Elucidating substituent effects in magnetic properties of redox active cobalt complexes and testing them as potential catalysts for HER.

We describe the synthesis of three cobalt complexes, C1-C3, featuring redox-active catechol ligands and a tetradentate salen ligand (L). Structural characterization using single-crystal X-ray diffraction at 120 K, along with UV-vis, infrared spectroscopy, and SQUID magnetometry, provided detailed insights into their electronic and geometric properties. Magnetic measurements revealed that the two tetrahalogenated catechol complexes (C1 and C2) remain diamagnetic up to 400 K, whereas the complex with 3,5-ditert butyl catechol (C3) shows radical characteristics and stays in semiquinone form strongly coupled with a Co(iii) centre. This radical behaviour was further confirmed by EPR spectroscopy in MeCN solution at room temperature. Cyclic voltammetry studies demonstrated the influence of catechol substituents on the electronic properties of these complexes, as reflected in shifts in oxidation potential. Preliminary investigations into their electrocatalytic activity for hydrogen production indicate that none of the complexes function effectively as catalysts under the tested conditions.