Attractive ligand–ligand interactions involving aromatic rings in five-coordinate ternary copper(II) complexes with a dipeptide and a bidentate N-donor ligand

Structures and stabilities of ternary copper(II) complexes, Cu(DP)(DA), where DP and DA denote dipeptides with an aromatic amino acid residue and bidentate nitrogen donor ligands, respectively, have been studied by synthetic, crystallographic, potentiometric, and spectroscopic methods. The X-ray crystal structure analysis of [Cu(gly·l-tyr)(bzp)] (gly·l-tyr=glycyl-l-tyrosinate, bzp=N-benzyl-N-2-pyridylmethylamine) revealed that the complex has a distorted square-pyramidal structure exhibiting an intramolecular edge-to-face stacking interaction between the pyridine ring of bzp and the phenol ring of gly·l-tyr around the Cu(II) center. [Cu(gly·l-tyr)(bzmp)] (bzmp=N-benzyl-N-6-methyl-2-pyridylmethylamine) was also revealed to have a similar structure and an intramolecular CH-π-type interaction between the methyl group of bzmp and the phenol ring of gly·l-tyr. The absorption spectra of the ternary Cu(II)–DP–DA systems in water showed that the ternary complexes have a shoulder peak near 900 nm, supporting that the Cu(II) center has a square-pyramidal geometry. From the stability constants for the ternary Cu(II) complexes determined by pH titration at 25 °C and I=0.1 M (KNO3), Cu(gly·l-tyr)(DA) and Cu(gly·l-trp)(DA) (DA=ligands with one or two aromatic rings) were found to be stabilized relative to Cu(gly·gly)(DA) (gly·gly=glycylglycinate), which has been concluded to be due to the contribution of the edge-to-face or CH-π interactions within the complexes in solution.