Oxidation of Ruthenium and Iridium Metal by XeF2 and Crystal Structure Determination

Salts containing [Xe2F3] + cations and [MF6] – anions (M = Ru, Ir) were synthesized by the oxidation of metal with excess of XeF2 in anhydrous hydrogen fluoride (aHF) as a solvent. Single crystals of [Xe2F3][RuF6]·XeF2, [Xe2F3][RuF6] and [Xe2F3][IrF6] were grown by slow evaporation of the solvent. [Xe2F3][RuF6]·XeF2 crystallizes in a triclinic P–1 space group (a = 8.3362(1) Å, b = 8.8197(2) Å, c = 9.3026(4) Å; α = 68.27(1)°, β = 63.45(1)°, γ = 82.02°, V = 568.09(9) Å (Z = 2)). Discrete [Xe2F3] , XeF2 and [RuF6] – units are found in the asymmetric unit. [Xe2F3][RuF6] and [Xe2F3][IrF6] compounds are isostructural and crystallize in a monoclinic Cc space group (a = 14.481(3) Å (Ru); 14.544(3) Å (Ir); b = 8.0837(8) Å (Ru), 8.0808(7) Å (Ir), c = 10.952(2) Å (Ru), 11.014(2) Å (Ir); β = 136.825(6)° (Ru), 139.954(7)°, V = 877.2(3) Å (Ru), 883.6(3) Å (Ir); Z = 4). The asymmetric unit in the [Xe2F3][MF6] (M = Ru, Ir) consists of one [Xe2F3] + and one [MF6] – unit.


Introduction
XeF 2 is the most stable and easily handled noble gas fluoride and therefore its chemistry is very extensive. Basic information about XeF 2 , its properties and the possibilities that it offers can be found in review paper and book and the references listed therein. 1,2 One of its unexpected and interesting abilities is also binding to metal centres in order to form coordination compounds. A large variety of such compounds has been found in previous years. 3 The formation of XeF 2 adducts with main-group and transitionmetal Lewis acidic pentafluoridometalates -MF 5 is known for decades. So far three types of such compounds were found: 2XeF 2 ·MF 5 , XeF 2 ·MF 5 and XeF 2 ·(MF 5 ) 2 . The degree of ionic character in these compounds varies depending on the Lewis acidity of the respective pentafluoride. Compounds were mainly characterized by vibrational spectroscopy and can be written as salts [ F 11 ], especially in the case of reactions with strong Lewis acids (for example AsF 5 , SbF 5 , BiF 5 ...). The formation of 2:1 compounds was found in the cases where M was As, Sb, Bi, Ta, Ru, Os and Ir. 4,5,6,7 From this type of compounds (2:1 composition) [Xe 2 F 3 ][MF 6 ] (M = As, Sb) 8,9 were also structurally char-acterized. The compounds with composition 1:1 (XeF + MF 6 -) and 1:2 (XeF + M 2 F 11 -) were obtained for the most of the MF 5  trafluoroethylene) valves. The manipulations of the nonvolatile materials were carried out in a glove-box (M. Braun). The residual water in the atmosphere within the glove-box never exceeded 1 ppm. The reactions were carried out in FEP (tetrafluoroethylene-hexafluoropropylene; Polytetra GmbH, Germany) reaction vessels (height 250-300 mm with inner diameter 16 mm and outer diameter 19 mm) equipped with PTFE valves and PTFE coated stirring bars. T-shaped reaction vessels from PTFE, which were constructed as described earlier, were used for the crystallization process. 15 Prior to their use all reaction vessels were passivated with elemental fluorine. Fluorine was used as supplied (Solvay Fluor and Derivate GmbH, Germany). Anhydrous HF (Linde, 99.995%) was treated with K 2 NiF 6 (Advance Research Chemicals, Inc.) for several hours prior to use. XeF 2 was synthesized by photochemical reaction between Xe and F 2 . 16 Caution: aHF, F 2 and XeF 2 must be handled with great care in a well-ventilated fume hood, and protective gear must be worn at all times.

