Structure of the β-Cyclodextrin·p-Hydroxybenzaldehyde Inclusion Complex in Aqueous Solution and in the Crystalline State

Abstract

β-Cyclodextrin (β-CD) and p-hydroxybenzaldehyde (p-HB) were studied by 1H-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 β-CD was confirmed by the observation of NMR shifts for the inside H5 protons of the β-CD cavity. In the solid state X-ray analysis was carried out and revealed the detailed structure of the inclusion complex. Two β-CDs cocrystallize with four p-HB and 9.45 water molecules[2(C6H10O5)7·4C7H6O2·9.45H2O] in the triclinic space group P1 with unit cell parameters: a = 15.262(2), b = 15.728(1), c = 16.350(1) A, α = 92.67(1)°, β = 96.97(1)°, γ = 103.31(1)°. The anisotropic refinement of 1973 atomic parameters converged at an R-factor = 0.066 for 10157 data with Fo2 > 2σ (Fo2). The 2:4 stoichiometry for the β-CD inclusion complex with p-HB in the crystalline state is different from that obtained in solution. β-CD forms dimers stabilized by direct O2(m)1O3(m)1·O2(n)2O3(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 β-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 13C CP-MAS NMR spectroscopy. The results obtained enabled us to structurally characterize the β-CD inclusion complex with p-HB.