Systematic Study of Podand Molecules for Synergistic Halogen and Hydrogen Bond-Driven Anion Recognition in the Solid State.

The increasing demand of species for the efficient capture and sensing of anions benefits from a systematic study of anion binding capabilities in the solid state. This work reports a detailed crystallographic study of ten structurally related podands and shows that these charged receptors bind anions with a combination of charge-assisted halogen and hydrogen bonds. Computational tools helped in highlighting the role of the different involved interaction and afforded possible design principles for the design of improved podands.

Tuning of Ionic Liquid Crystal Properties by Combining Halogen Bonding and Fluorous Effect.

We report halogen-bonded complexes between 1-polyfluoroalkyl-3-alkylimidazolium iodides and mono-iodoperfluoroalkanes of different chain lengths or di-iodoperfluorooctane. 19 F NMR analyses revealed that the preferred stoichiometry between the donors and acceptors is 1 : 1 in the cases of the mono-iodoperfluoroalkanes, and 2 : 1 with di-iodoperfluorooctane, as a result of the monodentate behavior of the iodide anion (halogen bond acceptor). Single crystal X-ray diffraction analyses showed the presence of a perfluorinated superanion, which interdigitates with the cation fluorinated chains, favoring the formation of lamellar structures. All of the obtained supramolecular complexes exhibit enantiotropic liquid crystalline phases over a broad range of temperatures. Most of the obtained complexes show melting points lower than 100 °C, two of them being liquid at room temperature, thus representing a new family of fluorinated ionic liquid crystals.

The diiodomethyl-sulfonyl moiety: an unexplored halogen bond-donor motif.

Herein, we report the crystal structures of the antimicrobial agent diiodomethyl-p-tolylsulfone and of three halogen bonded co-crystals demonstrating that the bioactive moiety -SO2CHI2 can function as a quite effective halogen bond based motif in the solid state and in solution, namely demonstrating that α-iodosulfones may become a new entry in the quite small group of alkyl-iodides functioning as reliable halogen bond-donors.

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.

Connectivity and Topology Invariance in Self-Assembled and Halogen-Bonded Anionic (6,3)-Networks.

We report here that the halogen bond driven self-assembly of 1,3,5-trifluorotriiodobenzene with tetraethylammonium and -phosphonium bromides affords 1:1 co-crystals, wherein the mutual induced fit of the triiodobenzene derivative and the bromide anions (halogen bond donor and acceptors, respectively) elicits the potential of these two tectons to function as tritopic modules (6,3). Supramolecular anionic networks are present in the two co-crystals wherein the donor and the acceptor alternate at the vertexes of the hexagonal frames and cations are accommodated in the potential empty space encircled by the frames. The change of one component in a self-assembled multi-component co-crystal often results in a change in its supramolecular connectivity and topology. Our systems have the same supramolecular features of corresponding iodide analogues as the metric aspects seem to prevail over other aspects in controlling the self-assembly process.

Halogen bonding in hypervalent iodine and bromine derivatives: halonium salts.

Halogen bonds have been identified in a series of ionic compounds involving bromonium and iodonium cations and several different anions, some also containing hypervalent atoms. The hypervalent bromine and iodine atoms in the examined compounds are found to have positive σ-holes on the extensions of their covalent bonds, while the hypervalent atoms in the anions have negative σ-holes. The positive σ-holes on the halogens of the studied halonium salts determine the linearity of the short contacts between the halogen and neutral or anionic electron donors, as usual in halogen bonds.

Halogen bonded Borromean networks by design: topology invariance and metric tuning in a library of multi-component systems.

A library of supramolecular anionic networks showing Borromean interpenetration has been prepared by self-assembly of crypt-222, several metal or ammonium halides, and five bis-homologous α,ω-diiodoperfluoroalkanes. Halogen bonding has driven the formation of these anionic networks. Borromean entanglement has been obtained starting from all the four used cations, all the three used anions, but only two of the five used diiodoperfluoroalkanes. As the change of the diiodoperfluoroalkane, the cation, or the anion has a different relative effect on the metrics and bondings of the self-assembled systems, it can be generalized that bonding, namely energetic, features play here a less influential role than metric features in determining the topology of the prepared tetra-component cocrystals. This conclusion may hold true for other multi-component systems and may function as a general heuristic principle when pursuing the preparation of multi-component systems having the same topology but different composition.

