BINUCLEAR COMPLEXES .3. SYNTHESIS AND REACTIVITY OF A FAMILY OF COPPER MONOOXYGENASE MODEL SYSTEMS

A series of binuclear copper(I)complexes with the bis(imines) derived from the condensation of benzene-1,3-dicarboxaldehyde and two molecules of histamine (1), L-histidine methyl ester (2), N.tau.-methylhistamine (3), and N.tau.-methyl-L-histidine methyl ester (4) have been synthesized and characterized. The ligands provide two nitrogen donors, from an imine and an imidazole group, to each copper(I) center, but a molecule of solvent is additionally coordinated to the metal in solution. The complexes 3 and 4 undergo ready aromatic hydroxylation at position 2 of the phenyl nucleus when reacted with molecular oxygen in solution. The stoichiometry of the reaction is 1:2 O2/Cu, and the products formed are the corresponding binuclear, four-coordinate .mu.-phenoxo and .sbd.hydroxo copper(II) complexes. This reaction mimics the reactivity of the copper monooxygenase tyrosinase. For complexes 1 and 2 such a hydroxylation reaction can be observed only in protic solvents and is in competition with simple copper(I) oxidation. When 1 and 2 are reacted with O2 in nonprotic solvents, only copper(I) oxidation occurs, to form .mu.-imidazolato copper(II) compounds, with a stoichiometry of 1:4 O2/Cu. The ratio between hydroxylation and oxidation can be shifted in favor of the former reaction by addition of small amounts of acid. The products resulting from the oxygenation of 1-4 have also been isolated and characterized. The copper(I) complexes form weak three-coordinate adducts when reacted with CO (.nu.(CO) at 2088-2096 cm-1) and four-coordinate adducts when the carbonylation reaction is carried out in the presence of excess imidazole (.nu.(CO) at 2069-2073 cm-1). These .nu.(CO) data are discussed in relation to those of other Cu(I) systems and carbonyl hemocyanin.