Cocrystals of bis(4-hydroxy-1-methylpiperidine betaine) hydrochloride

Bis(4-hydroxy-1-methylpiperidine betaine) hydrochloride, [bis(1-carboxymethyl-4-hydroxy-1-methylpiperidinium) hydrochloride, (HO-MPB)2HCl], has been prepared from stoichiometric amounts of β-4-hydroxy-1-methylpiperidine betaine hydrochloride with the OH group in an equatorial position and α-4-hydroxy-1-methylpiperidine betaine inner salt with the OH group in an axial one. Cocrystals of (HO-MPB)2HCl belong to monoclinic system with C2/c space group. Piperidinium ring has a chair conformation with the CH2COO group in the equatorial position and the CH3 group in the axial one, while the OH group occupies the equatorial or axial position in the disordered structure. There are three types of substituted piperidinium molecules A, B and C in the asymmetric unit. These isomers are distributed randomly yielding partially occupied sites of the hydroxyl groups. The disorder of molecules has been confirmed by the X-ray diffraction experiments at low temperature. The piperidinium molecules form two independent homoconjugated cations, A-B and C-C, with short asymmetrical (2.457(2) Å) and symmetrical (2.440(2) Å) OHO hydrogen bonds, respectively. The hydroxyl groups interact with the Cl− anions by the O-H⋯Cl hydrogen bonds. The A-B cation is linked with C-C by the O-H⋯O-H hydrogen bond, while C-C cations are joined together by the O-H⋯O=C hydrogen bonds, forming infinite zigzag chains. The FTIR spectrum shows an intense νOH band in the 3300-3100 cm−1 region and a broad and intense ν(OHO) absorption in the 1500-400 cm−1 region, which confirm the presence of the O-H⋯Cl and O·H·O hydrogen bonds. The 1H and 13C NMR spectra, in the aqueous solution, prove the inequivalence of the piperidinium ring caused by two conformations of the OH group at the ring.