Tribenzoatobismuth(III): a new polymorph

A new polymorph (β) was obtained for an active pharmaceutical ingredient, bismuth tribenzoate, [Bi(C6H5CO2)3]. The new β-polymorph is 1.05 times denser than the previously known polymorph [Rae et al. (1998 ▶). Acta Cryst. B54, 438–442]. In the β-polymorph, the Bi atom is linked with three benzoate anions, each of them acting as a bidentate ligand, and these assemblies with C 3 point symmetry can be considered as ‘molecules’. The structure of the β-polymorph has no polymeric chains, in contrast to the previously known polymorph. The ‘molecules’ in the β-polymorph are stacked along [001], so that the phenyl rings of the neighbouring molecules are parallel to each other. Based on the pronounced difference in the crystal structures, one can suppose that two polymorphs should differ in the dissolution kinetics and bioavailability.

A new polymorph () was obtained for an active pharmaceutical ingredient, bismuth tribenzoate, [Bi(C 6 H 5 CO 2 ) 3 ]. The new -polymorph is 1.05 times denser than the previously known polymorph [Rae et al. (1998). Acta Cryst. B54, [438][439][440][441][442]. In the -polymorph, the Bi atom is linked with three benzoate anions, each of them acting as a bidentate ligand, and these assemblies with C 3 point symmetry can be considered as 'molecules'. The structure of the -polymorph has no polymeric chains, in contrast to the previously known polymorph. The 'molecules' in the -polymorph are stacked along [001], so that the phenyl rings of the neighbouring molecules are parallel to each other. Based on the pronounced difference in the crystal structures, one can suppose that two polymorphs should differ in the dissolution kinetics and bioavailability.

Comment
Bismuth tribenzoate, as other bismuth salts, is an active pharmaceutical ingredient, which has anti-infective and analgesic effects on lymphatic tissue and mucous tunic (Kislichenko, 1999), (Goddard et al., 2003). Besides, it can be used as precursor for fine-dispersed powders of the metallic bismuth and its oxides (Mehring, 2007).
While developing a new method of synthesis of the title salt (Timakova et al., 2010), we have discovered, that the powder diffraction pattern of the compound obtained in our experiments I did not match the one calculated based on the single-crystal diffraction data for the known monoclinic polymorph of bismuth tribenzoate, which was prepared by anionic exchange of bismuth triacetate with benzoic acid (Rae et al., 1998). At the same time, the powder diffraction pattern of I agreed well with the powder diffraction pattern reported in a much earlier publication (Hanawalt et al., 1938). We have managed to select a small crystal from the batch of the synthesized compound I of quality suitable for a single-crystal diffraction study. The powder diffraction pattern calculated based on the results of the structure solution using this single-crystal agreed well with the diffraction pattern of the whole polycrystalline batch, thus proving, that the crystal was representative for the whole sample. We report herein the crystal structure of the β-polymorph of bismuth tribenzoate.
The new β-polymorph I is 1.05 times denser than the previously known polymorph II with the space group P2 1 /m (Rae et al., 1998): the density of I at room temperature is 2.098 g/cm 3 , and that of II at 173 K is 1.99 g/cm 3 . The structure of I is isomorphous to the structure of antimony tribenzoate (Rosmann et al., 1995). In I each bismuth atom is linked to three benzoate anions, which act as bidentate ligands (Bi1-O1 distance is 2.254 (5)Å) and Bi1-O2 distance is 2.513 (5)Å). These assemblies with C 3 point symmetry can be considered as `molecules' (Fig. 1). The `molecules' in I are stacked along [0 0 1], so that the phenyl rings of the neighbouring `molecules' are parallel to each other (Fig. 2). The carboxylate group in a ligand is rotated at 13.3 (3)° relative to phenyl ring. The neighbouring `molecules' can be supposed to interact noticeably with each other, as far as one can judge from the intermolecular Bi1···O1 i [symmetry code: (i) x, y, z-1] distances (3.110 (6)Å), which are shorter, than the sum of van der Waals radii of Bi and O (3.67Å; Alcock, 1972). Still, the structure of I has no polymeric chains, in contrast to the II. Phenyl ring of each benzoic anion is placed between two benzoic cations of neighbouring molecules' (Fig. 3). In the structure of II each bismuth atom is also linked to three benzoate ligands, however, O atoms act as bridges between the neighbouring Bi atoms, thus forming infinite polymeric chains along a axis. No individual molecules can be selected in II. Based on the pronounced difference in the crystal structures, one can suppose that two polymorphs should differ in the dissolution kinetics and bioavailability.

Experimental
The title salt was precipitated from bismuth perchloric acid solution (prepared from 217 ml of distilled water, 483 ml of concentrated perchloric acid and 850 g of bismuth oxide, then diluted to 1:10 with water) by benzoic acid with the mole ratio of benzoate-ions to bismuth being equal to 3. The reaction was carried out at 343 K during 1 h. The obtained white powder of bismuth tribenzoate was filtered and washed once with water and dried at room temperature.
The typical particle size in the powder sample was in the range 0.002-0.01 mm, but a few larger crystals could be found. The largest of the selected crystals (0.07× 0.01×0.01 mm) was fixed at a Mitigen MicroMesh holder with cryoil and used for a single-crystal X-ray diffraction study. Powder X-ray diffraction (Stoe Stadi MP, Cu Kα 1 , curved germanium monochromator (111), linear PSD) has proved that the selected crystal was representative for the whole powder sample batch.

Refinement
After the positions of Bi atoms were determined by direct methods, carbon and oxygen atoms could be located from difference Fourier maps one after another in several cycles of refinement. Hydrogen atoms of the aromatic ring were placed geometrically with C-H distance 0.93Å with U iso (H) = 1.2U eq (C). The highest peak at the electron density map is located at 1.23Å distance from Bi1 atom and could be a consequence of the Fourier sum truncation. Fig. 1. An ellipsoid plot of the title compound, showing the atom-numbering scheme and 50% probability displacement ellipsoids. Hydrogen atoms were shown as s small spheres of arbitrary radii and were not labeled for clarity. Symmetry codes: (i) -1-x+y, 1-x, z; (ii) 1-y, 2+x-y, z.

Tribenzoatobismuth(III)
Crystal data [Bi(C 7   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.