Structural studies metal-silyl complexes

H, 13C and 29SiNMR spectroscopy have been used extensively to assign structures and study fluxional processes in transition-metal silyl complexes. For example, cobalt silyls (CO)4CoSiR3 (R = Ph, F, Cl) are fluxional in solution as determined by variable-... 

ABSTRACT. Polysilanes, (-SiRR -)n, represent a class of inorganic polymers that have unusual chemical properties and a number of potential applications. Currently the most practical synthesis is the Wurtz-type coupling of a dihalosilane with an alkali metal, which suffers from a number of limitations that discourage commercial development. A coordination polymerization route to polysilanes based on a transition metal catalyst offers a number of potential advantages. Both late and early metal dehydrogenative coupling catalysts have been reported, but the best to date appear to be based on titanocene and zirconocene derivatives. Our studies with transition metal silicon complexes have uncovered a number of observations that are relevant to this reaction chemistry, and hopefully important with respect to development of better catalysts. We have determined that many early transition metal silyl complexes are active catalysts for polysilane synthesis from monosilanes. A number of structure-reactivity correlations have been established, and reactivity studies have implicated a new metal-mediated polymerization mechanism. This mechanism, based on step growth of the polymer, has been tested in a number of ways. All proposed intermediates have now been observed in model reactions. 

Further support for the presence of 7r-bonding in the above compounds comes from structural studies with d° silyl complexes, which should not exhibit dn-dn backbonding since d orbitals on the transition metal are unoccupied. As can be seen from Table 2, M-Si distances in d° complexes are close to, or even greater than, the predicted values. Note that the reported Zr-Si distances (2.81 A) are remarkably greater than the M-Si distances found in other second transition series silyl complexes, such as the d6 RhHCl(SiCl3)(PPh3)2 [2.203 (4) A]150 and the d4 Cp Rh(SiEt3)2H2 [2.379 (2) A]27. 

For Class B (substitution labile) metal complexes, reequilibration to more thermodynamically favorable coordination modes will be very rapid relative to immobilization. Such behavior is typical of first-row TM complexes. In addition, these ions are usually very oxophilic, so the metal complexes are typically subject to ICC interactions with oxide materials. Since these metal ions are generally immobilized under conditions of thermodynamic control, all pertinent speciation equilibria, including ICC reactions (Section III.B), must be considered in order to understand or predict the outcome of immobilization reactions. It is essential to understand the relevant equilibria if direct imprinting of active site structures is to be successful. The studies of Klonkowski et al. (210-213), for example, underscore this point Sol-gel immobilization of copper complexes bearing silylated amine and ethylenediamine ligands were shown by EPR to result in multiple copper environments, suggesting competition between immobilization and ICC reactions. 

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