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Planar Tetracoordination of Carbon in Groups 4 and 5 Organometallic

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Planar Tetracoordination of Carbon in Groups 4 and 5 Organometallic Acc. Chem. Res. 1999, 32, 494-502 Planar Tetracoordination of Scheme 1 Carbon in Groups 4 and 5 Organometallic Chemistry ROBERT CHOUKROUN* AND PATRICK CASSOUX Equipe PreÂcurseurs MoleÂculaires et MateÂriaux, Laboratoire de Chimie de Coordination du CNRS, 205 Route de Narbonne, 31077 Toulouse CeÂdex, France Received July 29, 1998 dentally. Several recent important papers and exhaustive and comprehensive reviews have appeared at different Introduction stages of development of this research,5 and the reader is especially directed to the most recent one by Erker et al.6 Nowadays, when so many things are questioned again and Examination of the structural and electronic features of again, tetrahedral sp3 hybridization of tetravalent carbon the various involved compounds indicates that the sta- seemed to be a rock solid concept since its proposal by bilization required for obtaining ptC compounds may van't Hoff and Le Bel.1 Forget this! Planar tetracoordinate result from the incorporation of a carbon atom into carbon is not as odd as a three-dollar bill and does exist. sterically rigid, small ring or polycyclic systems, or from In 1970, Hoffman et al. were the first to predict, on the the increase of the electron density in the σ system and basis of a theoretical approach, the possible existence of its decrease in the π system. In the second case, variation planar methane with an electronic structure involving a of the electronic density may be obtained by use of six-electron four-bond electron-deficient σ system and a appropriate σ-donor/π-acceptor substituents, such as, for two-electron-containing p -based orbital perpendicular to z example, transition metals. As appears in the literature,6 the σ plane.2 Naturally, as the energy of planar methane most ptC compounds were obtained that way, and it may is higher than that of tetrahedral methane,3 some kind of seem ironic that organometallic chemistry provides the stabilization was needed if compounds containing planar most efficient route toward planar tetracoordinate carbon. tetracoordinate carbon (ptC) were to be prepared. Indeed, this prediction was experimentally confirmed,4 when Seidel et al. reported in 1976 the synthesis,4a and Cotton Planar Tetracoordinate Carbon in Groups 4 4b ‚ et al. in 1977 the structure, of V2(2,6-dimethoxyphenyl)4 and 13 Dimetallic Complexes THF, a divanadium complex in which a ptC may be From a general point of view, the preparation of dimetallic identified. Following on this seminal result, a number of complexes associating two transition metals has recently other ptC compounds have been prepared, often inci- attracted an increasing amount of attention. This is due to a new chemical reaction behavior resulting from the Robert Choukroun was born in Alger (in the former French department of Algeria), synergistic combination of the properties of both metals on June 24, 1944. He received all of his degrees from the Universite´ Paul Sabatier, which may be utilized in stoichiometric or catalytic in Toulouse, where he joined the CNRS in 1968 (Doctorat d'Etat, in 1975, under reactions.7 It happens that most of the ptC-containing the Supervision of Professor Danie`le Gervais). After a postdoctoral stay in 1975- 1976 at the Unit of Nitrogen Fixation, University of Sussex (Brighton, U.K.) under transition metal complexes reported to date are dimetallic the direction of Professor Chatt, he joined the Laboratoire de Chimie de complexes of groups 4 and 13 (I in Scheme 1).6 Coordination de Toulouse, where he has remained to this day, becoming Directeur de Recherche in 1996. His current research interests are concerned with the All the synthetic procedures described for preparing I organometallic chemistry of the early transition metals of groups 4 and 5 and are believed to involve an (alkyne)-metallocene interme- their development and use as catalysts for homogeneous catalysis and molecular diate compound II which reacts with another metal- precursors to ceramic materials and colloids. containing species to afford the dimetallic complex I in Patrick Cassoux was born in Hanoõ¨, former French Indochina (Viet Nam), on which a C1dC2 bond is connected to both metals. The C2 September 5, 1941. He received the ªInge´nieurº degree from the ªEcole Nationale atom is the ptC in this system as shown by the planarity Supe´rieure de Chimie de Toulouseº, the master degree from the Paul Sabatier 1 2 ′ ° University at Toulouse, in 1965, and the ªDoctorat d'Etatº (Ph.D.) from the same of the MC C M X cycle and the value close to 360 of the University, in 1971, under the direction of F. Gallais. He was a postdoctoral sum of the bond angles around C2. The preparation of associate at the University of Michigan, Ann Arbor, with R. L. Kuczkowski in ptC-containing, group 4 homodimetallic complex cations - 1973 74, and a visiting research fellow at the General Electric R&D Center, seems also to involve an (alkyne)-metallocene intermedi- Schenectady, NY, with L. V. Interrante in 1979. He joined the French National Center for Research (CNRS) in 1966, where he rose through the ranks to become ate.6,8,9 Thus, the presence of this (alkyne)-metallocene ªDirecteur de Recherche Iº in 1989. His research interests include molecular coordination compounds with unusual solid-state properties and molecular * Phone: +(33) [0]5 61333161. FAX: +(33) [0]5 61553003. E-mail: organometallic precursors to ceramic materials. [email protected]. 