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

Oxidative addition of bulky primary, secondary, and tertiary silanes to PNP (PNP = [N(2-PiPr2-4-Me-C6H3)2]−) iridium complexes (PNP)IrH2 and (PNP)Ir(COE) (11) afforded iridium silyl hydride complexes (PNP)Ir(H)(SiRR′R′′) (3−8). Addition of 2 equiv of PhSiH3 or (3,5-Me2C6H3)SiH3 to (PNP)IrH2 or 11 yielded disilyl complexes (PNP)Ir(SiH2R)2 (R = Ph (9), 3,5-Me2C6H3 (10)). Hydride abstraction from (PNP)Ir(H)(SiH2R) (R = Trip (5), Dmp (6)) by [Ph3C][B(C6F5)4] afforded iridium silylene complexes [(PNP)(H)IrSiR(H)][B(C6F5)4] (R = Trip (12), Dmp (13)) exhibiting downfield 29Si NMR resonances (234 ppm (12), 226 ppm (13)) and downfield 1H NMR resonances for the Si−H group (10.76 ppm (12), 9.76 ppm (13)). Thermally stable disubstituted silylene complexes [(PNP)(H)IrSiPh2][A] (A = −B(C6F5)4 (14), −CB11H6Br6 (16)) were isolated via hydride abstraction from (PNP)Ir(H)(SiHPh2). The X-ray structure of 16 confirmed sp2 hybridization at silicon and revealed a short Ir−Si bond of 2.210(2) Å. Catalytic hydrosilation of alkenes by hydrogen-substituted silylene complexes [(PNP)(H)IrSiMes(H)][B(C6F5)4] (1) and 14 exhibited anti-Markovnikov regioselectivity with an array of alkene substrates. Addition of H3SiMes to complex 1 afforded [(PNP)(SiH(Mes)(Hex))IrH(SiH2Mes)][B(C6F5)4] (19), featuring a β-agostic interaction demonstrated by a JSiH of 102 Hz for the N−SiH hydrogen. Similarly, addition of H2SiPh2 to 16 afforded the structurally characterized Ir(V) disilyl complex [(PNP)(SiPh2)Ir(SiPh2H)(H)2][CB11H6Br6] (20). Complex 20 was found to be catalytically active for the hydrosilation of alkenes, which is consistent with its intermediacy in the catalytic cycle.