Novel building blocks for oligonuclear copper complexes derived from beta-ketoenamines of histidine

Condensation products of L-histidine with the 3-oxoenolethers diethyl-ethox ymethylene-malonate (1) and ethyl-ethoxymethylene-cyanoacetate (2) react wi th copper(II) as di-anionic ligands to give neutral 1:1 complexes Cu-His1 a nd Cu-His2. Both complexes crystallize as oligonuclear units, even from str ongly donating solvents like N-methylimidazole (Meim) (Cu-His1) and pyridin e (Cu-His2). X-ray structure analyses show supramolecular structures, forme d of two (Cu-His1) or four (Cu-His2) formula units of the complex, which ar range to macrocycles by means of intermolecular coordination of the imidazo le-N. Strong H-bridges result in a face-to-face orientation of the hydrophi lic sites of two,great rings. ESI-MS investigations in pyridine solution gi ve evidence for the existence of dimeric, tetrameric and - in case of Cu-Hi s2 - trimeric units, besides the monomeric adducts with one pyridine. In co ntrast to the dimeric or tetrameric ("cubane-like") copper(II) complexes of amino alcohols and their beta-ketoenamines, the complexes Cu-His1 and Cu-H is2 show no significant spin coupling from room temperature down to 4 K. Th e complexes Cu-His1 and Cu-His2 give no electrochemically reversible Cu-II/ I reduction in pyridine. However, the isolation of a stable diamagnetic cop per(I) complex of the methylester derivative, Cu-I-HisMe1, supports the ass umption, that similar histidine-derived copper complexes should display rev ersible redox behaviour and catalytic activity in reactions with O-2.