaleic acid has been demonstrated to be the main product of this aromatic ring cleavage, with chlorofumaric, maleic and fumaric acids Fig. 1 Structure of the iron complex of tetrasulfophthalocyanine (FePcS) being minor products.2 The proposed mechanism of this aromatic ring degradation involves a nucleophilic iron(III) peroxo complex, PcSFeIIIOOH, as the key active species responsible for the epoxidation of intermediate quinones and for the attack of carbonyl groups of the oxidized quinones to explain the aromatic CwC cleavage via a Grob-type fragmentation.4 By using 14C-labeled TCP substrate, carbon dioxide was demonstrated to be the ultimate degradation product in this catalytic oxidative degradation of such a recalcitrant compound. 3 In this catalytic oxidation of TCP two out of the three chlorine substituents of the starting material were recovered as chloride ions, one carbon out of the six carbon atoms ended as carbon oxides and four other carbon atoms .nished as water-soluble diacids.3 The .nal goal of this approach is not to burn the recalcitrant molecule to carbon dioxide, as in incineration processes, but to initiate a degradation of the pollutant to more biodegradable products. Here we wish to report new data on the key factors controlling the catalytic activity of the system in the oxidation of TCP: FePcS¡íH2O2 the sequential addition of and the pH and solvent H2O2 eUects on the monomer/dimer equilibria of the catalyst precursor(s).Results and Discussion Sequential addition of H2O2 A typical reaction mixture (.nal volume of 2 mL) contained the following .nal concentrations : 10~2 M of substrate (corresponding to 2000 ppm of TCP), 3.7]10~4 or 10~4 M New J.Chem., 1998, Pages 45¡í51 45of catalyst (corresponding to 3.7 or 1% [FePcS]-to-[TCP] ratios, respectively) and 5]10~2 M of oxidant All (H2O2). oxidations, performed in an acetonitrile-buÜered water mixture (v/v\1 : 3) to dissolve the hydrophobic 2,4,6- trichlorophenol, were initiated by a single addition of 5 equiv.of In homogeneous conditions, a relatively high H2O2 . catalyst-to-substrate ratio of 3.7% was necessary to obtain a full conversion of the pollutant within a few minutes. In the early stage of reaction we observed a gas release.GC analyses of the gas phase over the reaction mixture before and after reaction showed a slight increase in the dioxygen concentration, indicating a catalase-like dismutation of Conse- H2O2 . quently, two main reactions, namely the substrate oxidation and the dismutation, are competing. Previously, iron H2O2 and manganese porphyrin complexes have been shown to be catalase models.5 Because of the second reaction order in hydrogen peroxide in catalase-type dismutation (two molecules of are required to produce one molecule of a H2O2 O2) decrease of the local concentration should strongly dis- H2O2 favor the dismutation reaction with respect to the catalytic oxidation. In fact, we observed a reduction of the gas release when reducing the rate of the oxidant addition.Therefore we decided to add the oxidant sequentially : –ve 20 lL aliquots of an aqueous 3.5% solution were added every 5 min to H2O2 the reaction mixture (initial volume: 2 mL). The results obtained with this method at diÜerent catalyst : substrate ratios are reported in Table 1. The two important criteria for optimal degradation of the pollutant are fast conversion of the substrate and a high number of chloride ions released per converted TCP molecule ; the latter is expected for an advanced degree of oxidation of the substrate.Complete substrate conversions were obtained with only 1% and 0.5% FePcS (runs 2 and 3) and the –nal conversion in the presence of only 0.1% FePcS was still 50% (run 5). These results show that fast and complete substrate conversion with a high dechlorination (2 Cl~ per converted TCP) was obtained using 1% of catalyst (run 2).Sequential addition of the solution remarkably H2O2 improved the substrate conversion at low FePcS-to-TCP ratios and led to complete substrate conversion and a higher number of released Cl~ with only one-fourth of the catalyst, as compared to an addition of all at once.High oxidant H2O2 concentrations favor the dismutation reaction ; is thus H2O2 consumed and no longer available for the substrate oxidation. Furthermore, we noted that a very large excess of hydrogen peroxide (1 M –nal concentration ; i.e., a H2O2 ratio of 100 : 1) inhibited both TCP conver- [H2O2] : [TCP] sion and dismutation, although the green-blue color of H2O2 the reaction mixture remained, indicating no FePcS degrada- Table 1 Oxidation of 2,4,6-trichlorophenol by FePcS»H2O2 Catalyst : TCP TCP substrate conversion conversion Dechlorinationa Run ratio (%) at 14 min (%)b at 60 min (%) at 60 min 1 3.7 100 (27) 100 1.9 2 1.0 100 (100) 100 2.0 3 0.5 83 (166) 100 1.8 4 0.25 64 (256) 84 n.d. 5 0.1 44 (440)c 50 n.d. 6 3.7 100 (27) 100 1.7 7 1.0 38 (38) 38 n.d.Runs 1»5: sequential addition of (5 aliquots), runs 6»7: one- H2O2 portion addition of H2O2 a Cl~ ions released per converted TCP molecule. n.d.\not determined. b Number of catalytic cycles. For very fast reactions (run 1 and run 6), the number of catalytic cycles cannot be compared with the other runs, sincfull conversion was achieved before 14 min. c Based on 14 min, the turnover rate corresponds to 31 cycles min~1.Table 2 Quantitative analysis of oxidation productsa Chloromaleic Chlorofumaric Maleic Fumaric Coupling Run acid 1 acid 2 acid 3 acid 4 products 1b 10 \1 \1 \1 20 2c 21 3 1 2 24 3c 13 2 1 2 37 4c 7 4 1 2 25 Run 1: 1% FePcS, one-portion addition of run 2: 1% FePcS, H2O2; 5 aliquots of run 3: 3.7% FePcS, one-portion addition of H2O2; run 4: 3.7% FePcS, 5 aliquots of H2O2; H2O2.a Products were quanti–ed by integration of the vinylic proton signals at 6.67 ppm (for 1), 7.30 ppm (for 2), 6.39 ppm (for 3), 6.75 ppm (for 4) and of the aromatic protons at 7.9»8.1 ppm for coupling products. All these data are yields in % with respect to TCP. b TCP conversion was only 80% in this run. c TCP conversion was 100% in runs 2»4.tion (the blue color of the starting iron phthalocyanine changes to blue-green after addition of the oxidant and gradually to a deep purple when quinone derivatives are generated). Quantitative analysis of the oxidation products formed by the diÜerent addition methods allowed us to compare H2O2 the efficiency of the catalytic system to oxidize TCP. The products were quanti–ed by NMR and the results are reported in Table 2. The catalytic TCP oxidations were performed using 1 and 3.7% catalyst-to-substrate ratios with diÜerent modes of addition.With a 1% catalyst-to-substrate ratio the H2O2 sequential addition of gave a signi–cant increase in the H2O2 yields of generated acids, from 11% in the case of a oneportion addition of (run 1, Table 2) up to 27% (run 2, H2O2 Table 2) indicating a more complete TCP degradation. It should be noted that only the sequential-addition method allowed full substrate conversion at low catalyst loading to be reached.In the case of a 3.7% catalyst-to-substrate ratio, we observed a signi–cant decrease in the yield of coupling products, from 37% in the case of the one-portion addition of (run 3, Table 2) to 25% (run 4, Table 2), suggesting H2O2 again that the sequential addition of the solution H2O2 favored the formation of ring-cleavage products in this catalytic TCP oxidation.The lower yields of dicarboxylic acids could be explained by their further oxidation in the reaction mixture because of a higher catalyst concentration compared to run 2, Table 2. In—uence of the pH value We also studied the in—uence of pH value of the buÜered reaction mixture on the conversion of TCP at 60 min; the data are reported in Fig. 2. The catalytic reaction is strongly dependent on the pH value of the solution. No catalytic activity was detected at pH 4 after 60 min, but for the same reaction time the substrate conversion at pH 5 and pH 6 increased to 24 and 66%, respectively.Full conversion of TCP was obtained at pH 7 and pH 8, which correspond to the optimal pH values of the catalytic reaction mixture. At pH 9 we observed a dramatic loss of catalytic activity. The –rst step of the TCP oxidation is the one-electron oxidation of the phenolate anion.3,4 Since the value of TCP pKa is 6.2,6 only small amounts of TCP are present in the reaction mixture as phenolate (a more oxidizable form compared to the phenol itself) at pH 5 and 6, leading to poor substrate conversions.At pH 7 and pH 8, 100% of the TCP is present as phenolate, thus explaining the full substrate conversions as well as the better oxidation rates. The absence of catalytic activity at pH 9 can probably be explained by the basefavored formation of l-oxo dimeric species of FePcS.It should be noted that the buÜer concentration in—uenced the substrate conversion as well. Final buÜer concentrations of 0.125 M or 0.25 M were necessary to keep the catalyst in an 46 New J. Chem., 1998, Pages 45»51Conversion of TCP (%) pH Fig. 2 In—uence of the reaction mixture pH on the oxidation of 2,4, 6-trichlorophenol (TCP) by the system FePcS»H2O2 active form and to neutralize the acids formed during TCP oxidation.In—uence of the solvent on the catalytic oxidation reaction In order to have a homogeneous solution when we worked with a reaction mixture containing 10 mM of TCP (2000 ppm), we used acetonitrile as a co-solvent to dissolve the hydrophobic TCP (acetonitrile : buÜered water ratio of 1 : 3). Keeping in mind the possible development of the system for the oxidative depollution of aqueous FePcS»H2O2 effluents, we decided to study the in—uence of acetonitrile content using 1 mM TCP (200 ppm) and a 3.7% FePcS-to- TCP molar ratio.Increasing the acetonitrile : water ratio leads to an increase in the substrate conversions (Table 3). The conversions of TCP after one hour were 13, 54, 96 and 100% in reaction mixtures containing 2.5, 10, 25 and 