Acta Cryst. (2004). B60, 502-511 [ doi:10.1107/S0108768104018051 ]
Abstract: The hydrogen-bond patterns in hydrogen phthalates (HP) have been elucidated by the determination of the crystal structures of hydrogen phthalate salts of [Mg(H2O)4-(CH3OH)2]2+, 2-(2'-pyridylamine)pyridinium and diethylammonium. The stoichiometry of the latter salt corresponds to a hydrogen phthalate salt; however, it contains the phthalic acid in its three possible acid-base forms. The hydrogen phthalate ions in the three salts display the two common hydrogen-bonding motifs. One has a very short intramolecular O-H-O hydrogen bond (Intra-H), as seen in the magnesium salt [O
O 2.397 (2) Å]. In the other two salts the hydrogen phthalate ions are connected by hydrogen bonds into infinite chains (Chain-H) with OO distances of 2.460 (1) Å in the diethylamine salt and 2.610 (1) Å in the 2,2'-dipyridylamine salt. This difference in hydrogen-bond pattern was further elucidated by a search in the Cambridge Structural Database for other hydrogen phthalate salts. Intra-H possesses the shortest OO distances and an almost planar hydrogen phthalate ion, whereas the hydrogen phthalate ions show significantly more variation in the Chain-H salts. The two hydrogen-bonding motifs are reflected in the molecular geometry of the hydrogen phthalate ion.
Keywords: crystal packing; geometry; phthalic acid; hydrogen bonding; hydrogen phthalate.
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