Proton in a Confined Space: Structural Studies of H+ ⊂Crypt-111 Iodide and Some Halogen-Bonded Derivatives.

Experimental observations and modeling data are reported on the solid-state structural features of crypt- 111⋅HI (1) and the three-component co-crystals that 1 forms with α,ω-diiodoperfluoroalkanes 2 a-d. X-ray analyses indicate that, in all five systems and at low temperature, the caged proton is covalently bonded to a single nitrogen atom and is involved in a network of intramolecular hydrogen bonds. In contrast, room-temperature, solid-state 15 N NMR spectroscopy suggests magnetic equivalency of the two N atoms of crypt-111 in both 1 and co-crystals of 1 with diiodoperfluoroalkanes. Computational modelling confirms that the acidic hydrogen inside the cavity preferentially sits along the internitrogen axis and is covalently bonded to one nitrogen. The computed energy barriers suggest that the hopping of the encapsulated proton between the two N atoms of the cage can occur in the halogen-bonded co-crystals of 1⋅2, but it is hardly possible in the pure H+ ⊂crypt-111 iodide 1. These different pictures of the proton position and dynamics obtained by using different techniques and conditions confirm the unique characteristics of the confined space within the cavity of crypr-111 and the distinctive features of processes occurring therein.

Superfluorinated Ionic Liquid Crystals Based on Supramolecular, Halogen-Bonded Anions.

Unconventional ionic liquid crystals in which the liquid crystallinity is enabled by halogen-bonded supramolecular anions [Cn F2 n+1 -I⋅⋅⋅I⋅⋅⋅I-Cn F2 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.

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.

Dynamic Characterization of Crystalline Supramolecular Rotors Assembled through Halogen Bonding.

A modular molecular kit for the preparation of crystalline molecular rotors was devised from a set of stators and rotators to gain simple access to a large number of structures with different dynamic performance and physical properties. In this work, we have accomplished this with crystalline molecular rotors self-assembled by halogen bonding of diazabicyclo[2.2.2]octane, acting as a rotator, and a set of five fluorine-substituted iodobenzenes that take the role of the stator. Using variable-temperature (1)H T1 spin-lattice relaxation measurements, we have shown that all structures display ultrafast Brownian rotation with activation energies of 2.4-4.9 kcal/mol and pre-exponential factors of the order of (1-9) × 10(12) s(-1). Line shape analysis of quadrupolar echo (2)H NMR measurements in selected examples indicated rotational trajectories consistent with the 3-fold or 6-fold symmetric potential of the rotator.

Halogen bond: a long overlooked interaction.

Because of their high electronegativity, halogen atoms are typically considered, in most of their derivatives, as sites of high electron density and it is commonly accepted that they can form attractive interactions by functioning as the electron donor site (nucleophilic site). This is the case when they work as hydrogen bond acceptor sites. However, the electron density in covalently bound halogens is anisotropically distributed. There is a region of higher electron density, accounting for the ability of halogens to function as electron donor sites in attractive interactions, and a region of lower electron density where the electrostatic potential is frequently positive (mainly in the heavier halogens). This latter region is responsible for the ability of halogen atoms to function as the electron-acceptor site (electrophilic site) in attractive interactions formed with a variety of lone pair-possessing atoms, anions, and π-systems. This ability is quite general and is shown by a wide diversity of halogenated compounds (e.g., organohalogen derivatives and dihalogens). According to the definition proposed by the International Union of Pure and Applied Chemistry, any attractive interactions wherein the halogen atom is the electrophile is named halogen bond (XB). In this chapter, it is discussed how the practice and the concept of XB developed and a brief history of the interaction is presented. Papers (either from the primary or secondary literature) which have reported major experimental findings in the field or which have given important theoretical contributions for the development of the concept are recollected in order to trace how a unifying and comprehensive categorization emerged encompassing all interactions wherein halogen atoms function as the electrophilic site.

The 1:1 co-crystal of triphen-yl(2,3,5,6-tetra-fluoro-benz-yl)phospho-nium bromide and 1,1,2,2-tetra-fluoro-1,2-di-iodo-ethane.

