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DALTON PERSPECTIVE J. Chem. Soc., Dalton Trans., 1999, 1703–1709 1703 The heterogenization of homogeneous metallocene catalysts for olefin polymerization Helmut G. Alt Laboratorium für Anorganische Chemie, Universität Bayreuth, Universitätsstr. 30, D-95440 Bayreuth, Germany Received 11th November 1998, Accepted 10th February 1999 Metallocene complexes of titanium, zirconium and hafnium are very active and versatile catalysts for olefin polymerization and are already contributing to commercial production.Owing to a variety of catalyst parameters a wide range of polymers with various properties are accessible. Since metallocene complexes are homogeneous in solution they can easily be studied by spectroscopic methods. However, for industrial application they must be heterogenized. 1 Introduction Metallocene complexes have revolutionized the world of polyolefins 1–3 and they are going to contribute polymers with new properties and applications. They represent a new generation of olefin polymerization catalysts and they oVer many advantages compared with the Ziegler–Natta and Phillips catalysts.Owing to their homogeneous nature every molecule has an active site and thus metallocene catalysts can be many more times as active as established Ziegler–Natta catalysts. One of the record holders in terms of activity for ethylene polymerization is the bridged bis(fluorenyl) complex [Zr(C13H8C2H4- C13H8)Cl2] that can produce 300 tons of polyethylene (g Zr)21 h21.4 The variation of the aromatic ligands, the bridge and the metal provides an enormous amount of parameters to control the polymerization reactions in terms of stereospecificity (when prochiral olefins such as propylene are applied), long chain and short chain branching and the generation of block copolymers (oscillating catalysts).In other words, metallocene catalysts can produce tailored polyolefins for nearly every purpose and with Helmut G. Alt was born in 1944.He received his chemical education at the Technische Universität München. After his postdoctoral period with M. D. Rausch at the University of Massachusetts in 1973/1974 he returned to Munich. In 1978 he joined his “Doktorvater”, Professor M. Herberhold, at the newly founded University of Bayreuth and finished his Habilitationsarbeit in 1980. There he is an extraordinary professor in chemistry. He has published more than 200 papers and is the inventor/ coinventor of numerous patents.His research interests include transition metal complexes and their application in catalysis and synthesis. Helmut G. Alt these “new materials” they generate new markets, like LLDPE (linear low density polyethylene). The first application of a metallocene catalyst goes back to the ’50s when the Breslow5 and Natta 6 groups independently found that [TiCp2Cl2] (Cp = cyclopentadienyl) can be activated with mixed aluminium alkyl halides to polymerize ethylene in homogeneous solution but with poor activity.The next breakthrough came in the late ’70s when Sinn and Kaminsky 7–9 applied methylalumoxane [(MeAlO)n] (MAO) as a much better cocatalyst for activation than AlXR2. In the ’80s Brintzinger and co-workers 10 contributed the first ansa-bis(indenyl) complexes with fixed symmetry (rac form) to produce isotactic polypropylene. In 1988 Razavi and co-workers synthesized the mixed ansa-metallocene complex [ZrC5H4CMe2C13H8Cl2] that opened the door to syndiotactic polypropylene.11 My group succeeded with the synthesis of the first bridged bis(fluorenyl) complexes like [M(C13H8C2H4C13H8)Cl2] (M = Zr or Hf ) 4 that proved excellent catalyst precursors for the polymerization of ethylene.Today numerous reviews are available dealing with the application of various metallocene catalysts.12–19 In the past ten years the Alt research group at the University of Bayreuth have synthesized more than 600 diVerent metallocene and halfsandwich catalysts and tested them for olefin polymerization.From the very beginning it was our goal to cover all possible applications for olefin polymerization and not only concentrate on specialities like isotactic polypropylene, syndiotactic1704 J. Chem. Soc., Dalton Trans., 1999, 1703–1709 polypropylene or block copolymers deriving from the stereospecific polymerization of propylene. It turned out that metallocene complexes can also be used as attractive catalysts for the polymerization of ethylene generating short or long chain branching and thus providing materials with superior mechanical and optical properties.Some of these new resins are already in the market like the linear low density polyethylenes (LLDPE) “mPact” (Phillips Petroleum Company) or “Elite” (Dow). 2 Preparation and activation of metallocene catalyst precursors The most common catalyst precursors are metallocene dichloride complexes. This class of compounds is air-stable but very sensitive to moisture due to the high oxophilicity of zirconium or hafnium. Depending on the nature of the aromatic ligands several methods for the preparation of unbridged and bridged metallocene complexes are available (Schemes 1–4).A special route allows the preparation of an ansa-metallocene complex with an Si–N–Si backbone in the bridge.23 3 Activation of the catalyst precursors and mechanism of the polymerization In order to obtain active metallocene catalysts it is necessary to activate the catalyst precursor (Scheme 5).For this purpose methylalumoxane is the most commonly used reagent to generate a cationic metallocene monomethyl cation that is supposed Scheme 1 Preparation of unbridged metallocene dichloride complexes. Scheme 2 Preparation of ansa-metallocene dichloride complexes via the “fulvene method”.20,21 to be the actual catalyst. Other potential cocatalyst anions can be various borates, especially [B(C6F5)4]2.24 The chemical nature of MAO is still not quite clear.The partially hydrolysed trimethylaluminium seems to consist of linear –[MeAlO]n– Scheme 3 C,C Coupling reactions for the preparation of ligand precursors.22 Scheme 4 Preparation of a metallocene complex with a bridging Si–N–Si unit.23 Scheme 5 Activation of a metallocene dichloride complex.J. Chem. Soc., Dalton Trans., 1999, 1703–1709 1705 units, n ranging from 5 to 20. However, cyclic species can also exist that aggregate to cages.Such a cage could accommodate a monomeric AlMe3 molecule which accomplishes the necessary activation steps:25,26 methylation of the metallocene dichloride complex, and the subsequent carbanion abstraction to generate the catalytically active metallocene monomethyl cation. The resulting ion pair is the active catalyst. Indeed, it is possible to increase the activity of such catalysts by magnitudes when the cation and the anion can be separated as in the case of substituted bis(fluorenyl) complexes. This can be achieved by substituting the 4 and 5 positions of the fluorenyl ligands of the ansa-complex [Zr(C13H8C2H4C13H8)Cl2].27 After activation with MAO the activity for ethylene polymerization increases by a factor of five.The reaction mechanism of these “single-site” catalysts comprises three essential steps: co-ordination of the olefin, olefin insertion, i.e. alkyl migration, to generate the polymer chain and generation of a new co-ordination site by inversion. Each step can be influenced by changing parameters like the nature of the ligand, the metal, the cocatalyst and the solvent. 4 The heterogenization of metallocene catalysts Though metallocene complexes possess excellent activities and stereospecificities for the polymerization of prochiral olefins as well as narrow molecular weight distributions of the generated polymers they are not suitable for technical application. Since metallocene catalysts are of homogeneous nature they cannot be applied in the conventional gas phase or slurry reactors because they would cause “fouling”.This means the formed polyolefin is deposited at the reactor walls and causes all the problems that are known from the “boiler scale eVect”; a continuous process is not possible. Since the established Ziegler–Natta and Phillips catalysts are all heterogeneous it is necessary to support metallocene catalysts for industrial application. 4.1 Organic support materials One approach is the use of organic support materials like crosslinked polystyrene.28–30 It is also possible to fix a ligand precursor on a polymeric support and then build up the catalyst 31 (Scheme 6). This approach avoids polar components on the support surface that could decrease the catalyst activity. Scheme 6 Fixation of a catalyst on polystyrene. 4.2 Inorganic support materials Inorganic support materials are widely used for Ziegler–Natta and Phillips catalysts: silica, alumina, magnesium dichloride and mixtures thereof are representative examples.32,33 The fixation of the catalyst can either be performed by an absorption process at the surface of the solid particles or by a chemical bond, for instance with silica (Scheme 7).It is interesting that a stereospecific catalyst for the polymerization of syndiotactic polypropylene, such as [Zr(C5H4CMe2C13H8)Cl2]/MAO, can change its stereospecificity from syndiotactic to isotactic because the bulky support no longer allows the inversion step in the process (change of the active sites of the catalyst molecules).35 These inorganic support materials can also be used to control the morphology of the resulting polymer particles so that free flowing powders are obtained and reactor fouling is prevented.Another method is to immobilize the cocatalyst methylalumoxane. This can be accomplished by the reaction of MAO and wet silica and provides a universal heterogeneous cocatalyst.36 4.3 The self-immobilization of metallocene catalysts All these methods have the disadvantage that the catalysts can lose magnitudes of their activities 17 because their metal centres (Lewis acids) are not accessible at the surface or they can be blocked by oxygen functions (Lewis bases).Thus we tried a completely diVerent approach that would avoid all these problems. The idea was to synthesize metallocene catalysts with an olefin or alkyne function that can be used as a comonomer in the polymerization process.The following complexes are typical examples for this approach.37 Such a metallocene dichloride complex can be activated in solution with MAO to give a homogeneous catalyst (Scheme 8). Scheme 7 Fixation of a catalyst on silica.341706 J. Chem. Soc., Dalton Trans., 1999, 1703–1709 Scheme 8 Proposed mechanism for the “self-immobilization” of a homogeneous ansa-metallocene complex.37 As soon as an olefin like ethylene is applied the olefin is polymerized and simultaneously catalyst molecules are incorporated into the growing polymer chain due to their olefin function.As a consequence a precipitate is formed that consists of polyethylene and incorporated active catalyst. When the homogeneous catalyst solution is coloured the formed precipitate adopts the same colour and when the suspension is allowed to stand for a while the supernatant solvent becomes colourless (Fig. 1). In other words the homogeneous catalyst is transferred to a heterogeneous system without using any support.This catalyst system can provide its own support. As a very pleasant side eVect the excess of MAO in the washing liquid can be used again for activation processes and can be recycled. In this way the enormous excess of MAO can be reduced from 10000 to ca. 500. Schemes 9–11 describe the synthesis of some typical catalyst precursors.38–40 The structure of [Zr{C13H8C(Me)(C4H7)C5H3 tBu}Cl2] is shown in Fig. 2.37 The mechanism of this self-immobilization is still not quite clear. It is possible that several diVerent intramolecular and intermolecular mechanisms take place, e.g. as in Scheme 12. Another alternative is an intermolecular hydrozirconation reaction to form dinuclear species (Scheme 13). The necessary hydrido complexes could be provided by b-H-elimination termination steps during the polymerization process. The formation of a metallacyclic system could also be responsible for the catalytic process. We have an experimental indication for this step: in a 1H NMR experiment we have demonstrated that the olefin protons of an alkenyl substituent disappear as soon as only one equivalent of C2H4 is applied.Scheme 9 Preparation of fulvenes with w-alkenyl substituents as ligand precursors.J. Chem. Soc., Dalton Trans., 1999, 1703–1709 1707 Scheme 10 Synthesis of various ligand precursors via C,C coupling reactions. Scheme 11 Synthesis of various w-alkenyl substituted ansa-metallocene complexes. 4.4 Ethylene polymerization and eVect of substituents The length of the alkenyl substituent determines the activity of the catalyst and the molecular weight of the formed polymer. We studied this with a whole variety of catalysts in order to optimize the parameters (Figs. 3 and 4).38 It turned out that an alkenyl substituent with five carbon atoms gave the highest activity. The catalyst molecule is fixed in a polymer chain and it needs a certain amount of freedom to become available to the monomers.This behaviour can also be explained with the “dog on a leash” phenomenon. The diVerent molecular weights of the polymers can be explained in a similar way: the length of the alkenyl substituent must have some steric influence on the rate of the b-hydrogen elimination reaction that terminates the growth of the polymer chain. The length of the alkenyl substituent could influence this process due to various steric requirements.This method of self-immobilization oVers another very interesting aspect: since the active catalyst is a cationic species we need an anion for compensation. In the preparation process we can isolate the heterogeneous ion pair. This means from the aluminium content of the catalyst we can figure out how many aluminium atoms contribute to the hitherto unknown MAO anion. We found the number 80 by atomic absorption methods. Now we can speculate what such an MAO anion could look like.1708 J.Chem. Soc., Dalton Trans., 1999, 1703–1709 Fig. 1 (a) Metallocene dichloride complex in toluene solution. (b) Metallocene dichloride complex activated with MAO in toluene solution. (c) Generated heterogeneous catalyst in toluene suspension after prepolymerization with ethylene. Scheme 12 Intramolecular cyclization reactions.37–39 Scheme 13 Formation of a dinuclear metallocene complex by a “hydrozirconation” reaction. 5 Conclusion The self-immobilization of metallocene complexes provides an elegant tool to heterogenize homogeneous metallocene catalysts in order to apply them for industrial processes.In addition the position and the chain length of the w-alkenyl substituents that are needed for this process determine the activity of the catalysts and the molecular weight of the formed polyethylenes in a wide range. 6 Acknowledgements This work has been supported by Phillips Petroleum Company, USA and the Deutsche Forschungsgemeinschaft. I am also very grateful to my enthusiastic co-workers who contributed to this project.Fig. 2 Molecular structure of [Zr{C13H8C(Me)(C4H7)C5H3 tBu}Cl2]. Fig. 3 Influence of the w-alkenyl substituent chain length R on the catalyst activity and the molecular weight of the produced polyethylene (C3 = = 3-propenyl etc.). Conditions: 1 mg catalyst precursor, 7 ml MAO in toluene solution (30%), 500 ml pentane solvent in a 1 l reactor, 10 bar ethylene pressure, 60 8C, 1 h reaction time.J.Chem. Soc., Dalton Trans., 1999, 1703–1709 1709 7 References 1 K. B. Sinclair and R. B. Wilson, Chem. Ind., 1994, 857. 2 W.-W. du Mont, M. Weidenbruch, A. Grochman and M. Bochmann, Nachr. Chem. Techn. Lab., 1995, 43, 115. 3 R. Beckhaus, Nachr. Chem. Techn. Lab., 1998, 46, 611. 4 H. G. Alt and S. J. Palackal, J. Organomet. Chem., 1994, 472, 113. 5 D. S. Breslow and N. Newburg, J. Am. Chem. Soc., 1957, 79, 5072. 6 G. Natta, P.Pino, G. Mazzanti and R. Lanzo, Chim. Ind., 1957, 39, 1032. 7 A. Anderson, H.-G. Cordes, J. Herwig, W. Kaminsky, A. Merck, R. Mottweiler, J. Pein, H. Sinn and H.-J. Vollmer, Angew. Chem., 1976, 88, 689. 8 H. Sinn and W. Kaminsky, Adv. Organomet. Chem., 1980, 18, 99. 9 H. Sinn, W. Kaminsky, H.-J. Vollmer and R. Woldt, BASF AG, U.S. Pat., 4 404 344, 1983. 10 F. R. W. P. Wild, L. Zsolnai, G. Huttner and H.-H. Brintzinger, J. Organomet. Chem., 1982, 232, 233. 11 J. A. Ewen, R. L. Jones, A.Razavi and J. D. Ferrara, J. Am. Chem. Soc., 1988, 110, 6255. Fig. 4 Influence of the position and the chain length of the w-alkenyl substituent on the activity of a catalyst and the molecular weight of the produced polyethylene. Conditions as in Fig. 3. 12 H.-H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger and R. M. Waymouth, Angew. Chem., Int. Ed. Engl., 1995, 34, 1143. 13 M. Aulbach and F. Küber, Chem. Unserer Zeit, 1994, 4, 197. 14 P. C. Möhring and N. J.Coville, J. Organomet. Chem., 1994, 479, 1. 15 W. Kaminsky, J. Chem. Soc., Dalton Trans., 1998, 1413. 16 W. Kaminsky and M. Arndt, Applied Homogeneous Catalysis with Organometallic Compounds, eds. B. Cornils and W. A. Herrmann, VCH, Weinheim, New York, Basel, Cambridge, Tokyo, 1996, vol. 1, p. 220. 17 W. Kaminsky and M. Arndt, Adv. Polym. Sci., 1997, 127, 143. 18 H. G. Alt and E. Samuel, Chem. Soc. Rev., 1998, 27, 323 and refs. therein. 19 C. Janiak, Metallocenes, Vol. II, eds.A. Togni and R. L. Haltermann, Wiley-VCH, Weinheim, New York, Basel, Cambridge, Tokyo, 1998, p. 547. 20 A. Razavi and J. D. Ferrara, J. Organomet. Chem., 1992, 435, 299. 21 H. G. Alt and R. Zenk, J. Organomet. Chem., 1996, 518, 7. 22 S. J. Palackal, Dissertation, Universität Bayreuth, 1991. 23 H. G. Alt, K. Föttinger and W. Milius, J. Organomet. Chem., 1998, 564, 109. 24 X. Yang, C. L. Stern and T. J. Marks, J. Am. Chem. Soc., 1991, 113, 3623. 25 H. Sinn, Makromol. Chem., Makromol. Symp., 1995, 97, 27. 26 M. R. Mason, J. M. Smith, S. G. Bott and A. R. Barron, J. Am. Chem. Soc., 1993, 115, 4971. 27 P. Schertl and H. G. Alt, J. Organomet. Chem., in press. 28 R. H. Grubbs, C. Gibbons, L. C. Kroll, W. D. Bonds, Jr. and C. H. Brubaker, Jr., J. Am. Chem. Soc., 1973, 95, 2373. 29 L. Sun, C. C. Hsu and D. W. Bacon, J. Polym. Sci. Part A, 1994, 32, 2127. 30 S. B. Roscoe, J. M. Fréchet, J. F. Walzer and A. J. Dias, Science, 1998, 280, 270. 31 B. Peifer and H. G. Alt, unpublished results. 32 R. Jackson, J. Ruddlesden, D. J. Thompson and R. Whelan, J. Organomet. Chem., 1977, 125, 57. 33 W. Kaminsky, Macromol. Chem. Phys., 1996, 197, 3907 and refs. therein. 34 H. G. Alt and K. Patsidis, Ph.D. thesis, Universität Bayreuth, 1993. 35 W. Kaminsky and F. Renner, Makromol. Chem. Rapid Commun., 1993, 14, 239. 36 M. Chang, Exxon Chemical Co., U.S. Pat., 4 912 075, 1990; 5 529 965, 1996. 37 B. Peifer, W. Milius and H. G. Alt, J. Organomet. Chem., 1998, 553, 205. 38 H. G. Alt, M. Jung and G. Kehr, J. Organomet. Chem., 1998, 562, 153. 39 H. G. Alt and M. Jung, J. Organomet. Chem., 1998, 562, 229. 40 H. G. Alt and M. Jung, J. Organomet. Chem., 1998, 568, 87. Paper 8/08812H

