Catechol oxidase activity of dinuclear copper(II) complexes of robson type macrocyclic ligands: syntheses, X-ray crystal structure, spectroscopic characterization of the adducts and kinetic studies

Abstract

Five dinuclear copper(II) complexes,[Cu₂L¹(N₃)₂ × 2H₂O] (1), [Cu₂L²(N₃₂× 2H₂O] (2), [Cu₂L³ (N₃)₂ × 2H₂O] (3), [Cu₂L⁴(N₃₂ × 2H₂O] (4) and [Cu₂L⁵(N₃)₂×2H₂O] (5) of Robson type macrocyclic Schiff-base ligands derived from [2 + 2] condensation of 4-methyl-2,6-diformylphenol with 1,3-diaminopropane (H₂L¹),1,2-diaminoethane (H₂L²),1,2-diaminopropane (H₂L³), 1,2-diamino-2-methylpropane (H₂L⁴) and 1,2-diaminocyclohexane (H₂L⁵), respectively have been synthesized and characterized. Catecholase activity of those complexes using 3,5-di-tert-butylcatechol as substrate has been investigated in two solvents, methanol and acetonitrile. The role of the solvent and of the steric properties of the macrocyclic ligand of these complexes on their catecholase activity has been examined thoroughly. Acetonitrile is observed to be a better solvent than methanol as far as their catalytic activity is concerned. However, methanol reveals to be a better choice to identify the enzyme-substrate adduct. The investigation also prompted that chelate ring size does affect on the catalytic efficiency: 6-membered ring (as in H₂L¹) exhibits better activity than its 5-membered counterpart (as in H₂L²). The activity of the 5-membered counter parts also depend upon the steric factor. Moreover, the catalytic activity of the complexes is enhanced to a significant extent by increasing the bulkiness of the substituents on the backbone of macrocyclic H₂L² ligands.