Isolation and characterization of carbene-supported air stable Co(ii)-radical complexes with bileptic redox non-innocent ligands: stability, bonding, ring-opening copolymerization studies and photo-catalytic ring cyclization activity

Abstract

Two redox-active Co(II)-radical complexes (3a and 3b), featuring a high-spin tetrahedral Co(II) ion, have been isolated as dark blue needles and remain stable in air for over a week. These complexes contain two redox non-innocent ligands (a carbene and a dithiolene), each with different coordinating atoms (two sulfur atoms and one carbon atom). Cyclic voltammetry (CV) reveals that complex 3a and 3b can form their corresponding mono-cation at potentials above −0.61 V. The structure of the complexes has been determined by single-crystal X-ray diffraction, and it has been further characterized by UV-vis, IR, Raman, EPR, and spectroscopy. DC/AC magnetometry measurements of complex 3a show antiferromagnetic coupling between the high-spin tetrahedral Co(II) ion and the dithiolene-centered radical electron, as concluded from DFT calculations, with support from EPR measurements. Quantum mechanical magnetic model fitting to the experimental data sets suggested that complex 3a exists in equilibrium between S = 1 and S = 2 states, depending on temperature, as reflected in the M vs. H plots. The stability, bonding, and electron density distribution of this air-stable complex 3a have also been studied using various quantum chemical calculations. This dark-colored, photo-redox active complex (3a) was found to catalyse the synthesis of the well-known alkaloid natural product, N-isopentylcrinasiadine (5b), under photocatalytic conditions using a 427 nm Kessil light source. Additionally, complex 3a also catalyses a copolymerization reaction in the presence of a chloride ion.