Theoretical investigation of Ni(II) complexes for hydrogen oxidation and hydrogen production

We present a detailed theoretical study of [Ni(P^R₂N^R′₂)₂]²⁺ electrocatalysts for H₂ oxidation and H₂ formation developed at the EFRC Center for Molecular Electrocatalysis at PNNL. P^R₂N^R′₂ denotes cyclic diphosphine ligands that incorporate an amine base in each ligand chelate ring. Density functional theory-based calculations and hybrid QM/MM molecular dynamics simulations have been employed to efficiently map the complex chemistry of these catalysts. Results are rationalized in terms of the P and N substituents and their capability to tune the pKa of the pendant amines and the hydricity of the metal center. A detailed analysis of the conformational dynamics of the reaction intermediates and their role on the catalytic activity is also presented, along with a discussion on the role of solvent (acetonitrile) and of small amounts of water, which have been shown to have a dramatic effect on rates of H₂ production.