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

Abstract

A series of Ni complexes bearing a redox and acid-base noninnocent tetraamido macrocyclic ligand, H sub(4-)(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 sup(II), Ni sup(III), and Ni sup(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 sup(IV) was characterized to be [Ni sup(III)(TAML-4 sup(.+))] with the oxidation state of the Ni sup(III) ion and the one-electron oxidized TAML-4 ligand, TAML-4 sup(.+). The Ni sup(III) oxidation state and the TAML-4 radical cation ligand, TAML-4 sup(.+), were supported by X-ray absorption spectroscopy and density functional theory calculations. The reversible interconversions between [Ni sup(II)(TAML-4)] sup(2-) and [Ni sup(III)(TAML-4)] sup(-) and between [Ni sup(III)(TAML-4)] sup(-) and [Ni sup(III)(TAML-4 sup(.+))] were demonstrated in spectroelectrochemical measurements as well as in chemical oxidation and reduction reactions. The reactivities of [Ni sup(III)(TAML-4)] sup(-) and [Ni sup(III)(TAML-4 sup(.+))] were then investigated in hydride transfer reactions using NADH analogs. Hydride transfer from 9,10-dihydro-10-methylacridine (AcrH sub(2)) to [Ni sup(III)(TAML-4 sup(.+))] was found to proceed via electron transfer (ET) from AcrH sub(2) to [Ni sup(III)(TAML-4 sup(.+))] with no deuterium kinetic isotope effect (k sub(H)/k sub(D) = 1.0(2)). In contrast, hydride transfer from AcrH sub(2) to [Ni sup(III)(TAML-4)] sup(-) proceeded much more slowly via a concerted proton-coupled electron transfer (PCET) process with k sub(H)/k sub(D) = 7.0(5). In the latter reaction, an electron and a proton were transferred to the Ni sup(III) center and the TAML-4 ligand, respectively. The mechanisms of the ET by [Ni sup(III)(TAML-4 sup(.+))] and the concerted PCET by [Ni sup(III)(TAML-4)] sup(-) were ascribed to the different redox potentials of the Ni complexes.