Experimental and Theoretical Investigation of Structures, Stoichiometric Diversity, and Bench Stability of Cocrystals with a Volatile Halogen Bond Donor

We present a combined experimental and theoretical study of the structures and bench stability of halogen-bonded cocrystals involving the volatile halogen bond donor octafluoro-1,4-diiodobutane, with phenazine and acridine as acceptors. Cocrystallization experiments using mechanochemistry and solution crystallization revealed three chemically and structurally distinct cocrystals. Whereas only one cocrystal form has been observed with acridine, cocrystallization with phenazine led to two stoichiometrically different cocrystals, in which phenazine employs either one or two nitrogen atoms per molecule as halogen bond acceptor sites. Cocrystal stability was evaluated experimentally by simultaneous thermogravimetric analysis and differential thermal analysis or differential scanning calorimetry, real-time powder X-ray diffraction monitoring of cocrystals upon storage in open air, and theoretically by using dispersion-corrected periodic density functional theory. The use of real-time powder X-ray diffraction enabled the comparison of rates of cocrystal decomposition, and the observed trends in cocrystal stability were reproduced by the ranking of theoretically calculated cocrystal decomposition enthalpies. Whereas all cocrystals eventually lose the volatile halogen bond donor upon storage in open air or by heating, these experimental and theoretical studies show that the cocrystal of acridine is the most stable, in agreement with its more basic properties. The stoichiometric variations of the phenazine cocrystal also exhibit a notable difference in stability, with the cocrystal containing the halogen bond acceptor and donor in a 1:1 stoichiometric ratio being of particularly low stability, decomposing in open air within minutes.