Helene P. A. Mercier

The chemistry of krypton

Abstract Krypton is the only noble gas other than xenon to give rise to isolable compounds in macroscopic amounts, although the chemistry of krypton is presently limited to the +2 oxidation state. The strong oxidant-fluorinator properties and thermal instabilities of krypton(II) compounds have posed considerable challenges to determining the extent to which the chemistries of krypton(II) and xenon(II) are analogous. Krypton(II) compounds possessing Kr–F, Kr–O and Kr–N bonds have been prepared and structurally characterized by X-ray crystallography, spectroscopic means (NMR, vibrational, Mossbauer), and electron structure calculations. The strong oxidative fluorinators, KrF 2 and KrF + , have found application in the syntheses of new examples of fluorides and oxide fluorides of main-group, transition metal, lanthanide, and actinide elements in their highest oxidation states.

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.

[C6F5Xe]+ and [C6F5XeNCCH3]+ salts of the weakly coordinating borate anions, [BY4]- (Y = CN, CF3, or C6F5).

New examples of [C6F5Xe]+ salts of the weakly coordinating [BY4]- (Y = CN, CF3, or C6F5) anions were synthesized by metathesis of [C6F5Xe][BF4] with MI[BY4] (MI = K or Cs; Y = CN, CF3, or C6F5) in CH3CN at -40 degrees C, and were crystallized from CH2Cl2 or from a CH2Cl2/CH3CN solvent mixture. The low-temperature (-173 degrees C) X-ray crystal structures of the [C6F5Xe]+ cation and of the [C6F5XeNCCH3]+ adduct-cation are reported for [C6F5Xe][B(CF3)4], [C6F5XeNCCH3][B(CF3)4], [C6F5Xe][B(CN)4], and [C6F5XeNCCH3][B(C6F5)4]. The [C6F5Xe]+ cation, in each structure, interacts with either the anion or the solvent, with the weakest cation-anion interactions occurring for the [B(CF3)4]- anion. The solid-state Raman spectra of the [C6F5Xe]+ and [C6F5XeNCCH3]+ salts have been assigned with the aid of electronic structure calculations. Gas-phase thermodynamic calculations show that the donor-acceptor bond dissociation energy of [C6F5XeNCCH3]+ is approximately half that of [FXeNCCH3]+. Coordination of CH3CN to [C6F5Xe]+ is correlated with changes in the partial charges on mainly Xe, the ipso-C, and N, that is, the partial charge on Xe increases and those on the ipso-C and N decrease upon coordination, typifying a transition from a 2c-2e to a 3c-4e bond.

[H(OXeF2)n][AsF6] and [FXeII(OXeIVF2)n][AsF6] (n = 1, 2): Examples of Xenon(IV) Hydroxide Fluoride and Oxide Fluoride Cations and the Crystal Structures of [F3Xe---FH][Sb2F11] and [H5F4][SbF6]·2[F3Xe---FH][Sb2F11]

The xenon(IV) hydroxide fluoride and oxide fluoride salts, [H(OXeF2)n][AsF6] and [FXe(II)(OXe(IV)F2)n][AsF6] (n = 1, 2), have been synthesized as the natural abundance and the (18)O- and (2)H-enriched salts and structurally characterized by low-temperature Raman spectroscopy. Quantum-chemical calculations have been used to arrive at vibrational assignments. The experimental vibrational frequencies and isotopic shift trends are reproduced by the calculated gas-phase frequencies at several levels of theory. The cation chain is limited to one or two OXeF2 subunits, which are oxygen-bridged and strongly ion-paired with the AsF6(-) anion. The reaction of XeOF2·xHF with a superacidic HF/SbF5 solvent mixture yielded crystals of [H5F4][SbF6]·2[XeF3·HF][Sb2F11], [XeF3·HF][Sb2F11], and [XeF3][SbF6]. The XeF3(+) cations of each salt are very similar, displaying T-shaped primary coordination of xenon to three fluorine atoms (AX3E2 VSEPR arrangement) and a secondary contact to the fluorine atom of HF in [H5F4][SbF6]·2[XeF3·HF][Sb2F11] and [XeF3·HF][Sb2F11] or to a fluorine atom of SbF6(-) in [XeF3][SbF6]. The secondary coordination spheres of xenon in [H5F4][SbF6]·2[XeF3·HF][Sb2F11] and [XeF3·HF][Sb2F11] are similar; however, the F3Xe---FH(+) cation of the latter salt is disordered. An additional contact between the XeF3(+) cation and the SbF6(-) anion in [H5F4][SbF6]·2[XeF3·HF][Sb2F11] presumably alters the crystal packing sufficiently to give an ordered F3Xe---FH(+) cation, a rare example in which HF is coordinated to a noble gas. The latter compound also provides the first documentation of the H5F4(+) acidium ion, which forms a zigzag F4-chain terminated by SbF6(-) anions. Enthalpies and Gibbs free energies of reaction obtained from Born-Fajans-Haber thermochemical cycles support the proposed decomposition pathways.

