Anion⋯anion self-assembly under the control of σ- and π-hole bonds

The electrostatic attraction between charges of opposite signs and the repulsion between charges of the same sign are ubiquitous and influential phenomena in recognition and self-assembly processes. However, it has been recently revealed that specific attractive forces between ions with the same sign are relatively common. These forces can be strong enough to overcome the Coulomb repulsion between ions with the same sign, leading to the formation of stable anion⋯anion and cation⋯cation adducts. Hydroden bonds (HBs) are probably the best-known interaction that can effectively direct these counterintuitive assembly processes. In this review we discuss how σ-hole and π-hole bonds can break the paradigm of electrostatic repulsion between like-charges and effectively drive the self-assembly of anions into discrete as well as one-, two-, or three-dimensional adducts. σ-Hole and π-hole bonds are the attractive forces between regions of excess electron density in molecular entities (e.g., lone pairs or π bond orbitals) and regions of depleted electron density that are localized at the outer surface of bonded atoms opposite to the σ covalent bonds formed by atoms (σ-holes) and above and below the planar portions of molecular entities (π-holes). σ- and π-holes can be present on many different elements of the p and d block of the periodic table and the self-assembly processes driven by their presence can thus involve a wide diversity of mono- and di-anions. The formed homomeric and heteromeric adducts are typically stable in the solid phase and in polar solvents but metastable or unstable in the gas phase. The pivotal role of σ- and π-hole bonds in controlling anion⋯anion self-assembly is described in key biopharmacological systems and in molecular materials endowed with useful functional properties.

How Photogenerated I2 Induces I-Rich Phase Formation in Lead Mixed Halide Perovskites

Bandgap tunability of lead mixed halide perovskites (LMHPs) is a crucial characteristic for versatile optoelectronic applications. Nevertheless, LMHPs show the formation of iodide-rich (I-rich) phase under illumination, which destabilizes the semiconductor bandgap and impedes their exploitation. Here, it is shown that how I2 , photogenerated upon charge carrier trapping at iodine interstitials in LMHPs, can promote the formation of I-rich phase. I2 can react with bromide (Br- ) in the perovskite to form a trihalide ion I2 Br- (Iδ- -Iδ+ -Brδ- ), whose negatively charged iodide (Iδ- ) can further exchange with another lattice Br- to form the I-rich phase. Importantly, it is observed that the effectiveness of the process is dependent on the overall stability of the crystalline perovskite structure. Therefore, the bandgap instability in LMHPs is governed by two factors, i.e., the density of native defects leading to I2 production and the Br- binding strength within the crystalline unit. Eventually, this study provides rules for the design of chemical composition in LMHPs to reach their full potential for optoelectronic devices.

Ionic Organic Solid 1,3-Bis(sulfomethyl)imidazoliumate as an Effective Metal-Free Catalyst for Sustainable Organic Syntheses

The 1,3-bis(sulfomethyl)imidazole (bsmim) was effectively prepared by a multicomponent reaction, employing aminomethanesulfonic acid, glyoxal, and formaldehyde. The catalytic activity of bsmim was tested in the synthesis of quinoline derivatives, by means of the Friedländer reaction, and in the allylic substitution of alcohols with nitrogen-containing heterocycles. The performance of sulfo-imidazole derivative (bsmim) resulted in higher comparison with the carboxyimidazole analogs (bcmim and bcmimCl), under the same reaction conditions. This type of ionic organic solid allows the promotion of reactions in the absence of solvent and mild reaction conditions, which improves the sustainability of organic synthetic processes.

