Synthesis and Structure of PNP-Supported Iridium Silyl and Silylene Complexes: Catalytic Hydrosilation of Alkenes

Abstract

Oxidative addition of bulky primary, secondary, and tertiary silanes to PNP (PNP = [N(2-P(i)Pr(2)-4-Me-C(6)H(3))(2)](-)) iridium complexes (PNP)IrH(2) and (PNP)Ir(COE) (11) afforded iridium silyl hydride complexes (PNP)Ir(H)(SiRR'R'') (3-8). Addition of 2 equiv of PhSiH(3) or (3,5-Me(2)C(6)H(3))SiH(3) to (PNP)IrH(2) or 11 yielded disilyl complexes (PNP)Ir(SiH(2)R)(2) (R = Ph (9), 3,5-Me(2)C(6)H(3) (10)). Hydride abstraction from (PNP)Ir(H)(SiH(2)R) (R = Trip (5), Dmp (6)) by [Ph(3)C][B(C(6)F(5))(4)] afforded iridium silylene complexes [(PNP)(H)Ir=SiR(H)][B(C(6)F(5))(4)] (R = Trip (12), Dmp (13)) exhibiting downfield (29)Si NMR resonances (234 ppm (12), 226 ppm (13)) and downfield (1)H NMR resonances for the Si-H group (10.76 ppm (12), 9.76 ppm (13)). Thermally stable disubstituted silylene complexes [(PNP)(H)Ir=SiPh(2)][A] (A = (-)B(C(6)F(5))(4) (14), (-)CB(11)H(6)Br(6) (16)) were isolated via hydride abstraction from (PNP)Ir(H)(SiHPh(2)). The X-ray structure of 16 confirmed sp(2) hybridization at silicon and revealed a short Ir-Si bond of 2.210(2) A. Catalytic hydrosilation of alkenes by hydrogen-substituted silylene complexes [(PNP)(H)Ir=SiMes(H)][B(C(6)F(5))(4)] (1) and 14 exhibited anti-Markovnikov regioselectivity with an array of alkene substrates. Addition of H(3)SiMes to complex 1 afforded [(PNP)(SiH(Mes)(Hex))IrH(SiH(2)Mes)][B(C(6)F(5))(4)] (19), featuring a beta-agostic interaction demonstrated by a J(SiH) of 102 Hz for the N-SiH hydrogen. Similarly, addition of H(2)SiPh(2) to 16 afforded the structurally characterized Ir(V) disilyl complex [(PNP)(SiPh(2))Ir(SiPh(2)H)(H)(2)][CB(11)H(6)Br(6)] (20). Complex 20 was found to be catalytically active for the hydrosilation of alkenes, which is consistent with its intermediacy in the catalytic cycle.