Co-crystal sustained by π-type halogen-bonding inter-actions between 1,4-di-iodo-perchloro-benzene and naphthalene.

The formation and crystal structure of a co-crystal based upon 1,4-di-iodo-perchloro-benzene (C6I2Cl4) as the halogen-bond donor along with naphthalene (nap) as the acceptor is reported. The co-crystal [systematic name: 1,2,4,5-tetra-chloro-3,6-di-iodo-benzene-naphthalene, (C6I2Cl4)·(nap)] generates a chevron-like structure that is held together primarily by π-type halogen bonds (i.e. C-I⋯π contacts) between the components. In addition, C6I2Cl4 also inter-acts with the acceptor via C-Cl⋯π contacts that help stabilize the co-crystal. Within the solid, both aromatic components are found to engage in offset and homogeneous face-to-face π-π stacking inter-actions. Lastly, the halogen-bond donor C6I2Cl4 is found to engage with neighboring donors by both Type I chlorine-chlorine and Type II iodine-chlorine contacts, which generates an extended structure.

Halogen-bonded co-crystal containing 1,3-di-iodo-perchloro-benzene and the photoproduct rtct-tetra-kis-(pyridin-4-yl)cyclo-butane resulting in a zigzag topology.

The formation and crystal structure of a zigzag network held together by I⋯N halogen bonds is reported. In particular, the halogen-bond donor is 1,3-di-iodo-perchloro-benzene (C6I2Cl4 ) while the acceptor is the photoproduct rtct-tetra-kis-(pyridin-4-yl)cyclo-butane (TPCB). Curiously, within the resulting co-crystal (C6I2Cl4 )·(TPCB), the photoproduct accepts only two halogen bonds between neighbouring 4-pyridyl rings and as a result behaves as a bent two-connected node rather than the expected four-connected centre. In addition, the photoproduct, TPCB, is also found to engage in C-H⋯N hydrogen bonds, forming an extended zigzag chain.

Halogen-bonded zigzag mol-ecular network based upon 1,2-di-iodo-perchloro-benzene and the photoproduct rctt-1,3-bis-(pyridin-4-yl)-2,4-di-phenyl-cyclo-butane.

The formation and crystal structure of a zigzag mol-ecular network held together by I⋯N halogen bonds is reported. In particular, the halogen-bond donor is 1,2-di-iodo-perchloro-benzene (1,2-C6I2Cl4 ) while the acceptor is a head-to-tail photoproduct, namely rctt-1,3-bis-(pyridin-4-yl)-2,4-di-phenyl-cyclo-butane ( ht -PP). In this co-crystal (1,2-C6I2Cl4 )·( ht-PP), the donor acts as a bent two-connected node while the acceptor behaves as a linear linker to form the extended solid. Neighbouring chains pack in a tongue-and-groove-like pattern that engage in various Cl⋯π inter-actions to both the phenyl and pyridyl rings resulting in a supra-molecular two-dimensional sheet.

Controlling Thermal Expansion in Supramolecular Halogen-Bonded Mixed Cocrystals through Synthetic Feed and Dynamic Motion.

Control over thermal expansion (TE) behaviors in solid materials is often accomplished by modifying the molecules or intermolecular interactions within the solid. Here, we use a mixed cocrystal approach and incorporate molecules with similar chemical structures, but distinct functionalities. Development of mixed cocrystals is at a nascent stage, and here we describe the first mixed cocrystals sustained by one-dimensional halogen bonds. Within each mixed cocrystal, the halogen-bond donor is fixed, while the halogen-bond acceptor site contains two molecules in a variable ratio. X-ray diffraction demonstrates isostructurality across the series, and SEM-EDS shows equal distribution of heavy atoms and similar atomic compositions across all mixed cocrystals. The acceptor molecules differ in their ability to undergo dynamic motion in the solid state. The synthetic equivalents of motion capable and incapable molecules were systematically varied to yield direct tunabililty in TE behavior.