Synthesis and Characterization Procedures
Synthetic procedures for the ruthenium and iridium compounds were the same. Metal powder (Ru: 0.215 g, 2.13 mmol, Ir: 0.430 g, 2.20 mmol) was added into a reaction vessel inside the glove-box. The aHF was condensed into the reaction vessel at -196 °C at the vacuum line. Large excess of XeF 2 (mole ratio M : XeF 2 was approximately 1:10) was weighed into another reaction vessel inside the glove-box. Anhydrous HF was added to the XeF 2 and the reaction vessel was warmed up to room temperature. These two reaction vessels (one with the suspension of the metal, and another with dissolved XeF 2 ) were attached in a T-shape manner and additional valve was used in order to provide completely closed system. The XeF 2 solution was then poured into cold reaction vessel (-196°C ) with suspension of the metal powder (Ru, Ir). The reaction vessel was left to slowly warm up to room temperature. The solution turned immediately green in the case of ruthenium but the reaction with iridium proceeded at room temperature for several days (light gray solid product). Products of the oxidation were isolated by removal of aHF and excessive XeF 2 under dynamic vacuum at room temperature. Several crystallization experiments were performed. In some cases the product of the oxidation of the metal with XeF 2 was dissolved in aHF in a wider arm of the T-shaped crystallization vessel, while in the other additional XeF 2 was added. Solution was then poured into narrow arm of the crystallization vessel and left to crystallize by a small temperature gradient used for slow evaporation of aHF.
The Raman spectra were recorded at room temperature with a Horiba Jobin Yvon LabRam-HR spectrometer equipped with an Olympus BXFM-ILHS microscope and CCD detector. The samples were excited by the 632.8 nm emission line of a He-Ne laser. Samples for measurement were transferred into the quartz capillary inside glovebox.
The crystallographic parameters and summaries of data collection for all compounds are presented in Table 1. Single-crystal data were collected on a Rigaku AFC7 diffractometer using graphite monochromatized MoKα radiation at 200 K. Crystals were immersed into perfuorinated oil in glove-box and further on selected under the microscope. An empirical multi-scan absorption correction was applied. All structures were solved by direct methods using SIR-92 17 and SHELXS-97 programs (teXan crystallographic software package of Molecular Structure Corporation) 18 and refined with SHELXL-97 software, 19 implemented in program package WinGX. 20 Full-matrix least-squares refinements based on F 2 were carried out for the positional and thermal parameters for all non-hydrogen atoms. The figures were prepared using DIAMOND 3.1 software. 21

