X-ray Crystal Structures of [XeF][MF6] (M = As, Sb, Bi), [XeF][M2F11] (M = Sb, Bi) and Estimated Thermochemical Data and Predicted Stabilities for Noble-Gas Fluorocation Salts using Volume-Based Thermodynamics

Fluorination/Dearomatization of C6F5 Groups: An FLP Route to an Electrophilic Borane and Non-Coordinating Anions

B(C6F5)3 and the corresponding anion [B(C6F5)4]- are ubiquitous in main group and transition metal chemistry. Known derivatives are generally limited to the incorporation of electron donating substituents. Herein we describe electrophilic fluorination and dearomatization of such species using XeF2 in the presence of BF3 or Lewis acidic cations. In this fashion the anions [HB(C6F5)3]-, [B(C6F5)4]- and [(C6F5)3BC≡NB(C6F5)3]-, are converted to [FB(C6F7)3]-, [B(C6F7)4]-, and [(C6F7)3BC≡NB(C6F7)3]-, respectively. Similarly, the borane adducts (L)B(C6F7)3 (L=MeCN, OPEt3) are produced. These rare examples of electrophilic attack of electron deficient rings proceed as [XeF][BF4] acts as a frustrated Lewis pair effecting fluorination and dearomatization of C6F5 rings.

Noble-Gas Difluoride Complexes of MOF4 (M = Mo, W); Syntheses, Structures and Bonding of NgF2·MOF4 and XeF2·2MOF4 (Ng = Kr, Xe)

The NgF 2 ∙MOF 4 (Ng = Kr, Xe; M = Mo, W) and XeF 2 ∙2MOF 4 complexes were synthesized in anhydrous HF (aHF) solvent and melts, respectively. Their single-crystal X-ray diffraction (SCXRD) structures show NgF 2 ∙MOF 4 and XeF 2 ·2MOF 4 have F t -Ng-F b ---M arrangements, in which the NgF 2 ligands coordinate to MOF 4 through Ng-F b ---M bridges. The XeF 2 ligands of XeF 2 ·2MOF 4 also coordinate to F 3 OM-F b '---M'OF 4 moieties through Xe-F b ---M bridges to form F t -Xe-F b ---M(OF 3 )-F b '---M'OF 4 , where XeF 2   coordinates trans to the M=O bond and F b ' coordinates trans to the M'=O bond. The Ng-F t , Ng-F b , and M---F b bond lengths of NgF 2 ∙ n MOF 4 are consistent with MOF 4 and F 3 OM-F b '---M'OF 4 fluoride-ion affinity trends: CrOF 4 < MoOF 4 < WOF 4 ≈ F 3 OMo-F b '---Mo'OF 4 < F 3 OW-F b '---W'OF 4 . The [--(F 4 OMo)( µ 3 -F)H---H( µ -F)--] ∞ and [H 2 F][W 2 O 2 F 9 ] solvates were also synthesized in aHF and characterized by SCXRD. Quantum-chemical calculations show the M---F b bonds of NgF 2 ·MOF 4 and XeF 2 ·2MOF 4 are predominantly electrostatic, σ-hole type bonds.

Syntheses and Characterizations of the Mixed Noble-Gas Compounds, [FKrII FXeII F][AsF6 ]⋅0.5 KrII F2 ⋅2 HF, ([KrII 2 F3 ][AsF6 ])2 ⋅XeIV F4 , and XeIV F4 ⋅KrII F2

