Comparative Crystal Structure Studies on Distortion of iso-Butyl(aqua) cobaloxime Induced by α-Cyclodextrin and β-Cyclodextrin Hosts

The crystal and molecular structures of iso-butyl(aqua)cobaloxime (I) and its hydrated inclusion complexes with α-cyclodextrin (II) and β-cyclodextrin (III) were determined by X-ray diffraction analyses. Crystal data for I: P2₁/c, a = 30.450(1) Å; b = 9.659(2) Å; c = 19.200(2) Å; β= 107.23(3)°; Z = 12. Crystals of II·H₂O and III·9H₂O both crystallize in space group P2₁2₁2₁, with Z = 4; a = 13.847(1) Å, b = 19.418(1) Å, c = 24.593(1) Å for II·8H₂O, and a = 28.188(6) Å, b = 15.190(3) Å, c = 19.017(4) Å for III·9H₂O. In the inclusion complexes, the Co(DH)₂ unit is located near the wide opening of the cyclodextrin with its iso-butyl group inserted into the cyclodextrin cavity. The structure and conformation changes of the iso-butyl(aqua)cobaloxime are probably related to the noncovalent interaction between the iso-butyl group and cyclodextrin cavity, and steric interaction between cobaloxime and cyclodextrin rims. Our results also show that distortions of iso-butyl(aqua)cobaloxime/cyclodextrin induced by α-CD and β-CD occur in different extent, which assumedly is due to the different cavity diameters of the cyclodextrins.