Unravelling the Reaction Mechanism of Formic Acid Dehydrogenation by CpRh(III) and CpCo(III) Catalysts with Proton-Responsive 4,4′- and 6,6′-Dihydroxy-2,2′-Bipyridine Ligands: A DFT Study

The catalytic mechanism of hydrogen production via formic acid decomposition by pentamethylcyclopentadienyl (Cp*) rhodium­(III) and cobalt­(III) catalysts with proton-responsive 4,4′-dihydroxy-2,2′-bipyridine (4L) and 6,6′-dihydroxy-2,2′-bipyridine (6L) ligands ([Cp*M­(4L)­(H 2 O)] 2+ and [Cp*M­(6L)­(H 2 O)] 2+ ; M = Rh and Co) were explored using density functional theory calculations. The effect of pH on the protonation state of M­(4L) and M­(6L) ligands was studied using the speciation approach, and the fully protonated dihydroxy-2,2′-bipyridine ligand was found to be the dominated species throughout the catalytic mechanism of formic acid decomposition at pH 2.5. For both Cp*Rh­(III) and Cp*Co­(III) catalysts with 4L or 6L ligands, the β-hydride elimination step was found to be the rate-determining step irrespective of the position of the hydroxyl group on the bipyridine ligand. In the case of M­(6L), both formic acid- and water-assisted hydrogen evolution transition states were considered, and from the computed free energy profile, the water-assisted H2 generation was found to be the most favorable pathway. The electronic origin of the difference in the catalytic efficiency of the chosen catalysts was traced by performing natural bonding orbital analysis. These analyses reveal that the second-order stabilizing interactions and hydricity in the reaction intermediates and transition states play a significant role in altering the energetics of the formic acid decomposition reaction. Furthermore, the calculated activation free energies for the β-hydride elimination step catalyzed by the chosen catalysts were in the range of 15.8 to 20.3 kcal/mol, signifying that these catalysts are promising candidates for hydrogen generation with catalytic activities comparable to its Ir analogue. Especially, Co­(6L) with a relatively low activation energy barrier of 15.8 kcal/mol can be considered as an efficient low-cost catalyst for achieving fast dehydrogenation of formic acid. Overall, the present study paves the way for designing novel catalysts for hydrogen generation via formic acid dehydrogenation.