Single crystals are typically brittle, inelastic materials. Such mechanical responses limit their use in practical applications, particularly in flexible electronics and optical devices. Here we describe single crystals of a well-known coordination compound-copper(II) acetylacetonate-that are flexible enough to be reversibly tied into a knot. Mechanical measurements indicate that the crystals exhibit an elasticity similar to that of soft materials such as nylon, and thus display properties normally associated with both hard and soft matter. Using microfocused synchrotron radiation, we mapped the changes in crystal structure that occur on bending, and determined the mechanism that allows this flexibility with atomic precision. We show that, under strain, the molecules in the crystal reversibly rotate, and thus reorganize to allow the mechanical compression and expansion required for elasticity and still maintain the integrity of the crystal structure.

Elastically flexible crystals form an emerging class of materials that exhibit a range of notable properties. The mechanism of thermal expansion in flexible crystals of bis(acetylacetonato)copper(II) is compared with the mechanism of molecular motion induced by bending and it is demonstrated that the two mechanisms are distinct. Upon bending, individual molecules within the crystal structure reversibly rotate, while thermal expansion results predominantly in an increase in intermolecular separations with only minor changes to molecular orientation through rotation.

1,4-Diazabicyclo[2.2.2]octane (DABCO) forms well-defined co-crystals with 1,2-diiodotetrafluorobenzene (1,2-DITFB), [(1, 2 DABCO], and 1,3,5-triiodotrifluorobenzene, [(1,3, 2 DABCO]. Both systems exhibited lowerthan-expected supramolecular connectivity, which inspired a search for polymorphs in alternative crystallization solvents. In dichloromethane solution, the Menshutkin reaction was found to occur, generating chloride anions and quaternary ammonium cations through the reaction between the solvent and DABCO. The controlled in situ production of chloride ions facilitated the crystallization of new halogen bonded networks, DABCO− CH 2 Cl[(1,2-DITFB)Cl] (zigzag X-bonded chains) and (DABCO−CH 2 Cl) 3 [(1,3,5-TITFB) 2 Cl 3 ]•CHCl 3 (2D pseudo-trigonal X-bonded nets displaying Borremean entanglement), propagating with charge-assisted C−I•••Cl − halogen bonds. The method was found to be versatile, and substitution of DABCO with triethylamine (TEA) gave (TEA-CH 2 Cl) 3 [(1,2-DITFB)Cl 3 ]•4(H 2 O) (mixed halogen bond hydrogen bond network with 2D supramolecular connectivity) and TEA-CH 2 Cl[(1,3,5-TITFB)Cl] (tightly packed planar trigonal nets). The co-crystals were typically produced in high yield and purity with relatively predictable supramolecular topology, particularly with respect to the connectivity of the iodobenzene molecules. The potential to use this synthetic methodology for crystal engineering of halogen bonded architectures is demonstrated and discussed.

In this article, we report the crystal structures of five halogen bonded co-crystals comprising quaternary ammonium cations, halide anions (Cl − and Br − ), and one of either 1,2-, 1,3-, or 1,4-diiodotetrafluorobenzene (DITFB). Three of the co-crystals are chemical isomers: 1,4-DITFB-[TEA-CH 2 Cl]Cl, 1,2-DITFB[TEA-CH 2 Cl]Cl, and 1,3-DITFB[TEA-CH 2 Cl]Cl (where TEA-CH 2 Cl is chloromethyltriethylammonium ion). In each structure, the chloride anions link DITFB molecules through halogen bonds to produce 1D chains propagating with (a) linear topology in the structure containing 1,4-DITFB, (b) zigzag topology with 60°angle of propagation in that containing 1,2-DITFB, and (c) 120°angle of propagation with 1,3-DITFB. While the individual chains have highly distinctive and different topologies, they combine through π-stacking of the DITFB molecules to produce remarkably similar overall arrangements of molecules. Structures of 1,4-DITFB[TEA-CH 2 Br]Br and 1,3-DITFB[TEA-CH 2 Br]Br are also reported and are isomorphous with their chloro/chloride analogues, further illustrating the robustness of the overall supramolecular architecture. The usual approach to crystal engineering is to make structural changes to molecular components to effect specific changes to the resulting crystal structure. The results reported herein encourage pursuit of a somewhat different approach to crystal engineering. That is, to investigate the possibilities for engineering the same overall arrangement of molecules in crystals while employing molecular components that aggregate with entirely different supramolecular connectivity.

Combining [Ni­(phen)3]­I2 or [Ni­(phen)3]­Cl2 (phen = 1,10-phenanthroline) with the iodoperfluorobenzenes (IPFBs), 1,2-, 1,3-, 1,4-diiodotetrafluorobenzene (1,2-, 1,3-, and 1,4-DITFB, respectively), or 1,3,5-triiodotrifluorobenzene (1,3,5-TITFB) resulted in the formation of six different cocrystalline materials featuring halogen-bonded networks encapsulating [Ni­(phen)3]2+ ions. The cocrystals have the general formula [Ni­(phen)3]­[(IPFB) n (X2)­(L) m ]·solvate (n = 2 or 3; X = Cl– or I–; L = halogen-bonded H2O and/or MeOH; solvate = isolated H2O and/or MeOH). The halide ions balance the charge of the metal complexes and simultaneously act as halogen bond acceptors for the electronically polarized iodine atoms of the IPFB donors. The structures display a wide variety of supramolecular motifs in the context of both the aggregation of the metal complexes and the topology and connectivity of the halogen bond networks. The well-known supramolecular “aryl embrace” motifs of [Ni­(phen)3] complexes are present but are structurally compromised to varying degrees in the crystals of [Ni­(phen)3]­[(1,2-DITFB)2I2]·MeOH, [Ni­(phen)3]­[(1,3-DITFB)2I2]·2MeOH, and [Ni­(phen)3]­[(1,3-DITFB)2(H2O)2Cl2]·1.5MeOH. In [Ni­(phen)3]­[(1,3-DITFB)3I2], the [Ni­(phen3]2+ complexes are so thoroughly enclosed in halogen-bonded networks that the metal complexes have no significant supramolecular contact. In contrast, in [Ni­(phen)3]­[(1,4-DITFB)3I2(MeOH)0.5] and [Ni­(phen)3]­[(1,3,5-TITFB)2Cl2] the complexes are arranged in typical aryl embrace motifs (pairwise and 1D chains, respectively), but with adjacent complexes held in closer proximity to each other than they reside in crystals of the pure metal complex. The interplay between the supramolecular chemistry of the halogen-bonded networks and the metal complexes was examined in detail, and the results demonstrate that it is possible to significantly influence the aggregation of metal complexes by encapsulation in different halogen bond networks.