The Interhalogen Cations [Br2 F5 ]+ and [Br3 F8 ]+

[Br4F21]- - a unique molecular tetrahedral interhalogen ion containing a μ4-bridging fluorine atom surrounded by BrF5 molecules

The reaction of [NMe4][BrF6] with an excess of BrF5 leads to the compound [NMe4][Br4F21]·BrF5. It features molecular [(μ4-F)(BrF5)4]- anions of tetrahedron-like shape containing central μ4-bridging F atoms coordinated by four BrF5 molecules. It is the most BrF5-rich fluoridobromate anion by mass. Quantum-chemical calculations showed that the μ4-F-Br bonds within the anion are essentially ionic in nature. The compound is the first example where F atoms bridge μ4-like neither to metal nor to hydrogen atoms. It was characterized by Raman spectroscopy and by single-crystal X-ray diffraction. The latter showed surprisingly that its crystal structure is related to the intermetallic half-Heusler compound and structure type MgAgAs.

[(μ3 -F)(BrF5 )3 ]- - An Unprecedented Molecular Fluoridobromate(V) Anion in Cs[Br3 F16 ]

The reaction of Cs[BrF6 ] with BrF5 gave the compound Cs[Br3 F16 ] with the unprecedented propeller-shaped, C3 -symmetric [(μ3 -F)(BrF5 )3 ]- anion. All other currently known fluoridobromates(V) contain only octahedral [BrF6 ]- anions, which, unlike the related [IF6 ]- anions, never exhibited stereochemical activity of the lone pair on the Br atoms. Despite the same coordination number of six for the Br atom in the [BrF6 ]- and [(μ3 -F)(BrF5 )3 ]- anions, the longer μ3 -F-Br bonds provide additional space, allowing the lone pairs on the Br atoms to become stereochemically active. Cs[Br3 F16 ] was characterized by single-crystal X-ray diffraction, Raman spectroscopy, and quantum-chemical calculations for both the solid-state compound and the isolated anion at 0 K. Intrinsic bond orbital calculations show that the μ3 -F-Br bond is essentially ionic in nature and also underpin the stereochemical activity of the lone pairs of the Br(V) atoms.

Structural proof of a [C-F-C]+ fluoronium cation

Organic fluoronium ions can be described as positively charged molecules in which the most electronegative and least polarizable element fluorine engages in two partially covalent bonding interactions to two carbon centers. While recent solvolysis experiments and NMR spectroscopic studies on a metastable [C–F–C]⁺ fluoronium ion strongly support the divalent fluoronium structure over the alternative rapidly equilibrating classical carbocation, the model system has, to date, eluded crystallographic analysis to confirm this phenomenon in the solid state. Herein, we report the single crystal structure of a symmetrical [C–F–C]⁺ fluoronium cation. Besides its synthesis and crystallographic characterization as the [Sb₂F₁₁]⁻ salt, vibrational spectra are discussed and a detailed analysis concerning the nature of the bonding situation in this fluoronium ion and its heavier halonium homologues is performed, which provides detailed insights on this molecular structure.

Unlike other halogen atoms, the ability for fluorine to exist in a [C–X–C]⁺ connectivity pattern has only been shown in spectroscopic studies. Here the authors present a single crystal structure of a fluoronium cation, characterized by X-ray diffraction.

Cs[Cl3 F10 ]: A Propeller-Shaped [Cl3 F10 ]- Anion in a Peculiar A[5] B[5] Structure Type

Reaction of CsF with ClF3 leads to Cs[Cl3 F10 ]. It contains a molecular, propeller-shaped [Cl3 F10 ]- anion with a central μ3 -F atom and three T-shaped ClF3 molecules coordinated to it. This anion represents the first example of a heteropolyhalide anion of higher ClF3 content than [ClF4 ]- and is the first Cl-containing interhalogen species with a μ-bridging F atom. The chemical bonds to the central μ3 -F atom are highly ionic and quite weak as the bond lengths within the coordinating XF3 units (X = Cl, and also calculated for Br, I) are almost unchanged in comparison to free XF3 molecules. Cs[Cl3 F10 ] crystallizes in a very rarely observed A[5] B[5] structure type, where cations and anions are each pseudohexagonally close packed, and reside, each with coordination number five, in the trigonal bipyramidal voids of the other.

