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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. [ABSTRACT FROM AUTHOR]

The reactivity between 2-(p-tolyl)selenopheno[2,3-b]pyridine (L) and ICl in MeCN is presented. Single crystal structure analysis revealed the formation of two distinct crystalline materials both featuring the protonated 2-(p-tolyl)selenopheno[2,3-b]pyridinium cation [HL]+. Colourless crystals were identified as the dihydrate salt [HL]Cl·2H2O (1), and dark-red crystals as compound {[HL]+[I2Cl]−}3·1/2I2 (2) that features the peculiar presence of the rare non-classical [I2Cl]− interhalide. Halogen Bonds (XBs) between the cocrystallized I2 molecule and [I2Cl]− anions led to the novel overall discrete [I10Cl4]4−H-shaped polyinterhalide formation, which is stabilized through hydrogen bonds (HBs) with the templating [HL]+ cations. The experimental observations are supported by extensive theoretical calculations at the DFT level that were employed to study the role played by non-covalent interactions in the formation of the presented unique polyinterhalide. [ABSTRACT FROM AUTHOR]

The synthesis and characterisation of a library of acyclic antimony(III) and bismuth(III) triaryl pnictogen bonding (PnB) receptor systems are reported. In the first‐generation receptor series, quantitative 1H NMR chloride titration experiments in THF solvent media reveal halide anion binding potency is intimately correlated with both the electronic‐withdrawing nature of the aryl‐ substituent and the polarisability of the PnB donor. Further extensive anion binding investigations with the most potent Sb‐ and Bi‐based PnB receptors: 1⋅Sb2CF3 and 1⋅Bi2CF3, reveal novel selectivity profiles, both displaying Cl− selectivity relative to the heavier halides and, impressively, to a range of highly basic oxoanions. The synthesis and preliminary chloride anion binding studies of a series of novel tripodal tris‐proto‐triazole triaryl Sb(III) and Bi(III) mixed PnB‐HB receptor systems are also described. Whereas parent triphenyl Sb(III) and Bi(III) compounds are incapable of binding Cl− in THF solvent media, the PnB‐triazole HB host systems exhibit notable halide affinity. [ABSTRACT FROM AUTHOR]

A series of neutral tetradentate halogen bonding (XB) macrocycles, comprising of two bis-iodotriazole XB donors were synthesised in 60–70% yields via a stepwise CuAAC-mediated cyclisation strategy. Extensive 1H NMR anion titration experiments reveal halide binding affinities are critically dependent on the substitution pattern of the xylyl spacer unit. The meta-substituted macrocycle remarkably displays cooperative tetradentate XB-halide anion recognition in highly competitive 40% aqueous-organic D2O/acetone-d6 (40 : 60, v/v) solvent mixtures. Integration of para-xylyl and naphthyl spacer units generates extended macrocyclic cavities, capable of selective oxalate recognition. Furthermore, preliminary fluorescence exeperiments reveal dicarboxylate specific sensing can be achieved through monitoring of the naphthylene centred emission. [ABSTRACT FROM AUTHOR]

This Review deals with the evolving field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides). In addition to a historical outline, current progress in synthetic approaches towards the formation of polyfluorides, polychlorides, polybromides, and polyinterhalides is also illustrated. The structural diversity of polyhalides has substantially increased in the past decade, especially for polychlorides and polybromides, which are commonly characterized by single‐crystal X‐ray diffraction, Raman spectroscopy, and quantum‐chemical calculations. Polyfluorides have been examined by sophisticated state‐of‐the‐art quantum‐chemical calculations and investigated spectroscopically in noble gas matrix‐isolation experiments under cryogenic conditions at 4 K. The bonding in such polyhalide systems is also discussed. The last Section deals with applications of polyhalides in halogenation reactions and electrochemistry as well as their use as reactive ionic liquids, emphasizing the promising future of polyhalogen chemistry. [ABSTRACT FROM AUTHOR]

Methyl-bambus[6]uril may form a variety of inclusion compounds, especially with anionic species. This communication reports the formation of the non-classical [I2Cl]− interhalide ion when chloride-bearing methyl-bambus[6]uril interacts with iodine, both in solution and in the solid state. [ABSTRACT FROM AUTHOR]

