Three hydrogen-bonded complexes of trigonelline with squaric acid: Crystallographic, spectroscopic and theoretical studies

Trigonelline (N-methyl-3-carboxypyridinium inner salt, TRG) forms with squaric acid (H2SQ) and water three complexes with TRG:H2SQ:H2O ratios of 3:3:1 (1), 2:2:1 (2) and anhydrous 1:1 (3), which stoichiometry depends on the solvent used for recrystallization. Their structures have been determined by X-ray diffraction, DFT calculations and characterized by FTIR and NMR spectroscopy. Crystals 1 contain three hydrogen-bonded TRG–H2SQ complexes and one water molecule in the asymmetric unit of triclinic space group P 1 ¯ . Crystals 2 are monoclinic P21/c space group and contain two hydrogen-bonded TRG–H2SQ complexes and one water molecule. In complexes 1 and 2 water molecule play a role of proton-donors and proton-acceptors in hydrogen bonds. Crystal 3, obtained from 98% ethanol belongs to the monoclinic P21/n space group and has one TRG–H2SQ complex in the asymmetric unit. In the structures of 1–3 complexes there are several short asymmetric OH···O hydrogen bonds (2.428(1)–2.542(1) Å). FTIR spectra are dominated by a broad and intense absorption in the 1600–400 cm−1 region, typical of such short hydrogen bonds. The structures of hydrate 2a and anhydrous 3a have been optimized at the B3LYP/6-31G(d,p) level of theory.