Reaction of [XeF][AsF₆ ] with excess KrF₂ at -78 °C in anhydrous HF (aHF) solvent has yielded the first mixed Kr^II /Xe^II noble-gas compound, [FKrFXeF][AsF₆ ] ⋅0.5 KrF₂ ⋅2 HF, a salt of the [FKrFXeF]⁺ cation. The potent oxidative fluorinating properties of Kr^II fluoride species resulted in oxidation of Xe^II to Xe^IV in aHF at -60 °C to form the mixed Kr^II /Xe^IV cocrystals, ([Kr₂ F₃ ][AsF₆ ])₂ ⋅XeF₄ and XeF₄ ⋅KrF₂ . Further decomposition at 22 °C resulted in oxidation of Xe^IV to Xe^VI to give the recently reported Kr^II /Xe^VI complexes, [F₅ Xe(FKrF)_n ][AsF₆ ] (n=1, 2), [F₅ Xe][AsF₆ ], and a new Kr^II /Xe^VI complex, [(F₅ Xe)₂ (μ-FKrF)(AsF₆ )₂ ], which was characterized by low-temperature (LT) Raman spectroscopy. The [FKrFXeF][AsF₆ ]⋅0.5 KrF₂ ⋅2 HF, ([Kr₂ F₃ ][AsF₆ ])₂ ⋅XeF₄  , and XeF₄ ⋅KrF₂ compounds were characterized by LT Raman spectroscopy and single-crystal X-ray diffraction. Quantum-chemical calculations were used to assess the bonding in [FKrFXeF]⁺ , [Kr₂ F₃ ]⁺ , and [Xe₂ F₃ ]⁺ and to aid in their vibrational assignments.

Coloring and distortion variants of the bcc packing and for the aristotypes BaAl4 and CeMg2Si2

The huge number of intermetallic structure types with many representatives calls for structural systemization. The combination of crystal chemistry with group theory is an efficient tool for such systemization and can be displayed in a concise and compact way via group-subgroup schemes. The present overview deals with such group-subgroup schemes (Bärnighausen trees) for coloring and distortion variants of the bcc packing as well as superstructures that derive from the aristotypes BaAl4 and CeMg2Si2.

Syntheses, Structures, and Bonding of NgF2 ⋅CrOF4 , NgF2 ⋅2CrOF4 (Ng=Kr, Xe), and (CrOF4 )∞

The noble-gas difluoride adducts, NgF₂ ⋅CrOF₄ and NgF₂ ⋅2CrOF₄ (Ng=Kr and Xe), have been synthesized and structurally characterized at low temperatures by Raman spectroscopy and single-crystal X-ray diffraction. The low fluoride ion affinity of CrOF₄ renders it incapable of inducing fluoride ion transfer from NgF₂ (Ng=Kr and Xe) to form ion-paired salts of the [NgF]⁺ cations having either the [CrOF₅ ]^- or [Cr₂ O₂ F₉ ]^- anions. The crystal structures show the NgF₂ ⋅CrOF₄ adducts are comprised of F_t -Ng-F_b - - -Cr(O)F₄ structural units in which NgF₂ is weakly coordinated to CrOF₄ by means of a fluorine bridge, F_b , in which Ng-F_b is elongated relative to the terminal Ng-F_t bond. In contrast with XeF₂ ⋅2MOF₄ (M=Mo or W) and KrF₂ ⋅2MoOF₄ , in which the Lewis acidic, F₄ (O)M- - -F_b - - -M(O)F₃ moiety coordinates to Ng through a single M- - -F_b -Ng bridge, both fluorine ligands of NgF₂ coordinate to CrOF₄ molecules to form F₄ (O)Cr- - -F_b -Ng-F_b - - -Cr(O)F₄ adducts in which both Ng-F_b bonds are only marginally elongated relative to the Ng-F bonds of free NgF₂ . Quantum-chemical calculations show that the Cr-F_b bonds of NgF₂ ⋅CrOF₄ and NgF₂ ⋅2CrOF₄ are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr- - -F_b -Ng bridge angles and staggered O-Cr- - -F_b -Ng-F_t dihedral angles. The crystal structures and Raman spectra of two CrOF₄ polymorphs have also been obtained. Both are comprised of fluorine-bridged chains that are cis- and trans-fluorine-bridged with respect to oxygen.