50% acetonitrile, respectively.Several hypotheses can be proposed to explain the strong in—uence of the acetonitrile content on this catalytic oxidation : (i) MeCN might act as an axial ligand on the iron center of FePcS to enhance its catalytic activity ; (ii) might H2O2 react with MeCN to form a new peroxide able to react quickly with FePcS to give a highly active species. (Alkalicatalyzed epoxidations by using nitriles as co-reactant H2O2 have been published7,8 and peroxycarboximidic acid generated by addition of to acetonitrile has been proposed as H2O2 an active species in ole–n epoxidations.) ; and (iii) acetonitrile Table 3 Oxidation of TCP by In—uence of the FePcS»H2O2 .acetonitrile : buÜered water ratio over the substrate conversiona Acetonitrile content TCP conversion Dechlorinationb Run (%) at 60 min (%) at 60 min 1 2.5 13 0.2 (1.5) 1c 2.5 17 n.d. 2 10 54 1.2 (2.2) 3 25 96 2.0 (2.1) 4 50 100 2.4 (2.4) a [TCP]\1 mM, [FePcS]\0.037 mM, [phosphate buÜer]\50 mM (pH 7), mM, one-portion addition. b Cl~ ions rel- [H2O2]\5 eased per initial TCP molecule; the data in parentheses correspond to the number of Cl~ ions released per converted TCP molecule.c Addition of 3-cyanobenzoic acid ; 3-cyanobenzoic acid : FePcS ratio of 100 : 1. Table 4 Solvent in—uence on TCP conversiona Conversion of TCP Co-solvent at 60 min (%) Polarity ET(30)b Acetone 100 42.2 Acetonitrile 96 45.6 Ethanol 86 51.9 Formamide 69 56.6 Waterc 13 63.1 a [TCP]\1 mM, [FePcS]\0.037 mM, [phosphate buÜer]\50 mM (pH 7), mM, one-portion addition, solvent : buÜer [H2O2]\5 ratio\1 : 3.b Solvent parameter developed by Dimroth and Reichardt, 9a see ref. 9(b) for a recent review article on solvent properties. c We used 2.5% acetonitrile in this reaction mixture to completely solubilize TCP. might be able to dissociate an inactive dimer of FePcS to a monomeric complex, which is catalytically active.To check the role of acetonitrile, we replaced it by diÜerent solvents. The conversions obtained after one hour in the presence of acetone, acetonitrile, ethanol and formamide were 100, 96, 86 and 69%, respectively (solvent : buÜered water ratio of 1 : 3) (Table 4). The total dechlorination values decreased in the same order. The data in Table 4 also indicate that TCP conversions decrease when the solvent polarity9 increases, suggesting that these organic solvents might have a strong in—uence on the dissociation of inactive dimeric forms of the catalyst precursors.To verify the hypothesis of an organic solvent molecule acting as axial ligand to FePcS, we added 100 equivalents of 3-cyanobenzoic acid, a potentially good axial ligand, to 1 equivalent of FePcS (Table 3, run 1c).We did not observe a signi–cant diÜerence in the substrate conversions in the presence or in the absence of 3-cyanobenzoic acid (17% and 13%, respectively). This lack of in—uence of the 3-cyanobenzoic acid suggests that the eÜect of the diÜerent organic solvents is not related to their possible coordination on the axial position of FePcS. The possible formation of a reactive intermediate between acetonitrile or formamide and by analogy to the Payne H2O2 reaction8 was excluded, since a solvent unable to form this kind of peroxidic intermediate, such as ethanol, also gave a high conversion (86% within 60 min, Table 4).In addition, this experiment con–rmed that free hydroxyl radicals (usually trapped by ethanol) do not play a key role in this oxi- H2O2 dation reaction catalyzed by iron sulfophthalocyanine. In—uence of the solvent on the catalyst precursor(s) The organic solvents caused a noticeable change in the UV-vis spectrum of FePcS.In pure water, we observed only one absorption maximum in the visible range at 632 nm. When adding an aqueous solution of this complex to a solvent mixture with a –nal organic solvent : water ratio of 1 : 3, we observe a decrease of the absorption maximum at 632 nm and the appearance of two absorption bands at 636 and 668 nm within 30 min (the co-solvents used were the same as in the catalytic reaction : acetonitrile, acetone or ethanol).The changes observed in the UV-vis spectra in the presence of acetonitrile with isobestic points at 554, 644 and 714 nm might be due to the dissociation of a dimer of FePcS to a monomeric complex.The dependence of the spectrum of FePcS on acetonitrile content is shown in Fig. 3. The nature of the FePcS structure in aqueous solutions has already been the subject of many diÜerent studies.10h18 There is a general agreement that FePcS exists in a dimeric form in aqueous solutions with an absorption maximum near 630 nm, whereas a monomeric form exhibits a peak in the region of 670 nm.10,13,14 FePcS obtained from diÜerent preparations, New J.Chem., 1998, Pages 45»51 47Wavelength / nm Absorbance Fig. 3 Modi–cation of the UV-vis spectrum of FePcS in water by addition of acetonitrile by the Weber»Busch method19 using iron(II) or iron(III) template compounds, or by the insertion of an iron(II) salt with in the presence of air, can be separated on Sephadex H2PcS G10 into two main fractions.18 The –rst fraction, a dark green product, showed a strong peak at 670 nm with minimal absorption at ca. 630 nm. The second fraction, containing a FeIII low-spin product, was dark blue with a strong absorption at 632 nm and minimal absorption at 670 nm. The ratio between these two components was dependent on the preparation method.Materials prepared by these diÜerent methods gave the same complex in the presence of imidazole. The exact nature of these green and blue products is still not available in the literature on metallophthalocyanine chemistry. The magnetic moments of the solid FePcS obtained from the Weber»Busch preparation were in the low-spin d5 region and Moé ssbauer parameters were consistent with a mixture of ferric S\1/2, 3/2, 5/2 spin states, with the low-spin species being the major component.12 FePcS has been prepared by the Weber»Busch method in diÜerent forms (H`, Na`, K`, Cs`) and studied by diÜerent spectroscopic methods.17 The authors concluded that FePcS contained a wide range of iron complexes with ratios depending on each preparation. The sodium salt of FePcS was a low-spin FeIII species in the solid state, whereas high-spin FeIII compounds were detected in buÜer solutions (pH 2.5»12.0). Fanning et al.found no evidence for l-oxo complexes in the solid state or in solution.17 One or two molecules of water were proposed to be axial ligands susceptible to be bound to the stacked high-spin FeIII complexes at high ionic strengths.By lowering the ionic strength and increasing the concentration a strong water interaction can occur to form a low-spin FeIII complex.17 However, the preparation and characterization of l-oxobis- [tetraalkylsulfonamidophthalocyanine iron(III)] complexes have been reported by Lever et al.11 The identity of this l-oxo dimer has been con–rmed by UV-vis spectroscopy, magnetism and FTIR (the band at 810 cm~1 being attributed to the FewOwFe antisymmetric deformation).Gupta et al. published a FTIR spectrum for the solid FePcS complex in a KBr matrix having a band at 837 cm~1, in the typical frequency region of the FewOwFe deformation mode (800»900 cm~1).10 However, a similar band is also present in the FTIR spectra of CoPcS (831 cm~1) and (850 cm~1), indicat- H2PcS ing that the identi–cation of the FePcS l-oxo dimer by IR is still a matter of debate.Using the hydrophobic FePc complex, Ercolani et al. published the preparation and a well-documented characterization of two interchangeable l-oxo dimers.15 The –rst l-oxo dimer showed two antisymmetric FewOwFe frequencies at 852 and 824 cm~1, which can be shifted to 806 cm~1 upon treatment, while the second l-oxo dimer did not show 18O2 any band for the FewOwFe vibration. These authors proposed a bent FewOwFe structure for the –rst l-oxo species and a quasi-linear one for the second l-oxo dimer.In fact, the proposed bent l-oxo dimer might be a hydroxo bridge, as recently proposed by Scheidt and co-workers for iron and manganese porphyrin complexes.20,21 Hanack and coworkers have also prepared and characterized by UV-vis, FD mass, Moé ssbauer, NMR and EPR spectroscopy two isomeric l-oxo compounds for various substituted iron phthalocyanines. 16 However, the nature of these two isomeric l-oxo compounds remains unknown.16 One can conclude that despite a large number of studies the chemistry of l-oxo complexes of iron phthalocyanines, and in particular of FePcS, in aqueous solutions and the solid state is not presently fully understood.We tried to evidence a l-oxo dimer structure of FePcS by using FTIR spectroscopy. We observed a band at 833 cm~1, which can be attributed to the FewOwFe deformation mode. Previously, a FewOwFe band at 810 cm~1 has been evidenced for the iron(III) complex of l-oxo-bis-tetra(dodecylsulfonamido) phthalocyanine.11 In order to check the hypothesis that the band at 833 cm~1 is the signature of a l-oxo dimer, we incubated FePcS in in the presence of H218O 0.1 M acid in order to cleave the l-oxo dimer; then the solution was neutralized with KOH to generate a Few18OwFe l-oxo dimer.We obtained practically the same IR spectrum to that of the starting FePcS with only a small shift of the 833 cm~1 band to 839 cm~1, which cannot be attributed to an isotopic shift.However, this disappointing experiment should not be taken as proof of the de–nitive absence of a l-oxo dimer for the FePcS material that we are using as a catalytic precursor in the FePcS-mediated oxidation of TCP because of the complex nature of water-soluble metallophthalocyanines.Nevertheless, the formation of strongly associated FePcS dimers in water is highly favorable since this process should reduce the unfavorable contacts between water and the apolar core of this aromatic macrocyclic molecule. In contrast, less polar organic solvents should disfavor the hydrophobic interactions between two diÜerent planes of phthalocyanines, which