ISSN:1477-9226    

DOI:10.1039/a808812h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON PERSPECTIVE J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1707 Molecular architecture of cyclic nanostructures: use of co-ordination chemistry in the building of supermolecules with predefined geometric shapes Bogdan Olenyuk, Andreas Fechtenkötter and Peter J. Stang * Department of Chemistry, The University of Utah, Salt Lake City, Utah 84112, USA Rapid growth and recent breakthroughs in the field of molecular manufacturing have resulted in the development of an entirely new synthetic strategy for the preparation of organized nanostructures.This strategy is based on molecular self-assembly, a phenomenon in which the individual subunits are quickly driven together and held in place by multiple, accurately positioned non-covalent interactions. The use of transition metals and coordination- based design allows the formation of a variety of selforganized nanosystems in a few highly convergent synthetic steps. Molecular architecture utilizes the large diversity of available transition metals and their co-ordination chemistry to create complex geometric shapes.This article explores some of its most interesting aspects, beginning with the construction of simple selfassembled structures that have the shapes of various convex polygons, such as squares, rectangles and triangles to more complex assemblages with shapes of polyhedra and three-dimensional nets. An attempt is also made to provide insight on how this strategy can be used to create advanced materials with properties and functions determined by their structure. 1 Introduction The challenge facing the future of modern nanotechnology is molecular manufacturing, a process that is designed to synthesize advanced materials with specific properties and functions. These are determined by controlling the form, shape and distribution of each individual building block and their precise placement. Such intermolecular control imposes strict requirements on the nature, type and directionality of the bonding forces that operate within the entire aggregated structure.The chemical bonding of the subunits must be relatively weak, thermodynamically stable, yet kinetically labile to allow the self-rearrangement of the subunits within the entire structure, thereby enabling the self-correction of possible defects. Another important requirement is the conformational rigidity of the building blocks in order to reduce entropic factors upon self-organization.This is only an illustrative, not an exhaustive list of important considerations, which has triggered enormous interest and growth in modern supramolecular Peter J. Stang was born in Nürnberg, Germany, raised in Hungary until 1956, and educated in the USA. He earned a B.S. in chemistry from DePaul University in Chicago in 1963 and a Ph.D. from U.C.-Berkeley in 1966. After NIH postdoctoral work at Princeton he joined the faculty at the University of Utah in 1969 where since 1992 he has held the rank of Distinguished Professor of Chemistry, and served as Department Chair from 1989 to 1995.From 1982 until the present he has been an Associate Editor of the Journal of the American Chemical Society. He has received numerous awards and honors, including the 1998 ACS James Flack Norris Award in Physical-Organic Chemistry. His current research interests are in molecular architecture via self-assembly using co-ordination as the motif.Bogdan Olenyuk was born in Western Ukraine in 1970, studied in Kiev Shevchenko University and received his honors Diploma in Chemistry in 1992. He is currently finishing his doctoral studies at the University of Utah Department of Chemistry. His research interests include the design of novel highly symmetrical and stereochemically controlled supramolecular entities and study of the dynamics of the self-assembly process. His interests also include molecular modeling, computational chemistry and computer graphics. Peter J.Stang Bogdan Olenyuk Andreas Fechtenkötter Andreas Fechtenkötter was born in Osnabrück, Germany in 1971. He received his Pre-Diploma at the Universität Marburg in 1994. He is presently pursuing his Diploma in Chemistry at the Technische Universität Braunschweig. As part of an exchange program with the University of Utah (USA), he carried out research in Professor P. J. Stang’s group in 1995/96. He returned to work with Professor Stang in June 1997 to conduct his Diploma research, which is supervised by Professor R.Schmutzler in Braunschweig. After receiving his Diploma, he intends to start his Ph.D. studies in Germany.1708 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 chemistry. A recent novel synthetic protocol in the construction of organized nanoscopic assemblies from multiple building blocks in a single step, namely self-assembly, relies on critical information about the shape and properties of the resulting structure being preprogrammed into each individual building block. Although this approach was initiated by the artificial mimicking of natural receptors that utilize weak hydrogen bonds, it has now resulted in an entirely diVerent ‘unnatural’ strategy, molecular architecture, that employs transition metals and dative bonding to achieve structurally well defined, highly ordered assemblages.This approach relies on the fact that fewer metal–ligand bonds may be used in place of several hydrogen bonds owing to their greater strength. Another advantage lies in the existence of a large variety of transition metals with diVerent co-ordination numbers, thus facilitating the building of diverse nanoscopic entities with tremendous variations in shapes and sizes.In this article we present an overview of this molecular architecture paradigm and demonstrate the application of transition metals and co-ordination chemistry in the rational design of artificial nanoscopic objects with desired forms and shapes.What is self-assembly and why is it gaining importance in the construction of complex macromolecules and organized nanosystems? Throughout the evolution of chemistry as a discipline, the preparation of various chemical compounds via the stepwise formation of covalent bonds between appropriate precursors has been the most widely used method. Although this method is useful for the synthesis of relatively small organic molecules, it becomes burdensome when applied to the synthesis of large macromolecules or molecular assemblies since it possesses several inherent limitations.Among its serious drawbacks are the inordinate amount of time required for the linear step by step synthesis of complex macromolecules composed of hundreds or even thousands of subunits and the drastic reduction of the overall yield in such a multistep synthetic process. Since most covalent bonds are kinetically inert, even a single assembly error can jeopardize the integrity and functionality of the entire structure.Self-assembly oVers some important advantages over stepwise bond formation. Since it proceeds via the simultaneous assembly of predetermined building blocks, the resulting synthesis is highly convergent and thus requires fewer steps than the corresponding covalent synthesis. Also, since non-covalent interactions are usually established very rapidly, final product formation is fast and facile. The presence of kinetically labile non-covalent interactions between the constituents results in relatively defect-free assemblies with self-maintained integrity since the usual equilibria between the constituents and the final products contribute to the self-rearrangement of components and correction of defects. Nature has been exploiting these advantages of self-assembly for a long time.Various cell components, such as ribosomes, mitochondria, chromosomes and others, are almost exclusively made via self-assembly and non-covalent interactions, such as hydrogen bonds.During their formation the individual components are quickly driven together and held in place by thousands of accurately positioned non-covalent interactions, creating incredibly complex and exquisite patterns of life. In light of this tremendous complexity, it is diYcult to imagine how these components could have been created if only covalent forces were in Nature’s arsenal.In marked contrast to biological supramolecules, the design of artificial self-assembling systems is still in its early stages. It requires consideration of many factors, such as type and strength of dative bonds between various components, the symmetry of both the constituents and the entire self-assembled structure, the precise positioning of the coordination sites of the components, temperature and solvent polarity, and possibly many others. The first step in solving this complex problem is the development of relatively simple, selfassembling structures of the desired shape and symmetry that mimic at least a single function of an appropriate biomolecule.Such functions may be catalytic, receptive, transportive or others. It is also important for such structures to exhibit some active property that diVers from the properties of its constituent components. These tasks are being addressed by supramolecular chemistry, a relatively new field that is concerned with the design and structure of large and complex nanoscalesized macromolecules.1 Molecular design is based on the basic principles of ‘molecular informatics’ in which the structure and function of the final product can be preprogrammed within its individual building blocks, giving the chemist enormous control over the intermolecular bond.Modern supramolecular chemistry 1 emerged from studies of covalent systems, such as cyclophanes, crown ethers, calixarenes and cryptands, although it is now dominated by studies of non-covalent assemblies.In less than a decade the novel motif of transition metals and coordination bonds in assemblies has emerged as a strong and viable alternative to hydrogen-bonded aggregates, patterned after natural macromolecules. 2 Principles and Design Strategies The structural and functional features of self-assembled supramolecular entities result from the information stored in their components and the components’ intrinsic properties that are dictated by the presence of functional groups.A simple and general concept for generating ordered structures is based on the recognition-driven spontaneous assembly of complementary subunits. Since transition metals have coordination sites with specific geometries that depend upon their electronic structure they can serve as acceptor subunits. These can be linked together via donor building blocks that form the rigid frame of the assembled entity.Both of these types of subunits must possess specific geometries and remain multidentate or at least bidentate, i.e. they must have at least two co-ordination sites that cannot be capped, to form the desired cyclic structure. For the synthesis to be eYcient and convergent, it is important for these building blocks to be readily available. From the above considerations it is obvious that these units can be nitrogen-containing heteroaryls, cyanosubstituted aromatic ligands, as well as bis(o-catecholates) and some thiocatechols. The construction of almost any entity that contains a transition metal requires the assessment of the angles between the binding sites of each donor and acceptor subunit.Hence, the subunits can be classified into two types based on the value of this angle: linear subunits that possess these reactive sites with a 1808 orientation relative to each other and angular subunits that have other, smaller angles.2 When these types of building blocks are combined the structure of the resulting species will solely depend on the symmetry and the number of binding sites within each subunit.The symmetry of the resulting assembly will be the combination and spherical distribution of the main symmetry axis of each building block. Thus, monocyclic entities can be built by combining subunits with symmetry axes not higher than twofold, while the construction of polycyclic frameworks requires at least one subunit to possess a symmetry axis higher than twofold.2 Thus, the shapes of monocyclic entities will resemble convex polygons and those of polycyclic frameworks will resemble canonical polyhedra. Hence, the assembly of a planar triangle requires the combination of three linear building blocks and three angular ones with a 608 turning angle.A molecular square can be assembled in several diVerent ways, either by combining four linear with four angular building blocks, or by combining two diVerent angular subunits.Molecular pentagons can be built by combining five linear components with five angular ones that possess a 1088 angle between their binding sites. Likewise, molecular hexagons can be constructed via the combination of six linear components and six angular subunits with a 1208 directingJ. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1709 angle or via the combination of two diVerent types of angular subunits with a 1208 angle between the binding sites (Fig. 1). The design of three-dimensional polyhedra is more complex since it requires the interaction of many more subunits and at least one type of building block to be multidentate with more than two binding sites (Fig. 2).2 Thus, the preparation of a triangular prism can involve the combination of two tridentate subunits with three linear spacers. If the spacers do not have linear geometry then the formation of a triple helix is expected.Likewise, twelve linear subunits combined with eight tridentate subunits with 908 angles between each of the co-ordinating sites will yield a cube whereas six angular bidentate units in combination with four angular tridentate subunits will result in the shape of an octahedron and so on. It is important to note that this approach only accounts for the angles between the binding sites within each free subunit and extrapolates them into the final self-assembled entity. Therefore, it can be assumed that the value of the directing angle within each such subunit does not change significantly upon its incorporation into the self-assembled structure.This assumption is based on the initial requirement of conformational rigidity of the subunits. In reality, however, distortions of the binding angle up to several degrees may occur, but in most cases they can be neglected as the weak dative bonding to the transition metals is likely to prevent the formation of highly distorted structures. 3 Cyclic Binuclear Molecular Systems: Polygons, Rectangles and Strands One of the first cyclic self-assembled host molecules was reported by Maverick et al.3 This work was based on the formation of a cofacial binuclear structure generated from the Cu(NH3)4 21 complex and a bis(b-diketone) ligand, which on mixing in an aqueous solution generated assembly 1. The ability of 1 to function as a host was tested by measuring its binding constants with pyridine, pyrazine, quinuclidine and DABCO.For example, DABCO could be selectively bound inside the macrocyclic host over the other potential guests with a binding constant of 220 m21. The internal co-ordination of DABCO was established by X-ray studies of the inclusion complex. These investigations were among the early observations of the intermolecular co-ordination of bifunctional Lewis bases to binuclear transition metal-based hosts.3 Another example of binuclear assemblies with potential receptor abilities is the work of Hartshorn and Steel.4 They employed polyheteroaryl-substituted arenes in combination Fig. 1 A combinatorial library of cyclic molecular polygons may be created via the systematic combination of building blocks with predetermined angles with silver(I) salts to prepare binuclear frameworks. When 2 equivalents of 1,4-bis(2-pyridoxy)benzene were allowed to react in acetone with 2 equivalents of silver nitrate assembly 2 was formed, as determined by X-ray crystallographic studies.In this assembly each silver atom is co-ordinated to two pyridine nitrogens and less strongly to a water oxygen. This distorts its geometry to T-shaped with the N]Ag]N9 angle being close to 1578. Another interesting feature of this assembly is the close p–p stacking of the two benzene rings that are coplanar and separated only by 3.33 Å.4 Hannon et al.5 employed an interesting strategy. They incorporated both chelating ligands into the reactive donor unit, thereby eliminating the need for the additional preparation of the transition metal complex and allowing the use of transition metals with multiple, open co-ordination sites.The ligand 4-methylsulfanyl-6-(3-pyridyl)-2,29-bipyridine acts as a bidentate chelate for one transition metal and as a monodentate binding site for the other. The preparation of assembly 3 was achieved by mixing 1 equivalent of copper(I) salt with 1 equivalent of ligand in acetonitrile.Mass spectrometry and singlecrystal X-ray studies allowed the elucidation of its structure. Interestingly, a topologically similar assembly could also be obtained by using an octahedral Group II metal, such as cadmium. An elegant example of a binuclear structure was prepared by utilizing the copper(I) or zinc(II) 2,29-bipyridyl co-ordination.6 Since this type of co-ordination chemistry is well known, it allowed the authors to use two 2,29-bipyridyls connected to each side of the aromatic spacers, such as 2,2-naphthalene and 2,7-pyrene.Treatment of these ligands with either copper(I) or zinc(II) salts produced the binuclear assemblies 4. Fast atom bombardment and ES mass spectrometry were used to establish the stoichiometry of these complexes and variable temperature NMR spectra to study their dynamics. Binuclear triple helicates can be formed by combining the transition metals that prefer octahedral co-ordination, such as iron or Group IVa metals, with three subunits that contain one bidentate binding site at each end.Caulder and Raymond7 employed this motif to produce highly ordered helical assemblies in solution that are formed from a mixture of predesigned building blocks. The three ligands, 5–7 (Scheme 1), contain a systematically increased distance between the binding sites. The reaction of 3 equivalents of any of these ligands with 2 equivalents of trivalent M(acac)3 (M = Fe, Al, Ga) 8 in methanol and KOH results in the formation of triple-helical assemblies 9–11 in high yields.7 The remarkable degree of selfrecognition in this self-assembly process is illustrated by the Fig. 2 A part of the combinatorial library of canonical molecular polyhedra1710 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 O O Cu O O O O Cu O O 1 O O O O Ag Ag 2 2+ H2O OH2 N SMe N N [Cu(MeCN)4]PF6 N N N N N N Cu NCMe Cu MeCN SMe MeS 2+ 3 2 NO3 – 2 PF6 – 2+ O N O N Cu 4 O N Cu N O N N 2 PF6 – N N Scheme 1 NH NH O O OH OH OH OH O NH NH O OH OH OH OH O NH OH OH NH O OH OH 5 6 7 M(acac)3 (M = Fe, Ga, Al) 8 9 10 11 following two examples.When these complexes were prepared in the presence of an excess of ligand NMR spectra only indicated the formation of the final products and the excess of the free ligand remained intact.7 When mixtures of all three or any two ligands were allowed to react with [Ga(acac)3] 8 under these conditions only complexes containing one type of ligand were formed with no mixed ligands or oligomers formed in solution.Both NMR spectroscopy and electrospray mass spectrometry indicated a mixture of three final helical products in equal amounts with no traces of oligomeric or mixed species. The main reasons for this self-recognition are the high degree of conformational rigidity of each donor subunit combined with diVerent steric requirements due to the diVerent distances between the binding sites in the assembled product.By introducing additional tetrahedral carbons between the pyridine rings, Fujita et al.8 assembled several water-soluble binuclear macrocycles. When ligand 12 was mixed with an aqueous solution of [Pd(en)(NO3)2] 13 the formation of assembly 14 was observed (Scheme 2). Since it contains the electron-deficient subunit with the perfluorinated phenylene, it was capable of recognizing electron-rich compounds such as naphthalene, in an aqueuous medium.8 Using a slightly modified subunit, 1,4-bis(4-pyridylmethyl)- benzene 15 (Scheme 3), it was possible to prepare the corresponding palladium-based bimetallic species 16.Its interesting property is the ability to form the catenated dimer 17.9 At ambient temperature, the palladium catenane 17 is in equilibrium with its monomeric form 16, as confirmed by spectroscopic studies. At lower concentrations this equilibrium favors the single-ring assembly but at higher concentrations the catenane is the dominant species.The reaction of a platinum precursor 18 with 15 produced an analogous assembly 19 (Scheme 3). Due to the greater Pt]N bond strength, it is exclusively formed as the monocyclic structure at room temperature. However, when the reaction mixture was heated to 100 8C in the presence of NaNO3, the formation of the catenated dimer 20 was observedJ. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1711 due to the increased lability of the platinum–pyridine bond.10 The irreversible formation of this catenane can be achieved by cooling the solution to room temperature.The self-assembly of molecular rectangles may be accomplished by using modified organic angular subunits.11 Two types Scheme 2 N F F N F F 12 + NH 2 Pd H2 N N N 13 F F F F F F F F NH2 Pd H2 N N N NH 2 Pd H2 N ONO2 ONO2 14 4+ 4 NO3 – Scheme 3 NH 2 M H2 N N N NH 2 M H2 N N N NH 2 M H2 N ONO2 ONO2 4+ 4 NO3 – + N N 15 13 M = Pd 18 M = Pt 16 M = Pd 19 M = Pt (M = Pd) 8+ H2N M NH2 N H2N M NH2 N N N H2N M NH2 H2N M NH2 N N N 8 NO3 – 17 M = Pd 20 M = Pt of rectangles were constructed: one which contains two identical angular subunits in combination with [Pd(en)(NO3)2] 13 and the other composed of this transition metal complex combined with both linear and angular units.When the unsymmetrical angular block 21 was mixed with 13 it quantitatively formed the catenated rectangular assembly 22 (Scheme 4). This molecule is remarkably stable as the dissociation of 22 into its components was not observed even at low concentrations.11 Reaction of the same palladium complex with two disparate heteroaryls, such as substituted biphenyl 23 and 4,49-bipyridyl 24 formed catenated assembly 25 (Scheme 4). In the solid state catenane 22 consists of two crystallographically independent structures whereas catenane 25 is a stable single isomer.11 The preparation of a double helix can be carried out by utilizing the co-ordination chemistry of four-co-ordinate transition metals, as illustrated in the following two examples. Sauvage and co-workers 12 used functionalized bis(1,10-phenanthroline) 26 which reacted with appropriate copper(I) salts (Scheme 5).The assembly 27, after being subjected to further chemical transformations, resulted in the formation of a unique trefoil knot.13 Williams et al.13 prepared a triple-helical binuclear assembly by using octahedral cobalt(II) salts and an organic heterocyclic subunit bis[2-(29-pyridyl)benzimidazoyl]methane 28.The product helicate 29 (Scheme 5) was isolated as a per- Scheme 4 NH 2 Pd H2 N ONO2 ONO2 13 + N N 21 N N N Pd NH2 H2N NH2 Pd N N H2N NH2 Pd H2N N N NH2 Pd H2N 22 NH 2 Pd H2 N ONO2 ONO2 13 + N 23 N N N 24 H2O H2O N N N N NH2 Pd H2N N NH2 Pd H2N H2N NH2 Pd N N H2N NH2 Pd + 25 8 NO3 – 8+ 8 NO3 – 8+1712 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 chlorate salt and its crystal structure indicates an 8 Å separation between the transition metals which are interconnected by three helical strands.The current literature contains hundreds of examples of unique helical assemblies that are prepared by using the co-ordination motif and which, due to the restrictive nature of this article, we were forced to omit. Interested readers may wish to pursue some recent reviews14 which provide further insights into the chemistry of this interesting class of supramolecular assemblies. 4 Molecular Polygons: Triangles, Squares, Pentagons and Hexagons Although the de novo construction of a molecular triangle seems not to be a very synthetically complicated task, a surpris- Scheme 5 N N N N OR Cu N N N N OR RO 26 N N N N OR RO Cu RO 27 Cu+ N N N N O Cu N N N N O O Cu O Trefoil Knot N N N N N N Me Me Me Me 28 29 Co(ClO4)2 3 2+ 2+ 4+ 4 ClO4 – ingly small number of attempts have been reported in the literature.From the general design standpoint, any cyclic combination of three linear building blocks with three angular ones that possess a 608 directing angle should result in a molecular triangle. One possible reason for the relative rarity of such assemblies may be the value of its turning angle, which is quite uncommon and relatively diYcult to attain, both in transition metals and organic linking subunits.Fujita et al.15 observed the formation of two types of equilibrium products when [Pd(en)- (NO3)2] 13 was mixed with selected linear linking components 30–32 resulting in molecular squares 33 and additional highly symmetrical assemblies that were assigned as molecular triangles (Scheme 6).A triangular entity 34 dominated the equilibrium products when the linear linking components were extended or were more constitutionally flexible. The assignment of one equilibrium product to the triangle is strongly supported by the eVect of concentration on the equilibrium ratio. At higher concentrations this ratio shifts towards the formation of a molecular triangle.This equilibrium results from a thermodynamic balance, where the less strained molecular square is more stable in terms of enthalpy while entropy favors selfassembled entities with fewer number of components, i.e. molecular triangles. The presence of steric eVects can also play a vital role in the equilibrium shift towards the molecular square or the molecular Scheme 6 NH Pd HN ONO2 ONO2 +N X N 30 X = 31 X = 32 X = C6H4 – C C C C 13 H2O N X N Pd NH2 H2N Pd H2N NH2 N N X X N N N X N Pd NH2 H2N Pd H2N NH2 8 –OSO2CF3 33 8+ N Pd NH2 H2N X N Pd H2N NH2 N N X N Pd NH2 H2N N X 34 6+ 6 –OSO2CF3J. Chem.Soc., Dalton Trans., 1998, Pages 1707–1728 1713 triangle. It was found that, if for instance, the ethylenediamine groups on Pd were replaced with the more sterically demanding 2,29-bipyridine, the resulting squares 35 were also in equilibrium with the molecular triangles 36 (Scheme 7), probably due to steric repulsion between two subunits.15 Scheme 7 N N Pd N N Pd N N N N N N N N Pd N N Pd N N N Pd N N N Pd N N N N N Pd N N N 8+ 8 –OSO2CF3 35 6+ 6 –OSO2CF3 36 Scheme 8 r.t.= Room temperature N N M PPh2 Ph2P M Ph2P PPh2 N N N N N N M PPh2 Ph2P M Ph2P PPh2 PPh2 M PPh2 OTf OTf N N 24 37 M = Pd 38 M = Pt + CH2Cl2 r.t. 39 M = Pd, 96% 40 M = Pt, 87% 8 –OSO2CF3 8+ Self-assembly of molecular squares can be achieved via the interaction of four bidentate angular units with four linear components where the angular and linear subunits have a cyclic arrangement.Transition metals with specific geometries and co-ordination numbers are best suited to become angular subunits for these assemblies. Divalent four-co-ordinate complexes of Pd and Pt are square planar species with adjacent bond angles of about 908. Co-ordination with a bis(phosphane) results in a cyclic chelated complex with a constrained cis geometry. If the remaining two adjacent co-ordination sites are occupied by weakly co-ordinated ligands, such as the triflate ion, then on interaction with a linear nitrogen-containing bis(heteroaryl) (i.e.with the lone pairs of the nitrogen atoms oriented 1808 relative to each other) a square assembly will be formed. Indeed, when bis(phosphane) complexes 37 and 38 reacted with an equivalent amount of 4,49-bipyridine 24 in dichloromethane at room temperature the resulting species were identified as molecular squares 39 and 40 (Scheme 8) based on multinuclear NMR, physical properties and later by single-crystal X-ray diVraction studies.16,17 A space-filling model based on the X-ray structure of 40 is shown in Fig. 3. A number of interesting geometric features are worthy of mention. The geometry about the PtII is square planar with only a slight deviation of the angles from 908. The shape of the molecule is undoubtedly that of a square despite the deviation from planarity of about 4 Å. This assembly has a large, moleculesized cavity. There is a high degree of p stacking between the phenyls of the chelated bis(phosphane) and one of the 4,49-bipyridine rings.16 This kind of interaction may be responsible for the high stability of squares 39 and 40.Experiments have shown that it is impossible to prepare and isolate only a ‘corner’ or a ‘side’ of these assemblies. Even with a 100-fold excess of 4,49-bipyridine in solution only complete squares were formed, with the rest of the free bipyridine left intact. It was also not possible to obtain an isolable intermediate when the reaction was carried out with a large excess of transition metal bis(triflate). In the last few years various tetranuclear molecular squares have been prepared and reported in the literature by us 3 as well as by Fujita et al.,18 who assembled a water-soluble square 41 from the complex 13 in combination with 4,49-bipyridyl 24 (Scheme 9).The assemblies 42–46 contain various linking subunits, such as 4,49-bipyridyl, 1,4-dicyanobenzene, 4,49-dicyano- 1,19-biphenyl, diazapyrene and diazaperylene combined with both chelated and non-chelated bis(phosphines).Molecular squares 47 are built by using ferrocene-containing units.19 The versatility of this self-assembly strategy also allowed the preparation of a series of multicomponent squares 48–51 that Fig. 3 Space-filling model of molecular square 40, based on its X-ray coordinates1714 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 N N M M N Et3P Et3P Et3P N Et3P N N M M PEt3 N PEt3 N PEt3 PEt3 8+ 8 –OSO2CF3 42 Et3P Et3P N PEt3 N PEt3 43 M N N M Et3P N Et3P N PEt3 PEt3 M N N M 8 –OSO2CF3 N N N N N N N N M M M M Et3P Et3P PEt3 PEt3 PEt3 PEt3 Et3P Et3P 8 –OSO2CF3 44 8+ 8+ C C N N C C N N Pd C C N N C C N N PPh2 P Ph2 Pd Ph2P P Ph2 PPh2 Ph2P Pd Ph2P PPh2 Pd 8+ 8 –OSO2CF3 45 C C N N C C N N Pd C C N N C C N N PPh2 P Ph2 Pd Ph2P P Ph2 PPh2 Ph2P Pd Ph2P Ph2 P Pd 46 8+ 8 –OSO2CF3 N N N N N N N M M M M PPh2 P Ph2 Ph2P PPh2 Ph2 P Ph2P PPh2 Ph2 P Fe Fe Fe Fe 8+ 8 –OSO2CF3 47 M = Pd, Pt contain crown ethers or calixarenes as angular units 20 or squares 52 which contain porphyrins as linear linking components.21 The self-assembly of water-soluble squares of similar topology was completed by Fujita et al.18 Their approach utilized the co-ordination of 4,49-bipyridyl 24 to the ethylenediamine complexes of palladium(II) and platinum(II) dinitrates.Assemblies of this type are soluble in water and serve as hosts for various aromatic guests, such as naphthalene, 1,4-dimethoxybenzene and others.Hupp and co-workers 22 recently utilized the co-ordination chemistry of octahedral rhenium complexes to prepare an interesting family of luminescent molecular squares. By heating a mixture of [Re(CO)5Cl] and selected ligands 24 and 53, 54 (Scheme 10) in toluene for 2 d, the resulting assemblies 55–57 were isolated in excellent yields. Usual characterization and X-ray crystallographic studies have established the structure of these interesting macrocycles.Molecular squares 55 and 57 were found to be luminescent and have been further subjected to time-resolved luminescence measurements. All the molecular squares described above were isolated inJ. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1715 PPh2 Ph2P N Ph2P N PPh2 Pt N N Pt Ph2P N PPh2 N Ph2 P Ph2P Pt N N Pt O O O O O O O O O O O O N Pt N I N I N Pt PPh2 P Ph2 O O O Ph2 P Ph2P O O O But But But But O But But But But PPh2 Ph2P N Ph2P N PPh2 Pt N N Pt Ph2P N PPh2 N Ph2 P Ph2P Pt N N Pt O O O O O O O O O O O O N Pt Pt Pt N N Pt PPh2 P Ph2 O O O O Ph2 P Ph2P O O O But But But But O But But But But O O O O But But But But P Ph2 PPh2 O O O O But But But But P Ph2 Ph2P N N N N N 48 8+ 8+ 8 –OTf 8 –OTf 8+ 8 –OTf 6+ 6 –OTf 49 50 51 O N N N N N N X R R R R N M N N N N N X R R R R N N N N N N X R R R R N N N N N N X R R R R PPh2 Ph2 P M Ph2P PPh2 PPh2 Ph2P M P Ph2 PPh2 M 8+ 8 –OSO2CF3 52 M = Pd, Pt; X = 2H, Zn R = n-C6H13 high yields as robust, air-stable, microcrystalline solids, which decompose at their melting points.Most of them are prone to crystallize with either solvent of crystallization or water that cannot be removed even by prolonged heating under vacuum. Interesting examples of photoactive, porphyrin-based molecular squares were reported by Drain and Lehn.23 They prepared tetranuclear assemblies by two diVerent methods. One of them involved the cis-[Pt(PhCN)2Cl2] complex as an angular building block which, when combined with the linear porphyrin 58 resulted in molecular square 59 (Scheme 11).Another1716 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 Scheme 9 N N H2 NPd NH2 ONO2 ONO2 + H2O N N H2 NPd NH2 NH2 Pd H2N N N N N N N H2 NPd NH2 NH2 Pd H2N 41 8 NO3 – 8+ 13 24 Scheme 10 N N 24 L = N N N N 53 54 [Re(CO)5Cl] + L THF/Toluene, Heat OC Re CO CO Cl CO Re L OC CO Cl OC Re L CO CO Cl CO Re L L OC CO Cl 55 L = 4,4¢-bipyridyl 56 L = 1,2-di(4-pyridyl)ethene 57 L = pyrazine Scheme 11 N N N N X N N+ PhCN Pt Cl Cl PhCN 58 X = 2H, Zn N N N N X N N Pt Pt Cl Cl Cl Cl N N N N X N N Pt Pt Cl Cl Cl Cl N N N N X N N N N N N X N N Toluene, reflux 59 X = 2H, ZnJ.Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1717 Scheme 12 N N N N X N N + PhCN Pd NCPh Cl Cl CH2Cl2 60 X = 2H, Zn N N N N X N N N N N N X N N N N N N X N N N N N N X N N Pd Cl Cl Pd Cl Cl Pd Pd Cl Cl Cl Cl 61 X = 2H, Zn approach utilized 5,10-bis(49-pyridyl)porphyrins 60 as the angular building block and trans-[Pd(PhCN)2Cl2] as the linear linking subunit in the preparation of square 61 (Scheme 12).Both the original squares and their zinc porphyrin complexes were studied by UV/VIS, NMR spectroscopy and mass spectrometry. 23 The luminescence studies, including the fluorescence polarization spectra of these assemblies, confirmed that the electronic interactions in these molecules are greater than those in just the porphyrin subunits or simple porphyrin dimers.The photochemical properties of these assemblies may prove to be a valuable addition in the arsenal of modern photochemistry and provide insights into the structural and physico-chemical properties of some natural photoreceptors. An interesting type of molecular square can be prepared by utilizing a diVerent strategy: two types of bidentate angular components can be combined in a co-operative manner, resulting in a cyclic polygon.An angular component can be prepared by taking advantage of the T-shaped, pseudo-trigonal bipyramidal geometry of the iodonium moiety with its near 908 angles.24 The preparation of such iodonium-containing corner units can be easily accomplished in several simple synthetic steps.25 Since the hypervalent iodine plays the role of one of the 908 turns, only two transition metal centers are present in the resulting square, while two alternating corners are occupied by the hypervalent iodine moiety.26,27 In the building blocks 62, 63 (Scheme 13) the lone pairs of the nitrogen atoms are located perpendicular to each other.The interaction of equimolar amounts of these bis(heteroaryl)iodonium triflates with the reactive bis(triflate) complexes of transition metal (PdII or PtII) bis(phosphanes) 37, 38 and 64, 65 results in the ready formation of the hybrid molecular squares 66–73.26,27 These assemblies are air-stable microcrystalline solids with high decomposition points.Due to their relatively high polarity, they are soluble in polar organic solvents such as acetone and methanol and only slightly soluble in dichloromethane. The solid state structure of the Et3P]Pd containing square 68 was also confirmed by single-crystal X-ray diVraction studies.27 As in the case of the previously discussed structure of square 40, the N]PdII]N valent angle is less than 908 and is close to 848, while the C]I]C bond angle opens to 98.78 resulting in the rhomboid shape.One of the major advantages of modular self-assembly is the ability to vary corner units, and therefore charge density, via diVerent oxidation states or transition metals along with the1718 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 Scheme 13 N N N I L2M N ML2 I + + ++ ++ 6 –OSO2CF3 N I N + –OSO2CF3 Acetone r.t. 2-14h 62 + L M OTf OTf L 37 M = Pd, L = 1/2 dppp 38 M = Pt, L = 1/2 dppp 64 M = Pd, L = Et3P 65 M = Pt, L = Et3P + N I N + –OSO2CF3 63 Acetone r.t. 2-14h N N L2M ++ N N ML2 ++ I I + + 6 –OSO2CF3 66 M = Pd, L = 1/2 dppp 67 M = Pt, L = 1/2 dppp 68 M = Pd, L = Et3P 69 M = Pt, L = Et3P 70 M = Pd, L = 1/2 dppp 71 M = Pt, L = 1/2 dppp 72 M = Pd, L = Et3P 73 M = Pt, L = Et3P Scheme 14 Pt PPh2 Ph2P C N C N Pt PPh2 Ph2P C N C N Pt Ph2P PPh2 C N C N ML2 L2M ++ ++ 4 –OSO2CF3 76 M = Pd, Pt, L = 1/2 dppp, Et3P 74 + L M OTf OTf L 37 M = Pd, L = 1/2 dppp 38 M = Pt, L = 1/2 dppp 64 M = Pd, L = Et3P 65 M = Pt, L = Et3P + Pt PPh2 Ph2P N N 75 CH2Cl2 r.t.Pt PPh2 Ph2P N L2M ++ Pt Ph2P Ph2 P N N N ML2 ++ 4 –OSO2CF3 77 M = Pd, Pt, L = 1/2 dppp, Et3P CH2Cl2 r.t. cavity size. This feature is quite valuable since it can be finetuned by using diVerent connector ligands with the appropriate charge. All these considerations are of importance if the resulting assemblies are to be exploited as possible molecular hosts. The simplest way to prepare mixed neutral–charged or heterobimetallic molecular squares is by using the already described palladium(II) and platinum(II) bis(triflate) complexes 37, 38 and 64, 65 for the charged portion of the assembly along with specially designed building units which contain the covalently bound and thus neutral, late transition metal bis(phosphane).28 Reaction of equimolar amounts of these transition metal bis- (triflates) in an appropriate solvent with either 74 or 75 (Scheme 14) aVorded the mixed heterobimetallic squares 76, 77 in high isolated yields.28,29 Mass spectrometry, along withJ.Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1719 other spectroscopic techniques, was successfully used in the characterization of these squares.30 Building blocks can be potentially useful if they contain early transition metals, since the incorporation of these metals can allow one further to vary the charge density on each individual subunit as well as the physical dimensions and shapes.Titanocene complexes were chosen as an example of early transition metal modules due to the favorable valent angle between the metal and the attached ligands as well as their rich and versatile chemistry. Interaction of complex 78 with the platinum( II) bis(phosphane) complex 38 or 65 in nitromethane at room temperature for 5 h produced the macrocyclic assemblies 79 (Scheme 15) in excellent yields. Both assemblies were isolated as stable microcrystalline orange solids.31 They were characterized by a variety of spectroscopic methods, including liquid secondary ion mass spectrometry (LSIMS).These mass spectroscopic data show the doubly charged ion [M 2 2OTf]21 at m/z = 1002.6 whose isotope pattern matches the calculated value very closely, thereby confirming the tetranuclear nature of this macrocycle. Unlike all previously discussed assemblies, squares 79 contain relatively flexible oxygen links and are therefore significantly more conformationally flexible.The energy- Scheme 15 N O O O O N Ti N O O O O N Ti N O O O O N Ti L2M ML2 ++ ++ 38 or 65 CH3NO2 r.t. 4 h 78 79 M = Pd, Pt, L = 1/2 dppp, Et3P Fig. 4 Space-filling model of molecular square 79 (M = Pd) minimized model of one of these squares is presented in Fig. 4, illustrating the shape and unusual cavity of this interesting macrocyclic assembly. Further examples of modular selfassembly were reported by Hupp and co-workers 32 who prepared the luminescent heterobimetallic molecular square 80.Although a variety of naturally occurring biomolecules are chiral and in many instances comprised of a large number of chiral subunits, the understanding of the exact mechanisms of formation of such asymmetric entities is a relatively new field of inquiry. Thus, the design of artificial chiral self-assembled species that mimic some biomolecules represents a formidable challenge. In marked contrast to the chemistry of covalent organic molecules, where asymmetric synthesis may be done routinely, the stereochemical control of non-covalent selfassembly processes is still in its early stages of development.This task can be addressed by preparing a family of selfassembled, cyclic tetranuclear entities, in which chirality is introduced in several diVerent ways. One possibility is the use of a chiral auxiliary, such as a chiral bis(phosphine), co-ordinated to the transition metal. Another approach can employ diazaligands which lack rotation symmetry about their linkage axis, which will result in an overall ‘twist’ of the square, thereby introducing the elements of a cyclic helicate in its assembly.In this situation, however, the formation of several (six for a tetranuclear assembly) stereoisomers is possible. The use of inherently chiral octahedral metal centres is also possible; the separation of the individual enantiomers may be a problem in this case. Finally, very interesting results may be achieved by combining some of the above principles.Thus, a chiral metal auxiliary may be used in conjunction with the elements of helicity or with optically active diaza-ligands, and so on. Optically active hybrid molecular squares can be prepared via the interaction of bis[4-(49-pyridyl)phenyl]iodonium triflate 62 and [Pd{R-(1)-BINAP}(H2O)][OTf]2 81 or [Pt{R-(1)- BINAP}(H2O)][OTf]2 82 (Scheme 16) [BINAP = 2,29-bis- (diphenylphosphino)-1,19-binaphthyl].33 In this case, the diazaligands of the iodonium species possess rotation symmetry about their linkages and therefore molecular squares 83 are chiral exclusively due to the chiral transition metal auxiliary (BINAP) in the assembly.Another interesting example of chiral hybrid molecular squares was prepared via the reaction of bis(3-pyridyl)iodonium triflate 84 and chiral palladium(II) and platinum(II) complexes 81 and 82. Interaction of a chiral square planar complex of PdII or PtII with an iodonium precursor where the heterocyclic ring lacks rotation symmetry about its linkages can result in the formation of six diastereomers.33 However, it was anticipated that the use of a chiral auxiliary such as BINAP would, on self-assembly, reduce the complexity of the stereochemical outcome via asymmetric induction.Indeed, when chiral transition metal complexes reacted with bis(3-pyridyl)iodonium triflate in acetone the result was the formation, in excellent isolated yields, of an excess of one of N N Ph2P Pd PPh2 CO Re CO N N N N N N OC Re CO PPh2 Pd Ph2P 80 4 –OTf 4+ Cl CO Cl CO1720 J.Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 Scheme 16 N N N N I I M PPh2 Ph2P M Ph2P PPh2 6+ 6 –OSO2CF3 83 M = Pd, Pt acetone r.t. N I+ N –OTf 62 M P P OSO2CF3 OSO2CF3 Ph Ph Ph Ph 81 M = Pd 82 M = Pt N I+ N –OTf 84 acetone r.t. P Ph2P M N N M N I N I Ph2P PPh2 6+ 6 –OSO2CF3 85 M = Pd, Pt + + each of the preferred diastereomers of assembly 85 (Scheme 16) as assessed by NMR, physical and mass spectrometric data.The self-assembly of all-metal chiral molecular squares was also carried out using the above-mentioned chiral bis(triflate) complexes of PdII and PtII and the C2h-symmetrical diazaligands 2,6-diazaanthracene (DAA) 86, and 2,6- diazaanthracene-9,10-dione (DAAD) 87.34 When either 81 or 82 was mixed with DAA in acetone at room temperature, the formation of a single diastereomer of each of the squares 88 was observed (Scheme 17).A space-filling model of the palladium square is presented in Fig. 5. These assemblies, essentially cyclic tetranuclear helicates, possess a large, molecular-size cavity. When DAAD 87 was employed as a connector ligand the reaction mixture consisted of a significant excess of one diastereomeric product, 89, along with minor amounts of other diastereomers, as demonstrated by 31P NMR spectra. The macrocyclic nature of these species was established by multinuclear NMR and confirmed by mass-spectroscopic data.34 The interesting fact that both these types of assemblies are formed either as a single diastereomer or a significantly enriched diastereomeric mixture is attributable to a significant degree of asymmetric induction by the chiral bis(phosphane) complexes.In the absence of such induction a mixture of six isomers may be formed.34 Such a mixture was indeed observed when an achiral transition metal bis(phosphane) complex was used instead of BINAP.34 The synthesis of larger organometallic squares was achieved by applying a diVerent self-assembly strategy: the organoplatinum linear linking unit 90 was employed as the sides of the square, while the corners were the already described angular subunits 62 or 75 which possess roughly 908 geometries (Scheme 18).35 Unit 90 can be prepared from 4,49-diiodo- Fig. 5 Molecular model of a chiral square 88 (M = Pd), derived from force-field calculationsJ. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1721 Scheme 17 P P Ph Ph Ph M Ph N N N N N P P Ph Ph Ph Ph N M M PPh2 PPh2 N N M PPh2 PPh2 N N 86 acetone P P Ph Ph Ph M Ph N N N N N P P Ph Ph Ph Ph N M M PPh2 PPh2 N N M PPh2 PPh2 O O O O O O O O N N 84 acetone O O P P Ph Ph Ph M Ph OSO2CF3 OSO2CF3 •H2O 81 M = Pd 82 M = Pt + + 8+ 8+ 8 –OSO2CF3 8 –OSO2CF3 88 M = Pd, Pt 89 M = Pd, Pt biphenyl and [Pt(PPh3)4] via oxidative addition followed by treatment of the resulting product with AgOTf.Selfassembly of both macrocyclic assemblies 91 and 92 was achieved by simply mixing either 62 in acetone or 75 in dichloromethane with 90 (Scheme 18).Both products were isolated in good yields and characterized by a variety of physical and spectroscopic techniques. In addition, the structure of assembly 92 was confirmed both by MALDI (matrix-assisted laserdesorption ionization) and ESI-FTICR (Fourier transform ion cyclotron resonance) mass spectrometric techniques.35 Both these macrocyclic assemblies belong to the category of ultrafine particles, because of their large size with estimated dimensions of about 3.4 nm along the edge and 4.8 nm across the diagonal for assembly 91 and 3.0 and 4.3 nm for 92, respectively.In the light of their unique structure, they may become useful in the construction of nanoscale molecular devices. A strikingly diVerent approach was recently developed by Lehn and co-workers.36 They utilized a template eVect to prepare three diVerent types of metallacyclic polygons via exactly the same synthetic route.When the tris(2,29-bipyridine) ligand 93 was added to FeCl2 in ethylene glycol at 170 8C the formation of pentanuclear assembly 94 was detected (Scheme 19).371722 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 Scheme 18 Ph2P Pt PPh2 N Pt PPh3 PPh3 PPh2 Pt Ph2P N Pt PPh3 PPh3 Ph2P Pt PPh2 N Pt PPh3 PPh3 PPh2 Pt Ph2P N Pt PPh3 PPh3 N Pt PPh3 Ph3P N Pt PPh3 Ph3P N Pt Ph3P PPh3 N Pt Ph3P PPh3 8 –OSO2CF3 92 N Pt PPh3 PPh3 N Pt PPh3 PPh3 I I N Pt PPh3 PPh3 N Pt PPh3 PPh3 I N Pt PPh3 Ph3P N Pt PPh3 Ph3P I N Pt PPh3 Ph3P N Pt PPh3 Ph3P 91 12 –OSO2CF3 N I+ N – OSO2CF3 Pt Pt TfO PPh3 Ph3P OTf PPh3 Ph3P Ph2P Pt PPh2 N N 75 90 + + CH2Cl2, 4 h, r.t.Acetone, 4 h, r.t. 62 8+ 12+ When the iron dichloride was replaced with iron sulfate the hexanuclear complex 95 was obtained. It is worth noting that detailed studies of these complexes readily revealed the presence of a chloride ion within the central cavity of the assembly 94.This ion may ultimately be responsible for the preferred formation of the molecular pentagon in the first reaction. The replacement of the (CH2)2 bridge in ligand 93 with the more flexible CH2OCH2 had a profound impact on the structure of the resulting assembly. Reaction of 96 with FeCl2 under the above conditions (Scheme 19) resulted in the formation of a tetranuclear assembly 97.36 Even when this reaction was repeated with FeBr2, FeSiF6 or FeSO4 it resulted only in the formation of a tetranuclear assembly without even traces ofJ.Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1723 Scheme 19 N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe N N N N N N Fe O O N N N N N N Fe N N N N N N Fe O O N N N N N N Fe O N N N N N N Fe O O O N N N N N N N N N N N N N N N N N N O O FeSO4 Ethylene glycol, 170°C FeSO4 Ethylene glycol, 170°C FeSO4 Ethylene glycol, 170°C 94 95 97 93 93 96 9+ 9 –Cl 12+ 6 SO4 2– Cl– 8+ 4 SO4 2– the higher nuclearity metallacycles (Scheme 19).This example demonstrates an interesting and important point: the assembly of a specific architecture results from substrate binding eVects, such as interactions with counter ions and ligand-specific features, such as size and flexibility. Some parallels to this can be taken from natural structures, such as proteins.In the case of the latter the self-assembled metallacycle represents a tertiary structure. Each of them is unique in a structural sense, but all share the same secondary structure, i.e. co-ordination mode to the iron centers.1724 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 5 Three-dimensional Discrete Assemblies: Nanoscopic Polyhedra If molecular building blocks of various nanostructured materials are to be connected in a three-dimensional fashion, one needs to find rigid three-dimensional molecules which could serve as skeletons, thus providing the core of such nanostructures.Apart from the fact that the overwhelming bulk of molecules are of a flexible chain-like nature, the few rigid and compact cages (cubane, adamantane, dodecahedrane, the norbornanes, and the fullerenes) are very diYcult to functionalize in a useful and systematic way as they are extremely inert once synthesized. Their stepwise syntheses often face severe steric problems at one or more steps.Aso, lengthy syntheses and low yields are involved along with the fact that the harsh conditions often employed may be intolerable for many functional groups. Even norbornanes, which are usually assembled in one step by a Diels–Alder cycloaddition reaction, are not totally free from these limitations. Another serious drawback of these organic precursors is their relatively small size. The synthesis of conformationally rigid large cages with multiple functional binding sites by using only the tools of classical synthetic organic chemistry becomes more diYcult with the increasing number of carbon atoms in the cage.As any individual molecular building subunit has to be incorporated into the skeleton in a confined, rigid fashion, it usually will have to be attached by at least three bonds. If this building block then provides one additional functional group that will actually appear on its periphery, then in the overall analysis three functional groups have been consumed for the one that has been delivered.If one looks at the totally assembled structure one finds that essentially nothing has been gained in terms of providing more potential functionalization sites which would have been desirable for design and which was the reason why one wanted larger cages in the first place. The functional groups needed for holding together the subunits in the skeleton are lost because usually the atoms in such a bond cannot engage in any further function other than the bond formation itself.Similarly, no additional functionality can be gained by using the sp-hybrid carbons of acetylene, except larger overall dimensions. Hence, the formation of organic, carbon-based nanostructures suVers from the one fundamental unalterable limitation of the carbon atom, its maximum number of valences cannot exceed four. Transition metal-based nanosystems, on the other hand, are free from such limitations because it is relatively easy to find, for each specific task, a suitable metal with a co-ordination number higher than four.Moreover, due to multiple binding sites the resulting cages can be of significantly larger size and can still retain their conformational rigidity. An example of a tetranuclear adamantanoid cage was prepared by Saalfrank et al.38 They used the co-ordination chemistry of trivalent iron complexes that are bound to an anionic ligand 98.The three-dimensional complex 99 contains four iron atoms interconnected by six anionic subunits in a cyclic manner and has a large, open cavity. Another interesting example involved ligand 100 and mixed-valence iron complexes to prepare cage 101 (Scheme 20).38 They are formed by self-assembly via the reaction of dialkyl malonates with methyllithium and FeCl2 in tetrahydrofuran at low temperatures. Both assemblies are capable of encapsulating ammonium ions, as confirmed by mass spectrometry and X-ray crystallographic studies.38 The mixed-valence character of both assemblies was also established by studying their Mössbauer spectra and further proven by measurements of cyclic voltammograms.Due to their ability to hold cationic species, these complexes may be found valuable in the design of artificial cages that are capable of encapsulating various alkali metals. The self-assembly of nanoscopic three-dimensional molecules with the shape of an octahedron was carried out by Fujita et al.39 utilizing ethylenediamine palladium(II) dinitrate 13 in combination with the tridentate aromatic ligands 102.The near-quantitative yields of assemblies 103–105 (Scheme 21) and the high thermodynamic stability of the final products were noted. These assemblies were formed even in the presence of an excess of the transition metal-containing subunit. These watersoluble molecules are capable of encapsulating up to four aliphatic guests of similar symmetry and shape, such as adamantanecarboxylate. 39 The final products were reported to be very stable: when the transition metal complex 13 and the ligand 102 were mixed in a 2 : 1 ratio only the assembly with a 3 : 2 stoichiometry was observed while the excess of free transition metal complex remained intact. The inclusion complex of assembly 103 with adamantanecarboxylate was studied by X-ray crystal- Scheme 20J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1725 Scheme 21 NH 2 Pd H2 N ONO2 ONO2 N N N N N N 6 13 X X X 102 X = none, + N X N N X N N X N N N X N X X N N N X N N N X N N X N N N X N X X N N H2O OR H2O- MeOH 12 NO3 – 12+ NH 2 Pd H2 N = 103 X = none 104 X = 105 X = 4 , lography.39 Thus, assembly 103 formed a host–guest complex with four molecules of adamantanecarboxylate and no intermediate inclusion complexes with one, two or three guest molecules were observed.This is probably due to allosteric eVects: the hydrophobicity of the cavity increases and the complexation between 103 and the guest becomes more eVective with increasing number of the guests in the cavity.In work reported by Hartshorn and Steel 40 a topologically similar three-dimensional cage 106 was prepared by utilizing the co-ordination chemistry of 1,3,5-triethyl-2,4,6-tris(pyrazol- 1-ylmethyl)benzene and the square-planar palladium(II) complexes; NMR studies have confirmed the highly symmetrical nature of 106.Single crystals of this compound were also ana- R N N N N R N N R Pd N Cl Cl N R N R Pd Cl Cl N Cl N Pd Cl R R N N N N N Pd Cl Cl N N N N N R N Pd Cl Cl N N Pd Cl Cl R R R R 106 R = H, Et lysed by X-ray crystallography. This macromolecule possesses a relatively large cavity: the diagonally opposite palladium atoms are separated by 13–15 Å, thus the cavity size is suYcient to encapsulate relatively large guests. An example of a chiral three-dimensional structure was also recently reported.41 The previously mentioned R-(1)-BINAP bis(triflate) complexes of PdII 81 and PtII 82 were chosen as the bidentate angular units.Since BINAP is conformationally rigid the loss of conformational entropy is minimized on binding to the nitrogen-based ligand. Such a ligand, 1,3,5-tris(49- pyridylethynyl)benzene 107 was prepared from 1,3,5-tristriethynylbenzene and 4-bromopyridine via the Hagihara crosscoupling. The addition of ligand 107 to a dichloromethane solution of the transition metal bis(triflates) resulted in the self-assembly of the highly symmetrical entities 108, 109 (Scheme 22) with the stoichiometry of the reactants being 3 : 2 as observed by NMR spectroscopy.41 The stoichiometry of assembly 108 was also firmly established by mass spectrometry: the ESI-FTICR mass spectrum obtained from a dichloromethane solution resolved the peak centered at m/z = 1768.19 with an m/z peak spacing of 1– 4 corresponding to the [M 2 42OTf]41 ion, corresponding to the cyclic assembly with loss of four tri- flate counter ions.The observed molecular weight and close match of the calculated and observed isotopic distribution patterns of the 41 charge state (M 7092.76) are in agreement with the theoretical molecular weight of 7092.89 (error = 13 ppm). Since both molecules belong to the T-symmetry group, they are the first examples of highly symmetrical, yet chiral threedimensional macrocyclic cages prepared by using a rational, coordination- directed self-assembly strategy.The construction of a rigid metallacyclic cage from two preorganized cavitands and four square-planar transition metal bis(triflates) was also reported recently.42 When specifically designed tetracyanocavitand 110 reacted with bis(triflates) 37,1726 J. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 Scheme 22 P Ph Ph M P PhPh O3SCF3 O3SCF3 81 M = Pd 82 M = Pt + N N N 107 CH2Cl2 or CH3NO2, r.t. P Ph Ph M P Ph Ph N N N N N M N N N N N M Ph2P PPh2 M PPh2 Ph2P P Ph Ph P Ph Ph M P Ph Ph P Ph Ph M N N PPh2 Ph2P 12+ 12 –O3SCF3 108 M = Pd 109 M = Pt 38 at room temperature they produced rigid dimeric assemblies 111 (Scheme 23).Spectroscopic studies revealed one triflate counter ion being trapped within their rigid three-dimensional cavities. 6 Three-dimensional Infinite Assemblies: Nets The self-assembly of infinite co-ordination networks is of special interest to material science. The materials constructed by utilizing such an approach contain precisely arranged subunits and therefore depending upon the type of subunits and strategy employed, may exhibit interesting optical, magnetic or electronic properties.Construction of such infinite assemblies may be accomplished by applying the experience gained in the preparation of discrete assemblies, if they share the same building blocks. One example of a molecular grid that contains 4,49- bipyridine subunits was reported by Fujita et al.43 They used the co-ordination chemistry of divalent cadmium.When two components were mixed in aqueous ethanol solution a twodimensional grid was formed (Scheme 24). Each metal ion in this structure is surrounded by four 4,49-bipyridyl molecules; its repeated unit shape and size thus resembles the molecular squares discussed previously. Similar to their discrete parent squares, this grid was shown to encapsulate aromatic guests such as o-dibromobenzene with high shape selectivity.Interestingly, unlike the discrete squares, this complex also shows catalytic activity; it accelerated cyanosilylation reactions. AJ. Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 1727 topologically similar grid containing 4,49-bipyridyl and zinc(II) was also reported by Robson and co-workers,44 but without catalytic or host–guest observations. The application of preformed angular building blocks in Scheme 23 CN R CN R R R O O O O O O O O NC CN PPh2 M PPh2 OSO2CF3 OSO2CF3 + 37 M = Pd 38 M = Pt 110 CH2Cl2, r.t.O O O O O O O O R R R R C C C N M L L N C N M L L N C O O O O O O C N N M C C N N M L L L L R O O R R R –OSO2CF3 111 M = Pd, Pt, L = 1/2 dppp 7 –OSO2CF3 7+ Scheme 24 N N N N N N Cd N N N N N N Cd N N N N Cd N N Cd N N Cd Cd N N Cd N N Cd Cd H2O-EtOH N N n Cd(NO3)2 + n n+ n NO3 – 24 combination with cadmium(II) ions to construct infinite selfassembled arrays was also reported.12 When ligand 15 was mixed in solution with cadmium(II) nitrate it yielded an infinite assembly structurally similar to the discrete assemblies 16 or 19.This network also demonstrated interesting inclusion properties as was demonstrated by X-ray crystallographic studies of its inclusion complex with p-nitroaniline. Construction of a three-dimensional network using tridentate organic ligands in combination with a silver(I) salt was reported by Moore.45 These networks have a honeycomblike structure and utilize a tris(p-cyanobenzene)ethynylbenzene linking unit 112 co-ordinated to AgI in a triangular planar manner.The two other co-ordination sites of silver are occupied by the triflate counter ions. This network was capable of encapsulating benzene and some other small molecules within its porous structure.45 The crystals of such assemblies may also exhibit auxetic properties, such as a negative coeYcient of thermal expansion, due to the nearly barrierless transition from one type of crystal ordering (honeycomb) into another (flat hexagonal). 7 Outlook As discussed in this article, the molecular architecture paradigm allows the creation of a remarkable variety of artificial self-assembling systems of diVerent shapes and sizes. This selfassembly method is very versatile: it allows the ready formation of a great number of discrete supramolecular species with well defined, predesigned shapes and geometries. The most important advantages of this method are its wide applicability and the large and diVerent number of potentially suitable transition metal complexes and multidentate nitrogen-containing ligands that are available as building units.The excellent product yields that are inherent to such self-assembly processes have been observed in many cases. This method also allows the ready control of the polarity of the assembly and hence its solubility properties via controlled formation of charged and neutral macrocycles. Its synthetic versatility is in the possible use of covalent macrocycles as subunits in larger assemblies.Overall, it allows the precise control of the shape of the formed assembly via simple variations of the dative bond angles of the transition metal or linker units. The list of novel assemblies that have been presented in this article is by no means exhaustive. The applicability of the transition metal-mediated self-assembly process, employing coordination as the motif, allows the manipulation of the macroscopic properties of materials at the nanoscopic level and the creation of future supramolecular species and nanoscale-sized devices with specific, precisely tuned properties, functions and microenvironments. Today’s manufacturing methods are very crude at the molecular level.Casting, grinding and even lithography move atoms in great statistical numbers. In the future, nanotechnology will allow the development of new ‘post-lithographic’ NC CN CN 1121728 J.Chem. Soc., Dalton Trans., 1998, Pages 1707–1728 manufacturing processes which will allow one to, for example, build materials that are molecular in both size and precision and are interconnected in complex and highly specific patterns. It will be able to put together the fundamental building blocks of nature easily, inexpensively and in almost any arrangement that one desires. This will be essential if we are to continue the revolution in material science beyond the next decade, and will also let one fabricate an entire new generation of products that are stronger, lighter, cleaner and more precise. 8 References 1 J.-M. Lehn, J. L. Atwood, J. E. D. Davis, D. D. MacNicol and F. Vögtle (Editors), Comprehensive Supramolecular Chemistry, Pergamon Press, Oxford, 1990–1996, vols. 1–11; J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH, Weinheim, 1995; H. Schneider and H. Dürr (Editors), Frontiers in Supramolecular Chemistry, VCH, Weinheim, 1991; J.F. Stoddard (Editor), Monographs in Supramolecular Chemistry, Royal Society of Chemistry, Cambridge, 1989, 1991, 1994–1996, vols. 1–6; V. Balzani and L. DeCola (Editors), Supramolecular Chemistry, Kluwer, Dordrecht, 1992. 2 P. J. Stang and B. Olenyuk, Acc. Chem. Res., 1997, 30, 502. 3 A. W. Maverick, S. C. Buckingham, Q. Yao, J. R. Bradbury and G. G. Stanley, J. Am. Chem. Soc., 1986, 108, 7430. 4 C. M. Hartshorn and P. J. Steel, Inorg. Chem., 1996, 35, 6902. 5 M. J. Hannon, C. L. Painting and W. Errington, Chem. Commun., 1997, 307. 6 A. Bilyk and M. M. Harding, J. Chem. Soc., Dalton Trans., 1994, 77. 7 D. L. Caulder and K. N. Raymond, Angew. Chem., Int. Ed. Engl., 1997, 36, 1440. 8 M. Fujita, S. Nagao, M. Iida, K. Ogata and K. Ogura, J. Am. Chem. Soc., 1993, 115, 1574; M. Fujita, J. Yazaki, T. Kuramachi and K. Ogura, Bull. Chem. Soc. Jpn., 1993, 66, 1837. 9 M. Fujita, F. Ibukuro, H. Hagihara and K. Ogura, Nature (London), 1994, 367, 720. 10 M. Fujita, F. Ibukuro, K. Yamaguchi and K. Ogura, J. Am. Chem. Soc., 1995, 117, 4175. 11 M. Fujita, J. Synth. Org. Chem. Jpn., 1996, 54, 957. 12 C. O. Dietrich-Buchecker, J. Guilhem, C. Pascard and J.-P. Sauvage, Angew. Chem., Int. Ed. Engl., 1990, 29, 1154. 13 A. F. Williams, C. Piguet and G. Bernardinelli, Angew. Chem., Int. Ed. Engl., 1991, 30, 1490. 14 A. F. Williams, Chem. Eur. J., 1997, 3, 15; E. C. Constable, Tetrahedron, 1992, 48, 10 013; Angew. Chem., Int. Ed. Engl., 1991, 30, 1450. 15 M. Fujita, O. Sasaki, T. Mitsuhashi, T. Fujita, J. Yazaki, K. Yamaguchi and K. Ogura, Chem. Commun., 1996, 1535. 16 P. J. Stang, D. H. Cao, S. Saito and A. M. Arif, J. Am. Chem. Soc., 1995, 117, 6273. 17 P. J. Stang and D. H. Cao, J. Am. Chem. Soc., 1994, 116, 4981. 18 M. Fujita, J. Yazaki and K. Ogura, J. Am. Chem. Soc., 1990, 112, 5645. 19 P. J. Stang, B. Olenyuk, J. Fan and A. M. Arif, Organometallics, 1996, 15, 904. 20 P. J. Stang, D. H. Cao, K. Chen, G. M. Gray, D. C. Muddiman and R. D. Smith, J. Am. Chem. Soc., 1997, 119, 5163. 21 P. J. Stang, J. Fan and B. Olenyuk, Chem. Commun., 1997, 1453. 22 R. V. Slone, J. T. Hupp, C. L. Stern and T. E. Albrecht-Schmitt, Inorg. Chem., 1996, 35, 4096. 23 C. M. Drain and J.-M. Lehn, J. Chem. Soc., Chem. Commun., 1994, 2313. 24 A. Varvoglis, The Organic Chemistry of Polycoordinated Iodine, VCH, Weinheim, 1992; P. J. Stang and V. V. Zhdankin, Chem. Rev., 1996, 96, 1123. 25 P. J. Stang, B. Olenyuk and K. Chen, Synthesis, 1995, 937. 26 P. J. Stang and K. Chen, J. Am. Chem. Soc., 1995, 117, 1667. 27 P. J. Stang, K. Chen and A. M. Arif, J. Am. Chem. Soc., 1995, 117, 8793. 28 P. J. Stang and J. A. Whiteford, Organometallics, 1994, 13, 3776. 29 J. A. Whiteford, C. V. Lu and P. J. Stang, J. Am. Chem. Soc., 1997, 119, 2524. 30 J. A. Whiteford, E. M. Rachlin and P. J. Stang, Angew. Chem., Int. Ed. Engl., 1996, 35, 2524. 31 P. J. Stang and N. E. Persky, Chem. Commun., 1997, 77. 32 R. V. Slone, D. I. Yoon, R. M. Calhoun and J. T. Hupp, J. Am. Chem. Soc., 1995, 117, 11 813. 33 B. Olenyuk, J. A. Whiteford and P. J. Stang, J. Am. Chem. Soc., 1996, 118, 8221. 34 P. J. Stang and B. Olenyuk, Angew. Chem., Int. Ed. Engl., 1996, 35, 732. 35 J. Manna, J. A. Whiteford, P. J. Stang, D. C. Muddiman and R. D. Smith, J. Am. Chem. 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Venkataraman, J. S. Moore and S. Lee, Nature (London), 1995, 374, 792. Received 5th February 1998; Paper 8/01057I

