International audienceThe distinction between cocrystals and salts is usually investigated in hydrogen-bonded systems as A?H···B ? [A]?···[H?B]+, where the position of the hydrogen atom actually defines the ionicity of the complex. The same distinction, but in halogen-bonded systems, is addressed here, in complexes formed out of N-iodoimide derivatives as halogen bond donors, and pyridines as halogen-bond acceptors, anticipating that the position of the iodine atom in these A?I···B ? [A]?···[I?B]+ systems will also define their degree of ionicity. We show that the crystalline halogen-bonded complexes of N-iodosuccinimide (NIS) with pyridine, 4-methylpyridine, and 4-dimethylaminopyridine can be described as ?close-to-neutral? cocrystals while the crystalline halogen-bonded complex of N-iodosaccharin (NISac) with 4-dimethylaminopyridine adopts a ?close-to-ionic? structure. Theoretical calculations were performed (i) in gas phase on isolated NIS···Py-NMe2 and NISac···Py-NMe2 complexes, and (ii) on the periodic crystal phases, and combined with the topological analysis of the electron density distribution ?(r). We demonstrate unambiguously that the crystal environment actually plays a crucial role in the stabilization of the ?close-to-ionic? structure of the NISac···Py-NMe2 complex. An external homogeneous electric field ε applied to this complex (all atoms frozen at the crystalline geometry, except iodine) in either gas phase (ε = 3.7 GV m?1) or periodic pseudo-isolated configuration (ε = 2.8 GV m?1) is able to shift the iodine atom at the crystal geometry, miming the polarization effect induced by the local crystal electric field. The strong influence of the crystalline environment on the iodine position is demonstrated by using plane wave DFT periodic calculations on optimized NIS·Py-NMe2 and NISac·Py-NMe2 crystal structures, as well as by applying this plane wave basis set formalism to a hypothetical solid where the halogen-bonded complexes are pushed apart from each other within a periodic syste

Chalcogen bonding has been investigated in terms of the electron density distribution ρ(r) around chalcogen atoms. The evolution of ρ(r) along the series of chalcogen atoms is shown based on ab initio calculations on chalcogenophthalic anhydrides C8O2H4Chal (Chal = O, S, Se, and Te), where the Chal atom is in its sp 3 hybridization. From a detailed analysis of the experimental and theoretical electron density and the L(r) = −∇2ρ(r) function in the crystal phase of C8O2H4Se, we characterize directionality and strength of chalcogen bonding (Se···O and Se···Se) and hydrogen bonding (Se···H) interactions. In addition, several isolated dimers and a trimer of C8O2H4Se have been also studied at the X-ray geometry in order to compare interaction energies with those estimated from the measured electron density. Similarly to halogen atoms in halogen bonding interactions, the anisotropic distribution of ρ(r) around the Chal atoms was found to be at the origin of chalcogen bonding. Therefore, the concepts, developed earlier for halogen bonding, are extended here to chalcogen bonding interactions. From the results of this work, the L(r) function proves to be more precise than the σ-hole concept to identify electrophilic sites of Se-atoms in sp 3 hybridization.

Halogen bonding interactions between halide anions and neutral polyiodinated linkers are used for the elaboration of anion organic frameworks, by analogy with well-known MOF derivatives. The extended, 3-fold symmetry, 1,3,5-tris(iodoethynyl)-2,4,6-trifluorobenzene (1) cocrystallizes with a variety of halide salts, namely, Et3S(+)I(-), Et3MeN(+)I(-), Et4N(+)Br(-), Et3BuN(+)Br(-), Me-DABCO(+)I(-), Bu3S(+)I(-), Bu4N(+)Br(-), Ph3S(+)Br(-), Ph4P(+)Br(-), and PPN(+)Br(-). Salts with 1:1 stoichiometry formulated as (1)·(C(+),X(-)) show recurrent formation of corrugated (6,3) networks, with the large cavities thus generated, filled either by the cations and solvent (CHCl3) molecules and/or by interpenetration (up to 4-fold interpenetration). The 2:1 salt formulated as (1)2·(Et3BuN(+)Br(-)) crystallizes in the cubic Ia3 space group (a = 22.573(5) Å, V = 11502(4) Å(3)), with the Br(-) ion located on 3 site and molecule 1 on a 3-fold axis. The 6-fold, unprecedented octahedral coordination of the bromide anion generates an hexagonal three-dimensional network of Pa3 symmetry, as observed in the pyrite model structure, at variance with the usual, but lower-symmetry, rutile-type topology. In this complex system, the I centering gives rise to a 2-fold interpenetration of class Ia, while the cations and solvent molecules are found disordered within interconnected cavities. Another related cubic structure of comparable unit cell volume (space group Pa3̅, a = 22.4310(15) Å, V = 11286.2(13) Å(3)) is found with (1)2·(Et3S(+)I(-)).

