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

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^IIITAML^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(center dot)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(center dot)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.