Synthesis, characterization, and spectroscopic properties of three novel pentadentate copper(II) complexes related to the metal-chelating inhibitors against DNA binding with HIV-EP1

Three potentially five-coordinate peptide ligands having a pyridine and two histidine moieties, were synthesized to study their copper(II) complexation. Blue copper(II) complexes with deprotonated amide groups were isolated from methanolic solutions of the corresponding ligands with equimolar Cu(OAc)2·H2O. The structures of two of them were determined by X-ray crystallography. The Cu is coordinated to five nitrogen atoms in both complexes; the coordination geometry was a distorted square pyramid in one, and intermediate between a square pyramid and trigonal bipyramid in the other. EPR spectra in frozen methanol solutions at 77 K as well as visible absorption spectra indicate that the distortion of the geometry around the copper is reduced by the introduction of an alkylamine substituent on the pyridine of the ligand and that the substituted complexes distort toward trigonal bipyramidal geometry compared to the unsubstituted one in solution.