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

Four copper(II) complexes with Schiff base ligands N[(5-methylimidazol-4-yl )methylene]-beta-alanine (HL1) and N-[(1-methylimidazol-2-yl)methylene]-bet a-alanine (HL2) and their reduced forms N-[(5-methylimidazol-4-yl)methyl]-b eta-alanine (HL3) and N,N-bis[(5-methylimidazol-4-yl)methyl]-beta-alanine ( HL4) have been synthesized. The crystal structures of [CuL1(H2O)(ClO4)] 1, [CuL2(H2O)(ClO4)] 2, [CuL3(Py)(ClO4)] 3 and [Cu-2(L-4)(2)][ClO4](2). 2H(2)O 4 have been determined. Complexes 1 and 2 are structurally very similar, t he copper(II) atom being tridentately chelated by the Schiff base using one carboxy oxygen atom and two nitrogen atoms at the equatorial positions, wi th the fourth equatorial position being occupied by another carboxy oxygen atom from an adjacent Schiff base; one aqua ligand and one perchlorate oxyg en 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 brid ges two adjacent copper(II) atoms via two equatorial positions, resulting i n 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 L-3 liga nd 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 L-3 ligand, tridentately chelating an adjacent co pper(II) atom, resulting in an elongated octahedral geometry. Complex 4 con tains a dimeric cation with two very similar square-pyramidally co-ordinate d copper(II) atoms. Each L-4 ligand chelates a copper(II) atom in a tetrade ntate mode with the three nitrogen atoms occupying the equatorial positions and the carboxy oxygen atom the apical position. The fourth equatorial pos ition is taken by a carboxy oxygen atom from another L-4 ligand chelating a nother copper(II) atom, resulting in a bis(carboxylato-O)-bridged dimeric s tructure. The electronic and EPR spectra and redox properties of 1-4 are al so discussed.