The Structure of the Cyclodextrin Complex. VI. The Crystal Structure of α-CycIodextrin–m-Nitrophenol (1 : 2) Complex

Abstract α-Cyclodextrin (α-CDx) forms a 1 : 2 crystalline complex with m-nitrophenol (m-NP). The crystal structure was determined by the X-ray method. The crystal is orthorhombic, and the space group is P21212 with Z=2. The cell dimensions are a=22.231(7), b=16.865(6), and c=8.152(1) Å. The structure was solved on the basis of the nearly isomorphous structure of the α-CDx-sodium benzenesulfonate complex by using 2246 reflections, and refined by the block-diagonal least-squares method to the final R-value of 0.10. The framework of the crystal is built up of a stack of α-CDx molecules with a head-to-tail arrangement along the two-fold axis, showing a channeltype structure. Two m-NP molecules are statistically disordered on the two-fold axis. One is located in the channel, while the other is sandwiched between the α-CDx molecules. In the channel, the nitro group of m-NP is inserted into the α-CDx ring from the secondary hydroxyl side, and the phenolic hydroxyl group is hydrogen-bonded to the primary hydroxyl group of the next α-CDx molecule. The enthalpy change was calculated for the complex formation in an aqueous solution. The calculated results are in qualitatively agreement with the observed values. The catalytic ability of α-CDx in the hydrolysis of substituted phenyl acetates was discussed on the basis of the structural data.