Structure of the β-cyclodextrin•p-hydroxybenzaldehyde inclusion complex in aqueous solution and in the crystalline state

beta-Cyclodextrin (beta-CD) and p-hydroxybenzaldehyde (p-HB) were studied by H-1-NMR in deuterated aqueous solution and the stoichiometry of the resulting complex (1: 1) was determined by the continuous variation method. Inclusion of p-HB in beta-CD was confirmed by the observation of NMR shifts for the inside H5 protons of the beta-CD cavity. In the solid state X-ray analysis was carried out and revealed the detailed structure of the inclusion complex. Two beta-CDs cocrystallize with four p-HB and 9.45 water molecules [2(C6H10O5)(7).4C(7)H(6)O(2).9.45H(2)O] in the triclinic space group P 1 with unit cell parameters: a = 15.262(2), b = 15.728(1), c = 16.350(1) Angstrom, = alpha = 92.67(1)degrees, beta = 96.97(1)degrees, gamma =103.31(1)degrees. The anisotropic refinement of 1973 atomic parameters converged at an R-factor = 0.066 for 10157 data with F-o(2) > 2sigma(F-o(2)). The 2:4 stoichiometry for the beta-CD inclusion complex with p-HB in the crystalline state is different from that obtained in solution. beta-CD forms dimers stabilized by direct O-2(m)_1/O3(m)_1...O-2( n)_2/O3(n)_2 hydrogen bonds (intradimer) and by indirect O6(m)_1...O6(n)_2 hydrogen bonds with one or two bridging water molecules joined in between (interdimer). These dimers are stacked like coins in a roll constructing infinite channels where the p-HB molecules are included. The p-HB molecules direct their polar CHO and OH groups into the nonpolar beta-CD cavities and are hydrogen bonded to each other, yielding infinite, antiparallel chains. In addition, crystals of the complex were also investigated with thermogravimetry, vibrational spectroscopy (FTIR), and C-13 CP-MAS NMR spectroscopy. The results obtained enabled us to structurally characterize the beta-CD inclusion complex with p-HB.