Supramolecular structures of metal complexes containing barbiturate and 1,2-bis(4-pyridyl)-ethane

This work describes the synthesis, thermal, spectroscopic properties (Raman and infrared), and crystal structures of five new supramolecular compounds [Mn(bpa)(H2O)4]B2 center dot 4H2O (1), [Fe(bpa)(H2O)4]B2 center dot 4H2O (2), [Co(bpa)(H2O)4]B2 center dot 4H2O (3), [Zn(bpa)(H2O)4]B2 center dot 4H2O (4), and Co2mal2bpa center dot 2H2O (5), where B is the anion of barbituric acid, bpa is 1,2-bis(4-pyridyl)-ethane, and mal is malonate ion. Compounds 1-4 are isostructural, showing covalent linear 1-D [M(bpa)(H2O)4]2+ chains, which interact by hydrogen-bonding and -stacking interactions with barbiturate and crystallization water molecules resulting in a 3-D arrangement, belonging to Pbcn space group. Compound 5 has been obtained from the opening of the barbituric acid ring, with the formation of malonate, coordinated simultaneously to three cobalts in a 1-D chain along the c-axis, whereas bpa ligand gives rise to another 1-D chain along the a- and b-axes, resulting in a 3-D coordination polymer containing cavities. The vibrational spectra of 1-4 are also very similar; Raman spectra display two intense bands related to bpa at 1616 and 1020 cm-1, assigned to the (CC/CN) and ring stretching modes, respectively. The barbiturate is also confirmed by a band at 684 cm-1; the interesting point to be emphasized is this vibrational mode is not observed for 5, corroborating the absence of this building block in the structure.