Halogen Bonds of Halogen(I) Ions─Where Are We and Where to Go?

Halenium ions, X+, are particularly strong halogen-bond donors that interact with two Lewis bases simultaneously to form linear [D···X···D]+-type halonium complexes. Their three-center, four-electron halogen bond is both fundamentally interesting and technologically valuable as it tames the reactivity of halogen(I) ions, opening up new horizons in a variety of fields including synthetic organic and supramolecular chemistry. Understanding this bonding situation enables the development of improved halogen(I) transfer reactions and of advanced functional materials. Following a decade of investigations of basic principles, the range of applications is now rapidly widening. In this Perspective, we assess the status of the field and identify its key advances and the main bottlenecks. Clearing common misunderstandings that may hinder future progress, we aim to inspire and direct future research efforts.

Crystal structures of sixteen phosphane chalcogenide complexes of gold(I) chloride, bromide and iodide

The structures of 16 phosphane chalcogenide complexes of gold(I) halides, with the general formula R 1 3- nR 2 nPEAuX (R 1 = t-butyl; R 2 = isopropyl; n = 0 to 3; E = S or Se; X = Cl, Br or I), are presented. The eight possible chlorido derivatives are: 1a, n = 3, E = S; 2a, n = 2, E = S; 3a, n = 1, E = S; 4a, n = 0, E = S; 5a, n = 3, E = Se; 6a, n = 2, E = Se; 7a, n = 1, E = Se; and 8a, n = 0, E = Se, and the corresponding bromido derivatives are 1b-8b in the same order. However, 2a and 2b were badly disordered and 8a was not obtained. The iodido derivatives are 2c, 6c and 7c (numbered as for the series a and b). All structures are solvent-free and all have Z' = 1 except for 6b and 6c (Z' = 2). All mol-ecules show the expected linear geometry at gold and approximately tetra-hedral angles P-E-Au. The presence of bulky ligands forces some short intra-molecular contacts, in particular H⋯Au and H⋯E. The Au-E bond lengths have a slight but consistent tendency to be longer when trans to a softer X ligand, and vice versa. The five compounds 1a, 5a, 6a, 1b and 5b form an isotypic set, despite the different alkyl groups in 6a. Compounds 3a/3b, 4b/8b and 6b/6c form isotypic pairs. The crystal packing can be analysed in terms of various types of secondary inter-actions, of which the most frequent are 'weak' hydrogen bonds from methine hydrogen atoms to the halogenido ligands. For the structure type 1a, H⋯X and H⋯E contacts combine to form a layer structure. For 3a/3b, the packing is almost featureless, but can be described in terms of a double-layer structure involving borderline H⋯Cl/Br and H⋯S contacts. In 4a and 4b/8b, which lack methine groups, Cmeth-yl-H⋯X contacts combine to form layer structures. In 7a/7b, short C-H⋯X inter-actions form chains of mol-ecules that are further linked by association of short Au⋯Se contacts to form a layer structure. The packing of compound 6b/6c can conveniently be analysed for each independent mol-ecule separately, because they occupy different regions of the cell. Mol-ecule 1 forms chains in which the mol-ecules are linked by a Cmethine⋯Au contact. The mol-ecules 2 associate via a short Se⋯Se contact and a short H⋯X contact to form a layer structure. The packing of compound 2c can be described in terms of two short Cmethine-H⋯I contacts, which combine to form a corrugated ribbon structure. Compound 7c is the only compound in this paper to feature Au⋯Au contacts, which lead to twofold-symmetric dimers. Apart from this, the packing is almost featureless, consisting of layers with only translation symmetry except for two very borderline Au⋯H contacts.

Halogen Bond of Halonium Ions: Benchmarking DFT Methods for the Description of NMR Chemical Shifts

