Glycine coupled to a bis(imidazole): a tetradentate ligand rendering dinuclear copper(II) compounds through ligand sharing

Abstract

The tetradentate ligand (carboxymethylamino)bis(imidazole-2-yl)methane (Hglyim), obtained through the reaction of glycine with bis(imidazol-2-yl)nitromethane, rendered several dinuclear copper(II) compounds, viz. [Cu₂(glyim)₂Cl₂]·6H₂O 1, [Cu₂(glyim)₂Br₂]·5H₂O 2, [Cu₂(glyim)₂Cl(H₂O)][CF₃SO₃]·2H₂O 3 and [Cu₂(glyim)₂Cl(H₂O)][ClO₄]·2H₂O 4. The single crystal structure of 1 was determined. It consists of [Cu₂(glyim)₂Cl₂] units and several, mostly disordered, water molecules. The ligands and the anions are grouped in dimers of formula [Cu₂(glyim^–)₂(Cl^–)₂] in which the two copper(II) ions are entangled by two ligands. Each copper is in a square pyramidal N₃OCl environment with the chloride ion occupying the apical position. Two of the co-ordinating nitrogens are from the imidazole groups of one ligand, while an amine nitrogen and a carboxyl oxygen are donated by the other ligand. This 2+2 ligand-sharing arrangement imposes a rather short distance on the two copper(II) ions, i.e. 3.249(2), 3.357(2) and 3.373(2) Å. A well resolved EPR spectrum of 10^–3M 3 in MeOH was obtained at 77 K with g_eff values for H_z1, H_xy1, H_z2 and H_xy2 at 2.69, 2.26, 1.89 and 1.86 respectively, an A_|| value of 87 G and a half-field signal at g = 4.24, indicating the preservation of the dinuclearity in solution (D = 0.047 cm^–1). At concentrations of 10^–4M and lower the dinuclear signals break down and the complex decomposes into monomeric species (g_|| = 2.27 and g_⊥ = 2.10).