Centrosymmetric dimer of quinuclidine betaine and squaric acid complex

The complex of squaric acid (3,4-dihydroxy-3-cyclobuten-1,2-dion, H2SQ) with quinuclidine betaine (1-carboxymethyl-1-azabicyclo[2.2.2]octane inner salt, QNB), 1, has been characterized by single-crystal X-ray analysis, FTIR and NMR spectroscopies and by DFT calculations. In the crystal of 1, monoclinic space group P21/n, one proton from H2SQ is transferred to QNB. QNBH+ and HSQ− are linked together by a OH⋯O hydrogen bond of 2.553(2) Å. Two such QNBH+·HSQ− complexes form a centrosymmetric dimer bridged by two OH⋯O bonds of 2.536(2) Å. The FTIR spectrum is consistent with the X-ray results. The structures of monomer QNBH+·HSQ− (1a) and dimer [QNB·H2SQ]2 (2) have been optimized at the B3LYP/6-311++G(d,p) level of theory. Isolated dimer 2 optimized back to a molecular aggregate of H2SQ and QNB. The calculated frequencies for the optimized structure of dimer 2 have been used to explain the frequencies of the experimental FTIR spectrum. The interpretation of 1H and 13C NMR spectra has been based on the calculated GIAO/B3LYP/6-311++G(d,p) magnetic isotropic shielding constants for monomer 1a.

► Proton transfer from squaric acid to quinuclidine betaine.
► OH⋯O hydrogen bonded centrosymmetric dimer.
► Two νOH frequencies appear in the calculated IR spectrum of the optimized dimer.
► Chemical shifts of the methylene group in the CH2COOH substituent evidence the protonation of quinuclidine betaine.