Synthesis, structural studies and vibrational spectroscopy of Fe2+ and Zn2+ complexes containing 4,4′-bipyridine and barbiturate anion

Two new coordination polymers of general formula FeB2bpy·10H2O (1) and ZnB2bpy·10H2O (2) (where B = barbiturate anion; bpy = 4,4′-bipyridine) were synthesized and characterized by X-ray diffraction, vibrational spectroscopy (Raman and infrared) and thermal analysis. The two complexes are isostructural and crystallize in a hexagonal crystal system with space group P6422. The compound contains the 4,4′-bipyridine ligand coordinated to the metal sites in a bis-monodentate mode, and also four water molecules, in a slightly distorted octahedral geometry, giving rise to the cationic unit with two barbiturate anions. Each structure shows one covalent linear [M(bpy)(H2O)4]2+ one-dimensional chain, which interacts via hydrogen bond with barbiturate anions and crystallization water molecules, resulting in a three-dimensional arrangement. The vibrational spectra of both compounds are very similar; in all infrared spectra is observed an intense band at 1686 cm−1 assigned to the νCO stretching mode of the barbiturate anion, as well as a medium band around 1610 cm−1, attributed to the νCC/νCN stretching modes of the 4,4′-bipyridine ligand. The Raman spectra show intense and conclusive bands for the nitrogen ligand at 1615, 1295 and 1020 cm−1, assigned to the νCC/νCN, νring + δCH and νring modes, respectively. The barbiturate anion is observed through the presence of a marker band at 680 cm−1, assigned to the ring breathing mode.