ISSN:1477-9226    

DOI:10.1039/a801057i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1711–1712 1711 The first structurally characterized 3,49-bipyridine copper(I) coordination polymer with an approximately rectangular molecular box Hoong-Kun Fun,*a S. Shanmuga Sundara Raj,a Ren-Gen Xiong,*b Jing-Lin Zuo,b Zhi Yu,b Xiao-Lei Zhu b and Xiao-Zeng You b a X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia. E-mail: hkfun@usm.my b Coordination chemistry Institute, State key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, P.R. China. E-mail: xyz@netra.nju.edu.cn Received 16th March 1999, Accepted 16th April 1999 The first metal coordination polymer containing 3,49- bipyridine (3,49-bpy) was prepared by the hydrothermal treatment of Cu(NO3)2?2.5H2O, Me3SnBr and 3,49-bpy and its crystal structure shows a novel infinite double chain composed of two 3,49-bpy and Br as well as copper(I) ions with an approximately rectangular molecular box of 4.0 × 10.68 Å. 4,49-Bipyridine (4,49-bpy) metal coordination chemistry has received extensive attention mainly due to the ligand being used to bridge metal centers to form one-, two-, and threedimensionally connected polymer networks.1–4 Furthermore, supramolecular chemistry and self-assembly with 4,49-bpy and its derivatives are at the frontiers of the molecular sciences, as demonstrated by the intense interest and the near exponential growth of publications in this area in just the last decade.5 However, novel structural types of metal coordination compounds using 4,49-bpy as a linear spacer are limited owing to its symmetry and linearity. More recently, a variety of attempts to synthesize 4,49-bpy derivatives, such as 3,39-bipyridine and 2,49-bipyridine, the former of which is not commercially available, were made and their metal coordination chemistry also appears in the current literature, showing that unprecedented structural types were observed, in comparison with the 4,49-bpy.6,7 The use of 2,29- bipyridine as a bidentate chelating ligand has also been extensively investigated in coordination chemistry.7c A search of the CCDC database gave no hits for any metal complex containing 3,49-bipyridine or 2,39-bipyridine ligands.Consequently, it is of interest to study the bipyridine system’s coordination chemistry. Herein, we report the synthesis, structure and fluorescence of [(3,49-bpy)(Br)CuI]n 1 which represents the first example of a metal coordination polymer containing the 3,49-bpy ligand.Complex 1† was prepared by hydrothermal treatment of Cu(NO3)2?2.5H2O, Me3SnBr and 3,49-bpy for one day at 110 8C. Perfect one-phase yellow-orange rectangular crystals were harvested. There is no broad absorption peak at ca. 1100 cm21 in the IR spectrum of 1, suggesting that the nitrate group does not persist in 1 Single crystal X-ray diVraction analysis ‡ indicates that the crystal structure of 1 (Fig. 1) contains an unprecedented double chain involving a twenty-membered ring with an approximately rectangular molecular box of 4.0 × 10.68 Å (a space-filling diagram is depicted in Fig. 2) which consists of two 3,49-bpy, Br atoms as well as copper(I) ions, unlike Cu–quinoxaline–Cu chains in which Cl atoms act as one of the two bridging ligands.8 To the best of our knowledge, 1 is the first example of a double chain structure with a bipyridine system in MX2 (X = halide) complexes.The structure is formed mainly due to the bent angle (about 1208) between the two nitrogen atoms of the 3 and 49 positions (the dihedral angle between the two pyridine rings is about 348 after the formation of 1). Furthermore, the polymer is composed of Cu2(m-Br)2(3,49-bpy)2 units in which two 3,49-bpy ligands bridge between four copper pairs to form a polymeric chain of tetramers. The local environment around the Cu(I) ion is a slightly distorted tetrahedron with a Cu–Cu distance of 2.798(1) Å, slightly smaller than the sum of the van der Waals radii of copper(I) (2.8 Å), suggesting that there is a degree of metal–metal bonding character.The Cu–Cu distance is significantly shorter than those in the polymers [(Ph3P)2Cu2(m-Cl)2(m-pyz)]n [3.095(1)] (pyz = pyrazine), [Cu2(m-Cl)2(m-phz)]n [3.258(1)] (phz = phenazine) and [Cu2(m-Br)2(m-phz)]n [3.391(2)] but longer than that in [Cu2(m-I)2(m-phz)]n [2.525(1)], metallic copper (2.56) and [Cu- (m-I)(NCR)]n (2.54–2.66 Å), respectively.9 The metal–ligand distances are typical for copper(I) complexes.In comparison with distances in similar polymers, the range of the Cu–N distances of 1.991(2)–2.105(2) Å is basically in agreement with 2.044(3), 1.993(8) and 1.97(2) Å in [(Ph3P)2Cu2(m-Cl)2(m-pyz)]n, [Cu(m- Cl)(py)] and [Cu(TTA)(4,49-bpy)]n [TTA = 4-(3-thienyl)-1,1,1- trifluorobutane-2,4-dionate], respectively.10 The range of the Cu–Br distances of 2.522–2.570 Å is also comparable to that of [Cu2(m-Br)(m-phz)]n [2.515(1)–2.614(1) Å].9 The molecules run along the c-axis as a one-dimensional layer in which the same laminar one-dimensional polymers are stacked together in a zigzag mode by van der Waals interactions.The diVuse reflectance UV-vis spectrum of 1 shows a high energy band at ca. 275 nm and a low-energy band at ca. 345 nm, respectively (Fig. 3). The former may be assigned to the intraligand transition of the free ligand due to the 3,49-bpy as it shows a band at a similar wavelength.With reference to previous spectroscopic work on related systems,11 the low-energy band is assigned to a metal-to-ligand charge transfer (MLCT). However, the metal center d to s orbital transition is not ruled out.12 The strong emission spectrum of 1 in the solid state at room temperature, shown in Fig. 3, has a maximum at ca. 580 nm, very similar to that of Cu4I4(py)4 (py = pyridine) with lmax 580 nm at 294 K.13 In [Re(CO)3(SR)]4(m-4,49-bpy)2,14 although the rectangular framework is of dimensions 3.81 × 11.57 Å as defined by the rhenium center, the dihedral angle between the two pyridyl ring Fig. 1 An ORTEP15 view of [(3,49-bpy)(Br)CuI]n showing a double chain and a rectangular molecular box along the b-axis.1712 J. Chem. Soc., Dalton Trans., 1999, 1711–1712 planes is about 18. As a result, once van der Waals radii are taken into account, the box fails to be an acceptor site, even for planar molecules, such as benzene.However, in 1, even if one of the dimensions in the rectangular box is 4 Å, guest molecules, such as phenol, may be sited in the bow surrounded by the pyridyl rings because the dihedral angle between the two pyridyl planes is large (ca. 348). As a result, the luminescence characteristics of the rectangular box makes 1 a candidate for sensing applications on the basis of recognition and inclusion of appropriate guest molecules.14 Acknowledgements This work was supported by a grant for a key research project from the State Science and Technology Commission and the National Nature Science Foundation of China.The authors would like to thank Universiti Sains Malaysia and the Malaysian Government for the research grant R&D No. 190- Fig. 2 A space-filling diagram of [(3,49-bpy)(Br)CuI]n. Fig. 3 (a) The diVuse reflectance UV-vis and (b) fluorescence spectra of [(3,49-bpy)(Br)CuI]n. 9609-2801. SSSR thanks Universiti Sains Malaysia for a Visiting Post Doctoral Fellowship.Notes and references † Preparation of [(3,49-bpy)(Br)CuI]n 1: hydrothermal treatment of Cu(NO3)2?2.5H2O (1.2 mmol), trimethyltin bromide (1 mmol), 3,49-bpy (1 mmol) and water (10 ml) for one day at 110 8C yielded an orangeyellow rectangular crystalline product (only one pure phase). The yield of 1 was 45% based on 3,49-bpy (Calc.: C, 40.07; H, 2.67; N, 9.35. Found for C20H16Br2Cu2N4: C, 39.82; H, 2.43; N, 10.10%). IR (KBr/ cm21): 1602vs, 1540vw, 1470vs, 1428m, 1401vs, 1324w, 1220w, 1123vw, 1067vw, 1023m, 1015m, 844m, 796vs, 692vs, 636w, 539m.‡ Crystal data for 1: C20H16Br2Cu2N4, monoclinic, P21/n, a 9.4944(1), b 12.9445(2), c 16.9514(1) Å, b 90.924(1)8, V 2083.06(4) Å3, Z 4, M 599.27, Dc 1.911 Mg m23, R1 0.032, wR 0.070 (4759 reflections). T 293 K, m 5.891 mm21. CCDC reference no. 186/1428. See http:// www.rsc.org/suppdata/dt/1999/1711/ for crystallographic files in .cif format. 1 (a) R. W. Gable, B.F. Hoskins and R. Robson, J. Chem. Soc., Chem. Commun., 1990, 1677; (b) B. F. Abrahams, B. F. Hoskins and R. Robson, J. Am. Chem. Soc., 1991, 113, 3603; (c) B. F. Abrahams, B. F. Hoskins, D. M. Michail and R. Robson, Nature (London), 1994, 369, 27; (d ) S. R. Batten, B. F. Hoskins and R. Robson, Angew. Chem., Int. Ed. Engl., 1995, 34, 820. 2 (a) M. Fujita, Y. J. Kwon, O. Sasaki, K. Yamaguchi and K. Ogura, J. Am. Chem. Soc., 1995, 117, 7287; (b) M. Fujita, Y. J. Kwon, S.Washizu and K. Ogura, J. Am. Chem. Soc., 1994, 116, 1151. 3 (a) S. Subramanian and M. J. Zaworotko, Angew. Chem., Int. Ed. Engl., 1995, 34, 2127; (b) M. J. Zaworotko, J. Chem. Soc. Rev., 1994, 23, 284; (c) L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, J. Am. Chem. Soc., 1995, 117, 4562; (d ) L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, J. Chem. Soc., Chem. Commun., 1994, 2755; (e) O. M. Yaghi and H. Li, J. Am. Chem. Soc., 1995, 117, 10401; ( f ) O. M. Yaghi and G.Li, Angew. Chem., Int. Ed. Engl., 1995, 34, 207. 4 (a) D. Venkataraman, G. B. Gardner, S. Lee and J. S. Moore, J. Am. Chem. Soc., 1995, 117, 11600; (b) D. Venkataraman, S. Lee, J. Zhang and J. S. Moore, Nature (London), 1994, 371, 591; (c) S. D. Huang and R.-G. Xiong, Polyhedron, 1997, 16, 3929. 5 (a) P. J. Stang, Chem. Eur. J., 1998, 4, 19; (b) B. Olenyuk, A. Fechtenkotter and P. J. Stang, J. Chem. Soc., Dalton Trans., 1998, 1707; (c) C. J. Jones, Chem. Soc. Rev., 1998, 27, 289. 6 (a) S. W. Keller, Angew. Chem., Int. Ed. Engl., 1997, 36, 247; (b) S. Lopez, M. Kahraman, M. Harmata and S. W. Keller, Inorg. Chem., 1997, 36, 6138; (c) J. R. Hall, S. J. Loeb, G. K. H. Shimizu and G. P. A. Yap, Angew. Chem., Int. Ed., 1998, 37, 121; (d ) R.-D. Schnebeck, L. Randaccio, E. Zangrando and B. Lippert, Angew. Chem., Int. Ed., 1998, 37, 119. 7 (a) R.-G. Xiong and W. Lin, work to be published, in which only the nitrogen atoms of the pyridine ring take part in coordination to the metal ion, such as Cd21 and Co21 (Inorg.Chim. Acta, in the press); (b) M.-L. Tong, X.-M. Chen, B.-H. Ye and S. W. Ng, Inorg. Chem., 1998, 37, 5269; (c) J. A. McCleverty and M. D. Ward, Acc. Chem. Res., 1998, 31, 842. 8 (a) S. Lindroos, P. Lumme, Acta Crystallogr., Sect. C, 1990, 45, 2039; (b) J. Lu, C. Yu, T. Niu, T. Paliwala, G. Crisci, F. Somosa and A. J. Jacobson, Inorg. Chem., 1998, 37, 4637. 9 (a) M. Henary, J. L. Wootton, S. I. Khan and J. I. Zink, Inorg. Chem., 1997, 36, 796; (b) M. Munakata, T. Kuroda-Sowa, M. Maekawa and A. Honda, J. Chem. Soc., Dalton Trans., 1994, 2771; (c) P. C. Healy, J. D. Kildea, B. W. Skelton and A. H. White, Aust. J. Chem., 1989, 42, 79. 10 M. Li, Z. Xu, X. You, Z. Dong and G. Guo, Polyhedron, 1993, 12, 921. 11 (a) C. E. A. Palmer and D. R. McMillin, Inorg. Chem., 1987, 26, 3837; (b) V. W.-W. Yam, Y.-L. Pui, W.-P. Li, K. K.-W. Lo and K.-K. Cheung, J. Chem. Soc., Dalton Trans., 1998, 3615. 12 D. Li, H. K. Yip, C. M. Che, Z. Y. Zhou, T. C. W. Mak and S. T. Liu, J. Chem. Soc., Dalton Trans., 1992, 2445. 13 K. R. Kyle, C. K. Ryu, J. A. DiBenedetto and P. C. Ford, J. Am. Chem. Soc., 1991, 113, 2954. 14 (a) R. V. Slone, J. T. Hupp, C. L. Stern and T. E. Albrecht-Schmit, Inorg. Chem., 1996, 35, 4096; (b) Z. Pikramenou and D. G. Nocera, Inorg. Chem., 1992, 31, 532; (c) K. D. Benkstein, J. T. Hupp and C. L. Stern, Inorg. Chem., 1998, 37, 5404. 15 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976. Communication 9/02054C

ISSN:1477-9226    

DOI:10.1039/a902054c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 1713–1715 1713 Easy ring expansion and contraction in Pt–Sn bonded metallacycles Michael C. Janzen, Hilary A. Jenkins, Michael C. Jennings, Louis M. Rendina and Richard J. Puddephatt * Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7. E-mail: pudd@uwo.ca Received 14th April 1999, Accepted 22nd April 1999 The alkyne RCCR, R 5 CO2Me, reacts with 5-membered metallacycles [PtMe2{SnMe2ESnMe2E}(bu2bpy)], E 5 S, Se or Te, bu2bipy 5 4,49-di-tert-butyl-2,29-bipyridine, yielding the corresponding 7-membered metallacycles [PtMe2- {SnMe2ESnMe2CR]] CRE}(bu2bpy)] which slowly eliminate “Me2SnE” to give new 5-membered metallacycles [PtMe2- {SnMe2CR]] CRE}(bu2bpy)]; further reaction of which with excess RCCR gives [PtMe2(CR]] CRH)(CCR)(bu2bpy)], a complex which contains alkyl, alkenyl and alkynyl functionalities in the same molecule.A key property of metallacyclic compounds is their ability to undergo easy ring expansion/contraction reactions and these reactions are central to such useful catalytic reactions as alkene or alkyne metathesis, dimerization or trimerization.1 This article reports that easy ring expansion and contraction can occur in reactions of an alkyne with the metallacycles [PtMe2- (SnMe2ESnMe2E)(bu2bpy)]; bu2bpy = 2,29-di-tert-butyl-4,49- bipyridine, E = S, 2; Se, 3; or Te, 4, which are easily prepared by reaction of (Me2SnE)3 with [PtMe2(bu2bpy)] 1, as shown in Scheme 1.2 These appear to be unique examples of such reactions in heteronuclear metallacycles; the closest analogy appears to be the single chalcogen atom abstraction, which converts the 6-membered PtIIENC(Ph)NE (E = S or Se) ring to a 5-membered ring.3 The electrophilic alkyne dimethyl acetylenedicarboxylate inserts regioselectively into a Sn–E bond of the 5-membered metallacycle 2, 3, or 4 to form the corresponding 7-membered metallacycle 5, 6, or 7.These reactions are complete in about 1 h at room temperature and the products are yellow and airstable when E = S or Se, but dark brown and air sensitive when E = Te.They were characterized by their NMR spectra 4 and, for complex 6 by an X-ray structure determination.5 The 1H NMR spectra of 5–7 contain four MeSn, two MePt and two MeO resonances, each corresponding to three protons, and so demonstrate that one equivalent of alkyne has been added. The 119Sn NMR spectrum of 5 contained two resonances, one of which displayed a coupling 1J(PtSn) = 11 860 Hz, thus showing that the Pt–Sn bond was still present, and both resonances exhibited a coupling 2J(Sn1–Sn2) = 151 Hz, thus showing that the PtSnMe2SSnMe2 unit was still present.Final proof that insertion occurred into the remaining PtE–Sn bond of 2–4 was obtained from the structure determination for complex 6 (Fig. 1).5 The conformation of the 7-membered ring leads to relatively short transannular distances Sn2 ? ? ? Se1 = 3.42 Å, Sn1 ? ? ? Se1 = 3.48 Å, perhaps indicating weak secondary bonding between these atoms.The stereochemistry at the C]] C bond is cis and one CO2Me group stacks below the bipyridyl ligand. It is interesting that the Sn–E bonds in the precursor molecules (Me2SnE)3 are unreactive towards this alkyne, and so the Sn–E bond is activated within the platinum complex. We suggest that the reaction is initiated by nucleophilic attack from a lone pair of electrons of the PtE group on the electrophilic alkyne, and that the nucleophilicity of E is increased by donation of electron density from the trans MePt group.The complexes 2–4 are unreactive towards less electrophilic alkynes such as PhCCPh. The complexes 5–7 decompose, over a period of about 8 hours at room temperature in solution in CH2Cl2 by elimination of (Me2SnE)3 (identified by its NMR spectrum) 2,3 to form the 5-membered metallacycles 8–10, Scheme 1. The 1H NMR of complexes 8–10 each contained two MeSn, two MePt and two MeO resonances, and the 119Sn NMR spectra each contain only one resonance with a large coupling due to 1J(PtSn).4 The structure of 9 is shown in Fig. 2.5 The 5-membered PtSnC]] CSe ring is only slightly distorted from planarity (torsional angle Pt–Se–C]] C = 28.68; Pt–Sn–C]] C = 9.38), in contrast to the twisted conformation adopted by the 7-membered ring in 6 and the envelope conformations of 2–4.2,3 In both 6 and 9, the Pt–N distance trans to tin is longer than that trans to methyl, as a result of the very high trans influence of tin.Complex 9 reacts catalytically with excess (Me2SnSe)3 and RCCR to give a mixture of products. When reactions were monitored by 1H NMR, 9 was shown to remain as the catalyst Scheme 1 R = CO2Me.1714 J. Chem. Soc., Dalton Trans., 1999, 1713–1715 “resting state”. The organoselenium complexes were separated chromatographically and identified as a mixture of the known6 selenole 11 and the bis(Z-alkenyl)selenium compound 12,4,7 but an organotin product, 13, shown to be present in the reaction mixture by its NMR spectra,4 was decomposed on the column and so was not isolated in pure form or structurally characterized. The same organoselenium and organotin compounds were formed stoichiometrically by reaction of 9 with excess alkyne but, in this case, a new organoplatinum complex 14 was also formed as shown in Scheme 2.Complex 14 is stable and fails to react with either (Me2SnSe)3 or excess alkyne; it is a unique organoplatinum(IV) complex in that it contains two methyl groups, an alkenyl and an alkynyl group and it has been characterized by its 1H and 13C NMR spectra and by a structure determination (Fig. 3).4,5 One methyl group and the alkenyl group are trans to nitrogen donors while the other methyl group and the alkynyl group are mutually trans. The formation of the alkenyl groups present in compounds 12 and 14 requires that an H-atom abstraction step must occur and, since the alkenyl proton is still observed in the 1H NMR when the reaction is carried out in deuteriated solvents such as CD2Cl2 or C6D6, the source of the H-atom in the CR]] CRH group must be one of the reagents used.The formation of 14 also requires cleavage of a C–C bond of the alkyne and the fate of the CO2Me fragment Fig. 1 The molecular structure of 6. Selected bond distances (Å): Pt– N(2) 2.156(3), Pt–N(1) 2.228(4), Pt–C(1) 2.085(5), Pt–C(2) 2.065(5), Pt– Sn(2) 2.5625(4), Sn(2)–Se(2) 2.5671(6), Se(2)–Sn(1) 2.5137(6), Sn(1)– C(3) 2.161(6), C(3)–C(6) 1.323(7), C(6)–Se(1) 1.919(4), Se(1)–Pt 2.5380(5).Bond angles (8): Pt–Sn(2)–Se(2) 114.52(2), Sn(2)–Se(2)–Sn(1) 103.19(2), Se(2)–Sn(1)–C(3) 112.4(1), Sn(1)–C(3)–C(6) 122.2(4), C(3)– C(6)–Se(1) 121.4(4), C(6)–Se(1)–Pt 106.4(1), Se(1)–Pt–Sn(2) 84.12(1). Fig. 2 The molecular structure of 9. Selected bond distances (Å): Pt– C(1) 2.062(9), Pt–C(2) 2.091(8), Pt–Sn(1) 2.5578(7), Pt–Se(1) 2.5303(9), Pt–N(1) 2.144(6), Pt–N(2) 2.244(7).Bond angles (8): Se(1)–Pt–Sn(1) 88.60(3), Pt–Sn(1)–C(7) 99.0(2), Sn(1)–C(7)–C(8) 120.6(6), C(7)–C(8)– Se(1) 109.4(5), C(8)–Se(1)–Pt 104.5(2). that is eliminated is unknown. It had been envisioned that the platinum complex 9 might catalyze the reaction of (Me2SnSe)3 with RCCR to give organotin metallacycles of the form {(Me2- SnSe)n(RCCR)m}, but the actual catalytic reactions are clearly more complex.The nature of the organoselenium products suggests that reaction of 9 with alkyne may be initiated by nucleophilic attack by selenium on the electrophilic alkyne, but the mechanisms of subsequent steps are still to be determined. This work is significant in showing that Sn–E bonds are strongly activated within organoplatinum metallacycles, that easy ring expansion and contraction can occur in reactions with an electrophilic alkyne, and that catalytic reactions may be developed. Acknowledgements We thank the NSERC (Canada) for financial support.Notes and references 1 See for example: P. W. Jennings and L. L. Johnson, Chem. Rev., 1994, 94, 2241. 2 L. M. Rendina, J. J. Vittal and R. J. Puddephatt, Organometallics, 1996, 15, 1749; M. C. Janzen, H. A. Jenkins, L. M. Rendina, J. J. Vittal and R. J. Puddephatt, Inorg. Chem., in the press. Fig. 3 The molecular structure of 14. Selected bond distances (Å): Pt–C(35) 2.01(1), Pt–C(28) 2.07(1), Pt–C(21) 2.09(1), Pt–C(27) 2.10(1), Pt–N(1) 2.134(8), Pt–N(12) 2.114(9).Scheme 2 R = CO2Me.J. Chem. Soc., Dalton Trans., 1999, 1713–1715 1715 3 N. Feeder, R. J. Less, J. M. Rawson and J. N. B. Smith, J. Chem. Soc., Dalton Trans., 1998, 4091. 4 Selected spectroscopic data: NMR in CD2Cl2 (refs. SiMe4, Me4Sn, K2PtCl4). 5: d(1H) 3.41 [s, 3H, b-CO2Me]; 2.80 [s, 3H, a-CO2Me]; 0.86 [s, 3H, 2J(PtH) = 59 Hz, Pt–Me]; 0.76 [s, 3H, 2J(SnH) = 46 Hz, Pt–Sn–Mea]; 0.59 [s, 3H, 2J(SnH) = 52 Hz, Sn–Mea]; 0.39 [s, 3H, 3J(PtH) = 4 Hz, 2J(SnH) = 46 Hz, Pt–Sn–Meb]; 0.31 [s, 3H, 2J(SnH) = 55 Hz, Sn–Meb]; 0.11 [s, 3H, 2J(PtH) = 62 Hz, Pt–Me]; d(119Sn) 6.81 [2J(SnSn) = 151 Hz, Pt–S–C]] C–Sn]; 281.45 [1J(PtSn) = 11 860 Hz, Pt–Sn]; d(195Pt) 21770 [1J(SnPt) = 11 860 Hz]. 6: d(1H) 3.43 [s, 3H, b-CO2Me]; 2.83 [s, 3H, a-CO2Me]; 0.87 [s, 3H, Pt–Sn–Me]; 0.81 [s, 3H, 2J(PtH) = 59 Hz, Pt–Me]; 0.70 [s, 3H, 2J(SnH) = 51 Hz, Sn–Mea]; 0.46 [s, 3H, 2J(SnH) = 44 Hz, Pt–Sn– Meb]; 0.34 [s, 3H, 2J(SnH) = 53 Hz, Sn–Meb]; 0.20 [s, 3H, 2J(PtH) = 62 Hz, Pt–Me]. 7: d(1H) 3.46 [s, 3H, b-CO2Me]; 2.88 [s, 3H, a-CO2Me]; 0.98 [s, 3H, 2J(SnH) = 56 Hz, Sn–Me]; 0.92 [s, 3H, Sn– Me]; 0.74 [s, 3H, 2J(PtH) = 60 Hz, Pt–Me]; 0.55 [s, 3H, 2J(SnH) = 45 Hz, Sn–Me]; 0.41 [s, 3H, 2J(SnH) = 52 Hz, Sn–Me]; 0.30 [s, 3H, 2J(PtH) = 61 Hz, Pt–Me]. 8: d(1H) 3.65 [s, 3H, CO2Me]; 3.61 [s, 3H, CO2Me]; 0.94 [s, 3H, 2J(PtH) = 61 Hz, 3J(SnH) = 6 Hz, Pt–Me]; 0.60 [s, 3H, 2J(SnH) = 49 Hz, 3J(PtH) = 6 Hz, Sn–Me]; 0.34 [s, 3H, 2J(SnH) = 55 Hz, 3J(PtH) = 4 Hz, Sn–Me]; 0.29 [s, 3H, 2J(PtH) = 57 Hz, Pt–Me]; d(119Sn) 22.1 [1J(SnPt) = 9904 Hz]. 9: d(1H) 3.66 [s, 3H, CO2Me]; 3.62 [s, 3H, CO2Me]; 1.03 [s, 3H, 2J(PtH) = 60.3 Hz, 3J(SnH) = 5 Hz, Pt–Me]; 0.58 [s, 3H, 2J(SnH) = 48 Hz, 3J(PtH) = 5 Hz, Sn–Me]; 0.34 [s, 3H, 2J(SnH) = 54 Hz, 3J(PtH) = 4 Hz, Sn–Me]; 0.32 [s, 3H, 2J(PtH) = 57 Hz, Pt–Me]; d(119Sn) 25.8 [1J(SnPt) = 10 031 Hz]. 10: d(1H) 3.65 [s, 3H, CO2Me]; 3.62 [s, 3H, CO2Me]; 1.03 [s, 3H, 2J(PtH) = 61 Hz, Pt–Me]; 0.55 [s, 3H, 2J(SnH) = 48 Hz, 3J(PtH) = 5 Hz, Sn–Me]; 0.34 [s, 3H, 2J(PtH) = 55 Hz, Pt–Me]; 0.32 [s, 3H, 2J(SnH) = 53 Hz, 3J(PtH) = 4 Hz, SnMe]. 12: d(1H) 6.46 [s, 2H, 3J(SeH) = 5 Hz, Se–C]] C–H]; 3.83 [s, 6H, CO2Me]; 3.75 [s, 6H, CO2Me]; MS: m/z = 366. 13: d(1H) 0.54 [s, 2J(SnH) = 66 Hz, MeSn]; no other proton resonances. 14: d(1H) 6.54 [s, 1H, 3J(PtH) = 80 Hz, Pt–C]] C–H]; 3.79 [s, 3H, CO2Me]; 3.68 [s, 3H, CO2Me]; 3.51 [s, 3H, CO2Me]; 1.30 [s, 3H, 3J(PtH) = 69 Hz, Pt–Me]; 20.04 [s, 3H, 3J(PtH) = 51 Hz, Pt–Me]; d(13C) 25.38 [1J(PtC) = 575 Hz, PtMe trans to N]; 3.88 [1J(PtC) = 477 Hz, PtMe trans to C]; 119.26 [1J(PtC) = 780 Hz, PtC(alkenyl)]; 142.17 [1J(PtC) = 901 Hz, PtC(alkynyl)]. 5 Crystal data: 6: C30H48N2O4PtSe2Sn2, M = 1091.09, triclinic, space group P1� , a = 10.8110(3), b = 12.5192(2), c = 18.975(3) Å, a = 78.895(1), b = 70.862(1), g = 86.721(1)8, V = 1889.60(7) Å3, Dc = 1.918 g cm23, Z = 2, T = 294 K, R = 0.0267, Rw = 0.0621, m = 6.96 mm21, 6616 independent reflections. 9: C28H42N2O4PtSeSn, M = 863.38, orthorhombic, Pca21, a = 30.899(6), b = 12.156(2), c = 17.311(4) Å, V = 6502(2) Å3, Dc = 1.764 g cm23, Z = 8, T = 150(2) K, R = 0.0293, Rw = 0.0630, m = 6.22 mm21, 6852 independent reflections. 14: C30H40N2O6Pt, M = 719.73, monoclinic, P2(1)/n, a = 10.2375(6), b = 16.035(1), c = 18.889(1) Å, b = 102.624(3)8, V = 3025.9(3) Å3, Dc = 1.580 g cm23, Z = 4, T = 150(2) K; R = 0.039, Rw = 0.0959, m = 4.68 mm21, 3057 independent reflections. CCDC reference number 186/1434. See http://www.rsc.org/suppdata/dt/1999/1713/ for crystallographic files in .cif format. 6 M. R. J. Dorrity, J. F. Malone, C. P. Morley and R. R. Vaughan, Phosphorus, Sulfur Silicon Relat. Elem., 1992, 68, 37. 7 The Z stereochemistry for 12 is defined from the magnitude of 3J(SeH) = 5 Hz; the coupling would be ca. 20 Hz in the E-isomer. C. Paulmier, Selenium Reagents and Intermediates in Organic Synthesis, Pergamon, Toronto, 1986. Communication 9/0299

