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

Abstract

The ruthenium complex TpRu(PPh 3)(CH 3CN)H (Tp = hydrotris(pyrazolyl)borate) catalyzes H/D exchange between CH 4 and some deuterated organic solvents-benzene-d 6, tetrahydrofuran-d 8, diethyl ether-d 10, and dioxane-d 8. Preferential cleavage of the α-C-D and the β-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 σ-complexes TpRu(PPh 3)(η 2-H-R)H are active species in the exchange processes. During the exchange processes, the reversible transformations of TpRu(PPh 3)(η 2-H-R)H to TpRu(PPh 3)(η 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(PPh 3)(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 (η 2-H-R) species to the (η 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 (η 2-H 2) species and lowers the transformation barrier. The complex TpRu(PPh 3)(CH 3CN)H catalyzes H/D exchange between H 2 and the deuterated solvents too.