Hydrogen bonds and electrostatic interactions in the 1:1 complex of DABCO di-betaine with squaric acid: Crystallographic, theoretical and spectroscopic studies

Abstract

► Hydrogen squarate anions are bonded to homoconjugated DABCO di-betaine cations. ► O H···O and C H···O hydrogen bonds and N + ···O electrostatic interactions are present. ► C H···O hydrogen bonds are confirmed by the νC H···O vibrations in the IR spectrum. ► DABCO moiety has a propeller conformation. ► The crystal structure is more extended than optimized ones. The molecular structure of the 1:1 complex of DABCO di-betaine (1,4-dicarboxymethyl-1,4-diazabicyclo[2.2.2]octane inner salt) with squaric acid ( 1 ) has been characterized by single-crystal X-ray diffraction, infrared spectroscopy and DFT calculations. Crystals 1 are monoclinic, space group P 2 1 / c . One proton of squaric acid is transferred to one of the carboxylate groups of DABCO di-betaine. The mono-protonated DABCO di-betaine cation is further engaged in the COOH⋯OOC hydrogen bond of 2.526(2) Å with the neighboring cations, linking them into a zigzag chain. The hydrogen squarate anion interacts with the oxygen atom of the carboxylate group of DABCO di-betaine cation, through the O H⋯O C hydrogen bonds of 2.600(2) Å. The structures of monomer 1a , dimer 2 and cation 3 have been optimized at the B3LYP/6-31G(d,p) level of theory. The FTIR spectrum is consistent with the X-ray results. The second-derivative spectrum of 1 and calculated frequencies for the optimized structure 1a are used to explain the frequencies in the experimental FTIR spectrum.