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 (POCsp2OP = 2,6-(i-Pr2PO)2C6H3; POCsp3OP
= (i-Pr2POCH2)2CH).
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 σ-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 σ-bond
metathesis pathways.