Structure of copper(II) complexes with L-leucyl-D- or L-leucyl-L-phenylalanine and molecular orbital calculations on their stabilization

Abstract

The complexes [CuL(H₂O)]·H₂O 1(H₂L =L-Leu-D-Phe) and [CuL]2(H₂L =L-Leu-L-Phe) have been prepared and structurally characterized. In both the co-ordination of the Cu is a distorted square pyramid with the dipeptide occupying three basal positions, the fourth being occupied by a carboxyl oxygen of an adjacent molecule. A water molecule and a peptide oxygen of another complex complete the co-ordination of copper in the apical position in 1 and 2, respectively. Thus, the dipeptide acts also as a bridging ligand between the metal centres so that both compounds exhibit a polymeric structure in the solid state. In both the phenyl group of the dipeptide is far apart from the metal, since it is on the same side as the apical ligand. In contrast, an interaction between the phenyl residue with Cu in 2 is found in solution. The behaviour is interpreted as due to breaking of the polymeric structure detected in the crystal. The interaction between the copper(II) ion and the aromatic ring is evaluated by means of a molecular orbital approach carried out on the simplified model [Cu(OH)₂(NH₃)₂] interacting with a benzene molecule.