Ennobling an old molecule: thiazyl trifluoride (N≡SF3), a versatile synthon for Xe-N bond formation.

The fields of sulfur-nitrogen-fluorine chemistry and noble-gas chemistry have been significantly extended by the syntheses and characterizations of four new Xe-N-bonded cations derived from N≡SF(3). The adduct-cation, F(3)S≡NXeF(+), has provided the entry point to a significant chemistry through HF solvolysis of the coordinated N≡SF(3) ligand and HF-catalyzed and solid-state rearrangements of F(3)S≡NXeF(+). The HF solvolyses of [F(3)S≡NXeF][AsF(6)] in anhydrous HF (aHF) and aHF/BrF(5) solutions yield the F(4)S═NXe(+) cation, which likely arises from an HF-catalyzed mechanism. The F(4)S═NXe(+) cation, in turn, undergoes HF displacement to form F(4)S═NH(2)(+) and XeF(2), as well as HF addition to the S═N bond to form F(5)SN(H)Xe(+). Both cations undergo further solvolyses in aHF to form the F(5)SNH(3)(+) cation. The F(4)S═NXe(+) and F(4)S═NH(2)(+) cations were characterized by NMR spectroscopy and single-crystal X-ray diffraction and exhibit high barriers to rotation about their S═N double bonds. They are the first cations known to contain the F(4)S═N- group and significantly extend the chemistry of this ligand. The solid-state rearrangement of [F(3)S≡NXeF][AsF(6)] at 22 °C has yielded [F(4)S═NXe][AsF(6)], which was characterized by Raman spectroscopy, providing the first examples of xenon bonded to an imido nitrogen and of the F(4)S═N- group bonded to a noble-gas element. The rearrangement of [F(3)S≡NXeF][AsF(6)] in a N≡SF(3) solution at 0 °C also yielded [F(4)S═NXe−N≡SF(3)][AsF(6)], which represents a rare example of a N−Xe−N linkage and the first to be characterized by X-ray crystallography. Solvolysis of N≡SF(3) in aHF was previously shown to give the primary amine F(5)SNH(2), whereas solvolysis in the superacid medium, AsF(5)/aHF, results in amine protonation to give [F(5)SNH(3)][AsF(6)]. Complete structural characterizations were not available for either species. Isolation of F(5)SNH(2)·nHF from the reaction of N≡SF(3) with HF has provided a structural characterization of F(5)SNH(2) by Raman spectroscopy. Crystal growth by sublimation of F(5)SNH(2)·nHF at -30 to -40 °C has resulted in the X-ray crystal structure of F(5)SNH(2)·2[F(5)SNH(3)][HF(2)]·4HF and structural characterizations of F(5)SNH(2) and F(5)SNH(3)(+). The redox decomposition of [F(4)S═NXe−N≡SF(3)][AsF(6)] in N≡SF(3) at 0 °C generated Xe, cis-N(2)F(2), and [F(3)S(N≡SF(3))(2)][AsF(6)].

F5SN(H)Xe+; a Rare Example of Xenon Bonded to sp3-Hybridized Nitrogen; Synthesis and Structural Characterization of [F5SN(H)Xe][AsF6]

