NH⋯O and OH⋯O interactions of glycine derivatives with squaric acid

Abstract

Four new hydrogen-bonded complexes of a simple amino acid glycine (GLY) and its methyl derivatives – sarcosine (N-methylglycine, SAR), dimethylglycine (DMG) and betaine (N,N,N-trimethylglycinium, BET) with squaric acid (3,4-dihydroxy-3-cyclobuten-1,2-dione, H₂SQ) are synthesized and characterized by X-ray diffraction, FTIR and NMR spectroscopy. The complexes differ in stoichiometry, interaction with H₂SQ and the hydrogen-bonding system. Methylation of glycine gradually reverses the dissociation of squaric acid in co-crystals. This process is correlated with the number of N–H⋯O bonds to the squaric acid oxygens and the N–H donor capability of glycine derivatives. The effect of the presence of methyl groups on the proton and carbon-13 chemical shifts is studied. The DFT calculations are performed for one unit of complexes and stable zwitterionic structures of SAR, DMG and BET in complex with H₂SQ, except the GLY unit, are suggested, which appears as an uncharged (neutral) form. The experimental and computed vibrational spectra of complexes studied are reported.