Catecholase activity of a self-assembling dimeric Cu(II) complex with distant Cu(II) centers

Catecholase activity and its mechanistic investigation of a self-assemblying dimeric Cu(II) complex, Cu₂L₂, where L is N,N′-(ethane-1,2-diyldi-o-phenylene)-bis(pyridine-2-carboxamidide), is presented. The complex was constructed by a spontaneous self-assembly of the organic ligand and Cu(II) ion. X-ray crystallography of Cu₂L₂(CH₃OH)₂·2CH₃OH, found that the coordination geometry of the Cu(II) site was a distorted square pyramid and two Cu(II) ions were apart by 6.992 Å. Though the metal centers are distant, Cu₂L₂ showed high catecholase activity when 3,5-di-tert-butylcatechol (3,5-H₂dtbc) was treated with Cu₂L₂ in the presence of air at basic condition with k_cat of 720 h^-1 in Michaelis-Menten model. Information collected from the UV-vis kinetic, electron paramagnetic resonance (EPR), and high-resolution electrospray ionization mass spectrometry (ESI MS) measurements on the reaction mixtures could lead the suggestion of the mechanistic pathway of the catecholase activity of Cu₂L₂ in which each Cu(II) site oxidized 3,5-H₂dtbc to 3,5-di-tert-butyl-1,2-benzosemiquinonate anion radical (3,5-dtbq^-). Subsequently, 3,5-di-tert-butyl-o-quinone (3,5-dtbq) was formed via both disproportionation and air oxidation of 3,5-dtbq^-. Cu₂L₂ could be reduced to either one- or two-electron reduced state. The reduced complexes were reoxidized by dioxygen molecule accompanying with the generation of hydrogen peroxide.