Titanium-catalyzed dehydrocoupling of silanes: direct conversion of primary monosilanes to titanium(0) oligosilane complexes with agostic alpha-Si-H...Ti interactions.

Polysilane high polymers have been attracting increasing interest owing to their potential applications as photoresists, photoinitiators, thermochromic materials, and precursors to silicon carbide.1 In 1985, Harrod and co-workers discovered that group 4 metallocenes such as Cp2TiMe2 could catalyze the dehydrogenative coupling of primary organosilanes to linear poly(hydrosilanes), H-(SiHR)n-H with ∼140 SiHR units.2 Since then, this reaction has been intensively studied, and many new catalysts have been discovered. The Si-Si bond-forming step in these dehydrocoupling reactions is thought to involve σ-bond metathesis reactions of metal silyl complexes with an entering hydrosilane.2,3 Several group 4 silyl complexes have been isolated that serve as models of the intermediates responsible for the catalysis.2,4 Silane dehydrocoupling reactions have found additional use in the derivatization of silicon surfaces.5 In the course of investigating the reaction chemistry of the 14electron titanium(II) alkyl TiMe2(dmpe)2, we found that it is a catalyst for the dehydrogenative oligomerization of primary arylsilanes. More importantly, we have been able to isolate several of the titanium-containing species that are formed in this reaction; these species document consecutive steps in the dehydrocoupling reaction and thereby provide intriguing insights into the inner workings of this process. Specifically, we find that treatment of solutions of TiMe2(dmpe)2 with 4 equiv of PhSiH3 -20 °C for 4 h quantitatively yields a diamagnetic species of stoichiometry Ti(Si3H5Ph3)(dmpe)2 (1), which can be isolated as a purple solid.7 The conversion of TiMe2(dmpe)2 to the trisilane complex 1 is accompanied by the formation of ∼1 equiv each of methane and PhSiMeH2. When solutions of 1 are incubated with PhSiH3 in Et2O at -20 °C for several weeks, a new compound of stoichiometry Ti(Si4H6Ph4)(dmpe)2 (2) is generated nearly quantitatively.9 This tetrasilane species can also be isolated as a pure material.