MOLECULAR-STRUCTURES OF HOST-GUEST COMPLEXES WITH REBEKS DIACID

The crystal and molecular structures of 1:1 complexes of Rebek's diaci d (1) with pyrazine (2) and quinoxaline (3) have been determined by si ngle crystal X-ray diffraction. The unit cells of the crystalline 1:2 and 1:3 each contain two crystallographically independent molecules of their respective complexes; all four complexes exhibit nearly symmetr ic, syn, two-point binding of the diamine guests by the convergent car boxyl groups of 1. These data confirm the original structural proposal of Rebek et al. (J. Am. Chem. Soc. 1987, 2426-2431). and stand in con trast to the central conclusion (one-point binding) of Jorgensen et al .'s computational study of the complex of Rebek's diacid hydrochloride (1:HCl) has been determined. In this complex, compound 1 is protonate d on the acridine nitrogen, and the chloride counterion participates i n a ''three-point binding'', T-shaped, hydrogen-bonded network within the diacid cleft.