Synthesis, vibrational spectroscopy and crystal structures of polymers involving transition metals with squarate ions and 1,3-bis(4-pyridyl)propane

Two new coordination polymers of general formula [Mn(C4O4)(BPP)2(H2O)2] and [Co(C4O4)(BPP)2(H2O)2] [where BPP = 1,3-bis(4-pyridyl)propane; C4O4 = squarate ion] were synthesized and characterized by vibrational spectroscopy (Raman and infrared), X-ray diffraction and thermal analysis. These two compounds are isostructural, containing BPP and squarate ligands coordinated to the metal sites, as well as two water molecules, in a distorted octahedral geometry. The squarate ion adopts the 1,3-bis(monodentate) coordination mode binding two metallic sites, extending the chain along the a crystallographic axis; this chain interacts with other adjacent chains by means of hydrogen bonds through the free oxygen atoms of the squarate ion and also the water molecule, as well as the π-stacking of the pyridine rings of the BPP ligand, giving rise to a two-dimensional arrangement where the unidimensional chains are interdigitated. The vibrational spectra of both compounds are very similar and are supportive of the crystallographic data. In the infrared spectra of both complexes an observed band of medium intensity around 1620 cm−1 and another strong band at 1535 cm−1, assigned to CC/CN and CO stretching modes, respectively, are indicative of the coordination of BPP and squarate ions to the metal sites. In the Raman spectra, the strong band at 1769 cm−1, assigned to the CO stretching mode of uncoordinated carbonyl groups of the squarate ion, is noteworthy.