Metal complexes of dipeptides containing the α-aminoisobutyric residue (Aib): preparation, characterization and crystal structures of copper(II) complexes with H–Aib–Aib–OH

Abstract

A systematic investigation of the reactions between metal ions and dipeptides containing the α-aminoisobutyric residue (Aib) was initiated. The solid complexes [Cu(H −1L)] n ( 1, 2) and (LH 2) n [CuCl 3] n ( 3) were isolated and characterized by single-crystal X-ray crystallography, solid-state IR spectra and UV–Vis spectroscopy in solution (H −1L 2− is the dianionic form of H–Aib–Aib–OH). Of remarkable interest is the isolation of two complexes which have the same chemical formula, [Cu(H −1L)] n , but different structures. Complex 1 is a chain (1D) compound, while complex 2 is a 3D coordination polymer. The doubly deprotonated dipeptide behaves as a N(amino), N(peptide), O(carboxylate), O′(carboxylate) η 1:η 1:η 1:η 1:μ 2 ligand in 1. In 2 the H −1L 2− ligand binds to one Cu II atom at its amino and peptide nitrogens and at one carboxylate oxygen, and to a second metal at the other carboxylate oxygen, while a third Cu II atom is attached to the peptide oxygen. The structure of 3 consists of zigzag (CuCl 3) n n− chains and protonated dipeptides (H 2L +) which remain uncoordinated. Two neighbouring Cu II atoms are bridged by two chloro ligands, while a terminal chloro ligand completes five coordination at each metal center. The IR data are discussed in terms of the nature of bonding and known structures. The UV–Vis spectra show that the solid-state structures of 1 and 2 do not persist in H 2O.