The salt [F5SN(H)Xe][AsF6] has been synthesized by the reaction of [F5SNH3][AsF6] with XeF2 in anhydrous HF (aHF) and BrF5 solvents and by solvolysis of [F3S triple bond NXeF][AsF6] in aHF. Both F5SN(H)Xe(+) and F5SNH3(+) have been characterized by (129)Xe, (19)F, and (1)H NMR spectroscopy in aHF (-20 degrees C) and BrF5 (supercooled to -70 degrees C). The yellow [F5SN(H)Xe][AsF6] salt was crystallized from aHF at -20 degrees C and characterized by Raman spectroscopy at -45 degrees C and by single-crystal X-ray diffraction at -173 degrees C. The Xe-N bond length (2.069(4) A) of the F5SN(H)Xe(+) cation is among the shortest Xe-N bonds presently known. The cation interacts with the AsF6(-) anion by means of a Xe---F-As bridge in which the Xe---F distance (2.634(3) A) is significantly less than the sum of the Xe and F van der Waals radii (3.63 A) and the AsF6(-) anion is significantly distorted from Oh symmetry. The (19)F and (129)Xe NMR spectra established that the [F5SN(H)Xe][AsF6] ion pair is dissociated in aHF and BrF5 solvents. The F5SN(H)Xe(+) cation decomposes by HF solvolysis to F5SNH3(+) and XeF2, followed by solvolysis of F5SNH3(+) to SF6 and NH4(+). A minor decomposition channel leads to small quantities of F5SNF2. The colorless salt, [F5SNH3][AsF6], was synthesized by the HF solvolysis of F3S triple bond NAsF5 and was crystallized from aHF at -35 degrees C. The salt was characterized by Raman spectroscopy at -160 degrees C, and its unit cell parameters were determined by low-temperature X-ray diffraction. Electronic structure calculations using MP2 and DFT methods were used to calculate the gas-phase geometries, charges, bond orders, and valencies as well as the vibrational frequencies of F 5SNH3(+) and F5SN(H)Xe(+) and to aid in the assignment of their experimental vibrational frequencies. In addition to F5TeN(H)Xe(+), the F5SN(H)Xe(+) cation provides the only other example of xenon bonded to an sp (3)-hybridized nitrogen center that has been synthesized and structurally characterized. These cations represent the strongest Xe-N bonds that are presently known.

Xenon(IV)–Carbon Bond of [C6F5XeF2]+; Structural Characterization and Bonding of [C6F5XeF2][BF4], [C6F5XeF2][BF4]·2HF, and [C6F5XeF2][BF4]·nNCCH 3 (n = 1, 2); and the Fluorinating Properties of [C6F5XeF2][BF4]

The [C6F5XeF2](+) cation is the only example of a Xe(IV)-C bond, which had only been previously characterized as its [BF4](-) salt in solution by multi-NMR spectroscopy. The [BF4](-) salt and its new CH3CN and HF solvates, [C6F5XeF2][BF4]·1.5CH3CN and [C6F5XeF2][BF4]·2HF, have now been synthesized and fully characterized in the solid state by low-temperature, single-crystal X-ray diffraction and Raman spectroscopy. Crystalline [C6F5XeF2][BF4] and [C6F5XeF2][BF4]·1.5CH3CN were obtained from CH3CN/CH2Cl2 solvent mixtures, and [C6F5XeF2][BF4]·2HF was obtained from anhydrous HF (aHF), where [C6F5XeF2][BF4]·1.5CH3CN is comprised of an equimolar mixture of [C6F5XeF2][BF4]·CH3CN and [C6F5XeF2][BF4]·2CH3CN. The crystal structures show that the [C6F5XeF2](+) cation has two short contacts with the F atoms of [BF4](-) or with the F or N atoms of the solvent molecules, HF and CH3CN. The low-temperature solid-state Raman spectra of [C6F5XeF2][BF4] and C6F5IF2 were assigned with the aid of quantum-chemical calculations. The bonding in [C6F5XeF2](+), C6F5IF2, [C6F5XeF2][BF4], [C6F5XeF2][BF4]·CH3CN, [C6F5XeF2][BF4]·2CH3CN, and [C6F5XeF2][BF4]·2HF was assessed with the aid of natural bond orbital analyses and molecular orbital calculations. The (129)Xe, (19)F, and (11)B NMR spectra of [C6F5XeF2][BF4] in aHF are reported and compared with the (19)F NMR spectrum of C6F5IF2, and all previously unreported J((129)Xe-(19)F) and J((19)F-(19)F) couplings were determined. The long-term solution stabilities of [C6F5XeF2][BF4] were investigated by (19)F NMR spectroscopy and the oxidative fluorinating properties of [C6F5XeF2][BF4] were demonstrated by studies of its reactivity with K[C6F5BF3], Pn(C6F5)3 (Pn = P, As, or Bi), and C6F5X (X = Br or I).

Syntheses and Structures of F6 XeNCCH3 and F6 Xe(NCCH3)2.

