An I 6 2 − anion in the crystal structure of theophyllinium triiodide monohydrate, C₇H₁₁I₃N₄O₃

Introduction

The term polyiodides – formerly named periodides [6] – describes a class of compounds that is defined as the anionic part of a salt-type structure, which fullfills the general formula I 2 m − n n − (n = 2–5, m = integer). Almost all of the currently known polyiodides consist of I⁻, I 3 − and I₂ subunits. These two simple ions and the I₂ molecule show a strong tendency to form extended, halogen bonded structures [7], [8], [9], [10], [11]. Polyiodides are of interest not only because of their structures, but also because of reported applications. Some important applications are listed in our preceeding article [12]. We have already shown that planar, nitrogen-based heterocyclic cations support the formation of iodine rich compounds [13, 14]. Especially using different methylxanthinium cations, some polyiodide salts have already been characterised by us [15, 16] and others [17], [18], [19]. The bonding properties of short-chain polyiodides like I 4 2 − [12, 20, 21] and especially I 6 2 − [22], [23], [24] are still of general interest.

Structural comments

The asymmetric unit of the title structure contains two crystallographically independent theophyllinium cations, one I 3 − anion in a general position, two halves of I 3 − anions on inversion centers (Wyckoff sites 1a and 1e) and two water molecules in general positions (see the figure). Thus, the overall composition can be summerized as theophyllinium triiodide monohydrate [systematic name: 1,3-dimethyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-9-ium triiodide—water (1/1)].

Each of the two crystallographically independent theophyllinium cations forms a dimer with a symmetry related theophyllinium cation by NH⃛O hydrogen bonds (see the left part of the figure). These dimers are furthermore connected via O–H⃛O hydrogen bonds between the water molecules and the cations to furnish a wavy layer. The bond lengths and angles within the cations as well as the geometric parameters in the various hydrogen bonds are in the expected ranges [16]. The two I 3 − anions located on inversion centers (see the middle section of the Figure; I2−I1−I2′; I4−I3−I3″; ′ = 1−x 1−y, −z, ″ = 2−x, −y, −z) are not connected to any other triiodide anions and feature only weak hydrogen bonds to water molecules and the cations. The triiodide anion located in a general position is attached to its symmetry-related triiodide anion (inversion symmetry; Wyckoff site 1b) (see middle part of the figure; I7⃛I7‴ distance = 3.6605(3) Å; symmetry operation: 2−x, 2−y, 1−z). This secondary I⃛I bonding interaction is weak, but significantly shorter than any van der Waals distances in various scales [25]. To classify this secondary halogen bond in more detail a comparison with some literature known I 6 2 − anions is needed. Structures of catenated triodides have not been considered for this comparison. To the best of our knowledge, the shortest halogen bonded triiodide—triiodide distance of 3.5017(2) Å is found in a structure, which is characterised by strong charge supported hydrogen bonds between the coutercation and both terminal I atoms of the I 6 2 − anion [22]. A distance of 3.6317(4) Å is found for the I 6 2 − anion trapped in the van der Waals void of a hydrogen bonded framework [23]. Recently even in the case of a distance of 3.848 Å [24] a halogen bonding interaction was discussed by the authors. Thus the I7⃛I7‴ distance of 3.6733(12) Å in the title structure is in the upper part of the scope of such halogen bonds, but we think that the motif must be discussed as another I 6 2 − anion. In detail, each triiodide part within the I 6 2 − anion shows a bonding angle of 175.08(2)° and the angle between the two I 3 − ions (I8−I7⃛I7‴) is 160.74(1)°. In general all I−I bonding parameters of the triiodide anions are in the expected ranges (I1−I2 = 2.9307(2) Å; I3−I4 = 2.9296(2) Å; I5−I6 = 2.9463(2) Å; I6−I7 = 2.9183(2) Å) [13].

A packing diagram with a view against the c axis is shown in the right part of the figure. Classical hydrogen bonding interactions only occur in the aforementioned layers consisting of the cations and the water molecules. These layers are stacked along the b axis. The hydrogen bonded layers are connected via van der Waals interactions only (green line in the right part of the figure). The triiodide anions as well as the I 6 2 − anions fill the voids within and between these layers. One of the triiodide anions (right part of the figure, I4−I3−I4″) and the I 6 2 − anions are oriented along [110], whereas the triiodide anion I2−I1−I2′ is almost oriented along [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].

Conclusion

Recently we have shown that hydrogen bonded motifs constructed by theophyllinium cations form layered and more complex frameworks, that are able to stabilize interesting couter anions [[26] and references cited there]. The title structure is one more example to show the performance of the theophyllinium cation to act as a tecton in crystal engineering.