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We explore the halogen bond acceptor potential of the morpholinyl oxygen atom in the synthesis of cocrystals involving organic and metal−organic units, by using N-aminomorpholine either as a potential halogen bond acceptor or as a reagent to insert a morpholine moiety into larger organic and copper(II)-based metal−organic building blocks. Challenged against four well-known halogen bond donor molecules differing in binding geometry and composition, these three morpholine containing units have yielded a total of seven previously not reported cocrystals, of which six (86%) exhibited the formation of I···O or Br···O halogen bonds to the morpholinyl oxygen atom. The results illustrate the possibility to systematically insert and use a morpholine group as an efficient halogen bond acceptor into organic and metal−organic structures, thus enabling cocrystal formation.

True trimorphic cocrystals, i.e. multi-component molecular crystals of identical composition that exhibit three polymorphic structures, are exceedingly rare and so far no halogen-bonded cocrystal system has been reported to exhibit trimorphism. Here we describe a unique example of a trimorphic cocrystal exhibiting both hydrogen and halogen bonds in which the differences between polymorphs reveal their orthogonality, evident by the apparently independent variation of well-defined hydrogen- and halogen-bonded motifs. peerReviewed

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.

We provide a systematic investigation of the structures and composition of halogen-bonded cocrystals involving the bent (meta) halogen bond donor 1, 3-diiodotetrafluorobenzene (1, 3-tfib), in comparison to analogous systems based on its linear (para) 1, 4-isomer. In particular, whereas 1, 4-tfib has now been established as an archetypal, ubiquitous example of a halogen bond donor, its meta-isomer has remained almost completely unexplored. This study investigates the structures of 14 new cocrystals of 1, 3-tfib and 9 new, analogous cocrystals of 1, 4-tfib, with 9 monotopic and 7 ditopic nitrogen-based aliphatic and aromatic halogen bond acceptors. Combined with previously reported structures of cocrystals of 1, 4-tfib, the results of our study indicate that whereas halogen bond lengths and the thermal stability of the investigated cocrystals are similar, the change in molecular shape between the two halogen bond donors brings about important consequences in supramolecular architectures and preferred stoichiometric compositions of otherwise analogous cocrystals. These preliminary results suggest that the principal factor responsible for such differences might be the different abilities of the cocrystals based on 1, 3-tfib and 1, 4-tfib to form close-packed structures.

We demonstrate a design for halogen-bonded metal–organic cocrystals involving coordinatively unsaturated square-planar Cu(II) and Ni(II) centers, by utilizing a Schiff base ligand whose pendant acetyl group enables halogen bonding. The robustness of this design is evident by the assembly of a large family of eight cocrystals based on zero-, one-, and two-dimensional halogen bonded architectures involving mono- or ditopic halogen bond donors based on iodine or bromine.