Large Telluroxane Bowls Connected by a Layer of Iodine Ions

Phenyltelluroxane clusters of the composition [{(PhTe)₁₉O₂₄}₂I₁₈(solv)] (1) are formed during the hydrolysis of [PhTeI₃]₂ or the oxidation of various phenyltellurium(II) compounds with iodine under hydrolytic conditions. The compounds consist of two half‐spheres with a {(PhTe)₁₉O₂₄}⁹⁺ network, which are connected by 18 iodine atoms. The spherical clusters can accommodate solvent molecules such as pyridine or methanol in the center of two rings formed by iodine atoms. The presence of other metal ions during the cluster formation results in a selective replacement of the central {PhTe}³⁺ units of each half‐sphere as has been demonstrated with the isolation of [{(PhTe)₁₈({Ca(H₂O)₂}O₂₄}₂I₁₆] (2) and [{(PhTe)₁₈({Y(NO₃)(H₂O)}O₂₄}₂I₁₆] (3). A crownether‐like coordination by six oxygen atoms of the telluroxane network is found for the {Ca(H₂O}₂}²⁺ and {Y(NO₃)(H₂O)}²⁺ building blocks. Mass spectrometric studies show that considerable amounts of the intact clusters are transferred to the gas phase without dissociation.

Words in supramolecular chemistry: the ineffable advances of polyiodide chemistry

Polyiodide chemistry has a rich history deeply intertwined with the development of supramolecular chemistry. Technological and theoretical interest in polyiodides has not diminished in the last decade, quite the contrary; yet the advances this perspective intends to cover are muddled by the involution of supramolecular vocabulary, preventing their unbiased discussion. Herein we discuss the pressing necessity of ordering the current babel of novel - and less so - supramolecular terms. Shared decisions at the community level might be required to shape the field into a harmonious body of knowledge, dominated by concepts rather than words. Secondary, σ-hole and halogen bonding schools of thought are all addressed here, together with their respective impact on the field. Then, on the basis of a shared vocabulary, a discussion of polyiodide chemistry is presented, starting with a revisited view of triiodide. The contemporary fields of supramolecular caging and polyiodide networks are then discussed, with emphasis on how the terms we choose to use deeply affect scientific progress.

Synthesis and supramolecular organization of the iodide and triiodides of a polycyclic adamantane-based diammonium cation: the effects of hydrogen bonds and weak I⋯I interactions

Adamantane-like divalent building blocks and iodide or polyiodide anions combine into supramolecular architectures with the help of various noncovalent forces ranging from strong hydrogen bonds to secondary and weak I⋯I interactions.

One dimensional halogen bond design: Br⋯N versus I⋯N with fluoroarenes

29 structures of 1,4-dibromo- and 1,4-diiodotetrafluorobenzene, as well as 4,4′-dibromo- and 4,4′-diiodooctafluorobiphenyl with bifunctional amines were synthesized, exposing correlations of halogen bond length with both electrostatic potential and percent buried volume.

The Relevance of Size Matching in Self-assembly: Impact on Regio- and Chemoselective Cocrystallizations

Decamethonium diiodide is reported to perform the chemo- and regioselective encapsulation of para-dihalobenzenes through the competitive formation of halogen-bonded cocrystals starting from solutions that also contain ortho and meta isomers. Selective caging in the solid occurs even when an excess ortho or meta isomers, or even a mixture of them, is present in the solution. A prime matching between the size and shape of the dication and the formed dianions plays a key role in enabling the selective self-assembly, as proven by successful encapsulation of halogen-bond donors as weak as 1,4-dichlorobenzene and by the results of cocrystallization trials involving mismatching tectons. Encapsulated para-dihalobenzenes guest molecules can be removed quantitatively by heating the cocrystals under reduced pressure and be recovered as pure materials. The residual decamethonium diiodide can be recycled with no reduction in selectivity.

Genesis of Complex Polyiodide Networks: Insights on the Blue Box/I−/I2 Ternary System

The Stoddart’s blue box (BB) (cyclobis(paraquat-p-phenylene))/iodide binary system was recently demonstrated to give rise to porous three-dimensional networks which can hardly be classified as common XOF-type materials (X = M, C, S, i.e., metal, covalent, or supramolecular organic frameworks), leading to the definition of permutable organized frameworks (POFs). The ternary BB/iodide/iodine system was reported to generate pentaiodide-based structures constituted by the most complex interlocked polyiodides so far isolated (up to an infinite supramolecular pseudopolyrotaxane with a poly[3]catenane axle). The missing link, i.e., the XRD structure of the BB/triiodide complex, is herein reported: structural similarities and novel Raman evidence, opening perspectives in the genesis of solid-state BB-based complex polyiodide networks from solution.