Results and Discussion
The oxidation power of XeF 2 was used in order to prepare previously mentioned Ru(V) and Ir(V) compounds. XeF 2 dissolved in anhydrous hydrogen fluoride (aHF) was also used as selective inorganic fluorinating reagent for oxidation and fluorination of Ir and RuF 3 almost three decades ago with final products being IrF 5 and RuF 5 . 22 Ruthenium metal was oxidized rapidly with vigorous reaction being observed during the warming of the reaction vessel from -196 °C to room temperature. A clear, slightly green solution was obtained. We used a slightly modified procedure for the preparation of the ruthenium compound ([Xe 2 F 3 ][MF 6 ]·XeF 2 ). The product was further used for the synthesis of [Ba(XeF 2 ) 5 ][RuF 6 ] 2 . 23 The same reaction with iridium proceeded for several weeks. The colour of the solution was red-brown at the beginning and after several days became slightly yellow with some grey precipitatesame as the solid product after its isolation. The grey solid obtained by the oxidation of Ir metal with XeF 2 was re-dissolved in aHF and a clear, slightly green solution was obtained. Products of the reactions were also monitored by Raman spectroscopy, which shows ( Figure 1) that in both cases the compound with composition [Xe 2 F 3 ][MF 6 ] ·nXeF 2 (M = Ru, Ir, n is approx. 1 according to the mass balance of the reaction) were obtained. An alternative method used for the preparation of related compounds with Ru(V) (KRuF 6 , LiRuF 6 ) and Ir(V) (KIrF 6 , LiIrF 6 ) with al-kaline metals is the room temperature oxidation with elemental fluorine in the presence of Lewis base (KF) in aHF. 24,25 One of the recently published ways to prepare soluble iridium compounds, which seems to be important for modern "urban mining", is the reaction of the metal with tetrafluorobromates (MBrF 4 ; M = K, Rb, Cs). 26 For solid XeF 2 the band at 497 cm -1 is characteristic. 27 The bands at 506 cm -1 , 515 cm -1 in ruthenium compound and 506 cm -1 and 516 cm -1 in iridium compound can be attributed to the XeF 2 weakly associated with [Xe 2 F 3 ] + cations and [MF 6 ]anions in the [Xe 2 F 3 ] [MF 6 ]·XeF 2 product. Similar positions and assignment of these bands were observed in some other cases in the system XeF 2 -MF 5 (M = Sb, Ta, Nb). The weakly associated XeF 2 was found in the melt of the compounds. 28 "Free" XeF 2 was also found in the compounds XeF 2 ·XeF 6 ·AsF 5 and XeF 2 ·2(XeF 6 )·2(AsF 5 ), where the Raman bands depend on the interaction of the so called "free" XeF 2 molecule with cations and anions and consequential distortion of its shape. "Free" XeF 2 in XeF 2 ·2(XeF 6 )·2(AsF 5 ) is probably in a completely symmetric environment, therefore the band assigned to it coincides with the symmetric stretching frequency in molecular XeF 2 (497 cm -1 ). On the other hand Raman spectrum of XeF 2 ·XeF 6 ·AsF 5 doesn't show a symmetric vibration of molecular XeF 2 but two bands at 557 cm -1 and 429 cm -1 which represent a distorted XeF 2 molecule meaning that XeF 2 in this compound can be far from "free". 29 Linear distortion of XeF 2 was found also in the Raman spectrum of XeF 2 · [XeF 5 ] [RuF 6 ]. 30 Bands at 578 cm -1 and 586 cm -1 in the ruthenium compound and bands at 578 cm -1 and 587 cm -1 in iridium compound can be confidently assigned to the Xe-F t stretch vibrations of the [Xe 2 F 3 ] + cation. They are in the region that is characteristic for such vibrations (from ca. 575 cm -1 to 600 cm -1 ). The bands at 646 cm -1 , 667 cm -1 and 265 cm -1 for the ruthenium compound and 662 cm -1 and 243 cm -1 for the iridium can be assigned to the vibration of the   6 ] (M = Ru, Ir) salts seem to be stable at room temperature under dynamic vacuum. Raman analysis of the slow removal of XeF 2 under dynamic vacuum in the case of ruthenium is shown in Figure 2. In the spectrum shown on the Figure 2a ( (Figure 3). XeF 2 molecules and [Xe 2 F 3 ] + cations are oriented roughly perpendicularly to each other. When viewed along (-3 -4 3) direction alternating cationic and anionic layers could be seen. Anions are separated by XeF 2 molecules (Figure 4). The compound is structurally related to [Kr 2 F 3 ][SbF 6 ]·KrF 2 in which the crystal packing consists of alternating cation and equally populated anion/KrF2 layers. 31 XeF 2 molecules are nearly linear with distances being 1.980(7) and 1.992(7) Å and angle F10-Xe-F11 of 178.9(4)°. [Xe 2 F 3 ] + cation exhibit a planar, V shape configuration with nearly symmetrical Xe-F t bonds (2.139(7) and 2.152(7) Å) and a Xe-F b -Xe angle of 154.3(4)°. The [RuF 6 ]anions are slightly distorted octahedra with Ru-F bond distances in the range from 1.834(8) to 1.861(7) Å.   The Xe centres from both XeF 2 and [Xe 2 F 3 ] + moieties interact with fluorine atoms from another structural units. The shortest F9···Xe2 iii and F9···Xe1 distances of 3.190(7) and 3.216(7) Å respectively correspond to slightly elongated Ru1-F9 bond (1.861(7) Å). There are four XeF 2 molecules bound to one anion via Xe···F contacts of 3.301(7)-3.373(7) Å. Terminal F2 and F3 atoms from each [Xe 2 F 3 ] + also form longer contacts with Xe centres from other cations. These contacts are slightly longer (from 3.219(7) to 3.278(8) Å, correspondingly) than those in the case of XeF 2 molecules.
Weak Xe···F(Xe) and Xe···F(Ru) interactions connect above mentioned units into three-dimensional network ( Figure 4) if contacts shorter than 3.4 Å are taken into consideration. The sum of the van der Waals radius for xenon (2.16 Å) and fluorine (1.35 Å) is 3.51 Å. 32   6 ]anion ( Figure 5). In both cases sev-eral Xe···F contacts in the range above 3 Å can be found. A packing diagram along the b-axis is presented in Figure  6.  14 Strong dependence of the Xe···F b ···Xe bridge angle on the crystal packing and on the nature of the counter anion was demonstrated in a previous study. Calculation (Christiansen-Ermler ECP) in the same study also predicted non-linear structure of the [Xe 2 F 3 ] + cation with a bridging angle of 168°. 9

Conclusions
The oxidizing power of XeF 2 was demonstrated by oxidation of ruthenium and iridium metal in aHF as a solvent. Products of the oxidation belong to the family of no-