Overlap and Covalency in a κN-Arylnitrosyl Cu(II) Metalloradical Formed by Hydroxylamine Oxidative Binding

Redox noninnocent ligands present opportunities to access reactive metal-radical complexes, but competing redox pathways often hinder their selective formation. Here, a pentadentate arylhydroxylamine-containing (ArNHOH) ligand directs copper salts to form a single, well-defined Cu II –arylnitrosyl (ArNO •– ) radical complex via an unprecedented oxidative binding pathway. To the best of our knowledge, this is the first crystallographically characterized mononuclear κN-bonded Cu II –(ArNO •– ) complex. Magnetometry, NMR, and density functional theory (DFT) reveal a pronounced difference in copper-radical antiferromagnetic coupling (AFC) between the solid state and solution (−1290 vs −428 cm –1, respectively). This phase-dependent magnetism is associated with different geometries that impart different d σ (Cu)−π*(ArNO) overlaps of the magnetic orbitals. Quantitative L-edge X-ray absorption spectra (XAS) together with DFT shows that the Cu II –(ArNO •– ) interaction is strongly covalent, consistent with its robust antiferromagnetism. Mechanistic studies, including electrochemistry, establish conditions where oxidative binding is operative and show that product selectivity is governed by Cu oxidation state, base concentration, and the order and rate of addition of constituents. Taken together, this work introduces oxidative binding as a distinct route to access Cu-arylnitrosyl radical species and highlights how ligand design and geometric tuning dictate electronic structure and spin-state energetics in metal-radical complexes.