Crystal structure of β-cyclodextrin–benzoic acid inclusion complex Original Research Article

The inclusion complex of β-cyclodextrin (β-CD) with benzoic acid (BA) has been characterized crystallographically. Two β-CDs cocrystallize with two BAs, 0.7 ethanol and 20.65 water molecules [2(C6H10O5)7·2(C7H6O2)·0.7(C2H6O)·20.65H2O] in the triclinic space group P1 with unit cell constants: a=15.210(1), b=15.678(1), c=15.687(1) Å, α=89.13(1), β=74.64(1), γ=76.40(1)°. The anisotropic refinement of 1840 atomic parameters against 16,201 X-ray diffraction data converged at R=0.078. In the crystal lattice, β-CD forms dimers stabilized by direct O-2(m)_1/O-3(m)_1⋯O-2(n)_2/O-3(n)_2 hydrogen bonds (intradimer) and by indirect O-6(m)_1⋯O-6(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 endless channels where the guest molecules are included. The BA molecules protrude with their COOH groups at the β-CD O-6-sides and are maintained in positions by hydrogen bonding to the surrounding O-6H groups and water molecules. Water molecules (20.65) are distributed over 30 positions in the interstices between β-CD molecules, except the water sites W-1, W-2 that are located in the channel of the β-CD dimer. Water site W-2 is hydrogen bonded to the disordered ethanol molecule (occupancy 0.7).