Reductive Coupling of H - H , H - C , and C - C Bonds from Pd Complexes

Reductive coupling to form H-H, C-H, and C-C bonds from transition-metal complexes is of fundamental importance in many catalytic processes. However, despite numerous experimental' and theoretical2 studies, there remain a number of puzzles concerning these processes. For example, reductive elimination of methane from Pt(H)(CH3)(PPh3), is quite facile at -25 'C,ld while Pt(CH3)2(PPh,)2 is qkite stable (it decomposes at 237 OC!)." This puzzle is exacerbated by the theoretical results of Siegbahn and co-workers2e~f whose calculations lead to essentially the same barriers for C-H and C-C reductive elimination. In this work we show that the activation barrier for reductive elimination is sensitive to the nature of the bond being formed. Hydrogen has a spherically symmetric 1 s valence orbital allowing it to simultaneously form H-H bonds while breaking M-H bonds, leading to small (1.55 kcal) intrinsic barriers. The directionality of the methyl sp3 hybrid orbital makes it more difficult to convert from M-C to form C-C or C-H bonds during reductive elimination. Thus the CH3 group needs to have a different orientation for the M-C and C-C bonds, and in the transition state a compromise must be reached that is not optimal for either bond. The result is an intrinsic barrier of 10.4 kcal for C-H coupling and 22.0 kcal for C-C coupling. Studies on a prototypical oxidative addition/reductive elimination process3