Scientific paper
Reactions of Xenon with Iridium-and Osmiumhexafluoride
Farhad Tamadon,1 Stefan Seidel2 and Konrad Seppelt1'*
1 Freie Universität Berlin, Institut für Chemie und Biochemie, Fabeckstraße 34-36, 14195 Berlin
2 Thüringer Str. 29, 34212 Melsungen * Corresponding author: E-mail: seppelt@ zedat.fu-berlin.de Received: 14-05-2012 Dedicated to Professor Boris Zemva
Abstract
Xenon and Iridiumhexafluoride react at temperatures above room temperature forming XeF+IrF6-. In presence of SbF5 FXe+IrSbFjj- is formed. Xenon and Osmiumhexafluoride form in solution a blue charge transfer complex that cannot be isolated as a solid.
Keywords: Xenon oxidation, iridium hexafluoride, osmium hexafluoride, fluoro xenon hexafluoroiridate crystal structure
1. Introduction
The reaction between Xenon and PtF6 is of historical importance since it marked the beginning of noble gas chemistry.1 Ironically it is not even now clear what the mustard colored amorphous, paramagnetic powder, »XePtF6«, is.2 After warming to room temperature the IrF6 also fluorinates NO up to NOF3, while OsF6 gives on-
count.7 Precise experimental data for the electron affinities are not available and seem very difficult to be obtained, at least with a certain minimum accuracy.
Qualitatively it is obvious that IrF6 is a stronger oxi-dator than OsF6. The former can oxidize Cl2 to Cl4+IrF6-, while OsF cannot do the same under similar conditions.8
material converts into XeF+PtF6 or XeF+Pt2F11.2
In the beginning of noble gas chemistry it has been shown that some other metal hexafluoride promote a similar redox reaction with xenon, namely RuF63 and RhF6,4 whereas other do not, e.g. IrF6, OsF6, ReF6, WF6, and MoF6. It is striking that the deeply colored hexafluo-rides PtF6, RhF6, RuF6 have an electron affinity high enough to oxidize xenon, while lightly colored (yellow, orange) IrF6, OsF6, ReF6 TcF6 or colorless WF6 and MoF6 have a too low electron affinity. The absorptions in the visible region are assigned to fluorine to metal charge transfer bands5 and therefore reflect the extreme electron affinity of the Pt, Ru, and Rh centers in the hexaf-luorides.
But the actual electron affinities of these metal he-xafluorides are till under discussion. N. Bartlett has proposed a qualitative sequence: WF6 < ReF6 < OsF6 < IrF6 < PtF6 5 Classical theory would clearly rank OsF6 (d2) higher in electron affinity than IrF6 (d3). This sequence can be exchanged if spin orbit coupling is taken into ac-
ly NOF.6
2. Results and Discussion
Xenon and OsF6 or IrF6, if condensed into a quartz tube9 and held at -78 °C give an intense blue solution. If these samples are warmed to room temperature a slow reaction sets in. In case of Xe/IrF6 yellow crystals are formed. No reaction is observed in the Xe/OsF6 samples.
The blue color is certainly due to a charge transfer absorption of a Xe/MF6 complex which exists in solution, but cannot be crystallized. Similar charge transfer colors have been observed in the Cl2/OsF6, Cl2/IrF6, and Xe2+/ Xe/SbF5 mixtures.9
The identity of the crystalline material is XeF+IrF6-, as is established by a single crystal structure determination, see figure 1 and table 1.
The crystal structure reveals that there are three different XeF+IrF6- unit in the asymmetric unit. But with one
Figure 1. XeF+IrF6-, Ortep representation, 50% probability ellipsoids. There are three crystallographically independant, but chemically quite similar units in the asymmetric unit.
exception their bond lengths and angles are so close that they can be discussed altogether, see Table 1.
The ionic formulation XeF+IrF6- should be extended by calling this compound an ion pair with a directed attraction of the bridging fluorine atom between the iridium and the xenon atom, giving an almost linear fluorine environment around the xenon atom. This is a well known behaviour of Xe(II), e.g. observed in XeF+Sb2F11- for the first time,10 and in many cases later on. The three crystallographic different
Figure 2. FXe+ IrSbFu-, Ortep representation, 50% probability ellipsoids. The overall structure is very similar to FXe+Sb2Fu- and other salts of that stoichiometry. To our knowledge it is the first time that a mixed anion ABF,is identified.
ion pairs FXe+IrF6- differ essentially only in the angle of the bridging fluorine atom, with values of 119.3, 126, and 141.9. In compounds of a similar composition FXe+AF6- A=As, Sb, Bi, and Ru this angle at the bridging fluorine atom varies between 133.6(2)° (A=As and 156.1(4)° A=Bi).11-14
The reaction between Xe and IrF6 can be speeded up dramatically if it is performed in anhydrous HF and in presence of SbF5. The reaction sets in at temperatures below 20 °C and is completed quickly at this temperature. No blue charge transfer complex is observed as an intermediate. The outcome is the compound FXe+IrSbF11-, see figure 2 and tables 1 and 2.
