Ligand Redox Non-Innocence in [Coᴵᴵᴵ(TAML)]0/‒ Complexes Affects Nitrene Formation

The redox non-innocence of the TAML scaffold in cobalt-TAML (Tetra-Amido Macrocyclic Ligand) complexes has been under debate since 2006. In this work we demonstrate with a variety of spectroscopic measurements that the TAML backbone in the anionic complex [Co^III(TAML^red)]^- is truly redox non-innocent, and that one-electron oxidation affords [Co^III(TAML^sq)]. Multi-reference (CASSCF) calculations show that the electronic structure of [Co^III(TAML^sq)] is best described as an intermediate spin (S = 1) cobalt(III) center that is antiferromagnetically coupled to a ligand-centered radical, affording an overall doublet (S = ½) ground-state. Reaction of the cobalt(III)-TAML complexes with PhINNs as a nitrene precursor leads to TAML-centered oxidation, and produces nitrene radical complexes without oxidation of the metal ion. The ligand redox state (TAML^red or TAML^sq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [Co^III(TAML^sq)] or [Co^III(TAML^red)]^- with PhINNs results in formation of [Co^III(TAML^q)(N^•Ns)] and [Co^III(TAML^q)(N^•Ns)₂]^-, respectively. Herein, ligand-to-substrate single-electron transfer results in one-electron reduced Fischer-type nitrene radicals (N^•Ns^-) that are intermediates in catalytic nitrene transfer to styrene. These nitrene radical species were characterized by EPR, XANES, and UV-Vis spectroscopy, high resolution mass spectrometry, magnetic moment measurements and supporting CASSCF calculations.