ISSN:1477-9226    

DOI:10.1039/a902991e

出版商:RSC    

年代:1999

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DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 1717–1727 1717 Co-ordination chemistry of the [Re(NO)2(PR3)2]1 fragment: crystallographic and computational studies† Heiko Jacobsen, Katja Heinze, Angela Llamazares, Helmut W. Schmalle, Georg Artus and Heinz Berke Anorganisch-chemisches Institut, Universität Zürich, Winterthurerstr. 190, CH-8057 Zürich, Switzerland Received 19th February 1999, Accepted 8th April 1999 The cationic complexes [Re(NO)2(PCy3)2]1 I1 and [Re(NO)2(PR3)2L]1 [L = CO, R = Cy III1; L = C6H5CHO, R = Cy IV1; L = ONRe(NO)(PR3)2H, R = iPr V1] have been synthesized and their structures determined. The counter ion in all cases is [B{3,5-(F3C)2C6H3}4]2.Complex I1 adopts the C2v butterfly geometry, whereas III1 takes on a trigonal bipyramidal (TBP) co-ordination. In IV1 and V1 one of the nitrosyl ligands is strongly bent, and a shape analysis suggests that the co-ordination geometry of the [Re(NO)2(PR3)2L]1 core is best described as tetragonal pyramidal (TP).A computational study based on density functional theory showed how steric eVects due to the ligand L induce the NO bend, and subsequently lead to the change in co-ordination from TBP to TP. Examination of a series of model compounds [Re(NO)2(PH3)2L]1 showed further how the p donor and acceptor properties of the ligand L are reflected in the P–Re–P and N–Re–N angles of the complexes. The nitrosyl ligand 1 plays a special role in transition metal chemistry.It is capable of supporting diVerent oxidation states of the metal center via diVerent co-ordination modes, and has the capability to activate metal–ligand bonds. Prominent examples of the latter are nitrosyl substituted transition metal hydride complexes,2 in which the M–H bond shows an increased reactivity toward alkyne insertion and carbonyl reduction. In the context of structural chemistry and reactivity exploration in this class of compounds, we have prepared a series of mononitrosyl hydrido complexes containing various phosphorus donor ligands, as well as chromium,3 tungsten 4 and rhenium5 centers.The increased hydridicity 2 of these compounds has been probed by the interaction with acidic substrates. 6 We also directed our eVorts towards the synthesis of dinitrosyl hydride derivatives, which should possess even more activated metal–hydrogen bonds. In contrast to their carbonyl analogues [Mn(CO)3(PR3)2H], manganese complexes of the general formula [Mn(NO)2(PR3)2H] undergo facile insertions of polar unsaturated molecules.7 We then set out to extend this chemistry to the related rhenium complexes, and provided synthetic access to compounds of the type [Re(NO)2(PR3)2H].8 During the course of this work we were also able to isolate and characterize the 16 electron fragment [Re(NO)2(PCy3)2]1, as well as a variety of complexes of the type [Re(NO)2(PR3)2L]1.The present paper is mainly concerned with structural aspects of [Re(NO)2(PR3)2L]n1 complexes (n = 0 or 1).In particular, we want to address the question of how structural changes in the [Re(NO)2(PR3)2]1 fragment under co-ordination of a ligand L might provide information about the nature of the Re–L bond. The variation in the co-ordination geometry might further influence the reactivity of the species. The experimental part is complemented by a computational study based on density functional theory (DFT).9 The molecular and electronic structure of 12 model compounds were investigated, as displayed in Fig. 1. The calculations were implemented to support † Supplementary data available: optimized geometries and eigenvalues. For direct electronic access see http://www.rsc.org/suppdata/dt/1999/ 1717/, otherwise available from BLDSC (No. SUP 57540, 4 pp.) or the RSC Library. See Instructions for Authors, 1999, Issue 1 (http:// www.rsc.org/dalton). the results obtained from the X-ray crystallographic analyses, and to provide explanations for the observed structural features.Results and discussion Crystallographic studies We determined the crystal structures of [Re(NO)2(PCy3)2]- [BArF 4], and of the three [Re(NO)2(PR3)2L][BArF 4] complexes with L = CO, C6H5CHO, or ONRe(NO)(PR3)2H. Here, [BArF 4] stands for [B{3,5-(F3C)2C6H3}4]2. The anion is excluded from our discussion, which is focussed on the structural elements of the rhenium fragments which will be analysed together with those of Re(NO)2(PR3)2H.8 We shall refer to the metal fragments as [Re(NO)2(PCy3)2]1 I1, Re(NO)2(PiPr3)2H II, [Re(NO)2- (PCy3)2(CO)]1 III1, [Re(NO)2(PCy3)2(C6H5CHO)]1 IV1, and [Re(NO)2(PiPr3)2{ONRe(NO)(PiPr3)2H}]1 V1.Selected structural parameters for these complexes are presented in Table 1. For in-depth background information a reader should refer to the deposited crystallographic data. [Re(NO)2(PCy3)2]1 I1. A view of the molecular structure of complex I1 in the crystal is displayed in Fig. 2. It can be Fig. 1 The [Re(NO)2(PH3)2L]n1 model complexes n = 0 or 1. L Re ON ON PH3 n+ 1+ 2 3+ 4+ 5+ 6 7+ 8 9+ 10+ 11+ 12+ L – H – CO CN-CH3 PH3 CNNC- CH3 Cl- ON H2CO ONH ON-R' x z y R' = Re(NO)(PH3)2H PH31718 J. Chem. Soc., Dalton Trans., 1999, 1717–1727 Table 1 Selected bond lengths and angles a for [Re(NO)2(PR3)2L]n1 complexes (n = 0 or 1) I1 II III1 IV1 V1 R Cy iPr Cy Cy iPr L — H2 CO C6H5CHO ONR9 b Re–L — 177.93(2) 197.9(6) 218.8(3) 219.9(9) Re–P 245.4(3) 246.2(3) 242.8(2) 242.1(2) 247.4(2) 248.8(1) 248.4(1) 248.8(1) 247.2(4) 248.2(4) Re–N 173.5(10) 176.6(8) 180.4(7) 178.0(7) 179.0(7) 182.5(5) 175.8(4) 181.1(4) 178(1) 180(2) N–O 122.5(12) 118.0(11) 119.3(9) 122.7(9) 119.1(9) 117.6(1) 119.9(5) 120.4(5) 120(1) 117(2) P–Re–P 159.93(8) 153.89(6) 169.62(5) 158.40(4) 162.8(1) Re–N–O 166.9(9) 165.7(9) 173.1(8) 175.4(7) 174.0(6) 176.3(6) 150.9(3) 175.9(4) 158(1) 172(1) N–Re–N 115.9(4) 127.4(3) 121.5(3) 108.8(2) 111.4(6) Other L–Re–N C–O L–Re–N C–O L–Re–N O–N L–Re–N 122.3(3) 110.2(2) 114.6(8) 129.6(3) 108.9(3) 123.6(5) 91.6(1) 159.5(1) 125(2) 97.4(5) 150.7(5) a Distances in pm, angles in (8).b R9 = Re(NO)(PiPr3)2H. obtained almost quantitatively by the reaction of II with [(C6H5)3C][BArF 4] in benzene. The complex can be described as a distorted C2v butterfly fragment, which is obtained on removing one equatorial ligand from an ideal trigonal bipyramidal arrangement. Important geometric features are a P–Re–P angle some 208 smaller than the ideal value of 1808, and an N–Re–N angle close to 1208.The phosphorus atoms are bent away from the NO groups; the nitrosyl ligands themselves are not linearly co-ordinated but show a cisoid bend of about 158. Structures of a variety of [M(NO)2(PR3)2]n1 compounds (n = 0 or 1; M = Fe, Ru, Os, Co, Rh or Ir), are described in the literature.10 All these complexes having electron counts of 18 (or 17),10m exhibit the co-ordination geometry of a distorted tetrahedron,‡ and therefore cannot be compared with I1.However, the crystal structures of two isoelectronic carbonyl compounds are known, namely [Rh(CO)2{P(2,4,6-(MeO)3C6H2)3}2]1 and [Ru(CO)2(PtBu2Me)2].12,13 The latter complex 13 possesses the same C2v butterfly geometry as that of I1, whereas the former12 shows square planar co-ordination. Thus, it was not initially clear which geometry the fragment I1 might adopt. Fig. 2 Molecular structure of complex I1. Displacement ellipsoids are shown at the 30% level.Hydrogen atoms are omitted for clarity. Not shown are the counter ion and solvate molecules. ‡ For an orbital analysis of MX2(NO)2 systems by extended Hückel theory compare ref. 10(m). See also ref. 11. The main aspects of the Walsh diagram for the planar D4h into bent C2v transformation are established for ML4 complexes, 14a and Caulton and co-workers 13 have adapted this analysis for compounds of the type M(CO)2(PR3)2. The dz2 orbital is stabilized under bending, because of diminished overlap with the sCO lone pair, and because back bonding into p*CO is now possible.15 The dxz orbital is also stabilized by back donation in the bent geometry.On the other hand, the dyz orbital is strongly destabilized in the bent structure, due to diminished overlap with p*CO, and due to antibonding overlap with the sCO lone pair. The important interactions are shown below [adopted from ref. 13(a)] (see also Fig. 7). The antibonding interaction between dyz and sCO or sNO, respectively, can be reduced by a cisoid bend of the M–C–O or the M–N–O angle.This explains the observed non-linear coordination of the nitrosyl ligands in complex I1. The preferred geometry will be non-planar if the stabilization due to back donation outweighs the destabilizing interactions. The important criterion is the energetic match between the metal donor orbitals and the ligand p*XO (X = C or N) acceptor orbitals.13 In I1 the electron rich metal center Re2I possesses d orbitals which are at relatively high energies.These are energetically well suited for an interaction with the p*NO orbitals. Thus, I1 prefers the C2v butterfly geometry. The same holds for the neutral ruthenium complex [Ru(CO)2(PtBu2Me)2].13 In contrast, the low energy of the d orbitals of RhI in [Rh(CO)2{P(2,4,6- (MeO)3C6H2)3}2]1 decreases the role of back donation. This complex therefore adopts the square planar geometry.12 In a DFT calculation we have tried to optimize the square planar geometry of the model complex [Re(NO)2(PH3)2]1 11.This could only be achieved by employing angular constraints, and enforcing a planar co-ordination environment, which indicates that planar 11 is not a local minimum on the potential energy surface. This hypothetical molecule should have a triplet state, since one of the rhenium d-based orbitals and one combination of NO p* orbitals are accidentally degenerate.[Re(NO)2(PiPr3)2H] II. The preparation and the structure of complex II, as shown in Fig. 3, have already been discussed,8 d z 2 - p*CO y z d yz - sCOJ. Chem. Soc., Dalton Trans., 1999, 1717–1727 1719 and we will only briefly comment on its geometry. The structure is that of a distorted trigonal bipyramidal (TBP). Compared to I1, we observe that the N–Re–N angle opens up by about 118 under co-ordination of the hydride ligand. At the same time, P–Re–P becomes narrower by 68.The bending distortion of the phosphorus donor ligands is well understood.14 Bending of the PR3 groups of [Re(NO)2(PR3)2L]n1 towards the ligand site polarizes the dxz orbital of the metal in the direction of the p accepting nitrosyl ligands, providing better dxz–p*NO overlap, and enhancing the amount of back donation to NO. The degree of back bending of the PR3 is limited by steric repulsion between PR3 and L, and between the phosphorus ligands themselves. The small hydride ligand does not provide much steric hindrance for the bulky PiPr3 group, but it does increase the electron density on the rhenium center.Back bonding to the nitrosyl ligands becomes stronger, and as a consequence the P–Re–P angle decreases. [Re(NO)2(PCy3)2(CO)]1 III1. Reaction of complex I1 with the prototypical p acceptor ligand CO leads to formation of III1. Its molecular geometry in the crystal is displayed in Fig. 4. One of the PCy3 ligands is highly disordered, but was resolved in the course of the structure refinement.The N–Re–N angle is still larger than that in the free fragment I1, but the P–Re–P angle opens up by 108 (see Table 1). Since CO is competing with the NO ligands for back donation, a strong polarization of dxz away from the L site is no longer favorable, and consequently P–Re–P opens up. The fact that CO is competing for electron density manifests itself also in a small N–O bond contraction Fig. 3 Molecular structure of complex II.Displacement ellipsoids are shown at the 40% level. Hydrogen atoms are omitted for clarity, except for the hydride ligand, which is displayed as a sphere with arbitrary size. PH3 PH3 PH3 PH3 x z y [compare d(N–O) in I1 and III1], and a small C–O bond elongation [compare to d(C–O) = 112.8 pm in the gas phase 16]. [Re(NO)2(PCy3)2(C6H5CHO)]1 IV1. Compound IV1 is instantaneously formed when I1 is treated with benzaldehyde. In this complex a new structural motif is introduced.The benzaldehyde L does not bind in a symmetrical, but rather asymmetrical fashion, as can be seen in Fig. 5. The pseudo C2 rotational axis is removed, and only idealized Cs symmetry is retained. Characteristic bond lengths such as Re–N and Re–P are very similar in III1 and IV1, but the co-ordination geometry is very diVerent. The N–Re–N angle is now smaller than that of the free fragment I1. Furthermore, one of the nitrosyl ligands (N2O2 in Fig. 5) is strongly bent forming a Re–N–O angle of 1518.This falls right between the linear co-ordination of the 3e2 donor NO1 (M–N–O 1808) and the bent co-ordination of the 1e2 donor NO2 (M–N–O 1208). Two very diVerent L–Re–N angles are observed, one being close to 908 and the other being about 1608. Thus, the co-ordination geometry of IV1 resembles Fig. 4 Molecular structure of complex III1. Details as in Fig. 2. Fig. 5 Molecular structure of complex IV1. Details as in Fig. 2.1720 J. Chem. Soc., Dalton Trans., 1999, 1717–1727 Table 2 Observed d angles a for [Re(NO)2(PR3)2L]1 complexes, together with values b for idealized polyhedra.Also given are the standard deviations s(G) (see text for definition) Complex Ideal TBP (D3h) III1 IV1 V1 Ideal TP (C4v) d(a1) 101.5 115.5 124.7 120.3 119.8 d(a2) 101.5 111.9 79.1 85.2 75.7 d(a3) 101.5 98.3 127.6 123.8 119.8 d(a4) 101.5 115.3 126.1 122.8 119.8 d(a5) 101.5 110.9 77.1 80.9 75.7 d(a6) 101.5 99.7 127.7 123.5 119.8 d(e1) 53.1 26.6 68.3 65.5 75.7 d(e2) 53.1 52.7 59.0 52.4 75.7 d(e3) 53.1 44.0 5.6 5.9 0.0 s(D3h) 0.0 12.4 26.1 23.2 26.9 s(C4v) 26.9 30.4 7.9 9.7 0.0 a In 8.b From ref. 18(b). more closely that of a tetragonal pyramid (TP) than that of a trigonal bipyramid (TBP). The infrared spectrum shows however, both in solution and in the solid state, a group of peaks in the carbonyl–nitrosyl region that could not be assigned (see Experimental section). We can envisage this structural change as follows:§ co-ordination of benzaldehyde L to the open coordination site of the d8-Re(NO)2(PR3)2 fragment induces a bend in one nitrosyl ligand.This subsequently leads to a formal oxidation of the metal center, resulting in a d6-Re(NO)2(PR3)2L species, which is still co-ordinatively and electronically unsaturated. The preferred geometric arrangement of a d6-MX5 fragment is tetragonal pyramidal. The origin of this distortion will be analysed at a later point. [Re(NO)2(PiPr3)2{ONRe(NO)(PiPr3)2H}]1 V1.The last complex we include in this section can be described as an adduct of the type [I1]V1[II]V1 (the nomenclature [X]Y stands for a fragment having the structure of X but with the geometric parameters as found in the molecule Y). It is prepared by the reaction of [(C6H5)3C][BArF 4] on II in a 1 : 2 ratio. The geometry in the crystal is displayed in Fig. 6. The oxygen of one of the nitrosyl groups of II is apparently a Lewis base strong enough to interact with other Lewis acids, such as I1 or BF3.8 The geometry of the [I1]V1 fragment is similar to that of IV1, and might also be described as a tetragonal pyramid.The bend of one of the NO ligands, however, is not as prominent as in IV1, and the L–Re– N angles are also somewhat closer to the value of 1208 of the ideal trigonal bipyramid (see Table 1). The geometry of the [II]V1 fragment is related to that of II. A major diVerence is an even smaller P–Re–P angle of 1418. One of the nitrosyl oxygen Fig. 6 Molecular structure of complex V1. Displacement ellipsoids are shown at the 20% level. Hydrogen atoms are omitted for clarity, except for the hydride ligand, which is displayed as a sphere with arbitrary size. Not shown is the counter ion. § The principal orbital interactions for five- and six-co-ordination have been investigated by HoVmann and co-workers in a classical series of papers, see refs. 15 and 17. atoms of [II]V1 functions as a Lewis base, which leads to electron depletion at this particular nitrosyl ligand.This in turn can be counteracted by an eVective back donation, which is made possible by the narrowing of the P–Re–P angle (see above). We have found a similar eVect for the complex [Re(H)(NO)- (NOBF3)(PiPr3)2].8 The Re and the bridging NO are not coplanar; the Re1–O3–N3–Re2 dihedral angle amounts to 1398. It was mentioned that the co-ordination geometries of both complexes IV1 and V1 are closer to a TP than to a TBP coordination. To put this argument on more quantitative grounds, we follow the approach of Muetterties,18 and obtain a measure of shape for these aggregates by means of the dihedral angles d formed by the normals to adjacent faces of a given polytopal form.The three five-co-ordinated molecules described for the first time in this work can then be compared to the idealized geometries of a D3h trigonal bipyramid and a C4v tetragonal pyramid, as shown below [adopted from ref. 18(b)]. The molecules are oriented such that the phosphorus ligands occupy the A1 and A2 positions. For III1, the CO ligand is chosen to occupy the E2 position, whereas for IV1 and V1 the bent nitrosyl ligand is placed at E2. The results of the shape analysis for the rhenium complexes together with values for the ideal co-ordination polyhedra as defined by Muetterties 18 are collected in Table 2. The values of the shape determining angles d(en), especially that of d(e3), and the fact that two of the d(an) angles, namely d(a2) and d(a5), are significantly smaller than the remaining members of the set, all indicate that IV1 and V1 are indeed closer to the C4h-TP in coordination geometry.For III1, the d(an) angles span a smaller range of values, and its co-ordination geometry is related to that of the D3h-TBP. Also, the standard deviations s(G), eqn. (1), lead to the same conclusion that the co-ordination s(G) =÷1 9 o9 n = 1 ((dn)exp 2 (dn)G)2 (1) polyhedra for IV1 and V1 match closer the tetragonal pyramid, and that III1 can be described as a trigonal bipyramid (compare Table 2).Computational studies We divide the twelve model complexes as presented in Fig. 1 into two groups. Symmetric complexes 11–91 are characterized by ligands L, which possess higher symmetry than Cs, whereas E3 E1 E2 A1 A2 E1 A2 E3 a1 a3 a6 a4 e3 e1 e2 a5 a2 C4v D3h A1 E2J. Chem. Soc., Dalton Trans., 1999, 1717–1727 1721 Table 3 Optimized geometries a for Cs-symmetric [Re(NO)2(PH3)2L]n1 model complexes (n = 0 or 1) Complex 11 23 1 41 51 67 1 89 1b L —H 2 CO CNCH3 PH3 CN2 NCCH3 Cl2 ON Re–L — 172.4 200.9 206.5 248.8 210.7 213.8 249.2 216.6 Re–P 245.7 239.1 247.0 245.4 245.6 242.5 245.7 243.3 247.9 247.7 Re–N 180.3 182.5 183.9 183.0 182.4 182.2 181.5 180.7 181.1 180.7 N–O 118.3 120.3 117.6 118.3 118.4 119.9 118.7 120.4 118.2 118.1 P–Re–P 160.8 151.7 174.0 173.2 176.1 160.9 173.5 161.2 172.6 Re–N–O 161.9 174.1 175.2 172.8 169.9 171.7 168.3 166.6 166.5 167.0 N–Re–N 118.5 126.6 125.9 123.1 121.6 121.6 116.9 115.1 110.9 Other C–O C–N H–P–H C–N N–C O–N Re–O–N 115.3 117.2 98.2 117.6 116.3 119.4 179.5 a Distances in pm, angles in 8.b Unrestricted calculation without symmetry constraints on the doublet state. Table 4 Optimized geometries a for asymmetric [Re(NO)2(PH3)2L]1 complexes Complex 101 trans-111 cis-111 121 L H2CO ONH ONH ONR9 b Re–L 222.0 211.2 211.2 222.2 Re–P 246.1 247.4 247.6 245.0 Re–N 179.3 184.6 182.5 181.7 180.8 185.9 179.3 183.9 N–O 118.2 119.3 118.2 118.6 119.1 118.3 119.1 119.5 P–Re–P 167.7 176.8 169.9 160.6 Re–N–O 176.9 147.8 164.6 172.6 145.8 180.0 177.7 149.4 N–Re–N 108.5 121.5 109.6 108.2 Other C–O L–Re–N O–N O–N–H L–Re–N O–N O–N–H L–Re–N O–N L–Re–N O–N–R9 123.5 95.3; 156.2 126.6 104.4 115.6; 122.9 125.2 107.1 156.9; 93.5 126.0 154.3 97.5 126.0 a Distances in pm, angles in 8.b R9 = Re(NO)(PH3)2H. in the asymmetric complexes 101–121 the ligands are considered to be mirror symmetric.Selected geometric parameters are presented in Tables 3 and 4, respectively. If we compare the optimized geometries of compounds 11, 2 and 31 with the crystal structures of I1, II and III1 we find reasonable agreement between experiment and theory. The general trends are well reproduced in the calculations, e.g. a shortening of Re–P and an increase in N–Re–N when going from I1(11) to II(2). The Re–N separation is generally overestimated in the calculations by about 6 pm.As a consequence, due to a reduced back bonding, the N–O distance falls somewhat short in comparison to the experiment. Surprisingly, the simple model phosphine PH3 reproduces the co-ordination geometry of the phosphorus ligands extremely well, especially where the Re–P distances are concerned. Also, 101 and 121 seem to be good models for complexes IV1 and V1, respectively. The calculation predicts the asymmetric co-ordination with two diVerent nitrosyl ligands as found in the experiment. The co-ordination geometry of the nitrosyl ligand is in satisfactory accordance to the crystal structure, and the angles P–Re–O and N–Re–N are also close to within 28.Influence of the phosphorus donor. The reasonable close agreement between the calculated and observed P–Re–P angle of the symmetric complexes suggests that this parameter is not overly dependent on the nature of the R group of the PR3 ligand. Instead, the right polarization of the dxz orbital needed to achieve an optimum ratio of back bonding between the NO and L ligands to first order determines the degree of PR3 bending (see above).The diVerent donor capability however influences the electron densities at the Re, and therefore to a certain extent the geometric arrangement of the ligands in the yz plane. This might explain the somewhat larger deviation between theory and experiment in the Re–N distances. We further checked the influence of the P–Re–P angle on the co-ordination of the NO and L ligands by restricted geometry optimizations for complexes 11–8, in which P–Re–P was fixed at 150 and 1708, respectively. In all cases, only marginal geometric diVerences compared to the fully optimized species were found.The potential energy surface for the P–Re–P bend is very shallow, and the angle bending does not require much energy. As an example, we provide two cases, beginning with 2, the angle P–Re–P fixed at 1708.For the 188 distortion from the calculated equilibrium geometry, an energy of only 11 kJ mol21 is needed. The average deviation between selected bond distances and angles amounts to 0.4 pm and 1.08. For 51 fixed at 1508, narrowing the P–Re–P requires 38 kJ mol21. The selected angles change on average by 0.68, and the bond distances (Re–L excluded) by 0.3 pm. The Re–L bond in 51 (1508) is elongated by 3.2 pm. This is easily explained by keeping in mind that diminishing the P–Re–P angle leads to a polarization of dxz away from L, and thus to a reduced back bonding to the PH3 ligand in equatorial position.This in turn weakens and lengthens the P–Re bond. Additional information on structures and energies of the restricted geometry complexes can be found in SUP 57540. The P–Re–P size allows us to weigh the importance of pxz back bonding to L. For [Re(NO)2(PR3)2L] complexes in which P–Re–P is about the same size or smaller than in the Re(NO)2(PR3)2 fragment, this back-bonding interaction is of no or only minor importance.This is naturally the case for L = H2, 2, and Cl2, 8, but also for CN2, 6. On the other hand, when P–Re–P is substantially larger than in the free fragment, pxz back bonding is of importance, as it is for L = CO, 31, CNCH3, 41, NCCH3, 71, and also for PH3, 51. This argument is based on qualitative considerations, and it does not allow one to infer direct correlation between the amount of p back bonding and the P–Re–P angle.Re-dxz Interactions with other ligand based orbitals, as well as interactions involving Re-dyz, further influence P–Re–P and the amount of p back donation to the equatorial ligands. Dependence of the angle N–Re–N on the nature of L. We already mentioned the important orbital interactions which determine the size of N–Re–N in relation to the problem of the ground state geometry of [Re(NO)2(PCy3)2]1 I1. We will now1722 J. Chem. Soc., Dalton Trans., 1999, 1717–1727 Table 5 Composition of the three highest occupied orbitals of the complexes 2, 31 and 8 at their equilibrium geometry Complex Symmetry Re (%) NO (%) L (%) 2 31 8 1b1 1a1 1b2 1b1 1a1 1b2 1b1 1a1 1b2 px 5 pz 11 py 9 pz 9 py 9 pz 6 py 7 dxz 48 dz2 7 dyz 31 dxz 60 dz2 18 dyz 34 dxz 37 dz2 19 dyz 19 dx2 2 y2 8 dx2 2 y2 5 dx2 2 y2 7 N px 9 N py 8 N pz 15 N px 5 N py 8 N pz 12 N px 7 N py 9 N py 6 N pz 13 N pz 13 N pz 6 N pz 15 O px 24 O py 16 O pz 29 O px 14 O py 15 O pz 22 O px 19 O py 25 O py 11 O pz 21 O pz 22 O pz 13 O pz 22 H s 9 C px 3 C s 3 C py 5 Cl px 32 Cl pz 12 Cl py 13 O px 6 C pz 1 O py 7 discuss this structural parameter in more detail, and address the question how diVerent types of ligands L may influence N–Re– N in [Re(NO)2(PR3)2L]n1 complexes.We have chosen L to be a s donor, p acceptor, or a p donor ligand. The representative model compounds which we will analyse in detail are then 2, 31 and 8. The highest three occupied molecular orbitals for these complexes are displayed in Fig. 7. The basic composition of these MOs is similar in all three compounds, and a detailed breakdown is presented in Table 5. The metal contribution to the HOMO-2, 1b1, is mainly Re-dxz. Back bonding to the p*NO,xz orbitals increases, when P–Re–P is diminished, and 1b1 is lowered in energy. There is no contribution from H2 to 1b1 in 2. For 31 a p*CO,xz acceptor orbital is combined with Re-dxz in a bonding fashion, whereas for 8 we have antibonding interaction between the metal based orbital and a filled px,Cl2 orbital.Not shown in Fig. 7 are contributions of the PH3 ligand to 1b1. Their importance has been discussed in the previous sections. In orbital 1a1 back bonding occurs from the metal Re-dz2 to the p*NO,yz orbitals. Again, the overlap increases when P–Re–P becomes smaller. The contributions from L to 1a1 are in all cases s antibonding; the ligand orbitals involved are sH2 2, sCO 31 and pz,Cl2 8.Lastly, back bonding to p*NO,yz is also possible from Re-dyz, as realized in 1b2. In contrast to 1a1, the overlap is now lessened when P–Re–P decreases. As in the case of 1b1, there is no con- Fig. 7 Sketches of the three highest molecular orbitals of complexes 2, 31 and 8. Not shown are contributions due to the phosphorus donor ligands. H Re N N O O CO Re N N O O Cl Re N N O O 1 b1 1 a1 1 b2 y z 2 1 b1 1 a1 1 b2 3+ 1 b1 1 a1 1 b2 8 tribution from H2, 2, a bonding interaction with p*CO,yz, 31, and an antibonding interaction with py,Cl2, 8.Our analysis shows that two metal d orbitals compete for back bonding to the p*NO,yz orbitals, namely dz2 in 1a1 and dyz in 1b2.¶ However, these interactions show a diVerent nature in their dependence on the angle N–Re–N. In the former case the metal–ligand overlap increases when N–Re–N decreases, whereas for the latter the opposite trend holds. The relative importance of these two interactions will determine the size of N–Re–N.The Walsh diagram along the N–Re–N bending mode for the three highest occupied orbitals for complexes 2, 31 and 8 is displayed in Fig. 8. The energy curve for orbital 1b1 looks similar in all three cases; the interaction of dxz with p*NO,xz is mainly influenced by the phosphorus donors, and only to a minor degree by the nature of the ligand L. This orbital serves as a reference point for the comparison of the relative energies of the orbitals amongst the diVerent systems (due to the cationic nature of 31, its orbitals are at considerably lower energies than those of 2 and 8).The energy dependence of 1a1 and 1b2 follows the expected trend in all three cases, but there are some important diVerences. For 2, we find orbital crossing of 1a1 and 1b2 around 1208, close to the value of N–Re–N in the “free” fragment 11. Distortion of this angle leads to a stabilization of the HOMO-1, which is 1a1 when N–Re–N decreases, or 1b2 when N–Re–N increases.The orbital coeYcient of the metal based d orbitals in 1a1 is smaller when compared to Re-dyz in 1b2. In the case of 1a1, this is due to the antibonding interaction between the d orbitals and sH2. Consequently, back bonding is more eYcient in 1b2, and when complex 2 is formed from the fragments N–Re–N will open up to lower the energy of 1b2, and to increase this particular interaction. We encounter a similar situation for complex 31. Again, we see the destabilizing s interaction in 1a1, which leads to orbital crossing at around 1208.Again, the metal d contributions are smaller in 1a1 than in 1b2, so that an increase in N–Re–N maximizes the bonding energy. The picture emerged so far changes, when considering the p donor Cl2. In complex 8 both orbitals 1a1 and 1b2 undergo antibonding interaction with occupied pCl2 orbitals; 1b2 is significantly destabilized when compared to 1a1, and the orbital crossing occurs at an angle of around 1358, far from the free fragment.At the N–Re–N value of 11, orbital 1a1 now provides the main backbonding interaction, so that in this case N–Re–N is diminished, to maximize overlap and bonding energy. To sum up our analysis, we might say that in [Re(NO)2- (PR3)2L]n1 complexes, when L is a pure s donor or a p acceptor, the value of N–Re–N is larger than that of the free fragment [Re(NO)2(PR3)2]1. In contrast, if L is a p donor, we expect to find a decrease in N–Re–N. This might allow us to judge the relative importance of p acceptor vs.p donor interaction. From ¶ Strictly speaking, the metal d contribution in 1a1 is a mixture of dz2 and dx2 2 y2, and in complex 2 both components are of equal importance. For the sake of convenience, we keep referring to dz2 in the 1a1 case since only this orbital participates in the s antibonding interaction with L; further details are to be found in Table 4.J. Chem. Soc., Dalton Trans., 1999, 1717–1727 1723 the values presented in Table 2, we see that, in addition to H2 and CO, NCCH3, PH3 and CN2 also show an increase in N–Re–N.Interestingly, for the acetonitrile complex we find a smaller value for this angle, which might indicate that in this case p-acceptor interaction is of only minor importance. The same holds true for the isonitrosyl ligand NO. Before continuing our discussion, we should explain why we included the somewhat unusual isonitrosyl ligand in the list of our model compounds. Initially, we wanted to find a simple model for complex V1 in order to investigate the nature of the NO bend and the unusual co-ordination geometry.To probe the influence of p donation on the co-ordination geometry of the nitrosyl ligands, we provided for starting geometries 8 and 91, in which the Re(NO)2(PR3)2 fragment adapted a similar arrangement to that found in the crystal structures of IV1 and V1. All attempts to optimize such an asymmetric structure, however, converged to the symmetric co-ordination geometry of 8 or 91.This was a first indication that no orbital eVect is probably responsible for the particular co-ordination geometry of V1. We then extended our calculations to the asymmetric complexes 101–121, and also considered steric eVects in our analysis. These results are presented in the next paragraph. Fig. 8 Walsh diagram along the N–Re–N bending mode for (a) complex 2, (b) 31 and (c) 8. Origin of the NO bend.The formaldehyde compound 101 already provides a good model of the benzaldehyde complex IV1. The calculation satisfactorily reproduces the main structural features of the experimentally determined structure. One of the NO ligands is bent by about 308, and the co-ordination geometry falls between TBP and TP (see data in Tables 1 and 4). The calculations on the hypothetical nitroso hydride 19 complex 111 provide an initial clue as to why one of the NO ligands deviates from a linear co-ordination geometry.For HNO two diVerent co-ordination geometries are possible, the first in which the hydrogen points away from the metal fragment, trans- 111, a second in which it is directed toward one of the NO ligands, namely cis-111. As can be seen from the data in Table 4, trans-111 adopts a co-ordination geometry close to that of TBP, Fig. 9 Molecular structures along the transformation pathway trans- 111 æÆ cis-111. The PH3 groups are omitted for clarity. See text for further details.1724 J.Chem. Soc., Dalton Trans., 1999, 1717–1727 whereas cis-111 displays the distorted TBP–TP arrangement, as found in 101 or IV1 (see also Fig. 9). To analyse the origin of this distortion we performed calculations for hypothetical molecules on the pathway trans-111Æ cis-111. Beginning with the fully optimized geometry of trans- 111, we introduce a hydrogen flip by a 1808 rotation around the ON axis of the nitroso hydride ligand, while keeping all other geometric parameters fixed.We then allow for the NO bend to adapt to the value of cis-111. Finally we let the complex relax to the fully optimized asymmetric geometry cis-111. This transformation is illustrated in Fig. 9. The corresponding orbital energy diagram of the highest three occupied orbitals is presented in Fig. 10. In light of this analysis it appears as though the symmetric cis structure should be the most stable one, and the geometry distortion to the final structure of cis-111 should not seem obvious.As anticipated, no orbital eVect is clearly responsible for the observed modification in complex geometry when the coordination of the HNO ligand is changed from trans to cis. We extended our analysis also to include steric eVects, and essentially decomposed the total bonding energy TBE of a given molecule into components due to repulsive steric interaction, DE0, and attractive orbital interaction, DEint.20 The energy decomposition along the pathway trans-111Æcis-111 is presented in Fig. 11. The energy contributions of trans-111 are set at zero. Hydrogen flipping leads to an energetic stabilization due to electronic interactions. As can be observed in Fig. 10, all the three dxz, dyz and dz2 orbitals are lowered in energy.|| However, we also find a considerable increase in steric repulsion, so that, as a net eVect, the hydrogen flip destabilizes the molecular arrangement by 20 kJ mol21. The NO bend now decreases the steric repulsion from 78 to 66 kJ mol21.The orbital interaction energy however is diminished, since the now partially oxidized metal center has an unfavorable TBP co-ordination geometry. In the last step the geometry relaxes from TBP to TP, which eVectively enhances the electronic interaction and further reduces the steric repulsion. Our analysis shows that the hydrogen of the HNO ligand, when pointing towards one nitrosyl group, leads to an increase in DE0. Bending of the aVected NO minimizes steric repulsion, and further rearrangement to the TP geometry maximizes electronic interaction.The same structural element, a hydrogen pointing toward a nitrosyl ligand, can be found in the case Fig. 10 Orbital energy diagram for the three highest occupied orbitals along the path trans-111 æÆ cis-111. || We adopt a simplified classification of the orbitals according to the Re-d contributions. To not confuse the reader, we prefer to keep the nomenclature as it was established for C2v symmetry, although the correct classification for the HOMO to HOMO-2 should be dx2 2 y2, dxy and dyz.Furthermore, in some cases we have substantial mixing between dx2 2 y2 and dxy. of the formaldehyde or benzaldehyde ligands in 101 or IV1, respectively. This hydrogen then induces the same structural changes discussed for the hypothetical nitroso hydride complex 111. The last question we want to address is whether or not steric repulsion is also responsible for the geometric distortion encountered in complex V1.To this end, we performed a bonding analysis of the model compound 121, by building up the final complex from the constituting fragments 11 and 2, eqn. (2). The energy associated with eqn. (2) is the so-called H(NO)(PH3)2ReNO 1 [Re(NO)2(PH3)2]1 æÆ 2 11 [H(NO)(PH3)2ReNOÆRe(NO)2(PH3)2]1 (2) 121 bond snapping energy BEsnap,21 since the fragments have already been promoted from their ground state geometry to the one they adopt in the final complex; BEsnap can again be broken down into steric and electronic contributions, eqn.(3). The BEsnap = 2[DE0 1 DEint] (3) bond analysis was performed not only for 121, but for a symmetrical compound sym-121 as well, which was constructed by adopting structural features from 121 [geometry of the H(NO)(PH3)2ReNO fragment and the phosphorus donor ligands, N–Re–N] and 91 (O–N–Re). The geometries of both model complexes are shown in Fig. 12, and the results of the bonding analysis are collected in Table 6.For the two coordination geometries the electronic interaction energy is virtually identical. Again, a reduced steric repulsion in 121 favors the Re–L bond in the asymmetric compound by 16 kJ mol21. In this section we have elucidated the role of DE0 in the co-ordination geometry of [Re(NO)2(PR3)2L]n1 complexes. In asymmetric co-ordination geometries one of the NO ligands bends to reduce steric repulsion between [Re(NO)2(PR3)2]1 and the ligand L.Noteworthy is the fact that this bending distortion does not require much energy; DEbend can be estimated as about 20 kJ mol21. The NO ligand seems to be very flexible in adapting to the right co-ordination geometry and eVectively minimizing steric repulsion; this is evident not only in the TBE–TP geometries of IV1 and V1, but also in the strong cisoid bends encountered in I1. Conclusion The co-ordination chemistry of the 16e2 fragment [Re(NO)2- (PR3)2]1 11 has been explored by means of crystal structure analyses and DFT calculations.The ion possesses a C2v butterfly ground state geometry. This arrangement could be Fig. 11 Energy decomposition along the path trans-111 æÆ cis-111. The total bonding energy (d, TBE) is divided into steric (j, DE0) and electronic (r, DEint) contributions. See text for further details.J. Chem. Soc., Dalton Trans., 1999, 1717–1727 1725 rationalized by simple arguments based on orbital interactions, similar to those employed for the isoelectronic compound [Ru(CO)2(PtBu2Me)2].13 The structural changes of the [Re(NO)2(PR3)2]1 under formation of [Re(NO)2(PR3)2L]n1 complexes (n = 0 or 1) have been used to characterize the nature of the Re–L bond.The angle P–Re–P is determined by the competition for p-back bonding between the nitrosyl groups and the ligand L. In symmetric complexes a new orbital eVect was found to determine the size of the N–Re–N angle.When L is a pure s donor or p acceptor the value of N–Re–N is larger than that of 11. In contrast, if L is a p donor, we expect to find a decrease in N–Re–N. In asymmetric complexes it was shown that the driving force in bending of one of the NO groups and the subsequent distortion from a TBP to a TBP–TP is the minimization of steric repulsion. We have also seen that this rearrangement is accompanied by only small changes in the bonding energy, and that the NO ligand is very flexible in adapting its co-ordination geometry to changes in electronic structure or steric influences. This might entail important implications for the chemistry and reactivity of 11.In this work we have investigated the structural and static features of the co-ordination chemistry of the [Re(NO)2- (PR3)2]1 fragment. This study is intended to provide a basis for a better understanding of the reactivity and dynamic features of this transition metal complex. We are currently investigating the potential of 11 as an eVective catalyst in hydrogenation and hydrosilation reactions.22 From a theoretical point of view, the nature of the intramolecular interaction between the bending nitrosyl groups and the ligand L provides an interesting challenge.Further investigations might reveal whether or not intramolecular hydrogen bonding can indeed be related to the phenomenon of NO bending. Fig. 12 Molecular structures of complex 121 and sym-121 in the plane of the NO ligands; PH3 groups are omitted for clarity.Table 6 Bond analyses a for the model complexes 121 and sym-121 DE0 DEint BEsnap 121 38 2213 175 sym-121 54 2212 158 a In kJ mol21. Experimental All operations were carried out under a nitrogen atmosphere using standard Schlenk and glove-box techniques. Solvents were dried over sodium diphenylketyl [THF, Et2O, O(SiMe3)2, hydrocarbons] or P2O5 (CH2Cl2) and distilled under N2 prior to use. The deuteriated solvents used in the NMR experiments were dried over sodium diphenylketyl (C6D6, toluene-d8, THF-d8) or P2O5 (C6D5Cl, CD2Cl2) and vacuum transferred for storage in Schlenk flasks fitted with Teflon stopcocks.All NMR experiments were carried out on a Varian Gemini 300 spectrometer. Chemical shifts are given in ppm. The 1H and 13C-{1H} NMR spectra were referenced to the residual proton or 13C resonances of the deuteriated solvent, 31P chemical shifts externally referenced to 85% H3PO4 sealed in a capillary and inserted into a standard 5 mm NMR tube filled with the deuteriated solvent.The IR spectra were recorded on a Bio-Rad FTS-45 spectrometer. The complex [Re(NO)2(PiPr3)2H] II was prepared according to a reported procedure.9 Benzaldehyde was purchased from Fluka (puriss.), degassed and used without further purification. For the crystal structure analyses, the diVraction data were collected on an image plate detector system (STOE IPDS) for complexes I1 and III1, and on a four circle diVractometer (upgraded Nicolet R3) for IV1 and V1.The X-ray generators were equipped with sealed tubes and graphite monochromators (Mo-Ka, l = 0.71073 Å). All crystals were mounted on glass rods or on top of glass capillaries using silicon grease (IV1, V1) or covered with perfluoro polyether oil (I1, III1). Programs used for cell refinement, data collection and data reduction: CELL,23 EXPOSE,23 INTEGRATE,23 XRED23 and XDISK;24 for absorption correction, numerical,25 XRED (I1, III1), and semiempirical based on y-scan data, XEMP (V1).24 Structure solution was done with SHELXS 9726 (I1, III1) and SIR 92 27 (IV1, V1).Structure refinement was done with SHELXL 9728 (I1, III1) and CRYSTALS 9629 (IV1, V1). All positions of the hydrogen atoms, except for the hydride of V1, were calculated at distances relevant for the measuring temperature, and were placed geometrically for each refinement cycle. Complexes I1 and III1 were refined on Fo 2 using all unique reflections, applying an empirical weighting scheme;28 IV1 and V1 were refined on Fo using reflections with I > s(I ), and a Chebyshev polynomial weighting scheme.30 Molecular graphics were done with PLATON 97.31 CCDC reference number 186/1421.See http://www.rsc.org/suppdata/dt/1999/1717/ for crystallographic files in .cif format. Preparations [Re(NO)2(PCy3)2][BArF 4]. A heterogeneous mixture containing [Re(NO)2(PCy3)2H] (150 mg, 0.189 mmol) and [Ph3C]- [BArF 4] (209 mg, 0.189 mmol) in C6H6 (15 mL) was stirred for 2 h.During this period a dark red oily solid forms. The solvent was removed in vacuo until ca. 3 mL of C6H6 were left, and then pentane was added (15 mL). The liquid was discharged and the solid washed with additional pentane (3 × 15 mL) and dried in vacuo to give 290 mg of I1[BArF 4] (91.9%). Crystals for the X-ray diVraction study were grown by cooling slowly, starting at 90 8C, a saturated C6H6 solution of the complex.IR(Nujol): nNO 1711m and 1649s cm21. 31P-{1H} NMR (C6D5Cl): d 46.5 (s). 1H NMR (C6D5Cl): d 8.10 (m, br, 8 H, BArF 4), 7.47 (m, br, 4 H, BArF 4) and 2.30–0.6 (m, 66 H, PCy3) (Calc. for C68H78BF24- N2O2P2Re: C, 48.90; H, 4.71; N, 1.68. Found: C, 48.72; H, 4.65; N, 1.57%). Crystal structure determination. The compound crystallizes with one molecule of C6H6 and one molecule of (C2H5)2O per unit cell, which are both disordered via a center of symmetry. Thus, the solvent molecules were refined isotropically.C73H86- BF24N2O2.5P2Re, M = 1746.39, triclinic, space group P1� (no. 2), a = 13.4230(14), b = 17.641(2), c = 17.946(2) Å, a = 101.790(13),1726 J. Chem. Soc., Dalton Trans., 1999, 1717–1727 b = 109.224(12), g = 92.608(13)8, V = 3898.4(0.8) Å3 (5000 reflections used for cell parameter refinement), T = 193 K, Z = 2, m(Mo-Ka) = 1.7 mm21, 218 images exposed using a f oscillation scan mode at constant times of 3.0 min per image. 35186 Reflections measured (qmax = 268), 13856 unique (Rint = 0.0463) which were used in all calculations, 934 parameters in full matrix refinement, final R1 = 0.0738, wR2(F2) = 0.1864.[Re(NO)2(PCy3)2(CO)][BArF 4]. The complex [Re(NO)2- (PCy3)2][BArF 4] (48 mg, 0.0287 mmol) was introduced in a 100 mL flask and C6H6 (10 mL) added. The mixture was placed under 950 mbar of CO and heated at 80 8C for 10 min. Upon cooling to room temperature small yellow crystals started to be formed. The solvent was removed until ca. 1 mL of C6H6 was left, and then pentane was added (10 mL). The solid was subsequently washed with pentane (2 × 10 mL) and dried under vacuum to give 35 mg of III1[BArF 4] (71.3%). Suitable crystals for the X-ray diVraction study and elemental analyses were obtained by recrystallization in CH2Cl2–pentane. IR(CD2Cl2): nCO 2025m; nNO 1717m and 1675s cm21. 31P-{1H} NMR (CD2Cl2): d 23.6 (s). 1H NMR (CD2Cl2): d 7.73 (m, br, 8 H, BArF 4), 7.60 (m, br, 4 H, BArF 4) and 2.40–0.8 (m, 66 H, PCy3). 13C-{1H} NMR (CD2Cl2): d 202.4 (t, CO, JCP = 9.4 Hz) (Calc for C69H78BF24N2O3P2Re: C, 48.80; H, 4.63; N, 1.65. Found: C, 49.11; H, 4.42; N, 1.58%). Crystal structure determination The compound crystallizes with one molecule of CH2Cl2 per unit cell, which is disordered via a center of symmetry. For the solvent molecule, the split Cl atoms were refined anisotropically, whereas the remaining atoms were treated isotropically. C69.5H79BClF24N2O3P2Re, M = 1740.75, triclinic, space group P1� (no. 2), a = 12.9909(12), b = 16.6700(16), c = 19.0701(19) Å, a = 79.633(12), b = 71.952(11), g = 79.440(11)8, V = 3826.4(0.6) Å3 (5000 reflections used for cell parameter refinement), T = 193 K, Z = 2, m(Mo- Ka) = 1.768 mm21, 200 images exposed using a f rotation scan mode at constant times of 1.8 min per image. 48209 Reflections measured (qmax = 308), 20707 unique (Rint = 0.0506) which were used in all calculations, 993 parameters in full matrix refinement.All three cyclohexyl groups bound to P2 are disordered (from diVerence electron density maps), and were refined using the PART option.27 Final R1 = 0.0616, wR2(F2) = 0.1977. [Re(NO)2(PCy3)2(C6H5CHO)][BArF 4]. A slurry of [Re(NO)2- (PCy3)2][BArF 4] (50 mg, 0.0299 mmol) in C6H6 (1 mL) was treated with benzaldehyde (10 mL, 0.0984 mmol). In a few minutes the starting material dissolved and a brown solution was obtained. Pentane was layered over this solution and after 24 h red-brown crystals were collected, washed with pentane (2 × 10 mL) and dried in vacuo yielding 40 mg of IV1[BArF 4](C6H6) (72.2%).IR(CD2Cl2): nCO,NO 1704w, 1668s, 1651 (sh), 1617s, 1611s, 1593s and 1575m cm21. 31P-{1H} NMR (CD2Cl2): d 32.5 (s). 1H NMR (CD2Cl2): d 9.92 (s, 1 H, C6H5CHO), 8.04–7.78 (m, 5 H, C6H5CHO), 7.73 (m, br, 8 H, BArF 4), 7.60 (m, br, 4 H, BArF 4) and 2.30–0.8 (m, 66 H, PCy3). 13C-{1H} NMR (CD2Cl2): d 206.9 (s, br, C6H5COH) [Calc. for C75H84BF24- N2O3P2Re (recrystallized in CH2Cl2–pentane): C, 50.71; H, 4.77; N, 1.58.Found: C, 50.67; H, 4.59; N, 1.58%]. Crystal structure determination. The compound crystallizes with three molecules C6H6 per asymmetric unit. Formula C93- H102BF24N2O3P2Re, M = 2010.79, triclinic, space group P1� (no. 2), a = 13.892(3), b = 18.768(3), c = 19.569(3) Å, a = 97.76(2), b = 107.96(2), g = 102.88(2)8,615.8(1.2) Å3, T = 153 K, Z = 2, m(Mo-Ka) = 1.46 mm21, w scan width 1.68, variable scan speed 2–298 min21, 16060 reflections measured (qmax = 258), 15223 unique (Rint = 0.030) which were used in all calculations, 1194 parameters in full matrix refinement, final R1 = 0.0539, wR(Fobs) = 0.0385.y-Scan reflections for absorption correction were measured, but did not lead to further improvement of the results. Therefore, the uncorrected data set was used in structure refinement. The F atoms for two of the trifluoromethyl groups had to be refined isotropically. One of the cyclohexyl groups appeared to be disorderd as well, and the four C atoms involved had to be split and refined with isotropic displacement parameters. [Re(NO)2(PiPr3)2{ONRe(NO)(PiPr3)2H}]BArF 4].A heterogeneous mixture of [Re(NO)2(PiPr3)2H] (165 mg, 0.299 mmol) and [Ph3C][BArF 4] (163 mg, 0.147 mmol) in C6H6 (15 mL) was stirred for 2 h. During this period an orange solid was formed. The solvent was removed in vacuo until ca. 3 mL of C6H6 were left and then pentane (15 mL) was added.The residue was washed with pentane (3 × 15 mL) and dried in vacuo to give 250 mg of V1[BArF 4] (83.7%). Crystals for the X-ray diVraction study were grown by recrystallization of a diluted solution of the complex from C6H6–pentane. IR(Nujol): nNO 1659s, 1645m, 1627s and 1609s cm21. 31P-{1H} NMR (C6D5Cl): d 54.9 (s, br, 2P) and 43.3 (s, br, 2P). 1H NMR (C6D5Cl): d 8.12 (m, br, 8H, BArF 4), 7.47 (m, br, 4 H, BArF 4), 3.39 (t, br, JHP = 46.8 Hz, Re), 2.30 [m, br, 6 H, P(CHMe2)3], 2.05 [m, br, 6 H, P(CHMe2)3] and 0.95 [m, 72 H, P(CHMe2)3] (Calc.for C68H97BF24N4- O4P4Re2: C, 40.89; H, 4.89; N, 2.80. Found: C, 40.95; H, 4.84; N, 2.78%). Crystal structure determination. The very small crystal size caused high residual electron density of 7.68 e Å23, 0.94 Å away from Re2. C68H97BF24N4O4P4Re2, M = 1997.61, triclinic, space group P1� (no. 2), a = 14.256(2), b = 16.859(2), c = 17.771(2) Å, a = 97.22(1), b = 93.87(1), g = 96.11(1)8, V = 4198.9(0.8) Å3, T = 183 K, Z = 2, m(Mo-Ka) = 3.09 mm21, w scan width 1.28, variable scan speed 2–298 min21, 15369 reflections measured (qmax = 258), 14579 unique (Rint = 0.020), 10254 reflections used in all calculations.Isopropyl groups are disordered; 962 parameters in full matrix refinement, final R1 = 0.1104, wR(Fobs) = 0.081. Owing to the small crystal dimensions, five C atoms of four isopropyl groups, as well as one C atom of the BArF 4 anion, had to be refined isotropically. Computational details All calculations were based on the local density approximation (LDA) in the parameterization of Vosko et al.32 with the addition of gradient corrections due to Becke 33 and Perdew34 (BP86), which were included self-consistently (NL-SCF). The calculations utilized the Amsterdam Density Functional package ADF,35 release 2.3.Use was made of the frozen core approximation, and the ns, np, nd and (n 1 1)s shells of the transition metal were described by a triple z-STO basis augmented by one (n 1 1)p function (ADF database IV).The valence shells of the main group atoms were described by a double z-STO basis plus one polarization function (ADF database III). The numerical accuracy 35b,d was set to 5.0, and final gradients were 2.0 × 1023 au Å21 and better. If not mentioned otherwise, calculations were performed under C2v or Cs symmetry constraints. Relativistic eVects were included using a quasi-relativistic approach.36 Acknowledgements This work was supported by the Swiss National Science Foundation (SNSF). Access to the computing facilities of the Rechenzentrum der Universität Zürich is gratefully acknowledged.References 1 G. B. Richter-Addo and P. Legdins, Metal Nitrosyls, Oxford University Press, New York, NY, 1992; M. Feelisch and J. S. Stammler (Editors), Methods in Nitric Oxide Research, Wiley, Chichester, 1996. 2 H. Berke and P. Burger, Comments Inorg. Chem., 1994, 16, 279. 3 A. A. H. van der Zeijden, T.Bürgi and H. Berke, Inorg. Chim. Acta, 1992, 201, 131. 4 A. A. H. van der Zeijden, C. Sontag, H. W. Bosch, V. Shklover,J. Chem. Soc., Dalton Trans., 1999, 1717–1727 1727 H. Berke, D. Nanz and W. von Philipsborn, Helv. Chim. Acta, 1991, 74, 1194; A. A. H. van der Zeijden, V. Shklover and H. Berke, Inorg. Chem., 1991, 23, 4393; A. A. H. van der Zeijden, H. W. Bosch and H. Berke, Organometallics, 1992, 11, 563; 2051. 5 H.-U. 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Ross and T.Ziegler, ACS Symp. Ser., 1996, 629. 10 (a) D. M. P. Mingos and J. A. Ibers, Inorg. Chem., 1970, 9, 1105; (b) A. P. Gaughan Junior, B. J. Corden, R. Eisenberg and J. A. Ibers, Inorg. Chem., 1974, 13, 786; (c) V. G. Albano, A. Araneo, P. L. Bellon, G. Ciani and M. Manassero, J. Organomet. Chem., 1974, 67, 413; (d ) B. L. Haymore and J. A. Ibers, Inorg. Chem., 1975, 14, 2610; (e) J. A. Kaduk and J. A. Ibers, Inorg. Chem., 1975, 14, 3070; ( f ) S. Bhaduri and G. M. Sheldrick, Acta Crystallogr., Sect.B, 1975, 31, 897; ( g) B. E. Reichert, Acta Crystallogr., Sect. B, 1976, 32, 1934; (h) J. A. Kaduk and J. A. Ibers, Inorg. Chem., 1977, 16, 3283; (i) A. M. M. Lanfredi, A. Tiripicchio and M. Tiripicchio Camellini, Acta Crystallogr., Sect. C, 1983, 39, 1633; ( j) G. Le Borgne, L. Mordenti, J. G. Riess and J.-L. Roustan, New J. Chem., 1986, 10, 97; (k) H. Li Kam Wah, M. Postel and M. Pierrot, Inorg. Chim. Acta, 1989, 165, 215; (l) V. Munyejabo, J.-P. Damiano, M.Postel, C. Bensimon and J.-L. Roustan, J. Organomet. Chem., 1995, 491, 61; (m) F. L. Atkinson, H. E. Blackwell, N. C. Brown, N. G. Connelly, J. G. Crossley, A. G. Orpen, A. L. Rieger and P. H. Rieger, J. Chem. Soc., Dalton Trans., 1996, 3491. 11 R. H. Summerville and R. HoVmann, J. Am. Chem. Soc., 1976, 98, 7240. 12 S. C. Haefner, K. R. Dunbar and C. Bender, J. Am. Chem. Soc., 1991, 113, 9540. 13 M. Ogasawara, S. A. Macgregor, W. E. Streib, K. Folting, O. Eisenstein and K. G. Caulton, J. Am. Chem. Soc., (a) 1995, 117, 8869; (b) 1996, 118, 10189. 14 T. A. Albright, J. K. Burdett and M.-H. Whangbo, Orbital Interactions in Chemistry, Wiley, New York, NY, 1985, (a) ch. 19.1; (b) ch. 15.4. 15 M. Elian and R. HoVmann, Inorg. Chem., 1975, 14, 1058. 16 K.-P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, Van Nostrand Reinhold, New York, NY, 1979, vol. IV. 17 R. HoVmann, L. M. Chen, M. Ellian, A. R. Rossi and D. M. P. Mingos, Inorg. Chem., 1974, 13, 2666; A. R. Rossi and R. HoV- mann, Inorg. Chem., 1975, 14, 365; R. HoVmann, J. M. Howell and A. R. Rossi, J. Am. Chem. Soc., 1976, 98, 2484. 18 (a) E. L. Muetterties, Tetrahedron, 1974, 30, 1595; (b) E. L. Muetterties and L. J. Guggenberger, J. Am. Chem. Soc., 1974, 96, 1748. 19 D. H. Mordaunt, H. Flöthmann, M. Stumpf, H.-M. Keller, C. Beck, R. Schinke and K. Yamashita, J. Chem. Phys., 1997, 107, 6603. 20 T. Ziegler and A. Rauk, Theor. Chim. Acta, 1977, 46, 1; Inorg. Chem., 1979, 18, 1558; T. Ziegler, NATO ASI, Ser. C, 1992, 378, 367; F. M. Bickelhaupt, N. M. M. Nibbering, E. M. van Wezenbeek and E. J. Baerends, J. Phys. Chem., 1992, 96, 4864. 21 H. Jacobsen and T. Ziegler J. Am. Chem. Soc., 1994, 116, 3667. 22 A. Llamazares, H. W. Schmalle and H. Berke, in preparation. 23 DS Software, Stoe and Gie, Darmstadt, Germany, 1997. 24 SHELXTL PLUS, Siemens Analytical X-Ray Instruments, Madison, WI, 1994. 25 P. Coppens, L. Leiserowitz and D. Rabinovich, Acta Crystallogr., 1965, 18, 1035. 26 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467. 27 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori and M. Camalli, SIR 92, University of Bari, 1992. 28 G. M. Sheldrick, SHELXL 97, University of Göttingen, 1997. 29 D. J. Watkin, C. K. Prout, J. R. Carruthers and P. W. Bettridge, CRYSTALS, Issue 10, Chemical Crystallography Library, University of Oxford, Oxford, 1996. 30 J. R. Carruthers and D. J. Watkin, Acta Crystallogr., Sect. A, 1979, 35, 698. 31 A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46, C34. 32 S. J. Vosko, M. Wilk and M. Nussair, Can. J. Phys., 1980, 58, 1200. 33 A. D. Becke, Phys. Rev. A, 1988, 38, 3098. 34 J. P. Perdew, Phys. Rev. B, 1986, 33, 8822. 35 (a) E. J. Baerends, D. E. Ellis and P. E. Ros, Chem. Phys., 1973, 2, 41; (b) G. te Velde and E. J. Baerends, J. Comput. Phys., 1992, 99, 84; (c) C. Fonseca Guerra, O. Visser, J. G. Snijders, G. te Velde and E. J. Baerends, In Methods and Techniques in Computational Chemistry: METECC-95, eds. E. Clementi and G. Corongiu, STEF, Cagliari, 1995, p. 305; (d ) G. te Velde, ADF 2.1 User’s Guide, Vrije Universiteit, Amsterdam, 1996. 36 T. Ziegler, V. Tschinke, E. J. Baerends, J. G. Snijders and W. Ravenek, J. Phys. Chem., 1989, 93, 3050; G. Schreckenbach, J. Li and T. Ziegler, Int. J. Quantum. Chem., 1995, 56, 477. Paper 9/01384I

