Copper(II) complexes of aminocarbohydrate beta-ketoenaminic ligands: efficient catalysts in catechol oxidation

Copper(II) complexes of tridentate dianionic beta-ketoenaminic ligands derived from differently functionalized amino-deoxyglucoses were synthesized and characterized with respect to their structural, spectroscopic, and catalytic properties. The (probably dimeric) complex [1,2-O-isopropylidene-6-N-(3-acetyl-2-oxobut-3-enyl)amino-6-deoxyglucofuranoso)copper(II) Cu(3a) was a highly efficient catalyst for the catechol-oxidase-like oxidation of 3,5-di-tert-butylcatechol (dtbc) into 3,5-ditert-butylquinone (dtbq) by molecular oxygen (kcat=2.63s(-1)). In contrast to this magnetically "normal" complex Cu(3a), the analogous dinuclear complex [[Cu(2a)]2], derived from the isomeric amino sugar 5-amino-5-deoxyglucofuranose, forms six-membered chelate rings with the sugar moiety and has very strong antiferromagnetic-coupled copper atoms (resulting in a diamagnetic ground state). It has a rather insignificant activity (kcat < 10(-3)s(-1)). The ligand H2 1a, derived from a (protected) 6-amino-6-deoxyglucopyranose, forms a trinuclear complex [[Cu(1a)]]2.Cu(OAc)2] in which two basic formula units are bridged by one copper acetate. This compound and the complex derived from an (isomeric) aminodeoxyglucopyranose ([Cu(4a)]: kcat approximately equals 0.03 s(-1)) show moderate activity. All complexes with a peripheral ethoxycarbonyl group instead of the acetyl substituent R2, Cu(1b)-Cu(3b) and Cu(4c), are inactive. The complexes derived from 2-hydroxocyclohexylamine, Cu(5a) and Cu(5c), which were used as models of the active complex Cu(3a), have the typical "cubane-like" tetranuclear structure known from many copper complexes with derivatives of saturated 2-aminoalcohols. They are inactive with respect to the activation of dioxygen.