2,3-Dimethoxy-10-oxostrychnidinium 2-carboxy-4,5-dichlorobenzoate

The structure of the title compound, C₂₃H₂₇N₂O₄⁺·C₈H₃Cl₂O₄⁻, a 1:1 proton-transfer compound of brucine with 4,5-dichlorophthalic acid, has been determined at 130 K. The brucinium cations and the hydrogen phthalate anions associate through single N—H...O_carboxylate hydrogen bonds [2.639 (3) Å], giving dimers which are extended via weak head-to-tail C—H...O_methoxy associations into chains forming down the 2₁ screw axis of the unit cell. The carboxyl proton of the anion gives a short intramolecular O—H...O_carboxylate hydrogen bond [2.441 (3) Å].