Because of their anisotropic electron distribution and electron deficiency, halonium ions are unusually strong halogen-bond donors that form strong and directional three-center, four-electron halogen bonds. These halogen bonds have received considerable attention owing to their applicability in supramolecular and synthetic chemistry and have been intensely studied using spectroscopic and crystallographic techniques over the past decade. Their computational treatment faces different challenges to those of conventional weak and neutral halogen bonds. Literature studies have used a variety of wave functions and DFT functionals for prediction of their geometries and NMR chemical shifts, however, without any systematic evaluation of the accuracy of these methods being available. In order to provide guidance for future studies, we present the assessment of the accuracy of 12 common DFT functionals along with the Hartree-Fock (HF) and the second-order Møller-Plesset perturbation theory (MP2) methods, selected from an initial set of 36 prescreened functionals, for the prediction of 1H, 13C, and 15N NMR chemical shifts of [N-X-N]+ halogen-bond complexes, where X = F, Cl, Br, and I. Using a benchmark set of 14 complexes, providing 170 high-quality experimental chemical shifts, we show that the choice of the DFT functional is more important than that of the basis set. The M06 functional in combination with the aug-cc-pVTZ basis set is demonstrated to provide the overall most accurate NMR chemical shifts, whereas LC-ωPBE, ωB97X-D, LC-TPSS, CAM-B3LYP, and B3LYP to show acceptable performance. Our results are expected to provide a guideline to facilitate future developments and applications of the [N-X-N]+ halogen bond.

O-I-O halogen bond of halonium ions

The reactivity of halonium ions is conveniently modulated by three-center, four-electron halogen bonds. Such stabilized halonium complexes are valuable reagents for oxidations and halofunctionalization reactions. We report the first example of the stabilization of a halenium ion in a three-center, four-electron halogen bond with two oxygen ligands. The influence of electron density and solvent on the stability of the complexes is assessed. O-I-O halogen bond complexes are applicable as synthetic reagents and as supramolecular synthons.

Halogen bonds of halonium ions

Halonium ions are particularly strong halogen bond donors, and are accordingly valuable tools for a variety of fields, such as supramolecular and synthetic organic chemistry.

Phosphanchalkogenide und ihre Metallkomplexe. V. Derivate von [2.2]Paracyclophanylphosphanena

The known compound diphenyl([2.2]paracyclophanyl)phosphane 1 reacted smoothly with elemental sulfur or selenium to give the phosphane chalcogenides 3 and 4. The corresponding chlorido- or bromido-gold(I) complexes were however not obtained by the usual reaction with (tht)AuCl or (tht)AuBr. For the latter, direct oxidation of the reaction mixture with elemental bromine led to small quantities of {(PCP)PPh2Br}+ [AuBr4]− 5 (PCP = [2.2]paracyclophanyl). Attempts to obtain the alkyl phosphane di-isopropyl([2.2]paracyclophanyl)phosphane 2 were at first unsuccessful because of contamination by the phosphonium derivatives [ i Pr2(PCP)PH]+X− (X = Cl 6, X = Br 7), but the mixture was found to react with elemental sulfur or selenium to give the phosphane chalcogenides 8 and 9. The gold(I) complexes (PCP) i Pr2PEAuX [E = S, X = Cl (10), Br (11); E = Se, X = Cl (12), Br (13)] were obtained by the reactions of 8 and 9 with (tht)AuX. The chlorido complexes 10 and 12 were oxidized by PhICl2 to the gold(III) complexes (PCP) i Pr2PEAuCl3 14 (E = S) and 15 (E = Se). An excess of PhICl2 led to the fully oxidized compound {(PCP) i Pr2PSeCl}+[AuCl4]− 16. The bromido complexes 11 and 13 were oxidized by elemental bromine to (PCP) i Pr2PEAuBr3 17 (E = S) und 18 (E = Se), the latter however with a poor yield. Further oxidation was not achieved. The reactions of the chalcogenides 3, 4, 8 and 9 with elemental iodine led to the products 19, 20, 21 (1:1 adducts) and 22 (1:1 adduct with additional disordered diiodine), respectively.

