Evaluation of the Role of Water in the H₂ Bond Formation by Ni(II)-Based Electrocatalysts

We investigate the role of water in the H–H bond formation by a family of nickel molecular catalysts that exhibit high rates for H₂ production in acetonitrile solvent. A key feature leading to the high reactivity is the Lewis acidity of the Ni­(II) center and pendant amines in the diphosphine ligand that function as Lewis bases, facilitating H–H bond formation or cleavage. Significant increases in the rate of H₂ production have been reported in the presence of added water. Our calculations show that molecular water can displace an acetonitrile solvent molecule in the first solvation shell of the metal. One or two water molecules can also participate in shuttling a proton that can combine with a metal hydride to form the H–H bond. However the participation of the water molecules does not lower the barrier to H–H bond formation. Thus these calculations suggest that the rate increase due to water in these electrocatalysts is not associated with the elementary step of H–H bond formation or cleavage but rather with the proton delivery steps. We attribute the higher barrier in the H–H bond formation in the presence of water to a decrease in direct interaction between the protic and hydridic hydrogen atoms forced by the water molecules