Interactions of squaric acid with DABCO mono-betaine: Structural, spectroscopic and calculation studies

The 1:1 complex of squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dion) with DABCO mono-betaine (1-carboxymethyl-1,4-diazabicyclo[2.2.2]octane inner salt), 1, has been characterized by single-crystal X-ray analysis, FTIR and NMR spectroscopies and by DFT calculations. The crystal of 1 is triclinic, space group P  1¯. Short COOH⋯O and NH⋯O hydrogen bonds of 2.445(2) and 2.582(2) Å, respectively, link the diprotonated DABCO mono-betaine dications and squarate dianions into chain. The structure is additionally stabilized by the N+⋯O electrostatic interactions and weak CH⋯O hydrogen bonds. The FTIR spectrum shows two broad absorptions in the 3000–2000 cm−1 region assigned to the νNH⋯O vibration and in the 1900–500 cm−1 region attributed to the ν(OHO) and γ(OHO) vibrations of the short hydrogen bonds.

► Diprotonated DABCO mono-betaines and squarate dianions form chains.
► Molecules are linked by short COOH⋯O and NH⋯O hydrogen bonds.
► There are two broad absorptions at ca. 2470 and 1245 cm−1 in IR spectrum.
► Structures optimized by DFT calculations.
► Structures are dominated by OH⋯O, NH⋯O and CH⋯O hydrogen bonds and electrostatic interactions.