is a driving force for dimer formation.Consequently, the presence of an organic solvent should diminish the tendency of dimerization. So co-solvents with a low polarity should strongly contribute to shift the dimer/monomer equilibrium towards monomer formation. In fact, this correlation was observed in the oxidation of TCP catalyzed by FePcS: acetone[acetonitrile[ethanol[formamide[water (see data in Table 4).Modi–cations of FePcS spectra in the presence of TCP, acids or bases Addition of TCP to a water solution of FePcS resulted in the same changes in the UV-vis spectra as observed after an organic solvent addition (see above) with isobestic points at 552, 650 and 714 nm, suggesting the formation of a FePcS monomer.4 Spectrophotometric titration of a 20 lM FePcS aqueous solution with 50 mM HCl revealed a decrease of the band at 632 nm and the appearance of two new absorbances at 636 and 680 nm with quasi-isobestic points at 340, 564 and 650 nm (Fig. 4). Thus spectral changes (positions of new bands and isobestic points) are similar in all three cases : addition of acid, acetonitrile or TCP to a water solution of FePcS.These data strongly suggest the formation of (a) monomeric form(s) of FePcS under all these conditions. We also noted a signi–cant diÜerence of 12 nm between the absorption bands in the presence of acetonitrile and TCP (668 nm) and HCl (680 nm). Upon HCl titration the spectrum reached a saturation level with maxima at 636 and 680 nm when the acid concentration was 2.5 mM.A further increase in acid 48 New J. Chem., 1998, Pages 45»51Absorbance Wavelength / nm Wavelength / nm Absorbance PcSFeIII–O–FeIIIPcS organic solvent + H2O 2 PcSFeIII–OH 2 [PcSFeIII(H2O)]+ B 668 nm C 680 nm A 632 nm + H+ + HO– n PcSFeIII–OH n [PcSFeIII(H2O)]+ Cstack 636 nm Bstack 636 nm + H+ + HO– Fig. 4 Modi–cation of the UV-vis spectrum of FePcS in water by addition of an HCl solution concentration up to 250 mM did not change the UV-vis spectrum.When hydrochloric acid was replaced by sulfuric or phosphoric acid, the same phenomenon was observed. Therefore we concluded that the presence of diÜerent anions of these acids, chloride, sulfate or phosphate, were not at the origin of the changes observed in the visible spectra of FePcS.To check the reversibility of an equilibrium between diÜerent forms of FePcS, a FePcS solution previously acidi–ed by was titrated with NaOH. In the beginning we H2SO4 , observed the regular disappearance of the maximum at 680 nm without an increase of the absorption bands at 636 and 632 nm. At pH 6, the maximum at 680 nm nearly disappeared and further addition of NaOH produced a spontaneous reappearance of the initial maximum at 632 nm (Fig. 5). Further Fig. 5 Modi–cation of the UV-vis spectrum of an acidic aqueous solution of FePcS by addition of an NaOH solution Scheme 1 Schematic representation of the diÜerent forms of FePcS in aqueous solution. All these diÜerent proposed structures must be considered as working hypotheses (see text). The four negative charges of the sulfonato groups have been omitted for clarity titration with NaOH up to pH 9 provoked the appearance of a shoulder at approximately 670 nm.The hypothesis that a dimeric form of FePcS is transformed into a single monomer is not sufficient to explain all these diÜerent experimental results. In the presence of a co-solvent or TCP, we propose that the l-oxo dimer (632 nm, compound A in Scheme 1) can be cleaved to a FePcS monomer with a hydroxide or phenolate residue as an axial ligand (668 nm, compound B in Scheme 1).Titration of FeIII(PcS)OH with an acid leads to another monomer Mcompound C, probably with a water axial ligand as pre- [FeIII(PcS)H2O]`N, viously observed for sulfonated iron porphyrin complexes.22 The absorption band at 636 nm, observed with all the diÜerent acids, can be attributed to the corresponding stacked monomers (compounds and Bstack Cstack ).When a 20 lM FePcS solution is titrated with acid, the ratio between the absorption bands at 680 and 636 nm is always equal to 0.8 and does not change when adding more acid. But when we titrate a 4 lM FePcS solution under the same conditions, the ratio between the two absorption maxima changes and is equal to 1.0.The higher monomer content at lower FePcS concentration supports the hypothesis of an equilibrium between the monomer form C and the corresponding stacked monomer Finally, the formation of Cstack . a monomeric FePcS is a prerequisite for catalytic activity of the metallophthalocyanine in TCP oxidations and the key role of an organic solvent is to shift the dimer/monomer equilibrium towards a monomeric FePcS complex, the true catalyst precursor.Lifetime of the FePcS catalyst in the oxidation of TCP H2O2 One of the key questions in homogeneous catalytic oxidations is the lifetime of the catalyst. In order to answer this question we decided to monitor the catalytic activity in batch reactions after addition of new portions of pollutant and oxidant (in a one-portion addition or –ve-aliquot addition fashion, see Fig. 6).These experiments were performed under the conditions of run 4 of Table 3. To our surprise, we found that the TCP conversion was still 90% within 10 min in the third run [see Fig. 6(a)]. The half-conversion time of the third run was 3 min, very close to that measured for the –rst two runs (2 min) and the total dechlorination was 5.0 Cl~ for 2.9 converted TCP molecules or an average value of 1.7 Cl~ per TCP.The kinetics were slightly diÜerent in the case of the –ve-aliquot addition of hydrogen peroxide (half-conversion times were 9, 5 and 7 min for runs 1, 2 and 3, respectively), but the total dechlorination was 5.2 Cl~ for 2.9 converted TCP molecules or an average value of 1.8 Cl~ per TCP [see Fig. 6(b)]. It must be noted that the initial blue color of FePcS changed to green or violet after addition of hydrogen peroxide by aliquots or by the one-portion mode, respectively. But the color turned to brown during the second and third runs due to the formation of quinone derivatives, making the monitoring of the catalyst concentration by UV-visible spectrophotometry more difficult. Finally, these data are highly encouraging for a catalyst that is easily accessible and cheap.Conclusion Gradual addition of to the reaction mixture in oxida- H2O2 tions of trichlorophenol, catalyzed by FePcS, results in a more ììeconomicœœ consumption of the oxidant due to a decrease of the wasteful dismutation of As a consequence, higher H2O2 .conversions occurred with more advanced TCP oxidations using a smaller charge of catalyst. The catalyst precursor is probably a non-stacked monomeric iron sulfophthalocyanine able to react with hydrogen peroxide to generate an ironperoxo complex, previously proposed as the active species in the oxidative degradation of chlorophenols. The key role of New J. Chem., 1998, Pages 45»51 49Conversion of TCP (%) Time / min Conversion of TCP (%) Time / min (b) (a) Fig. 6 Conversion of TCP in three consecutive oxidation reactions by in the same batch; (a) one-portion addition of the FePcS»H2O2 oxidant, (b) –ve-aliquot addition the miscible organic solvent used in these aqueous catalytic mixtures is related to the shift of the dimer/monomer equilibrium towards a non-stacked monomeric form of FePcS.At the present stage of development, this catalytic system requires an organic co-solvent to be efficient. This fact makes the system more adapted for the treatment of FePcS»H2O2 process waters still containing 10 to 20% of organic solvents than for the –nal treatment of aqueous effluents. Repetitive batch experiments con–rmed that the catalyst is not destroyed at the end of substrate conversion, making this catalytic system based on a ììgreen oxidantœœ and a low cost catalyst very attractive.A further step in the development of the FePcS»H2O2 system would be to prepare a phthalocyanine complex that would exist in a monomeric form in water solution without addition of miscible organic solvents and to support this catalyst on materials compatible with water treatments.We are currently working in this direction. Experimental Materials The conversion of TCP was monitored by HPLC (Waters 510 pump, Waters 486 detector) equipped with a l-Bondapak C18 column, a methanol : water mixture (7 : 3, v/v) as the eluent at 1 mL min~1 and detection at 294 nm. The UV-visible absorption spectra were recorded on a Hewlett Packard 8452 A spectrophotometer. 1H NMR spectra were obtained on a Bruker AM 250 MHz spectrometer. All solvents used were of analytical grade. 2,4,6-Trichlorophenol was purchased from Janssen. Hydrogen peroxide was obtained from Acros as a 35 wt.% solution. 3-Cyanobenzoic acid was purchased from Aldrich. The concentrations of Cl~ were determined by the mercuric thiocyanate method.23 Preparation of the iron tetrasulfophthalocyanine FePcS Iron tetrasulfophthalocyanine (FePcS) was prepared according to the method of Weber and Busch.19 We found that the yield is highly dependent on the addition rate of the solid reagent mixture to the hot nitrobenzene solution.Thus the well-ground mixture of the monosodium salt of 4- sulfophthalic acid (17.28 g, 64.8 mmol), ammonium chloride (1.88 g, 36 mmol), urea (23.2 g, 388 mmol), ammonium molybdate (272 mg, 0.24 mmol) and (3.44 g, 19.2 FeSO4 … 1.5H2O mmol) was added to 24 mL of nitrobenzene at 180 °C over a period of 1 h.The resulting mixture was then heated for 7 h. The isolation and puri–cation of FePcS was performed according to the published protocol. Yield 68%. Anal. calcd for C 33.78 ; H 2.57 ; N 9.85 ; C32H12N8O12S4Na4Fe … 8H2O: Fe 4.91.Found: C 33.52 ; H 2.49 ; N 9.82 ; Fe 5.05%. UV-vis (water), (e/mol L~1 cm~1) : 328 (55 700), 632 (65 200). jmax/nm IR (KBr pellet), 1717, 1634, 1511, 1399, 1330, 1192, lmax/cm~1: 1146, 1110, 1074, 1056, 1030, 930, 833, 761, 747, 698, 649, 636, 594. Treatment of FePcS in the presence of H2 18O Two milligrams of FePcS were dissolved in 200 lL ofH218O (97 atom%, Eurisotop, Gif-sur-Yvette) and 2.3 lL of 48% HBr were added (–nal HBr concentration of 0.1 M).The resulting mixture was stirred for 1 h. Then HBr was neutralized with 2.3 lL of 8.84 M KOH and the reaction mixture was again stirred for 1 h. The sample was dried in vacuum. The IR spectrum of a KBr pellet showed the same bands as the initial FePcS material except that the 833 cm~1 band shifted to 839 cm~1.Oxidation of TCP by FePcS and H2O2 DiÜerent concentrations of FePcS (Table 1). The reaction mixture of 2 mL contained 20 lmol of 2,4,6-trichlorophenol (500 lL of a 40 mM stock solution in acetonitrile), phosphate buÜer (1 mL of a 500 mM stock solution at pH 7) and diÜerent catalyst concentrations : 740, 200, 100, 50, 20 nmol of FePcS for a 3.7, 1.0, 0.5, 0.25, 0.1% catalyst-to-substrate ratio (i.e., 500 lL of a 1.48, 0.4, 0.2, 0.1, 0.04 mM stock solution of FePcS in water, respectively).