Oxidant/complexing properties of the methimazole (MeImHS)/iodine system towards palladium and gold metals. Crystal structure of the complex cation [PdII(MeImHS)4]2+ balanced by a tetraiodide/iodide mixture

The oxidative dissolution of palladium is easily and safely achievable both in dichloromethane and water by the methimazole/I₂ mixture.

Stabilization of polyiodide networks with Cu(ii) complexes of small methylated polyazacyclophanes: shifting directional control from H-bonds to I⋯I interactions

Modulation of hydrogen bonds and iodine–iodine interactions, both covalent and supramolecular, unlocks novel possibilities for the construction of transition metal-polyiodide hybrid networks.

Iodine Clathrated: A Solid-State Analogue of the Iodine-Starch Complex

Co-crystallizing iodine with a simple dicationic salt (1,8-diammoniumoctane chloride) results in the clathration of the iodine (I₂ ) molecules inside trigonal and hexagonal helical channels of the crystal lattice with 72 wt % overall I₂ loading. The I₂ inside the bigger trigonal channel forms a I-I⋅⋅⋅I-I⋅⋅⋅I-I halogen-bonded infinite helical chain, while the I₂ in the smaller hexagonal channel is disordered. In both channels the I₂ interaction with the channel wall happens through I-I⋅⋅⋅Cl^- halogen bonds. The helical channels in the crystal lattice are constructed via the strong charge-assisted H₂ N⁺ H⋅⋅⋅Cl^- hydrogen bonds between the dications and the chloride anions. The structure shows a marked similarity with the well-known starch-I₂ system, and thus may provide insight for the yet unresolved structure of the I₂ in the helical starch channel.

Stabilization of Supramolecular Networks of Polyiodides with Protonated Small Tetra-azacyclophanes

Polyiodide chemistry is among the first historically reported examples of supramolecular forces at work. To date, owing to the increasingly recognized role of halogen bonding and the incorporation of iodine-based components in several devices, it remains an active field of theoretical and applied research. Herein we re-examine azacyclophanes as a class of ligands for the stabilization of iodine-dense three-dimensional networks, showing how we devised novel possible strategies starting from literature material. The new set of azacyclophane ligands affords novel crystal structures possessing intriguing properties, which develop on a double layer. At a macroscopic level, the obtained networks possess a very high iodine packing density (less than 2 times more diluted than crystalline I2): a simple parameter, IN, is also introduced to quickly measure and compare iodine packing density in different crystals. On the microscopic level, the present study provides evidence about the ability of one of the ligands to act as a three-dimensional supramolecular mold for the template synthesis of the rarely observed heptaiodide (I7−) anion. Therefore, we believe our approach and strategy might be relevant for crystal engineering purposes.

Why are reactions of 2- and 8-thioquinoline derivatives with iodine different?

The crystal structures of 1,2-dihydro-1,1′-bi[thiazolo[3,2-a]quinoline]-10a,10a′-diium diiodide hemihydrate, C22H16N2S2 2+·2I−·0.5H2O, and 1,2-dihydro-1,1′-bi[thiazolo[3,2-a]quinoline]-10a,10a′-diium iodide triiodide, C22H16N2S2 2+·I−·I3 −, obtained during the reaction of 1,4-bis(quinolin-2-ylsulfanyl)but-2-yne (2TQB) with iodine, have been determined at 120 K. The crystalline products contain the dication as a result of the reaction proceeding along the iodocyclization pathway. This is fundamentally different from the previously observed reaction of 1,4-bis(quinolin-8-ylsulfanyl)but-2-yne (8TQB) with iodine under similar conditions. A comparative analysis of the possible conformational states indicates differences in the relative stabilities and free rotation for the 2- and 8-thioquinoline derivatives which lead to a disparity in the convergence of the potential reaction centres for 2TQB and 8TQB.

