Mixed-ligand complexes of zinc(II) with α-hydroxycarboxylates and aromatic N-N donor ligands:: Synthesis, crystal structures and effect of weak interactions on their crystal packing

Several new two-ligand complexes of zinc(II) with the aromatic N,N-donor ligands 2,2 '-bipyridine or 1,10-phenanthroline and one of three different alpha-hydroxycarboxylates (HL ') derived of the alpha-hydroxycarboxylic acids (H2L1) (2-methyllactic, H(2)mL; mandelic, H2M or benzilic, H2B) were prepared. The compounds of formula [Zn(HL ')(2)(NN)](.)nH(2)O (HL ' = HM, HB) were isolated as white powders and characterized by elemental analysis, IR spectroscopy and thermogravimetric analysis. The complexes of general formula [Zn(HL ')(NN)(2)](HL ')(.)nH(2)O (HL ' = HmL, HM) and [Zn(HB)(2)(NN)(2)], were obtained as single crystals and were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis and X-ray diffractometry. In all cases, the zinc atom is in a distorted octahedral environment. In [Zn(HL ')(NN)(2)]-(HL ')(.)nH(2)O the alpha-hydroxycarboxylato ligands behave as bidentate chelating monoanion and an alpha-hydroxycarboxylate as counterion is also present. In [Zn(HB)(2)(NN)(2)], the monoanionic benzilato ligand behaves as monodentate through one oxygen atom of the carboxylate function. The effect of the classical and no-classical hydrogen bonding and of the pi-pi and C-H-...pi interactions in the 3D supramolecular arrangement of these molecular complexes is analyzed.