X-ray, NMR and DFT studies of the complex of 1,4-dimethylpiperazine mono-betaine with p-hydroxybenzoic acid

Crystal structure of the 1:1 complex of 1,4-dimethylpiperazine mono-betaine (MBPZ) with p-hydroxybenzoic acid (HBA) has been determined by X-ray diffraction. The acid molecule is involved in hydrogen bonds with two betaine molecules forming infinite zigzag chains along the [ 1 ¯ -01] direction, which interact with the other ones only by van der Waals contacts. In this structure, two symmetry-independent hydrogen bonds are formed: the shorter is the O–H⋯O bond of 2.573(2) Å between the hydroxyl group of the acid and the carboxylate group of betaine, and the longer one is the OH⋯N bond of 2.736(3) Å between the carboxylic group and the N(4) atom of piperazine. The piperazine ring has a chair conformation with the N(4)-methyl and N+(1)–CH2COO− groups in the equatorial positions, while the N+(1)-methyl group is in the axial position. The optimized geometry of the complex investigated in the B3LYP/6–31G(d,p) approach is consistent with that in the crystal structure. The 1H and 13C NMR spectra have been analyzed to elucidate the complex structure in the solution.