Dicarboxylic Acid Separation by Dynamic and Size-Matched Recognition in Solution and in the Solid State

Bis(trimethylammonium) alkane diiodides dynamically encapsulate dicarboxylic acids through intermolecular hydrogen bonds between the I^- anions of the hosts and the carboxylic OH groups of the guests. A selective recognition is realized when the size of the I^- ⋅⋅⋅HOOC(CH₂ /CF₂ )_n COOH⋅⋅⋅I^- superanion matches the dication alkyl chain length. Dynamic recognition is also demonstrated in solution, where the presence of the size-matching organic salt boosts the acid solubility profile, thus allowing efficient mixture separation.

Halogen bonds in 2,5-dihalopyridine-copper(ii) chloride complexes

In the (2,5-dihalopyridine)₂·CuCl₂ complexes varying the C5-position halogens affects the halogen bonding properties so that the C5–X5⋯Cl–Cu halogen bonds follow the order F5 < Cl5 < Br5 < I5 when the substituent at 2-position is chlorine and Cl5 < Br5 < I5 when it is bromine.

Factors affecting charge transfer in tetraiodide dianions

Thirty-one examples of crystal structures containing discrete tetraiodide I₄²⁻dianions were identified from the Cambridge Structural Database (CSD) and analyzed in detail in order to find the factors influencing the geometry of this rare fragment. The intermolecular interactions are at least partially responsible for the changes in the geometry of the dianion.

Cyanine dyes: synergistic action of hydrogen, halogen and chalcogen bonds allows discrete I42− anions in crystals

Discrete tetraiodide dianions (I₄²⁻) are formed in crystals via halogen bond coordination of I₂ by iodide anions which are pinned in their positions by a network of hydrogen bonds involving a benzoselenazole cyanine dye.

Formation and distortion of iodidoantimonates(III): the first isolated [SbI6]3- octahedron

The ability to intentionally construct, through different types of interactions, inorganic-organic hybrid materials with desired properties is the main goal of inorganic crystal engineering. The primary deformation, related to intrinsic interactions within inorganic substructure, and the secondary deformation, mainly caused by the hydrogen bond interactions, are both responsible for polyhedral distortions of halogenidoantimonates(III) with organic cations. The evolution of structural parameters, in particular the Sb-I secondary- and O/N/C-H...I hydrogen bonds, as a function of temperature assists in understanding the contribution of those two distortion factors to the irregularity of [SbI₆]^3- polyhedra. In tris(piperazine-1,4-diium) bis[hexaiodidoantimonate(III)] pentahydrate, (C₄H₁₂N₂)₃[SbI₆]₂·5H₂O (TPBHP), where the isolated [SbI₆]^3- units were found, distortion is governed only by O/N/C-H...I hydrogen bonds, whereas in piperazine-1,4-diium bis[tetraiodidoantimonate(III)] tetrahydrate, (C₄H₁₂N₂)[SbI₄]₂·4H₂O (PBTT), both primary and O-H...I secondary factors cause the deformation of one-dimensional [{SbI₄}_n]^n- chains. The larger in spatial dimensions piperazine-1,4-diium cations, in contrast to the smaller water of crystallization molecules, do not significantly contribute to the octahedral distortion, especially in PBTT. The formation of isolated [SbI₆]^3- ions in TPBHP is the result of specific second coordination sphere hydrogen bond interactions that stabilize the hybrid structure and simultaneously effectively separate and prevent [SbI₆]^3- units from mutual interactions. The temperature-induced changes, further supported by the analysis of data retrieved from the Cambridge Structural Database, illustrate the significance of both primary and secondary distortion factors on the deformation of octahedra. Also, a comparison of packing features in the studied hybrids with those in the non-metal containing piperazine-1,4-diium diiodide diiodine (C₄H₁₂N₂)I₂·I₂ (PDD) confirms the importance and hierarchy of different types of interactions.