Bromination and iodination of diphosphane dichalcogenides

The diphosphane dichalcogenides dppmSe₂, dppeSe₂, dⁱprpmSe₂, dⁱprpeSe₂, dppmS₂ and dppeS₂, [dppm = bis(diphenylphosphano)methane, dppe = bis(diphenylphosphano)ethane, dⁱprpm = bis(diisopropylphosphano)methane, dⁱprpe = bis(diisopropylphosphano)ethane] have been treated with elemental bromine or iodine and the products 1-15 characterized by single crystal X-ray diffraction. DppeSe₂ with bromine or iodine gives the simple (and previously known) adducts dppe(SeBr₂)₂1 or dppe(SeI₂)₂2 respectively; dⁱprpeSe₂ reacts similarly to give dⁱprpe(SeBr₂)₂3 (at 0 °C) or dⁱprpe(SeI₂)₂4. The bromine derivatives display a T-shaped geometry at selenium, whereas the iodine derivatives involve linear Se-I-I groups. With dppmSe₂ and bromine, a product was formed at -50 °C, but could not be isolated; at room temperature the reaction leads to a mixture of products, from which only [dppmBr₂]²⁺ [Se₂Br₁₀]^2-5 (the anion involving a dibromine molecule coordinating to a Se centre) could be isolated. Reaction of dppeSe₂ with excess bromine at room temperature leads to [dppeBr₂]²⁺·½[SeBr₆]^2-·[SeBr₂·Br₂·Br₃]^-6. The reaction of dppmSe₂ with iodine gives the 1 : 2 adduct [dppmSe₂I]⁺ [I₃]^-7, the cation of which contains a novel five-membered CP₂Se₂ heterocycle. In a 1 : 1 ratio the same reaction gives another 1 : 2 adduct, [I₂Se(dppm)Se-I-Se(dppm)SeI₂]⁺ [I₃]^-8. DⁱprpmSe₂ with iodine gives [dⁱprpmSe₂I]⁺ [I₃]^-9, analogous to 7, and with bromine at 0 °C the corresponding product [dⁱprpmSe₂Br]⁺ [Br₃]^-10. At room temperature, dⁱprpeSe₂ reacts with bromine to give a mixture of [dⁱprpe(SeBr₂)Br]⁺ Br^-11 and [dⁱprpeBr₂]²⁺ [SeBr₄]^2-12, whereas dⁱprpmSe₂ forms [dⁱprpmBr₂]²⁺ [SeBr₄]^2-13, analogous to 12. The disulfides dppmS₂ and dppeS₂ react with bromine to give [dppmSBr]⁺ [Br₃]^-14 and [dppeSBr]⁺ [Br₃]^-15.

Structural diversity in the products formed by the reactions of 2-arylselanyl pyridine derivatives and dihalogens

The presence of competing donor sites in L1–L4 influences their reactivity towards dihalogens and interhalogens.

Hydrogen- and halogen-bond cooperativity in determining the crystal packing of dihalogen charge-transfer adducts: a study case from heterocyclic pentatomic chalcogenone donors

A synergic cooperation between HB and XB interactions determines the supramolecular architectures in dihalogen CT adducts of hydantoin-like chalcogen donors.

Phosphanchalkogenide und ihre Metallkomplexe. III. Halogenierungsprodukte der Gold(I)komplexe Ph3PEAuX (E = S oder Se; X = Cl, Br oder I)

The complexes Ph3PEAuI (E = S, Se; 1, 2) were obtained from the reaction of Ph3PEAuCl with KI; they are appreciably less stable than their chloro and bromo analogues. The X-ray structures were determined, whereby 1 proved to be contaminated by a small amount of Ph3PS·I2. Oxidation of 1 and 2 with elemental iodine led to the adducts Ph3PEAuI·0.5I2 (3 and 4), but X-ray investigation of a crystal initially assumed to be 3 proved it to be a 1:1 mixture of 3 with the adduct Ph3PS·1.5I2, while in 4 the iodine molecule was severely disordered, which prevented successful refinement of the structure. Decomposition of 4 by loss of gold led to Ph3PSeI2·1.5I2 4a. Complexes Ph3PEAuX (E = S, Se; X = Br, Cl) were oxidized by elemental bromine (X = Br) or PhICl2 (X = Cl) to Ph3PEAuX 3 (5, 6, 9, 10); none of these X-ray structures could be refined satisfactorily because of diffuse scattering phenomena. Further oxidation led to the ionic compounds [Ph3PEX]+ [AuX 4]– (X = Br, E = S, Se: 7, 8; X = Cl, E = S, 11) or [Ph3PSeCl]+ 0.5[Au4Cl10Se2]2– (12), containing the novel groupings P–E–X. X-ray structures confirmed the nature of all four of these compounds, which display longer P–E bonds than the gold(I) starting materials and short X···X and/or E···X contacts between cations and anions.

