Zinc barbiturate complexes with bidentate N-donor ligands:: Syntheses, crystal structures, spectroscopic, thermal and voltammetric studies

Two new bis(5,5-diethylbarbiturato) (barb) complexes of zinc, [Zn(barb)2(en)] (1) and [Zn(barb)(2)(bpy)] center dot H2O (2) [en = ethylenediamine, bpy = 2,2'-bipyridine], have been prepared and characterized by elemental analysis, IR spectroscopy, thermal analysis and single crystal X-ray diffraction. Complexes 1 and 2 crystallize in monoclinic space groups. The zinc(II) ions in complex 1 are tetrahedrally coordinated by two barb and an en ligand. The barb ligands are N-coordinated while the en ligand acts as a bidentate chelating ligand. One carbonyl oxygen atom of each barb ligands in complex 2 participates in the bonding with the zinc ion with remarkably long Zn-O bonds resulting in a highly distorted octahedral geometry. Molecules of complex 1 are connected via N-H center dot center dot center dot O hydrogen bonds, involving hydrogen atoms of both barb and en ligands, while molecules of complex 2 are linked by N-H center dot center dot center dot O and O-H center dot center dot center dot O hydrogen bonds and also aromatic pi(bpy)center dot center dot center dot pi(bpy) stacking interactions. The voltammetric behavior of complexes 1 and 2 was investigated in aqueous solution by cyclic voltammetry using a NH3/NH4Cl buffer. The cyclic voltammogram of 1 shows a cathodic peak at -1.260 V and an anodic peak at -1.068 V due to a quasi-reversible two-electron process. Complex 2 yields two reduction peaks at -1.312 and -1.412 V. The former corresponds to a quasi-reversible electrode process of the zinc(II) ions in complex 2, while the latter is attributed to the reduction of the coordinated bpy ligands.