ISSN:1477-9226    

DOI:10.1039/a901384i

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 1729–1733 1729 1,1,1,2,2-Pentaiododiphosphanium cations, P2I5 1EI4 2 (E 5 Al, Ga or In): synthesis and characterisation by 31P MAS NMR, IR and Raman spectroscopy Christoph Aubauer,a Günter Engelhardt,b Thomas M. Klapötke *a and Axel Schulz a a Institut für Anorganische Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13(Haus D), D-81377 Munich, Germany. E-mail: tmk@cup.uni-muenchen.de b Institut für Technische Chemie I, Universität Stuttgart, D-70550 Stuttgart, Germany Received 10th February 1999, Accepted 7th April 1999 The compound P2I5 1AlI4 2 and the novel compounds P2I5 1GaI4 2 and P2I5 1InI4 2 have been prepared in two diVerent ways either from PI3 and EI3 or from P2I4 and I2/EI3 (E = Al, Ga or In).The products have been characterised by solid-state 31P MAS NMR, Raman and IR spectroscopy. The solid-state 31P MAS NMR spectra are compared with NMR studies of related PI4 1 salts and alkylphosphorus tetraiodides.Vibrational assignments for the normal modes for the solid-state species P2I5 1EI4 2 (E = Al, Ga or In) have been made on the basis of their Raman and IR spectra. These results are in excellent agreement with density functional calculations for the P2I5 1 cation and are consistent with the ionic formulation P2I5 1EI4 2 (E = Al, Ga or In). Introduction As early as in 1964 Baudler and Wetter 1 obtained a deeply coloured co-ordination compound (2 PI3?AlI3) from the reaction of PI3 and AlI3 in CS2.They assumed a monomeric structure having a symmetric trigonal bipyramidal arrangement (symmetry D3h) with sp3d bond hybridisation and a coordination number of 5 at the aluminium atom, analogous to (Me3N)2AlH3.2 The 2 : 1 adduct of PI3 and AlI3 was structurally characterised by X-ray crystallography and identified as P2I5 1AlI4 2 by Pohl 3 in 1983. Pohl suggested as reaction mechanism the intermediate formation of “PI2 1” via I2 abstraction by the Lewis acid AlI3 yielding AlI4 2.In the next step the “PI2 1” reacts with a second molecule of PI3 forming the P2I5 1 cation via P–P linkage.3 Similar to PI4 1AlI4 2,4 the cation and anion are connected by bridging I ? ? ? I interactions. The P–P distance in the P2I5 1 cation (2.218 Å) is very similar to that in P2I4 (2.21 Å).5 Like the related alkylphosphorus tetraiodides RPI4 (R = Me,6 i -Pr,7 t -Bu7 or Me3SiCH2 7) and the tetraiodophosphonium cations in PI4 1AlI4 24 and PI4 1GaI4 2,8 the P2I5 1 cation exists only in the solid state.Recently, we published a combined theoretical and experimental study on the PI4 1 cation.8 It has an extremely large negative 31P chemical shift which quantum chemical calculations and solid-state NMR spectroscopy showed is due to spin–orbit coupling. This investigation naturally led to further work on binary phosphorus–iodine cations. A recent MAS 31P NMR study on related cationic di- and tri-tertiary phosphines in the solid state and in solution can be found in the literature. 9a,b In this paper we report on the preparation and properties of the structurally and chemically interesting P2I5 1 cation. Results and discussion All the compounds reported here were prepared from the reaction of 2 equivalents PI3 with 1 equivalent of EI3 (E = Al 1a, Ga 2a or In 3a), eqn. (1), or from the reaction of P2I4 with I2 2 PI3 1 EI3 CS2 P2I5 1EI4 2 (1) and EI3 E = Al 1b, Ga 2b or In 3b) in a 1 : 1 : 1 molar ratio, eqn.(2), in CS2 under nitrogen. P2I4 1 I2 1 EI3 CS2 P2I5 1EI4 2 (2) The P2I5 1 cation exists only in the solid state. Dissolving P2I5 1EI4 2 (E = Al 1, Ga 2 or In 3) in CS2 gives a solution that contains essentially only PI3 (d31P 1178 in CS2 9c) and the corresponding triodides EI3. Evaporation of the solvent allows recovery of the P2I5 1 compound. Attempts to prepare P2I5 1 compounds, containing an iodine free anion, such as AsF6 2 or SbF6 2, by treating P2I4 with I3 1AsF6 2 or I3 1SbF6 2 in diVerent solvents, were unsuccessful.The reactions were carried out analogously to the preparation of PI4 1AsF6 210 and PI4 1SbF6 2.8 However, these reactions led only to decomposition into the thermodynamically more stable products PF3, I2, AsI3 and SbI3, respectively, which were identi- fied by Raman spectroscopy. We conclude that the P2I5 1 cation can only be stabilised with anions like EI4 2 (E = Al, Ga or In). Fig. 1 shows the B3LYP-optimised minimum structure of the free P2I5 1 cation.In addition, the geometry and frequencies of the PI4 1 cation were calculated for comparison [B3LYP: d(P–I) = 2.431 Å, n1 (A1, 0.0) 165, n2 (E, 0.0) 62, n3 (T2, 67) 385, n4 (T2, 0.0) 96 cm21]. Both cations were shown to possess stable minima at the B3LYP level (no imaginary frequencies). As expected for steric reasons, P2I5 1 displays a staggered configuration with an I1–P1–P2–I5 dihedral angle of 54.38, and three essentially equal P–I distances for the PI3 part and two eVectively equal, but longer, P–I separations for the PI2 unit.The Fig. 1 Fully optimised geometry of P2I5 1 [bond lengths in Å, partial charges (italics) in e].1730 J. Chem. Soc., Dalton Trans., 1999, 1729–1733 Table 1 Raman and IR wavenumbers (cm21) for P2I5 1EI4 2 (E = Al 1a, Ga 2a or In 3a) P2I5 1AlI4 2 1a Raman 390 (1) 384 (2) 348 (0.5) 324 (1) 210 (10) 147 (2) 129 (6) 97 (4) 89 (2) IR 393m 382m 346 (sh) 317 (br) 209m P2I5 1GaI4 2 2a Raman 389 (1) 382 (2) 347 (0.5) 321 (2) 209 (10) 143 (3) 129 (8) 97 (5) 88 (2) IR 392m 380m 343m 314 (br) 231s 224s 209vs P2I5 1InI4 2 3a Raman 389 (1) 381 (1) 346 (0.5) 323 (2) 210 (10) 138 (4) 127 (9) 98 (5) 85 (2) IR 389m 380s 347m 314 (br) 211vs Calculation a 394 (A9, 50.0) 382 (A0, 37.9) 354 (A9, 9.4) 350 (A0, 61.6) 325 (A9, 24.9) 209 (A9, 25.1) 128 (A9, 0.0) 97 (A0, 0.6) 94 (A0, 0.0) 80 (A9, 0.1) 65 (A0, 0.0) 58 (A9, 0.2) 42 (A0, 0.1) 39 (A9, 0.1) 16 (A0, 0.0) Assignment w1 w2 w3 w4 w5 n3 (EI4 2) n3 (EI4 2) w6 n1 (EI4 2) w7 w8 w9 w10 w11 w12 w13 w14 w15 a See Fig. 4. b In parentheses: symmetry, IR intensity [km mol21]. experimentally not observed eclipsed configuration (Fig. 2) represents a transition state (number of imaginary frequencies=1, 2i14 cm21) describing the internal rotation about the P–P axis. The rotation barrier was calculated to be of the order Ea (B3LYP, 298 K) = 2.4 kcal mol21. The calculated NBO (natural bond orbital) partial charges (Fig. 1) reveal fairly covalent P–I bonds with the positive charge almost evenly distributed amongst all atoms. Vibrational spectroscopy Table 1 summarises the computed and experimentally observed vibrational frequencies of the P2I5 1 compounds synthesised (1a, 2a, 3a) in this study. Fig. 3 shows the Raman spectra of these compounds. Raman and IR spectra of all three sets of compounds (1a, 1b, 2a, 2b, 3a, 3b) are very similar regardless of the method of synthesis.The measured vibrational spectra agree excellently with our theoretical calculation (B3LYP) for Fig. 2 Transition state for the internal rotation [bond lengths in Å]. Fig. 3 Raman spectra of P2I5 1EI4 2 (E = Al 1a, Ga 2a or In 3a). the isolated P2I5 1 cation. The most intense peak, observed at ca. 210 cm21, can be assigned to a mixture of P1–P2 stretching (w6, Fig. 4) and P1–I1 stretching. The P1–I1 stretching mode (w1) at ca. 390 cm21 and the asymmetric P1–I2,3 stretching mode (w2) at ca. 382 cm21 can be observed in all IR and Raman spectra. Similar vibrational frequencies are reported for the asymmetric P–I stretching mode, n3 (T2), in PI4 1AlI4 211 and PI4 1GaI4 28 at ca. 380 cm21. The signals at ca. 346 cm21 can be described as a mixture of the P1–P2 stretching vibration (w3) and the asymmetric P2–I4,5 stretching vibration (w4; the resolution was not suYcient enough). The predicted value for the symmetric P–I stretching vibration of the P2–I4,5 unit (w5), 325 cm21 (Fig. 4), is most closely associated with the experimental values at ca. 323 (Raman) and ca. 314 cm21 (IR), respectively. Similar P–I stretching frequencies are reported for the vibrational spectra of P2I4 12 (IR: 330 cm21), PI3 12 (IR: 310 cm21. Raman: 325 cm21) and the adduct PI3?BI3 13 (IR: 304 and 329 cm21). w7, w8 and w9 represent P–I deformation modes (Fig. 4) with w7 (ca. 128 cm21) being one of the most intensive peak in all Raman spectra. The presence of the anions EI4 2 (E = Al 1, Ga 2 or In 3) is confirmed by the symmetric stretching mode, n1 (A1), at 147 (1), Fig. 4 Pictorial description of the calculated normal modes of P2I5 1.J. Chem. Soc., Dalton Trans., 1999, 1729–1733 1731 143 (2) and 138 cm21 (3). They are consistent with literature values [n1 (AlI4 2) 146;14 n1 (GaI4 2) 145;15 n1 (InI4 2) 138 cm21 15]. The IR spectra of 2 show two strong peaks at 231 and 224 cm21 that can be assigned to the asymmetric stretching mode, n3 (T2), of GaI4 2 [n3 (GaI4 2) 222 cm21 15].Solid-state NMR spectroscopy Fig. 5 shows the 31P MAS NMR spectra of the three compounds P2I5 1EI4 2 with E = Al 1, Ga 2, or E = In 3. Table 2 summarises the isotropic chemical shifts (d), chemical shift anisotropies (DdCSA), asymmetry parameters (hCSA), and relative signal intensities obtained from spectra simulations. As an example, the details of the simulation for P2I5 1AlI4 2 1 are presented in Fig. 6. Besides the resonance of a PI3 impurity (component 3, see below, d31P 1160 to 1185 in solution,9c 1237 in the solid state 16), two partially overlapping side band patterns can be identified (components 1 and 2) which correspond to the two P atoms in the P2I5 1 cation.The diVerent intensity distributions of the spinning side bands indicate diVerent chemical shift anisotropies of the two resonances, but the total intensities are in the expected ratio of 1 : 1 (Table 2). The spectrum of the PI3 impurity could be unequivocally identified by the 31P MAS NMR spectrum measured at a pure solid PI3 sample (see Table 2).The weak narrow lines in the ranges of d 0 to 40 and 2340 to 2360 are due to very small amounts of other impurities. It should be noted that no indications of scalar 1J(PP) Fig. 5 31P MAS NMR spectra (no = 162.96 MHz, nrot = 12.5 kHz) of P2I5 1EI4 2 with E = Al 1, Ga 2 or In 3. The central lines are indicated by arrows; all the other broad peaks are spinning side bands.The sharp lines originate from impurities. Table 2 The 31P NMR isotropic chemical shift (d), chemical shift anisotropy (DdCSA), asymmetry parameter (hCSA) and relative peak intensity (I) of the compounds P2I5 1EI4 2 and PI3 P2I5 1EI4 2 d (±0.5 ppm) DdCSA a (±5 ppm) hCSA b (±0.2) I (%) (±3%) 1, E = Al P1 (I3P) P2 (I2P) 2142 114 2216 2124 0 0.4 48 52 2, E = Ga P1 (I3P) P2 (I2P) 2142 112 2217 2117 0 0.4 49 51 3, E = In P1 (I3P) P2 (I2P) PI3 2139 112 237 2225 2116 2111 0 0.4 0.3 48 52 — a DdCSA = d33 2 diso.b hCSA = |d22 2 d11|/|d33 2 diso|. couplings of the two inequivalent P atoms of the P2I5 1 cation could be observed. Obviously, these splittings cannot be resolved due to the large linewidths (half-width about 3500– 4000 Hz for P1 and 2000–2500 Hz for P2). The spectra of the corresponding P2I5 1GaI4 2 2 and P2I5 1InI4 2 3 compounds are very similar to that of P2I5 1AlI4 2 1 (see Fig. 5) and were simulated in the same way. No impurities were found in the 31P NMR spectrum of 2.The two central peaks of the P2I5 1 cation in each of the three compounds were located by variation of the spinning frequency and are consistently observed at d ca. 2140 (P1) and 1112 (P2), respectively (Table 2). The resonance at d 1112 (P2) with a non-axial anisotropy tensor (hCSA = 0.4) can be assigned to a three-coordinated phosphorus atom, i.e. to the I2P fragment of the P2I5 1 cation. The shift is very close to that of P2I4 (d 1106 in solution 17 and 1127 in the solid state 18).The 31P chemical shift d ca. 2140 (P1) is attributed to the four-co-ordinated phosphorus atom in the I3P fragment of the P2I5 1 cation. In accord with the C3v symmetry of this fragment,3 this resonance shows an axially symmetric anisotropy tensor (hCSA = 0). The strong low-frequency (high-field) 31P shift of d ca. 2267 from the P2I4 resonance can be interpreted as a result of spin–orbit interactions due to the presence of three iodine substituents at the four-co-ordinated phosphorus atom in the I3P fragment.Using density functional calculations, Kaupp et al.8 have recently shown that the extremely large low-frequency shift observed for the related compounds PI4 1AlI4 2 (d 2304), PI4 1GaI4 2 (d 2295), PI4 1AsF6 2 (d 2519), and PI4 1SbF6 2 (d 2517) 8 is entirely due to spin–orbit contributions from the four heavy iodine substituents, transmitted to the phosphorus nucleus by a very eVective Fermi-contact mechanism. Equally, for t-BuPI4 (d 249.2) the 31P NMR resonance in the solid state is shifted by 2215 ppm to high field from the adduct t-BuPI2 (d 1165).7 Owing to the close similarity of the isotropic chemical Fig. 6 Experimental and simulated 31P MAS NMR spectra of P2I5 1AlI4 2. The central lines of the spinning side band patterns are indicated by arrows in the experimental spectrum. The simulated spectrum is the sum of the three component spectra.1732 J. Chem. Soc., Dalton Trans., 1999, 1729–1733 shifts and anisotropies of the P2I5 1 cation of all three P2I5 1EI4 2 compounds, it can be concluded that the structures of P2I5 1GaI4 2 2 and P2I5 1InI4 2 3 are very similar to the known structure of P2I5 1AlI4 2 with weak I ? ? ? I interactions between cation and anion.3 However, no crystals suitable for X-ray diffraction analyses could be grown up to now.Conclusion The results indicate that compounds such as P2I5 1AlI4 2, P2I5 1GaI4 2, P2I5 1InI4 2, including the 1,1,1,2,2-pentaiododiphosphanium cation, can be prepared in two diVerent ways.The solid-state 31P MAS NMR results show two diVerent peaks, which can be assigned to a PI2 and a PI3 1 fragment, respectively. Our calculations agree excellently with the experimental wavenumbers of Raman and IR spectra. Solid-state 31P MAS NMR and vibrational results suggest that these compounds exist as ionic species in which the P2I5 1 cations possess only weak I ? ? ? I interactions to the anions.It can be considered that the novel compounds P2I5 1GaI4 2 and P2I5 1InI4 2 have similar structures to that found in P2I5 1AlI4 2.3 Experimental General All experiments were carried out in a dry-box under dry nitrogen. The compounds PI3, P2I4, AlI3, GaI3, InI3 (Aldrich) and I2 (Merck) were used as received; CS2 was refluxed with P4O10 and distilled before use. The 31P NMR spectra were measured at 161.96 MHz with a Bruker MSL-400 NMR spectrometer under magic angle spinning (MAS) conditions. A standard doublebearing MAS probe for 4 mm rotors was used with spinning frequencies up to 12.5 kHz.Single pulse acquisition with 1 ms pulse width (corresponding to a 258 flip angle) and 5 s pulse repetition was applied. The samples were filled in 4 mm zirconia rotors under a nitrogen atmosphere in a glove-box. The spectra were referenced to 85% aqueous phosphoric acid. Spectral simulations were carried out with the PC program WINFIT of the Bruker WINNMR software package.Raman spectra were obtained on powdered solid samples contained in glass capillary tubes with a Perkin-Elmer 2000 NIR spectrometer in the range 500–50 cm21, IR spectra on Nujol mulls between CsI plates in the range 500–200 cm21 on a Nicolet 520 FT IR spectrometer. For the determination of decomposition points, samples were heated in sealed glass capillaries in a Büchi B450 instrument. Preparations The compounds P2I5 1EI4 2 (E = Al, Ga or In) were prepared by addition of PI3 in CS2 to CS2 solutions of EI3 (E = Al, Ga or In) (1a, 2a, 3a) or by addition of P2I4 and I2 in CS2 to CS2 solutions of EI3 (E = Al, Ga or In) (1b, 2b, 3b) by stirring at room temperature.After 24 h the solvent was removed under dynamic vacuum, leaving the solid compound. P2I5 1AlI4 2 1a. Starting materials: 0.84 g of PI3 (2.00 mmol), 0.41 g of AlI3 (1.00 mmol). Yield: 1.13 g (92%) of dark red crystals, mp 90 8C (decomp.). P2I5 1AlI4 2 1b. Starting materials: 0.57 g of P2I4 (1.00 mmol), 0.25 g of I2 (1.00 mmol), 0.41 g of AlI3 (1.00 mmol).Yield: 1.08 g (88%) of dark red crystals, mp 90 8C (decomp.). P2I5 1GaI4 2 2a. Starting materials: 0.84 g of PI3 (2.00 mmol), 0.45 g of GaI3 (1.00 mmol). Yield: 1.10 g (86%) of red solid, mp 111 8C (decomp.). P2I5 1GaI4 2 2b. Starting materials: 0.57 g of P2I4 (1.00 mmol), 0.25 g of I2 (1.00 mmol), 0.45 g of GaI3 (1.00 mmol). Yield: 1.10 g (86%) of red solid, mp 111 8C (decomp.).P2I5 1InI4 2 3a. Starting materials: 0.84 g of PI3 (2.00 mmol), 0.50 g of InI3 (1.00 mmol). Yield: 1.28 g (99%) of dark red solid, mp 84 8C (decomp.). P2I5 1InI4 2 3b. Starting materials: 0.57 g of P2I4 (1.00 mmol), 0.25 g of I2 (1.00 mmol), 0.50 g of InI3 (1.00 mmol). Yield: 1.28 g (99%) of dark red solid, mp 84 8C (decomp.). Computational methods The structure and vibrational data for PI4 1 and P2I5 1 were calculated by using density functional theory with the program package GAUSSIAN 94.19 For phosphorus a standard 6-31G(d,p) basis set was used and for I a quasi-relativistic pseudopotential (ECP46MWB)20 and a (5s5p1d)/[3s3p1d]- DZ1P basis set.21 The computations were carried out at the DFT level using the hybrid method B3LYP which includes a mixture of Hartree–Fock exchange with DFT exchange correlation.Becke’s 3 parameter functional where the non-local correlation is provided by the LYP expression (Lee, Yang, Parr correlation functional) was used which is implemented in GAUSSIAN 94.For a concise definition of the B3-LYP functional see ref. 22. Acknowledgements We gratefully acknowledge the support of the Fonds der Chemischen Industrie and the University of Munich. We also thank Dr M. Kaupp for helpful advice and the Leibnitz Rechenzentrum for a generous allocation of time on the Cray T 90 computer. References 1 M. Baudler and G. Wetter, Z. Anorg. Allg. Chem., 1964, 329, 3. 2 O. Stecher and E. Wiberg, Ber. Dtsch.Chem. Ges., 1942, 75, 2003; E. Wiberg, H. Graf, M. Schmidt and R. Usón, Z. Naturforsch., Teil B, 1952, 7, 578; E. Wiberg, H. Graf and R. Usón, Z. Anorg. Allg. Chem., 1953, 275, 221; G. W. Fraser, N. N. Greenwood and B. P. Straughan, J. Chem. Soc., 1963, 3742. 3 S. Pohl, Z. Anorg. Allg. Chem., 1983, 498, 20. 4 S. Pohl, Z. Anorg. Allg. Chem., 1983, 498, 15. 5 Y. C. Leung and J. Waser, J. Phys. Chem., 1956, 60, 539. 6 V. A. Ginsburg and N. F. Privezentsva, Zh. Obshch. Khim., 1958, 28, 736. 7 W.-W. duMont, V. Stenzel, J. Jeske, P. G. Jones, A. Sebald, S. Pohl, W. Saak and M. Bätcher, Inorg. Chem., 1994, 33, 1502. 8 M. Kaupp, Ch. Aubauer, G. Engelhardt, T. M. Klapötke and O. L. Malkina, J. Chem. Phys., 1999, 110, 3897. 9 (a) N. Bricklebank, S. M. Godfrey, C. A. McAuliVe, P. Deplano, M. L. Mercuri and J. M. Deplano, J. Chem. Soc., Dalton. Trans., 1998, 2379; (b) N. Bricklebank, S. M. Godfrey, H. P. Lane, C. A. McAuliVe, R. G. Pritchard and J. M. Moreno, J.Chem. Soc., Dalton. Trans., 1995, 2421; (c) cf. e.g. K. B. Dillon, M. G. C. Dillon and T. C. Waddington, J. Inorg. Nucl. Chem., 1976, 38, 132. 10 I. Tornieporth-Oetting and T. M. Klapötke, J. Chem. Soc., Chem. Commun., 1990, 132. 11 Ch. Aubauer, Diploma Thesis, Ludwig-Maximilians-Universität, München, 1998. 12 S. G. Frankiss, F. A. Miller, H. Stammreich and Th. T. Sans, Spectrochim. Acta, Part A, 1967, 23, 543. 13 A. H. Cowley and S. T. Cohen, Inorg. Chem., 1965, 4, 1200; G. W.Chantry, A. Finch, P. N. Gates and D. Steele, J. Chem. Soc. A, 1966, 896. 14 G. M. Begun, C. R. Boston, G. Torsi and G. Mamantov, Inorg. Chem., 1971, 10, 886. 15 L. A. Woodward and G. H. Singer, J. Chem. Soc., 1958, 716. 16 M. Kaupp, Ch. Aubauer, G. Engelhardt and T. M. Klapötke, unpublished results. 17 K. B. Dillon, A. W. G. Platt and T. C. Waddington, Inorg. Nucl. Chem. Lett., 1981, 17, 201. 18 Ch. Aubauer, G. Engelhardt and T. M. Klapötke, unpublished results. 19 GAUSSIAN 94, Revision B.2, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Peterson, J. A. Montgomery,J. Chem. Soc., Dalton Trans., 1999, 1729–1733 1733 K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. A. Ortiz, J. B. Foresman, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzales and J. A. Pople, Gaussian Inc., Pittsburgh, 1995. 20 P. Schwertfeger, M. Dolg, W. H. E. Schwarz, G. A. Bowmaker and P. D. W. Boyd, J. Chem. Phys., 189, 91, 1762. 21 M. Kaupp, P. v. R. Schleyer, H. Stoll and H. Preuss, J. Am. Chem. Soc., 1991, 113, 1602. 22 C. W. Bauschlicher and H. Partridge, Chem. Phys. Lett., 1994, 231, 277; A. D. Becke, J. Chem. Phys., 1993, 98, 5648; Phys. Rev. A, 1988, 38, 3098; C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785; S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1980, 58, 1200. Paper 9/01133A

ISSN:1477-9226    

DOI:10.1039/a901133a

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1998, Pages 1731–1733 1731 Single protonation labilises but double protonation inhibits substitution of [Fe4S4Cl4]22 Richard A. Henderson and Kay E. Oglieve John Innes Centre, Nitrogen Fixation Laboratory, Norwich Research Park, Colney, Norwich, UK NR4 7UH Successive protonation of [Fe4S4(SPh)4]22 progressively labilises the cluster towards substitution of the thiolate ligands, whereas single protonation of [Fe4S4Cl4]22 catalyses, but diprotonation inhibits, substitution of the chloro-ligands. The acid catalysis of substitution reactions is a recurring theme in organic, inorganic and biological chemistry.In this paper we report an unusual eVect of protonation on substitution reactions: single protonation of [Fe4S4Cl4]22 catalyses the rate of substitution of the chloro-ligands, but diprotonation inhibits substitution. The acid-catalysed substitution reaction [typified by equation (1)] is entirely general for both synthetic and extracted bio- [Fe4S4X4]22 RSH [NHR3]1 [Fe4S4X3(SHR)]2 1 X2 (1) logical iron–sulfur-based clusters.1,2 Earlier studies 2,3 in MeCN, with the relatively weak acid [NHEt3]1 (pKa = 18.46) 3 showed the following general mechanistic features.(i) Protonation of a thiolate ligand is not labilising.1 (ii) Labilisation is a consequence of protonation of the cluster core, probably at a m3-S site.4 (iii) The mechanism is dissociative when X = RS,5 but associative when X = halide.1 (iv) Protonation catalyses substitution irrespective of whether X = RS or halide.1 With the stronger acid [lutH]1 (lut = 2,6-dimethylpyridine; pKa = 14.13) we observe (not unexpectedly) that diprotonation of [Fe4S4X4]22 (X = PhS or Cl) occurs, but the eVect on the substitution labilities of the two clusters is dramatically diVerent.The kinetics of the substitution reaction between [Fe4S4- (SPh)4]22 and EtSH, in the presence of an excess of [lutH]1, follow the same pattern as with [NHEt3]1, except that the reaction is faster (Fig. 1). Thus, the rate of reaction exhibits a firstorder dependence on the concentration of cluster and a nonlinear dependence on [lutH1]/[lut]. In the reactions reported in this paper the free thiol acts only as the nucleophile. The thiol is a much weaker acid than [NHEt3]1 or [lutH]1 in MeCN and does not significantly contribute to the protonation of the cluster. Thus in the reaction of [Fe4S4(SPh)4]22, varying the concentration of EtSH ([EtSH] = 1–10 mmol dm23), whilst maintaining [lutH1]/[lut] = 4.0, does not aVect the rate of the reaction (kobs = 0.25 ± 0.01 s21). Similar behaviour was observed in the earlier studies 5 with [NHEt3]1.The data in Fig. 1 are consistent with the dissociative pathways shown in Scheme 1 and described by the general rate law of equation (2). From the earlier studies 1 with [NHEt3]1 it is 2d[Fe4S4] dt = {K1 Sk3 S[lutH1]/[lut] 1 K1 SK2 Sk4 S[lutH1]2/[lut]2}[Fe4S4] 1 1 K1 S[lutH1]/[lut] 1 K1 SK2 S[lutH1]2/[lut]2 (2) known that protonation of the first m3-S is associated with pKa = 18.6 for the cluster. Hence, in the studies with [lutH1], we can calculate K1 S = 3.2 × 104. Consequently, under the conditions reported herein, K1 S[lutH1]/[lut] @ 1 and equation (2) simplifies to equation (3), with k3 S = 0.085 ± 0.003 s21, 2d[Fe4S4] dt = {k3 S 1 K2 Sk4 S[lutH1]/[lut]}[Fe4S4] 1 1 K2 S[lutH1]/[lut] (3) k4 S = 0.39 ± 0.02 s21 and K2 S = 0.38 ± 0.02; pKa S = 13.7.The sequence of protonation and substitution steps for the acidcatalysed substitution reaction of [Fe4S4(SPh)4]22 is as follows. At all concentrations of [lutH]1, it seems likely that a thiolate ligand is protonated but, as we have pointed out before,1 protonation at this site is not labilising. At low values of [lutH1]/[lut], additional protonation of a single m3-S occurs and this labilises the thiol to dissociation. Consistent with this interpretation, we find that under these conditions, the rate of substitution is the same as that observed in earlier studies 5 using [NHEt3]1 (k3 S = 0.080 ± 0.001 s21; Fig. 1 insert). At high values of [lutH1]/ [lut] further protonation occurs and (by analogy with the earlier studies) this probably occurs at another m3-S atom. This further labilises the co-ordinated thiol. Subsequent rapid attack by EtSH completes the substitution. Fig. 1 The kinetics of the first substitution reaction of [Fe4S4(SPh)4]22 with EtSH, in the presence of acid in MeCN at 25.0 8C.INSERT. EVect of single protonation: dependence of kobs on [NHEt3 1]/[NEt3]. Curve and data from ref. 5. MAIN. EVect of diprotonation: dependence of kobs on [lutH1]/[lut]. Curve drawn is that defined by equation (3) and the values in the text. In MeCN, the protolytic equilibrium between [lutH]1 and RS2 lies to the right hand side of [lutH]1 1 RS2 lut 1 RSH. With an excess of [lutH]1 the concentrations can be calculated as follows: [lutH1] = [lutH1] 2 [RS2] and [lut] = [RSH] = [RS2].The thiolate is supplied as the [NEt4]1 salt, and the acid as the [BPh4]2 salt1732 J. Chem. Soc., Dalton Trans., 1998, Pages 1731–1733 The substitution mechanisms of [Fe4S4(SPh)4]22 are dissociative and thus the eVect protonation has on the rate primarily reflects changes to the Fe]SPh bond strength. Initial protonation of the thiolate ligand weakens this Fe]S s-bond but strengthens Fe-to-S p-back bonding.Consequently, this protonation has little eVect on the rate of cluster substitution.6 Additional protonation of one m3-S makes this atom a good pelectron acceptor which competes with the thiol for the electron density on Fe, thus labilising the thiol to dissociation. Further protonation, at another m3-S, additionally competes for the pelectron density of Fe and consequently further weakens, and labilises, the Fe–SHPh bond. The kinetics of the substitution reaction between [Fe4S4Cl4]22 and PhSH in the presence of an excess of [lutH]1 shows two distinct diVerences from those of [Fe4S4(SPh)4]22.(1) The reaction exhibits a first-order dependence on the concentration of PhSH. Thus, when 3.0 < [lutH1]/[lut] < 11.0, kobs varies linearly as the concentration of PhSH is changed (kobs/[PhSH] = 4.0 ± 0.5 × 102 dm3 mol21 s21). (2) The rate of the reaction is inhibited by increasing [lutH1]/[lut] (Fig. 2). This behaviour is consistent with the mechanism shown in Scheme 2.This mechanism is analogous to that shown in Scheme 1, except that the act of substitution is associative, involving attack of PhSH at Fe Scheme 1 Mechanism for the dissociative acid-catalysed substitution reactions of [Fe4S4(SPh)4]22 (Fe = d; S = s). For clarity, only the PhS group undergoing substitution is shown prior to chloride dissociation. The general rate law for this mechanism is shown in equation (4). The only diVerence 2d[Fe4S4] dt = {K1 Ck5 C[lutH1]/[lut] 1 K1 CK2 Ck6 C[lutH1]2/[lut]2}[PhSH][Fe4S4] 1 1 K1 C[lutH1]/[lut] 1 K1 CK2 C[lutH1]2/[lut]2 (4) between equations (2) and (4) is the dependence on the concentration of PhSH in the numerator of the latter, consistent with the associative mechanism. Earlier studies 1 showed that the first protonation of [Fe4S4Cl4]22 is associated with pKa = 18.8.1 Hence, K1 C = 5.0 × 104 can be calculated and thus, under the conditions studied in this paper, K1 C[lutH1]/[lut] @ 1, and equation (4) simplifies to equation (5), with k5 C = 1.5 ± 0.2 × 104 dm3 mol21 s21, 2d[Fe4S4] dt = {k5 C 1 K2 Ck6 C[lutH1]/[lut]}[PhSH][Fe4S4] 1 1 K2 C[lutH1]/[lut] (5) k6 C = 4.0 ± 0.5 × 102 dm3 mol21 s21 and K2 C = 3.3 ± 0.2 × 102; pKa C = 16.6.[The value of k5 C determined in these studies is in excellent agreement with that determined earlier 4 using [NHEt3]1 (k5 C = 1.5 ± 0.2 × 104 dm3 mol21 s21).] The dramatically diVerent eVects of single and double protonation, on the lability of [Fe4S4Cl4]22 are not a consequence of the two protons binding to diVerent sites.We have already Fig. 2 The kinetics of the first substitution reaction of [Fe4S4Cl4]22 with PhSH, in the presence of acid in MeCN at 25.0 8C. EVect of single protonation (m): dependence of kobs/[PhSH] on [NHEt3 1]/[NEt3]. Curve and data from ref. 5. EVect of diprotonation (d): dependence of kobs/[PhSH] on [lutH1]/[lut]. Curve drawn is that defined by equation (5) and the values in the text.For the studies where [lutH1]/[lut] < 1.0, the calculated amount of lut was added to a solution containing [lutH1] = 10.0 mmol dm23 and [PhS2] = 5.0 mmol dm23J. Chem. Soc., Dalton Trans., 1998, Pages 1731–1733 1733 shown (with [NHEt3]1) that protonation of [Fe4S4Cl4]22 occurs exclusively at the cluster core (probably m3-S).4 Even with [lutH]1 protonation of the chloro-ligand is thermodynamically unfavourable (pKa HCl = 8.9).3 Why successive protonations Scheme 2 Mechanism for the associative acid-catalysed substitution reactions of [Fe4S4Cl4]22 (Fe = d; S = s).For clarity, only the Cl group undergoing substitution is shown aVect the lability of [Fe4S4Cl4]22 so diVerently is a consequence of this cluster undergoing substitution by an associative mechanism. The protonation of m3-S residues will have two eVects on the reaction. First, protonation will increase the Fe]Cl bond strength (the chloro-ligand is predominantly a s-donor and weak p-donor or -acceptor).This eVect alone would result in a decreased rate of substitution. However, protonation will also decrease the electron density on Fe thus facilitating attack by the PhSH nucleophile. Experimentally, we observed that the nett eVect of protonating one m3-S is to increase the rate of substitution (Fig. 2). This must be because the dominant eVect of single protonation is to facilitate nucleophilic attack. Protonation of two m3-S groups will compound these electronic eVects. However, the major eVect of the second protonation must be to further strengthen the Fe]Cl bond without significantly increasing the rate of nucleophilic attack, resulting in inhibition of the substitution.Although the behaviour described in this paper is unusual, its mechanistic origins indicate that it may operate in other systems. We have observed that the substitution reactions of [Fe4S4(SEt)4]22 (which reacts by a dissociative mechanism) are catalysed by the addition of one or two protons, whilst those of [Fe4S4Br4]22 and [{MoFe3S4Cl3}2(m-SPh)3]32 (which react by an associative mechanism) are catalysed by the addition of one proton, but inhibited by the addition of the second proton. Although this paper concerns the reactivity of clusters, in principle, other compounds could show this behavior.It appears that the only requirements are that the species undergoing substitution is capable of binding two protons and that the mechanism of the substitution step is associative. Acknowledgements We thank the BBSRC for funding this research. References 1 K. L. C. Grönberg and R. A. Henderson, J. Chem. Soc., Dalton Trans., 1996, 3667, and refs. therein. 2 K. L. C. Grönberg, C. A. Gormal, B. E. Smith and R. A. Henderson, Chem. Commun., 1997, 713. 3 K. Izutsu, Acid-Base Dissociation Constants in Dipolar Aprotic Solvents, Blackwell Scientific, Oxford, 1990. 4 R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Chem. Commun., 1994, 377. 5 R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1993, 1467. 6 For a discussion of the structural eVects of protonating co-ordinated sulfur atoms see, D. Sellmann and J. Sutter, Acc. Chem. Res., 1997, 30, 460, and refs. therein. Received 24th March 1998; Communication 8/02303D

