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

A series of Ni complexes bearing a redox and acid-base noninnocent tetraamido macrocyclic ligand, H₄-(TAML-4) {H₄-(TAML-4) = 15,15-dimethyl-5,8,13,17-tetrahydro-5,8,13,17-tetraaza-dibenzo[a,g]cyclotridecene-6,7,14,16-tetraone}, with formal oxidation states of Ni^II, Ni^III, and Ni^IV 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^IV was characterized to be [Ni^III(TAML-4^•+)] with the oxidation state of the Ni^III ion and the one-electron oxidized TAML-4 ligand, TAML-4^•+. The Ni^III 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^II(TAML-4)]^2- and [Ni^III(TAML-4)]⁻ and between [Ni^III(TAML-4)]⁻ and [Ni^III(TAML-4^•+)] were demonstrated in spectroelectrochemical measurements as well as in chemical oxidation and reduction reactions. The reactivities of [Ni^III(TAML-4)]⁻ and [Ni^III(TAML-4^•+)] were then investigated in hydride transfer reactions using NADH analogs. Hydride transfer from 9,10-dihydro-10-methylacridine (AcrH₂) to [Ni^III(TAML-4^•+)] was found to proceed via electron transfer (ET) from AcrH₂ to [Ni^III(TAML-4^•+)] with no deuterium kinetic isotope effect (k_H/k_D = 1.0(2)). In contrast, hydride transfer from AcrH₂ to [Ni^III(TAML-4)]⁻ proceeded much more slowly via a concerted proton-coupled electron transfer (PCET) process with k_H/k_D = 7.0(5). In the latter reaction, an electron and a proton were transferred to the Ni^III center and the TAML-4 ligand, respectively. The mechanisms of the ET by [Ni^III(TAML-4^•+)] and the concerted PCET by [Ni^III(TAML-4)]⁻ were ascribed to the different redox potentials of the Ni complexes.