Structure and properties of the 1:2 complex of 1,4-dicarboxymethyl-1,4-dimethylpiperazinium inner salt (di-betaine) with p-hydroxybenzoic acid studied by X-ray, DFT, FTIR and NMR methods

Crystal structure of the complex of 1,4-dicarboxymethyl-1,4-dimethylpiperazinium inner salt (1,4-dimethylpiperazine di-betaine, DBPZ) with two p-hydroxybenzoic acids (HBA) has been determined by X-ray diffraction. The crystals are monoclinic, space group P21/c, with the DBPZ molecules located at the inversion centers. Both COO− groups of DBPZ form COO⋯HOOC hydrogen bonds of 2.645(3) Å with two HBA molecules, linking them into a centrosymmetric trimolecular complex. The OH group of the p-hydroxybenzoic acid is further involved in the O–H⋯OC hydrogen bonds of 2.643(3) Å with the carboxylate groups of betaines of the neighboring complexes, linking them into infinite chains. The piperazine has a chair conformation with the CH2COO− groups in the axial and the methyl groups in the equatorial positions. Both conformers of DBPZ(HBA)2 and DBPZ, with CH2COO in axial and equatorial positions, were analyzed at the B3LYP/6-31G(d,p) level of theory. The conformer of DBPZ(HBA)2 with axial CH2COO has lower energy than that with this equatorial CH2COO, while in betaine (DBPZ) the situation is reverse. The optimized geometry of the complex investigated in the B3LYP/6-31 G(d,p) approach is consistent with that in the crystal structure. The broad band in the 3000–2400 cm−1 region assigned to the νOH mode, and the two bands attributed to the νCO (1688 cm−1) and νasCOO (1628 cm1) vibrations in the FTIR spectrum confirm the structure of the title complex. The 1H–13C HMBC spectrum has been analyzed in order to assign the carbon-13 chemical shifts.