Charge-assisted halogen bonding is unambiguously revealed from structural and electronic investigations of a series of isostructural charge-transfer complexes derived from iodinated tetrathiafulvalene and tetracyanoquinodimethane derivatives, (EDT-TTFI2)2(TCNQF(n)), n=0-2, which exhibit variable degrees of ionicity. The iodinated tetrathiafulvalene derivative, EDT-TTFI2, associates with tetracyanoquinodimethane (TCNQ) and its derivatives of increasing reduction potential (TCNQF, TCNQF2) through highly directional C-I⋅⋅⋅N≡C halogen-bond interactions. With the less oxidizing TCNQ acceptor, a neutral and insulating charge-transfer complex is isolated whereas with the more oxidizing TCNQF2 acceptor, an ionic, highly conducting charge-transfer salt is found, both of 2:1 stoichiometry and isostructural with the intermediate TCNQF complex, in which a neutral-ionic conversion takes place upon cooling. A correlation between the degree of charge transfer and the C-I⋅⋅⋅N≡C halogen-bond strength is established from the comparison of the structures of the three isostructural complexes at temperatures from 300 to 20 K, thus demonstrating the importance of electrostatics in the halogen-bonding interaction. The neutral-ionic conversion in (EDT-TTFI2)2(TCNQF) is further investigated through the temperature dependence of its magnetic susceptibility and the stretching modes of the C≡N groups.

The formation of charge-transfer salts with an iodinated tetrathiafulvalene derivative, EDT-TTF-I (1), has been investigated with a series of TCNQ acceptors of different oxidative strengths, namely, TCNQ, TCNQF, TCNQF2, and TCNQF4. These series of compounds have been prepared in order to investigate the effect of the charge in these complexes on the C–I···NC halogen bond interactions which can take place between 1 and the various TCNQs. With the most oxidizing TCNQF4 acceptor, a 1:1 compound formulated as (1)(TCNQF4) was obtained with a charge ρ(1) = +1 on 1, while with TCNQF2, a 2:1 conducting salt formulated as (1)2(TCNQF2) was characterized with ρ(1) ≈ +0.5. With TCNQ itself, both a 1:1 conducting phase, (1)(TCNQ) with ρ(1) ≈ +0.5, and a 2:1 (1)2(TCNQ) compound with ρ(1) = 0 were crystallized. Transport and magnetic properties were rationalized, based on the ρ(1) value and band structure calculations. CTTF–I···NC halogen bond interactions were observed in the mixed valence [ρ(1) = +0.5] salts and even in the neutral [ρ(1) = 0] compound, while they are surprisingly absent from the full charge transfer TCNQF4 salt. It is shown that TTF oxidation also activates the TTF sp2 hydrogen atom located α to the iodine atom, toward the preferential formation of CTTF–Hα···NC hydrogen bonds, also present in the mixed-valence salts. These series provide an opportunity to evaluate the relative strength of competing C–H···N hydrogen and C–I···N halogen bonds and their relative sensitivity to charge.

International audienceChirality and halogen bonding ability are combined in a single tetrathiafulvalene molecule (1) with iodo and dimethyl(propylenedithio) substituents, with II interactions developing along a 21 screw axis. The chiral mixed-valence conducting salt (1)2Cl exhibits a strongly 1D electronic structure

International audienceChiral, ditopic, halogen bond donor molecules are prepared from the reaction of C6F5I with three different enantiopure chiral diols, namely, (R,R)-2,3-butanediol, (R,R)-hydrobenzoin and S-binaphthol, with displacement of the fluorine atom para to the iodine atom in C6F5I, to give (R,R)-1, (R,R)-2 and (S)-3, respectively. Chiral, halogen-bonded networks with halide anions are investigated upon co-crystallisation of (R,R)-1 with (Et3S+, I−), (Et4N+, Br−), (n-Bu4N+, Br−) and (n-Pe4N+, Br−). In the first three salts with 1 : 1 stoichiometry, the halide anions are coordinated by two iodine atoms with short IX− (X− = I−, Br−) distances and acute (75–80°) IX−I angles, leading to the formation of chains, eventually organized into helices around twofold screw axes as in [(R,R)-1]·[Bu4NBr]. Co-crystallisation with tetrapentylammonium bromide affords a 2 : 1 stoichiometry salt, [(R,R)-1]2·[Pe4NBr], with a fourfold coordination around the Br− anion and the formation of a square lattice network built out of interconnected helices

Conducting and chiral [Ni(dmit)(2)] dithiolene salts were obtained by electrocrystallization of the radical [n-Bu(4)N][Ni(dmit)(2)] salt in the presence of chiral, enantiopure trimethylammonium cations. Three different cations were investigated, namely, (R)-Ph(Me)HC*-NMe(3)(+), (S)-((t)Bu)(Me)HC*-NMe(3)(+), and (S)-(1-Napht)MeHC*-NMe(3)(+), noted (R)-1, (S)-2, and (S)-3. Salts of 1:3 stoichiometry were obtained with (R)-1 and (S)-2, formulated as [(R)-1][Ni(dmit)(2)](3) and [(S)-2][Ni(dmit)(2)](3)·(CH(3)CN)(2). They both crystallize in the P2(1)2(1)2(1) chiral space group, with three crystallographically independent complexes exhibiting different oxidation degrees. Another salt with 2:5 stoichiometry was isolated with (S)-3. The semiconducting character of the three salts (σ(RT) = 20-30 × 10(-3) S cm(-1)) finds its origin in a strong electron localization, favored by the large number of crystallographically independent [Ni(dmit)(2)] complexes in these chiral structures and their association into weakly interacting dimeric or trimeric motifs. Racemic salts with the same cations, obtained only with difficulties with the tert-butyl-containing (rac)-2 cation, afforded similar trimerized structures. The observed unusual stoichiometry and strong charge localization is tentatively assigned to the size and anisotropic charge distribution of the cations.