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In situ PXRD monitoring the mechanosynthesis of metal-organic halogen-bonded cocrystals

In the last decade, in situ powder X-ray diffraction (PXRD) monitoring of mechanochemical reactions has become a prominent technique for studying the course and mechanisms of organic and metal-organic solid formation [1]. Following our previous study on mechanochemical syntheses of metal-organic halogen-bonded cocrystals [2], in this work we have investigated mechanosynthesis of cocrystals containing CoCl2bzpy2 (bzpy = 2- benzoylpyridine) and a halogen bond donor, 1, 4- diiodotetrafluorobenzene (14tfib), by in situ PXRD. First, we performed experiments in solution, and have unexpectedly obtained three different products: two cocrystals with different metal- organic unit isomers, trans- (CoCl2bzpy2)(14tfib)2 and cis-(CoCl2bzpy2) (14tfib)2, and a cocrystal of 1:1 stoichiometry, cis-(CoCl2bzpy2)(14tfib). In order to determine whether single phases could be prepared, mechanosynthesis of obtained cocrystals was studied by in situ PXRD using synchrotron X- ray radiation. Three different liquid-assisted grinding experiments were monitored: grinding of CoCl2bzpy2 and 14tfib in the 1:2 molar ratio, one- pot grinding of CoCl2·6H2O, bzpy and 14tfib in the 1:2:2 molar ratio and grinding of CoCl2bzpy2 and 14tfib in the 1:1 molar ratio. All three monitored reactions revealed presence of a cocrystal with cis isomer as an intermediate and fast conversions to a final, thermodynamically more stable product, trans-(CoCl2bzpy2)(14tfib)2 , in less than 10 min. Single crystal X-ray diffraction experiments reveal that dominant supramolecular interactions in all obtained solids are I···Cl halogen bonds. In the trans- cocrystal halogen bonds form 2D networks while in cis- cocrystals 1D chains are formed. [1] T. Friščić et al., Nat. Chem., 2013, 5, 66. [2] K. Lisac and D. Cinčić, CrystEngComm., 2018, 20, 5955.

metal-organic halogen-bonded cocrystals, mechanochemistry, in situ PXRD monitoring

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