Testing the tools for revealing and characterizing the iodine-iodine halogen bond in crystals

To understand what tools are really suitable to identify and classify the iodine-iodine non-covalent interactions in solid organic polyiodides, we have examined the anisotropy of the electron density within the iodine atomic basin along and across the iodine-iodine halogen bond using the Laplacian of electron density, one-electron potential and electron localization function produced by Kohn-Sham calculations with periodic boundary conditions. The Laplacian of electron density exhibits the smallest anisotropy and yields a vague picture of the outermost electronic shells. The one-electron potential does not show such a deficiency and reveals that the valence electron shell for the halogen-bond acceptor iodine is always wider than that for the halogen-bond donor iodine along its σ-hole direction. We have concluded that the one-electron potential is the most suitable for classification of the iodine-iodine bonds and interactions in complicated cases, while the electron localization function allows to distinguish the diiodine molecule bonded with the monoiodide anion from the typical triiodide anion.

Photoresponsive ionic liquid crystals assembled via halogen bond: en route towards light-controllable ion transporters

We demonstrate that halogen bonding (XB) can offer a novel approach for the construction of photoresponsive ionic liquid crystals. In particular, we assembled two new supramolecular complexes based on 1-ethyl-3-methylimidazolium iodides and azobenzene derivatives containing an iodotetrafluoro-benzene ring as XB donor, where the iodide anion acted as an XB acceptor. DSC and X-ray diffraction analyses revealed that the preferred stoichiometry between the XB donors and acceptors is 2 : 1, and that the iodide anions act as bidentate XB-acceptors, binding two azobenzene derivatives. Due to the high directionality of the XB, calamitic superanions are obtained, while the segregation occurring between the charged and uncharged parts of the molecules gives rise to a layered structure in the crystal lattice. Despite the fact that the starting materials are non-mesomorphic, the halogen-bonded supramolecular complexes exhibited monotropic lamellar liquid-crystalline phases over broad temperature ranges, as confirmed with polarized optical microscopy. Due to the presence of the azobenzene moieties, the LCs were photoresponsive, and a LC-to-isotropic phase transition could be obtained by irradiation with UV light. We envisage that the light-induced phase transition, in combination with the ionic nature of the LC, provides a route towards light-induced control over ion transport and conductance in these supramolecular complexes.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

σ-Hole Bond vs π-Hole Bond: A Comparison Based on Halogen Bond

The σ-hole and π-hole are the regions with positive surface electrostatic potential on the molecule entity; the former specifically refers to the positive region of a molecular entity along extension of the Y-Ge/P/Se/X covalent σ-bond (Y = electron-rich group; Ge/P/Se/X = Groups IV-VII), while the latter refers to the positive region in the direction perpendicular to the σ-framework of the molecular entity. The directional noncovalent interactions between the σ-hole or π-hole and the negative or electron-rich sites are named σ-hole bond or π-hole bond, respectively. The contributions from electrostatic, charge transfer, and other terms or Coulombic interaction to the σ-hole bond and π-hole bond were reviewed first followed by a brief discussion on the interplay between the σ-hole bond and the π-hole bond as well as application of the two types of noncovalent interactions in the field of anion recognition. It is expected that this review could stimulate further development of the σ-hole bond and π-hole bond in theoretical exploration and practical application in the future.