Abstract: Due to a more distinct σ‐hole, BrCl is able to form stronger halogen bonds than those in polyhalogen anions based on Cl2 and Br2. This stabilization allows the crystallographic characterization of a variety of new polyinterhalides, in which chloride functions as the central ion as shown by the molecular structures of [AsPh4][Cl(BrCl)3] and [CCl(NMe2)2][Cl(BrCl)5]. Furthermore, the solid‐state structure of an octahedrally coordinated nonclassical interhalide is reported for the first time. The tridecainterhalide monoanion [Cl(BrCl)6]− consists of a central chloride ion, which is coordinated by six BrCl molecules in a slightly distorted octahedral structure. All new compounds were characterized by single‐crystal X‐ray diffraction (XRD), NMR and Raman spectroscopy, as well as quantum‐chemical calculations. [ABSTRACT FROM AUTHOR]

In tyrosinium polyiodide hydrate, cations and anions aggregate in layers. The cation layers are stabilized by classical hydrogen bonds. The anionic part of the structure consists of parallel infinite polyiodide strands; the distance pattern along these chains suggests the presence of smaller subunits I3−, I2 and I−. Comparative calculations for small fragments and longer chains in frozen geometries indicate that this unexpected arrangement is favoured by local stabilizing anion…anion interactions and partial charge transfer between the subunits. The topological analyses of the electron density ρ, its negative Laplacian L = −蜑2ρ and the electrostatic potential φ functions have been used to study the intrachain I–I and I…I interactions. Thorough analysis carried out with L indicates the successive arrangement of generalized charge concentration and charge depletion sites (for either L ＞ 0 or L ＜ 0 regions) along bond paths, and permits to distinguish iodides from iodine atoms even when they are involved in intermediate situations where interatomic distances and net charges are not conclusive. [ABSTRACT FROM AUTHOR]

Unconventional ionic liquid crystals in which the liquid crystallinity is enabled by halogen-bonded supramolecular anions [C nF2 n+1-I⋅⋅⋅I⋅⋅⋅I-C nF2 n+1]− are reported. The material system is unique in many ways, demonstrating for the first time 1) ionic, halogen-bonded liquid crystals, and 2) imidazolium-based ionic liquid crystals in which the occurrence of liquid crystallinity is not driven by the alkyl chains of the cation. [ABSTRACT FROM AUTHOR]

Two different dicationic N-donors, based on the DABCO diamine, have been studied as templates for polyiodides. The results present a new strategy for polyiodide stabilization, which involves both N…I halogen bonding and cation–anion interactions. This is highlighted by the self-assembly of an unprecedented discrete pseudo-linear dodecaiodide species. [ABSTRACT FROM AUTHOR]

Simple organic salts based on aniline-derived cations and D-tartrate anions formed organogels and helical nanofibers. The organic salt ( p-fluoroanilinium)( D-tartrate) was found to generate an organogel despite the absence of a hydrophobic alkyl chain, whereas ( p-iodoanilinium)( D-tartrate) formed helical nanofibers in braided ropelike structures through a rolling-up process. The helicity of these nanofibers could be reversed by changing the growth solvent. The driving forces responsible for the formation of the nanofibers were determined to be 1D OH⋅⋅⋅O− hydrogen-bonding interactions between D-tartrate anions and π stacking of anilinium cations, as well as steric hindrance between the hydrogen-bonded chains. [ABSTRACT FROM AUTHOR]

An imidazole urea displays switchable hydrogelation behaviour that is turned off by metal ion coordination to give anion-bound coordination complex arrays. [ABSTRACT FROM AUTHOR]

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 sp2 hybridisation at the phenate oxygen atom, there is strong preference for the formation of two non-covalent interactions with the oxygen sp2 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 sp2 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. [ABSTRACT FROM AUTHOR]

The article presents a study on organocatalysis through halogen-bond donors. It states that 2,6-diiodo-3,4,5-trifluoropheny group was usually selected as building block due to electron-withdrawing fluorine substituents, high rigidity to be achieved through aryl moieties coupling, and avoidance from atrop-isomers formation. It adds that neutral multidentate halogen-bond donors were able to catalyze the chemical reaction between silyl enol ethers and ketene silyl acetals with 1-chloroisochroman.

This critical reviewdescribes how halocarbons can function as effective binding sites of anions viahalogen bonding, the noncovalent interaction whereby halogen atoms accept electron density. The focus is on the binding and coordination of oxyanions, by far the most numerous class of anions in organic chemistry. It is shown how a large variety of inorganic and organic oxyanions can form discrete adducts and 1D, 2D, or 3D supramolecular networks with chloro-, bromo-, and iodocarbons. Specific examples are discussed in order to identify new supramolecular synthons based on halogen bonding and to outline some general principles for the design of effective and selective receptors based on this interaction. The interaction allows for several other anions to self-assemble with halocarbons and mention is also given to the halogen bonding-based coordination of halides, polycyano- and polyoxometallates (72 references). [ABSTRACT FROM AUTHOR]