Absolute ion hydration enthalpies and the role of volume within hydration thermodynamics

This paper reports that various thermodynamic properties in aqueous media for certain individual ions and for compounds are linear functions of the inverse cube root of the solid respective ionic and compound solid state volumes, V_m^−1/3.

Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the - naturally subjective -"reactive"-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

High-pressure stabilization of argon fluorides

On account of the rapid development of noble gas chemistry in the past half-century both xenon and krypton compounds can now be isolated in macroscopic quantities. The same does not hold true for the next lighter group 18 element, argon, which forms only isolated molecules stable solely in low temperature matrices or supersonic jet streams. Here we present theoretical investigations into a new high-pressure reaction pathway, which enables synthesis of argon fluorides in bulk and at room temperature. Our hybrid DFT calculations (employing the HSE06 functional) indicate that above 60 GPa ArF2-containing molecular crystals can be obtained by a reaction between argon and molecular fluorine.

The saturation of the gas phase acidity of nHF/AlF3 and nHF/GeF4 (n = 1–6) superacids caused by increasing the number of surrounding HF molecules

The acidic strength of selected Brønsted/Lewis superacids is evaluated on the basis of theoretical calculations carried out at the QCISD/6-311++G(d,p) level.

Prediction of neutral noble gas insertion compounds with heavier pnictides: FNgY (Ng = Kr and Xe; Y = As, Sb and Bi)

Neutral noble gas insertion compounds involving arsenic, antimony and bismuth atoms wherein the triplet electronic state is the ground state are predicted for the first time.

Predictive thermodynamics for ionic solids and liquids

Thermodynamic properties of ionic solids and liquids may reliably be predicted using volume-based thermodynamics (VBT) and thermodynamic difference rules (TDR).

Design of stereoelectronically promoted super lewis acids and unprecedented chemistry of their complexes

A new family of stereoelectronically promoted aluminum and scandium super Lewis acids is introduced on the basis of state-of-the-art computations. Structures of these molecules are designed to minimize resonance electron donation to central metal atoms in the Lewis acids. Acidity of these species is evaluated on the basis of their fluoride-ion affinities relative to the antimony pentafluoride reference system. It is demonstrated that introduced changes in the stereochemistry of the designed ligands increase acidity considerably relative to Al and Sc complexes with analogous monodentate ligands. The high stability of fluoride complexes of these species makes them ideal candidates to be used as weakly coordinating anions in combination with highly reactive cations instead of conventional Lewis acid-fluoride complexes. Further, the interaction of all designed molecules with methane is investigated. All studied acids form stable pentavalent-carbon complexes with methane. In addition, interactions of the strongest acid of this family with very weak bases, namely, H2, N2, carbon oxides, and noble gases were investigated; it is demonstrated that this compound can form considerably stable complexes with the aforementioned molecules. To the best of our knowledge, carbonyl and nitrogen complexes of this species are the first hypothetical four-coordinated carbonyl and nitrogen complexes of aluminum. The nature of bonding in these systems is studied in detail by various bonding analysis approaches.

Noble-gas difluoride complexes of mercury(II): the syntheses and structures of Hg(OTeF5)2·1.5NgF2 (Ng = Xe, Kr) and Hg(OTeF5)2