(5]20 lL or 1]100 lL H2O2 of a 3.5% solution in water, which was prepared by ten H2O2 fold dilution of a commercial 35 wt.% solution) was H2O2 added to the reaction medium and stirred at 18 °C. Aliquots of the reaction mixture were taken at de–ned times, diluted with an acetonitrile»water mixture to stop the oxidation reaction and analyzed by HPLC.Quantitative analysis of the oxidation products by NMR (Table 2). The NMR analysis of the oxidation products was performed as for runs 1, 2 and runs 6, 7 of Table 1; the scale was 50 times larger (100 mL reaction volume). Acetonitrile and water were evaporated under vacuum at 50 °C and the residue was dried for 1 h at room temperature.Seven milliliters of 1 M HCl saturated with NaCl were added to the residue ; the solution was adjusted to pH 2 by dropwise addition of 12 M HCl. The products were extracted with diethyl ether (8]60 mL). After evaporation of ether, the dark brown residue was dried under vacuum for 2 h at room temperature and dissolved in deuterated dimethyl sulfoxide for NMR analysis.We added 6 lL of (75 lmol) as an internal CHCl3 standard to quantify the oxidation products. The total yields of coupling products were based on two protons per aromatic ring. 50 New J. Chem., 1998, Pages 45»51DiÜerent pH values. The reaction mixture of 2 mL contained 20 lmol of 2,4,6-trichlorophenol (500 lL of a 40 mM stock solution in acetonitrile), 0.2 lmol of FePcS (500 lL of a 0.4 mM stock solution in water) and 500 lmol of buÜer (1 mL of the respective stock buÜer solution : pH 4 (0.25 M citric acid»0.5 M disodium hydrogen phosphate); pH 5, 6, 7, 8 (0.5 M phosphate buÜer); pH 9 (0.5 M boric acid»sodium hydroxide).(5]20 lL of a 3.5% solution in H2O2 H2O2 water) was added to the reaction mixture and stirred at 18 °C.Variation of the acetonitrile : water ratio (Table 3). The reaction mixture of 10 mL contained 10 lmol of 2,4,6-trichlorophenol (250 lL of a 40 mM stock solution in acetonitrile), 0.37 lmol FePcS (250 lL of a 1.48 mM aqueous stock solution) and 500 lmol of phosphate buÜer (1 mL of a 500 mM stock solution at pH 7). This solution was adjusted with the required volumes of acetonitrile and water to obtain the desired ratio (e.g., 4.75 mL of acetonitrile and 3.75 mL of water to obtain an acetonitrile : water ratio equal to 1 : 1).(50 lL of a 3.5% solution) was added and the H2O2 H2O2 reaction mixture stirred at 18 °C. In the presence of diÜerent co-solvents (Table 4). The concentrations used were the same as given above for Table 3.The stock solutions of TCP were prepared in the respective co-solvents. To obtain a co-solvent : water ratio of 1 : 3, the reaction mixture was adjusted with 2.25 mL of a co-solvent (acetone, acetonitrile, ethanol, formamide) and 6.25 mL of water. Titration of FePcS With diÜerent co-solvents. A solvent mixture of 730 lL of water and 250 lL of co-solvent (e.g., acetone, acetonitrile, ethanol or formamide) was prepared in a cuvette with a volume of 1 mL.Twenty microliters of a 1.48 mM stock solution of FePcS in water were added and the mixture was quickly shaken. The –rst UV-vis spectrum was recorded immediately; further spectra were recorded every 2 min. With TCP. One milliliter of a 20 lM FePcS solution in water was titrated every 5 min with 10 lL of a 40 mM TCP solution in acetonitrile.The UV-vis spectra were recorded 4 min after every addition. To control the eÜect of acetonitrile, the TCP stock solution was replaced by acetonitrile. With acetonitrile. DiÜerent samples of acetonitrile»water mixtures with 0, 5, 10, 15, 20, 25, 30 or 35% of acetonitrile containing 20 lmol of FePcS were prepared and the spectra recorded 10 min after preparation.With HCl, or One hundred microliters of a H2SO4 H3PO4 . 0.2 mM FePcS stock solution in water were added to a mixture of 5 lL of 0.05 M HCl or and 895 (H2SO4 H3PO4) lL of To obtain diÜerent acid concentrations, we H2O. changed the 0.05 M HCl : water ratio to 10 : 890, 15 : 885, 20 : 880, 25 : 875, 30 : 870, 50 : 850. The spectra were recorded 15 min after preparation. Titration of an acidic solution of FePcS with NaOH.One hundred microliters of a 0.2 mM FePcS stock solution in water were added to a solution of 850 lL and 50 lL of H2O 0.05 M HCl (or in a cuvette of 1 mL and the spec- H2SO4) trum was recorded after 15 min. The acidi–ed FePcS solution was then titrated every 5 min with 10 lL of 0.05 M NaOH solution.The UV-vis spectra were recorded 4 min after every addition. Lifetime of the FePcS catalyst in the oxidation of TCP H2O2 Batch experiments were performed under the conditions used for run 4 in Table 3 (i.e., 3.7% FePcS vs. TCP and 50% acetonitrile). The preparation of the reaction mixture is the same as mentioned above in the experimental part of Table 3 (the total volume being equal to 10 mL).New 10 lmol portions of 2,4,6-trichlorophenol (250 lL of a 40 mM stock solution in acetonitrile) were added at 24 and 49 min. In the case of a one-portion addition of corresponding to Fig. 6(a), H2O2 50 lL of a 3.5% solution (i.e., 3 aliquots of 50 lmol of H2O2 were added at 0, 25 and 50 min. In the case of a H2O2) sequential addition of the oxidant corresponding to Fig. 6(b), 15]10 lL of a 3.5% solution (i.e., 15 aliquots of 10 H2O2 lmol of were added every 5 min. In both cases, the H2O2) dechlorination values were determined at 22, 47 and 90 min. Acknowledgements thanks the European Community for a PhD fellowship A.H. (TMR grant No. ERBFMBICT 950030). A.S. is indebted to ELF-Atochem for a postdoctoral fellowship. We are grateful to Christian Forquy and Laurent Fraisse (ELF-Atochem, Lacq) for helpful discussions.Financial support was provided by CNRS, Elf-Atochem and EC (Training and Mobility of Researchers Programme). References 1 D. Henscher, Angew Chem., Int. Ed. Engl., 1994, 33, 1920. 2 A. Sorokin, J.-L. Seç ris and B. Meunier, Science, 1995, 268, 1163. 3 A. Sorokin, S. De Suzzoni-Dezard, D. Poullain, J.-P. Noeé l and B. Meunier, J. Am. Chem. Soc., 1996, 118, 7410. 4 A. Sorokin and B. Meunier, Chem. Eur. J., 1996, 2, 1308. 5 A. Robert, B. Loock, M. Momenteau and B. Meunier, Inorg. Chem., 1991, 30, 706. 6 P. G. Tratnyek and J. Holgneç , Environ. Sci. T echnol., 1991, 25, 1596. 7 G. B. Payne and P. H. Williams, J. Org. Chem., 1961, 26, 651. 8 G. B. Payne, P. H. Deming and P. H. Williams, J. Org. Chem., 1961, 26, 659. 9 (a) K. Dimroth, A. Reichardt, T. Siebmann and F. Bohlmann, L iebigs. Ann. Chem., 1963, 661, 1. (b) Y. Marcus, Chem. Soc. Rev., 1993, 22, 409. 10 S. Gupta, H. Huang and E. Yeager, Electrochim. Acta, 1991, 36, 2165. 11 A. B. P. Lever, S. Licoccia and B. S. Ramaswany, Inorg. Chim. Acta, 1982, 64, L87 and references therein. 12 B. R. Saunders, K. S. Murray, R. J. Fleming and Y. Korbatieh, Chem. Mater., 1993, 5, 809. 13 D. V. Stynes, S. Lui and H. Marcus, Inorg. Chem., 1985, 24, 4335. 14 W. A. Nevin, W. Lui, M. Melnik and A. B. P. Lever, J. Electroanal. Chem., 1986, 213, 217. 15 C. Ercolani, M. Gardini, F. Monacelli, G. Pennesi and G. Rossi, Inorg. Chem., 1983, 22, 2584. 16 R. Dieing, G. Schmid, E. Witke, C. Feucht, M. J. Pohmer Dreêen, and H. Hanack, Chem. Ber., 1995, 128, 589 and references therein. 17 J. C. Fanning, G. B. Park, C. G. James and W. R. Heatley Jr., J. Inorg. Nucl. Chem., 1980, 42, 343. 18 G. McLedon and A. E. Martell, Inorg. Chem., 1977, 16, 1812. 19 J. H. Weber and D. H. Busch, Inorg. Chem., 1965, 4, 469. 20 W. R. Scheidt, B. Cheng, M. K. Safo, F. Cukiernik, J. C. Marchon and P. G. Debrunner, J. Am. Chem. Soc., 1992, 114, 4420. 21 B. Cheng, P. H. Fries, J. C. Marchon and W. R. Scheidt, Inorg. Chem., 1996, 35, 1024. 22 T. C. Bruice, Acc. Chem. Res., 1991, 24, 243. 23 T. M. Florence and Y. J., Anal. Chim. Acta, 1971, 54, 373. Received 24th April 1997; Paper 7/06741K New J. Chem., 1998, Pages 45»51 51

ISSN:1144-0546    

DOI:10.1039/a706741k

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

