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Application of thermal analysis and Raman microscopy in the study of cadmium(II) coordination polymers’ flexibility

Mechanical properties of crystalline materials are closely related with their structure and their potential applications in various areas of industry. Crystals are generally considered to be stiff and inflexible, and can crack or break due to the action of mechanical force. [1] By studying the crystals of organic compounds, it was established that the application of external mechanical force can lead to plastic or elastic deformation of a crystal. [2] Elastic materials undergo reversible bending (returning to their initial position when mechanical force is removed) upon applied mechanical stimuli, while plastic materials exhibit a permanent change in their shape (irreversible deformation). In the last few years, research on the mechanical flexibility of metal-organic crystalline solids has been carried out, but it is still, compared to crystals of organic compounds, much less explored and understood. As a significant contribution to the hitherto poorly researched mechanical flexibility of metal-organic crystals, the most recent research on cadmium(II) coordination polymers’ flexibility should be mentioned. [3–6] By systematic study of those crystals, different flexible responses upon application of mechanical force (from elastic, elastic→plastic up to plastic) was established. The crystal structure of coordination polymers is based on coordination bonds and much weaker non-covalent interactions and our goal was to elucidate the relative role and importance of these two features on mechanical properties of the crystals. Detailed structural studies revealed that small differences in the strength and directionality of non- covalent interactions can significantly impact on type and degree of mechanical response of such crystals. In order to examine possibilities of the thermal analysis methods and Raman microscopy in the study of the crystals with plastic response, we prepared coordination polymers of the cadmium(II) halides with 3-chloropyridine (3-Clpy) and 3-bromopyridine (3-Brpy). The crystals of required morphology (needle-like) of the coordination polymers were prepared by a liquid diffusion method. The effect of the bending on the strength of the intermolecular interactions was examined by Raman spectroscopy are correlated with results obtained by means of thermal analysis methods (TGA, DSC and Hot stage microscopy). [1] A. Worthy, A. Grosjean, M. C. Pfrunder, Y. Xu, C. Yan, G. Edwards, J. K. Clegg, J. C. McMurtrie, Nature Chem. 2018, 10, 65. [2] S. Saha, M. K. Mishra, C. M. Reddy, G. R. Desiraju, Acc. Chem. Res. 2018, 51, 2957. [3] M. Đaković, M. Borovina, M. Pisačić, C. B. Aakeröy, Ž. Soldin, B.‐M. Kukovec, I. Kodrin, Angew. Chem. Int. Ed. 2018, 57, 14801. [4] M. Pisačić, I. Biljan, I. Kodrin, N. Popov, Ž. Soldin, M. Đaković, Chem. Mater. 2021, 33, 3360. [5] M. Pisačić, I. Kodrin, I. Biljan, M. Đaković, CrystEngComm 2021, 23, 7072. [6] M. Pisačić, I. Kodrin, A. Trninić, M. Đaković, Chem. Mater. 2022, 34, 2439.

cadmium(II) coordination polymers, thermal methods, Raman microscopy, flexible crystals

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