Reactivity of Heterobimetallic Ion Pairs in Formic Acid Dehydrogenation

The series of bimetallic complexes (tBuPXCYP)Pd-(mu-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 degrees 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(mu-OC)Mo(CO)2Cp (3), which gives complete conversion at 2 mol % loading in 30 min at 50 degrees 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 center dot center dot center dot 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 Bro''nsted or Lewis acids at the hydride abstraction/CO2 release step and unify the mechanisms suggested for different catalytic systems.