Quest for a Symmetric [C-F-C]+ Fluoronium Ion in Solution: A Winding Path to Ultimate Success

In this Account, we chronicle our tortuous but ultimately fruitful quest to synthesize a [C-F-C]⁺ fluoronium ion in solution, thus providing the last piece of the organic halonium ion puzzle. Inspiration for the project can be traced all the way back to the graduate career of the corresponding author, wherein the analogy between a [C-H-C]+ "hydrido" bridge and a hypothetical [C-F-C]⁺ bridge was first noted. The earliest attempt to construct a bicyclo[5.3.3]tridecane-based fluoronium ion (based on the analogous hydrido bridged cation) proved to be synthetically difficult. A subsequent attempt involving a 1,8-substituted naphthalene ring was theoretically naïve in retrospect, and it resulted in a classical benzylic carbocation instead. A biphenyl-based substrate, although computationally sound, proved to be kinetically untenable. At last, after some tweaking (including a dead-end detour into a fluoraadamantane skeleton), we finally achieved success with a highly rigid, semicage precursor based on the decahydro-1,4:5,8-dimethanonaphthalene system. This strained substrate possessed a triflate leaving group to enhance its solvolytic reactivity. Detailed isotopic labeling and kinetic studies supported the generation of a symmetrical [C-F-C]+ bridge; interesting solution behavior allowed the manipulation of the rate-determining step for solvolysis depending on solvent nucleophilicity. After initial generation as a transient intermediate, the fluoronium ion was later produced as a stable species in solution and was fully characterized by ¹⁹F, ¹H, and ¹³C NMR, with the resultant species displaying evident C_ssymmetry through coordination of a molecule of SbF₅. This remarkable ion proved stable to -30 °C. We also address a disagreement surrounding the nomenclature of fluoronium ions in particular and its potential impact upon the naming of onium ions in general. We strove to highlight the dangers of confusing the arbitrary concept of calculated partial charge with IUPAC nomenclature. Finally, we discuss future directions, for example, the synthesis of a fluoronium ion in which fluorine resides within an aromatic ring.

HKLF5Tools: a program for processing diffraction data of non-merohedrally twinned crystals

The HKLF5Tools program provides useful tools for structure refinement against non-merohedrally twinned datasets. The software shows statistical information on each twin component, can delete or rename twin components, merge reflection data, and add an inversion twin component to any of the existing twin components. Also, the software can convert ShelXL FCF files into HKL reflection files so that refinement against detwinned data in the final stages of the refinement becomes possible.

Synthesis and Characterization of [Br3 ][MF6 ] (M=Sb, Ir), as well as Quantum Chemical Study of [Br3 ]+ Structure, Chemical Bonding, and Relativistic Effects Compared with [XBr2 ]+ (X=Br, I, At, Ts) and [TsZ2 ]+ (Z=F, Cl, Br, I, At, Ts)

[Br₃ ][SbF₆ ] and [Br₃ ][IrF₆ ] were synthesized by interaction of BrF₃ with Sb₂ O₃ or iridium metal, respectively. The former compound crystallizes in the orthorhombic space group Pbcn (No. 60) with a=11.9269(7), b=11.5370(7), c=12.0640(6) Å, V=1660.01(16) ų , Z=8 at 100 K. The latter compound crystallizes in the triclinic space group P 1 ‾ (No. 2) with a=5.4686(5), b=7.6861(8), c=9.9830(9) Å, α=85.320(8), β=82.060(7), γ=78.466(7)°, V=406.56(7) ų , Z=2 at 100 K. Both compounds contain the cation [Br₃ ]⁺ , which has a bent structure and is coordinated by octahedron-like anions [MF₆ ]^- (M=Sb, Ir). Experimentally obtained cell parameters, bond lengths, and angles are confirmed by solid-state DFT calculations, which differ from the experimental values by less than 2 %. Relativistic effects on the structure of the tribromonium(1+) cation are studied computationally and found to be small. For the heaviest analogues containing At and Ts, however, pronounced relativistic effects are found, which lead to a linear structure of the polyhalogen cation.