Kinetic Analysis and Sequencing of Si–H and C–H Bond Activation Reactions: Direct Silylation of Arenes Catalyzed by an Iridium-Polyhydride

The saturated trihydride IrH3{κ3-P,O,P-[xant­(PiPr2)2]} (1; xant­(PiPr2)2 = 9,9-dimethyl-4,5-bis­(diisopropylphosphino)­xanthene) coordinates the Si–H bond of triethylsilane, 1,1,1,3,5,5,5-heptamethyltrisiloxane, and triphenylsilane to give the σ-complexes IrH3(η2-H-SiR3)­{κ2-cis-P,P-[xant­(PiPr2)2]}, which evolve to the dihydride-silyl derivatives IrH2(SiR3)­{κ3-P,O,P-[xant­(PiPr2)2]} (SiR3 = SiEt3 (2), SiMe­(OSiMe3)2 (3), SiPh3 (4)) by means of the oxidative addition of the coordinated bond and the subsequent reductive elimination of H2. Complexes 2–4 activate a C–H bond of symmetrically and asymmetrically substituted arenes to form silylated arenes and to regenerate 1. This sequence of reactions defines a cycle for the catalytic direct C–H silylation of arenes. Stoichiometric isotopic experiments and the kinetic analysis of the transformations demonstrate that the C–H bond rupture is the rate-determining step of the catalysis. As a consequence, the selectivity of the silylation of substituted arenes is generally governed by ligand–substrate steric interactions.