**4. Conclusions**

Knowledge of even highly unstable and transient solvates is crucial for understanding the formation of a specific solid form and for developing pharmaceuticals and needs to be considered. Solvates that desolvate readily as soon as they are removed from the mother liquor can easily be overlooked in polymorph screening programs, and wrong conclusions concerning the solvent effects on the nucleation of individual solid forms may result. The current study clearly indicates the existence of two elusive chloroform solvates of BL, the heterosolvate S3CHCl3-H2O and the monosolvate SCHCl3, both transforming into solvate SCHCl3-H2O when being exposed to ambient conditions, outside the solution of crystallization. Thus, three chloroform solvates of BL with diverse stoichiometry have been discovered. Solvate formation may be easily rationalized based on a mismatch of H-bonding donor or acceptor groups. The reason for solvate formation in BL is the lack of H-bonding donor groups. The analyses of their crystal structures when comparing with the anhydrous forms shows that SCHCl3-H2O, the more stable solvate, incorporates one water molecule and one chloroform molecule to satisfy the acceptor capacities of the benzimidazole and the carboxylate, respectively. Thus, this study adds more data about the structural reasons for hydrate formation. Interesting and novel, solvate SCHCl3-H2O can serve as a useful intermediate towards any of the three anhydrous forms of BL, provided the right conditions are chosen.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/cryst11040342/s1. Details of the solid form screening are shown in Tables S1–S14. Tables S15 and S16 summarize the crystal data of all the crystal structures solved in the present study for BL from SCXRD and from PXRD, respectively.

**Author Contributions:** Conceptualization, C.P.; methodology, C.P., A.P. and A.B.; software, C.P., M.F.-B. and O.V.; validation, C.P., A.P. and A.B.; formal analysis, C.P., A.P., A.B., M.F.-B. and O.V.; investigation, C.P., A.P. and A.B.; data curation, C.P., A.P. and A.B.; writing—original draft preparation, C.P.; writing—review and editing, C.P.; supervision, C.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** ALBA synchrotron is acknowledged for the provision of beamtime.

**Conflicts of Interest:** The authors declare no conflicts of interest.
