Heterolytic H2activation by rhodium thiolato complexes bearing the hydrotris(pyrazolyl)borato ligand and application to catalytichydrogenation under mild conditions

Abstract

Thiolato complexes of Rh(III) bearing a hydrotris(3,5-dimethylpyrazolyl)borato ligand (Tp^Me2) have been prepared, and their reactivity toward H₂ has been investigated. The bis(thiolato) complex [Tp^Me2Rh(SPh)₂(MeCN)] (1) reacted with 1 atm H₂ at 20 °C to produce the hydrido-thiolato complex [Tp^Me2RhH(SPh)(MeCN)] (2) and PhSH via heterolytic cleavage of H₂. This process is reversible and in equilibrium in THF and benzene. The bis(selenolato) complex [Tp^Me2Rh(SePh)₂(MeCN)] (4) was also converted to [Tp^Me2RhH(SePh)(MeCN)] and PhSeH under 1 atm H₂, but the equilibrium largely shifted to 4. Reaction of the dithiolato complex [Tp^Me2Rh(bdt)(MeCN)] (3; bdt = 1,2-C₆H₄S₂) with H₂ occurred in the presence of amine, giving the anionic hydrido complex [Tp^Me2RhH(bdt)]⁻ and an equimolar amount of ammonium cations. Catalytic activity for hydrogenation has been examined under 1 atm H₂ at 20–50 °C. While 1, 2, and 4 slowly hydrogenated styrene at similar rates at 50 °C, activities for the hydrogenation of N-benzylideneaniline increased in the order, 2 < 1 < 4. Complex 3 was found to be the most active and selective catalyst for hydrogenation of imines, and thus a variety of imines were reduced at 20 °C under 1 atm H₂, with the CC and CO bonds in the substrate molecules completely preserved. An ionic mechanism was involved to explain such high chemoselectivity.