Intriguing I2 Reduction in the Iodide for Chloride Ligand Substitution at a Ru(II) Complex: Role of Mixed Trihalides in the Redox Mechanism

The compound [Ru(CN(t)Bu)4(Cl)2], 1, reacts with I2, yielding the halogen-bonded (XB) 1D species {[Ru(CN(t)Bu)4(I)2]·I2}n, (2·I2)n, whose building block contains I(-) ligands in place of Cl(-) ligands, even though no suitable redox agent is present in solution. Some isolated solid-state intermediates, such as {[Ru(CN(t)Bu)4(Cl)2]·2I2}n, (1·2I2)n, and {[Ru(CN(t)Bu)4(Cl)(I)]·3I2}n, (3·3I2)n, indicate the stepwise substitution of the two trans-halide ligands in 1, showing that end-on-coordinated trihalides play a key role in the process. In particular, the formation of ClI2(-) triggers electron transfer, possibly followed by an inverted coordination of the triatomic species through the external iodine atom. This allows I-Cl separation, as corroborated by Raman spectra. The process through XB intermediates corresponds to reduction of one iodine atom combined with the oxidation of one coordinated chloride ligand to give the corresponding zerovalent atom of I-Cl. This redox process, explored by density functional theory calculations (B97D/6-31+G(d,p)/SDD (for I and Ru atoms)), is apparently counterintuitive with respect to the known behavior of the corresponding free halogen systems, which favor iodide oxidation by Cl2. On the other hand, similar energy barriers are found for the metal-assisted process and require a supply of energy to be passed. In this respect, the control of the temperature is fundamental in combination with the favorable crystallizations of the various solid-state products. As an important conclusion, trihalogens, as XB adducts, are not static in nature but are able to undergo dynamic inner electron transfers consistently with implicit redox chemistry.

Iodine uptake and enhanced electrical conductivity in a porous coordination polymer based on cucurbit[6]uril

Iodine uptake and enhanced electrical conductivity have been observed in a solid material built by cucurbit[6]uril units and an iodide matrix.

I5 – polymers with a layered arrangement: synthesis, spectroscopy, and structure of a new polyiodide salt in the nicotine/HI/I2 system

The reaction of S-nicotine with hydroiodic acid in the presence of iodine gave the new polyiodide-containing salt nicotine-1,1′-diium bis(triiodide)-diiodine (1/1) (C10H16N2) [I3]2·I2 (1). The title compound has been characterised by spectroscopic methods (Raman and IR) and single-crystal X-ray diffraction. The asymmetric unit of the title structure consists of one dication, two triiodide anions, and one iodine molecule, all located in general positions in the non-centrosymmetric space group P1. One of the two crystallographically independent triiodide anions and the doubly protonated nicotinium dication form hydrogen-bonded chains along b, which are arranged parallel to each other in the ½bc plane. The second crystallographically independent triiodide anion and the iodine molecule form an I5 – moiety, which is end-on connected to two symmetry-related anions resulting in polyiode zig–zag chains along the [0 1 1̅] direction. These polyiodide chains are stacked parallel to each other in the 0bc plane. The Raman spectrum of the title compound shows characteristic lines in the 50–200 cm–1 range, which are in excellent agreement with the findings derived from the crystal structure.

A Halogen-Bonded Dimeric Resorcinarene Capsule

Iodine (I2) acts as a bifunctional halogen-bond donor connecting two macrocyclic molecules of the bowl-shaped halogen-bond acceptor, N-cyclohexyl ammonium resorcinarene chloride 1, to form the dimeric capsule [(1,4-dioxane)3@1(2)(I2)2]. The dimeric capsule is constructed solely through halogen bonds and has a single cavity (V=511 Å(3)) large enough to encapsulate three 1,4-dioxane guest molecules.

