Thermochemical Studies of Nickel Hydride Complexes with Cationic Ligands in Aqueous and Organic Solvents

Transition metal hydride complexes are key intermediates in a variety of catalytic processes. Transfer of a hydride, hydrogen atom, and proton are defined by the thermochemical parameters of hydricity, bond dissociation free energy (BDFE), and pKa, respectively. These values have been studied primarily in organic solvents to predict or understand reactivity. Despite growing interest in the development of aqueous metal hydride catalysis, BDFE measurements of transition metal hydrides in water are rare. Herein, we report two nickel hydride complexes with one or two cationic ligands that enable the measurement of BDFE values in both aqueous and organic solvents using their reduction potential and pKa values. The Ni(I/0) reduction potentials increase anodically as more charged groups are introduced into the ligand framework and are among the most positive values measured for Ni complexes. The complex with two cationic ligands, 2-Ni(II)–H, displays exceptional stability in water with no evidence of decomposition at pH 1 for at least 2 weeks. The BDFE of the nickel hydride bond in 2-Ni(II)–H was measured to be 53.8 kcal/mol in water, and between 50.9-56.2 kcal/mol in acetonitrile, consistent with prior work that indicates minimal solvent dependence for BDFEs of O-H and N-H bonds. These results indicate transition metal hydride BDFEs do not change drastically in water and inform future studies on highly cationic transition metal hydride complexes.