The title compound, C25H18F4P(+)·Br(-)·C2F4I2, is a 1:1 co-crystal of triphen-yl(2,3,5,6-tetra-fluoro-benz-yl)phospho-nium (TTPB) bromide and 1,1,2,2-tetra-fluoro-1,2-di-iodo-ethane (TFDIE). The crystal structure consists of a framework of TTPB cations held together by C-H⋯Br inter-actions. In this framework, infinite channels along [100] are filled by TFDIE mol-ecules held together in infinite ribbons by short F⋯F [2.863 (2)-2.901 (2)Å] inter-actions. The structure contains halogen bonds (XB) and hydrogen bonds (HB) in the bromide coordination sphere. TFDIE functions as a monodentate XB donor as only one I atom is linked to the Br(-) anion and forms a short and directional inter-action [I⋯Br(-) 3.1798 (7) Šand C-I⋯Br(-) 177.76 (5)°]. The coordination sphere of the bromide anion is completed by two short HBs of about 2.8 Š(for H⋯Br) with the acidic methyl-ene H atoms and two longer HBs of about 3.0 Šwith H atoms of the phenyl rings. Surprisingly neither the second iodine atom of TFDIE nor the H atom on the tetra-fluoro-phenyl group make any short contacts.

Azobenzene-based difunctional halogen-bond donor: towards the engineering of photoresponsive co-crystals.

Halogen bonding is emerging as a powerful non-covalent interaction in the context of supramolecular photoresponsive materials design, particularly due to its high directionality. In order to obtain further insight into the solid-state features of halogen-bonded photoactive molecules, three halogen-bonded co-crystals containing an azobenzene-based difunctional halogen-bond donor molecule, (E)-bis(4-iodo-2,3,5,6-tetrafluorophenyl)diazene, C12F8I2N2, have been synthesized and structurally characterized by single-crystal X-ray diffraction. The crystal structure of the non-iodinated homologue (E)-bis(2,3,5,6-tetrafluorophenyl)diazene, C12H2F8N2, is also reported. It is demonstrated that the studied halogen-bond donor molecule is a reliable tecton for assembling halogen-bonded co-crystals with potential photoresponsive behaviour. The azo group is not involved in any specific intermolecular interactions in any of the co-crystals studied, which is an interesting feature in the context of enhanced photoisomerization behaviour and photoactive properties of the material systems.

(4,7,13,16,21,24-Hexaoxa-1,10-di-aza-bicyclo-[8.8.8]hexa-cosa-ne)sodium iodide-1,1,2,2,tetra-fluoro-1,2-diiodo-ethane (2/3).

The title complex (CX1), [Na(C18H36N2O6)]I·1.5C2F4I2, is a three-component adduct containing a [2.2.2]-cryptand, sodium iodide and 1,1,2,2-tetra-fluoro-1,2-di-iodo-ethane. The di-iodo-ethane works as a bidentate halogen-bonding (XB) donor, the [2.2.2]-cryptand chelates the sodium cation, and the iodide counter-ion acts as a tridentate XB acceptor. A (6,3) network is formed in which iodide anions are the nodes and halocarbons the sides. The network symmetry is C 3i and the I⋯I(-) XB distance is 3.4492 (5) Å. This network is strongly deformed and wrinkled. It forms a layer 9.6686 (18) Šhigh and the inter-layer distance is 4.4889 (10) Å. The cations, inter-acting with each other via weak O⋯H hydrogen bonds, are confined between two anionic layers and also form a (6,3) net. The structure of CX1 is closely related to that of the KI homologue (CX2). The 1,1,2,2,-tetrafluoro-1,2-diiodoethane molecule is rotationally disordered around the I⋯I axis, resulting in an 1:1 disorder of the C2F4 moiety.

Memory of chirality approach to the enantiodivergent synthesis of chiral benzo[d]sultams.

The "memory of chirality" stereodivergent synthesis of polyfluorobenzo[d]sultams has been developed. The interest of this protocol resides in the possibility of using the chirality of a starting sulfonamide single enantiomer to synthesize the target sultams in both absolute configurations, by using different base systems, under homogeneous conditions.

Multinuclear Solid-State Magnetic Resonance as a Sensitive Probe of Structural Changes upon the Occurrence of Halogen Bonding in Co-crystals.

