A Halogen-Bonded Organic Framework (XOF) Emissive Cocrystal for Acid Vapor and Explosive Sensing, and Iodine Capture

There is a strong and urgent need for efficient materials that can capture radioactive iodine atoms from nuclear waste. This work presents a novel strategy to develop porous materials for iodine capture by employing halogen bonding, mechanochemistry and crystal engineering. 3D halogen-bonded organic frameworks (XOFs) with guest-accessible permanent pores are exciting targets in crystal engineering for developing functional materials, and this work reports the first example of such a structure. The new-found XOF, namely TIEPE-DABCO, exhibits enhanced emission in the solid state and turn-off emission sensing of acid vapors and explosives like picric acid in nanomolar quantity. TIEPE-DABCO captures iodine from the gas phase (3.23 g g-1 at 75 °C and 1.40 g g-1 at rt), organic solvents (2.1 g g-1 ), and aqueous solutions (1.8 g g-1 in the pH range of 3-8); the latter with fast kinetics. The captured iodine can be retained for more than 7 days without any leaching, but readily released using methanol, when required. TIEPE-DABCO can be recycled for iodine capture several times without any loss of storage capacity. The results presented in this work demonstrate the potential of mechanochemical cocrystal engineering with halogen bonding as an approach to develop porous materials for iodine capture and sensing.

Zwitterionic iodonium species afford halogen bond-based porous organic frameworks

Porous architectures characterized by parallel channels arranged in honeycomb or rectangular patterns are identified in two polymorphic crystals of a zwitterionic 4-(aryliodonio)-benzenesulfonate. The channels are filled with disordered water molecules which can be reversibly removed on heating. Consistent with the remarkable strength and directionality of the halogen bonds (XBs) driving the crystal packing formation, the porous structure is stable and fully preserved on almost quantitative removal and readsorption of water. The porous systems described here are the first reported cases of one-component 3D organic frameworks whose assembly is driven by XB only (XOFs). These systems are a proof of concept for the ability of zwitterionic aryliodonium tectons in affording robust one-component 3D XOFs. The high directionality and strength of the XBs formed by these zwitterions and the geometrical constraints resulting from the tendency of their hypervalent iodine atoms to act as bidentate XB donors might be key factors in determining this ability.

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.

Halogen-Bond Mediated [2+2] Photodimerizations: À la Carte Access to Unsymmetrical Cyclobutanes in the Solid State

The ditopic halogen-bond (X-bond) donors 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene (1,2-, 1,3-, and 1,4-di-I-tFb, respectively) form binary cocrystals with the unsymmetrical ditopic X-bond acceptor trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (2,4-bpe). The components of each cocrystal (1,2-di-I-tFb)·(2,4-bpe), (1,3-di-I-tFb)·(2,4-bpe), and (1,4-di-I-tFb)·(2,4-bpe) assemble via N···I X-bonds. For (1,2-di-I-tFb)·(2,4-bpe) and (1,3-di-I-tFb)·(2,4-bpe), the X-bond donor supports the C=C bonds of 2,4-bpe to undergo a topochemical [2+2] photodimerization in the solid state: UV-irradiation of each solid resulted in stereospecific, regiospecific, and quantitative photodimerization of 2,4-bpe to the corresponding head-to-tail (ht) or head-to-head (hh) cyclobutane photoproduct, respectively.

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 (SO₄^2-) and eight waters of hydration, namely, 4,4',4'',4'''-(cyclobutane-1,2,3,4-tetrayl)tetrapyridinium bis(sulfate) octahydrate, C₂₄H₂₄N₄⁴⁺·2SO₄^2-·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.

New node on the block for organic solid-state chemists: rtct-tetrakis(pyridin-4-yl)cyclobutane

We now know the symmetry and unit-cell parameters of the functionalized cyclo­butane rtct -TPCB , as well as a multicomponent solid containing it. What other networks and topologies are possible?

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.

Application of a tetrapyrimidyl cyclobutane synthesized in the organic solid state: a halogen-bonded supramolecular ladder

A supramolecular ladder sustained by halogen bonds with rungs based upon a photoproduct, namely rctt-tetrakis(5′-pyrimidyl)cylcobutane, generated in the solid state is reported.