The reactions of para-halo diaryl diselenides with halogens. A structural investigation of the CT compound (p-FC6H4)2Se2I2, and the first reported “RSeI3” compound, (p-ClC6H4)SeI·I2, which contains a covalent Se-I bond

The reactions of the diaryl-diselenides (p-FC(6)H(4))(2)Se(2) and (p-ClC(6)H(4))(2)Se(2) with diiodine have been investigated. Species of stoichiometry "RSeI" are formed when the ratio employed is 1:1. The solid-state structure of "(p-FC(6)H(4))SeI" has been determined, and shown to be a charge-transfer (CT) adduct, (p-FC(6)H(4))(2)Se(2)I(2), where the Se-Se bond is retained and the diiodine molecule interacts with only one of the selenium atoms. The Se-I bond in (p-FC(6)H(4))(2)Se(2)I(2) is 2.9835(12) Å, which is typical for a (10-I-2) Se-I-I CT system. When diiodine is reacted in a 3:1 ratio with (p-XC(6)H(4))(2)Se(2) (X = F, Cl) species of stoichiometry "RSeI(3)" are formed. The structure of "(p-ClC(6)H(4))SeI(3)" reveals that this is not a selenium(IV) compound, but is better represented as a selenium(II) CT adduct, (p-ClC(6)H(4))SeI·I(2). The Se-I bond to the diiodine molecule is typical in magnitude for a CT adduct, Se-I: 2.8672(5) Å, whereas the other Se-I bond is much shorter, Se-I: 2.5590(6) Å, and is a genuine example of a rarely observed covalent Se-I bond, which appears to be stabilised by a weak Se···I interaction from a neighbouring iodine atom. The reaction of (p-ClC(6)H(4))SeI with Ph(3)P results in the formation of a CT adduct, Ph(3)PSe(p-ClC(6)H(4))I, which has a T-shaped geometry at selenium (10-Se-3). By contrast, the reaction of (p-FC(6)H(4))SeI with Ph(3)P does not form an adduct, but results in the formation of Ph(3)PI(2) and (p-FC(6)H(4))(2)Se(2).

Halogenation of (phosphine chalcogenide)gold(I) halides; some unexpected products

The monoselenide of 1,8-bis(diphenylphosphino)naphthalene reacts with (tht)AuCl to give the gold(III) system [(dppnAuSe)(2)](2+) 2Cl(-) (1); bromination of the bromogold(I) complex of the 1,2-bis(diphenylphosphino)methane monosulfide ligand furnishes the tribromide salt (2a) of a gold(III) cation [LAuBr(2)](+); bromination of the bromogold(I) complex of the 1,2-bis(diphenylphosphino)benzene monosulfide ligand leads to a mixed bromide/tetrabromoaurate salt (3) of a heterocyclic dication involving a [-PPh(2)-S-PPh(2)-](2+) moiety; analogous reactions of triphenylphosphine sulfide and selenide complexes lead to tetrabromoaurate salts (4a and 4b) of the (bromochalcogeno)phosphonium cations Ph(3)PEBr(+).

Concomitant but disappearing: two polymorphs of 1,4-bis(tribromomethyl)benzene

The title compound, C(8)H(4)Br(6), (I), initially crystallized from deuterochloroform as the comcomitant polymorphs (Ia) (prisms, space group P2(1)/n, Z = 2) and (Ib) (hexagonal plates, space group C2/c, Z = 4). The molecules in both forms display crystallographic inversion symmetry. All further attempts to crystallize the compound led exclusively to (Ib), so that (Ia) may be regarded as a `disappearing polymorph'. Surprisingly, however, the density of (Ia) is greater than that of (Ib). The only significant difference between the molecular structures is the orientation of the CBr(3) groups. The molecular packing of both structures is largely determined by Br···Br interactions, although (Ia) also displays a C-H···Br hydrogen bond and both polymorphs display one Br···π contact. For (Ia), six of the eight contacts combine to form a tube-like substructure parallel to the a axis. For (Ib), the two shortest Br···Br contacts link `half' molecules consisting of C-CBr(3) groups to form double layers parallel to (001) in the regions z ≃ ¼, ¾.

2,5-Bis(bromomethyl)biphenyl

In the title compound, C₁₄H₁₂Br₂, the Br atoms lie on opposite sides of their ring plane. The biphenyl inter­planar angle is 53.52 (8)°. The packing is characterized by several H⋯Br contacts to each Br atom, but at long distances of 3.07–3.43 Å.