Polymorphs of 4-(dihydroxymethyl)pyridinium hydrogensquarate – Crystal structures and spectroscopic properties

The novel salt of a geminal diol 4-(dihydroxymethyl)pyridinium hydrogensquarate was derived from isonicotinaldehyde and squaric acid in water. Two polymorphs were characterized by means of single crystal X-ray diffraction and linear-polarized infrared (IR-LD) spectroscopy of oriented colloids in nematic host. UV spectroscopy was employed to confirm the stabilization of the geminal diol form in aqueous solution. Depending of the reaction conditions two polymorphs of 4-(dihydroxymethyl)pyridinium hydrogensquarate are determined – monoclinic (1) and a triclinic (2) polymorph. The monoclinic (1) crystallizes in the centrosymmetric space group P21/c, while (2) crystallizes in the centrosymmetric space group P  1¯, respectively. The hydrogensquarate anions in both forms form the α chain motif via interanionic hydrogen bonding interactions 2.664(7) Å (1) and 2.517(5) Å (2), respectively. The geminal diol group of the cations in (1) are directed to an adjacent hydrogensquarate ion via two O–H···O hydrogen bonds with an averaged donor–acceptor distances forming an eight membered ring. In contrast to (1), in (2) the infinite hydrogen bonded two-dimensional layers formed by cations and anions are connected via a hydrogen bond between hydroxyl groups of adjacent symmetry related cations.