Coordination chemistry of anions through halogen-bonding interactions.

While an IUPAC definition for hydrogen bonding was only released in 2011 after decades of discussions in the scientific community (Arunann et al., 2011), it did not take such a long time to come up with an analogous definition of halogen bonding (Desiraju et al., 2013), following a revival of this interaction in the literature, which can be traced back to the early 1990s. The identification of this interaction is, however, not recent, as illustrated by the isolation of the H3N I2 adduct (Colin, 1814). Two centuries later, several review articles (Cavallo et al., 2016; Gilday et al., 2015; Fourmigué, 2009) have gathered together most available data. The halogen-bonding interaction (noted XB) is essentially described as an electrostatic interaction between a charge concentration (Lewis base) and a chargedepleted area, called an -hole, that a covalently bound halogen atom exhibits in the extension of this bond. Note that a charge-transfer covalent contribution can be found in the strongest halogen bonds, as illustrated for example in the triiodide anion following: I2 + I ! (I—I—I) . The presence of a halogen bond is characterized by a shortening of the interatomic distance, relative to the sum of the van der Waals (or ionic) radii of the interacting atoms, which is defined as the reduction ratio. The halogen-bond donor character of halogenated molecules is modulated by the nature of the halogen with I > Br >> Cl >> F, by the hybridization of the substituted C atom with C C—I > C CH—I > C—CH2—I, and by the electron-withdrawing ability of the carbon substituents, with a strong activation in aliphatic and aromatic perfluorinated substrates such as F2n + 1Cn—I or C6F6 nIn. Besides, dihalogens (I2, Br2, Cl2), interhalogens (I—Cl, I—CN) and Niodoimides (N-iodosuccinimide, N-iodosaccharin) are also strong halogen-bond donors. All Lewis bases are potentially XB acceptors and among them, anionic species, and particularly halide anions (Cl , Br , I ) thanks to a maximal charge concentration (Metrangolo et al., 2008a; Cavallo et al., 2010). 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. Earlier reports (Metrangolo et al., 2008b) have indeed shown that ‘the mutual induced fitting process elicits the tridentate coordination profile of both 1,3,5-trifluoro-2,4,6triiodobenzene (1) and I ions resulting in cation-templated anionic (6,3) networks’, with the small cations (Me3S , Me4P , Et4N ) sitting within the pores of the honeycomb network. With Et4N I , Et4P I and solid solutions thereof, the adducts even crystallize in the trigonal space group R 3c, while with larger cations (Metrangolo et al., 2008b; Triguero et al., 2008), the (6,3) networks were maintained but strongly corrugated. 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. 1a), the adduct with the larger EtPh3P Br salt crystallizes in the cubic space group Pa 3, with sixfold coordination around the bromide anion (Fig. 1c), as already observed with the smaller Et3BuN Br salt (Lieffrig et al., ISSN 2052-5206