COMPARATIVE-STUDY OF THE CATALYTIC-OXIDATION OF CATECHOLS BY COPPER(II) COMPLEXES OF TRIPODAL LIGANDS

Copper(II) complexes of the ligands tris(2-pyridylmethyl)amine (tpyma) , tris(2-pyridylethyl)amine (tpyea), tris(3,5-dimethylpyrazol-1-ylmeth yl)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 spect roscopies and chemical analysis. The ability of the complexes to oxidi ze 3,5-di-tert-butylcatechol to 3,5-di-tert-butyl-o-benzoquinone has b een studied and the results show that the rate of reaction is dependen t on the nature of the heterocyclic donor, its basicity, steric consid erations, the chelate ring size and the type of exogenous donor presen t. Large variations in the rate were observed with the most effective catalysts being those with pyridine donors which formed six-membered c helate 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. Electro chemical data for the series of compounds show that there is a non-lin ear relationship between their ability to oxidize catechols and their reduction potentials. The most effective catalysts were those complexe s which exhibited reduction potentials close to 0.00 V, while those th at 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 define d their reactivity. Comparisons to the biological system tyrosinase ar e drawn.