Conformational Dynamics and Proton Relay Positioning in Nickel Catalysts for Hydrogen Production and Oxidation

The [Ni­(P^R₂N^R′₂)₂]²⁺ complexes (where P^R₂N^R′₂ is 1,5-R′-3,7-R-1,5-diaza-3,7-diphosphacyclooctane) are fast electrocatalysts for H₂ production and oxidation. Binding of a fifth ligand (CH₃CN or BF₄^–) or chair/boat isomerization has the potential to slow catalysis by blocking the addition of H₂ or by incorrectly positioning the pendant amines. We report the structural dynamics of a series of nickel complexes characterized by NMR spectroscopy and theoretical modeling to examine the effects of the fifth ligand for the Ni­(II) complexes, including CH₃CN, BF₄^–, Cl^–, and H^–, as well as the differences in dynamics between the Ni­(II) and Ni(0) oxidation states. A fast exchange process was observed for the [Ni­(CH₃CN)­(P^R₂N^R′₂)₂]²⁺ complexes, with rates ranging from 10⁴ to 10⁷ s^–1 depending on the phosphorus and nitrogen substituents on the P^R₂N^R′₂ ligand. This exchange process was identified to occur through a multistep mechanism, which consists of dissociation of the acetonitrile, boat/chair isomerization of each of the four rings (including nitrogen inversion), and reassociation of an acetonitrile on the opposite side of the complex. The rate of the chair/boat inversion was found to be influenced by varying the substituent on the nitrogen atom, but the rate of the overall exchange process is at least an order of magnitude faster than the catalytic rate in acetonitrile, demonstrating that the structural dynamics of the [Ni­(CH₃CN)­(P^R₂N^R′₂)₂]²⁺ complexes do not hinder catalysis. Possible catalytic implications of the coordination of a fifth ligand to the Ni­(II) complex are discussed.