Reactivity of electrogenerated polysul–de ions towards acyl thioanhydrides and anhydrides in N,N-dimethylacetamide Julie Robert, Meriem Anouti and Jacky Paris* L aboratoire de Physicochimie des Interfaces et des Milieux UFR Sciences et Reç actionnels, T echniques, Parc de Grandmont, 37200 T ours, France The reactivity of electrogenerated polysul–de ions in N,N-dimethylacetamide has been followed S3~~ (HS62~) by spectroelectrochemistry of a series of RC(O)X species : thioanhydrides X\SC(O)R 1a, 2a) (R\CH3 C6H5 and anhydrides X\OC(O)R 3b, 4b, 5b, 6b).With thioanhydrides two steps (R\CH3 n-C3H7 t-C4H9 C6H5 were evidenced: (i) formation of RC(O)S~ in equilibrium with from both fast substitution at the RC(O)S2~ trigonal carbon and exclusion from the nucleofugic anion X~: (ii) subsequent reaction of on RC(O)S2~ substrates leading to diacyl disul–des. With anhydrides the –rst step only occurs at a slower rate.The electrolysis of sulfur in the presence of 1a or 2a allowed the preparative scale formation of as RC(O)S2C(O)R isolated products from the ìelectrochemical insertion of sulfur œ in diacyl monosul–des. Reç activiteç des ions polysulfures e ç lectrogeç neç reç s dans le dimeç thylaceç tamide vis-a` -vis des thioanhydrides et anhydrides dœacides carboxyliques La reç activiteç des ions polysulfures eç lectrogeç neç reç s dans le N,N- S3~~ (HS62~) dimeç thylaceç tamide a eç teç suivie par spectroeç lectrochimie vis-a` -vis dœune seç rie de deç riveç s RC(O)X: thioanhydrides X\SC(O)R 1a, 2a), anhydrides X\OC(O)R 3b, 4b, 5b, 6b).(R\CH3 C6H5 (R\CH3 n-C3H7 t-C4H9 C6H5 Avec les thioanhydrides, deux eç tapes sont mises en eç vidence : (i) formation des ions RC(O)S~ et en RC(O)S2~ eç quilibre du fait de la substitution sur le carbone trigonal et de lœobtention de lœanion nucleç ofuge X~; (ii) reç action ulteç rieure des ions sur les substrats conduisant aux diacyldisulfures. Avec les anhydrides seule la RC(O)S2~ premie` re eç tape sœeÜectuant plus lentement est observeç e.Lœeç lectrolyse du soufre en preç sence des espe` ces 1a ou 2a reç aliseç e au niveau preç paratif a permis dœisoler les diacyldisulfures issus de ì lœinsertion eç lectrochimique du soufreœ sur les diacylmonosulfures. As reported recently,1 acyl chlorides ìinstantaneouslyœ react with ions in N,N-dimethylacetamide, a dipolar S3~~ (HS62~) aprotic medium, to produce diacyldisul–des (62»75% yield).Two successive steps were evidenced by spectroelectrochemistry : (i) initial substitution (eqn 1) of the leaving group, with concurrent equilibria (eqns 2 and 3) as established by direct addition of sulfur to thiocarboxylate ions :2 RC(O)Cl]2 S3~~]RC(O)S~]5/2 S2]Cl~ (1) 2 RC(O)S~]3 S2H[RC(O)]2S2]2 S3~~ (2) 2 RC(O)S~]S2H2 RC(O)S2~ (3) and (ii) subsequent reaction (eqn 4) of species : RC(O)S2~ RC(O)S2~]RC(O)Cl][RC(O)]2S2]Cl~ (4) Eqns 1 and 4 are analogous to those implied in the formation of diacylperoxides from RC(O)X [X\Cl, OC(O)R] and superoxide ions in aprotic media.3 O2~~ We report here on the relative reactivities of electrogenerated ions towards acylating agents : S3~~ ìthioanhydridesœ 1a, 2a) and [RC(O)]2S (R\CH3 C6H5 anhydrides 3b, 4b, 5b, [RC(O)]2O (R\CH3 n-C3H7 t-C4H9 6b).Reactions were followed at 20 °C by UV-vis absorp- C6H5 tion spectrophotometry coupled with stationary voltammetry. Results Sulfur-polysul–de ion characteristics in DMA The partial dissociation (eqn 5) of cyclooctasulfur into S8 S2 molecules was recently proposed by our group in dimethylacetamide: 4 S8H4 S2 (5) K1(297 K)\[S2]4/[S8]\10~7 mol3 dm~9 (6) In aprotic media such as DMA, sulfur reduces in two twoelectron steps with respect to the cyclic form [waves R1, S84 V vs. reference and R2, V, experi- E1@2\[0.40 E1@2\[1.10 mental value lA mmol~1 dm3] on a rotating i(R1)/[S8]0\34 gold-disc electrode.In the presence of excess of sulfur we expect the initial single-electron transfer to be S2]e~]S2~~ followed by the reaction of with the dimeric ions, up S2 S42~ to the formation of or species.4,5 The S62~ (HS3~~) S82~ stable product of the overall electrolysis of at controlled S8 potential on R1 (eqn 10) is the blue anion-radical S3~~ nm, dm3 mol~1 cm~1) through the (jmax\617 emax\4390 disproportionation (eqn 8) of the carmine red ions S82~ nm, dm3 mol~1 cm~1; (jmax1\515 emax1\3800 jmax2\360 nm, dm3 mol~1 cm~1) : emax2\9000 S8]2 e~]S82~ (7) S82~Hb f 2S3~~]S2 (8) K2(297 K)\[S3~~]2[S2]/[S82~]\1.7]10~6 mol2 dm~6 (9) S8]8/3 e~]8/3 S3~~ (10) ions are in equilibrium with their dimer S3~~ S62~ (jmax\ 465 nm, dm3 mol~1 cm~1) : emax\3100 S62~H2 S3~~ (11) New J.Chem., 1998, Pages 53»56 53l / nm l / nm ( a) ( b) A A E / V i / mA K3\[S3~~]2/[S62~]\0.043 mol dm~3 (12) UV-vis absorption spectra mol~1 cm~1) of (ei/dm3 S8 , ions between 250 and 750 nm were pre- S82~, S62~, S3~~ viously reported.5 In dilute solutions remains low with [S62~] respect to (i.e., 16% at total concentration [S3~~] [S3 ~~]0T\ mol dm~3). and [S3~~]]2 [S6 2~]\5.0]10~3 S82~ S([1/3) ions oxidize (O1) and reduce (R2) at the same potentials V; V].[E1@2(O1)\[0.20 E1@2(R2)\[1.10 Reactivity of ions with the thioanhydrides 1a, 2a S3 ~ó As observed with acyl chlorides,1 the addition of the thioanhydrides 1a, 2a to a sulfur solution greatly enhances the limiting current of the reduction wave R1: i(R1)exp./i(R1)th.\ 2.3 (1a) and 2.0 (2a) for This homoge- [(RCO)2S]/[S8]0\2.0. neous catalytic eÜect (eqns 7 and 13) agrees with the fast regeneration (eqn 13) of sulfur in the course of the reaction of polysul–de ions with substrates RC(O)X [X\Cl1, SC(O)R] in the diÜusion layer : RC(O)X]S82~]RC(O)S~]7/8 S8]X~ (13) Here the nucleofuge X~ and the substitution product would be the same species : RC(O)S~.This was veri–ed by the addition of a concentrated solution of thioanhydride 1a or 2a in DMA (2.0»7.0]10~2 mol dm~3) to ions of total con- S3~~ centrations close to 5.0]10~3 mol dm~3.Fig. 1 and [S3~~]0 T 2 show the evolution of A\f(j) and i\f(E) as a function of the ratio for the example y\[RC(O)X]/[S3~~]0 T R\CH3 with mol dm~3. As long as y remains [S3~~]0T\5.22]10~3 below B0.15 (Fig. 1a), decreases in favor of A617 (S3~~) A515 and with an isosbestic point at 540.5 nm; there is A360 (S82~) Fig. 1 (a) Evolution of UV-vis spectra during the addition of diacetyl sul–de 1a to an S([1/3) solution, mol [S3~~]0T\5.22]10~3 dm~3. The thickness of the cell was 0.1 cm; y\ (curve 1), 0.03 (2), 0.05 (3), 0.08 (4), 0.11 (5), [(RCO)2S]/[S3~~]0T\0 0.14 (6), 0.15 (7). (b) The same as (a) with y\0.15 (7), 0.24 (8), 0.33 (9), 0.50 (10), 0.71 (11), 0.84 (12), 1.27 (13) Fig. 2 Evolution of voltammograms during the reaction of diacetyl sul–de 1a with S([1/3) ions. Same conditions as for Fig. 1. Rotating gold-disc electrode, )\1000 rev min~1, diameter\2 mm; E vs. Ag/ AgCl, KCl satd in (0.1 mol dm~3) reference DMA»N(Et)4ClO4 no sign of R1 on any of the voltammograms. The stoichi- (S8) ometry (eqn 15) is the same as with acyl chlorides : sulfur coming from the substitution (eqn 14) totally reacts with S3~~ ions in excess according to eqn 8b: [RC(O)]2S]2 S3~~]RC(O)S~]5/2 S2]RC(O)S~ (14) [RC(O)2]2S]7 S3~~]2 RC(O)S~]5/2 S82~ (15) At the same time, the oxidation wave of RC(O)S~/RC(O)S2~ ions (the electroanalytic process eqns 16]3]17 previously described,2 V) increases at the expense of the E1@2\]0.09 one V).S82~/S3~~ (E1@2\[0.20 2 RC(O)S~][RC(O)]2S2]2 e~ (16) 2 RC(O)S2~][RC(O)]2S2]S2]2 e~ (17) The subsequent consumptions of the two ions and S3~~ S82~ by a shift in the equilibrium (eqn 8f) (0.15\y\0.5) permits the detection of sulfur by the growth of its cathodic wave R1 V).For y\0.5 (stoichiometry of eqn 14), the (E1@2\[0.40 equilibria (eqns 2 and 3) bear out the remaining presence of polysul–de ions in the solution ; the and concentra- S3~~ S82~ tions calculated from and the constants2 K1, K2 , K3 K4 , K5 lead to and values close to the experimental ones A617 A515 (^10%): K4\[RC(O)S2~][S3~~]2/[RC(O)S~]2[S2]3 \(12^2) dm6 mol~2 (18) K5\[RC(O)S2~]2/[RC(O)S~]2[S2]1\(48^4) dm3 mol~1 (19) With further additions of (0.5\y\1.0, curves [RC(O)]2S 10»13), and ions continue to be consumed S3~~ S82~ [decrease in and i(O)] but these species cannot be A617 , A515 totally eliminated because of the weak oxidation (eqn 2) of the nucleofugic RC(O)S~ ions.Low concentrations of ions (eqn 3) are revealed in the spectra (Fig. 1b, CH3C(O)S2~ curves 12, 13) by their characteristic absorbances2 (jmax1\ 336 nm, dm3 mol~1 cm~1; nm, emax1\4800 jmax2\467 dm3 mol~1 cm~1). i(R1) continues to rise with emax2\800 values greater than those of generated due to a catalytic S8 , eÜect analogous to eqns 7]13, which was previously noticed when diacyldisul–des were added to sulfur.2 At y\1, the oxidation current of ions is in agreement RC(O)S~/RC(O)S2~ with that resulting from the overall eqn 20. 2 [RC(O)]2S]2 S3~~][RC(O)]2S2]2 S2]2RC(O)S~ (20) 54 New J. Chem., 1998, Pages 53»56E/ V i / mA E / V i / mA Fig. 3 Evolution of voltammograms during the electrolysis of a solution with mol dm~3 in the presence of diben- [S8]0\1.05]10~3 zoylsul–de, mol dm~3 at E\[1.0 V vs. [2a]0\1.74]10~3 reference n F mol~1 2a\0 (curve 1), 0.37 (2), 0.75 (3), 1.12 (4), 1.49 (5), 1.86 (6), 2.24 (7), 2.62 (8) With the addition of sulfur, ions are not oxi- C6H5C(O)S~ dized in accordance with eqn 22 and the residual formation of ions (eqn 3) is only detected by their electro- RC(O)S2~ catalytic and kinetic oxidation wave2 (eqns 16, 3]17, E1@2\ ]0.35 V).The evolution of i\f(E) and A\f(j) for the reaction of with are the same as with [C6H5C(O)]2S S3~~ R\alkyl (0\y\1) except that ions totally dis- S82~/S3~~ appear at y\0.5. The electrochemical reduction of sulfur (EB[1.0 V) in the presence of the thioanhydrides 1a, 2a, which con–rms the preceding results, is illustrated in Fig. 3 with the experimental conditions : mol dm~3, [(C6H5CO)2S]0\1.74]10~3 mol dm~3. For 0\n F mol~1 2a\2 [S8]0\1.05]10~3 (curves 2»6) the decrease of the catalytic current i(R1) goes with the increase of the anodic waves of the RC(O)S2~ V) and RC(O)S~ V) ions.2 Two (E1@2\]0.35 (E1@2\]0.72 steps were observed when solutions were elec- RC(O)Cl]S8 trolyzed in the same way:1 (i) initial formation (eqn 21) of diacyldisul–de (0\n\1), with only appearance of the oxidation current of Cl~ ions on the voltammograms: 2 RC(O)X]S8]2 e~][RC(O)]2S2]3/4 S8]2 X~ (21) and (ii) reduction (eqn 22) of by polysul–de ions [RC(O)]2S2 (1\n\2), with the growth of the anodic wave of RC(O)S~ Fig. 4 Evolution of voltammograms during the addition of trimethylacetic anhydride 5b to an S([1/3) solution, [S3~~]0T\5.69 ]10~3 mol dm~3. (curve 1), 0.14 (2), 0.25 y\[RCO)2S]/[S3~~]0T\0 (3), 0.47 (4), 0.79 (5), 1.22 (6) ions : [RC(O)]2S2]S8]2 e~]2 RC(O)S~](S8) (22) In our particular case X~ species are RC(O)S~ ions, which are then generated on the basis 1 RC(O)S~/1 F.The overall process looks like the noteworthy ìelectrochemical insertion œ (eqn 23) of sulfur into thioanhydrides : 2 [RC(O)]2S]S2]2 e~][RC(O)]2S2]2 RC(O)S~ (23) Beyond n\2, ions result from the reduction of S82~/S3~~ sulfur (growth of and i(O) at V, A617 , A515 E1@2\[0.20 curves 7, 8). The electrolysis of 1a, 2a with sulfur [RC(O)]2S added as a ìmediatorœ at a ratio of 8 [S8]0/[RC(O)]2SB2.5 were performed on a preparative scale Mn\1 F mol~1 was the only product isolated [RC(O)]2SN.