The catalytic activity of metal-ligand complexes containing redox-active (or 'non-innocent') ligands can originate from the metal ion as well as the ligands as both are able to donate or accept electrons. In order to achieve more efficient or more selective catalysis, it is therefore very important to establish the electronic structure of the metal-ligand complexes in great detail and elucidate the catalytic mechanism.

In a current JACS paper, researchers of the Homogeneous, Supramolecular and Bio-Inspired Catalysis group at the University of Amsterdam's Van 't Hoff Institute for Molecular Sciences devote their attention to catalytically active [Co(TAML)] complexes, consisting of a cobalt ion and a tetra-amido macrocyclic ligand. Since 2006, there has been scientific debate regarding the oxidation state of the metal and ligand in these complexes. In a combined effort of the groups of Prof. Bas de Bruin (Bio-inspired Sustainable Catalysis) and Dr Jarl Ivar van der Vlugt (Bioinspired Ligand Design and Homogeneous Catalysis) it was established that oxidation of the anionic [Co^III(TAML^red)]^– (red = fully reduced) complex takes place at the ligand, resulting in the formation of a neutral [Co^III(TAML^sq)] (sq = one-electron oxidised) complex.

By means of spectroscopic studies (electron paramagnetic resonance, EPR; UV-Vis spectroscopy; determination of the magnetic moment) and computational studies (density functional theory, DFT;  complete active space self-consistent field, CASSCF) it was shown that after oxidation of [Co^III(TAML^red)]^– a ligand-centred radical (unpaired electron) is formed, which is antiferromagnetically coupled to a triplet (two unpaired electrons with parallel spin) cobalt centre. Additional proof for the +III oxidation state of cobalt was obtained by usage of XANES  (X-ray absorption near-edge spectroscopy) measurements in collaboration with the Sustainable Materials Characterization Group under supervision of Prof. Moniek Tromp.

Influence of the ligand on the formation of nitrene radicals

The formation of C–N bonds can be facilitated by usage of a metal-nitrene intermediate, which can transfer a nitrene (N–R group) to the desired place in the substrate molecule. The formed products are prevalent in drugs, chemical building blocks, plastics, etc.. A specific class of these metal-nitrene intermediates bears an unpaired electron on the nitrene (nitrene radical), which can influence the activity and selectivity of the reactive intermediate.

The neutral ([Co^III(TAML^sq)]) and anionic ([Co^III(TAML^red)]^–) cobalt complexes can be transformed into very reactive nitrene radical complexes ([Co^III(TAML^q)(N^●Ns)] and ([Co^III(TAML^q)(N^●Ns)₂]^–). Interestingly, the formation of the nitrene radicals(N^●Ns) is mediated by the redox-active ligand, as the ligand donates the required electrons, which allows cobalt to retain its original +III oxidation state. The nitrene radical complexes were completely characterized (EPR, UV-Vis, magnetic moment, XANES, high resolution mass spectrometry and supporting DFT and CASSCF calculations). Besides the electron donating ability of the ligand, it was also found that the unpaired electrons on the cobalt centre change from two unpaired electrons to solely paired. 

The nitrene radical complexes are active in the formation of aziridines from alkenes, and in follow up work the applicability and mechanism of this reaction is further studied. Herein, it appears that the redox active ligand is again playing a crucial role in the reactivity and selectivity of the catalyst.

Publication details

Nicolaas P. van Leest, Martijn A. Tepaske, Jean-Pierre H. Oudsen, Bas Venderbosch, Niels R. Rietdijk, Maxime A. Siegler, Moniek Tromp, Jarl Ivar van der Vlugt, Bas de Bruin. Ligand Redox Noninnocence in [Co^III(TAML)]^0/− Complexes Affects Nitrene Formation. J. Am. Chem. Soc. 2020, 142, 552–563 DOI: 10.1021/jacs.9b11715