Mechanism of formic acid dehydrogenation catalysed by Cp*Co(III) and Cp*Rh(III) complexes with N,N’-bidentate imidazoline-based ligands: A DFT exploration

Abstract

Homogenous formic acid dehydrogenation (FAD) is largely accomplished with the aid of Ir and Rh based catalysts. The use of earth-abundant transition-metal based catalysts for FAD are rather scarcely reported. This work computationally explores the mechanistic aspects of FAD catalysed by Cp*Co(III) and Cp*Rh(III) -based catalysts with pyridyl-imidazoline (L1) and 2,2’-biimidazoline (L2) ligand architectures. The computed free energy profiles substantiate that the β-hydride elimination is the slowest step in the FAD process. Further mechanistic analysis revealed that H3O+ is the energetically favoured proton source compared to formic acid (FA) for generating H2. NBO analysis signified that the proton from H3O+ ion is relatively more electrophilic than FA and that the metal-hydride bond in the transition state is more negatively charged in the transition state involving H3O+ ion. These factors make the protonation process easier and decreases the barrier for hydronium ion mediated protonation process. All the four designed catalysts were found to possess a moderate range of computed activation barrier (13.5 kcal/mol to 16.2 kcal/mol) at the rate-determining step which are comparable with their corresponding iridium analogues reported in the earlier studies. The Co and Rh catalysts designed in the present work with L1 ligand system were found to be relatively better candidates than L2. Particularly, the Co catalysts and the mechanistic details derived there in would help in the development of efficient earth-abundant transition metal-based catalysts for FAD.