Reactions of Phenylhydrosilanes with Pincer-Nickel Complexes: Evidence for New Si-O and Si-C Bond Formation Pathways

This contribution presents evidence for new pathways manifested in the reactions of the phenylhydrosilanes PhnSiH4-n with the pincer complexes (POCsp2OP)Ni(OSiMe3), 1-OSiMe3, and (POCsp3OP)Ni(OSiMe3), 2-OSiMe3 (POCsp3OP = 2,6-(i-Pr2PO)(2)C6H3; POCsp3OP = (i-Pr2POCH2)(2)CH). Excess PhSiH3 or Ph2SiH2 reacted with 1-OSiMe3 to eliminate the disilyl ethers PhnH3-nSiOSiMe3 (n = 1 or 2) and generate the nickel hydride species 1-H. Subsequent reaction of the latter with more substrate formed corresponding nickel silyl species 1-SiPhH2 or 1-SiPh2H and generated multiple Si-containing products, including disilanes and redistribution products. The reaction of 1-OSiMe3 with excess Ph2SiH2/Ph2SiD2 revealed a net KIE of ca. 1.3-1.4 at room temperature. Treating 1-OSiMe3 with excess Ph3SiH also gave 1-H and the corresponding disilyl ether Ph3SiOSiMe3, but this reaction also generated the new siloxide 1-OSiPh3 apparently via an unconventional sigma-bond metathesis pathway in which the Ni center is not involved directly. The reaction of excess PhSiH3 and 2-OSiMe3 gave polysilanes of varying solubilities and molecular weights; NMR investigations showed that these polymers arise from Ni(0) species generated in situ from the reductive elimination of the highly reactive hydride intermediate, 2-H. The stoichiometric reactions of 2-OSiMe3 with Ph2SiH2 and Ph3SiH gave, respectively, siloxides 2-OSiPh2(OSiMe3) and 2-OSiPh3. Together, these results demonstrate the strong influence of pincer backbone and hydrosilane sterics on the different reactivities of 1-OSiMe3 and 2-OSiMe3 toward PhnSiH4-n (dimerization, polymerization, and redistribution vs formation of new siloxides). The mechanisms of the reactions that lead to the observed Si-O, Si-C, and Si-Si bond formations are discussed in terms of classical and unconventional sigma-bond metathesis pathways.