C−H Bond Activation by a Hydrotris(pyrazolyl)borato Ruthenium Hydride Complex

Abstract

The ruthenium complex TpRu(PPh3)(CH3CN)H (Tp = hydrotris(pyrazolyl)borate) catalyzes H/D exchange between CH4 and some deuterated organic solventsbenzene-d6, tetrahydrofuran-d8, diethyl ether-d10, and dioxane-d8. Preferential cleavage of the α-C−D and the β-C−D bonds of THF-d8 and diethyl ether-d10, 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 σ-complexes TpRu(PPh3)(η2-H−R)H are active species in the exchange processes. During the exchange processes, the reversible transformations of TpRu(PPh3)(η2-H−R)H to TpRu(PPh3)(η2-H2)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 (η2-H−R) species to the (η2-H2) 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(sp2) bonding, which stabilizes the (η2-H2) species and lowers the transformation barrier. The complex TpRu(PPh3)(CH3CN)H catalyzes H/D exchange between H2 and the deuterated solvents too.