[RC(O)]2S2 (R\ yield 48%; 74%). CH3, R\C6H5 , Reactivity of ions with anhydrides 3bñ6b S3 ~ó An analogous study was carried out with anhydrides as substrates. Whatever the nature of R (3b»6b) the enhancement of the reduction current of sulfur was only observed at R2 potentials with the addition of [RC(O)]2O: i(R1]R2)exp./i(R1 for As noticed on the ]R2)th.B1.5 [(RCO)2O]/[S8]0\2.0.–rst wave R1 with thioanhydrides, this observation agrees with the catalytic eÜect (eqns 24]25), which implies here the more reducing agents These last species were not gen- S42~.4,6 erated in the present study by quantitative electrolysis of sulfur. S8]4 e~]2 S42~ (24) [RC(O)]2O]S42~]RC(O)S~]3/8 S8]RCO2~ (25) Thiocarboxylate ions (R\3»6) proved to be practically unreactive towards anhydrides at room temperature: the maximal absorbance and the oxidation wave of a solution A262\1.90 of mol dm~3 V) [CH3C(O)S~]0\2.90]10~3 (E1@2\]0.31 only decreased by 5% in the presence of [CH3C(O)]2O\3.0 ]10~3 mol dm~3 whereas no spectroelectrochemical changes were noticed for [(t- mol dm~3 C4H9CO)2S~]\2.70]10~3 with [(t- mol dm~3.In the C4H9CO)2O]ad\3.60]10~3 presence of sulfur, the reactions were limited : for mol dm3; 8 [CH3C(O)S~]0\2.27]10~3 [S8]0\11.0 ]10~3 mol dm~3; mol dm~3, [(CH3CO)2O]ad\8.5]10~3 the anodic current of ions RC(O)S2~/RC(O)S~ (E1@2\]0.05 V) retained 60% of its initial value at equilibrium after 10 min while i(R1) increased because of the catalytic eÜect (eqns 7]13) due to the partial formation of Under [RC(O)]2S2 .the same conditions solutions were unre- C6H5C(O)S~]S8 active towards benzoic anhydride. When anhydrides 3b»6b were added to ions (R\t- Fig. 4), the evolutions S3~~ C4H9 , of the spectra and voltammograms for 0\y\0.5 were identical to those observed with thioanhydrides (i.e., Figs. 1a and 1b, curves 1»10) or acyl chlorides ;1 however, except for R\ the reactions slowed down for y greater than 0.3 : as an C6H5 , example for mol dm~3, y\0.40, equi- [S3~~]0T\5.50]10~3 libria were attained after 1 min with and 8 min R\n-C3H7 with Beyond y\0.5, the addition of alkyl sub- R\t-C4H9 .strates only partially consumed ions and RC(O)S~/RC(O)S2~ at a slow rate : e.g., from curve 6 of Fig. 4 which was recorded at y\1.22 after 15 min, 80% of anionic species remained in solution. Discussion Diacyldisul–des are usually synthesized by chemical7 or electrochemical8 oxidation of thiocarboxylate ions and reactions of acyl chlorides with or under PTC con- Li2S29 Na2Sx ditions.10 Our results establish that these species are readily New J. Chem., 1998, Pages 53»56 55obtained by the reactions of thioanhydrides with poly- S3~~ sul–de ions at room temperature, as observed with acyl chlorides. 1 In both cases, thiocarboxylate ions coming from the fast nucleophilic substitution on the carbonyl carbon react in the presence of sulfur with the organic substrates RC(O)X. The formation of species can be explained by an [RC(O)]2S2 enhanced reactivity of intermediate ions compared RC(O)S2~ to RC(O)S~.This a eÜect,11 already displayed with RS2~ ions,5 probably competes with the displacement of the equilibrium (eqn 2f) by consumption of the stronger nucleo- S3~~ philes. The lower reactivity of anhydrides in general compared to that of acyl chlorides12 only allows access to RC(O)S~ ions. With respect to thioanhydrides, the same observation agrees with ìthe relative weakness of the overlapping of the C(2p) and S(3p) orbitals in the carbon»sulfur bondœ as noted by Cronyn et al.13 Thioanhydrides can be easily prepared by acylation of thiocarboxylate ions.14,15 These more stable species appear to be as efficient acylating agents as acyl chlorides in aprotic media.Experimental Materials and equipment Diacetyl sul–de 1a and anhydrides 3b»6b were obtained from Aldrich and used as received (purity[98%).Dibenzoyl sul–de 2a (mp 45»47 °C, lit.15 47»48 °C) was previously synthesized2 by addition of benzoyl chloride to electrogenerated thiobenzoate ions from thiobenzoic acid. Spectroelectrochemical experiments were carried out in DMA (Aldrich) with added tetraethylammonium perchlorate (Fluka, 0.1 mol dm~3) at 20 °C with equipment, electrodes and the —ow-through cell previously described.4 Potential values refer to Ag/AgCl, KCl satd in (0.1 mol dm~3).DMA/N(Et)4ClO4 Analysis of diacyl disul–des was performed by GC-MS (Hewlett-Packard 5989 A) and NMR spectroscopy (Bruker AC 200 spectrometer, as solvent, J values in Hz at CDCl3 200.132 and 50.323 mHz for 1H and 13C NMR, respectively).Generation of S(‘1/3) ions S([1/3) solutions (40 cm3) were prepared at concentrations near 5]10~3 mol dm~3 before addition of concentrated RC(O)X substrates in DMA cm3) by electro- (Vmax\4 reduction of sulfur at controlled potential (R2, EB[1.4 V) on a large gold-grid electrode.4 ions were the S3~~ (HS62~) only species in solutions when reached its maximum A617 value.Preparative electrolysis and were obtained by electro- [CH3C(O)]2S2 [C6H5C(O)]2S2 lysis of sulfur ([0.7 V\E\[0.5 V) with added homologous diacyl monosul–des up to 1 F mol~1 (eqn 23). [RC(O)]2S The experimental conditions (two-compartment cell, electrodes, procedure and puri–cation) were the same as with acyl chlorides.1 The intensity remained at a high value (200»250 mA) in the course of the electroreductions because of catalytic eÜects with both substrates and products [RC(O)]2S [RC(O)]2S2 .Diacetyl disul–de. Diacetyl sul–de: 1.18 g (10 mmol); S8: 0.77 g (24 mmol S). Product: diacetyl disul–de (0.36 g, 48%); 2.54 (6 H, s) ; 28.8 (2 C) and 189.5 (2 C); m/z 150 (M`, dH dC \2%) and 43 (100). Dibenzoyl disul–de. Dibenzoyl sul–de: 0.72 g (3 mmol); S8 : 0.24 g (7.5 mmol S).Product dibenzoyl disul–de (0.41 g, 74%); mp 135»136 °C (lit. :16 136»136.5 °C); 7.53»7.75 (6 H, m) dH and 8.13 (4 H, d, 7.4 Hz); 128.1 (4 C), 129 (4 C), 133.8 3J1H dC (2 C), 134 (2 C), and 186 (2 C); direct introduction mode m/z 274 (M`, 4%), 105 (50), 77 (100) and 51 (25). References 1 J. Robert, M. Anouti, M. Abarbri and J. Paris, J. Chem. Soc., Perkin T rans. 2, 1997, 1759. 2 J. Robert, M. Anouti and J. Paris, J. Chem. Soc., Perkin T rans. 2, 1997, 473. 3 (a) R. Johnson, T etrahedron L ett., 1976, 5, 331; (b) D. T. Sawyer, J. J. Stamp and K. A. Menton, J. Org. Chem., 1983, 48, 337; (c) J. P. Stanley, J. Org. Chem., 1980, 45, 1413; (d) A. Le Berre and Y. Berguer, Bull. Soc. Chim. Fr., 1966, 7, 2368. 4 G. Bosser and J. Paris, New J. Chem., 1995, 19, 391 and references therein. 5 G. Bosser, M. Anouti and J. Paris, J. Chem. Soc., Perkin T rans. 2, 1996, 1993. 6 J. Paris and V. Plichon, Electrochim. Acta, 1982, 27, 1501. 7 R. L. Franck and J. R. Blegen, Org. Synth. Coll., 1955, 3, 116»118. 8 Y. Hirabayashi and T. Mazume, Bull. Chem. Soc., Jpn., 1966, 39, 1971. 9 J. A. Gladysz, V. K. Wong and B. S. Jick, T etrahedron, 1979, 35, 2329. 10 (a) M. Kodomari, M. Fukuda and S. Yoshitomi, Synthesis, 1981, 8, 637; (b) J. X. Wang, W. Cui, Y. Hu and K. Zhao, Synth. Commun., 1995, 25, 889. 11 J. E. Dixon and T. C. Bruice, J. Am. Chem. Soc., 1972, 94, 2052 and references therein. 12 J. March, Advanced Organic Chemistry: Reactions, Mechanisms and Structure, Wiley, New York, 1992, p. 409. 13 M. W. Cronyn, M. Chang Pin and R. A. Wall, J. Am. Chem. Soc., 1955, 77 3031. 14 (a) E. E. Reid, Organic Chemistry of Bivalent Sulfur, Chemical Publishing, New York, 1962, vol. 4, pp. 11»58; (b) H. Boé hme and H. P. Steudel, L iebigs Ann. Chem., 1969, 730, 121. 15 M. Mikolajczyk, P. Kielbasinski and H. M. Schiebel, J. Chem. Soc., Perkin T rans. 1, 1976, 564. 16 C. Christophersen and P. Carlsen, T etrahedron, 1976, 32, 745. Received 28th May 1997; Paper 7/06743G 56 New J. Chem., 1998, Pages 53»56

ISSN:1144-0546    

DOI:10.1039/a706743g

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

O Cl Cl R R O Cl R R O Cl R R O Cl Cl R R O Cl Cl R R – R R O Cl 7 + 2e –Cl– O Cl R R – + 5 8 –Cl– 5 6 – Electrochemically induced Favorskii rearrangement. a,b-Unsaturated amides and esters in the electrochemical reduction of polyhaloketones A. Inesi,a L. Rossi,*,a M. Feroci*,b and M. Rizzutob a Dipartimento di Chimica, Ingegneria Chimica e Materiali, degli Studi, I-67040 Universita` Monteluco di Roio, L œAquila, Italy b Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia, degli studi di Roma ììL a Sapienzaœœ, V ia del Castro L aurenziano 7, I-00161 Roma, Universita` Italy Electrochemically reduced polyhaloketones react with amines and phenols aÜording the corresponding a,bunsaturated amides and esters in moderate yields.The formation of a-iminoketones and a-diimines (main products of the chemical reaction) is completely avoided.The stereochemistry of the a,b-unsaturated products is independent of the nature of the nucleophiles and haloketones. In the last years it has been highlighted that a,a@-dihaloketones are valuable substrates for the synthesis of many organic compounds.1 Particular attention has been devoted to the reactivity of polyhaloketones with amines.2 This reactivity is strongly in—uenced by solvent and temperature; moreover, the type of ketone and the type of amine play an important role in determining the reaction products.Sterically hindered a,a@- dihaloketones and hindered amines did not react (except under drastic reaction conditions, e.g., high temperature and very high excess of amine).Examining the reactivity of a,a@- dibromoketone with primary amines, De Kimpe and coworkers have brought out the possibility of selectively forming a-iminoketones and a-diimines. According to the proposed mechanisms,2 Favorskii-derived products could be obtained only under the conditions that allow the formation of halocyclopropanone A as an intermediate (Scheme 1).Recently many non-conventional synthetic