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

Abstract

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) Å). The piperazine ring adopts a chair conformation with the CH 2COOH 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 νCO band at 1734 cm −1. The p K a values of 3 and 6 were determined by potentiometric titration. The 1H and 13C NMR spectra in D 2O were analyzed.