1,10-Phenanthrolin-1-ium 2-carb­oxy-4,5-dichloro­benzoate

In the structure of the 1:1 proton-transfer compound of 1,10-phenanthroline with 4,5-dichloro­phthalic acid, C(12)H(9)N(2) (+)·C(8)H(3)Cl(2)O(4) (−), determined at 130 K, the 1,10-phenanthrolinium cation and the hydrogen 4,5-dichloro­phthalate anion associate through a single N—H⋯O(carbox­yl) hydrogen bond giving discrete units which have no extension except through a number of weak cation C—H⋯O(anion) associations and weak cation–anion aromatic ring π–π inter­actions [minimum centroid–centroid separation = 3.6815 (12) Å]. The anions are essentially planar "[maximum deviation 0.214 (1) Å (a carboxyl O)] with the syn-related H atom of the carboxyl group, forming a short intra­molecular O—H⋯O(carbox­yl) hydrogen bond.