processes, involving electrochemically generated intermediates, have been developed.3 These reactions, carried out by the electrochemical reductions or oxidations of suitable substrates, often occur under mild conditions and with considerable improvement in selectivity with respect to the corresponding classical chemical way.In this context, the reduction of halo compounds has turned out to be particularly important, especially because of the subsequent transformations undergone by the Scheme 1 corresponding intermediates, i.e. their reaction with the solvent, the parent molecule or with non-electroactive substrates purposely added to the solution, etc.4 Investigating the electrochemical behaviour of haloketones, we found that the electrochemical reduction of a,a@-dichloroketones 5, carried out at their –rst voltammetric peak, involves the two-electron cleavage of a carbon»chlorine bond to yield the corresponding carbanion 6.The chlorocyclopropanone 7 was also found to be one of the possible intermediates in the subsequent transformation of carbanion 8 (Scheme 2).5 In this paper we investigate the possibility of an electrochemical activation of a,a@-polyhaloketones with respect to primary and secondary amines in order to obtain the selective formation of the Favorskii-rearrangement products.Consequently, we have used the polyhaloketones 1a,b and 2a,b and amines 3a»e. Solutions containing polyhaloketone»amine systems were electrolysed at a potential negative enough to lead to the two-electron cleavage of the carbon»halogen bond so as to favour the formation of the halocyclopropanone intermediate.The investigation was also extended to the polyhaloketone»phenol 4a»c systems. Thus we intend to determine the eÜect of the nucleophile nature on the reactivity of the polyhaloketones and on the stereochemistry of the products. Results Solutions containing equimolar amounts of polyhaloketones 1a,b, 2a,b and amines 3a»e (see Scheme 3) did not react at Scheme 2 New J.Chem., 1998, Pages 57»61 57i / mA E / V (c) (d) (e) (a) (b) Scheme 3 room temperature, even over a one-to-two day interval. a- Iminoketones and a-diimines were both absent when DMF, THF and were used as solvents. The volt- CH3CN, CH2Cl2 ammetric curves of polyhaloketones 1a,b, 2a,b show two or more reduction peaks (Fig. 1) ; the –rst reduction peak can be related to the two-electron cleavage of a carbon»halogen bond V; V; V; (Ep(1b) @ \[1.53 Ep(2b) @ \[0.99 Ep(1a) @ \[1.08 V vs. SCE; solvent DMF with 0.1 mol dm~3 Ep(2a) @ \[0.6 TEAP; Hg cathode; c\1.0]10~3 mol dm~3; l\0.2 V s~1). The addition of amines 3a»e did not cause signi–cant variations in the voltammetric curves of 1a,b, 2a,b.On the contrary, the addition of phenols 4a,b caused the appearance of a new peak characterised, independently of the ketone, by the same potential value (E*\[2.27 V) (Fig. 1). Quite surprisingly, no new peak appeared when phenol 4c was used. Solutions of polyhaloketones 1a,b, 2a,b and amines 3a»e or phenols 4a»c in DMF with 0.1 mol dm3 TEAP were electrolysed at a potential negative enough to allow the two-electron cleavage of a carbon»halogen bond so as to obtain the formation of the halocyclopropanone 11 intermediate as indicated in Scheme 4.The voltammetric curves, recorded from the cathodic solutions at the end of the electrolyses, have no Fig. 1 Cyclic voltammetry curves at a Hg electrode of DMF with 0.1 mol dm~3 TEAP solutions of 2b in the absence (a) and in the presence of 4a (b) and of 1b in the absence (c) and in the presence 4a before the electrolysis (d) and at the end of the electrolysis (e) carried out at the potential E\[1.0 V, l\0.2 V s~1.Concentrations of 1b, 2b and 4a are c\1.0]10~3 mol dm~3 in all cases Table 1 Coulometric data and yields of the Favorskii products of the electrochemical reduction of solutions (DMF with 0.1 mol dm~3 TEAP) of ketones 1a,b and 2a,b in the presence of substrates 3a»d and 4a,b at a mercury cathode Favorskii product Z : E Entry Ketone Nucleophilea [E/V napp b Yields (%)c ratio 1 1a 3a 1.0 2.0 12a 29 67 : 33 2 1a 3b 1.0 2.0 12b 42 65 : 35 3 1a 3c 1.0 2.3 12c 41 66 : 34 4 1a 3d 1.0 1.8 12d 39 61 : 39 5 1a 4a 1.0 2.0 13a 33 58 : 42 6 1a 4b 1.0 2.1 13b 48 61 : 39 7 2a 3a 0.6 3.0 12a 47 65 : 35 8 2a 3b 0.6 3.0 12b 53 66 : 34 9 2a 3c 0.6 2.8 12c 59 59 : 41 10 2a 3d 0.6 1.7 12d 51 64 : 36 11 2a 4a 0.6 2.0 13a 44 63 : 37 12 2a 4b 0.6 2.8 13b 45 59 : 41 13 1b 3a 1.6 2.3 12a 68 54 : 46 14 1b 3b 1.6 1.9 12b 36 64 : 36 15 1b 3c 1.6 2.1 12c 44 66 : 34 16 1b 3d 1.6 2.0 12d 47 64 : 36 17 1b 4a 1.6 2.2 13a 64 77 : 23 18 1b 4b 1.6 2.7 13b 36 65 : 35 19 2b 3a 1.0 2.7 12a 74 73 : 27 20 2b 3b 1.0 2.8 12b 42 60 : 40 21 2b 3c 1.0 3.0 12c 58 62 : 38 22 2b 3d 1.0 1.6 12d 70 66 : 34 23 2b 4a 1.0 2.0 13a 42 64 : 36 24 2b 4b 1.0 2.9 13b 44 66 : 34 a The ratio between the concentrations of ketone and substrate is o\1.b Number of Faraday mol~1 obtained by coulometry. c Yields with respect to the initial amount of ketone. 58 New J. Chem., 1998, Pages 57»61N R1 R2 O O O R1 R2 R3 12a: R1 = CH2Ph; R2 = H 12b: R1 = Cyclohexyl; R2 = H 12c: R1 = CH2Ph; R2 = Me 12d: R1 = (CH2)3Ph; R2 = H 13a: R1 = R3 = H;R2 = OCH3 13b: R1 = R3 = CH3;R2 = H O O X X O O X O X X + 2e– –X– 9a,b 1a,b – O 2 + 2 + 10a,b – Scheme 4 peaks in the case of ketone»amine systems, whilst in the case of ketone-phenol (4a,b) systems there is still one peak at a potential E*\[2.27 V (Fig. 1). In addition, the voltammetric analysis shows that a,b-unsaturated amides 12a»d are not reducible at the potential preceding the discharge of the supporting electrolyte ; on the other hand a,b-unsaturated esters 13a,b are reducible at a potential of E*\[2.27 V. Accordingly, at the end of the electrolyses of polyhaloketone»amine 3a»d systems (Table 1, entries 1»4, 7»10, 13»16, 19»22), we were able to isolate the a,b-unsaturated amides 12a»d (see Scheme 5) from the cathodic solution, whereas polyhaloketone»phenol 4a,b systems (entries 6»7, 11»12, 17»18, 23»24) yielded the a,b-unsaturated ester 13a,b.Polyhaloketone»aniline 3e and polyhaloketone-4-nitrophenol 4c systems, on the contrary, did not provide the expected Favorskii-rearrangement products.In addition, in all the cathodic solutions both a-iminoketones and a-diimines were absent. These results suggest that the electrochemical reduction was able to activate polyhaloketones towards amines 3a»d and phenols 4a,b, inducing the formation of the halocyclopropanone 11 as described in Scheme 4. This intermediate is able to react only with the substrates that are nucleophilic enough to add to its carbonyl group.In fact, amines 3a»d and phenoxide anions of 4a,b with are nucleophiles that react 2\pKb\56,7 with the haloketones yielding the corresponding Favorskiirearrangement product; on the contrary aniline (pKb\9.4)6 and 4-nitrophenoxide anion are unreactive. (pKb\6.8)6 The formation of cyclopropanone 11 ensues from diÜerent routes according to the starting haloketones (Scheme 4) : a,a,- a@-trihaloketones 2a,b undergo the electrochemical cleavage of a carbon»halogen bond yielding the carbanion 10, which evolves to 11 via an intramolecular reaction.In the case of SN a,a@-dihaloketones 1a,b, the carbanion 10 is obtained both by the deprotonation of the parent molecule operated by the electrochemically generated carbanions 9a,b and by means of a more complex mechanism (Scheme 6),5 in which a single ion 9a,b allows the deprotonation of two molecules of 1a,b (in accordance with yields of 12 and 13 greater than 50%).Scheme 5 Scheme 6 Scheme 7 We also found that the Z/E ratio of the a,b-unsaturated amides 12a»d and esters 13a,b isolated from the cathodic solution was independent of the ketone»nucleophile system used (see Table 1).Moreover, it was pointed out8 that the Z/E ratio in a,b-unsaturated esters is solely dependent on the ratio of the two pathways for halide elimination from 10 (Scheme 7). The formation of 11 and its opening are stereospeci–c (Scheme 4). In any case the electrochemical activation of 1a,b and 2a,b leads to the formation of halocyclopropanones 11 with the same Z/E ratio.In addition we found that the Z/E ratio in halide elimination from 10 is independent of the nature of the halogen atom. Conclusion The electrochemical reduction allows the activation of polyhalogenoketones that do not otherwise react with primary and secondary amines and phenols. a,b-Unsaturated amides and esters were isolated, after electrolyses, from solutions containing polyhaloketones 1a,b, 2a,b and amines 3a»d or phenols 4a,b.Their Z/E ratio was found to be independent of the polyhaloketone»nucleophile system used. The formation of a-ketoimines and a-diimines was completely avoided. Experimental Methods Voltammetric measurements were carried out using an Amel 498 sessile mercury drop electrode with an AMEL 552 potentiostat equipped with an AMEL 566 function generator and an AMEL 563 multipurpose unit ; the curves were displayed on an AMEL 863 recorder assisted by a Nicolet 3091 digital oscilloscope. Coulometry and controlled-potential electrolysis were carried out with an AMEL 552 potentiostat equipped New J.Chem., 1998, Pages 57»61 59with an AMEL 721 integrator.The cells used for these techniques have already been described ;9 the cathode was a mercury pool, the counter electrode was a cylindrical platinum gauze and the reference electrode was the calomel type described by Fujinaga;10 its potential is [0.020 V vs. SCE (saturated calomel electrode). All the potentials are given with respect to this electrode.N,N-Dimethylformamide (DMF, Carlo Erba) and tetraethylammonium perchlorate (TEAP, Fluka) were puri–ed as previously described.11 All the experiments were carried out at 20.0^0.1 °C in DMF with 0.1 mol dm~3 TEAP solutions. The catholyte was degassed and preelectrolysed at the working potential before the addition of the substrate. Column chromatography was performed on Merck Silica gel (70»230 mesh, 60 g per 1 g of crude mixture).GC analyses were carried out on a Hewlett Packard 5890 gas chromatograph equipped with a —ame ionization detector, linear temperature programmer and a Hewlett Packard Model 3390A electronic integrator. The column used was a Supelco SP 2250 (30 mm]0.32 mm). GC-MS measurements were carried out on a SE 54 capillary column using a Fisons 8000 gas chromatograph coupled with a Fisons MD 800 quadrupole mass selective detector. 