3.5.1. Bilastine Anhydrates

Three anhydrous polymorphs of BL were obtained. Form I was solved by single crystal XRD. This anhydrate crystallizes in the monoclinic space group *P*21/*c* with Z = 4. This structure is zwitterionic as the piperidinyl N atom is protonated, revealing a proton transfer from the carboxylic group. For an amphoteric molecule with measured p*K*a values of 4.06 (carboxylic acid) and 9.43 (piperidine-N), application of the p*K*a rule of three would suggest that proton transfer from the carboxylic acid to the piperidine-N is essentially complete rendering BL zwitterionic. BL molecules are linked by hydrogen bonds (distance: 1.567 Å) involving the carboxylate oxygen of one BL molecule and the piperidinium nitrogen of another BL in an alternated mode, forming corrugated layer structures (Figure 9). π-π stackings are also observed between the benzene rings of the benzimidazoles (Figure 10). The observed lateral offset stack is 2.61 Å [29], the centroid to centroid distance, d(π··· π) is 4.25 Å and the interplanar distance is 3.35 Å.

**Figure 9.** Carboxylate/piperidinium H-bond interactions (NH···O distance in Å) between BL molecules of Form I.

**Figure 10.** π-π stacking (centroid–centroid distance in Å) observed in Form I of BL.

Attempts to grow quality crystals of forms II and III were unsuccessful. Crystals of form III were grown and observed under the microscope. The extinction of a crystal is observed in Figure 11; however, it was impossible to cut the extreme of this crystal in order to determine its structure by SCXRD.

**Figure 11.** Photomicrographs of form III while looking under (**a**) unpolarized light and (**b**) polarized light on a circular microscope stage (**c**) Extinction of the crystal is observed at the optimum angle to display maximum birefringence.

Usually form III crystallized as macles, as it is shown in Figure 12.

**Figure 12.** Photomicrographs of macles of form III observed under (**a**) unpolarized light and (**b**) polarized light microscope.

Thus, the resolution of the crystal structure of form III was achieved by using the direct space methodology using synchrotron X-ray powder diffraction data obtained in the high-resolution powder diffraction end station of the MSPD beam line in Alba. The powder diffraction pattern of form III was indexed to a monoclinic cell of about 2682 Å3, and the space group perfectly determined to be *P*21/*c* from the systematic absences. The crystal structure was solved with the direct-space strategy TALP, and the refinement was performed by the Rietveld method. Figure 13 depicts the final Rietveld plot.

**Figure 13.** Final Rietveld plot for the crystal structure refinement of form III. Agreement factor: Rwp = 2.4%. The plot shows the experimental powder XRD profile (red+marks), the calculated powder XRD profile (black solid line), and the difference profile (blue, lower line). Tick marks indicate peak positions.

Form III crystallizes in the same space group *P*21/*c* with four molecules in the unit cell. BL molecules in Form III are also linked by hydrogen bonds (distance: 1.518 Å) involving the carboxylate oxygen and the piperidinium nitrogen but not in the alternated mode shown in Form I. In Form III, all BL molecules are orientated in the same way. Therefore, a remarkable difference between both polymorphs is the different orientation of the selfassembled BL chains. While in Form I BL molecules are interdigitated, in Form III, they are organized as cascade layers (Figure 14). Moreover, in Form I benzimidazoles interact via π-π stacking while in Form III, the same benzimidazoles interact via CH-π with one hydrogen of the piperidinyl group (distance to the centroid: 2.93 Å, distance to the plane: 2.64 Å and H: 1.27 Å) and with one hydrogen of the methyl group (distance to the centroid: 3.41 Å, distance to the plane: 2.45 Å and H: 2.37 Å) (Figure 15).

**Figure 14.** Comparison between the crystal structures of (**a**) form I (corrugated layer) and (**b**) form III (cascade layer) of BL.

**Figure 15.** CH-π stacking (CH–centroid distances in Å) observed in Form III of BL.

The powder diffraction pattern of form II was indexed to a monoclinic cell (a = 28.3374 Å, b = 9.9384 Å, c = 19.7547 Å, β = 109.2696◦) of about 5251.8 Å3 and the space group perfectly determined to be *P*21/*c* from the systematic absences (Figure 16). Thus far, we have not managed to solve its crystal structure by using the direct space methodology, being an additional difficulty the presence of two different BL molecules in the asymmetric unit. Further work is still in progress.

**Figure 16.** Pawley pattern matching plot of form II. Agreement factor: Rwp = 1.3%. The plot shows the experimental powder XRD profile (red+marks), the calculated powder XRD profile (black solid line) and the difference profile (blue, lower line). Tick marks indicate peak positions.
