Catecholase activity of dinuclear copper(II) complexes with variable endogenous and exogenous bridge

Abstract

A group of a diverse family of dinuclear copper(II) complexes [Cu₂(L¹---O)(μ-OH)][ClO₄]₂ (1), [Cu₂(L²---O)(μ-OH)][ClO₄]₂ (2), [Cu₂(L³---O)(μ-C₃H₃N₂)(OClO₃)(H₂O)][ClO₄]·H₂O (3), [Cu₂(L⁴)(μ-OH)₂][ClO₄]₂·H₂O (4), [Cu₂(L⁵---O)₂(OClO₃)₂] (5) and [Cu₂(bpy)₂(μ-OH)(μ-OH₂)(μ-O₂CMe)][ClO₄]₂ (6) has been chosen to investigate their potential as catalysts for the aerial oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) (catecholase activity) [L¹---OH, 2,6-[N-methyl-N-(2-pyridylethyl)aminomethyl]phenol; L²---OH, 2,6-bis[N-(2-pyridylethyl)iminomethyl]phenol; L³---OH, 1,3-bis[N-methyl-N-(2-pyridylethyl)amino]propan---2-ol; L⁴, α,α'-bis[N-(6-methyl-2-pyridylmethyl)-N-(pyrazol-1-ylmethyl)amino]---m-xylene; L⁵---OH, 4-methyl-2,6-bis(pyrazol-1-ylmethy)phenol; bpy, 2,2'-bipyridine; C₃H₃N₂, pyrazolate ion; H, dissociable hydrogen atom]. The synthesis of a new m-xylyl-based dinucleating ligand L⁴ is described. Using this ligand a new dihydroxy-bridged copper(II) complex 4 has been synthesised and characterised by elemental analysis and physicochemical measurements. The types of bridges present are endogenous phenoxo- and exogenous hydroxo- (1 and 2), endogenous alkoxo- and exogenous pyrazolate- (3), exogenous hydroxo- (4), endogenous diphenoxo-bridge (5) and exogenous hydroxo-, aqua- and acetate- (6). Barring complex 6, which is ferromagnetically coupled, all other complexes are antiferromagnetically coupled and all reported complexes are structurally characterised. The reaction between the dicopper(II) complexes and tetrachlorocatechol was investigated spectrophotometrically to investigate whether or not, during catalysis, binding of 3,5-DTBC is occurring to the copper centre(s). Kinetic experiments have been performed to determine initial rate of reactions and the activity order follows: 1>2»3>4»6 (for 5 initial rate could not be determined). For a fixed 3,5-DTBC concentration, a linear relationship for the initial rates and the complex concentration is obtained, suggestive of a first-order dependence on the catalyst concentration. Analysis of Michaelis-Menten kinetic treatment has been done for complexes 1, 2, 3 and 4. Complex 1 is the most active catalyst to show catecholase activity.