The crystal structures of three isomorphous trimesic acid channel inclusion complexes of polyhalide ions. A second example of triple catenation in the solid state

Abstract

The three polyhalide complexes of trim esic acid (benzene-1, 3, 5-tricarboxylic acid; TMA) with compositions TMA.0.7H 2 O. 0.09 HI 5 (I), TMA. 0.7H 2 O. 0.103 HBr 5 (II), TMA. 0.7H 2 O. 0.167 HIBr 2 (III) are isomorphous (for example, for I, a = 21.945(7), b = 17.917(6), c = 16.711(6) Å, the space group is 1222, Z = 24; II and III have very similar cell dimensions). Crystal structure analysis (four-circle diffractometer, graphite-monochro-mated MoKa: I, 2273reflexions, R = 12.2% ; III, 1747 reflexions, R = 12.4%) shows that all three have the same TMA fram ework, which consists of two mutually-perpendicular infinite hexagonal planar networks of hydrogen-bonded TMA molecules. The two networks are parallel to (110) and (11̄0) respectively and are mutually triply catenated. The arrangement is such as to leave channels of square cross section along the z -direction, which contain the linear polyhalide ions. I an d II have pentahalide ions incommensurable with the TMA framework; the diffuse scattering pattern from I allows the structure of the I - 5 ion to be deter­mined as consisting of strongly interacting . . . I 2 - - - I - I 2 . . . units and it is inferred that Br - 5 has a similar structure. Compound III has IBr - 2 ions commensurable with the TMA framework, but some difficulties remain in the interpretration of these results. The TMA framework has additional voids in the region about 1/2, 0, 1/2 and these are occupied by the water molecules ( ca . sixteen per unit cell) in disordered fashion. The protons required to balance the charges of the polyhalide ions are pre­sumably attached to some of these water molecules. The materials are characterized as crystalline acid-molecule complexes.