Dynamic behaviour in the solid state

One of the more colourful remarks in the history of crystal engineering is “Ein Kristall ist ein chemischer Friedhof” (“a crystal is a chemical graveyard”), a somewhat desperate comment on the reactivity in solids attributed to the late Prof. Lavoslav Ruzička.¹ With one of us being educated as an experimental and the other as a computational solid-state chemist within the last 20 years, we are thoroughly convinced that nothing could be further from the truth. Perhaps the most important early evidence for this is in the observation that certain double bond-containing molecules in crystalline form can undergo a topochemical [2 + 2] photodimerisation. The dimerisation yields a cyclobutane ring whose stereochemistry is pre-determined by the crystal packing of reactant molecules. This reaction appears unavoidable in advancing our understanding of molecular solids in the past 40 years. Crystallographic studies in the 1960s and 1970s led Schmidt to propose his set of topochemical postulates and simultaneously impart the term “Crystal Engineering” with its modern meaning.² However, [2 + 2] photodimerisation is also associated with a landmark discovery in solid-state reactivity: the single crystal diffraction studies by Jones and Thomas in the 1980s established, for the first time, the possibility of single-crystal-to-single-crystal (SCSC) reactivity.³ This discovery demonstrated beyond doubt that transformations of covalent bonds can be compatible with a static appearance of a single crystal and hugely contributed to our understanding of intermolecular reactions and the development of time-resolved X-ray crystallography.

However, the dynamic behaviour of solid-state materials is a much wider area of research, as illustrated by the succinct overview kindly provided for this special issue by Professor John M. Thomas (DOI: 10.1039/C1CE90016A). Consequently, we felt that now, 30 years after this seminal report on the single-crystal-to-single-crystal formation of covalent bonds in an irradiated crystal of 2-benzyl-5-benzylidenecyclopentanone,³ it is the right time to take a look at the landscape and potential new opportunities for solid-state reactivity in the broadest sense. Our own interest in the dynamic behaviour of solids has been greatly inspired and supported by Professor William Jones, and we are taking advantage of this special issue to congratulate him on his 60th birthday. We also make use of this occasion to re-examine the status of research into the dynamic solid state. We believe it is significant that a considerable number of articles in this issue relate to mechanochemistry. Specifically, Adams et al. (DOI: 10.1039/C0CE00020E) report on solvent-free synthesis and characterization of coordination polymers, Cinčić et al. describe the mechanochemical complexation of Schiff bases (DOI: 10.1039/C0CE00421A), Užarević et al. explore the mechanochemical exchange of ligands on a robust organometallic cluster (DOI: 10.1039/C0CE00807A), while Vrdoljak and co-workers utilise grinding for solid-state transformations involving polyoxometallates (10.1039/C0CE00927J). The synthesis of solid solutions of coordination polymers through mechanochemical methods is addressed by Lusi et al. (DOI: 10.1039/C1CE05164D), while two articles from Boldyreva and co-workers (DOI: 10.1039/C1CE05178D, DOI: 10.1039/C1CE05189J) investigate transformations between polymorphs of organic compounds using mechanochemical methods.

Solid-state photochemistry is addressed in articles by Bakowicz et al. on intramolecular SCSC Norrish–Yang reactions (DOI: 10.1039/C0CE00795A), [4 + 4] photodimerisation of anthracenes by Zouev et al. (DOI: 10.1039/C0CE00739K), while Shang et al. have explored the solid-state dimerisation of lithium carboxylates using γ-radiation (DOI: 10.1039/C0CE00810A). How the solid state can control the formation of non-conventional or unexpected products is described in articles by Garcia et al. (DOI: 10.1039/C0CE00860E) in the context of halogen-bonded systems, and by Grobelny et al. (DOI: 10.1039/C0CE00842G) in the context of pharmaceutical cocrystals. Hamad and Catlow have theoretically addressed the solution organisation and dynamics of the polymorphic glycine system (DOI: 10.1039/C0CE00877J). Finally, reactivity involving solid and gas phases has been addressed by Bacchi et al. (DOI: 10.1039/C0CE00816H) by Mínguez Espallargas et al. (DOI: 10.1039/C1CE05222E) in the context of coordination compounds, by Meazza et al. for the formation of trihalide complexes (DOI: 10.1039/C1CE05050h) and by Halasz and Vančik (DOI: 10.1039/C0CE00870B) in solid-state dimerisation of organic molecules.

We feel that the exciting contributions to this issue illustrate the scope of this research field and demonstrate a breadth of techniques, materials and reaction types.

References

1. J. D. Dunitz, Trans. Am. Crystallogr. Assoc., 1984, 20, 1.
2. G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647.
3. (a) W. Jones, H. Nakanishi, C. R. Theocharis and J. M. Thomas, J. Chem. Soc., Chem. Commun., 1980, 610; (b) H. Nakanishi, W. Jones, J. M. Thomas, M. B. Hursthouse and M. Motevalli, J. Chem. Soc., Chem. Commun., 1980, 611.

---