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

摘要

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.