Amino Acid Binding to Copper(II) in (1,4,7-Triazacyclononane)(L-histidine)copper(II) Hexafluorophosphate Monohydrate

The synthesis, characterization, and single-crystal X-ray structure determination of [Cu(tacn)(L-His)](PF6)(2) . H2O (tacn = 1,4,7-triazacyclononane, L-His= L-histidine) are reported. Spectroscopic studies indicated the formation of the square pyramidal copper(II) complex [Cu(tacn)(L-His)](2+). This was confirmed through structure determination, which also revealed that the distorted square-pyramidal copper(II) coordination sphere is comprised of three nitrogen donors from the tacn macrocycle plus one carboxylate oxygen atom and the primary nitrogen atom from L-histidine. The L-histidine ligand is zwitterionic, with the negative charge on the carboxylate balanced by protonation at one nitrogen atom on the imidazole ring. It binds to the copper centre, forming a five-membered chelate ring, common in amino acid coordination.