Acetonitrile and the potent oxidative fluorinating agent XeF6 react at -40 °C in Freon-114 to form the highly energetic, shock-sensitive compounds F6XeNCCH3 (1) and F6Xe(NCCH3)2⋅CH3CN (2⋅CH3CN). Their low-temperature single-crystal X-ray structures show that the adducted XeF6 molecules of these compounds are the most isolated XeF6 moieties thus far encountered in the solid state and also provide the first examples of Xe(VI)-N bonds. The geometry of the XeF6 moiety in 1 is nearly identical to the calculated distorted octahedral (C3v) geometry of gas-phase XeF6. The C2v geometry of the XeF6 moiety in 2 resembles the transition state proposed to account for the fluxionality of gas-phase XeF6. The energy-minimized gas-phase geometries and vibrational frequencies were calculated for 1 and 2, and their respective binding energies with CH3CN were determined. The Raman spectra of 1 and 2⋅CH3CN were assigned by comparison with their calculated vibrational frequencies and intensities.

Syntheses and Structures of Xenon Trioxide Alkylnitrile Adducts

The potent oxidizer and highly shock-sensitive binary noble-gas oxide XeO3 interacts with CH3 CN and CH3 CH2 CN to form O3 XeNCCH3 , O3 Xe(NCCH3 )2 , O3 XeNCCH2 CH3 , and O3 Xe(NCCH2 CH3 )2 . Their low-temperature single-crystal X-ray structures show that the xenon atoms are consistently coordinated to three donor atoms, which results in pseudo-octahedral environments around the xenon atoms. The adduct series provides the first examples of a neutral xenon oxide bound to nitrogen bases. Raman frequency shifts and Xe-N bond lengths are consistent with complex formation. Energy-minimized gas-phase geometries and vibrational frequencies were obtained for the model compounds O3 Xe(NCCH3 )n (n=1-3) and O3 Xe(NCCH3 )n ⋅[O3 Xe(NCCH3 )2 ]2 (n=1, 2). Natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), and molecular electrostatic potential surface (MEPS) analyses were carried out to further probe the nature of the bonding in these adducts.

Crystal structures and Raman spectra of imidazolium poly[perfluorotitanate(IV)] salts containing the [TiF6]2-, ([Ti2F9]-)∞, and [Ti2F11]3- and the new [Ti4F20]4- and [Ti5F23]3- anions.

Reactions between imidazole (Im, C3H4N2) and TiF4 in anhydrous hydrogen fluoride (aHF) in different molar ratios have yielded [ImH]2[TiF6]·2HF, [ImH]3[Ti2F11], [ImH]4[Ti4F20], [ImH]3[Ti5F23], and [ImH][Ti2F9] upon crystallization. All five structures were characterized by low-temperature single-crystal X-ray diffraction. The single-crystal Raman spectra of [ImH]4[Ti4F20], [ImH]3[Ti5F23], and [ImH][Ti2F9] were also recorded and assigned. In the crystal structure of [ImH]2[TiF6]·2HF, two HF molecules are coordinated to each [TiF6](2-) anion by means of strong F-H···F hydrogen bonds. The [Ti2F11](3-) anion of [ImH]3[Ti2F11] results from association of two TiF6 octahedra through a common fluorine vertex. Three crystallographically independent [Ti2F11](3-) anions, which have distinct geometries and orientations, are hydrogen-bonded to the [ImH](+) cations. The [ImH]4[Ti4F20] salt crystallized in two crystal modifications at low (α-phase, 200 K) and ambient (β-phase, 298 K) temperatures. The tetrameric [Ti4F20](4-) anion of [ImH]4[Ti4F20] consists of rings of four TiF6 octahedra, which each share two cis-fluorine vertices, whereas the pentameric [Ti5F23](3-) anion of [ImH]3[Ti5F23] results from association of five TiF6 units, where four of the TiF6 octahedra share two cis-vertices, forming a tetrameric ring as in [Ti4F20](4-), and the fifth TiF6 unit shares three fluorine vertices with three TiF6 units of the tetrameric ring. The [ImH][Ti2F9] salt also crystallizes in two crystal modifications at low (α-phase, 200 K) and high (β-phase, 298 K) temperatures and contains polymeric ([Ti2F9](-))∞ anions, which appear as two parallel infinite zigzag chains comprised of TiF6 units, where each TiF6 unit of one chain is connected to a TiF6 unit of the second chain through a shared fluorine vertex. Quantum-chemical calculations at the B3LYP/SDDALL level of theory were used to arrive at the gas-phase geometries and vibrational frequencies of the [Ti4F20](4-) and [Ti5F23](3-) anions, which aided in the assignment of the experimental vibrational frequencies of the anion series.