1H and 13C NMR spectra were recorded using a Bruker AC 200 spectrometer with as internal standard. All new compounds gave satisfac- CDCl3 tory elemental analyses (C^0.3%; H^0.2%; N^0.2%). Materials The amines 3a»e and phenols 4a»c were commercially available products of analytical grade. 1,3-Dibromo-3-methyl-2- pentanone (1a) and 1,1,3-tribromo-3-methyl-2-pentanone (2a) were prepared according to Rappe12 from the bromination of 3-methyl-2-pentanone (Aldrich) with two or three moles of bromine, respectively. 1,3-Dichloro-3-methyl-2-pentanone (1b) was prepared as described in the literature ;8 1,1,3-trichloro-3- methyl-2-pentanone (2b) was prepared using 3.0 equiv of sulfuryl chloride according to the procedure described by Sakai et al.13 1,3-Dibromo-3-methyl-2-pentanone (1a). 1H NMR: 4.43 (dd, AB, 2H, Hz, *l\10.8 Hz, d(CDCl3) JAB\15.6 2.18 (dq, 1H, J\14.6, 7.3 Hz, 2.03 CH2BrCO), CH3CH2), (dq, 1H, J\14.6, 7.3 Hz, 1.85 (s, 3H, CH3CH2), CH3CBr), 1.01 (t, 3H, J\7.3 Hz, 13C NMR: 197.6, CH3CH2) ; d(CDCl3) 69.2, 34.7, 30.6, 26.6, 10.1 ; GC-MS m/z : absent, 165 (4%), M`~ 163 (4), 137 (53), 135 (49), 123 (16), 121 (14), 95 (14), 93 (16), 55 (100). 1,1,3-Tribromo-3-methyl-2-pentanone (2a). 1H NMR: 6.64 (s, 1H, 2.27»1.99 (2H, m, d(CDCl3) CHBr2CO), CH3CH2), 1.89 (s, 3H, 1.05 (t, 3H, J\7.3 Hz, 13C CH3CBr), CH3CH2) ; NMR: 192.6, 67.8, 36.3, 34.5, 26.6, 10.1 ; GC-MS d(CDCl3) m/z : absent, 203 (1%), 201 (2), 199 (1), 175 (4), 173 (8), 171 M`~ (4), 165 (9), 163 (10), 137 (53), 135 (56), 55 (100). 1,1,3-Trichloro-3-methyl-2-pentanone (2b). 1H NMR: 6.70 (s, 1H, 2.10 (1H, dq, J\7.3 Hz, d(CDCl3) CHCl2CO), 1.97 (1H, dq, J\7.3 Hz, 1.72 (s, 3H, CH3CH2), CH3CH2), 1.04 (t, 3H, J\7.3 Hz, 13C NMR: CH3CCl), CH3CH2) ; 199.4, 75.5, 46.3, 34.7, 26.9, 8.9 ; GC-MS m/z : d(CDCl3) M`~ absent, 121 (4%), 119 (12), 93 (38), 91 (96), 87 (3), 85 (10), 83 (18), 55 (100).Reduction of haloketones in the presence of 3añe or 4añc General procedure. The controlled-potential electrolyses were carried out at the potential corresponding to the –rst voltammetric peak of the haloketones by stepwise addition of haloketones (1.0 mmol) and nucleophiles (1.0 mmol) to DMF with 0.1 mol dm~3 TEAP (30 mL) in such a way that its concentration never exceeded 10~2 mol dm~3.Each addition of ketone and nucleophiles was made when the current dropped from its initial value to that measured from the pre-electrolysis. At the end of the electrolysis, the catholyte was separated from the mercury, mixed with water (50 mL) and extracted with Et2O (5]30 mL). The organic extracts were washed with water, dried analysed by TLC and the solvent evaporated (Na2SO4), under reduced pressure.The residue was chromatographed on silica gel to aÜord the two pure isomers of the corresponding a,b-unsaturated carbonyl compound. The double-bond geometry was assigned by means of 1H NMR spectra. The yields are reported in Table 1. The Z/E ratio was determined by gas chromatography. The reduction of haloketones in the presence of 3e and 4c did not lead to the corresponding a,bunsaturated carbonyl compounds.N-Benzyl-3-methyl-2-pentenamide (12a). Z isomer: 1H NMR: 7.73»7.27 (5H, m, ar), 5.68 (1H, br s, NH), d(CDCl3) 5.51 (1H, d, J\1.3 Hz, CHCxC), 4.43 (2H, d, J\5.7 Hz, 2.65 (2H, q, J\7.5 Hz, 1.81 (3H, d, CH2Ph), CH3CH2), J\1.3 Hz, 1.06 (3H, t, J\7.5 Hz, CH3CxC), CH3CH2) ; 13C NMR: 166.4, 156.8, 138.6, 128.6, 127.7, 127.3, d(CDCl3) 117.7, 43.2, 26.1, 24.2, 12.7 ; GC-MS m/z : 204 2%), (M`~]1, 203 10), 188 (4), 97 (33), 91 (100), 41 (52).(M`~, E isomer: 1H NMR: 7.82»7.23 (5H, m, ar), 5.85 d(CDCl3) (1H, br s, NH), 5.53 (1H, d, J\1.0 Hz, CHCxC), 4.42 (2H, d, J\2.4 Hz, 2.64 (2H, q, J\7.6 Hz, 2.14 CH2Ph), CH3CH2), (3H, d, J\1.0 Hz, 1.01 (3H, t, J\7.6 Hz, CH3CxC), 13C NMR: 167.0, 156.1, 138.7, 128.6, CH3CH2) ; d(CDCl3) 127.7, 127.3, 116.7, 43.3, 29.3, 18.2, 12.0 ; GC-MS m/z : 204 2%), 203 9), 188 (4), 97 (43), 91 (100), 41 (58).(M`~]1, (M`~, N-Cyclohexyl-3-methyl-2-pentenamide (12b). Z isomer: 1H NMR: 5.45 (1H, s, CHCxC), 5.30 (1H, br s, NH), d(CDCl3) 3.74 (1H, m, NCH), 2.57 (2H, q, J\7.5 Hz, 1.76 CH3CH2), (3H, s, 1.06 (3H, t, J\7.5 Hz, 1.91» CH3CxC), CH3CH2), 0.98 (10H, m, 13C NMR: 165.8, 155.1, w(CH2)5w) ; d(CDCl3) 118.6, 47.8, 33.3, 29.3, 26.0, 24.9, 24.8, 12.7 ; GC-MS m/z : 196 (M`]1, 3%), 195 8), 180 (4), 97 (92), 41 (100).(M`~, E isomer: 1H NMR: 5.47 (1H, d, J\1.2 Hz, d(CDCl3) CHCxC), 5.35 (1H, br s, NH), 3.77 (1H, m, NCH), 2.57 (2H, q, J\7.5 Hz, 2.08 (3H, d, J\1.2 Hz, CH3CH2), CH3CxC), 1.02 (3H, t, J\7.5 Hz, 1.86»0.96 (10H, m, CH3CH2), 13C NMR: 166.4, 154.6, 117.5, 47.8, w(CH2)5w) ; d(CDCl3) 33.2, 26.1, 26.0, 24.9, 18.0, 12.0 ; GC-MS m/z : 196 (M`]1, 3%), 195 9), 180 (5), 97 (100), 41 (97).(M`~, N-Methyl-N-benzyl-3-methyl-2-pentenamide (12c). Z isomer: 1H NMR: 7.77»7.10 (5H, m, ar), 5.80 (1H, s, d(CDCl3) CHCxC), 4.53 (2H, s, 2.88 (3H, s, 2.35 (2H, CH2Ph), CH3N), q, J\7.5 Hz, 1.77 (3H, s, 1.05 (3H, t, CH3CH2), CH3CxC), J\7.5 Hz, 13C NMR: 166.8, 152.1, CH3CH2) ; d(CDCl3) 137.6, 128.8, 128.5, 126.9, 117.7, 54.0, 32.8, 29.6, 23.1, 12.5 ; GC-MS m/z : 217 4%), 202 (4), 97 (70), 91 (100), 41 (91).(M`~, E isomer: 1H NMR: 7.77»7.10 (5H, m, ar), 5.80 d(CDCl3) (1H, s, CHCxC), 4.60 (2H, s, 2.89 (3H, s, CH2Ph), CH3N), 2.35 (2H, q, J\7.5 Hz, 1.82 (3H, s, CH3CH2), CH3CxC), 1.05 (3H, t, J\7.5 Hz, 13C NMR: 167.2, CH3CH2) ; d(CDCl3) 151.5, 137.0, 128.7, 128.0, 126.6, 117.5, 54.4, 35.2, 26.8, 18.3, 12.0 ; GC-MS m/z: 217 5%), 202 (13), 97 (80), 91 (100), (M`~, 41 (98).N-(3-Phenylpropyl)-3-methyl-2-pentenamide (12d). Z isomer: 1H NMR: 7.27»7.10 (5H, m, ar), 6.14 (1H, d(CDCl3) br s, NH), 5.50 (1H, s, CHCxC), 2.60 (2H, q, J\7.7 Hz, 2.59 (4H, m, and 1.79 (3H, s, CH3CH2), CH2N CH2Ph), 1.76 (2H, m, 1.03 (3H, t, J\7.7 CH3CxC), CH2CH2CH2), Hz, 13C NMR: 166.6, 155.2, 141.4, 128.2, CH3CH2) ; d(CDCl3) 60 New J.Chem., 1998, Pages 57»61128.1, 125.7, 118.1, 42.0, 38.6, 33.1, 30.1, 24.7, 12.6 ; GC-MS m/z : 232 3%), 231 19), 127 (55), 97 (100), 91 (M`~]1, (M`~, (43), 41 (83). E isomer: 1H NMR: 7.23»7.07 (5H, m, ar), 6.16 d(CDCl3) (1H, br s, NH), 5.53 (1H, s, CHCxC), 2.60 (2H, q, J\7.7 Hz, 2.59 (4H, m, and 2.02 (3H, s, CH3CH2), CH2N CH2Ph), 1.76 (2H, m, 0.99 (3H, t, J\7.7 CH3CxC), CH2CH2CH2), Hz, 13C NMR: 167.2, 154.6, 141.4, CH3CH2) ; d(CDCl3) 128.13, 128.10, 125.7, 117.1, 41.4, 39.1, 33.2, 25.8, 17.9, 11.8 ; GC-MS m/z : 232 3%), 231 18), 127 (57), 97 (M`~]1, (M`~, (100), 91 (45), 41 (82). 4-Methoxyphenyl 3-methyl-2-pentenoate (13a). Z isomer: 1H NMR: 7.02»6.84 (4H, m, ar), 5.83 (1H, s, CHCxC), d(CDCl3) 3.77 (3H, s, 2.66 (2H, q, J\7.6 Hz, 1.55 OCH3), CH3CH2), (3H, s, 1.10 (3H, t, J\7.6 Hz, 13C CH3CxC), CH3CH2) ; NMR: 164.7, 164.3, 157.0, 144.3, 122.4, 114.6, 113.7, d(CDCl3) 55.5, 33.8, 24.7, 12.3 ; GC-MS m/z : 221 3%), 220 (M`~]1, 6), 124 (41), 97 (100), 41 (56).(M`~, E isomer: 1H NMR: 7.02»6.87 (4H, m, ar), 5.86 d(CDCl3) (1H, d, J\1.1 Hz, CHCxC), 3.77 (3H, s, 2.66 (2H, q, OCH3), J\7.6 Hz, 1.94 (3H, d, J\1.1 Hz, CH3CH2), CH3CxC), 1.07 (3H, t, J\7.6 Hz, 13C NMR: 165.4, CH3CH2) ; d(CDCl3) 164.8, 155.0, 144.3, 122.4, 114.6, 114.3, 55.5, 26.7, 18.9, 11.8 ; GC-MS m/z : 221 3%), 220 6), 124 (39), 97 (M`~]1, (M`~, (100), 41 (53). 3,5-Dimethylphenyl 3-methyl-2-pentenoate (13b). Z isomer: 1H NMR: 7.13»6.81 (3H, m, ar), 5.86 (1H, s, d(CDCl3) CHCxC), 2.68 (2H, q, J\7.5 Hz, 2.25 (3H, s, CH3CH2), 2.23 (3H, s, 1.96 (3H, s, 1.10 CH3Ph), CH3Ph), CH3CxC), (3H, t, J\7.5 Hz, 13C NMR: 164.9, CH3CH2) ; d(CDCl3) 164.8, 148.6, 130.2, 122.7, 118.8, 114.7, 29.7, 24.9, 19.1, 12.4 ; GC-MS m/z : 218 3%), 122 (11), 97 (100), 41 (52).(M`~, E isomer: 1H NMR: 7.14»6.81 (3H, m, ar), 5.89 d(CDCl3) (1H, d, J\1.1 Hz, CHCxC), 2.68 (2H, q, J\7.5 Hz, 2.25 (3H, s, 2.23 (3H, s, 2.19 (3H, CH3CH2), CH3Ph), CH3Ph), d, J\1.1 Hz, 1.10 (3H, t, J\7.5 Hz, CH3CxC), CH3CH2) ; 13C NMR: 165.4, 164.3, 148.6, 130.2, 122.7, 118.8, d(CDCl3) 113.7, 29.6, 24.8, 19.8, 11.8 ; GC-MS m/z : 218 4%), 122 (M`~, (11), 97 (100), 41 (40).Acknowledgements authors thank M. Di Pilato for his contribution to the The experimental part of this work.Financial support from the CNR and the Ministero dellœUniversita` e della Ricerca Scienti- –ca e Tecnologica are gratefully acknowledged. References 1 N. De Kimpe and R. Verheç , T he Chemistry of a-Haloketones, a- Haloaldehydes and a-Haloimines, eds S. Patai and Z. Rappoport, J. Wiley, 1988. 2 N. De Kimpe, L. DœHond and L. Moens, T etrahedron, 1992, 48, 3183. 3 Organic Electrochemistry, eds H. Lund and M. M. Baizer, Dekker, New York, 1991. 4 J. Casanova and V. P. Reddy, T he Chemistry of Halides, Pseudohalides and Azides, eds S. Patai and Z. Rappoport, Wiley, New York, 1995. 5 I. Chiarotto, M. Feroci, C. Giomini and A. Inesi, Bull. Soc. Chim. Fr., 1996, 133, 167. 6 CRC Handbook of Chemistry and Physics, ed. D. R. Lide, CRC, Boca Raton, 74th edn, 1993, pp. 8»43. 7 W. P. Jenks and J. Regenstein, Handbook of Chemistry, CRC, Cleveland, OH, USA, 1968, p. J-187. 8 N. Schamp, N. De Kimpe and W. Coppens, T etrahedron, 1975, 31, 2081. 9 A. Inesi, L. Rampazzo and A. Zeppa, J. Electroanal. Chem., Interfacial Electrochem., 1981, 122, 233. 10 T. Fujinaga, K. Izutsu and K. Takaota, J. Electroanal. Chem., 1966, 12, 203. 11 F. De Angelis, M. Feroci and A. Inesi, Bull. Soc. Chim. Fr., 1993, 130, 712. 12 C. Rappe, Archiv. Kemi, 1963, 21, 503. 13 T. Sakai, M. Ishikawa, E. Amano, M. Utaka and A. Takeda, Bull. Chem. Soc. Jpn., 1987, 60, 2295. Received 14th March 1997; Paper 7/06745C New J. Chem., 1998, Pages 57»61 61

ISSN:1144-0546    

DOI:10.1039/a706745c

出版商:RSC    

年代:1998

数据来源: RSC