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

A systematic investigation of the reactions between metal ions and dipeptides containing the alpha-aminoisobutyric residue (Aib) was initiated. The solid complexes [Cu(H L-1)](n) (1, 2) and (LH2)(n)[CuCl3](n) (3) were isolated and characterized by single-crystal X-ray crystallography, solid-state IR spectra and UV-Vis spectroscopy in solution (H L--1(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 L-1)](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) ligand in 1. In 2 the H L--1(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 (CuCl3)(n)(n-) chains and protonated dipeptides (H2L+) 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 H2O. (C) 2002 Elsevier Science Ltd. All rights reserved.