Unusual molecular structure of the 1:2.5:2 complex of DABCO di-betaine (1,4-diacetate-1,4-diazoniumbicyclo[2.2.2]octane) with p-hydroxybenzoic acid and water

The molecular structure of the 1:2.5:2 complex of DABCO di-betaine, p-hydroxybenzoic acid (HBA) and water (1) has been characterized by single-crystal X-ray diffraction, infrared spectroscopy and DFT calculations. The crystals 1 are triclinic, space group P  1¯. The carboxylate groups of DABCO di-betaine are engaged in the COO⋯HOOC and COO⋯HO hydrogen bonds with two HBA molecules of 2.750(2) and 2.621(2) Å, respectively. Two water molecules and one HBA, disordered over the inversion center in two orientations with half occupancies, play a role of the bridge between the DABCO di-betaine·(HBA)2 complexes. Three structures of DABCO di-betaine with HBA and water of different stoichiometry (2–4) have been optimized at the B3LYP/6-31G(d,p) level of theory. The νCO and νasCOO vibrations are distinguished in the solid-state FTIR spectrum of 1 and confirmed by the second-derivative spectrum of 1 and the calculated spectrum of 2 by the B3LYP/6-31G(d,p) approach.