Dinuclear copper(II) complexes: Solvent dependent catecholase activity

Four new dicopper(II) complexes of phenol based compartmental ligands, namely [Cu2(L1H)2(H2O)2(NO3)2](NO3)2 (1), [Cu2(L2)(OH)(H2O)(NO3)](NO3) (2), [Cu2(L3)2(H2O)(NO3)](NO3) (3) and [Cu2(L4)(H2O)2(NO3)](NO3)2 (4) [where L1 = 2-formyl-4-methyl-6-(4-(aminomethyl)-piperidine)iminomethyl-phenolato, L2 = 2,6-bis(2-amino-2-methyl-1-propanol)iminomethyl-4-methyl-phenolato, L3 = 2-formyl-4-methyl-6-(benzylamine)iminomethyl-phenolato and L4 = 2,6-bis(2-aminoethylpyridine)iminomethyl-4-methyl-phenolato] have been synthesized and structurally characterized. The single crystal X-ray analyses reveal that all four complexes are dinuclear in nature; complexes 2 and 4 comprise of one respective ligand, whereas 1 and 3 are contain two respective ligands, and the Cu–Cu separation in each case is ca. 3.0 Å. All four complexes are soluble in dichloromethane (DCM), methanol, acetonitrile (ACN), dimethylsulfoxide (DMSO), water–methanol (50:50, v/v), and this property has been exploited to access the solvent effect on the catecholase activity of the complexes towards the aerobic oxidation of 3,5-DTBC to 3,5-DTBQ. A UV–Vis spectral study in the different solvents, followed by a kinetic investigation, suggests that the change in spectral behavior follows a similar trend, being dependent on the coordinating ability of the solvent, irrespective of the complex used. The commonly known physical parameters of the solvents, like the dielectric constant, dipole moment, polarity, etc., do not seem to be a key factor in controlling the catecholase activity. However, protic solvents are observed to be a better choice than aprotic solvents for the oxidation of 3,5-DTBC.

Catechol oxidase activity of four structurally characterized dicopper(II) complexes of p-cresol based compartmental ligands have been investigated in five different solvent systems, viz. dichloromethane, methanol, water–methanol (50:50, v/v), acetonitrile and dimethylsulfoxide. Elaborate study establishes a linear relationship between the catecholase activity (in terms of the kcat value) and the coordinating power of the solvent, rather than with common physical parameters of the solvent, like dielectric constant, polarity index, dipole moment.Figure optionsDownload as PowerPoint slideHighlights
► Syntheses, characterization and catalytic efficiency of four new CuII complexes.
► Catecholase activity shown by the dinuclear complexes.
► Solvent dependent catecholase activity.
► X-ray crystal structure determination.