Pyridine aided progression from amorphous to crystalline bis([5-(aryl)-1-diazenyl]quinolin-8-olato)zinc(II) compounds - Solution and solid-state structural characterization, nanoparticle formation and antibacterial activity

Ten new zinc(II) compounds, viz [Zn(L-Q)(2)(4-MePy)(2)]center dot H2O (1), [Zn(L-MeAQ)(2)(Py)(2)]center dot C6H6 (2a), [Zn(L-MeAQ)(2)(H2O)(2)] (2b), [Zn(L-OMeAQ)(2)(H2O)(2)] (3), [Zn(L-EtAQ)(2)(3-MePy)(2)] (4), [Zn(L-OEtAQ)(2)(4-MePy)(2)]center dot 0.5C(6)H(7)N (5a), [Zn(L-OEtAQ)(2)(H2O)(2)] (5b), [Zn(L-NMe2AQ)(2)(Py)(2)]center dot 1.5C(6)H(6)center dot H2O (6), [Zn(L-CAQ)(2)(Py)] (7) and [Zn(L-BAQ)(2)(Py)] (8) (primary ligands: L-Q = quinolin-8-olate and L-XAQ = 4-substituted 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-olate; secondary ligands: Py = pyridine, 3-MePy = 3-methylpyridine, 4-MePy = 4-methylpyridine) have been synthesized and characterized by elemental analysis, IR, NMR, UV-vis and fluorescence spectroscopy and single-crystal X-ray diffraction analysis. The structural characterization revealed distorted octahedral geometries for 1-6, in which trans-disposed 8-quinolate ligands occupy the equatorial and the secondary ligands the axial positions. Compounds 7-8 comprise five-coordinate molecular structures with square-pyramidal coordination polyhedra based on trans-oriented quinolate ligands and a Py molecule in the apical position. In the crystal structures of 1-8, the molecules are assembled by pi-stacking interactions and for the aquo-compounds additionally by O-H center dot center dot center dot O hydrogen bonds. As representative example, compound 2a was used as starting material for the synthesis of ZnO nanoparticles in an average size range of 70-120 nm, as illustrated by PXRD analysis and TEM images. The antimicrobial activity of the pro-ligands and zinc(II) compounds studied herein was assessed by inhibition zone test in agar cultures against five indicator bacterial strains, i.e. Escherichia coli MTCC 730, Streptococcus pyogenes MTCC 1925, Klebsiella pneumoniae MTCC 109, Bacillus cereus MTCC 430 and Salmonella enterica MTCC 735.