MOLECULAR RECOGNITION INVOLVING AN INTERPLAY OF ENTER-DOT-O,C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-O AND I-CENTER-DOT-CENTER-DOT-CENTER-DOT-PI-INTERACTIONS - THE ANOMALOUS CRYSTAL-STRUCTURE OF THE 1 1 COMPLEX 3,5-DINITROBENZOIC ACID-4-(N,N-DIMETHYLAMINO)BENZOIC ACID/

The crystal structure of the 1:1 donor-acceptor complex of 3,5-dinitro benzoic acid and 4-(N,N-dimethylamino)benzoic acid contains the uncomm on O-H...O hydrogen-bonded carboxy homodimers rather than the heterodi mers found in nine other related complexes. The formation of these hom odimers contradicts the general principle that in hydrogen-bonded netw orks, the strongest proton donor hydrogen bonds to the strongest proto n acceptor. This unusual homodimer is obtained because of difficulties in C-H...O hydrogen bond formation, the consequent importance of pi.. .pi stacking interactions and the enhanced stability of homodimer stac ks over heterodimer stacks. Additionally, it is concluded that: (i) O- H...O hydrogen bonds can act as a conduit for charge transfer and may alter the polarization of atoms; (ii) C-H...O bonds can be used for mo lecular recognition and C-H...O patterns are sensitive to molecular st oichiometry and substituent positioning; (iii) stacking interactions i nfluence the nature of hydrogen bonding and vice versa. This study sho ws that for precise supramolecular construction, strong and weak inter molecular interactions must be considered together.