Ligand Redox Noninnocence in [Co(III)(TAML)](0/–) Complexes Affects Nitrene Formation

[Image: see text] The redox noninnocence 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 noninnocent and that one-electron oxidation affords [Co(III)(TAML(sq))]. Multireference (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 = (1)/(2)) 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 the 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.