Thermal Expansion Properties and Mechanochemical Synthesis of Stoichiometric Cocrystals Containing Tetrabromobenzene as a Hydrogen- and Halogen-Bond Donor.

The solution and mechanochemical synthesis of two cocrystals that differ in the stoichiometric ratio of the components (stoichiometric cocrystals) is reported. The components in the stoichiometric cocrystals interact through hydrogen or hydrogen/halogen bonds and differ in π-stacking arrangements. The difference in structure and noncovalent interactions affords dramatically different thermal expansion behaviors in the two cocrystals. At certain molar ratios, the cocrystals are obtained concomitantly; however, by varying the ratios, a single stoichiometric cocrystal is achieved using mechanochemistry.

Square network based upon the molecular salt of the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane and the sulfate anion.

The formation and crystal structure of a hydrated molecular salt that results in a square network is reported. The crystalline solid is based upon the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane (4H-rtct-TPCB)4+ along with two sulfate anions (SO42-) and eight waters of hydration, namely, 4,4',4'',4'''-(cyclobutane-1,2,3,4-tetrayl)tetrapyridinium bis(sulfate) octahydrate, C24H24N44+·2SO42-·8H2O. The fully protonated photoproduct acts as a four-connecting node within the square network by engaging in four charge-assisted N+-H...O hydrogen bonds to the sulfate anion. The observed hydrogen-bonding pattern in this square network is akin to T-silica, which is a metastable form of SiO2. The included water molecules and sulfate anions engage in numerous O-H...O hydrogen bonds to form various hydrogen-bonded ring structures.

Controlling Topology within Halogen-Bonded Networks by Varying the Regiochemistry of the Cyclobutane-Based Nodes.

The formation of a pair of extended networks sustained by halogen bonds based upon two regioisomers of a photoproduct, namely rctt-1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane (ht-PP) and rctt-1,2-bis(4-pyridyl)-3,4-bis(phenyl)cyclobutane (hh-PP), that have varied topology is reported. These networks are held together via I⋯N halogen bonds between the photoproduct and the halogen-bond donor 1,4-diiodoperchlorobenzene (C6I2Cl4). The observed topology in each solid is controlled by the regiochemical position of the halogen-bond accepting 4-pyridyl group. This paper demonstrates the ability to vary the topology of molecular networks by altering the position of the halogen bond acceptor within the cyclobutane-based node.

Honeycomb molecular network based upon a hydrate of 4,6-dichlororesorcinol and the photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane.

The formation of a self-interpenetrated honeycomb molecular network based upon 4,6-dichlororesorcinol (4,6-diCl res), a water molecule, and the photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane (rtct-TPCB) is reported. Interestingly, only three of the four pyridine rings on the central cyclobutane ring are found to engage in O-H...N hydrogen bonds with either the 4,6-diCl res or an included water molecule, resulting in a three-connected net. Notably, the solid (4,6-diCl res)·(rtct-TPCB)·(H2O), C6H4Cl2O2·C24H20N4·H2O, contains channels that run along the crystallographic b axis, which are found to be interpenetrated. Although rtct-TPCB has been employed as a bridging ligand in the formation of numerous metal-organic materials, surprisingly neither the single-component X-ray structure nor any multi-component molecular solids based upon this stereoisomer have been reported previously. Lastly, the single-crystal X-ray structure of the photoproduct rtct-TPCB is also reported.

Supramolecular construction of a cyclobutane ring system with four different substituents in the solid state.

Methods to form cyclobutane rings by an intermolecular [2 + 2] cross-photoreaction (CPR) with four different substituents are rare. These reactions are typically performed in the liquid phase, involve multiple steps, and generate product mixtures. Here, we report a CPR that generates a cyclobutane ring with four different aryl substituents. The CPR occurs quantitatively, without side products, and without a need for product purification. Generally, we demonstrate how face-to-face stacking interactions of aromatic rings can be exploited in the process of cocrystallization and the field of crystal engineering to stack and align unsymmetrical alkenes in CPRs to afford chiral cyclobutanes with up to four different aryl groups via binary cocrystals. Overall, we expect the process herein to be useful to generate chiral carbon scaffolds, which is important given the presence of four-membered carbocyclic rings as structural units in biological compounds and materials science.