Although the understanding of intermolecular interactions, such as hydrogen bonding, is relatively well-developed, many additional weak interactions work both in tandem and competitively to stabilize a given crystal structure. Due to a wide array of potential applications, a substantial effort has been invested in understanding the halogen bond. Here, we explore the utility of multinuclear ((13)C, (14/15)N, (19)F, and (127)I) solid-state magnetic resonance experiments in characterizing the electronic and structural changes which take place upon the formation of five halogen-bonded co-crystalline product materials. Single-crystal X-ray diffraction (XRD) structures of three novel co-crystals which exhibit a 1:1 stoichiometry between decamethonium diiodide (i.e., [(CH3)3N(+)(CH2)10N(+)(CH3)3][2 I(-)]) and different para-dihalogen-substituted benzene moieties (i.e., p-C6X2Y4, X=Br, I; Y=H, F) are presented. (13)C and (15)N NMR experiments carried out on these and related systems validate sample purity, but also serve as indirect probes of the formation of a halogen bond in the co-crystal complexes in the solid state. Long-range changes in the electronic environment, which manifest through changes in the electric field gradient (EFG) tensor, are quantitatively measured using (14)N NMR spectroscopy, with a systematic decrease in the (14)N quadrupolar coupling constant (CQ) observed upon halogen bond formation. Attempts at (127)I solid-state NMR spectroscopy experiments are presented and variable-temperature (19)F NMR experiments are used to distinguish between dynamic and static disorder in selected product materials, which could not be conclusively established using solely XRD. Quantum chemical calculations using the gauge-including projector augmented-wave (GIPAW) or relativistic zeroth-order regular approximation (ZORA) density functional theory (DFT) approaches complement the experimental NMR measurements and provide theoretical corroboration for the changes in NMR parameters observed upon the formation of a halogen bond.

Tetra-phenyl-phospho-nium iodide-1,3,5-tri-fluoro-2,4,6-tri-iodo-benzene-methanol (3/4/1).

The crystallization of a 1:1 molar solution of 1,3,5-tri-fluoro-2,4,6-di-iodo-benzene (TFTIB) and tetra-phenyl-phosponium iodide (TPPI) from methanol produced tetra-gonal needles of pure TPPI and tabular pseudo-hexa-gonal truncated bipyramids of the title compound, 3C24H20P(+)·3I(-)·4C6F3I3·CH4O or (TPPI)3(TFTIB)4·MeOH. The asymmetric unit is composed of six TPPI mol-ecules, eight TFTIB mol-ecules and two methanol mol-ecules, overall 16 constituents. The formation of the architecture is essentially guided by a number of C-I⋯I(-) halogen bonds (XB), whose lengths are in the range 3.276 (1)-3.625 (1) Å. Layers of supra-molecular polyanions are formed parallel to (10-1) wherein iodide anions function as penta-, tetra- or bidentate XB acceptors. The structure is not far from being P21/n, but the centrosymmetry is lost due to a different conformation of a single couple of cations and the small asymmetry in the formed supra-molecular anion. One methanol mol-ecule is hydrogen bonded to an iodide anion, while the second is linked to the first one via an O-H⋯O contact. This second methanol mol-ecule is more loosely pinned in its position than the first and presents very high anisotropic displacement parameters and a seeming shortening of the C-O bond length. The crystal studied was refined as a perfect inversion twin.

[5,11,17,23-Tetra-tert-butyl-25,27-(3,6-dioxaoctan-1,8-di-oxy)-26,28-bis-(pyridin-2-ylmeth-oxy)calix[4]arene]sodium iodide-1,2,4,5-tetra-fluoro-3,6-diiodo-benzene-methanol (2/3/4).

The title compound, [Na(C62H76N2O6)]I·1.5C6F4I2·2CH3OH, is composed of five components: a calix[4]arene derivative (hereinafter C4), a sodium cation, an iodide anion, a 1,2,4,5-tetra-fluoro-3,6-diiodo-benzene (tFdIB) mol-ecule and a methanol mol-ecule in a 1:1:1:1.5:2 ratio. The complex shows several inter-esting features: (i) the polyoxygenated loop of C4 effectively chelates a sodium cation in the form of a distorted octahedron and separates it from the iodide counter-ion, the shortest Na(+)⋯I(-) distance being greater than 6.5 Å; (ii) the cavity of C4 is filled by a methanol mol-ecule; (iii) a second methanol mol-ecule is hydrogen-bonded to the N atom of a pyridinyl substituent pendant of C4 and halogen-bonded to the I atom of a tFdIB mol-ecule; (iv) the two I atoms of another tFdIB mol-ecule are halogen-bonded to two iodide anions, which act as monodentate halogen-bond acceptorss; (v) one of the two tFdIB molecules is located about a centre of inversion.