Halogen-bonded zigzag molecular network based upon 1,2-diiodoperchlorobenzene and the photoproduct rctt-1,3-bis(pyridin-4-yl)-2,4-diphenylcyclobutane

The formation of a halogen-bonded zigzag molecular network based upon 1,2-diiodoperchlorobenzene and the head-to-tail photoproduct rctt −1,3-bis(pyridin-4-yl)-2,4-diphenylcyclobutane is reported. The co-crystal is sustained by I⋯N halogen bonds where the photoproduct acts as a linear linker while the donor behaves as a bent two-connected node within the zigzag chain.


Chemical context
A continued area of research within crystal engineering is the design and formation of supramolecular networks that have specific and targeted structures (Yang et al., 2015;Vantomme & Meijer, 2019). While the field is diverse and interdisciplinary, the self-assembly of small molecules to yield purely organic materials continues to be a main focus for materials scientists as well as solid-state chemists . Controlling the overall topology of these assembled supramolecular networks can easily be achieved by the careful selection of both the node and linker groups typified by metalorganic and supramolecular coordination frameworks (Jiang et al., 2018) as well as flexible organic frameworks (Huang et al., 2019). Halogen bonding continues to be a well-established and reliable non-covalent interaction in the formation of these supramolecular networks (Gilday et al., 2015). A continued goal within our research groups has been the design and construction of halogen-bonded molecular solids containing nodes generated by the [2 + 2] cycloaddition reaction (Dunning et al., 2021;Oburn et al., 2020;Sinnwell et al., 2020). In each example, the cyclobutane-based photoproduct accepts IÁ Á ÁN halogen bonds to form these extended solids. These functionalized photoproducts are ideal components, in the formation of these networks, due to the ability to control the number and position of halogen-bond accepting groups coming off the central cyclobutane ring (Gan et al., 2018). Recently, we reported the ability to vary the topology within a pair of halogen-bonded networks by controlling the regiochemistry of the pendant groups (Dunning et al., 2021). In that contribution, the resulting topology was dictated by the regiochemical position of the 4-pyridyl groups around the cyclobutane ring. In particular, the incorporation of the headto-tail photoproduct rctt-1,3-bis(pyridin-4-yl)-2,4-diphenylcyclobutane (ht-PP) or the head-to-head photoproduct rctt-1,2-bis(pyridin-4-yl)-3,4-diphenylcyclobutane resulted in either a linear or zigzag molecular topology, respectively. In both networks, the halogen-bond donor was 1,4-diiodoperchlorobenzene, which acted as a linear linker due to the para-position of the two I-atoms.
Using this as inspiration, a research project was undertaken to exploit the ability of 1,2-diiodoperchlorobenzene (1,2-C 6 I 2 Cl 4 ) to act as a halogen-bond donor  that would result in a similar zigzag structure when combined with ht-PP, a linear node-based photoproduct. To this end, we report here the synthesis and crystal structure of the co-crystal (1,2-C 6 I 2 Cl 4 )Á(ht-PP) that has a zigzag topology due to the ortho-position of the I atoms on the halogen-bond donor. This co-crystal is sustained by IÁ Á ÁN halogen bonds where neighbouring chains pack in a tongue-and-groove-like pattern. These neighbouring chains engage in various ClÁ Á Á interactions to both the phenyl and pyridyl rings on the photoproduct, resulting in a supramolecular two-dimensional sheet.
The various non-covalent interactions were also investigated and visualized by using a Hirshfeld surface analysis (Spackman et al., 2021) mapped over d norm (Fig. 4). The darkest red spots on the surface represent the IÁ Á ÁN halogen bonds while the lighter red spots are the ClÁ Á Á interactions. The ortho-position of the I atoms on the halogen-bond donor makes this molecule behave as a bent two-connecting node, which is required for the formation of a zigzag network.

Figure 1
The labelled asymmetric unit of (1,2-C 6 I 2 Cl 4 )Á(ht-PP). Displacement ellipsoids are drawn at the 50% probability level for non-hydrogen atoms while hydrogen atoms are shown as spheres of arbitrary size.

Figure 2
X-ray crystal structure of (1,2-C 6 I 2 Cl 4 )Á(ht-PP) illustrating the zigzag network held together by IÁ Á ÁN halogen bonds. The determined error in all measured angles is 0.1 . Halogen bonds are represented by yellow dashed lines.

Synthesis and crystallization
Materials and general methods. The solvents reagent grade ethanol (95%), methylene chloride, and toluene were all purchased from Sigma-Aldrich Chemical (St. Louis, MO, USA) and used as received. In addition, 4,6-dichlororesorcinol (4,6-diCl res), 4-stilbazole (SB), and sodium hydroxide pellets were also purchased from Sigma-Aldrich and were used as received. The [2 + 2] cycloaddition reaction was conducted in an ACE Glass photochemistry cabinet using UV radiation from a 450 W medium-pressure mercury lamp. The occurrence and yield of the [2 + 2] cycloaddition reaction was determined by using 1 H Nuclear Magnetic Resonance Spectroscopy on a Bruker Avance 400 MHz spectrometer with dimethyl sulfoxide (DMSO-d 6 ) as the solvent. The halogen-bond donor 1,2-diiodoperchlorobenzene (1,2-C 6 I 2 Cl 4 ) was synthesized utilizing a previously published method (Reddy et al., 2006).
Synthesis and crystallization. The formation of the photoreactive co-crystal (4,6-diCl res)Á(SB) was achieved using a previously published approach (Grobelny et al., 2018). In particular, co-crystals of (4,6-diCl res)Á(SB) were formed by dissolving 50.0 mg of SB in 2.0 mL of ethanol, which was then combined with a separate 2.0 mL ethanol solution containing 24.7 mg of 4,6-diCl res (2:1 molar equivalent). Then the resulting solution was allowed to slowly evaporate. After evaporation of the solvent, the remaining solid was removed and placed between Pyrex glass plates for irradiation. After 20 h of UV exposure, the [2 + 2] cycloaddition reaction occurred with a 100% yield. The formation of ht-PP was confirmed by 1 H NMR (Grobelny et al., 2018) by the complete loss of the olefin peak on SB at 7.57 ppm along with the appearance of a cyclobutane peak at 4.59 ppm (Fig. S1 in the supporting information). The 4,6-diCl res template was then removed by a base extraction with a 5.0 mL of a 0.2 M sodium hydroxide solution that was heated and stirred on a hot plate for 10 minutes. Afterwards, ht-PP was extracted by using three 10 mL aliquots of methylene chloride as the solvent. Then the methylene chloride was removed under vacuum to yield pure ht-PP. The formation of (1,2-C 6 I 2 Cl 4 )Á(ht-PP) was achieved by dissolving 25.0 mg of 1,2-C 6 I 2 Cl 4 in 2.0 mL of toluene and then combined with a 3.0 mL toluene solution containing 19.4 mg of ht-PP (1:1 molar equivalent). Within two days, single crystals suitable for X-ray diffraction were formed upon loss of some of the solvent by slow evaporation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Data collection at low temperature, namely 100 K, was facilitated using a Kryoflex system with an accuracy of 1 K. H atoms were included in the refinement at calculated positions. Hirshfeld surface of (1,2-C 6 I 2 Cl 4 )Á(ht-PP) mapped over d norm illustrating the IÁ Á ÁN halogen bonds and ClÁ Á Á interactions.

Funding information
RHG gratefully acknowledges financial support from Webster University in the form of various Faculty Research Grants. EB acknowledges the Missouri State University Provost Incentive Fund for the purchase of the X-ray diffractometer used in this contribution.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.