Comparative study of the catalytic oxidation of catechols by copper(II) complexes of tripodal ligands

Abstract

Copper(II) complexes of the ligands tris(2-pyridylmethyl)amine (tpyma), tris(2-pyridylethyl)amine (tpyea), tris(3,5-dimethylpyrazol-1-ylmethyl)amine (tpzma) and tris(3,5-dimethylpyrazol-1-ylethyl)-amine (tpzea) were prepared. The complexes, [Cu(ligand)Cl]Cl or [Cu(ligand)(H₂O)][BF₄]₂, were characterized by a combination of absorption and EPR spectroscopies and chemical analysis. The ability of the complexes to oxidize 3,5-di-tert-butylcatechol to 3,5-di-tert-butyl-o-benzoquinone has been studied and the results show that the rate of reaction is dependent on the nature of the heterocyclic donor, its basicity, steric considerations, the chelate ring size and the type of exogenous donor present. Large variations in the rate were observed with the most effective catalysts being those with pyridine donors which formed six-membered chelate rings; the complex [Cu(tpyea)(H₂O)][BF₄]₂ was the most active while [Cu(tpzea)(H₂O)][BF₄]₂ and [Cu(tpzea)Cl]Cl were inactive. Electrochemical data for the series of compounds show that there is a non-linear relationship between their ability to oxidize catechols and their reduction potentials. The most effective catalysts were those complexes which exhibited reduction potentials close to 0.00 V, while those that deviated from that potential by 200–300 mV in either direction were largely inactive. Within the range of complexes which were active, a steric match between the substrate and the complex also largely defined their reactivity. Comparisons to the biological system tyrosinase are drawn.