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Ligand redox noninnocence in [Co III (TAML)] 0/– complexes affects nitrene formation

DOI: 10.1021/jacs.9b11715 DOI Help

Authors: Nicolaas P. Van Leest (University of Amsterdam) , Martijn A. Tepaske (University of Amsterdam) , Jean-pierre H. Oudsen (University of Amsterdam) , Bas Venderbosch (University of Amsterdam) , Niels R. Rietdijk (University of Amsterdam) , Maxime A. Siegler (Johns Hopkins University) , Moniek Tromp (University of Amsterdam) , Jarl Ivar Van Der Vlugt (University of Amsterdam) , Bas De Bruin (University of Amsterdam)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society

State: Published (Approved)
Published: December 2019
Diamond Proposal Number(s): 22432

Open Access Open Access

Abstract: 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 [CoIII(TAMLred)]– is truly redox noninnocent and that one-electron oxidation affords [CoIII(TAMLsq)]. Multireference (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 = 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 (TAMLred or TAMLsq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [CoIII(TAMLsq)] or [CoIII(TAMLred)]– with PhINNs results in the formation of [CoIII(TAMLq)(N•Ns)] and [CoIII(TAMLq)(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.

Subject Areas: Chemistry

Instruments: B18-Core EXAFS