Synthesis, structure, magnetic properties, and catecholase-like activity of a phenoxo bridged dinuclear cobalt(II) complex

A dinuclear cobalt complex Co₂L₂ (H₂L = N,N'-bis(salicylidene)-2,2′-ethylenedianiline) have been synthesized and characterized. X-ray crystallography found that Co₂L₂ contains a planar Co₂O₂ core where two Co(II) ions are bridged by two phenoxo ligands with Co-Co distance of 3.221 Å and the coordination geometry of each cobalt(II) ion is a distorted trigonal bipyramid. The magnetic susceptibility measurements revealed that both Co(II) ions are in low-spin (S = 1/2) states and two Co(II) spin centers are antiferromagnetically coupled with the exchange coupling constant of J = −45 cm⁻¹. Co₂L₂ showed catecholase-like activity when 3,5-di-tert-butylcatechol (3,5-H₂dtbc) was treated with Co₂L₂ in the presence of O₂ and NaOH with the turnover number of k_cat = 803 h⁻¹. Electron paramagnetic resonance (EPR) on the reaction mixture observed a key reaction intermediates, 3,5-di-tert-butyl-1,2-benzosemiquinonate anion radical (3,5-dtsq^[rad]−) bound to Co(III)-center. This is the first time observation of Co(III)-3,5-dtsq^[rad]− intermediate during the oxidation of 3,5-H₂dtbc by a dicobalt(II/II) complex.