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

30 October 2019, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

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 [CoIII(TAMLred)]- is truly redox non-innocent, and that one-electron oxidation affords [CoIII(TAMLsq)]. Multi-reference (CASSCF) calculations show that the electronic structure of [CoIII(TAMLsq)] 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 (TAMLred or TAMLsq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [CoIII(TAMLsq)] or [CoIII(TAMLred)]- with PhINNs results in formation of [CoIII(TAMLq)(NNs)] and [CoIII(TAMLq)(NNs)2]-, respectively. Herein, ligand-to-substrate single-electron transfer results in one-electron reduced Fischer-type nitrene radicals (NNs-) 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.

Keywords

Ligand redox non-innocence
Ligand-to-substrate single-electron transfer
Nitrene radical
cobalt
TAML

Supplementary materials

Title
Description
Actions
Title
Supporting Information CoTAMLNitrenes Final ChemRXiv
Description
Actions

Comments

Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.