Base-Free Catalytic Hydrogen Production from Formic Acid Mediated by a Cubane-Type Mo3S4 Cluster Hydride

Abstract

Formic acid (FA) dehydrogenation is an attractive process in the implementation of a hydrogen economy. To make this process greener and less costly, the interest nowadays is moving toward non-noble metal catalysts and additive-free protocols. Efficient protocols using earth abundant first row transition metals, mostly iron, have been developed, but other metals, such as molybdenum, remain practically unexplored. Herein, we present the transformation of FA to form H-2 and CO2 through a cluster catalysis mechanism mediated by a cuboidal [Mo3S4H3(dmpe)(3)](+) hydride cluster in the absence of base or any other additive. Our catalyst has proved to be more active and selective than the other molybdenum compounds reported to date for this purpose. Kinetic studies, reaction monitoring, and isolation of the [Mo3S4(OCHO)(3)(dmpe)(3)](+) formate reaction intermediate, in combination with DFT calculations, have allowed us to formulate an unambiguous mechanism of FA dehydrogenation. Kinetic studies indicate that the reaction at temperatures up to 60 degrees C ends at the triformate complex and occurs in a single kinetic step, which can be interpreted in terms of statistical kinetics at the three metal centers. The process starts with the formation of a dihydrogen-bonded species with Mo-H center dot center dot center dot HOOCH bonds, detected by NMR techniques, followed by hydrogen release and formate coordination. Whereas this process is favored at temperatures up to 60 degrees C, the subsequent beta-hydride elimination that allows for the CO2 release and closes the catalytic cycle is only completed at higher temperatures. The cycle also operates starting from the [Mo3S4(OCHO)(3)(dmpe)(3)](+) formate intermediate, again with preservation of the cluster integrity, which adds our proposal to the list of the infrequent cluster catalysis reaction mechanisms.