Two coordination modes around the Cu(II) cations in complexes with benzo[b]furancarboxylic acids

Three Cu(II) complexes with derivatives of the benzo[b]furancarboxylic acid have been synthesized and characterized by the elemental and thermal analyses, and IR spectroscopy. The geometry of metal–ligand interaction for all compounds has been described using X-ray absorption spectroscopy and for one of them by X-ray crystallography. Two mononuclear Cu(II) complexes, with 7-acetyl-5-bromo-6-hydroxy-3-methylbenzo[b]furan-2-carboxylic and 6-acetyl-5-hydroxy-2-methylbenzo[b]furan-3-carboxylic acids, exhibit a tetra-fold coordination, CuO4. The Cu(II) cation in crystals with 7-acetyl-6-methoxy-3-methyl-benzo[b]furan-2-carboxylic acid is penta-coordinated; the bridging COO− groups and ethanol molecule stabilize the dinuclear center Cu2O10. The powdered form of this complex is based on the Cu2O8 units, indicating the absence of the ethanol molecules.