Charge-transfer complexes of 4-(dimethylamino)pyridine with 2,4-, 3,4- and 3,5-dinitrobenzoic acid.

In the three title crystal structures 4-(dimethylamino)pyridinium 2,4-dinitrobenzoate, (I), 4-(dimethylamino)pyridinium 3,4-dinitrobenzoate, (II), and 4-(dimethylamino)pyridinium 3,5-dinitrobenzoate, (III), all C(7)H(11)N(2)(+).C(7)H(3)N(2)O(6)(-), the ions are connected by an N-H.O hydrogen bond. Dihedral angles between the pyridine and phenyl rings are 69.9 (1), 26.7 (1) and 56.2 (1) degrees in (I), (II) and (III), respectively. Donor-acceptor pi-pi stacking is observed in (II) and (III), but not in (I).