Tullio Pilati

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.

2-Iodo-imidazolium receptor binds oxoanions via charge-assisted halogen bonding.

A detailed (1)H-NMR study of the anion binding properties of the 2-iodo-imidazolium receptor 1 in DMSO allows to fully attribute the observed affinities to strong charge-assisted C-I···X(-) halogen bonding (XB). Stronger binding was observed for oxoanions over halides. Phosphate, in particular, binds to 1 with an association constant of ca. 10(3) M(-1), which is particularly high for a single X-bond. A remarkably short C-I···O(-) contact is observed in the structure of the salt 1·H(2)PO(4)(-).

Hydrogen and halogen bonding drive the orthogonal self-assembly of an organic framework possessing 2D channels

Orthogonal self-assembly of an open organic framework showing 2D channels has been obtained by combining hydrogen and halogen bonding. The framework is able to host various guest molecules with a diverse set of steric demands and substitution patterns, and survives single-crystal-to-single-crystal guest exchanges from liquid and gas phases.

An Adaptable and Dynamically Porous Organic Salt Traps Unique Tetrahalide Dianions

Bis(I2) adducts of hexamethonium dihalides are pre-organized to respond dynamically to heating and reach a functional structure that favors the formation of the poorly stable and virtually unknown [I2Br2]2− and [I2Cl2]2− tetrahalides which could not be obtained in solution (see picture). The cavity-directed reactivity affords new opportunities for synthesis and interconversion of polyhalogen anions.

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

Abstract Unconventional ionic liquid crystals in which the liquid crystallinity is enabled by halogen‐bonded supramolecular anions [CnF2u2009n+1‐I⋅⋅⋅I⋅⋅⋅I‐CnF2u2009n+1]− are reported. The material system is unique in many ways, demonstrating for the first time 1)u2005ionic, halogen‐bonded liquid crystals, and 2)u2005imidazolium‐based ionic liquid crystals in which the occurrence of liquid crystallinity is not driven by the alkyl chains of the cation.

Photoalignment and Surface‐Relief‐Grating Formation are Efficiently Combined in Low‐Molecular‐Weight Halogen‐Bonded Complexes

It is demonstrated that halogen bonding can be used to construct low-molecular-weight supramolecular complexes with unique light-responsive properties. In particular, halogen bonding drives the formation of a photoresponsive liquid-crystalline complex between a non-mesogenic halogen bond-donor molecule incorporating an azo group, and a non-mesogenic alkoxystilbazole moiety, acting as a halogen bond-acceptor. Upon irradiation with polarized light, the complex exhibits a high degree of photoinduced anisotropy (order parameter of molecular alignment > 0.5). Moreover, efficient photoinduced surface-relief-grating (SRG) formation occurs upon irradiation with a light interference pattern, with a surface-modulation depth 2.4 times the initial film thickness. This is the first report on a halogen-bonded photoresponsive low-molecular-weight complex, which furthermore combines a high degree of photoalignment and extremely efficient SRG formation in a unique way. This study highlights the potential of halogen bonding as a new tool for the rational design of high-performance photoresponsive suprastructures.

Picturing the induced fit of calix[5]arenes upon n-alkylammonium cation binding

The serendipitous crystallization of a 1u2006:u20061 endo-cavity complex between calix[5]arene penta-ester 1a and n-butylammonium ions sheds light on the conformational rearrangement—from cone-in to cone-out—this calixarene family undergoes upon guest inclusion.

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.

Fluorination promotes chalcogen bonding in crystalline solids

Single crystal X-ray analysis shows that intermolecular C–S⋯F chalcogen bonds are by far the shortest contacts in the crystal structure of 2,2,4,4-tetrafluoro-1,3-dithietane. Analysis of the molecular surface electrostatic potentials indicates that these contacts result from the attraction between the negative potentials of fluorines and the positive regions resulting from the overlapping of sulfur and carbon positive σ-holes. In contrast, the lattice structure of 2,2,4,4-tetrachloro-1,3-dithietane is controlled by intermolecular C–Cl⋯Cl halogen bonds. The lattices of the two systems are thus controlled by two different σ-hole interactions.

Fluorine-induced J-aggregation enhances emissive properties of a new NLO push–pull chromophore

A new fluorinated push–pull chromophore with good second-order NLO properties even in concentrated solution shows solid state intermolecular aryl–fluoroaryl interactions leading to J-aggregates with intense solid state luminescence.