ISSN:1477-9226    

DOI:10.1039/a802303d

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 1735–1740 1735 Structure, spectroscopy and electrochemistry of the bis(2,29-bipyridine)(salicylato)ruthenium(II) complex Vera R. L. Constantino,* Henrique E. Toma, Luiz F. C. de Oliveira, Francisca N. Rein, Reginaldo C. Rocha and Denise de Oliveira Silva Instituto de Química, Universidade de São Paulo, Caixa Postal 26077, CEP 05599-970, São Paulo, SP, Brazil Received 1st September 1998, Accepted 16th March 1999 The bis(2,29-bipyridine)(salicylato)ruthenium(II) complex has been prepared and characterized by means of single crystal X-ray diVraction, electrochemistry and resonance Raman spectroscopy.The electronic bands in the visible region have been assigned to Ru–bipy charge-transfer transitions and discussed in terms of ZINDO/S semiempirical calculations. Spectroelectrochemical measurements have been performed in order to elucidate the nature of the electrochemical waves in the cyclic voltammograms.The green complex generated by oxidation of the complex at 0.25 V has been isolated, revealing substantial ruthenium–salicylate electronic mixing, as deduced from the corresponding resonance Raman spectra. Further oxidations at 1.2 and 1.4 V have been observed and ascribed to hydroxylation of the salicylate semiquinone ligand in the complex. Introduction Salicylic acid is a typical bidentate ligand for transition metal ions. In addition to a wide range of biological applications, it is also a commonly used radical scavenger, reacting rapidly with hydroxyl radicals and singlet oxygen species.1 The presence of the carboxy-phenolic group supports a number of analytical applications, as exemplified by the colorimetric detection of iron(III) species.In spite of its traditional use in co-ordination chemistry, to our knowledge, the expected similarities with the redox active, non-innocent quinonoid ligands 2–6 have never been exploited up to the present time.A strong covalence involving this type of ligands has been suggested for the corresponding ruthenium–polypyridine complexes, giving rise to a very interesting discussion concerning the assignment of the redox states and valence localization.2–6 Along this line, here we report a detailed investigation on the spectroscopic and electrochemical behavior of the bis(2,29-bipyridine)(salicylato)- ruthenium(II) complex. Experimental and computation Preparation Bis(2,29-bipyridine)(salicylato)ruthenium(II) tetrahydrate.The complex [Ru(bipy)2(sal)]?4H2O (bipy = 2,29-bipyridine, sal = salicylate ion) was synthesized by treating 1 mmol of [Ru- (bipy)2Cl2] 7 with 1.3 mmol of salicylic acid (Aldrich) and 4 mmol of NaOH, in a 2: 1 water–ethanol mixture (140 cm3), under reflux for 15 h, in the presence of an argon atmosphere. The reaction mixture was kept in a refrigerator for about five days until precipitation of the complex. The solid was collected on a filter and washed with a small amount of cold water and acetone (Found: C, 53.4; H, 4.5; N, 9.3.C27H28N4O7Ru requires C, 52.2; H, 4.5; N, 9.0%). The water content was determined thermogravimetrically as 10.9% of weight, corresponding to four water molecules per mol of ruthenium. Yield: 67%. Bis(2,29-bipyridine)(salicylato)ruthenium(III) hexafluorophosphate dihydrate. The complex [Ru(bipy)2(sal)]PF6?2H2O was obtained by treating equimolar amounts of [Ru(bipy)2(sal)]? 4H2O and AgNO3 in 10 cm3 of 1 : 1 water–ethanol (v/v) solution.After 15 min the green solution was filtered and the solvent removed in a rotary evaporator. The residue was dissolved with a small amount of water and a green solid precipitated by adding NH4PF6. The product was collected on a filter, washed with small amounts of cold water and diethyl ether, and dried under vacuum (Found: C, 44.1; H, 3.2; N, 7.6. C27H24F6- N4O5PRu requires C, 44.4; H, 3.3; N, 7.6%). Yield: 80%. Crystal structure determination of [Ru(bipy)2(sal)]?4H2O Single crystals of [Ru(bipy)2(sal)]?4H2O were obtained from a water–ethanol reaction mixture by cooling at refrigerator temperature and a purple crystal of dimensions 0.40 × 0.30 × 0.20 mm was mounted on the top of a glass fiber. Intensity data were collected on an Enraf-Nonius MACH-3S diVractometer using the CAD4-EXPRESS software.8 The measurements were carried out at room temperature using graphite monochromated Mo-Ka (l = 0.71069 Å) radiation.The data were corrected for absorption using the MOLEN software package.9 The positions of the metal atom were located by the Patterson method in SHELXS 8610 and the positions of the other non-hydrogen atoms through a sequence of Fourier-diVerence maps and leastsquares cycles. The refinement by full-matrix least-square procedures was carried out by standard methods with the use of SHELXL 93.11 No hydrogen atoms were placed on the water oxygen atoms except for O(4).Crystal data. C27H28N4O7Ru, M = 621.60, monoclinic, space group P21/n (no. 14), a = 9.888(3), b = 16.899 (2), c = 15.560 (2) Å, b = 90.90 (2) 8, V = 2599.7(9) Å3, T = 293 K, Z = 4, m(Mo- Ka) = 0.658 mm21, 4840 reflections collected, 4560 unique (Rint = 0.1372). R1 = 0.0582, wR2 = 0.1417 for I > 2s(I); R1, wR2 = 0.1055, 0.1733 (all data); goodness of fit (on F2) = 1.049. CCDC reference number 186/1395. Physical techniques The electronic spectra of the complexes were recorded on a Hewlett-Packard model 8452-A diode-array spectrophotometer or on a Guided-Wave model 260 fiber-optics instrument.Cyclic voltammetry measurements were carried out with a Princeton Applied Research instrument consisting of a model 173 potentiostat and a model 175 universal programmer. A platinum electrode was employed for the measurements using a1736 J. Chem. Soc., Dalton Trans., 1999, 1735–1740 Luggin capillary with Ag–AgNO3 (0.010 M) reference electrode (E8 = 0.503 V versus NHE)12 in dmf containing 0.10 M Et4NClO4.A platinum wire was used as the auxiliary electrode. The spectroelectrochemical measurements were carried out as previously described.13 The Raman spectra were obtained as reported previously using Jarrell-Ash 14,15 or Renishaw16 equipment, EPR spectra for the [Ru(bipy)2(sal)]PF6 solid complex on a Bruker EMX spectrometer, at room temperature and 220 K (working conditions: 2000–4000 G, 9.312 GHz microwave power = 20 mW, modulation frequency = 100 kHz, mod.amplitude 12 G). Thermogravimetric measurements were carried out using a Shimadzu model TGA-50 instrument. Molecular calculations Semiempirical molecular orbital calculations were carried out by using the INDO/S method 17 within the ZINDO 18 program from Molecular Simulation Inc., using default energy parameters but with b(4d) = 216 eV. As interaction factors, the values fps = 1.267 and fpp = 0.525 were used. SCF Molecular orbitals were obtained at the RHF (Restricted Hartree–Fock) and ROHF (Restricted Open-Shell Hartree–Fock) levels for the closed-shell (RuII) and open-shell (RuIII) ground state species, which correspond to the normal (t2g) 6 and (t2g) 5 configurations, respectively.Electronic spectra were generated by single CI excitations in a symmetric active space involving 20 frontier molecular orbitals (10 highest occupied and 10 lowest unoccupied MOs). The nuclear co-ordinates used were obtained from the crystallographic data for the [Ru(bipy)2(sal)] complex.Geometry optimizations were carried out as necessary, using the MM1 module, a modified MM2 force field 19 within the HyperChem 4.5 program. In this case, a gradient of 1 × 1025 kcal Å21 mol21 was used as a convergence criterion in a conjugate gradient method. All the calculations were processed on a SGI Indigo 2 R10000 workstation. Results and discussion Crystal structure of [Ru(bipy)2(sal)]?4H2O The molecular structure of [Ru(bipy)2(sal)] is shown in Fig. 1, and selected bond distances and angles are given in Table 1. The ruthenium(II) ion is chelated by two bipyridine and one salicylate ligand. The Ru–N(1) and Ru–N(4) distances are equivalent, as expected from their symmetric localization; however, the Ru–N(2) bond (2.018 Å) is significantly shorter than Ru–N(3) (2.038 Å). On the other hand, the Ru–O(2) bond distance (2.069 Å) is longer than Ru–O(1) (2.042 Å), indicating that the carboxylate oxygen is more weakly bound than the Fig. 1 An ORTEP20 drawing of the molecular structure and atom numbering scheme for [Ru(bipy)2(sal)]. The atoms are represented by thermal ellipsoids at 40% probability level. phenolate group. The bipyridine rings are approximately coplanar, exhibiting a dihedral angle of 5.68. The ruthenium– salicylate moiety is essentially planar. There are four molecules of water associated with each ruthenium complex in the crystal. Hydrogen bonds can be detected between the lattice water molecules and the non-coordinated carboxylic oxygen (typical O ? ? ? O distance = 2.69 Å), as well as between the water molecules themselves (typical O? ? ? O distance = 2.86 Å).Electronic and resonance Raman spectra of [RuII(bipy)2(sal)] The electronic spectrum of the [RuII(bipy)2(sal)] complex consists of three broad, composite bands around 590, 400 and 290 nm, as shown in Fig 2A. The low energy band is rather peculiar, since for ruthenium polypyridine complexes the ruthenium(II)- to-bipy charge-transfer bands are usually observed in the 400– 500 nm range.On the other hand, the salicylate ligand is a typical donor species which forms stable complexes with metal ions in relatively high oxidation states. Therefore, low energy ruthenium(II)-to-salicylate charge transfer bands are not expected, as would be the case of complexes with p-acceptor ligands. In order to elucidate this point, theoretical calculations were carried out using the spectroscopic implementation of the ZINDO semiempirical method, ZINDO/S (see Experimental Fig. 2 Electronic spectra of [Ru(bipy)2(sal)]: (A) experimental and (B) ZINDO/S calculated bands and theoretical simulation assuming a half-bandwidth of 2000 cm21 and Lorentzian lines.Table 1 Selected bond lengths (Å) and angles (8) for [Ru(bipy)2- (sal)]?4H2O Ru–N(1) Ru–N(2) Ru–N(3) Ru–N(4) N(2)–Ru–N(1) N(3)–Ru–N(4) N(2)–Ru–N(3) N(2)–Ru–N(4) N(3)–Ru–N(1) N(1)–Ru–N(4) O(1)–Ru–O(2) C(8)–N(1)–Ru C(12)–N(1)–Ru C(13)–N(2)–Ru C(17)–N(2)–Ru 2.042(5) 2.018(5) 2.038(5) 2.047(5) 79.5(2) 79.4(2) 96.9(2) 97.0(2) 99.0(2) 176.0(2) 90.2(2) 127.2(4) 114.8(4) 115.3(4) 126.3(4) Ru–O(1) Ru–O(2) N(1)–Ru–O(1) N(2)–Ru–O(1) N(3)–Ru–O(2) N(4)–Ru–O(2) N(1)–Ru–O(2) N(4)–Ru–O(1) N(2)–Ru–O(2) N(3)–Ru–O(1) C(1)–O(1)–Ru C(7)–O(2)–Ru C(18)–N(3)–Ru C(22)–N(3)–Ru C(23)–N(4)–Ru C(27)–N(4)–Ru 2.042(4) 2.069(4) 87.5(2) 87.1(2) 86.3(2) 88.7(2) 94.8(2) 94.3(2) 173.8(2) 172.8(2) 125.6(4) 129.2(4) 126.1(5) 115.4(4) 115.4(4) 125.8(5)J.Chem. Soc., Dalton Trans., 1999, 1735–1740 1737 Table 2 Electronic spectrum of [Ru(bipy)(sal)] Experimental Calculated (ZINDO/S) Transition a l/nm 290 400 c 590 d e/M21 cm21 3.0 × 104 8.1 × 103 7.3 × 103 l/nm 283, 289 365 390, 400 430 511 575 642 Osc. strength 0.686, 0.408 0.040 0.094, 0.121 0.110 0.028 0.245 0.048 MOi æÆ MOf 80, 81 æÆ 88 85, 87 æÆ 92; 86 æÆ 93 87 æÆ 92; 86 æÆ 91 86,87 æÆ 90; 87 æÆ 91 86 æÆ 89 85, 86 æÆ 88 86 æÆ 88; 87 æÆ 89 Assignment Internal bipy LLCT/MLCTb LLCT, MLCT MLCT/LLCTb MLCT MLCT MLCT/LLCTe a Only the main components of the transition. b Predominant character.c A broad composite band from 330 to 460 nm. d An envelope from 490 to near 800 nm containing at least 3 bands (maximum absorption around 590 nm). e MLCT and LLCT with equivalent character. and computational section for more details about the calculations). The results from the spectral simulation were surprisingly good, as shown in Fig. 2B and Table 2, supporting the assignment of the visible bands in terms of ruthenium(II)-topyridine charge-transfer transitions predicted at 365, 400, 511 and 575 nm, involving two occupied, predominantly dp metal levels (Table 3) split in a low symmetry field [MO numbers 85 (61.5% Ru) and 86 (53.5% Ru)] and three sets of nearly degenerate p*(bipy) empty levels [MO numbers 88 (LUMO), 89; 90, 91 and 92, 93 (all > 90% bipy)]. Although other bands show a minor LLCT character, the one at 430 nm may be ascribed predominantly to a salicylate-to-bipyridine charge transfer, since it involves primarily the MO 87 (HOMO; 63.0% sal) and the unoccupied MO 90 and 91 (bipy) levels.A low energy transition is expected at 642 nm, involving the occupied MOs 86 (Ru) and 87 (HOMO; sal) and the unoccupied MOs 88 and 89 (essentially bipy), exhibiting balanced ligand-toligand and metal-to-ligand charge-transfer characters (MLCT/ LLCT).This feature has also been recently observed in other related systems.21,22 The absorption band at 290 nm (calculated at 285 nm) can be assigned to p æÆ p* internal transitions in the bipyridine ligand (Table 2). According to the theoretical calculations, these transitions also exhibit a substantial chargetransfer character due to the strong mixing of the ruthenium and bipy p orbitals (see Table 3 for a complete description of the ordering and the fractional mixing of the molecular orbitals). Typical resonance Raman spectra of [RuII(bipy)2(sal)] are shown in Fig. 3. In addition to the intensity variations, which followed approximately the absorption profile, the enhanced peaks in the Raman spectra were characteristic of bipyridine Table 3 MO Energy order and fractional orbital mixing of [Ru(bipy)2- (sal)] MO 80 81 82 83 84 85 86 87 (HOMO) 88 (LUMO) 89 90 91 92 93 94 Energy/eV 28.893 28.853 28.176 27.872 27.726 27.035 26.684 26.415 21.351 21.160 20.671 20.531 20.294 20.242 0.434 Ru 0.009 0.003 0.012 0.059 0.351 a 0.615 b 0.535 c 0.285 d 0.054 0.092 0.008 0.047 0.023 0.029 0.002 bipy 0.974 0.991 0.005 0.021 0.125 0.178 0.327 0.085 0.939 0.894 0.991 0.948 0.975 0.967 0.997 sal 0.017 0.006 0.983 0.920 0.524 0.207 0.138 0.630 0.007 0.014 0.001 0.005 0.002 0.004 0.001 a 0.074 dxy 1 0.107 dxz 1 0.018 dyz 1 0.041 dx2 2 y2 1 0.110 dz2 (14s and 4p).b 0.027 dxy 1 0.002 dxz 1 0.125 dyz 1 0.431 dx2 2 y2 1 0.028 dz2 (14s and 4p).c 0.066 dxy 1 0.176 dxz 1 0.055 dyz 1 0.088 dx2 2 y2 1 0.145 dz2 (14s and 4p). d 0.039 dxy 1 0.080 d2xz 1 0.042 dyz 1 0.009 dx2 2 y2 1 0.113 dz2 (14s and 4p). vibrational modes, i.e. at 1595, 1544, 1470 (nCC,CN); 1318, 1260 (nCC,CN 1 dCCH); 1169, 1018 (dCCH 1 nCC); 662 (dCCC 1 nRuN); 373 cm21 (nRuN 1 aCCC). The Raman spectra were very similar to those observed for related ruthenium(II)–bipyridine complexes, 13–16,23 reinforcing our assignment of the charge-transfer bands in the visible region.Electrochemical and spectroelectrochemical behavior Characterization of the green oxidation product. Typical cyclic voltammograms for the [Ru(bipy)2(sal)] complex are shown in Fig. 4. By starting at 20.1 V and scanning in the direction of more positive potentials, a reversible wave (1) was observed at 0.24 V. Apparently, the position of this wave is unusual for ruthenium-(II)-(III) polypyridine complexes; however it fits the linear correlation between the redox potentials and the RuII–bipy MLCT wavenumbers previously reported for [Ru- (bipy)2L2] complexes,24 eqn.(1). For E(RuIII/II = 0.24 V, the expected wavenumber for the Ru–bipy MLCT band is 17300 cm21 (575 nm), while the observed one is 16700 cm21 (590 nm). Fig. 3 Typical resonance Raman spectra of [Ru(bipy)2(sal)] and the oxidized product [Ru(bipy)2(sal)]PF6 obtained at (A) lexc = 514.5 nm, (B) lexc = 632.8 nm, using pure solid samples (the salicylate peaks are indicated by an arrow).1738 J.Chem. Soc., Dalton Trans., 1999, 1735–1740 E (MLCT, cm21) = 5242 E (RuIII/II, V vs. NHE) 1 16130 (1) Therefore, the E(RuIII/II) value is consistent with the red shift of the Ru–bipy MLCT band, reflecting the influence of the donor properties of the salicylate ligand. The presence of a donor ligand should stabilize the higher oxidation state, decreasing E8, but in a limiting case the donor ligand can also be oxidized, competing with the metal center. By working under equivalent conditions, however, the first oxidation peak in the electrochemistry of free salicylate appeared at relatively high potentials (0.9 V), ruling out this hypothesis.Spectroelectrochemical measurements associated with this wave in the complex exhibited a reversible behavior, as shown in Fig. 5A. The oxidized product displayed a deep green color, and was stable enough to be isolated as a solid (see Experimental section). The spectrum of the oxidized product consisted of two absorption bands in the visible region, at 700 and 420 nm, departing from the typical spectra of the ruthenium-(II) or -(III) Fig. 4 (A) Cyclic voltammograms of [Ru(bipy)2(sal)], 1 mmol dm23 in dmf, showing (B) the reversible behavior of the starting complex at 100, 50 and 20 mV s21 and (C) the formation of a new, reversible couple in the reverse scan, after reaching 1.5 V. Table 4 MO Energy order and fractional orbital mixing of [Ru(bipy)2- (sal)]1 MO 80 81 82 83 84 85 86 87 (HOMO) 88 (LUMO) 89 90 91 92 93 94 Energy/eV 211.784 211.708 211.665 211.032 210.892 210.698 210.552 29.871 24.352 24.227 23.566 23.513 23.231 23.130 22.474 Ru 0.002 0.006 0.729 a 0.015 0.471 b 0.521 c 0.019 0.161 d 0.031 0.044 0.004 0.015 0.009 0.009 0.001 bipy 0.990 0.983 0.077 0.005 0.236 0.185 0.008 0.079 0.964 0.951 0.995 0.983 0.990 0.989 0.998 sal 0.008 0.011 0.194 0.980 0.293 0.294 0.973 0.760 0.005 0.005 0.001 0.002 0.001 0.002 0.001 a 0.031 dxy 1 0.012 dxz 1 0.061 dyz 1 0.295 dx2 2 y2 1 0.328 dz2 (14s and 4p).b 0.106 dxy 1 0.282 dxy 1 0.015 dyz 1 0.042 dx2 2 y2 1 0.023 dz2 (14s and 4p). c 0.068 dxy 1 0.020 dxz 1 0.164 dyz 1 0.190 dx2 2 y2 1 0.075 dz2 (14s and 4p). d 0.018 dxy 1 0.060 dxz 1 0.002 dyz 1 0.055 dx2 2 y2 1 0.025 dz2 (14s and 4p). polypyridine complexes, or from any kind of spectroelectrochemical correlation reported before. Theoretical calculations for the [Ru(bipy)2(sal)]1 complex (Table 4), based on the ZINDO/S method, revealed that: among the six highest occupied levels, one is mainly Ru (dp) [MO 82 (72.9% Ru)], two exhibit extensive Ru–bipy–salicylate mixing [e.g.MOs 84 (47.1% Ru 1 23.6% bipy 1 29.3% sal) and 85 (52.1% Ru 1 18.5% bipy 1 29.4% sal)] and three exhibit predominantly a salicylate character [MOs 83 (98.0% sal), 86 (97.3% sal) and 87 (76.0% sal)]. The fact that the frontier occupied orbital (HOMO; number 87) is made up of, in its majority, salicylate ligand and only 16% of RuIII (dp) suggests a description of the oxidized complex as [RuII(bipy)2(sal1)]1 better than [RuIII(bipy)2(sal)]1.On the other hand, the lowest unoccupied levels [MOs 88 (LUMO), 90 and 91)] are essentially bipy (pp*) (>95% bipy). The spectral simulations were not as good as in the case of the starting complex, however a series of transitions were located in the 350–700 nm range, from the HOMO 83–87 (donor) levels to the LUMO 88–91 (acceptor) levels. Considering the nature of the HOMO and LUMO levels involved in the electronic transitions, it should be noted that there is a strong electronic mixing of the ruthenium–bipy– salicylate p orbitals in the HOMO levels.The electronic transitions exhibit pronounced (salicylate 1 bipy) ligand-to-metal charge transfer character, as well as (salicylate-to-bipy) ligandto- ligand charge transfer, as observed in many quinonoid complexes.2–6,21,22 As an indication of strong electronic coupling, EPR measurements for the green solid at room temperature, as well as at 220 K, exhibited no evidence of the multiplet signals characteristic of aromatic semiquinone radicals, or of the typical ruthenium(III) signals.Fig. 5 Spectroelectrochemistry of the [Ru(bipy)(sal)] complex in dmf solutions, showing (A) the reversible behavior of the starting complex, (B) the decay of the visible absorption bands accompanying the oxidation processes at 1.38 V, (C) the formation of a new species after returning the potential from 1.38 to 0 V, and (D) the spectral changes associated with the reduction of the bipy ligand, at 21.5 V.J. Chem.Soc., Dalton Trans., 1999, 1735.1740 1739 Resonance Raman spectra for the green oxidized species revealed a simultaneous enhancement of the bipyridine and salicylate vibrations, as shown in Fig. 3. The bipyridine peaks at 1587, 1550, 1460, 1320, 1153, 1136, 1034, 662, and 370 cm21 (the 370 cm21 band occurs at low intensity) practically coincide with those observed for the reduced species.The salicylate peaks were identified based on a recent study on the resonance Raman spectra of the ligand, at 1441 (nCOO 1 n14), 1219 (nC.O 2), 879 (nC.COO 2 1 nC.O 2), 704 (n11), 582 (fCCC 1 rCOO 2), 490 (fCCC 1 dC.O 2) and 424 cm21 (n16) where the frequency notations refer to Wilsons numbering for benzene vibrational modes.25 The peak at 310 cm21 was ascribed to the Ru.O vibrational mode based on its strong enhancement, and on its absence in the spectra of the bipyridine and salicylate ligands.The enhancement of the bipyridine and salicylate ligand vibrations confirms the hypothesis of a strong mixing of the ligand orbitals in the oxidized complex, as is the case of the ruthenium polypyridine complexes with quinonoid and related ligands.2.6 Electrochemical oxidation of the green species. Further oxidation of the green [Ru(bipy)2(sal)]1 complex, generated at 0.24 V, was evidenced by two successive, irreversible waves at 1.2 and 1.4 V (waves 2 and 3 in Fig. 4A). The corresponding spectroelectrochemical changes (Fig. 5B) cannot be discriminated due to the close proximity of the redox waves and to their irreversible nature. The oxidation wave at 1.2 V can be ascribed to the monoelectronic oxidation of salicylate ligand, based on the electrochemical behavior of the sodium salicylate species, and on the consideration that by reversing the potential scan at this point the voltammogram of the starting complex is regenerated (see Fig. 4A). Therefore, the co-ordinated ligand remains intact up to this point. At 1.4 V, however, the oxidation process leads to an irreversible chemical change, generating a new species that exhibits, in the reverse scan, a reversible wave (4) at 0.55 V, as shown in the cyclic voltammograms of Fig. 4C. The isolation of this species has not been successful up to the present time; however, the spectroelectrochemical results shown in Fig. 5C-c resemble those associated with typical ruthenium(II).polypyridine complexes. Therefore, one can propose that the electrochemical process at 1.2 V involves the oxidation of the salicylate ligand to the semiquinone form, which is further oxidized at 1.4 V, generating a very reactive electrophilic species. In the presence of water molecules or OH2 ions, hydroxylation proceeds very fast,1 yielding the 4-hydroxysalicylate semiquinone complex as the most probable species, as shown in Scheme 1.The ZINDO/S spectral simulations for this complex reproduced the absorption profile centered around 500 nm, in Fig. 5C-c, showing, in addition, a series of ruthenium(II)-tobipy dp.pp charge-transfer transitions at 590 (weak), 500 (strong), 440 (weak), 400 (weak) and 380 nm (weak). The location of the main MLCT band at 500 nm is consistent with the expected value (520 nm, for E8 = 0.55 V) based on the reported spectroscopic.electrochemical correlation for [Ru- (bipy)2L2] complexes.24 Scheme 1 O IIRu O C H O O IIIRu O C H O O IIIRu O C H O O IIRu O C O O IIIRu O C O O IIIRu O C H O H OH H OH HO. 0.25 V 1.2 V 1.4 V 0.55 V It should be noted that, according to the cyclic voltammograms (Fig. 4), the expected dimerization of the radical species generated at 1.2 V (wave 2) does not compete with the oxidation process at 1.4 V (wave 3). In fact, the dimerization process does not require any additional electron transfer, and cannot be ascribed to the oxidation wave observed at 1.4 V.This process is actually responsible for the formation of the product absorbing at 500 nm. According to ZINDO/S calculations, the spectrum of the dimeric product would be very similar to that of the [Ru(bipy)2(sal)] complex, departing from the observed results shown in Fig. 5C(a,c). Electrochemical reduction of the [Ru(bipy)2(sal)]complex. An irreversible wave (5) was observed at 21.0 V, by scanning the potential in the direction of negative potentials (Fig. 4). The intensity and shape of this wave vary with the potential employed in the reverse scan (see Fig. 4). Since there were no detectable changes in the electronic spectra of the complex, the most plausible assignment would be the reduction of the water molecules, as well as of the protons released from the hydroxylation process. In contrast, the reduction of the bipyridine ligands can be detected at 21.5 V (wave 6, in Fig. 4), from the decay of the absorption band in the UV region, and the dramatic changes in the absorption bands in the visible region (Fig. 5D). Conclusion Salicylic acid forms a stable mixed-ligand complex with bis(bipyridine) ruthenium(II), displaying low energy electronic bands mainly associated with ruthenium-to-bipy charge-transfer transitions. Oxidation proceeds reversibly at 0.24 V generating an unusual green species involving a strong ruthenium.salicylate electronic delocalization. Further oxidation of the green complex leads to hydroxylation of the salicylate semiquinone ligand in the complex.Acknowledgements The support from FAPESP (Fundacao de Amparo a Pesquisa do Estado de Sa¢§o Paulo), CNPq (Conselho Nacional de Desenvoluimento Cientifico e Tecnologico) and PADCT (Programa de Apoio ao Desenvoluimento Cientifico e Tecnologico) is gratefully acknowledged. We thank the Laboratorio de Espectroscopia Molecular (Instituto de Quimica, Universidade de Sao Paulo) for the use of Raman equipment.We would also like to thank Dr Ana M. C. Ferreira (Instituto de Quimica, Universidade de Sao Paulo) for her assistance in EPR experiments. References 1 B. Kalyanaraman, S. Ramanujam, R. J. Singh, J. Joseph and J. B. Feix, J. Am. Chem. Soc., 1993, 115, 4007. 2 C. G. Pierpont and C. W. Lange, Prog. Inorg. Chem., 1994, 41, 331. 3 R. A. Metcalfe and A. B. P. Lever, Inorg. Chem., 1997, 36, 4762. 4 F. Hartl, T. L. Snoeck, D. J. Stufkens and A. B. P. Lever, Inorg. Chem., 1995, 34, 3887. 5 H.Masui, A. B. P. Lever and E. S. Dodsworth, Inorg. Chem., 1993, 32, 258. 6 H. Masui, A. B. P. Lever and P. R. Auburn, Inorg. Chem., 1991, 30, 2402. 7 B. P. Sullivan, D. J. Salmon and T. J. Meyer, Inorg. Chem., 1978, 17, 3334. 8 CAD4-EXPRESS, version 5.1, Delft Instruments X-Ray DiVraction, Delft, 1992. 9 L. H. Straver and A. J. Schierbeek, MOLEN Structure Determination System, Enraf-Nonius Corp., Delft, 1994. 10 G. M. Sheldrick, SHELXS 86, in Crystallographic Computing 3, eds. G. M. Sheldrick, C. Kruger and R. Goddard, Oxford University Press, Oxford, 1985, p. 175. 11 G. M. Sheldrick, SHELXL 93, in Crystallographic Computing 6, eds. H. D. Flack, L. Parkanyi and K. Simons, Oxford University Press, Oxford, 1993, p. 111. 12 B. Kratochvil, E. Lorah and C. Garber, Anal. Chem., 1969, 41, 1793.1740 J. Chem. Soc., Dalton Trans., 1999, 1735–1740 13 H. E. Toma and C. Cipriano, J. Electroanal. Chem., Interfacial Electrochem., 1989, 263, 313. 14 V. R. L. Constantino, L. F. C. Oliveira, P. S. Santos and H. E. Toma, Transition Met. Chem., 1994, 19, 103. 15 V. R. L. Constantino, H. E. Toma, L. F. C. Oliveira and P. S. Santos, J. Raman Spectrosc., 1992, 23, 629. 16 T. E. Chavez-Gil, D. L. A. de Faria and H. E. Toma, Vibrational Spectrosc., 1998, 16, 89. 17 M. C. Zerner, G. H. Loew, R. F. Kirchner and U. T. Mueller- WesterhoV, J. Am. Chem. Soc., 1980, 102, 589. 18 ZINDO, A comprehensive semiempirical SCF/CI package, M. C. Zerner and co-workers, University of Florida, Gainesville, FL, 1995. 19 N. L. Allinger, J. Am. Chem. Soc., 1977, 99, 8127; HyperChem 4.5, Hypercube, Inc., Gainesville, FL, 1995. 20 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge. National Laboratory, Oak Ridge, TN, 1976. 21 M. D. Ward, Inorg. Chem., 1996, 35, 1712. 22 W. Paw, W. B. Connick and R. Eisenberg, Inorg. Chem., 1998, 37, 3919. 23 P. K. Mallick, G. D. Danzer, D. P. Strommen and J. R. Kincaid, J. Phys. Chem., 1988, 92, 5628. 24 B. K. Ghosh and A. Chakravorty, Coord. Chem. Rev., 1989, 95, 239. 25 B. Humbert, M. Alnot and F. Quiles, Spectrochim. Acta, Part A, 1998, 54, 465. Paper 9/02219H

