Synthesis, structure, and reactivity of tantalum complexes containing organosilicon ligands

Two strategies have been developed for the synthesis of tantalum silyl complexes. The first method utilizes hexamethylsilacyclopropane (HMS) as a silylation agent, via a dimethylsilylene insertion into tantalum hydride or strained tantalum carbon bonds. The second method involves oxidative addition of Si-H bonds of hydrosilanes to thermally or photochemically generated 16e-, coordinatively unsaturated tantalum centers. A variety of tantalum(III) and tantalum(V) silyl complexes with the general formulas Cp$\sb2$Ta(L)(SiR$\sb3$) (Cp = $\eta\sp5$-C$\sb5$H$\sb5$, L = CO, PMe$\sb3$ and CH$\sb2$ = CH$\sb2$), Cp$\sb2$Ta(H)$\sb2$(SiR$\sb3$), and Cp$\sb2$Ta(H)(SiR$\sb3$)$\sb2$ have been prepared in good to excellent yields by these two methods. These compounds have been characterized by NMR, IR and single crystal X-ray diffraction studies. The tantalum silyldihydride complexes, Cp$\sb2$Ta(H)$\sb2$(SiR$\sb3$), exist as a mixture of two structural isomers: a symmetrical isomer Cp$\sb2$Ta(H)(SiR$\sb3$)(H)(a), and an unsymmetrical isomer Cp$\sb2$Ta(H)(H)(SiR$\sb3$)(b). The two isomers undergo facile interconversion under mild conditions. The rate of interconversion appears to depend on the strength of the Ta-Si bond. Studies have shown that the isomerization proceeds via an intramolecular rearrangement of the silyl and hydride ligands, most likely mediated by an $\eta\sp2$-silane complex. Tantalum(III) silyl complexes, Cp$\sb2$Ta(PMe$\sb3$)(SiR$\sb3$), are reactive towards the C-H bonds of unhindered arenes (benzene, toluene, and m-xylene), as a result of oxidative addition of the C-H bond to the 16e$\sp-$ intermediate Cp$\sb2$Ta(SiR$\sb3$) . In the case of the sterically hindered complex, Cp$\sb2$Ta(PMe$\sb3$)(Si(t-Bu)$\sb2$H), reaction with arenes quantitatively yields the corresponding aryl complexes, Cp$\sb2$Ta(PMe$\sb3$)(Ar). Thermodynamic studies of the system have permitted the first direct comparison of Si-H and C-H bond activations by the same metal center, and have resulted in the estimation of relative M-C and M-Si bond strengths. Cp$\sb2$Ta(PMe$\sb3$)(Si(t-Bu)$\sb2$H) also reacts with ethylene, yielding two kinds of products arising from a competition between C-H activation and ethylene insertion into the Ta-Si bond. In addition, this compound also reacts with BH$\sb3\cdot$ base adducts to form the bidentate silyl substituted borohydride complex, CP$\sb2$Ta($\eta\sp2$-BH$\sb3$(Si(t-Bu)$\sb2$H)), which adds PMe$\sb3$ to yield the monodentate silylborohydride complex, Cp$\sb2$Ta(PMe$\sb3$)($\eta\sp1$-BH$\sb3$(Si(t-Bu)$\sb2$H)). Addition of excess PMe$\sb3$ leads to the displacement of the silylborane, PMe$\sb3\cdot$BH$\sb2$(Si(t-Bu)$\sb2$H), from the tantalum center. This study provides the first example of the formation of a B-Si bond mediated by a transition metal