The Structure of the Cyclodextrin Complex. I. The Crystal and Molecular Structure of the α-Cyclodextrin– p-Iodoaniline Complex

The crystal structure of the α-cyclodextrin–p-iodoaniline complex [(C6H10O5)6·C6H6NI·3H2O] has been determined by the X-ray method. The space group is P212121 with unit cell dimensions of a=13.659(2), b=15.413(3), and c=24.486(5) Å. The structure was solved by the heavy-atom method and refined by least-squares techniques. The final R-value was 0.072. The p-iodoaniline molecule is included in the cavity with its axis coincident with the axis of α-cyclodextrin; the iodobenzene group is situated in the cavity, while the amino group sticks out from the cavity. The shortest contact between the α-cyclodextrin molecule and the “guest” p-iodoaniline molecule is 3.25 Å, indicating that the latter is rigidly fixed in the cavity of the former. The distances between the amino group and the hydroxyl group are greater than 4.0 Å; this fact well explains why the catalytic ability of α-cyclodextrin is low in the hydrolysis of para-substituted phenyl esters.