Hydrogenoxalate and squarate salts of (E)-4-(hydroxyiminomethyl)-pyridine – Crystal structures, spectroscopic and theoretical elucidation

(E)-4-(Hydroxyiminomethyl)-pyridinium hydrogenoxalate (1) and bis((E)-4-(hydroxyiminomethyl)-pyridinium) squarate (2) were synthesized and their crystal structures determined by means of single crystal X-ray diffraction. The structures of (1) and (2) consist of infinite layers and chains, respectively, formed by (1): N–H···O (2.852(2), 2.827(2) Å) and O–H···O (2.665(2) Å) and (2): N–H···O (2.620(2) Å) and O–H···O (2.600(1) Å) intermolecular hydrogen bonds between the cations and anions. Hydrogenoxalate ions form chains within the infinite layers by strong interanionic O–H···O interactions (2.583(2) Å). Antiperiplanar conformations of the cations for (1) and (2) (CCN–OH) dihedral angle of 178.9(1)° (1) and 179.4(1)° (2) are observed. Spectroscopic properties both in solution and in the solid state were elucidated by means of 1H- and 13C-NMR, UV-spectroscopy and conventional and linear polarized IR- (IR-LD) spectroscopy. Quantum chemical calculations at the DFT, MP2 and CIS level of theory and the 6-31++G∗∗ basis set were employed for predicting and supporting experimentally observed optical propertied and electronic structures of the protonated (E)-4-(hydroxyiminomethyl)-pyridine.