The Formation and Structure of Copper(II) Complexes with Cyclodextrins in an Alkaline Solution

Copper(II) forms a 2: 1 complex with cyclodextrin in an alkaline solution. The complexes with α- and β-cyclodextrins were isolated from solutions by the addition of ethanol. They are labile, and, upon heating, the Cu(II) is reduced to Cu(0). The role of the hydroxide ion was examined by means of potentiometrie and conductometric titrations; it was found that the reaction of 1 mol of α-cyclodextrin with 2 mol of Cu(OH)2consumes 2 mol of the hydroxide ion, whereas 3 mol of the hydroxide ion are consumed for the reaction of β-cyclodextrin. Polarimetric measurements showed that the optical rotation of cyclodextrin changes from dextrorotatory to levorotatory upon the formation of the Cu(II)-cyclodextrin complex. It was suggested, with a molecular model, that the two pairs of C2 and C3′ secondary hydroxyl groups of contiguous glucose units are cross-linked by the Cu(OH−)2Cu ion bridge in the α-cyclodextrin complex and by Cu(OH−)(O2−)Cu ion bridge in the β-cyclodextrin complex. Furthermore, the circular ring of cyclodextrin may be distorted to the ellipsoidal form by the complex formation, causing a pronounced change in the optical rotation.