Synthesis, Structure, and Redox Reactivity of Ni Complexes Bearing a Redox and Acid-Base Non-innocent Ligand with Ni, Ni, and Ni Formal Oxidation States.

A series of Ni complexes bearing a redox and acid-base noninnocent tetraamido macrocyclic ligand, H-(TAML-4) {H-(TAML-4) = 15,15-dimethyl-5,8,13,17-tetrahydro-5,8,13,17-tetraaza-dibenzo[]cyclotridecene-6,7,14,16-tetraone}, with formal oxidation states of Ni, Ni, and Ni were synthesized and characterized structurally and spectroscopically. The X-ray crystallographic analysis of the Ni complexes revealed a square planar geometry, and the [Ni(TAML-4)] complex with the formal oxidation state of Ni was characterized to be [Ni(TAML-4)] with the oxidation state of the Ni ion and the one-electron oxidized TAML-4 ligand, TAML-4. The Ni oxidation state and the TAML-4 radical cation ligand, TAML-4, were supported by X-ray absorption spectroscopy and density functional theory calculations. The reversible interconversions between [Ni(TAML-4)] and [Ni(TAML-4)] and between [Ni(TAML-4)] and [Ni(TAML-4)] were demonstrated in spectroelectrochemical measurements as well as in chemical oxidation and reduction reactions. The reactivities of [Ni(TAML-4)] and [Ni(TAML-4)] were then investigated in hydride transfer reactions using NADH analogs. Hydride transfer from 9,10-dihydro-10-methylacridine (AcrH) to [Ni(TAML-4)] was found to proceed via electron transfer (ET) from AcrH to [Ni(TAML-4)] with no deuterium kinetic isotope effect (/ = 1.0(2)). In contrast, hydride transfer from AcrH to [Ni(TAML-4)] proceeded much more slowly via a concerted proton-coupled electron transfer (PCET) process with / = 7.0(5). In the latter reaction, an electron and a proton were transferred to the Ni center and the TAML-4 ligand, respectively. The mechanisms of the ET by [Ni(TAML-4)] and the concerted PCET by [Ni(TAML-4)] were ascribed to the different redox potentials of the Ni complexes.