Design and reactivity of Tripodal Tetradentate mononuclear cobalt(II) complexes as biomimetic models for Phenoxazinone synthase and Catecholase activity

Abstract

On a quest to understand the effect of structural modification implied within outer coordination sphere, six mononuclear pentacoordinate cobalt complexes having the molecular framework [Co^II(Lⁿ)Cl]X {L¹: tris((1H-benzo[d]imidazol-2-yl)methyl)amine; n = 1 and L²: tris(N-methylbenzimidazol-2-ylmethyl)amine; n = 2 and X = Cl⁻, ClO₄⁻, PF₆⁻} were synthesized and tested for their catalytic activity towards o-aminophenol (OAP) and 3,5-di-tert-butyl catechol (3,5-DTBC) oxidation. For OAP oxidations, the k_cat values were 54.73 h⁻¹ for [1]Cl and 58.67 h⁻¹ for [2]Cl, while for catechol oxidation (CO), the values were 410.4 h⁻¹ and 483.09 h⁻¹, respectively. [1]ClO₄ and [1]PF₆ displayed less activity compared to the Cl⁻ counterpart due to change in outer coordination sphere. The complexes with methylated ligand (L²) displayed enhanced catalytic activity, with the chloride-containing complex remaining the most effective overall. The change in redox behaviour of [1]Cl and [2]Cl have been observed upon successive addition of OAP and 3,5-DTBC, and the progression was closely monitored over time using cyclic voltammetry (CV). Furthermore, the formation of free radical intermediates was revealed from EPR investigation. The formation of adducts, reaction intermediates, and the involvement of molecular oxygen likely play key roles in these oxidation reactions, as strongly supported by mass spectrometric analysis.