Synthesis and structure of interaction products of quinoline-2(1H)-thione with molecular iodine

The behavior of quinoline-2(1H)-thione, which is a potential antithyroid drug toward molecular iodine, was investigated. The ability of quinoline-2(1H)-thione to form the outer-sphere charge-transfer complex C9H7NS·I2 with iodine molecular in dilute chloroform solution has been studied by UV-vis spectroscopy (lgβ=3.85). The crystal structure of the new salt 2-(quinoline-2-yldisulfanyl)quinolinium triiodide - product of irreversible oxidation of quinoline-2(1H)-thione was determined by X-ray diffraction. The 2-(quinoline-2-yldisulfanyl)quinolinium cations form dimers through π-π-stacking interaction between quinoline rings. Strong intramolecular interactions are observed between iodine - sulfur atoms and iodine - hydrogen atoms with shortened contacts (less of sum of van der Waals contacts). It is noteworthy that two perfectly centrosymmetrical anions I3(-) form a very short contact I(3)⋯I(3') 3.7550(5) so we can state the formation of the dianion I(6)(2-). Therefore the formation and topology of polyiodide species depend on the characteristics, such as shape, size and charge, etc., of the counter cation, 2-(quinoline-2-yldisulfanyl)quinolinium, which is considered as templating agent.

N⋯I halogen bonding supported stabilization of a discrete pseudo-linear [I12]2−polyiodide

An unprecedented discrete nearly linear dodecaiodide, [I₁₂]²⁻, can be stabilizedviaN⋯I and cation⋯I⁻interactions using a dicationic template based on the 1,4-diazabicyclo[2.2.2]octane (DABCO) diamine.

Raman spectroscopy study of new thia- and oxazinoquinolinium triodides

New polyiodides of thia- and oxa-zinoquinolinium derivatives were characterized using Raman spectroscopy and periodic 3D calculations of the Raman intensities. Polarized Raman spectra of the oriented crystals revealed the features of spatial organization in the polyiodide-anion chains.

Halogen- and hydrogen-bonded salts and co-crystals formed from 4-halo-2,3,5,6-tetrafluorophenol and cyclic secondary and tertiary amines: orthogonal and non-orthogonal halogen and hydrogen bonding, and synthetic analogues of halogen-bonded biological systems

Co-crystallisation of, in particular, 4-iodotetrafluorophenol with a series of secondary and tertiary cyclic amines results in deprotonation of the phenol and formation of the corresponding ammonium phenate. Careful examination of the X-ray single-crystal structures shows that the phenate anion develops a C=O double bond and that the C-C bond lengths in the ring suggest a Meissenheimer-like delocalisation. This delocalisation is supported by the geometry of the phenate anion optimised at the MP2(Full) level of theory within the aug-cc-pVDZ basis (aug-cc-pVDZ-PP on I) and by natural bond orbital (NBO) analyses. With sp(2) hybridisation at the phenate oxygen atom, there is strong preference for the formation of two non-covalent interactions with the oxygen sp(2) lone pairs and, in the case of secondary amines, this occurs through hydrogen bonding to the ammonium hydrogen atoms. However, where tertiary amines are concerned, there are insufficient hydrogen atoms available and so an electrophilic iodine atom from a neighbouring 4-iodotetrafluorophenate group forms an I⋅⋅⋅O halogen bond to give the second interaction. However, in some co-crystals with secondary amines, it is also found that in addition to the two hydrogen bonds forming with the phenate oxygen sp(2) lone pairs, there is an additional intermolecular I⋅⋅⋅O halogen bond in which the electrophilic iodine atom interacts with the C=O π-system. All attempts to reproduce this behaviour with 4-bromotetrafluorophenol were unsuccessful. These structural motifs are significant as they reproduce extremely well, in low-molar-mass synthetic systems, motifs found by Ho and co-workers when examining halogen-bonding interactions in biological systems. The analogy is cemented through the structures of co-crystals of 1,4-diiodotetrafluorobenzene with acetamide and with N-methylbenzamide, which, as designed models, demonstrate the orthogonality of hydrogen and halogen bonding proposed in Ho's biological study.

Reversible iodine absorption of nonporous coordination polymer Cu(TCNQ)

Polycrystalline powders of Cu(TCNQ) absorb iodine to form Cu(TCNQ)I₄upon solid grinding with iodine or immersion in a hexane solution of iodine.