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Journal logoSTRUCTURAL

ISSN: 2053-2296

Halogen, chalcogen, pnictogen and tetrel bonds: structural chemistry and beyond

Edited by Lee Brammer, Anssi Peuronen and Thomas M. Roseveare

This virtual special issue of Acta Crystallographica Section C: Structural Chemistry brings together 12 articles that cover a variety of aspects associated with halogen, chalcogen, pnictogen and tetrel bonds. There are articles that focus on structure, bonding and bond strength, articles that investigate co-operation or competition between different classes of interactions, articles that explore the relationship between these interactions and chemical or physical properties of the compounds and materials involved, and, although all articles involve characterization by single-crystal X-ray diffraction, a number have a prominent focus on other experimental techniques or are combined with interpretation from theoretical calculations. Halogen bonding and chalcogen bonding dominate these articles, consistent with the wider literature.

Highlighted illustration

Cover illustration: A snapshot of current progress in the field of halogen bonds, chalcogen bonds, pnictogen bonds, tetrel bonds and other σ-hole interactions. Courtesy of Brammer et al. [Acta Cryst. (2023), 79, 204–216].

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A com­pilation of review articles on σ-hole inter­actions published since 2013 is presented alongside an overview of the 11 articles in the special issue on this topic.

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Three new chalco­gen-bonded cocrystals were synthesized and characterized by single-crystal X-ray diffraction, powder X-ray diffraction and 77Se/125Te magic-angle spinning NMR spectroscopy.

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Hexa- and octa­bromo­ferrocene, as well as a mercury derivative of deca­bromo­ferrocene, were isolated from mixtures of polybromo­ferrocenes and characterized by X-ray diffraction. The crystals show halogen and hydrogen bonding, which act co-operatively.

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The series of cocrystals of 1,4-di­aza­bicyclo­[2.2.2]octane (DABCO) with 1,2-di­iodo­acetyl­ene (C2I2), 1,2-di­iodo­ethene (1,2-C2H2I2) and 1,2-di­iodo­ethane (1,2-C2H4I2) illustrate the correlation between the strength of the α-C—I⋯N halogen bond and polarization of the I atom as a function of the α-C-atom hybridization.

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The crystal structure of 2,4,6-tri­methyl­pyrylium tetra­fluoro­borate reveals a significant contribution of fluorine-centred inter­actions, com­prising F⋯O, F⋯π and C—H⋯F contacts, which governs the crystal packing in the solid state. The qu­anti­tative and qualitative aspects of these contacts were also established.

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We have analyzed why single crystals of 4-chloro­benzo­nitrile and 4-bromo­benzo­nitrile are com­pliant but 4-iodo­benzo­nitrile is brittle. Geometry-corrected statistical analysis and DFT calculations both suggest that the C—I⋯N≡C halogen bond is stronger, as well as highly directional, com­pared to its bromo and chloro counterparts, which make the iodo crystal brittle.

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A series of 18 cocrystals were obtained through the combination of the heterocyclic mol­ecules imidazolidine-2-thione, 2-mercaptobenzimidazole, 2-mercapto-5-methyl­benz­imidazole, 2-mercaptobenzoxazole, and 2-mercaptobenzo­thia­zole with the common halogen-bond donors 1,2-, 1,3-, and 1,4-di­iodo­tetra­fluoro­benzene, 1,3,5-tri­fluoro­tri­iodo­benzene, and tetra­iodo­ethyl­ene. A rich series of hydrogen-, halogen-, and chalcogen-bonding inter­actions were observed.

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Four isostructural chalcogena­diazole derivatives have been synthesized and characterized using SC-XRD, MEPs and Hirshfeld surfaces. The structural influence and balance between halogen and chalcogen bonds have been systematically analyzed and rationalized.

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1,3-Bis(benzimidazoliumyl)benzene-based chalcogen-bonding catalysts were crystallized and their intermolecular interactions investigated. Depending on the chalcogen employed and the crystallization conditions, different binding motifs were found. With sulfur centres as Lewis acidic interaction sites, only weak chalcogen bonding was observed. For selenium-based compounds, however, different bidentate coordination motifs were found between the chalcogen-bonding donor and the respective counter-ions.

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Two binary cocrystals were successfully assembled from 1,4-di­iodo­tetra­fluoro­benzene and 1,3,5-tri­fluoro-2,4,6-tri­iodo­benzene with flexible 2-{[(naphthalen-2-yl)meth­yl]sulfan­yl}pyridine 1-oxide (NTPO), mainly by C—I⋯O—N+ halogen bonds and π-hole⋯π-bond inter­actions. The introduction of iodo­perfluoro­benzene has a significant influence on the conformation of the NTPO mol­ecule.

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The crystal structure of the pyridine analog of the selenium pharmaceutical ebselen is characterized by one-dimensional N—Se chalcogen-bonded chains where the pyridine N atom is the chalcogen-bond acceptor. Charge density analysis using high-resolution Mo Kα X-radiation and subsequent multipole refinement reveals a clear region of positive electrostatic potential at the antipode to the Se—N bond of the isoselenazole moiety. In the N-methylated iodide and tosylate salts, the iodide/tosylate counter-ions are strongly chalcogen bonded to the Se atom.

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Pyridine-4-thiol has been explored as an acceptor for a halogen-bond (HaB) inter­action. For this com­pound, it is the S atom instead of the pyri­dine group that is the moiety of choice for establishing the inter­actions, producing a betaine-type structure. Inter­estingly, when left in solution, this com­pound forms a methyl­ene-bridged sulfide that implies the activation of the CH2Cl2 solvent. The pre-organization imposed by the HaB seems to have a clear influence on the outcome.

Special and virtual issues

Acta Crystallographica Section C has recently published special issues on

Polyoxometalates (November 2018)

NMR Crystallography (March 2017)

Scorpionates: a golden anniversary (November 2016)

Full details are available on the special issues page.

The latest virtual issue features Coordination polymers, with an introduction by Len Barbour.

What are the 'most read' articles from the recent special issues?

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