1,3,5-Tri(iodoethynyl)-2,4,6-trifluorobenzene: halogen-bonded frameworks and NMR spectroscopic analysis

Abstract

Halogen bonding is the non-covalent interaction between the region of positive electrostatic potential associated with a covalently bonded halogen atom, named the σ-hole, and a Lewis base. Single-crystal X-ray diffraction structures are reported for a series of seven halogen-bonded cocrystals featuring 1,3,5-tris(iodoethynyl)-2,4,6-trifluorobenzene (1) as the halogen-bond donor, and bromide ions (as ammonium or phosphonium salts) as the halogen-bond acceptors: (1)·MePhPBr, (1)·EtPhPBr, (1)·acetonyl-PhPBr, (1)·PhPBr, (1)·[bis(4-fluorophenyl)methyl]triphenylphosphonium bromide, and two new polymorphs of (1)·EtBuNBr. The cocrystals all feature moderately strong iodine-bromide halogen bonds. The crystal structure of pure [bis(4-fluorophenyl)methyl]triphenylphosphonium bromide is also reported. The results of a crystal engineering strategy of varying the size of the counter-cation are explored, and the features of the resulting framework materials are discussed. Given the potential utility of (1) in future crystal engineering applications, detailed NMR analyses (in solution and in the solid state) of this halogen-bond donor are also presented. In solution, complex C andF multiplets are explained by considering the delicate interplay between various J couplings and subtle isotope shifts. In the solid state, the formation of (1)·EtBuNBr is shown through significant C chemical shift changes relative to pure solid 1,3,5-tris(iodoethynyl)-2,4,6-trifluorobenzene.

Coordination chemistry of anions through halogen-bonding interactions

“…It appears that in all these systems, the sizeable dispersion forces associated with the indirect IÁ Á ÁI contacts can favor these apparently surprising arrangements around the halide anion. Another remarkable point is also the coordination number around the halide anion, often found at 2 or 3, but easily extended to 6 as observed in the cubic structures described here (Szell et al, 2017;Lieffrig et al, 2013), and even to 8 in the co-crystal formed with CBr 4 as halogen-bond donor and Me 4 N + Br À (Lindeman et al, 2003). Note that in the latter and despite an apparent crowding around the bromide anion, the reduction ratio for the (Br 3 C-)BrÁ Á ÁBr À distance still amounts to 0.88, indicating a sizeable halogen-bond interaction.…”

“…Last but not least, the paper by Szell et al (2017) (Viger-Gravel et al, 2014). These last results illustrate how the halogen-bonding interaction, whose renewal in the 1990s was essentially rooted in crystal engineering studies, is now diffusing into all areas of chemistry (Cavallo et al, 2016), from theoretical and analytical investigations (in gas, liquid and solid states) to the optical, magnetic and conducting properties of crystalline materials, from soft matter issues (liquid crystals, polymers and gels) to biomolecular systems and their relevance to the pharmaceutical field.…”

“…The paper by Szell et al (2017) in this special issue of Acta Crystallographica Section B deals with such a crystal engineering approach through the formation of co-crystals associating ammonium or phosphonium halide salts with neutral, halogenated, activated halogen-bond donors such as sym-triiodotrifluorobenzene (1) or more specifically here its extended analog, 1,3,5-tris(iodoethynyl)-2,4,6-trifluorobenzene) (2). While ditopic halogen-bond donors such as diiodoacetylene (I-C C-I) or para-diiodotetrafluorobenzene most often lead to one-dimensional systems in the presence of halide anions, it was expected that threefold symmetric molecules such as (1) or (2) would provide higher symmetry structures (trigonal, hexagonal, cubic) that would express the molecular threefold symmetry of the halogen-bond donor.…”

“…In a search for larger pores, the extended halogen-bond donor (2) was recently designed (Lieffrig et al, 2013) and further used here with another set of cations of varying size and shape (Szell et al, 2017). Remarkably, besides the above-described honeycomb networks with tricoordinated halide anions (Fig.…”

Mechanochemical Preparations of Anion Coordinated Architectures Based on 3‐Iodoethynylpyridine and 3‐Iodoethynylbenzoic Acid

“…Halogen bonding has become an important interaction for anion coordination (C−I⋅⋅⋅X; X=Cl − , Br − , I − ), offering directional scaffolding for crystal engineering. Several crystal structures have been reported featuring anions coordinated by halogen bond donors, such as 1,4‐diiodotetrafluorobenzene and 1,3,5‐tri(iodoethynyl)‐2,4,6‐trifluorobenzene . This C−I⋅⋅⋅X − motif has been shown to be of use in several classes of functional materials, such as in halide recognition and rotaxanes .…”