MOLECULAR-STRUCTURES OF HOST-GUEST COMPLEXES WITH REBEKS DIACID

The crystal and molecular structures of 1:1 complexes of Rebek's diacid (1) with pyrazine (2) and quinoxaline (3) have been determined by single 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 symmetric, syn, two-point binding of the diamine guests by the convergent carboxyl groups of 1. These data confirm the original structural proposal of Rebek et al. (J. Am. Chem. Soc. 1987, 2426-2431). and stand in contrast 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 protonated on the acridine nitrogen, and the chloride counterion participates in a ''three-point binding'', T-shaped, hydrogen-bonded network within the diacid cleft.