High-Valent Ni and Cu Complexes of a Tetraanionic Bis(amidateanilido) Ligand

Abstract

High-valent metal species are often invoked as intermediates during enzymatic and synthetic catalytic cycles. Anionic donors are often required to stabilize such high-valent states by forming strong bonds with the Lewis acidic metal centers while decreasing their oxidation potentials. In this report, we discuss the synthesis of two high-valent metal complexes [ML]⁺ in which the Ni^III and Cu^III centers are ligated by a new tetradentate, tetraanionic bis(amidateanilido) ligand. [ML]⁺, obtained via chemical oxidation of ML, exhibits UV-vis-NIR, EPR, and XANES spectra characteristic of square planar, high-valent M^III species, suggesting the locus of oxidation for both [ML]⁺ is predominantly metal-based. This is supported by theoretical analyses, which also support the observed visible transitions as ligand-to-metal charge transfer transitions characteristic of square planar, high-valent M^III species. Notably, [ML]⁺ can also be obtained via O₂ oxidation of ML due to its remarkably negative oxidation potentials (CuL/[CuL]⁺: -1.16 V, NiL/[NiL]⁺: -1.01 V vs Fc/Fc⁺ in MeCN). This demonstrates the exceptionally strong donating nature of the tetraanionic bis(amidateanilido) ligation and its ability to stabilize high-valent metal centers..