Molecular structure, hydrogen bonding, basicity and spectroscopic propertis of N,N′-dimethylpiperazine betaines and their hydrohalides

Two N,N'-dimethylpiperazine betaines (mono (3) and double (6)) have been synthesized. Betaine 3 reacts with two equivalents of HCl or HBr, while 6 only with one. In the crystal structure of N,N'-dicarboxymethyl-N,N'-dimethylpiperazine monohydrochloride (N,N'-dimethylpiperazine doublebetaine monohydrochloride, 7-Cl) determined by X-ray diffraction, the piperazinium moieties form infinite chains bridged by very strong, symmetrical and linear hydrogen bonds (O...O 2.460(2) Angstrom). The piperazine ring adopts a chair conformation with the CH2COOH group in the axial and the methyl group in the equatorial positions. The N+ atoms interact electrostatically with the Cl- ion and the oxygen atoms of the carboxylate groups. The FTIR spectrum of 7-Cl shows an intense broad absorption in the 1500-400 cm(-1) region and a nuC=O band at 1734 cm(-1). The pK(a) values of 3 and 6 were determined by potentiometric titration. The H-1 and C-13 NMR spectra in D2O were analyzed. (C) 2002 Elsevier Science B.V. All rights reserved.