The Role of Solvent and the Outer Coordination Sphere on H2 Oxidation Using [Ni(PCy2NPyz2)2]2+

Hydrogenase enzymes are reversible catalysts for H-2 production/oxidation, operating with fast rates and minimal overpotentials in water. Many synthetic catalyst mimics of hydrogenase operate in organic solvents. However, recent work has demonstrated the importance of water in the performance of some model complexes. In this work, the H-2 oxidation activity of [Ni((P2N2(3-pyridazyl)methyl)-N-Cy)(2)](2+) (CyPyz) was compared as a function of acetonitrile, methanol, and water. The reactivity was compared under neutral and acidic conditions in all three solvents and improvement in catalytic activity, from 2 to 40 s(-1), was observed with increasing hydrogen bonding ability of the solvent. In addition, the overpotential for catalysis drops significantly in the presence of acid in all solvents, from as high as 600 mV to as low as 70 mV, primarily due to the shift in the equilibrium potential under these conditions. Finally, H-2 production was also observed in the same solution, demonstrating bidirectional (irreversible) homogeneous H-2 production/oxidation. A structurally and electronically similar complex with a benzyl instead of a pyridazyl group was not stable under these conditions, limiting the evaluation of the contributions of the outer coordination sphere. Collectively, we show that by tuning conditions we can promote fast, efficient H-2 oxidation and bidirectional catalysis.