C-H bond activation by a hydrotris(pyrazolyl)borato ruthenium hydridle complex

The ruthenium complex TpRu(PPh3)(CH3CN)H (Tp = hydrotris(pyrazolyl)borate) catalyzes H/D exchange between CH4 and some deuterated organic solvents-benzene-d(6), tetrahydrofuran-d(8), diethyl ether-d(10), and dioxane-d(8). Preferential cleavage of the alpha-C-D and the beta-C-D bonds of THF-d(8) and diethyl ether-d(10), respectively, is observed. The H/D exchange processes have been investigated by density functional theory calculations at the B3LYP level. Theoretical study on the reaction mechanism suggests that or-complexes TpRu(PPh3)(eta(2)-H-R)H are active species in the exchange processes. During the exchange processes, the reversible transformations of TpRu(PPh3)(eta(2)-H-R)H to TpRu(PPh3)(eta(2)-H-2)R are the crucial steps. The barriers for the transformations are in the range 10-13.4 kcal/mol. Interestingly, the transition states for the transformations correspond to the seven-coordinate TpRu(PPh3)-(R)(H)(H), which are species derived from the oxidative addition of H-R to the metal center. The exchange processes involve transformations of the (eta(2)-H-R) species to the (eta(2)-H-2) species followed by H-H rotation in the latter. The rotation barriers are calculated to be in the range 2-4 kcal/mol. The exchange process having an aromatic R group is found to be most favorable due to the strong Ru-C(sp(2)) bonding, which stabilizes the (eta(2)-H-2) species and lowers the transformation barrier. The complex TpRu(PPh3)(CH3CN)H catalyzes H/D exchange between H-2 and the deuterated solvents too.