Molecular recognition involving an interplay of O–H⋯O, C–H⋯O and π⋯π interactions. The anomalous crystal structure of the 1 : 1 complex 3,5-dinitrobenzoic acid–4-(N,N-dimethylamino)benzoic acid

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Abstract
The crystal structure of the 1:1 donor–acceptor complex of 3,5-dinitrobenzoic acid and 4-(N,N-dimethylamino)benzoic acid contains the uncommon O–H ⋯ O hydrogen-bonded carboxy homodimers rather than the heterodimers found in nine other related complexes. The formation of these homodimers contradicts the general principle that in hydrogen-bonded networks, the strongest proton donor hydrogen bonds to the strongest proton acceptor. This unusual homodimer is obtained because of difficulties in C–H ⋯ O hydrogen bond formation, the consequent importance of π⋯π stacking interactions and the enhanced stability of homodimer stacks 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 molecular recognition and C–H ⋯ O patterns are sensitive to molecular stoichiometry and substituent positioning; (iii) stacking interactions influence the nature of hydrogen bonding and vice versa. This study shows that for precise supramolecular construction, strong and weak intermolecular interactions must be considered together.

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