ISSN:1477-9226    

DOI:10.1039/a902219h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 1741–1751 1741 Redox reactions of the boron subhalide clusters BnCln 0/~2/22 (n 5 8 or 9) investigated by electrochemical and spectroscopic methods † Bernd Speiser,*a Carsten Tittel,a Wolfgang Einholz b and Ronald Schäfer b a Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany. E-mail: bernd.speiser@uni-tuebingen.de b Institut für Chemie, Universität Hohenheim, Garbenstraße 30, D-70593 Stuttgart, Germany Received 23rd November 1998, Accepted 31st March 1999 The redox properties of the electron hyperdeficient boron subhalide clusters octachlorooctaborane(8), B8Cl8, and nonachlorononaborane(9), B9Cl9, were investigated in solution by cyclic voltammetry at platinum or glassy carbon electrodes, and by 11B NMR as well as ESR spectroscopy.The neutral compounds undergo a spontaneous reduction by traces of moisture usually present even in dried solvents, and the voltammetric experiment starts from B8Cl8~2 or B9Cl9~2.The radical anions were identified by ESR spectroscopy. Their formation leads to line broadening in NMR spectra of BnCln. Electrochemically, they are quasireversibly reduced to the dianions, but oxidized in an ECcat (electrochemical step, catalytic chemical step) reaction with an essentially reversible electron transfer step to the neutral compounds. The potential ordering for the two redox processes is “normal” in both clusters, being in accordance with the fact that structural changes accompanying the electron transfer are minor.The radical anion B8Cl8~2 is even more stable against disproportionation than B9Cl9~2. Introduction Multiple-stage redox systems have extensively been studied in the case of organic (see, e.g. ref. 2) and organometallic (see, e.g. ref. 3) molecules. The spacing of the redox potentials for subsequent one-electron steps is an important factor which contributes to the behavior of compounds with several oxidation states.4 Usually one would expect that oxidation or reduction becomes increasingly diYcult with increasing or decreasing redox state of the molecule.It is thus common to find the diVerence in formal potentials in eqn. (1) for two-electron transfer DE0 = E2 0 2 E1 0 (1) reactions, eqns. (2) and (3), where the superscript for all species A0 A1± ± e2 E1 0 (2) A1± A2± ± e2E2 0 (3) indicates the diVerence in redox state relative to Ao (“1” for oxidations, “–” for reductions; often, but not always, the stable starting species is one of the “extreme” oxidation states, Ao or A2±; Ao is not necessarily neutral) to be positive for oxidations and negative for reductions. The symbol |DE0| will denote E2 0 2 E1 0 for an oxidation and 2(E2 0 2 E1 0) for a reduction.Then, the equilibrium (4) is characterized by an equilibrium constant A0 1 A2± 2 A1± (4) in eqn. (5). In aprotic solvents |DE0| often attains values of Kcomp = [A1±]2 [A0][A2±] = exp F F RT |DE0|G (5) approximately 0.4–0.5 V 4 (“normal potential ordering”).However, several examples have been identified where |DE0| is † Two-electron-transfer redox systems. Part 2.1 decreased to values <0.4 V (“potential compression”) 5 or the second electron transfer even occurs thermodynamically easier than the first one (“potential inversion”).6 In systems with potential inversion the intermediate redox state A1± is unstable with respect to disproportionation (4).Potential inversion is usually accompanied by a considerable change in the structure of the molecule during the redox process, for example conformational changes 1,7–10 or changes in cluster geometry.11 One class of chemical compounds which could undergo twoelectron transfers is the series of boron subhalide clusters with a 1 : 1 stoichiometry of boron vs. halogen. The chloroborane clusters BnCln (n = 4, 8–12), of which B8Cl8 and B9Cl9 are investigated in this study, are classified as electron hyperdeficient molecules 12 and sometimes are called hypercloso clusters,13 since the number of their framework electrons is 2n.The corresponding dianions closo-BnCln 22 (n = 6, 8–12) as well as the borate clusters closo-BnHn 22 possess 2n 1 2 cage bonding electrons and follow Wade’s rules of the framework electron count to structure correlation.14 Nevertheless, the structures both of hypercloso-BnCln (n = 8, 9) and closo-BnHn 22 are based upon the same n-vertex deltahedra: dodecahedron (D2d symmetry) for B8Cl8 15–17 and B8H8 22,18 tricapped trigonal prism (D3h or C3v symmetry, respectively) for B9Cl9 19,20 and B9H9 2221 (Fig. 1). For the system B9Br9 0/22 it has recently been confirmed by X-ray crystallographic analysis and ELF (electron localization function) calculations 19 that the cluster structure remains intact upon the redox conversion while changes in atomic distances and bond angles occur.Such a behavior is in sharp contrast to Fig. 1 Geometrical shapes of the deltahedral boron subhalide clusters BnCln (n = 8 or 9).1742 J. Chem. Soc., Dalton Trans., 1999, 1741–1751 the structural rearrangements found during reduction of [Os6- (CO)18], which changes from a bicapped tetrahedral structure (neutral; 2n framework electrons) to an octahedron (dianion; 2n 1 2 framework electrons).11 As well, the 6-vertex borate clusters B6X6 22 (X = Cl, Br, I or H) 22–25 show the expected geometry of an octahedron, whereas the hypothetical neutral B6H6 is suggested by an ab initio study26 to have a capped trigonal bipyramidal (bicapped tetrahedral) structure like [Os6(CO)18].Furthermore, B4Cl4 molecules are tetrahedral with nearly Td symmetry,19,22,27 but the hypothetical B4H4 22 ion is predicted by MNDO28 and ab initio calculations 29 to exhibit a puckered D2d conformation. The reasons for these structural features can be traced to the degeneracy or non-degeneracy of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbital of the polyhedrons:30–32 the 8-vertex D2d dodecahedron (B8X8 and B8X8 22) and the 9-vertex D3h tricapped trigonal prism (B9X9 and B9X9 22) have non-degenerate frontier orbitals (HOMO and LUMO), and thus can accommodate, n, n 1 1 or n 1 2 framework electron pairs.In contrast, the HOMOs and LUMOs of most of the other closo-borates BnHn 22 (n = 4–7, 10, 12) are degenerate. Removing two electrons from these clusters must result in a change of the structure according to Jahn– Teller theory.33 The 11B NMR spectra of B8Cl8 and B9Cl9 do not show two diVerent signals as would be expected by considering the molecular structure in the solid state, but only a single sharp resonance line (d11B 64.8 for B8Cl8 and 58.2 for B9Cl9, h1/2 ª 35 Hz).This eVect can be explained by the rapid fluctuation of these molecules in solution, which is described for the related eight-vertex cluster B8H8 22 by the diamond–square–diamond transformation.34 In contrast, the 11B NMR spectrum of B9Cl9 22 exhibits two peaks at d 21.5 and 25.5 in an intensity ratio of 1 : 2 representing the three boron atoms with a connectivity of 4 and the six boron atoms with a connectivity of 5 in the cage.19,35 Thus, there is no fluxional behavior or the transformation is very slow on the NMR timescale.The dianion of the eight-vertex polyhedron B8Cl8 22 is not yet known in the literature.Since the corresponding hydrogen substituted cluster B8H8 22, however, shows structural non-rigidity in solution, indicated by the appearance of only one 11B NMR signal at room temperature (d11B 25.8, doublet, JB–H = 128 Hz)34,36 we could expect the same structural features for B8Cl8 22. In earlier work, B9Cl9 was reduced chemically to both its paramagnetic mono- and its di-anion, and B9Cl9 22 oxidized by thallium(III) trifluoroacetate to the higher oxidation states.35 Bowden37 oxidized B9Cl9 22 electrochemically in CH2Cl2 and CH3CN, while Kellner 38 investigated the neutral B9Cl9 at a glassy carbon electrode in CH2Cl2.‡ In all cases, a stepwise redox reaction in the system B9Cl9 0/~2/22 was found, with all three redox states being stable in solution.The electron transfer chemistries of the smaller homologue B8Cl8 or its dianion do not seem to have been investigated. Our results of a cyclic voltammetric and spectroscopic study of nonachlorononaborane(9), B9Cl9, and octachlorooctaborane( 8), B8Cl8, are presented in this paper.Besides characterizing the redox chemistry of B8Cl8 for the first time, and determining the relative potential ordering of its formal potentials, we identified the starting species of the experiments to be diVerent from BnCln by means of rest potential measurements and ESR as well as NMR spectroscopy. Computer simulations ‡ After finishing electrochemical experimental work for the present manuscript we became aware that similar cyclic voltammetric investigations of B9Cl9 but not B8Cl8 had been conducted at the Universität Stuttgart, Germany, and that a manuscript was being prepared by the groups involved. Preliminary manuscripts were exchanged in September 1998.We refer to this version of the Stuttgart manuscript,39 which incorporates parts of the dissertation of Kellner.38 DiVerences and similarities will be discussed in the course of the present paper.of the cyclic voltammograms allowed the determination of kinetic constants. Results and discussion Overall electrochemistry of BnCln Earlier cyclic voltammetric work with B9Cl9 2237 and B9Cl9 38,39 indicates that the redox states of the nonachlorononaborane(9) cluster can be converted in two stepwise one-electron transfers. Based on these results, we expected that B9Cl9, and in analogy also B8Cl8 would be stable at suYciently positive electrode potentials E, and could be reduced to the respective dianions upon variation of E to less positive and finally negative values.Starting the voltammetric scan at rather positive potentials, both B9Cl9 and B8Cl8 in the dichloromethane electrolyte at Pt and glassy carbon (GC) electrodes indeed exhibit seemingly simple cyclic voltammograms with two separate peak couples (Fig. 2; the concentration of B8Cl8 used to record this voltammogram is only approximate due to some possible decay of the cluster during transfer to the cell).A close inspection of the current–potential curves, however, shows that at the starting potential of the voltammetric scan, where the BnCln were expected to be stable, an appreciable oxidation current flows, even though the electrode is held at this potential for a “quiet time” of 10 s before the scan is actually initiated. This indicates that at the beginning of the experiment a species is present which can be oxidized at the rather positive starting potentials.It should be noted that published voltammograms of B9Cl9 and B9Br9 exhibit the same feature.38,39 The rest potential, Erest, which is the potential at which no current flows through the working electrode in a particular electrolyte, provides a measure of the potential region where the initial species in the electrolyte is stable. Experimental determinations of Erest in the BnCln solutions immediately after dissolution of the neutral halides indeed result in values positive of the more anodic of the two peak couples in the voltammograms.However, Erest is not stable and decreases to less positive values (Fig. 3). After some time a stable state is reached, with Erest now located between the two peak couples of the respective voltammogram. Hence, the neutral clusters seem to undergo a reaction in the electrolyte to a product which is a less strong oxidant. If the cyclic voltammetric starting potential is selected close to the steady-state value of Erest, current–potential curves with a negligible current at Estart can be recorded for both clusters (Fig. 4). For the discussion below, only voltammograms recorded from such a starting potential were used. They show that the starting species is formed essentially quantitatively after dissolution and equilibration, and that it can be both oxidized and reduced in at least partially chemically and electrochemically reversible steps. In the case of B9Cl9 further, less intensive oxidation waves at more positive potentials were also observed.These will, however not be evaluated in the present paper. Fig. 2 Cyclic voltammograms of B9Cl9 (solid line) and B8Cl8 (dotted line) in CH2Cl2–0.1 M NBu4PF6 at a glassy carbon (GC) electrode with starting potentials located at values positive of both redox peak couples; c0(B9Cl9) = 1.7 mM, c0(B8Cl8) ª 2 mM, v = 0.2 V s21.J. Chem. Soc., Dalton Trans., 1999, 1741–1751 1743 Fig. 3 Temporal development of the rest potential Erest in solutions of B9Cl9 (a) and B8Cl8 (b) after dissolution at t = 0 s in CH2Cl2–0.1 M NBu4PF6 at a GC electrode; c0(B9Cl9) = 0.29 mM, c0(B8Cl8) = 0.44 mM.Fig. 4 Cyclic voltammograms of B9Cl9 [(a), c0 = 0.29 mM, platinum electrode v = 0.1 V s21] and B8Cl8 [(b), c0 ª 2 mM, GC electrode, v = 0.2 V s21] in CH2Cl2–0.1 M NBu4PF6 with starting potentials located at steady-state value of rest potential. Identification of starting species For the interpretation of the voltammograms of the boron subhalide clusters it is essential to identify the starting species formed after dissolution of the neutral compounds.The fact that these species are stable at potentials between the two respective redox waves indicates that they might correspond to a compound with an oxidation state intermediate between those of BnCln and BnCln 22, i.e. the radical anion BnCln~2. Such a radical had been prepared in the case of the nine-vertex cluster by reduction of B9Cl9 with a stoichiometric amount of NBu4I or oxidation of B9Cl9 22 with thallium(III) trifluoroacetate and its ESR spectrum was reported with g = 2.018.35 The neutral clusters, on the other hand, are diamagnetic.40,41 The analogous B9I9 cluster undergoes one-electron reduction with organic donor solvents to form B9I9~2 within minutes, but was stable in chlorinated hydrocarbon solutions.42 In contrast to earlier reports,43 ESR signals were observed in BCl3 solutions of B8Cl8 only with a very weak intensity or after addition of water, giving a diVerent g value of 2.031, and were attributed to hydrolysis products.40,41 The presence of B9Cl9~2 in the electrolyte after dissolution of B9Cl9 and equilibration is clearly shown by the ESR signal (g = 2.018, width 20 G, no hyperfine structure, Fig. 5) which is identical to the one reported earlier for the chemically prepared radical anion.35 In the case of B8Cl8 a similar ESR signal was found (g = 2.017, width 25 G, no hyperfine structure, Fig. 5). Proof Fig. 5 The ESR spectra of solutions of B9Cl9 (solid line) and B8Cl8 (dotted line) in CH2Cl2–0.1 M NBu4PF6, 30 min after dissolution, assigned to B9Cl9~2 and B8Cl8~2 respectively. that this ESR resonance is arising from the radical anion B8Cl8~2 follows from investigation of chemically prepared NBu4 1B8Cl8~2 which shows the same ESR spectrum. We thus conclude that B9Cl9 and B8Cl8 are reduced after dissolution in dichloromethane to their respective radical anions in a spontaneous redox process (6).The formation of B9Cl9~2 is BnCln 1 D BnCln~2 1 D~1 (6) observed in solutions of B9Cl9 in dichloromethane without supporting electrolyte to only a small extent, while the intensity of the ESR absorption is much stronger in the electrolyte containing NBu4PF6. After dissolution of B9Cl9 in the electrolyte, during ª30 min a deepening of the solution color to brown is observed. Simultaneously, the ESR intensity increases.After this time the intensity of the ESR signals remains essentially constant, even upon standing overnight. Note that the timescale for this development of the color and the ESR intensity coincides with that of the rest potential variation (see Fig. 3). Possibly, traces of moisture, coming either from the solvent, from the supporting electrolyte, or by diVusion of air into the electrochemical cell, are responsible for the formation of the radical anions BnCln~2. We thus investigated solutions with various concentrations of B8Cl8 in carbon tetrachloride, chloroform, or dichloromethane with diVerent contents of water by using dried and undried solvents.In each case we observed the ESR signal of B8Cl8~2. Only its intensity was varying depending on the contents of water. While in dried dichloromethane for example the intensity was low, it grew by a factor of about 15 after addition of undried, wet CH3Cl2. It is thus obvious that water is responsible for the formation of B8Cl8~2.The corresponding 11B NMR spectra of B8Cl8 solutions also reflect the influence of moisture on the half width and line shape of the B8Cl8 signal. When the dried solvent (CDCl3 or CCl4) and a relatively big amount of B8Cl8 was transferred to the NMR tube by means of vacuum or inert-gas techniques (concentration of B8Cl8 ª 0.03 M), the 11B resonance line was very sharp (h1/2 ª 35 Hz at d 64.8). When the NMR tube was opened to the atmosphere or when not well dried solvents were used the B8Cl8 signal was broadened (h1/2 = 100–200 Hz).This eVect was even stronger when using CD2Cl2 (h1/2 = 500–10001744 J. Chem. Soc., Dalton Trans., 1999, 1741–1751 Hz). Line broadening of the NMR signal can be explained by a rapid exchange of an electron between the radical anion B8Cl8~2 and the neutral cluster. When the concentration of B8Cl8 in dried CD2Cl2 was lower (0.003 M; closer to the situation as met in cyclic voltammetric experiments), no 11B NMR resonance for B8Cl8 could be detected probably because B8Cl8 was nearly quantitatively reduced to the paramagnetic anion B8Cl8~2.Only an extremely weak signal at d 58.2 (B9Cl9) was observed. This compound is probably present from the synthesis (see Experimental section). The intensity of the signal indicates that the concentration is so low that no peak in cyclic voltammograms should be visible. For B9Cl9 a similar eVect of line broadening in the 11B NMR spectrum caused by traces of water was found.By adding an excess of BCl3 under vacuum conditions the linewidth decreased. When NBu4I was added in an equivalent amount to B9Cl9 the 11B NMR signal disappeared. After condensing an excess of elemental bromine onto the mixture the sharp signal of B9Cl9 (h1/2 ª 50 Hz) in the NMR spectrum reappeared. Thus, the overall reversibility of the redox process B9Cl9 1 e2 B9Cl9~2 is proven. Since it is obvious that traces of water are responsible for the formation of the radical anions BnCln~2 (n = 8 or 9) we have to ask how this reduction process can occur.Water itself or in combination with the solvents CCl4, CHCl3 or CH2Cl2 can hardly act as an electron donor. Furthermore, there is no indication of a disproportionation of BnCln leading to BnCln~2 and BnCln~1. It is known that chloroborane clusters are cleaved by water to give B(OH)3, HCl, and H2.43 We did not, however, observe any evolution of hydrogen.Since the redox potentials E(BnCln/ BnCln~2) have rather high values (see Table 3), it could be expected that a neutral BnCln molecule should be reduced instead of H1. Since the voltammetric experiments show that most of the BnCln molecules are reduced to the anions BnCln~2 and because we could not find any other reaction products, it would be necessary that one molecule BnCln reacts completely or nearly completely with the appropriate quantity of water according to eqn. (7), so that only a small amount of BnCln will BnCln 1 3n H2O æÆ n B(OH)3 1 2nH1 1 nHCl 1 2ne2 (7) BnCln 1 e2 æÆ BnCln~2 n = 8 or 9 (8) be destroyed.At present, this hypothesis for the formation of the BnCln~2 seems to be the most reasonable one, based on the experimental facts discussed above. With this information we can explain the observations made during the reaction of B8Cl8 with CH2Cl2 which according to Morrison44 and Emery45 presumably gives the cluster molecules HB9Cl8, H2B9Cl7, and B9Cl9.They noticed that the 11B NMR spectrum does not show any resonance for B8Cl8 dissolved in dichloromethane. However, they found three signals at d 70, 63.7, and 58.5 and assigned them to H2B9Cl7, HB9Cl8, and B9Cl9, respectively along with a further signal at d 40.25 (B–H). In contrast to this, in our NMR experiments, we never observed a signal at d 40, which was supposed to indicate B–H groups. The other peaks we found as well when the least volatile fraction of B8Cl8 samples sublimed from the reaction mixture was used.The two downfield signals at d 70 and 63.7 can be assigned to B11Cl11 (d11B 69.5 44) and B10Cl10 (d11B 63.5 44) since traces of these compounds together with B9Cl9 (d11B 58.2) are present in B8Cl8 samples before adding dichloromethane if B8Cl8 is not separated well from the by-products of its synthesis. Hence, we conclude that B8Cl8 is not reacting with CH2Cl2 to give the clusters HB9Cl8 and H2B9Cl7, but that it is reduced to the paramagnetic radical anion B8Cl8~2 and this cannot be detected any more in the boron NMR spectrum. This result is in accordance with the cyclic voltammetric results, which indicate total disappearance of the neutral cluster upon dissolution in the electrolyte. The spontaneous formation of BnCln~2 from BnCln also explains the result of a bulk electrolysis experiment with B9Cl9.If B9Cl9 were the starting species and were reduced to the stable B9Cl9 22, 2 F were expected to be transferred upon reduction.Similarly, during reoxidation to B9Cl9 the charge should also correspond to 2 F. However, reduction used only ª0.7 F, while reoxidation at 11.8 V results in the transfer of a much larger charge than expected. Taking into account some loss of B9Cl9 during transfer to the cell, the reduction charge thus indicates that only a one-electron step occurs, starting from B9Cl9~2 and leading to B9Cl9 22. On the other hand, oxidation to B9Cl9 is followed by reaction (6) and reformation of the radical anion in a catalytic process (see also below, Electrochemical oxidation of BnCln~2) and a large quantity of charge is transported through the electrolyte.We thus conclude that the stable starting species present in the dichloromethane electrolyte is not BnCln, but BnCln~2 which can be reduced to BnCln 22 and oxidized to BnCln in heterogeneous electron transfers at the electrode surface (Scheme 1); BnCln~2 is formed from BnCln in a homogeneous redox reaction (6).Since essentially all BnCln is transformed into BnCln~2, we can assume the concentration of the radical anion to be practically identical to the initial concentration of the neutral cluster. The loss of 5–6% due to reaction (7) can probably not be detected in electrochemical experiments, since it is within the conventionally assumed current measurement reproducibility of experiments such as those performed here. Having established the starting species and the basic reaction steps of the BnCln 0/~2/22 system in dichloromethane electrolyte, we will now separately discuss the determination of mechanistic, kinetic and thermodynamic parameters for the reduction and oxidation processes of the BnCln~2 from electrochemical experiments. Electrochemical reduction of BnCln~2 Cyclic voltammograms and chronocoulograms of both B9Cl9 and B8Cl8 in CH2Cl2–0.1 M NBu4PF6 were recorded under variation of the concentration c0 of the clusters and the scan rate v or pulse duration t, respectively, in the potential range where reduction of the radical anions was observed.Both platinum and GC electrodes were used. Cyclic voltammetry. Features of cyclic voltammograms from a typical series of experiments are given in Tables 1 and 2 for the reduction of the two boron subhalides. The peak potentials Ep red and Ep ox for the reduction and oxidation peak on the forward and reverse scans of the voltammograms, respectively, are essentially independent of the scan rate and the concentration.The peak potential diVerence DEp is independent of v and close to 58 mV in all cases, indicating a situation close to electrochemical reversibility of the redox process. Independence of DEp from c0 demonstrates that compensation of the iR drop was eVectively performed. The midpoint potential, E� , calculated as the mean value of the two peak potentials, is again independent of v and c0.The electrochemal reversibility of the process is confirmed by the fact that the peak current function ip red/÷vc0 is independent of v and c0. Furthermore, proportionality between ip red and the square root of the scan rate clearly indicates the absence of adsorption of electroactive species. Chemical reversibility, i.e. stability of the BnCln 22 species with respect to Scheme 1 Homogeneous and heterogeneous electron transfers in the system BnCln 0/~2/22.J. Chem. Soc., Dalton Trans., 1999, 1741–1751 1745 Table 1 Typical cyclic voltammetric potential and current features for the reduction of B9Cl9~2 in CH2Cl2–0.1 M NBu4PF6 at a platinum electrode c0/mM 0.34 0.67 mean n/V s21 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 Ep red/V 0.029 0.028 0.026 0.027 0.027 0.028 0.026 0.026 0.027 0.025 0.031 0.031 0.029 0.033 0.031 0.031 0.029 0.028 0.031 0.028 0.029 ± 0.002 Ep ox/V 0.098 0.097 0.091 0.091 0.090 0.091 0.093 0.096 0.096 0.092 0.098 0.096 0.095 0.096 0.097 0.096 0.097 0.097 0.097 0.101 0.095 ± 0.003 DEp/mV 69 69 65 64 63 63 67 70 69 67 67 65 66 63 66 65 68 69 66 73 67 ± 3 E� a/V 0.064 0.063 0.059 0.059 0.059 0.060 0.060 0.061 0.062 0.059 0.065 0.064 0.062 0.065 0.064 0.064 0.063 0.063 0.064 0.065 0.062 ± 0.002 ip red/÷vc0 b 40.5 38.9 38.3 40.8 41.4 39.2 41.1 41.4 44.0 44.0 38.9 38.6 38.6 40.8 40.8 41.1 41.1 41.4 42.0 43.3 40.8 ± 1.6 ip ox/ip red 0.80 0.98 1.03 1.00 1.01 1.05 1.02 1.02 1.01 1.04 0.94 0.99 1.00 0.98 0.98 1.00 1.00 1.01 1.08 1.05 1.00 ± 0.06 a Midpoint potential E� = (Ep ox 1 Ep red)2.b In A cm3 s1/2 V21/2 mol21. Table 2 Typical cyclic voltammetric potential and current features for the reduction of B8Cl8~2 in CH2Cl2–0.1 M NBu4PF6 at a platinum electrode c0/mM 0.21 0.28 mean v/V s21 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 Ep red/V 0.082 0.081 0.079 0.081 0.079 0.079 0.079 0.078 0.076 0.078 0.082 0.083 0.081 0.084 0.083 0.081 0.083 0.083 0.078 0.080 0.081 ± 0.002 Ep ox/V 0.144 0.141 0.144 0.142 0.141 0.141 0.143 0.144 0.140 0.140 0.145 0.144 0.142 0.143 0.143 0.144 0.145 0.147 0.143 0.147 0.143 ± 0.002 DEp/mV 62 60 65 61 62 62 64 66 64 62 63 61 61 59 60 63 62 64 65 67 63 ± 2 E� a/V 0.113 0.111 0.112 0.112 0.110 0.110 0.111 0.111 0.108 0.109 0.114 0.114 0.112 0.114 0.113 0.113 0.114 0.115 0.111 0.114 0.112 ± 0.002 ip red/÷vc0 b 32.3 32.9 33.5 34.8 34.8 35.1 35.4 33.2 33.2 33.5 32.3 32.3 31.6 33.2 33.8 34.2 32.3 32.3 32.6 32.9 33.2 ± 0.9 ip ox/ip red 0.99 1.00 1.03 1.06 1.02 1.03 1.02 1.05 1.06 1.12 0.92 0.98 1.10 1.02 1.02 1.02 1.01 1.04 1.06 1.12 1.03 ± 0.05 a Midpoint potential E� = (Ep ox 1 Ep red)/2.b In A cm3 s1/2 V21/2 mol21. follow-up reactions, is indicated by the values of ip ox/ip red, which are close to 1.0. Only at scan rates below 0.02 V s21 the value of this ratio drops below unity. Under the experimental conditions of this work this could be due to some non-linear diVusion (“edge”) eVects, which become increasingly important at slow scan rates.Also, additional transport by convection may play a role. The peak currents at scan rates above 0.02 V s21, however, allow the determination of the diVusion coeYcient of BnCln~2 in the electrolyte used for the experiments.46 The midpoint potentials for reduction and the diVusion coeYcients of B9Cl9~2 and B8Cl8~2 are given in Table 3 as mean values from several independent experiments.All values are independent of the electrode material used. The standard deviations of the E� results show excellent reproducibility comparable to that within individual experiments (Tables 1 and 2). On the other hand, while ip red/÷vc0 is excellently reproducible within a series of experiments in a single cell set-up, even with variation of the concentration (Tables 1 and 2), the diVusion coe Ycients vary more strongly between set-ups. These variations may be due to problems with the determination of c0 and the limited stability of the neutral boron cluster starting compounds.The diVusion coeYcient of the B8Cl8 species, however, appears consistently higher than that of the larger B9Cl9 species. Table 3 Midpoint potentials E� and diVusion coeYcients D describing electrochemical reduction and transport of BnCln 0/~2/22a Redox process B9Cl9~2 1 e2 B9Cl9 22 B9Cl9~2 B9Cl9 1 e2 B8Cl8~2 1 e2 B8Cl8 22 B8Cl8~2 B8Cl8 1 e2 E� /V 10.064 ± 0.003 10.599 ± 0.003 10.114 ± 0.002 10.959 ± 0.002 106 D/cm2 s21 2 ± 2b 2 ± 1c 1 ± 1b 1 ± 1c 4 ± 1b 3 ± 2c 4 ± 1b 4 ± 2c a Mean values from several independent experiments under variation of scan rate v, concentration c0, and electrode material.b From cyclic voltammograms. c From chronocoulograms.1746 J. Chem. Soc., Dalton Trans., 1999, 1741–1751 Fig. 6 (a) Chronocoulograms for the reduction of B9Cl9~2 (solid line) and B8Cl8~2 (dotted line) in CH2Cl2–0.1 M NBu4PF6, t = 10 s, GC electrode; c0(B9Cl9) = 0.29 mM, c0(B8Cl8) = 0.44 mM.(b) Anson plot for reduction of B8Cl8~2; “time1/2” axis corresponds to t1/2 for the forward part (upper trace) and t1/2 1 (t 2 t)1/2 2 t1/2 for the reverse part (lower trace) of the chronocoulometric experiment. Chronocoulometry. The cyclic voltammetric data were complemented with chronocoulometric results (Fig. 6). Chronocoulometry confirms the electrochemical and chemical reversibility of the reduction of the radical anions by the almost linear plots of Q vs.t1/2 [“Anson plots”; Fig. 6(b)] 47 and the charge ratio 48 Q2t/Qt = 0.41 ± 0.03 for B9Cl9~2 (t is the pulse time, i.e. the time when the potential during the chronocoulometric experiment is switched). In the case of B8Cl8~2, Q2t/Qt is slightly larger (0.63 ± 0.04) than the expected value of 0.41, but does not increase with increasing pulse time. In accordance with the interpretation of the cyclic voltammetric data, we thus exclude a chemical follow-up reaction of B8Cl8 22.The Anson plots do show only a negligible intersection with the charge axis, thus confirming that none of the redox species is adsorbed at either electrode material used. From the slopes of the Anson plots values of the diVusion coeYcients are calculated in good agreement with the results of cyclic voltammetry, but again with rather high standard deviations. The mean values from several independent experiments are given in Table 3.Simulation. The information determined from these quantitative analyses of cyclic voltammograms and chronocoulograms was subsequently used as the basis for simulations of the experimental current–potential curves. A simple quasireversible one-electron transfer under planar diVusion conditions was assumed as the mechanistic model of the reduction. For each of the compounds a single set of system parameters (formal potential E0, diVusion coeYcient D, heterogeneous electron transfer rate constant ks, and transfer coeYcient, a; Table 4) was suYcient to simulate various series of voltammograms at diVerent v, c0 and electrode material.This set was found by varying E0, D, and ks, until an optimum fit was obtained. The diVusion coeYcients of the respective neutral, mono- and di-anionic species were assumed to be identical. The value of a was fixed in the calculations to 0.5 for both compounds. Variation of a did not significantly improve the fits.Comparisons of the simulations to the corresponding experimental curves for both BnCln at various scan rates and a single c0 are shown in Fig. 7. The fit between theory and experiment is excellent, except for the smallest scan rate used, where possibly non-ideal transport eVects are already visible in the experimental data. Thus, the simulations confirm the qualitative mechanistic picture gained so far. Also, the parameters E0 and D obtained from the fitting procedure compare very well Table 4 System parameter a sets used for simulations of the process BnCln~2 1 e2 BnCln 22 Parameter E0/V 106 D/cm2 s21 ks/cm s21 a n = 9 10.067 1 0.015 0.5 n = 8 10.112 4 0.05 0.5 a Parameters describing the detailsistic reaction steps.49 to the midpoint potentials E� and diVusion coeYcients determined before.For both the nine and the eight vertex cluster, values of ks close to the limit of electrochemical reversibility (ks ª 0.1 cm s21) 50 were found.Electrochemical oxidation of BnCln~2 In analogy to their reduction, the anodic oxidation of the BnCln radical anions was investigated in CH2Cl2–0.1 M NBu4PF6. Cyclic voltammetry. Cyclic voltammetric results for the oxidation of the BnCln~2 to the BnCln are collected in Tables 5 and 6. Only the first oxidation of B9Cl9~2 was analysed, and the switching potential for the voltammograms was adjusted accordingly. As in the case of the reduction of the BnCln~2, the peak potential features for the oxidation clearly indicate a oneelectron process close to electrochemical reversibility.Both the oxidation and the reduction peak potentials are independent of c0 and v; the peak potential diVerence is independent of these experimental parameters and close to the reversible limit of 58 mV. Also, the midpoint potential does not depend either on the experimental parameters or on the electrode material used (Pt or GC; for the mean values from several independent experiments see Table 3).On the other hand, the peak current data show that, at least at slow scan rates, some additional chemical reaction of BnCln must take place: for v £ 0.02 V s21 the peak current function ip ox/ ÷vc0 starts to increase, but, moreover, ip red/ip ox clearly decreases to values below 1.0 for v £ 0.5 (B9Cl9~2) or £0.2 V s21 (B8Cl8~2). Computer simulations of the cyclic voltammograms (see below) show that the homogeneous conversion of BnCln into BnCln~2 can explain this behavior.The interpretation of these features of the current–potential curves is hampered by the fact that at scan rates above v = 1 V s21 the reproducibility of the peak current data decreases. Also, in this experimental time regime the background correction leads to artifacts, in particular at the beginning of the voltammetric scan and close to the switching potential. These problems were much more severe for the octaboron cluster as compared to the B9Cl9 system, and also more pronounced for GC as compared to Pt as the electrode material.For these reasons, only data from the limited range of scan rates 0.01 £ v £1 V s21 were evaluated. Here, however, mean values of the voltammetric potential features are reproducible both within an experiment and within several sets of cell set-ups (Tables 5 and 6, as well as Table 3, respectively). The mean values of the diVusion coeYcients as derived from the oxidation peak currents over all independent experiments are also given in Table 3.Similar reasons as given in the case of reduction of the radical anions explain the relatively high standard deviations. Chronocoulometry. Chronocoulometric oxidations of the BnCln~2 cluster species met similar problems as those in the cyclic voltammetric experiments. In particular, shortly after switching back the potential (t � t), distortions of the charge vs. time curves were observed. For pulse lengths longer thanJ.Chem. Soc., Dalton Trans., 1999, 1741–1751 1747 Fig. 7 Simulated (solid lines) and experimental (dots) cyclic voltammograms for the reduction processes of B9Cl9~2 (left; c0 = 0.63 mM, Pt, v = 0.01, 0.05, 0.1, 0.5, 1.0 V s21, from top to bottom) and B8Cl8~2 (right; c0 = 0.28 mM, Pt, v = 0.01, 0.02, 0.1, 0.5, 2.0 V s21). several s, however, still reasonably linear Q vs. t1/2 plots were obtained with negligible intersections with the charge axis. We exclude adsorption of electroactive material at the electrode surface also for the oxidation process of the cluster radical anions.The slopes of these plots were evaluated in order to estimate values for the diVusion coeYcients, and again the results are presented in Table 3. Simulation. The cyclic voltammetric curves of the oxidation processes leading from the boron subhalide radical anions to their neutral redox partners were simulated starting from the parameters determined as discussed in the previous paragraphs (Fig. 8). In this case a more complex reaction model than a simple quasireversible electron transfer was used. We retained the assumption of planar diVusion. However, the homogeneous redox process converting BnCln produced at the electrode back to BnCln~2 was added to the one-electron oxidation (“catalytic” follow-up reaction; ECcat mechanism 46,51,52). Values of E0, D, ks, and the rate constant k for the homogeneous electron transfer step were varied until an optimum fit between experiment and theory was found for 0.01 £ v £ 10 V s21 and two concentrations of BnCln~2.The homogeneous step was assumed to follow first-order kinetics in this model. Again, a = 0.5 was used throughout. Close to the switching potential the fit between experimental and simulated curves is less satisfactory as compared to that for the reductive voltammograms discussed before. However, the changes in the shapes of the voltammograms at slow scan rates (decrease of reverse peak intensity, flattening of forward peak) which are characteristic for the ECcat mechanism46,51,52 are modelled adequately. Table 7 lists the optimum values of the system parameters as found from the best fitting simulations.The results for the E0 compare excellently to the E� determined from the peak potential analysis (Table 3). The diVusion coeYcients for the particular B9Cl9 experiments evaluated in the simulation are somewhat smaller than the mean values in Table 3, but we again attribute this to the low reproducibility of the concentration owing to the reactivity of the starting material.In the case of the B9Cl9~2 oxidation the fitting procedure of DigiSim converged on a value of 1400 cm s21 for the heterogeneous electron transfer rate constant. This indicates that the electron transfer is indeed fully diVusion controlled, i.e. electrochemically reversible. The numerical value, however, is not regarded as significant, since at such large rate constants the features of the cyclic voltammograms do no longer change with ks, and, consequently, the sensitivity of the curves with respect to this parameter becomes close to zero.53 The fitting routine will select a numerical value for this parameter which is strongly influenced by random errors in the data and is expected to have a large statistical uncertainty.54 The DigiSim software does not1748 J.Chem. Soc., Dalton Trans., 1999, 1741–1751 Table 5 Typical cyclic voltammetric potential and current features for the oxidation of B9Cl9~2 in CH2Cl2–0.1 M NBu4PF6 at a platinum electrode c0/mM 0.34 0.67 mean v/V s21 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 Ep ox/V 0.629 0.627 0.630 0.628 0.628 0.628 0.631 0.629 0.628 0.631 0.629 0.627 0.628 0.627 0.627 0.628 0.630 0.630 0.627 0.630 0.628 ± 0.001 Ep red/V 0.560 0.563 0.561 0.565 0.565 0.565 0.562 0.561 0.561 0.560 0.559 0.561 0.564 0.565 0.568 0.567 0.566 0.565 0.563 0.558 0.563 ± 0.003 DEp/mV 69 64 69 63 63 63 69 68 67 71 70 66 64 62 59 61 64 65 64 72 66 ± 2 E� a/V 0.595 0.595 0.596 0.597 0.597 0.597 0.597 0.595 0.595 0.596 0.594 0.594 0.596 0.596 0.598 0.598 0.598 0.598 0.595 0.594 0.596 ± 0.001 ip ox/÷vc0 b 42.4 37.3 39.5 37.3 37.9 36.7 36.0 ——— 40.5 37.6 35.7 37.0 35.1 35.1 34.5 ——— 36.6 ± 1.4 c ip red/ip ox 0.62 0.81 0.85 0.93 0.94 0.99 1.01 ——— 0.60 0.74 0.86 0.87 0.93 0.97 0.98 ——— — a Midpoint potential E� = (Ep ox 1 Ep red)/2.b In A cm3 s1/2 V21/2 mol21. c From values for v > 10 mV s21. Table 6 Typical cyclic voltammetric potential and current features for the oxidation of B8Cl8~2 in CH2Cl2–0.1 M NBu4PF6 at a platinum electrode c0/mM 0.21 0.28 mean v/V s21 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 0.01 0.02 0.05 0.1 0.2 0.5 1.0 2.0 5.0 10.0 Ep ox/V 0.991 0.996 0.992 0.987 0.987 0.988 0.987 0.998 0.994 0.993 0.997 0.996 0.994 0.989 0.991 0.991 0.987 0.990 0.996 0.996 0.992 ± 0.003 Ep red/V 0.925 0.924 0.0.927 0.926 0.927 0.925 0.930 0.923 0.926 0.924 0.926 0.929 0.928 0.929 0.932 0.926 0.926 0.924 0.927 ± 0.002 DEp/mV 66 72 63 61 60 62 60 64 64 70 71 72 68 60 63 62 55 64 70 72 65 ± 5 E� a/V 0.958 0.960 0.961 0.957 0.957 0.957 0.957 0.957 0.962 0.958 0.962 0.960 0.960 0.959 0.960 0.960 0.960 0.958 0.961 0.960 0.959 ± 0.002 ip ox/÷vc0 b 45.9 42.7 41.1 33.8 33.2 32.6 37.6 ——— 43.0 41.1 38.9 37.6 37.6 32.6 34.8 ——— 35.0 ± 2.2 c ip red/ip ox 0.59 0.72 0.72 0.91 0.96 1.04 0.96 ——— 0.62 0.78 0.82 0.87 0.88 1.07 1.07 ———— a Midpoint potential E� = (Ep ox 1 Ep red)/2.b In A cm3 s1/2 V21/2 mol21. c From values for v > 50 mV s21. provide direct quantitative measures of the standard deviations of the parameter values estimated by fitting. Consequently, no further analysis is possible. The heterogeneous electron transfer rate constant determined by the fitting procedure for the oxidation of B8Cl8~2 is still rather close to the reversibility/quasireversibility border.The characteristic changes in the shapes of the voltammograms at slow scan rates allow the determination of the rate of the homogeneous redox process (Table 7). Its value is similar for both clusters. Thus, simulation of these voltammograms confirms the reaction mechanism proposed and the system parameters derived from the peak features and chronocoulograms. Furthermore, it allows determination of the rate constants of the homogeneous redox processes under the assumption of first-order kinetics.Formal potentials and stability of the radical anions BnCln~2 In previous paragraphs we have shown that both the reduction and (at large scan rates) the oxidation of B9Cl9~2 and B8Cl8~2 are chemically reversible processes with rates of the electron transfers close to or in the region of electrochemical reversibility. Under such conditions and under the assumption of equal diVusion coeYcients for the three redox partners, respectively, the midpoint potentials, calculated as the mean values of the peak potentials, are good approximations for the formal potentials of the respective electron transfer processes.This is confirmed by the simulations which result in optimum E0 values identical to the E� within one standard deviation. The formal potentials thus determined for the BnCln systems in CH2Cl2 show a normal ordering, i.e. they increase with the oxidation state involved.The relative position of the E0 for the redox processes of B9Cl9~2 is very similar in CH2Cl2 (|DE0| = 0.533, this work; 0.51;38 0.53 V 39) and CH3CN (|DE0| = 0.540 V37). On the other hand, the absolute values determined here and in the work of other authors are not comparable due to the use of diVerent reference standards and the possibility of eVects of the halogenated substrates on the potential of the Ag–AgCl electrode used in the earlier work.37,38 If indeed the cluster radical anions are produced by hydrolysis of some of theJ.Chem. Soc., Dalton Trans., 1999, 1741–1751 1749 Fig. 8 Simulated (solid lines) and experimental (dots) cyclic voltammograms for the oxidation processes of B9Cl9~2 (left; c0 = 0.98 mM, GC, v = 0.01, 0.05, 0.1, 0.5, 1.0 V s21, from top to bottom) and B8Cl8~2 (right; c0 = 0.28 mM, Pt, v = 0.01, 0.02, 0.1, 0.5, 2.0 V s21). subhalide molecules, as formulated in eqns. (7) and (8), chloride ions are liberated which will shift the reference potential.Our reference system should not be aVected by such processes. Only the data in ref. 39 (E0 1 = 1 0.10 V and E0 2 1 0.63 V vs. Fc/Fc1) seem to have been determined with careful exclusion of such eVects. They diVer from our values by less than 40 mV. From the formal potentials the equilibrium constants Kcomp of reaction (4) follow through eqn. (5) as 1.1 × 109 (n = 9, in close agreement with Kcomp = 1.2 × 109 in ref. 39) and 1.9 × 1014 (n = 8).Both equilibria are strongly shifted to the side of the radical anions, which are thus rather stable with respect to disproportionation. Results for the B9Br9,37–39 B9I9,37,39 and B10Cl10 37 systems show a similar picture. It appears that the Table 7 System parameter a sets for simulations of the process BnCln~2 2 e2 BnCln Parameter E0/V D/cm2 s21 ks/cm s21 a k/s21 n = 9 10.600 6 × 1027 —b 0.5 0.09 n = 8 10.959 5.5 × 1026 0.13 0.5 0.07 a Parameters describing the details of the mechanistic reaction steps.49 b Electron transfer fully diVusion controlled.smaller cluster radical anion is even more stable in this respect than B9Cl9~2. Conclusion The electrochemical investigation of two electron hyperdeficient boron subhalides shows that both B9Cl9 and B8Cl8 and their respective radical anions and dianions can be interconverted at an electrode in a dichloromethane electrolyte through one-electron processes, well separated in potential.The reduction of the neutral clusters to the radical anions proceeds at rather positive potentials at the electrode, and additionally spontaneously with an electrolyte component. In view of the hypothetical rationalization of the “potential inversion” 6 phenomenon, the stepwise manner of electron transfer in the clusters investigated nicely mirrors the fact that probably all oxidation states attain the deltahedral closo type structure without drastic geometrical changes accompanying the redox process.Possibly, more pronounced structural rearrangements would be noticeable in smaller clusters of this series, such as the tetrahedral B4Cl4, where chemical reduction with trimethylstannane leads to the butterfly-shaped arachno-B4H10.44 However, reduction of B4Cl4 without simultaneous transfer of hydrogen has not yet been observed. It is thus planned to investigate the1750 J. Chem. Soc., Dalton Trans., 1999, 1741–1751 electrochemical reduction of such boron subhalide clusters of smaller size in future work.Experimental Solvents and supporting electrolyte Dichloromethane (Burdick & Jackson) was distilled to separate the stabilizing cyclohexene and dried by standing for several hours over activated basic Al2O3 (activation procedure: 4 h at a temperature of 400 8C and a pressure of 2 × 1023 mbar). Tetran- butylammonium hexafluorophosphate, NBu4PF6, was prepared from NBu4Br and NH4PF6 (Fluka) as described before.55 It was used in a concentration of 0.1 M.The electrolyte was degassed by three freeze–pump–thaw cycles before transferring it into the electrochemical cell under argon. Carbon tetrachloride (p.a., Merck) and the deuteriated solvents were dried over molecular sieves; NBu4I (puriss.) was purchased from Fluka. Syntheses The synthesis and all manipulations of the chloroboranes B8Cl8 and B9Cl9 were carried out by using standard high-vacuum or inert-atmosphere techniques as described by Shriver and Drezdzon.56 The compound B2Cl4 was obtained by the reaction of BCl3 with copper vapor 57 and purified by fractional condensation until it showed a vapor pressure of 59 mbar at 0 8C. Nonachlorononaborane(9).The compound B9Cl9 was prepared by heating B2Cl4 at 450 8C for 5 min under vacuum according to the procedure reported by Morrison.44,58 The product was purified by fractional sublimation into a long glass tube connected to a high-vacuum line. Octachlorooctaborane(8).The compound B8Cl8 was prepared similar to the synthesis described by Morrison.44,59 In a typical experiment, a solution of 3.7 g B2Cl4 (22.6 mmol) in 12.5 g of CCl4 was heated in a 100 ml flask under argon at 95 8C for 14 d. After evaporation of all volatile material (BCl3, B2Cl4, CCl4) at 0 8C (1024 mbar) a black residue remained, which contained, according to the 11B NMR spectrum, B8Cl8 (d 64.8 in CDCl3, 93 mol%), B9Cl9 (d 58.2, approximately 7 mol%), as well as traces of B10Cl10 (d11B 63.2, cf.ref. 44; 63.5) and an unidentified boron compound with d11B 51.7. The compound B8Cl8 was separated from the reaction mixture and purified by fractionated sublimation under vacuum (1024 mbar). Yield: 100 mg (0.27 mmol, 10% based upon B2Cl4). It should be noted that thick layers of B8Cl8 are nearly black, whereas thin layok dark green and become purple upon contact with traces of air. Electrochemical experiments All electrochemical experiments were performed with a Bioanalytical Systems (BAS, West Lafayette, IN, USA) 100 B/W electrochemical workstation controlled by a standard 80486 processor based personal computer (control program version 2.0).For electroanalytical experiments a BAS platinum or glassy carbon electrode tip was used as the working electrode. The electroactive area of the disks was determined from cyclic voltammograms, chronoamperograms, and chronocoulograms of ferrocene in dichloromethane under the assumption of a diVusion coeYcient D(Fc) = 2.32 × 1025 cm2 s21.60 The counter electrode was a platinum wire (diameter: 1 mm).A Haber–Luggin double reference electrode61 was used. The resulting potential values refer to Ag–Ag1 (0.01 M in CH3CN–0.1 M NBu4PF6). Ferrocene was used as an external standard.62 Its potential was determined by separate cyclic voltammetric experiments in CH2Cl2. All potentials reported in this paper are rescaled to E0(Fc–Fc1) = 10.226 V (vs.the Ag–Ag1 reference) and thus given vs. the Fc–Fc1 redox potential. For cyclic voltammetry, chronoamperometry and chronocoulometry a gas-tight full-glass three-electrode cell as described before 55 was used. It was purged with argon before being filled with the electrolyte. Background curves were recorded before adding the substrate to the electrolyte. The background currents were later subtracted from the experimental data measured in the presence of substrate. The uncompensated resistance in the cell was determined by the built-in procedure of the BAS 100 B/W instrument.For each scan rate a series of cyclic voltammograms was recorded with 70, 80, and 90% feed-back compensation of the iR drop. This was repeated for at least a second concentration in the same cell set-up. The resulting current–potential curves were compared and optimum compensation was assumed if the peak potential separation did not increase with concentration. The instability of the boron subhalides with respect to oxygen and traces of water required special precautions during weighing of the compounds and transfer of the samples to the electrochemical cell.Weighing was performed under argon. A concentrated stock solution was prepared with the degassed electrolyte and defined volumes of this solution were added to the blank electrolyte in the cell. After registration of all necessary voltammograms and chronocoulograms, further portions of the stock solution were added without changing the electrode arrangement.In this way at least two series of curves were recorded in each experiment with diVerent concentrations but otherwise identical conditions. For electrolysis experiments (bulk electrolysis), working and counter electrodes were Pt/Ir 90/10 nets (Degussa, Hanau, Germany), separated by a glass frit. The bulk electrolysis cell was also gas-tight and its temperature was controlled to be 17 8C. It was purged with argon prior to being filled with electrolyte.Rest potential measurements were performed using the standard experimental protocol of the BAS 100 B/W electrochemical workstation. Data analysis and simulations Cyclic voltammetric and chronocoulometric data were background corrected and evaluated with the BAS 100 B/W control program. Peak current ratios were determined according to Nicholson’s procedure.63 All error measures given in this paper are standard deviations. For simulations of the cyclic voltammograms the commercial simulator DigiSim 64 (Version 2.0) was used with standard numerical options.ESR and NMR spectra A Bruker ESP 300 spectrometer was used to record the ESR spectra. Preparation of the solution was similar to that for the electroanalytical experiments. Spectra were taken at various times after dissolution. For the determination of the g values the spectrometer was calibrated with Bruker “strong pitch” of g = 2.0028. The 11B NMR spectra at 80.25 MHz were obtained on a Bruker WM 250 spectrometer. All 11B NMR chemical shifts are referred to external F3B?OEt2 in CDCl3 or CD2Cl2, respectively.Investigation of B8Cl8 and NBu4 1B8Cl8~2 solutions. Solutions of B8Cl8 in CDCl3 or CCl4 were prepared by condensing the solvent (which was dried before with molecular sieves) onto B8Cl8 under vacuum. The solution was transferred under argon with a syringe to an NMR tube equipped with a polytetra- fluoroethylene (PTFE) valve.For the ESR measurements, B8Cl8 and the purified and dried solvent (CDCl3 or CH2Cl2) were condensed under vacuum into an ESR glass tube equipped with a PTFE valve or the components were condensed together in a flask connected to the vacuum line and transferred under argon with a syringe into the ESR tube. The NBu4 1B8Cl8~2 wasJ. Chem. Soc., Dalton Trans., 1999, 1741–1751 1751 prepared according to the synthesis of NBu4 1B9Cl9~235 by reduction of B8Cl8 with the equivalent amount of NBu4I in dried CDCl3. Acknowledgements The authors thank Paul Schuler for recording the ESR spectra, Stefan Dümmling for technical assistance and the Fonds der Chemischen Industrie, Frankfurt/Main, Germany, for financial support.References 1 B. Speiser and S. Dümmling, Part 1: DECHEMA-Monogr., in the press. 2 K. Deuchert and S. Hünig, Angew. Chem., 1978, 90, 927; Angew. Chem., Int. Ed. Engl., 1978, 17, 875. 3 N. G. Connelly and W. E. Geiger, Adv. Organomet.Chem., 1984, 23, 1. 4 J. Phelps and A. J. Bard, J. Electroanal. Chem. Interfacial Electrochem., 1976, 68, 313. 5 D. H. Evans, Acta Chem. Scand., 1998, 52, 194. 6 D. H. Evans and K. Hu, J. Chem. Soc., Faraday Trans., 1996, 3983. 7 K. Hu and D. H. Evans, J. Electroanal. Chem. Interfacial Electrochem., 1997, 423, 29. 8 K. Hu, M. E. Niyazymbetov and D. H. Evans, J. Electroanal. Chem. Interfacial Electrochem., 1995, 396, 457. 9 K. Hu and D. H. Evans, J. Phys. Chem., 1996, 100, 3030. 10 B. Speiser, M. Würde and C. Maichle-Mössmer, Chem. Eur. J., 1998, 4, 222. 11 B. Tulyathan and W. E. Geiger, J. Am. Chem. Soc., 1985, 107, 5960. 12 J. A. Morrison, in Advances in Boron and the Boranes, eds. J. F. Liebman, A. Greenberg, R. E. Williams, D. P. Loker and K. B. Loker, VCH, Weinheim, 1988, vol. 5, ch. 8, pp. 151–189. 13 R. L. Johnston and D. M. P. Mingos, Inorg. Chem., 1986, 25, 3321. 14 K. Wade, Adv. Inorg. Chem. 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ISSN:1477-9226    

