Reductive elimination/oxidative addition of carbon-hydrogen bonds at Pt(IV)/Pt(II) centers:: Mechanistic studies of the solution thermolyses of TpMe2Pt(CH3)2H

Reductive elimination of methane occurs upon solution thermolysis of kappa(3)-Tp(Me2)Pt(IV)(CH3)(2)H (1, Tp(Me2) = hydridotris (3,5-dimethylpyrazolyl)borate). The platinum product of this reaction is determined by the solvent. C-D bond activation occurs after methane elimination in benzene-d(6), to yield kappa(3)-Tp(Me2)Pt(IV)(CH3)-(C6D5)D (2-d(6)), which undergoes a second reductive elimination/oxidative addition reaction to yield isotopically labeled methane and kappa(3)-Tp(Me2)Pt(IV)(C6D5)(2)D (3-d(11)). In contrast, kappa(2)-Tp(Me2)Pt(II)(CH3)(NCCD3) (4) was obtained in the presence of acetonitrile-d(3), after elimination of methane from 1. Reductive elimination of methane from these Pt(IV) complexes follows first-order kinetics, and the observed reaction rates are nearly independent of solvent. Virtually identical activation parameters (DeltaH(obs)(not equal) = 35.0 +/- 1.1 kcal/mol, DeltaS(obs)(not equal) = 13 +/- 3 eu) were measured for the reductive elimination of methane from 1 in both benzene-d6 and toluene-as. A lower energy process (DeltaH(scr)(not equal) = 26 +/- 1 kcal/mol, DeltaS(scr)(not equal) = 1 +/- 4 eu) scrambles hydrogen atoms of 1 between the methyl and hydride positions, as confirmed by monitoring the equilibration Of kappa(3)- Tp(Me2)pt(IV)(CH3)(2)D (1-di) with its scrambled isotopomer, kappa(3)-Tp(Me2)Pt(IV)(CH3)(CH2D)H (1-d(1)'). The sigma-methane complex kappa(2)-Tp(Me2)Pt(II)(CH3)(CH4) is proposed as a common intermediate in both the scrambling and reductive elimination processes. Kinetic results are consistent with rate-determining dissociative loss of methane from this intermediate to produce the coordinatively unsaturated intermediate [Tp(Me2)Pt(II)(CH3)], which reacts rapidly with solvent. The difference in activation enthalpies for the H/D scrambling and C-H reductive elimination provides a lower limit for the binding enthalpy of methane to [Tp(Me2)Pt(II)(CH3)] Of 9 +/- 2 kcal/mol.