Controlling thermal expansion within mixed cocrystals by tuning molecular motion capability.

Controlling thermal expansion (TE) behaviors of organic materials is challenging because several mechanisms can govern TE, such as noncovalent interaction strength and structural motions. Here, we report the first demonstration of tuning TE within organic solids by using a mixed cocrystal approach. The mixed cocrystals contain three unique molecules, two of which are present in variable ratios. These two molecules either lack or exhibit the ability to undergo molecular motion in the solid state. Incorporation of higher ratios of motion-capable molecules results in larger, positive TE along the motion direction. Addition of a motion-incapable molecule affords solids that undergo less TE. Fine-tuned TE behavior was attained by systematically controlling the ratio of motion-capable and -incapable molecules in each solid.

Crystal structure and photoreactivity of a halogen-bonded cocrystal based upon 1,2-diiodoperchlorobenzene and 1,2-bis(pyridin-4-yl)ethylene.

The formation of a photoreactive cocrystal based upon 1,2-diiodoperchlorobenzene (1,2-C6I2Cl4) and trans-1,2-bis(pyridin-4-yl)ethylene (BPE) has been achieved. The resulting cocrystal, 2(1,2-C6I2Cl4)·(BPE) or C6Cl4I2·0.5C12H10N2, comprises planar sheets of the components held together by the combination of I...N halogen bonds and halogen-halogen contacts. Notably, the 1,2-C6I2Cl4 molecules π-stack in a homogeneous and face-to-face orientation that results in an infinite column of the halogen-bond donor. As a consequence of this stacking arrangement and I...N halogen bonds, molecules of BPE also stack in this type of pattern. In particular, neighbouring ethylene groups in BPE are found to be parallel and within the accepted distance for a photoreaction. Upon exposure to ultraviolet light, the cocrystal undergoes a solid-state [2 + 2] cycloaddition reaction that produces rctt-tetrakis(pyridin-4-yl)cyclobutane (TPCB) with an overall yield of 89%. A solvent-free approach utilizing dry vortex grinding of the components also resulted in a photoreactive material with a similar yield.

Diversifying molecular and topological space via a supramolecular solid-state synthesis: a purely organic mok net sustained by hydrogen bonds.

A three-dimensional hydrogen-bonded network based on a rare mok topology has been constructed using an organic molecule synthesized in the solid state. The molecule is obtained using a supramolecular protecting-group strategy that is applied to a solid-state [2+2] photodimerization. The photodimerization affords a novel head-to-head cyclo-butane product. The cyclo-butane possesses tetrahedrally disposed cis-hydrogen-bond donor (phenolic) and cis-hydrogen-bond acceptor (pyridyl) groups. The product self-assembles in the solid state to form a mok network that exhibits twofold interpenetration. The cyclo-butane adopts different conformations to provide combinations of hydrogen-bond donor and acceptor sites to conform to the structural requirements of the mok net.

Regioselective photoreactions within a series of mixed co-crystals containing isosteric dihalogenated resorcinols with 4-stilbazole.

The formation and photoreactivity of a series of mixed co-crystals containing 4-stilbazole along with both 4,6-dichlororesorcinol and 4,6-dibromoresorcinol at various ratios is reported. In all cases, the quantitative [2 + 2] cycloaddition reaction yields an identical head-to-tail photoproduct, namely rctt-1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane. The selectivity in the cycloaddition reaction occurred due to a shorter distance observed between two carbon-carbon double bonds that were found between and not within the hydrogen-bonded assemblies.

Cooling-rate dependent single-crystal-to-single-crystal phase transition in an organic co-crystal.

