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Supramolecular reactivity in the solid state: Step-wise assembly of ternary cocrystals through hydrogen and halogen bonding

In the field of crystal engineering, cocrystals have been among the most pursued topics in the last two decades.[1] Devising rational strategies for systematically assembling them has been one of the greatest challenges, even more so for cocrystals consisting of more than two components. Whereas both mechanochemical assembly and disassembly of the binary cocrystals have been studied[2, 3], the supramolecular reactivity of ternary cocrystals is still almost unexplored [4]. Given our interest in the orthogonality of hydrogen and halogen bonding in the context of supramolecular chemistry, we focused our interest on the combination of thioureas, phosphine oxides and various halogen bond donors. These types of molecules are known to form binary cocrystals through hydrogen (thiourea and phosphine oxide) or halogen bonds (thioureas or phosphine oxides with different halogen bond donors), and we hypothesized that the combination of all three might yield ternary cocrystals. Our initial studies revealed successful formation of a number of ternary cocrystals, primarily sustained by N−H···O hydrogen bonds between thioureas and phosphine oxides and C−I···S halogen bonds between thioureas and halogen bond donors. Next, reactivity studies were performed in the following way: First, for a given set of three components A, B, C, each two were ground together to probe for the formation of binary cocrystals AB, BC and AC. Subsequently, each of the resulting mixtures or cocrystals was ground with the third component from the set to probe for the formation of ternary solid ABC. These experiments revealed a spectrum of possible outcomes, from component sets where all of AB, BC, AC and ABC were successfully formed, to such cases where only binary cocrystal AB was detected, with all other combinations only yielding various mixtures. Structures of the obtained binary and ternary cocrystals, as well as the results of the mechanochemical experiments, will be discussed. References: [1] Aakeröy, C. B. Acta Cryst. 2015, B71, 387-391. [2] Cinčić, D., Friščić, T., Jones, W. J. Am. Chem. Soc., 2008, 130, 7524-7525. [3] Karki, S., Friščić, T., Jones, W. CrystEngComm, 2009, 11, 470-481. [4] Only briefly mentioned in: Bolla, G., Nangia, A. Chem.Commun. 2015, 11, 470-481.

crystal engineering, mechanochemistry, ternary co-crystals, hydrogen bonding, halogen bonding

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