Reactions of Cp*(PMe3)Ir(Me)OTf with silanes:: Role of base-free silylene complexes in rearrangements of the resulting silicon-based ligands

The mechanisms of silicon-hydrogen activation by Cp*(PMe3)Ir(Me)OTf (Cp* = eta(5)-C-5-Me-5, OTf = OSO2CF3; 1) and rearrangements of the resulting silyliridium complexes were investigated. The scope of this reaction has been studied for silanes with a variety of substituents. Silylene complexes of the type [Cp*(PMe3)Ir(SiR2)(H)] [X] (X = OTf, R = Mes, 6; X = B(C6F5)(4), R = Ph, 16) have been isolated, and the likelihood of their involvement in the rearrangements is discussed. The kinetics of the isomerization reaction of the cyclometalated iridium(V) complex {Cp*(PMe3)Ir(H)[kappa(2)-SiH(Mes)(2-CH2-4,6-Me2C6H2)]}[OTf] (5) to the iridium silylene [Cp*(PMe3)Ir(SiMeS2)(H)][OTf] (6) were examined using NMR spectroscopic techniques. The primary kinetic isotope effect (Si-H vs Si-D bond) for this process was determined to be 1.4+/-0.1, implying a rate-limiting hydride migration from silicon to iridium. The activation parameters for this isomerization have also been measured: DeltaH(not equal) = 23+/-2 kcal/mol and DeltaS(not equal) = 0.1+/-0.01 eu. Slow hydride migration to produce a silylene complex from either Cp*(PMe3)Ir(Me)OTf (1) or [Cp*(PMe3)Ir(Me)(CH2Cl2)][B(C6F5)(4)] (17) is observed for large substituents on silicon. However, production of the sterically less crowded complex [Cp*(PMe3)Ir(SiPh2)(H)][B(C6F5)(4)] (16) is extremely rapid upon reaction of 17 with H2SiPh2. This argues for the intermediacy of a three-coordinate silicon species.