Cocrystals of Kemp's triacid. Part II. Molecular structure of the complex of Kemp's triacid with 1,8-diazabicyclo[5.4.0]undec-7-ene

The 2:2 hydrogen-bonded complex between Kemp's triacid (KTA) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was synthesised and studied by X-ray diffraction and by FT-IR spectroscopy. Cocrystals of KTA-DBU (2:2) belong to the monoclinic system and crystallize in the space group which is P21/c with a = 9.286(2) Å, b = 23.512(4) Å, c = 11.300(2) Å, β = 96.92(1) and Z = 4. The ring of the deprotonated KTA molecule exhibits a chair conformation with all three carboxylic groups in the axial positions and all three CH3 groups in the equatorial positions. In the crystal of the complex cooperative systems involving inter- and intramolecular hydrogen bonds are formed. In the solid state two protonated DBU molecules and two deprotonated KTA molecules form a dimer in which three-dimensional hydrogen-bonded networks are found. Closer insights into the nature of these hydrogen bonds come from proton vibrations in the FT-IR spectra of the crystals studied.