Reactivity of Heterobimetallic Ion Pairs in Formic Acid Dehydrogenation

Abstract

The series of bimetallic complexes (tBuPXCYP)Pd(μ-OC)M(CO)2L (X, Y = CH2, O; M = W, Mo; L = Cp, Tp) catalyzes formic acid decomposition into H2/CO2 in fairly mild conditions (25–50 °C, toluene) without any organic base additives. The catalytic activity of bimetallic complexes increases with CH2-substitution of the O-bridges in the (PXCYP)-frame as well as with the proton-donating ability of the Mo/W hydride. The best result was obtained with (tBuPCP)Pd(μ-OC)Mo(CO)2Cp (3), which gives complete conversion at 2 mol % loading in 30 min at 50 °C (TOF = 100 h–1). During the catalysis, LM(CO)3H and (tBuPXCYP)Pd(OCHO) form as visible intermediates, while the palladium hydride species are also involved in the catalytic cycle. The experimental data show that hydride abstraction/CO2 release from palladium formates (tBuPXCYP)Pd(OCHO) is assisted by −OCHO···H–A hydrogen bonding with excess formic acid or acidic hydride LM(CO)3H. These findings highlight the pivotal role of the formate interaction with Brönsted or Lewis acids at the hydride abstraction/CO2 release step and unify the mechanisms suggested for different catalytic systems.