494 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 32, NO. 6, 1999 10.1021/ar970304z CCC: $18.00 1999 American Chemical Society Published on Web 01/29/1999 Planar Tetracoordination of Carbon Choukroun and Cassoux Scheme 2 and heterodimetallic complexes with various structural configurations. For example, Lang et al. have successfully used the reactivity of alkynes coordinated to titanium or zirconium to prepare heterodimetallic complexes in which the second metal is an element of group 11.17,18 These opportunities have also been exploited by Erker et al. who used Cp2Zr(butadiene) as a source of zirconocene. Reac- tion of ªCp2Zrº with Cp2M(CtCPh)2 (M ) Zr, Hf) is fol- Scheme 3 lowed by a redistribution process which affords either the dimeric [Cp2Zr(µ-CtCPh)]2 or the heterodimetallic Cp2- 19 Zr(µ-CtCPh)2HfCp2 complexes (Scheme 4). This re- search line was further explored by studying the proto- nation of [Cp2Zr(µ-CtCPh)]2, which led to the formation of ptC.19 intermediate, or, if available, an (alkyne)-metallocene- based reagent, seems crucial for obtaining group 4 and Synthesis and Characterization of Groups 4 13 dimetallic complexes containing one ptC. and 5 Dimetallic Complexes On the other hand, more recently, implication of All the ptC-containing dimetallic complexes reported until alkynyl moieties for incidental ptC generation was also recently had only one planar tetracoordinate carbon atom. observed by Rosenthal et al. in the reactivity of Ni0 We were able to prepare and characterize dimetallic complexes and titanocene (vide infra).10 Indeed, two complexes of groups 4 and 5 containing two ptCs.20 This - - heterobimetallic compounds (Ti Ni, Zr Ni) were pre- result was obtained in our group rather incidentally as our pared, and their crystal structure determination led to the primary goal was then the synthesis of dimetallic vana- identification of ptC in these compounds. dium-titanium complexes for use as molecular precursors in the organometallic chemical vapor deposition of car- Acetylide Chemistry. π Donation as the First bonitride ceramics containing both these metals.21,22 In Step many heterodimetallic complexes, an electron-poor metal The chemistry of acetylides is well suited for the prepara- (Ti, Zr) is associated with an electron-rich metal (Rh, 7,23 tion of new compounds of group 4 elements, either by Ru). The association in a single molecule of two oxidative addition of a diyne on the metal (initially TiII electron-poor metals is more unusual, and indeed, very and ZrII) of ligand-stabilized metallocene or by reaction few heterodimetallic compounds of groups 4 and 5 24-26 of an alkyne compound of a group 4 element with a Lewis elements have been described. The reaction behavior metal complex. Different situations may be encountered of alkyne triple bonds in substituted titanocene and depending on the nature of the metal, the substituents zirconocene compounds has recently been extensively 17,18,27-30 on the diyne or alkyne, and the stoichiometry, the simplest studied. In particular, the preparation by oxida- - - situation being the formation of the homodimetallic tive addition of titanium silver and titanium copper t complex [Cp M(µ-CtCPh)] (Cp ) cyclopentadienyl) heterodimetallic compounds from [(C5H4(SiMe3)]2Ti(C 2 2 ) ′ ′ ) (Scheme 2).11-14 Moreover, it has been shown by Rosenthal CSiMe3)2 and MCl2 (M Fe, Co, Ni) or M X(M Ag, Cu; X ) Cl, R) has been reported by Lang et al. (Scheme 5).18 et al. that the Cp2Ti(Me3SiCtCSiMe3) and Cp2Zr(THF)- t (Me3SiCtCSiMe3) compounds are convenient sources of Likewise, a similar oxidative addition of Cp2Hf(C CPh)2 15 transient Ti- and Zr-metallocene species (Scheme 3). by Cp2Zr(butadiene) has led to the preparation of Cp2Hf- t 19 Thus, the ªCp2Tiº titanocene and ªCp2Zrº zirconocene are (µ-C CPh)2ZrCp2. These results showed the way toward the reactive moieties on alkynes and dialkynes and can heterodimetallic [Ti, V] or [Zr, V] complexes involving the self-organize to afford [Ti, Ti] and [Zr, Zr] homodimetallic, use of acetylenic titanium or zirconium compounds. 16 2 1 2 as well as heterodimetallic, [Ti, Zr] complexes. Alter- Moreover, the synthesis of the [Cp2V(η -R CdCR ] com- nately, the reactivity toward diynes of these transient pound (Scheme 6)31 indicated that the oxidative addition ªCp2Mº species also allows the stabilization of these of vanadocene Cp2V (the oxidation state of vanadium species.
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