The structures of two anhydrous salt phases of theophylline, namely 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium tetrafluoroborate, C7H9N4O2(+)·BF4(-), and 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium chloride, C7H9N4O2(+)·Cl(-), are reported together with the structures of two monohydrate salt forms, namely 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium chloride monohydrate, C7H9N4O2(+)·Cl(-)·H2O, and 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium bromide monohydrate, C7H9N4O2(+)·Br(-)·H2O. The monohydrate structures are mutually isostructural, with the cations and anions lying on crystallographic mirror planes (Z' = ½). The main intermolecular interaction motif is a hydrogen-bonding network in the same mirror plane. The tetrafluoroborate structure is based on planar hydrogen-bonded theopylline cation dimers; the anions interact with the dimers in a pendant fashion. The anhydrous chloride structure has Z' = 2 and in contrast to the other species it does not form planar hydrogen-bonded constructs, instead one-dimensional chains of cations and anions propagate parallel to the crystallographic c direction. An earlier report claiming to describe an anhydrous structure of theophylline hydrochloride is re-examined in light of these results. It is concluded that the earlier structure has been reported in the wrong space group and that it has been chemically misidentified.
Keywords: 1,3-dimethyl-2,6-dioxo-7H-purin-9-ium; crystal structure; hydrogen bonding; theophylline salts.