Synthesis, crystal structures and properties of copper(II) complexes of Schiff base derivatives containing imidazole and β-alanine groups

Abstract

Four copper(II) complexes with Schiff base ligands N-[(5-methylimidazol-4-yl)methylene]-β-alanine (HL¹) and N-[(1-methylimidazol-2-yl)methylene]-β-alanine (HL²) and their reduced forms N-[(5-methylimidazol-4-yl)methyl]-β-alanine (HL³) and N,N-bis[(5-methylimidazol-4-yl)methyl]-β-alanine (HL⁴) have been synthesized. The crystal structures of [CuL¹(H₂O)(ClO₄)] 1, [CuL²(H₂O)(ClO₄)] 2, [CuL³(Py)(ClO₄)] 3 and [Cu₂(L⁴)₂][ClO₄]₂·2H₂O 4 have been determined. Complexes 1 and 2 are structurally very similar, the copper(II) atom being tridentately chelated by the Schiff base using one carboxy oxygen atom and two nitrogen atoms at the equatorial positions, with the fourth equatorial position being occupied by another carboxy oxygen atom from an adjacent Schiff base; one aqua ligand and one perchlorate oxygen atom ligate at the axial positions, resulting in an elongated octahedral geometry. Each carboxy group in 1 and 2 acts in the syn-anti mode and bridges two adjacent copper(II) atoms via two equatorial positions, resulting in one-dimensional helical (Cu–O–C–O–Cu)_n chains. In 3 the copper(II) atom is ligated by two nitrogen atoms, one carboxy oxygen atom from an L³ ligand and another nitrogen atom from pyridine at the equatorial position; the axial positions are occupied by one perchlorato oxygen atom and one carboxy oxygen atom from another L³ ligand, tridentately chelating an adjacent copper(II) atom, resulting in an elongated octahedral geometry. Complex 4 contains a dimeric cation with two very similar square-pyramidally co-ordinated copper(II) atoms. Each L⁴ ligand chelates a copper(II) atom in a tetradentate mode with the three nitrogen atoms occupying the equatorial positions and the carboxy oxygen atom the apical position. The fourth equatorial position is taken by a carboxy oxygen atom from another L⁴ ligand chelating another copper(II) atom, resulting in a bis(carboxylato-O)-bridged dimeric structure. The electronic and EPR spectra and redox properties of 1–4 are also discussed.