Structure of an inclusive compound of bis(piperidinium-4-carboxylate)hydrogen semi-tartrate with water and methanol studied by X-ray diffraction, NMR, FTIR and DFT methods

The complex consisting of two piperidine-4-carboxylic acid, l-tartaric acid, water and methanol molecules has been synthesized and characterized by X-ray diffraction, 1H, 13C NMR, 13C CP MAS NMR, FTIR spectra and DFT calculations. The title complex is composed of the following units: piperidinium-4-carboxylate (P4C), piperidinium-4-carboxylic acid (P4CH), semi-tartrate anion (TA), water and methanol; it crystallizes in orthorhombic space group P212121. TA anions form infinite chains through the COOH···OOC hydrogen bond of 2.503(5) Å. The zwitterionic P4C molecules are linked by the N+H···OOC hydrogen bond of 2.780(5) Å into chains. The P4CH cation is a bridge between the TA and P4C chains. P4CH and P4C form a homoconjugated cation through the COOH···OOC hydrogen bonds of 2.559(5) Å. Water interacts with TA and P4CH, while methanol interacts only with water. In the optimized molecule of the (P4C)2H·TA·H2O·HOCH3 complex, the components form a cyclic oligomer through four O–H···O and four N–H···O hydrogen bonds. The 1H and 13C NMR spectra elucidate the structure of the title complex in the aqueous solution. The 13C CP MAS NMR spectrum is consistent with the X-ray results. The FTIR spectrum confirms a very complex structure of the title compound.