We investigate a previously unobserved phase transition in an organic co-crystal containing the olefin trans-1,2-bis(4-pyridyl)ethylene. The olefin undergoes molecular motion in the crystalline state, and converts from a disordered to ordered phase upon cooling. Ordering causes a unit cell change to occur via a reversible single-crystal-to-single-crystal (SCSC) transformation. The ordered phase is only accessed via slow cooling; flash cooling locks the crystal in a kinetically trapped, disordered state, and SCSC reversibility is lost. The common practice of flash cooling may inhibit access to thermodynamic products and unique phases.

Metal-Organic Coordination versus Hydrogen Bonding: Highly Efficient Templated Photocycloadditions of Trisubstituted Isomeric Olefins in the Solid State.

Trisubstituted olefins are reactants in template-directed, solid-state [2+2] photocycloaddition reactions. Whereas hydrogen-bond-based templates afford crystalline assemblies with olefins that are photostable, AgI coordination results in carbon-carbon double bonds that react stereoselectively and result in quantitative yield.

Achieving dynamic behaviour and thermal expansion in the organic solid state via co-crystallization.

Thermal expansion involves a response of a material to an external stimulus that typically involves an increase in a crystallographic axis (positive thermal expansion (PTE)), although shrinking with applied heat (negative thermal expansion (NTE)) is known in rarer cases. Here, we demonstrate a means to achieve dynamic molecular motion and thermal expansions in organic solids via co-crystallizations. One co-crystal component is known to exhibit dynamic behaviour in the solid state while the second, when varied systematically, affords co-crystals with linear thermal expansion coefficients that range from colossal to nearly zero. Two co-crystals exhibit rare NTE. We expect the approach to guide the design of molecular solids that enable predesigned motion related to thermal expansion processes.

Investigation of the coordination interactions of S-(pyridin-2-ylmethyl)-L-cysteine ligands with M(CO)(3)(+) (M = Re, (99m)Tc).

Development of new ligands for fac-M(OH(2))(3)(CO)(3)(+) (M = Re, (99m)Tc) led the investigation with S-(pyridin-2-ylmethyl)-l-cysteine, 1. The ligand 1 has potential to coordinate with the metal through three different tridentate modes: tripodal through cysteine (O,N,S) and two linear involving the S-pyridyl and cysteine (O,S,N(Py), N,S,N(Py)). From the reaction with 1, two species were observed in the (1)H NMR, where the primary product was the linear fac-Re(N,S,N(Py)-1)(CO)(3)(+), 2a, complex. To identify the coordination mode of the minor product, functionalized analogues of 1 were prepared from S-(pyridin-2-ylmethyl)-Boc-l-cysteine-methyl ester, 3, with orthogonal protecting groups on the C terminus (methyl ester) in S-(pyridin-2-ylmethyl)-l-cysteine methyl ester, 4, or N terminus (Boc) in S-(pyridin-2-ylmethyl)-Boc-l-cysteine, 6, that specifically directed the coordination mode of fac-M(H(2)O)(3)(CO)(3)(+) to either N,S,N(Py) or O,S,N(Py), respectively. Two diastereomers [fac-Re(CO)(3)(N,S,N(Py)-4)](+), 5a and 5b, were observed and independently characterized by X-ray structure analysis and NMR in high yield with 4. Surprisingly, the O,S,N(Py) Re complex with ligand 6 was not observed and simplified versions, 3-(pyridin-2-ylmethylthio) propanoic acid, 7, and 2-(pyridin-2-ylmethylthio)acetic acid, 8, were investigated. Ligand 7 did not yield the desired linear tridentate O,S,N(Py) product. However, the shorter ligand 8 formed fac-Re(CO)(3)(O,S,N(Py)-8), 9, in high yield. (99m)Tc labeling studies were conducted and yielded similar results to the rhenium complex and effective (>99%) at 10(-5) M ligand concentration.