DOI:10.1039/a809134j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756 1751 Mono-, di- and poly-nuclear transition-metal complexes of a bis(tridentate) ligand: towards p-phenylenediamine-bridged co-ordination polymers Alan Hazell,a Christine J. McKenzie *,†,b and Lars Preuss Nielsen b a Department of Chemistry, Aarhus University, 8000 Århus C, Denmark b Department of Chemistry, Odense University, 5230 Odense M, Denmark The bis(tridentate) ligand N,N,N9,N9-tetrakis(2-pyridylmethyl)benzene-1,4-diamine (1,4-tpbd) is multifunctional in that mono-, di- and poly-nuclear transition-metal complexes as well as bis-co-ordinated complexes can be prepared.A prototype example of each class of complex has been characterized. In [ZnCl2(2,4-tpbd)] the 1,4-tpbd is co-ordinated via only one of its tridentate ends. Both ends of 1,4-tpbd are bound in a dipalladium complex. [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2. This structure constitutes the first example of a crystal structure of the counter anion [PdCl3(dmso)]2.The chloride salt of the [Pd2Cl2(tpbd)]21 cation has also been isolated. In the structures of both [ZnCl2(1,4-tpbd)] and [Pd2Cl2(tpbd)]21 quinoid character of the benzene linker of the 1,4-tpbd is evident. The compound [Ru(1,4-tpbd)2][PF6]2 is an example of a bis-co-ordinated complex. One tridentate end of each ligand is co-ordinated to the ruthenium(II) ion while the other end is unco-ordinated. These three contrasting complexes demonstrate the versatility of 1,4-tpbd as a ligand for transition-metal complexes and the series represent the structural elements required for the construction of homo- and hetero-nuclear co-ordination oligo- or poly-mers.The compositions of the products isolated from the reaction of 1,4-tpbd with iron and nickel are consistent with the polymeric formulations [M(1,4-tpbd)]nA2n (M = Fe, A = Cl; M = Ni, A = NO3). The last decade has witnessed a surge of interest in the discovery of simple building blocks capable of forming specific molecular arrays under certain chemical conditions.1 For example, the addition of a specific metal ion to appropriate ligands may induce the formation of one- and two-dimensional polymers or framework structures as a consequence of the constraints induced by co-ordination.2 The search for appropriate organic building blocks for the construction of onedimensional co-ordination polymers has led us to investigate tetra-N-functionalized p-phenylenediamines as potential bis(tridentate) bridging ligands.We have recently reported the preparation and characterization of the new potentially redoxactive hexadentate p-phenylenediamine-based ligand N,N,N9, N9-tetrakis(2-pyridylmethylbenzene)-1,4-diamine (1,4-tpbd) and its dicopper complexes with the general formulation [L2Cu(1,4-tpbd)CuL2]n1 (L = H2O, n = 4; L = Cl or NO3, n = 0).3 The compound 1,4-tpbd is easily oxidized in the presence of the redox-active metal ions Fe31, Cu21 and Mn31, and we have characterized, in solution, its one-electron oxidized form, the purple radical cation 1,4-tpbd~1, using cyclic voltammetry, ESR and UV/VIS spectroscopy.If 1,4-tpbd is treated with 2 or more equivalents of Cu21 radical formation is suppressed and the dinuclear copper complexes can be isolated. We have found no evidence to suggest any electronic redistribution (i.e. the stabilization of a charge-separated state) between the 1,4-tpbd and the copper ions to give, for example, formally mixed-valence radical-bridged complexes of the type [L2CuI- (1,4-tpbd~1)CuIIL2]n1.In order better to understand the redox behaviour of 1,4-tpbd we have extended this work to include the study of complexes of relatively redox-inert transitionmetal ions. Described here are zinc, palladium and ruthenium complexes. Each complex is a prototype for complexes of the following general formulations: [M(1,4-tpbd)Ln]n1 [LnM(1,4- tpbd)MLn]n1 and [(1,4-tpbd)M(1,4-tpbd)]n1.This series of complexes demonstrate that 1,4-tpbd shows ‘schizodentate’ character; co-ordination to transition-metal ions via either † E-Mail: chk@chem.ou.dk one or both of 1,4-tpbd’s tridentate ends is possible. Thus the elements necessary for the construction of one-dimensional co-ordination polymers using 1,4-tpbd are established. Such a polymer is depicted by Scheme 1 and our initial foray into the preparation of such compounds is also described.The co-ordination chemistry of 1,4-tpbd may be compared to that of the polypyridyl bis(tridentate) ligands 2,3,5,6-tetra- (2-pyridyl)pyrazine (tppz) 4 and the ‘back-to-back’ terpy-based systems,5 e.g. 69,60-di(2-pyridyl)-2,29:49,40:20,2--quaterpyridine (dpqtpy). To our knowledge crystal structures of metal complexes in which both ends of these ligands are simultaneously bound to a metal ion in either di- or poly-nuclear complexes have been reported in only the case of tppz, for the complexes [Cu2(tppz)(H2O)4][ClO4]4 and [{Zn2(m-tppz)(H2O)Cl(m-ZnCl4)- (m-ZnCl2)[m-ZnCl3(H2O)]}2][ClO4]4.4 Clearly co-ordination of both tppz and dpqtpy results in a meridional arrangement for the tridentate ends of these bis-chelating ligands. By contrast, the presence of a tertiary amine group in 1,4-tpbd gives the py N N py py M py 2 n+ n N py py M 2 n+ n py py N (a) (b) Scheme 1 View of a one-dimensional co-ordination polymer constructed of 1,4-tpbd and a divalent octahedral metal ion: (a) meridional co-ordination of the tridentate chelating end and (b) trans-facial coordination of the tridentate chelating end (cis-facial not shown)1752 J.Chem. Soc., Dalton Trans., 1998, Pages 1751–1756 potential for both meridional and facial co-ordination geometries. Thus we envision diVerent topologies in dinuclear and polymeric co-ordination complexes of these three linear bis(tridentate) ligands. With the present work we have achieved a more extensive collection of structurally characterized monoand di-nuclear transition-metal complexes of 1,4-tpbd compared with those known for tppz and dpqtpy.Experimental Infrared spectra were measured as KBr discs using a Hitachi 270-30 spectrometer, UV/VIS absorption spectra on a Shimadzu UV-3100 spectrophotometer, EI mass spectra on a Varian MAT311A spectrometer, FAB mass spectra on a Kratos MS-50 spectrometer and NMR spectra on a Bruker AC 250 spectrometer. Elemental analyses were carried out at the microanalytical laboratory of the H.C.Ørsted Institute, Copenhagen. N,N,N9,N9-Tetrakis(2-pyridylmethyl)benzene-1,4-diamine was synthesized as reported.3 CAUTION: Although no problems were encountered in the preparation of the perchlorate salt, suitable care should be taken when handling such potentially hazardous compounds. Preparations Dichloro[N,N,N9,N9-tetrakis(2-pyridylmethylbenzene)-1,4- diamine]zinc(II), [ZnCl2(1,4-tpbd)]. Zinc chloride (116 mg, 0.864 mmol) in dmso (5 cm3) was added to a solution of 1,4-tpbd (200 mg, 432 mmol) in dmso (25 cm3).After 7 d the product was deposited as pale pink crystals. These were collected and dried in vacuum, yield 132 mg, 50% (Found: C, 59.36; H, 4.79; N, 13.77. C30H28Cl2N6Zn requires C, 59.18; H, 4.64; N, 13.80%). FAB mass spectrum: m/z 707.9 {[Zn2Cl3(1,4-tpbd)H2]1, 12}, 671.0 {[Zn2Cl2(1,4-tpbd)H]1, 50} and 635 {[Zn2Cl(1,4-tpbd)]1, 100%}. 1H NMR [(CD3)2SO]: d 4.64 (s, 8 H, CH2), 6.55 (s, 4 H, C6H4), 7.28 (s, 8 H, py H3, H5) and 7.73 (s, 4 H, py H4); after addition of 3 equivalents of ZnCl2, d 4.62 (s, 8 H, CH2), 6.73 (s, 4 H, C6H4), 7.52 (m, 8 H, py H3), 7.99 (t, 4 H, py H4, J = 7.4) and 8.75 (d, 4 H, py H6, J = 4.4 Hz).Dichloro[N,N,N9,N9-tetrakis(2-pyridylmethyl)benzene-1,4- diamine]dipalladium(II) trichloro(dimethyl sulfoxide)palladate( II), [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2. Palladium(II) chloride (184 mg, 1.04 mmol) was heated in dmso (12 cm3) at ca. 100 8C for 30 min. The compound 1,4-tpbd (120 mg, 0.26 mmol) was added and the solution allowed to stand for 2 weeks, over which time orange crystals, in variable yields, were deposited. These were collected, washed with dmso and dried in vacuum (Found: C, 47.01; H, 4.03; Cl, 18.36. N, 10.85. C17H20Cl4N3O2Pd2S N N N N N N N N N N N N N N N N N N 1,4-tpbd tppz dpqtpy requires C, 46.34; H, 4.15; Cl, 18.24; N, 10.81%). FAB mass spectrum: m/z 791 ([M 1 Cl]1, 50), 756 ([M]1, 30), 721 ([M 2 Cl]1, 20) and 615 ([M 2 PdCl]1, 75%).Dichloro[N,N,N9,N9-tetrakis(2-pyridylmethyl)benzene-1,4- diamine]dipalladium(II) chloride, [Pd2Cl2(1,4-tpbd)]Cl2. Palladium( II) chloride (100 mg, 0.564 mmol) and KCl (84 mg, 1.127 mmol) were heated under reflux for 20 min; 1,4-tpbd (133 mg, 0.282 mmol) was added resulting in immediate precipitation of the light yellow product. Yield 174 mg, 74% (Found: C, 43.01; H, 3.58; N, 10.35. C15H14Cl2N3Pd requires C, 43.56; H, 3.41; N, 10.16%).FAB mass spectrum: m/z 791 {[Pd2Cl3(1,4-tpbd)]1, 36}, 756 {Pd2Cl2(1,4-tpbd)]1, 56}, 721 {[Pd2Cl(1,4-tpbd)]1, 24} and 615 {[PdCl(1,4-tpbd)]1, 51%}. UV/VIS (MeOH): l/nm (e/dm3 mol21 cm21): 234 (15 460), 261 (9030) and 354 (1010). Bis[N,N,N9N9-tetrakis(2-pyridylmethyl)benzene-1,4-diamine]- ruthenium(II) bis(hexafluorophosphate) [Ru(1,4-tpbd)2][PF6]2? 3H2O. The compounds 1,4-tpbd (200 mg, 0.424 mmol) and [RuCl2(C6H5CN)4] (62 mg, 0.104 mmol) in ethanol (100 cm3) were heated under reflux under argon for 48 h.The resulting yellow solution was evaporated to dryness and the residue redissolved in the minimum volume of water; 2 mol dm23 NaOH was added dropwise until pH 9. The precipitated unchanged ligand was filtered oV and the crude product precipitated by addition of a saturated aqueous solution of NH4PF6. Purification was achieved by column chromatography in silica gel using CH3CN–saturated aqueous KNO3–water (14:2:1). The main fraction containing [Ru(1,4-tpbd)2]21 eluted in the last, yellow band.Yellow [Ru(1,4-tpbd)2][PF6]2 was precipitated by addition of saturated aqueous NH4PF6 and recrystallized from water–acetonitrile (2 : 1). Yield 54 mg, 48% (Found: C, 51.72; H, 3.88; N, 11.93. C60H62F12N12O3RuP2 requires C, 51.84; H, 4.50; N, 12.09%). 1H NMR (CD3CN): d 4.08 (d, 2 H, CH2 co-ordinated, J = 20.1), 4.18 (d, 2 H, CH2 co-ordinated, J = 20.1), 4.27 (d, 2 H, CH2 co-ordinated, J = 19.1), 4.43 (d, 2 H, CH2 co-ordinated, J = 19.1), 4.77 (s, 8 H, CH2 unco-ordinated), 6.22 (m, 8 H, C6H4), 6.99–7.46 (m, 8 H, py H3 1 H5 co-ordinated 1 un-coordinated), 7.70–7.94 (m, 8 H, py H4 co-ordinated 1 unco-ordinated), 8.24 (d, 2 H, py H6 co-ordinated, J = 5.5), 8.66 (d, 4 H, py H6 unco-ordinated, J = 4.7) and 9.19 (d, 2 H, py H6 co-ordinated, J = 5.6). 13C NMR (CD3CN): d 57.88 (CH2 unco-ordinated), 64.76, 71.91 (CH2 co-ordinated), 112.84 (phenyl CH), 121.28, 122.93, 123.72, 123.95, 125.86, 126.60 (py C3 1 C5 co-ordinated 1 unco-ordinated), 138.09, 138.66, 140.96 (py C4 co-ordinated 1 unco-ordinated), 139.30, 146.12 (phenyl C, co-ordinated 1 unco-ordinated), 149.40, 153.04, 156.13 (py C6 coordinated 1 unco-ordinated), 159.34 (py C2 unco-ordinated), 162.03, 165.88 (py C2 co-ordinated).UV/VIS (CH3CN): l/nm (e/dm3 mol21 cm21): 255 (52 100) and 389 (13 750). Poly{[N,N,N9N9-tetrakis(2-pyridylmethylbenzene)-1,4- diamine]iron(II) chloride} [Fe(1,4-tpbd)]nCl2n. This synthesis was carried out under argon using standard Schlenk techniques.Anhydrous FeCl2 (32 mg, 0.26 mmol) in CH3CN (5 cm3) was added to a solution of 1,4-tpbd (120 mg, 0.26 mmol) in CH3CN (25 cm3) together with a few iron turnings. The stirred mixture was heated under reflux for 1 h, cooled and allowed to crystallize for 2 h and finally filtered under argon. The resulting yellow crystals were stable in air, yield 85 mg, 56% (Found: C, 60.90; H, 4.75; Cl, 11.63; N, 14.25. C30H28Cl2FeN6 requires C, 60.12; H, 4.71; Cl, 11.83; N, 14.02%).FAB mass spectrum: m/z 1035 {[FeCl(1,4-tpbd)2]1, 1}, 619 {[Fe2Cl(1,4-tpbd)]1, 20} and 563 {[FeCl(1,4-tpbd)]1, 60%}. Poly{[N,N,N9N9-tetrakis(2-pyridylmethylbenzene)-1,4- diamine]nickel(II) perchlorate}, [Ni(1,4-tpbd)]n[ClO4]2n?nH2O. The compound Ni(ClO4)2?6H2O (38.7 mg, 0.106 mmol) in hot absolute ethanol (5 cm3) was slowly added to a stirred solutionJ. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756 1753 of 1,4-tpbd (50 mg, 0.106 mmol) in hot absolute ethanol (10 cm3).The product precipitated immediately as a green solid, which was filtered oV using an extra fine filter, yield 62 mg, 80% (Found: C, 47.56; H, 4.12; Cl, 9.43; N, 11.47. C30H28Cl2N6NiO8) requires C, 48.16; H, 4.04; Cl, 9.48; N, 11.23%). FAB mass spectrum: m/z 1101 {[Ni(1,4-tpbd)2(ClO4)]1, 30}, 1002 {[Ni(1,4- tpbd)2]1, 25}, 887 {[Ni2(1,4-tpbd)(ClO4)3]1, 28}, 788 {[Ni2(1,4- tpbd)(ClO4)2]1, 27}, 629 {[Ni(1,4-tpbd)(ClO4)]1, 75} and 530 {[Ni(1,4-tpbd)]1, 100%}.X-Ray crystallography Crystals suitable for X-ray diVraction studies were isolated directly from reaction mixtures. Details of structure determinations are listed in Table 1. Intensities were measured using a Huber four-circle diVractometer, at room temperature for [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2. At room temperature crystals of [ZnCl2(1,4-tpbd)] decayed rapidly in the X-ray beam, and the crystal was therefore cooled to 120 K, by means of a Crystostream cooler,6 which reduced the problem.Cell dimensions were determined from reflections measured at ±2q. Data were corrected for background, Lorentz-polarization eVects, and absorption. The structures were determined using SIR 92 7 and from subsequent diVerence electron-density maps and were refined by the minimization of Sw(|Fo| 2 Fc|)2 using a modifi- cation of ORFLS.8 Crystals of [ZnCl2(1,4-tpbd)] were twinned on (001) so that reflections hkl with l = 2, 9 and 13 were partly overlapped and were rejected, those with l = 0, 11 were almost totally overlapped and could therefore be unscrambled, and the reflections of remaining layers were not overlapped and could be used unaltered.Non-hydrogen atoms were refined anisotropically; hydrogen atoms of the ligand were kept at calculated positions (C]H 0.95 Å) with isotropic displacement parameters 20% larger than the equivalent isotropic displacement parameters of the atoms to which they were attached. Atomic scattering factors and anomalous dispersion corrections (for Zn and Pd) were from ref. 9. CCDC reference number 186/924. See http://www.rsc.org/suppdata/dt/1998/1751/ for crystallographic files in .cif format. Results and Discussion Our investigations with the bis(tridentate) ligand 1,4-tpbd demonstrate that this ligand shows ‘schizodentate’ character; co-ordination to transition-metal ions via either one or both of the tridentate ends is possible in 1 : 1, 1 : 2 and 2 : 1 metal : ligand complexes.A mononuclear complex The reaction of 1,4-tpbd with an excess of ZnCl2 in dmso results in pale pink crystals of [ZnCl2(1,4-tpbd)]. This slight colouration is probably due to the presence of a trace amount of oxidized ligand (the one-electron oxidized form of the ligand, 1,4-tpbd~1, is purple 3). Preparations using other solvents gave products which precipitated much faster yielding white microcrystalline materials. The solubility properties of the neutral [ZnCl2(1,4-tpbd)] might be the reason for the precipitation of a mono- rather than a di- or poly-zinc complex.Preparations with large excesses of zinc did not alter the outcome. However electronic grounds cannot be excluded; co-ordination of one end of the ligand may withdraw electron density from the other end with the result that once the first metal is bound it is more diYcult to bind the second. The 1H NMR spectrum of [ZnCl2(1,4-tpbd)] was recorded in (CD3)2SO in both the presence and absence of extra zinc chloride.In the absence of additional ZnII the signals in the spectrum of [ZnCl2(1,4-tpbd)] are broad indicating, not surprisingly, rapid ligand exchange on the NMR timescale. When 3 equivalents of zinc chloride were added to the solution the signals sharpened and indeed multiplets were observed for the signals due to the aromatic protons. A singlet at d 4.64 due to the eight methylene protons indicates that they are chemically equivalent despite the crystal structure which shows that only one end of the ligand is co-ordinated.This result can be interpreted such that the complex is still labile in the presence of excess of zinc and/or dinuclear in solution. Insolubility in solvents appropriate for low-temperature NMR studies has prevented investigation of this issue. The crystal structure of neutral [ZnCl2(1,4-tpbd)] is shown in Fig. 1. Selected distances and angles are listed in Table 2. The zinc atom is five-co-ordinated to three nitrogen atoms of the ligand and two chlorine atoms, with Zn]Cl(1) 2.277(4), Zn]Cl(2) 2.300(4), average Zn]Npy 2.15(1) and Zn]Namine 2.26(1) Å. The co-ordination geometry is intermediate between that of a square pyramid and a trigonal bipyramid, but is closest to a tetragonal-pyramidal arrangement with Cl(1) at the apex.The sum of the C]N]C angles around the unco-ordinated phenylenediamine nitrogen, N(2), is 359.88 and C(4)]N(2) is only 1.37(1) Å. In contrast the sum of the C]N]C angles around the co-ordinated N(1) is 338.58 and C(1)]N(1) 1.49(1) Å.The planar geometry about the unco-ordinated amine nitrogen atom as well as the double-bond character for C(4)]N(2) indicates a p delocalization of this amine nitrogen atom lone pair with the aromatic system. A result of this delocalization is the quinoid character evident in the bond lengths of the aromatic linker. A dinuclear complex The reaction of 1,4-bpbd with PdCl4 22 in methanol–water yields the dinuclear palladium complex [Pd2Cl2(1,4-tpbd)]Cl2 as a relatively insoluble pale yellow microcrystalline material. However the cation of this complex could be structurally characterized in the orange crystalline compound [Pd2Cl2- (1,4-tpbd)][PdCl3(dmso)]2 which was obtained from the reaction of 1,4-tpbd with PdCl2 in dmso.The cation and the anion in the structure of [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2 are shown in Fig. 2. An ORTEP diagram of the counter anion [PdCl3(dmso)]2 is shown since to our knowledge this is the first report of a crystal structure containing this particular anion, although there are several examples known for its platinum analogue.11 Selected distances and angles for [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2 are listed in Table 3.In [Pd2Cl2(1,4-tpbd)]21 the 1,4-tpbd bridges the two palladium atoms which are each co-ordinated to three nitrogen atoms and one chlorine atom in a square-planar arrangement with Pd]Cl 2.290(1), Pd]Npyridyl 2.009(3) and Pd]Namine 2.047(4) Å.Even though meridional co-ordination of this ligand might be expected to produce a more strained system these bond distances are shorter than the corresponding metal–donor bonds in [ZnCl2(1,4-tpbd)]. The cation has approximate mm2 symmetry with one mirror plane coincident Fig. 1 An ORTEP10 drawing of the cation in Zn(1,4-tpbd)Cl21754 J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756 Table 1 Crystallographic data and experimental details * Formula M Space group a/Å b/Å c/Å b/8 U/Å3 ZT /K Dc/g cm23 F(000) Colour Crystal shape Crystal size/mm m(Mo-Ka)/mm21 Absorption correction Transmission factors Data collection range/8 No.reflections measured Rint No. unique reflections No. observed reflections No. variables RR 9 Goodness of fit (D/s)max rmin, rmax/e Å23 [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2?0.5H2O C34H41Cl8N6O2.5Pd4S2 1347.62 P21/n 8.822(1) 36.340(5) 14.125(1) 92.086(6) 4525(1) 4 294 1.978 2637.20 Orange Lath 0.50 × 0.15 × 0.03 2.170 Empirical 0.705–1.210 2 < 2q < 50; ±h, 1k, 1l 8892 0.032 7996 5575 [I > 3s(I)] 515 0.036 0.046 1.094 0.035 20.8(1), 0.7(1) [ZnCl2(1,4-tpbd)] C30H28Cl2N6Zn 608.88 P21 8.369(2) 13.448(3) 13.330(4) 111.19(1) 1399(1) 2 120 1.446 628 Pale pink Tabular 0.45 × 0.40 × 0.16 1.100 Integration 0.727–0.851 3 < 2q < 55; ±h, 1k, 1l 5288 0.170 3352 1976 [I > 2s(I)] 352 0.065 0.073 1.395 0.028 21.1(1), 1.2(1) * Details in common: monoclinic; 30 reflections centred; graphite-monochromated Mo-Ka radiation (l 0.710 73 Å); 0% decay of standards; R = S||Fo| 2 |Fc || /S|Fo|, R9 = [Sw2(|Fo| 2 |Fc|)2/Sw2(|Fo|]� �� , w = 1/{[sCS(F2) 1 1.03F2]� �� 2 |F|}.with the plane of the phenyl ring, and the other perpendicular to it and bisecting C(2)]C(3) and C(5)]C(6). The geometry of the palladium ion in the anion is square planar with Pd]Cl 2.300(1) and Pd]S 2.250(1) Å. There is a partially occupied water site.Again conjugation of the amine-based electrons with those of the linking benzene ring is reflected by a quinoid character for the bond distances of the phenylenediamine moiety and the planar geometry of the amine nitrogen atoms. The ‘bite’ angles of the tridentate ends of 1,4-tpbd in [Pd2Cl2(1,4- tpbd)]21 are very similar to those measured for the related terpy complex [PdCl(terpy)]1. The N]Pd]N angles reported for the distorted [PdCl(terpy)]1 cation are 79 and 828,12 similar to those for [Pd2Cl2(1,4-tpbd)]21 of 838.The similarities between these Table 2 Selected bond distances (Å) and angles (8) of [ZnCl2(tpbd)] Zn]Cl(1) Zn]Cl(2) Zn]N(21) Zn]N(11) Zn]N(1) N(1)]C(17) N(1)]C(1) N(1)]C(27) C(1)]C(6) N(11)]Zn]N(21) N(1)]Zn]N(21) Cl(1)]Zn]N(21) Cl(2)]Zn]N(21) N(1)]Zn]N(11) Cl(1)]Zn]N(11) Cl(2)]Zn]N(11) Cl(1)]Zn]N(1) Cl(2)]Zn]N(1) Cl(1)]Zn]Cl(2) C(1)]N(1)]C(17) C(17)]N(1)]C(27) Zn]N(1)]C(17) C(1)]N(1)]C(27) Zn]N(1)]C(1) 2.277(4) 2.300(4) 2.115(11) 2.178(9) 2.261(10) 1.466(16) 1.488(14) 1.489(16) 1.386(16) 150.9(4) 75.4(4) 99.7(3) 98.1(3) 76.5(4) 97.1(3) 96.1(3) 109.9(3) 135.3(3) 114.8(2) 111.5(10) 113.7(11) 105.7(8) 113.3(11) 112.0(7) C(1)]C(2) C(2)]C(3) C(3)]C(4) C(4)]N(2) C(4)]C(5) C(5)]C(6) N(2)]C(47) N(2)]C(37) Zn]N(1)]C(27) C(2)]C(1)]C(6) N(1)]C(1)]C(6) N(1)]C(1)]C(2) C(1)]C(2)]C(3) C(2)]C(3)]C(4) C(3)]C(4)]N(2) C(5)]C(4)]N(2) C(3)]C(4)]C(5) C(4)]C(5)]C(6) C(1)]C(6)]C(5) C(4)]N(2)]C(47) C(4)]N(2)]C(37) C(37)]N(2)]C(47) 1.411(15) 1.397(15) 1.401(15) 1.372(14) 1.424(18) 1.386(18) 1.416(19) 1.442(17) 99.9(8) 119.6(11) 121.7(11) 118.6(10) 120.4(10) 121.2(9) 121.8(11) 121.7(11) 116.4(10) 122.9(11) 119.2(11) 120.3(12) 118.6(11) 120.8(11) structures also support the less aliphatic nature of the tertiary amine donor in 1,4-tpbd.In the structures of the di- and tetra-positive cations [Pd2Cl2(1,4-tpbd)]21 (Fig. 2) and [Cu2(1,4-tpbd)(H2O)4]413 the Fig. 2 An ORTEP10 drawing of the cation (a) and the anion (b) in [Pd2Cl2(1,4-tpbd)][PdCl3(dmso)]2J. Chem.Soc., Dalton Trans., 1998, Pages 1751–1756 1755 donor nitrogen atoms and the metal atom to which they are coordinated are almost coplanar with the consequence of a trans arrangement of two pyridine groups at each metal ion. This pseudo-meridional co-ordination is expected in the case of the palladium complex, given the preference of palladium for square planar geometry. It is, however, a unusual arrangement for the bis(2-pyridylmethyl)amine ends of the ligand.The fact that a similar arrangement is present in the copper complex points towards the fact that a meridional-type arrangement may be preferred by the ligand. In contrast, the structures of complexes of bis(picolyl)amine and its N-substituted derivatives show almost exclusively facial co-ordination.13 A bis-coordinated complex The mononuclear bis-co-ordinated complex [Ru(1,4-tpbd)2]- [PF6]2 was prepared from the reaction of 1,4-tpbd and [RuCl2(C6H5CN)4] in 1 : 4 proportions in ethanol.The elemental analysis, mass and NMR spectra confirm the formulation. The presence of both co-ordinated and unco-ordinated picolyl groups is clearly evident in the NMR spectra of [Ru(1,4- tpbd)2][PF6]2. The 1H NMR spectrum is shown in Fig. 3. The four methylene protons of the unco-ordinated end of the Table 3 Selected bond distances (Å) and angles (8) of [Pd2Cl2(tpbd)]- [PdCl3(dmso)]2 Cation Pd(1)]N(11) Pd(1)]N(21) Pd(1)]N(1) Pd(1)]Cl(1) Pd(2)]N(31) Pd(2)]N(41) Pd(2)]N(2) Pd(2)]Cl(2) N(1)]C(1) N(1)]C(17) N(11)]Pd(1)]N(21) N(1)]Pd(1)]N(21) Cl(1)]Pd(1)]N(21) N(1)]Pd(1)]N(11) Cl(1)]Pd(1)]N(11) Cl(1)]Pd(1)]N(1) N(31)]Pd(2)]N(41) N(2)]Pd(2)]N(31) Cl(2)]Pd(2)]N(31) N(2)]Pd(2)]N(41) Cl(2)]Pd(2)]N(41) Cl(2)]Pd(2)]N(2) C(1)]N(1)]C(17) C(1)]N(1)]C(27) 2.005(5) 2.004(5) 2.051(5) 2.294(2) 2.011(5) 2.016(5) 2.042(5) 2.286(2) 1.480(7) 1.511(8) 166.4(2) 83.2(2) 97.1(1) 83.3(2) 96.5(1) 171.8(1) 167.1(2) 83.3(2) 96.7(2) 83.8(2) 96.2(2) 177.0(2) 111.6(4) 112.3(5) N(1)]C(27) N(2)]C(4) N(2)]C(37) N(2)]C(47) C(1)]C(2) C(1)]C(6) C(2)]C(3) C(3)]C(4) C(4)]6) C(17)]N(1)]C(27) C(4)]N(2)]C(37) C(4)]N(2)]C(47) C(37)]N(2)]C(47) C(2)]C(1)]C(6) N(1)]C(1)]C(6) N(1)]C(1)]C(2) C(1)]C(2)]C(3) C(2)]C(3)]C(4) C(3)]C(4)]C(5) N(2)]C(4)]C(3) N(2)]C(4)]C(5) C(4)]C(5)]C(6) C(1)]C(6)]C(5) 1.508(8) 1.480(7) 1.503(9) 1.504(8) 1.399(8) 1.376(8) 1.378(9) 1.374(9) 1.383(8) 1.395(8) 112.2(5) 111.4(5) 111.9(5) 112.4(5) 119.6(5) 121.2(5) 119.2(5) 119.5(6) 121.0(6) 119.7(5) 120.0(5) 120.3(5) 119.9(5) 120.2(5) Anion Pd(3)]S(1) Pd(3)]Cl(3) Pd(3)]Cl(4) Pd(3)]Cl(5) Pd(4)]S(2) Pd(4)]Cl(6) Pd(4)]Cl(7) Cl(4)]Pd(3)]S(1) Cl(5)]Pd(3)]S(1) Cl(3)]Pd(3)]S(1) Cl(4)]Pd(3)]Cl(5) Cl(3)]Pd(3)]Cl(4) Cl(3)]Pd(3)]Cl(5) Cl(8)]Pd(4)]S(2) Cl(7)]Pd(4)]S(2) Cl(6)]Pd(4)]S(2) Cl(7)]Pd(4)]Cl(8) Cl(6)]Pd(4)]Cl(8) Cl(6)]Pd(4)]Cl(7) 2.245(2) 2.311(2) 2.284(2) 2.296(2) 2.256(2) 2.312(2) 2.312(2) 94.01(7) 86.79(8) 176.66(6) 177.13(9) 88.01(8) 91.32(8) 90.8(1) 88.89(8) 178.02(9) 172.44(9) 90.3(1) 90.15(8) Pd(4)]Cl(8) S(1)]O(1) S(1)]C(7) S(1)]C(8) S(2)]O(2) S(2)]C(9) S(2)]C(10) Pd(3)]S(1)]O(1) Pd(3)]S(1)]C(7) Pd(3)]S(1)]C(8) Pd(4)]S(2)]O(2) Pd(4)]S(2)]C(9) Pd(4)]S(2)]C(10) O(1)]S(1)]C(8) O(1)]S(1)]C(7) C(7)]S(1)]C(8) O(2)]S(2)]C(9) O(2)]S(2)]C(10) C(9)]S(2)]C(10) 2.283(2) 1.467(5) 1.759(9) 1.757(8) 1.470(6) 1.763(8) 1.770(9) 114.9(2) 114.4(3) 109.3(4) 113.9(3) 114.5(4) 111.3(3) 109.7(5) 107.5(4) 100.1(5) 107.7(4) 108.8(5) 99.5(5) ligand are equivalent and appear as a singlet at d 4.77.The methylene protons of the co-ordinated end are constrained and show geminal coupling patterns. The four doublets due to these protons are shifted slightly upfield. The signals for the a-pyridine protons are split in a corresponding fashion. The a-protons of the unco-ordinated pyridines are seen as a doublet at d 8.66, whereas the a-protons of the co-ordinated pyridines are assigned to two doublets at d 8.24 and 9.19.From the NMR results it is impossible to determine the co-ordination geometry of the tridentate ends of the ligand in [Ru(tpbd)2]21, i.e. the ligand might be co-ordinated in either a cis- or transfacial or in a meridional fashion. The apparent trigonal geometry around the amine nitrogen atoms in the structures of the complexes of Zn, Pd and Cu3 suggest a preference for the tridentate end of the ligand for meridional co-ordination.However one indirect piece of evidence we have to support a possible facial co-ordination of the tridentate ends of the two 1,4-tpbd ligands in [Ru(1,4-tpbd)2]21 is the crystal structure of a mononuclear ruthenium(II) complex of a related phenyl-substituted bis(picolyl)amine ligand, [Ru(bpba)2][PF6]2 [bpba = N,N-bis- (2-pyridylmethyl)aniline] which shows cis-facial ligand coordination. 14 The 1H NMR signals arising from the aromatic and aliphatic protons of the co-ordinated end of [Ru(tpbd)2]21 show similar patterns to those assigned to the corresponding protons of bpba in the 1H NMR spectrum of [Ru(bpba)2]21,14 supporting the assignment of similar cis-facial co-ordination geometries.The cyclic voltammogram of [Ru(1,4-tpbd)2][PF6]2 shows one reversible wave centered at 1.196 V. This is assigned to a RuII–RuIII redox process rather than ligand oxidation. Thus ligand oxidation is suppressed upon co-ordination to ruthenium. (A reversible oxidation at 310 mV and an irreversible oxidation at 670 mV is observed for free 1,4-tpbd.3) Polynuclear complexes/co-ordination polymers The evidence so far suggests that polymeric or oligomeric nickel(II) and iron(II) complexes have been prepared, however in the absence of crystal structures we have found it diYcult unambiguously to characterize these materials.The relatively insoluble products obtained from the reaction of 1,4-tpbd with nickel(II) and iron(II) salts are microcrystalline, however crystals suitable for X-ray crystallography have so far eluded us.Elemental analyses are consistent with polymeric formulations. The pattern of the pyridine absorptions around 1600 cm21 in the IR spectra of [Fe(1,4-tpbd)]nCl2n and [Ni(1,4- tpbd)]n[ClO4]2n indicate co-ordination of all the pyridine groups since they bear greater resemblance to the spectra obtained for the complexes which show co-ordination of both tridentate ends of the ligand, i.e. the dinuclear 1,4-tpbdbridged palladium(II) and copper(II) complexes, compared to those of the monoco-ordinated zinc(II) and bis[ruthenium(II)]- complex.Mass spectrometry (FAB) supports the assignment of polymeric formulations. The spectra of [Fe(1,4-tpbd)]nCl2n and Fig. 3 Proton spectrum of [Ru(1,4-tpbd)2][PF6]21756 J. Chem. Soc., Dalton Trans., 1998, Pages 1751–1756 [Ni(1,4-tpbd)]n[ClO4]2n show several peaks consistent with the mass of the ions expected from decomposition of a polymer, e.g.peaks can be assigned to ions of the composition 1 : 2 metal : ligand and 2 : 1 metal : ligand ratios. In contrast and as expected, peaks assignable to 2: 1 metal:ligand combinations are observed while peaks for 1 : 2 metal : ligand combinations are absent in the mass spectra of the crystallographically characterized dinuclear complexes [Pd2(1,4-tpbd)Cl2][Pd(dmso)Cl3]2 and [Cu2(1,4-tpbd)(H2O)4][S2O6]2. An eVective magnetic moment of ca. 6 mB (mB ª 9.27 × 10224 J T21) per iron(II) at room temperature was obtained for [Fe(1,4- tpbd)]nCl2n at room temperature.The Mössbauer spectrum at room temperature shows a doublet with an isomer shift, d, of 0.803(2) mm s21 and a quadrupole splitting, DEQ, of 2.963(4) mm s21. These results are consistent for high-spin iron(II). If co-ordination polymers with 1,4-tpbd, tppz and dpqtpy are eventually structurally characterized they are expected to show quite diVerent topologies: the structures of the dinuclear complexes [Pd2Cl2(1,4-tpbd)]21 and [Cu2(1,4-tpbd)(H2O)4]41 reveal that the ligand exists in two diVerent conformations. This is reflected in the steric arrangement of the two ends; the metal planes in the dipalladium complex are located on the same side of the benzene linker, whilst a trans arrangement is evident in the case of the dicopper complex. The cause of these ‘cis’ and ‘trans’ arrangements is probably due simply to crystal-packing eVects.However the consequence of these conformations in oligomeric systems will be a puckering of the linear molecules.In the case of dpqtpy, a one-dimensional polymer is expected to be rod-like; only rotation about the interannular C]C bond linking the terpy-based ends is feasible. The ligand tppz is even less flexible, although it was shown to be highly twisted in the structures reported for the dinuclear copper and decanuclear zinc complex.4 Conclusion The isolation of a mononuclear complex [ZnCl2(1,4-tpbd)], and a bis complex, [Ru(1,4-tpbd)2][PF6]2, of 1,4-tpbd opens up fascinating possibilities, for example the co-ordination of a second and diVerent metal by the non-co-ordinated tridentate ends.Co-ordination of a second ligand to starting materials like the dinuclear [Pd2Cl2(1,4-tpbd)]21 and [Cu2(1,4-tpbd)(H2O)4]413 may lead to similar species. In fact these complexes represent well characterized examples of the types of building blocks needed to carry out the ‘complexes-as-metals, complexes-asligands’ approach to the assembly of oligomers proposed by Constable and Balzani and co-workers.15 Future work will include the reaction of [ZnCl2(1,4-tpbd)] and [Ru(1,4-tpbd)2]- [PF6]2 with a second transition-metal ion to give heterodinuclear complexes of the type [LnM(1,4-tpbd)ZnCl2]n1, [LnM(1,4-tpbd)Ru(1,4-tpbd)]n1 or heterotrinuclear complexes of the type [LnM(1,4-tpbd)Ru(1,4-tpbd)MLn]n1. In summary, we have now characterized transition-metal complexes with M: 1,4-tpbd ratios of 1 : 1, 2 : 1 and 1 : 2.Using combinations of these units the strategic build-up of linear heteronuclear complexes can be envisioned. Acknowledgements We are grateful for support from the Danish Natural Science Council (grant no. 9503162 to C. J. M.). Dr. Thomas Buchen, Johannes-Gutenberg-Universität-Mainz, Germany, is thanked for the magnetic susceptibility measurement and Mössbauer spectrum of the iron complex. References 1 F. Vögtle, Supramolecular Chemistry, Wiley, Chichester, 1991; Supramolecular Chemistry, eds.V. Balzani and L. De Cola, Kluwer, Dordrecht, 1992; Transition Metals in Supramolecular Chemistry, eds. L. Fabrizzi and A. Poggi, Kluwer, Dordrecht, 1994. 2 B. F. Hoskins and R. Robson, J. Am. Chem. Soc., 1989, 111, 5962; B. F. Abrahams, M. J. Hardie, B. F. Hoskins, R. Robson and G. A. Williams, J. Am. Chem. Soc., 1992, 114, 10 641; K. T. Potts, K. A. G. Raiford and M. Keshavarz-K, J. Am. Chem. Soc., 1993, 115, 2793; M.Fujita, Y. J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 1994, 116, 1151; S. Kawata, S. Kitagawa, M. Kondo, I. Furuchi and M. Munakata, Angew. Chem., Int. Ed. Engl., 1994, 33, 1759; S. Descurtins, H. W. Schmalle, P. Schneuwly, L. Zheng, J. Ensling and A. Hauser, Inorg. Chem., 1995, 34, 5501; M. 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Junis, M. Ciano and V. Balzani, in Perspectives in Coordination Chemistry, eds. A. F. Williams, C. Floriani and A. E. Merbach, VCH, Basel, 1992, p. 153; E. C. Constable, in Transition Metals in Supramolecular Chemistry, eds. L. Fabbrizzi and A. Poggi, NATO ASI Series, 1994, p. 81. Received 22nd January 1998; Paper 8/00602D

ISSN:1477-9226    

DOI:10.1039/a800602d

出版商:RSC    

年代:1998

数据来源: RSC