The Structure of the Cyclodextrin Complex. II. The Crystal Structure of α-Cyclodextrin-Methyl Orange (2 : 1) Complex

Abstract

Abstract α-Cyclodextrin(α-CDx) forms 2 : 1 complexes with Methyl Orange sodium salt (MONa) and Methyl Orange potassium salt (MOK). The structures were investigated by the X-ray method. The crystals of both complexes belongs to the orthorhombic space group P21212 with Z=2; the cell dimensions are a=22.099(6), b=16.359(3), and c=8.296(1) Å for the MONa complex, and a=22.120(4), b=16.419(4), and c=8.292(1) Å for the MOK complex. Weak streaks were observed between the layer lines in the oscillation photographs around the c* axis. The structures were determined by using 1566 reflections for the MONa complex and 2544 reflections for the MOK complex, and were refined by the least-squares method to the final R-values of 0.11 and 0.10 respectively. α-CDx is a doughnut-shaped molecule with a large cavity. The crystals have channel structures built up by the stacking of α-CDx rings along the c axes. The Methyl Orange anions are located in the channel with a statistical disorder of four different arrangements. The azo group and the benzene ring are included in the cavity of α-CDx, but the dimethylamino and sulfonato groups protrude from the cavity and are in contact with the adjacent α-CDx rings. The azo group is located at the “neck” of the cavity. The sulfonato group is hydrogen-bonded to the primary hydroxyl groups of α-CDx. No significant structural difference was observed between the MONa complex and the MOK complex.