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

Abstract

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 <b>[Co<sup>III</sup>(TAML<sup>red</sup>)]<sup>-</sup></b> is truly redox non-innocent, and that one-electron oxidation affords <b>[Co<sup>III</sup>(TAML<sup>sq</sup>)]</b>. Multi-reference (CASSCF) calculations show that the electronic structure of <b>[Co<sup>III</sup>(TAML<sup>sq</sup>)]</b> 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<sup>red</sup> or TAML<sup>sq</sup>) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of <b>[Co<sup>III</sup>(TAML<sup>sq</sup>)]</b> or <b>[Co<sup>III</sup>(TAML<sup>red</sup>)]<sup>-</sup></b> with PhINNs results in formation of <b>[Co<sup>III</sup>(TAML<sup>q</sup>)(N<sup>•</sup>Ns)]</b> and <b>[Co<sup>III</sup>(TAML<sup>q</sup>)(N<sup>•</sup>Ns)<sub>2</sub>]<sup>-</sup></b>, respectively. Herein, ligand-to-substrate single-electron transfer results in one-electron reduced Fischer-type nitrene radicals (N<sup>•</sup>Ns<sup>-</sup>) 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. <br>