The synthesis of high-purity Hg(OTeF5)2 has resulted in its structural characterization in the solid state by Raman spectroscopy and single-crystal X-ray diffraction (XRD) and in solution by (19)F NMR spectroscopy. The crystal structure of Hg(OTeF5)2 (-173 °C) consists of discrete Hg(OTeF5)2 units having gauche-conformations that interact through long Hg---O and Hg---F intramolecular contacts to give a chain structure. The Lewis acidity of Hg(OTeF5)2 toward NgF2 (Ng = Xe, Kr) was investigated in SO2ClF solvent and shown to form stable coordination complexes with NgF2 at -78 °C. Both complexes were characterized by low-temperature Raman spectroscopy (-155 °C) and single-crystal XRD. The complexes are isostructural and are formulated as Hg(OTeF5)2·1.5NgF2. The Hg(OTeF5)2 units of Hg(OTeF5)2·1.5NgF2 also have gauche-conformations and are linked through bridging NgF2 molecules, also resulting in chain structures. These complexes represent the only examples of coordination compounds where NgF2 coordinates to mercury in a neutral covalent compound and the only example of mercury coordinated to KrF2. Moreover, the Hg(OTeF5)2·1.5KrF2 complex is the only KrF2 complex known to contain a bridging KrF2 ligand. Energy-minimized gas-phase geometries and vibrational frequencies for the model compounds, [Hg(OTeF5)2]3 and [Hg(OTeF5)2]3·2NgF2, were obtained and provide good approximations of the local environments of Hg(OTeF5)2 and NgF2 in the crystal structures of Hg(OTeF5)2 and Hg(OTeF5)2·1.5NgF2. Assignments of the Raman spectra of Hg(OTeF5)2 and Hg(OTeF5)2·1.5NgF2 are based on the calculated vibrational frequencies of the model compounds. Natural bond orbital analyses provided the associated bond orders, valencies, and natural population analysis charges.

Amorphous/crystalline (A/C) thermodynamic "rules of thumb": estimating standard thermodynamic data for amorphous materials using standard data for their crystalline counterparts

Standard thermochemical data (in the form of Δ(f)H° and Δ(f)G°) are available for crystalline (c) materials but rarely for their corresponding amorphous (a) counterparts. This paper establishes correlations between the sets of data for the two material forms (where known), which can then be used as a guideline for estimation of missing data. Accordingly, Δ(f)H°(a)/kJ mol(-1) ≈ 0.993Δ(f)H°(c)/kJ mol(-1) + 12.52 (R(2) = 0.9999; n = 50) and Δ(f)G°/kJ mol(-1) ≈ 0.988Δ(f)H°(c)/kJ mol(-1) + 0.70 (R(2) = 0.9999; n = 10). Much more tentatively, we propose that S°(298)(c)/J K(-1) mol(-1) ≈ 1.084S°(298)(c)/J K(-1) mol(-1) + 6.54 (R(2) = 0.9873; n = 11). An amorphous hydrate enthalpic version of the Difference Rule is also proposed (and tested) in the form [Δ(f)H°(M(p)X(q)·nH(2)O,a) - Δ(f)H°(M(p)X(q),a)]/kJ mol(-1) ≈ Θ(Hf)n ≈ -302.0n, where M(p)X(q)·nH(2)O represents an amorphous hydrate and M(p)X(q) the corresponding amorphous anhydrous parent salt.

Freezing in resonance structures for better packing: XeF2 becomes (XeF+)(F-) at large compression

Recent high-pressure experiments conducted on xenon difluoride (XeF(2)) suggested that this compound undergoes several phase transitions up to 100 GPa, becoming metallic above 70 GPa. In this theoretical study, in contrast to experiment, we find that the ambient pressure molecular structure of xenon difluoride, of I4/mmm symmetry, remains the most stable one up to 105 GPa. In our computations, the structures suggested from experiment have either much higher enthalpies than the I4/mmm structure or converge to that structure upon geometry optimization. We discuss these discrepancies between experiment and calculation and point to an alternative interpretation of the measured cell vectors of XeF(2) at high pressure. At pressures exceeding those studied experimentally, above 105 GPa, the I4/mmm structure transforms to one of Pnma symmetry. The Pnma phase contains bent FXeF molecules, with unequal Xe-F distances, and begins to bring other fluorines into the coordination sphere of the Xe. Further compression of this structure up to 200 GPa essentially results in self-dissociation of XeF(2) into an ionic solid (i.e., [XeF](+)F(-)), similar to what is observed for nitrous oxide (N(2)O) at high pressure.