Table 2. Crystallographic Data
	FXe+IrF6-	FXe+IrSbF11-
a(pm)	794.9(2)	589.8(1)
b(pm)	1085.8(3)	2091.1(1)
c(pm)	2132.0(6)	787,9(1)
P(°)	91.637(7)	90.02(2)
V(pm3 • 106)	1839.4	971.7
lattice	monoclinic	monoclinic
space group	P21/c	C c
size (mm)	0.05 x 0.05 x 0.04	0.1 x 0.03 x 0.003
color	yellow	yellow
Pcalcd	4.945	4.602
|(mm-1)	27.27	20.04
reflexions		
total	21942	6566
unique	5478	3192
R-1 int	0.044	0.088
26 (°) maxv 7	62.28	65.11
parameters	245	140
R1	0.0323	0.0571
wR2	0.0619	0.1617
Goof	1.127	1.053
3. Experimental Section
3. 1. Preparation of FXe+IrF6-
A quartz tube is connected to a metal vacuum line and cooled to -196 °C. 300 mg (0.98 mmol) IrF6 and 1 ml
Table 1. Important bond lengths (pm) and angles (°) of FXe+IrF6 and FXe+IrSbFu . The three values for FXe+IrF6- correspond to the three crystallographic different molecules in the asymmetric unit.
	FXe+IrF6-	FXe+IrSbF11-
XeF	185.4(5), 186.7(5), 185.4(4)	187.9(10)
Xe-Fbra	227.2(4), 222.0(4), 224.5(5)	228.8(8)
Ir-V	195.6(5), 196.1(4), 195.9(5)	197.3(9)
Ir-Fb	185.4(5), 186.9(9), 186.3(5)	202.0(9)c
Ir-F	185.4-187.3(5), 185.8(5)-187.1(5), 185.7-186.1(5)	180.8-186.4(10)
Sb-F		183.5-185.6(10), 201.8(8)
F"Xe-Fbr	177.9(2), 177.4(2), 179.3(2)	178.1(4)
Xe-F-Ir	119.3, 126, 141.9	122.0(4)
a Fbr: bridging fluorine atom between Ir and Xe Fax: fluorine atom opposite to the bridging fluorine atom c bridging fluorine atom between Ir and Sb
c
xenon are condensed into it. The tube is sealed and warmed to room temperature.
Caution: high pressure! A dark blue, almost black solution is formed. The tube is further warmed to 45 °C, and then cooled to -196 °C and connected again to the metal vacuum line. By slow warming to room temperature the volatiles, mostly excess xenon, are pumped off. A yellow microcrystalline solid remains. 200 mg of it are filled into a PFA tube, and 600 mg anhydrous HF are condensed on it. Recrystallisation by cooling from 80 °C to room temperature bring out large, cubic shaped, yellow crystals.
3. 2. Attempted Reaction Between Xenon and OsF6
The same procedure as described above is tried with xenon and OsF6. Also a dark blue-black solution is formed. After evaporation no solid remains. If the OsF6 solution in xenon is cooled to -78 °C, yellow OsF6 crystallizes out, and the blue color of the charge transfer complex disappears.
3. 3. Preparation of Xe+IrSbFn-
1.2 g (5.5 mmol) SbF5 are given into a PFA tube, and the tube is connected to a metal vacuum line. 680 mg anhydrous HF are condensed into it, the sample is warmed to room temperature and well mixed. At -196 °C 1.06 g (3,5 mol) IrF6 are condensed in, warmed to room temperature, mixed, a yellow solution is formed, and cooled again to -196 °C, when 1,5 g (11 mol) xenon are condensed in. The tube is sealed and warmed slowly to room temperature. The orange solution is cooled slowly to -60 °C, yellow crystals of FXe+IrSbF11- are formed. The crystal structure suffers from racemic and a non-me-rohedral twinning, resulting only in moderate crystallo-graphic characteristics.
Crystals are selected and mounted with help of a special device16 on a Bruker CCD Smart diffractometer (MoKa irradiation, ®-scan in 0.3° steps, 2400 frames). Absorption correction equilibration of symmetry selected reflections. Structure solution and refinement by Shelx programs.17 Further details of the crystal structure determinations may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (FAX +49-7247-808-666; E-Mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request) for deposited data.html on quoting the depository numbers CSD 424640 (FXe+IrF,-) and 424641 (FXe+IrSbF,,-).
4. Conclusion
While the quantitative difference in oxidation potential and/or electron affinity between OsF6 and IrF6 is still
unclear, it is obvious that IrF6 has considerable higher values than OsF6. The difference in electron affinity as suggested by N. Bartlett, namely 0.5-1.0 eV may be too high, but the recently calculated value of this difference, 0.07 eV7 should be regarded as a lower limit, since converted into a chemical reaction enthalpy under otherwise identical conditions comes out only at 1.6 kcal/Mol. The assumption of a kinetic difference of the oxidation reaction of IrF6 and OsF6 with xenon has been disapproved with our result.
5. Acknowledgements
This work was done as part of the Graduiertenkolleg »Fluorine as a Key Element«, funded by the Deutsche Forschungsgesellschaft.
6. References
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15.	PFA: Tetrafluoroethene-perfluorovinylether copolymer.
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Povzetek
XeF+IrF6- je produkt reakcije med ksenonom in iridijevim heksafluoridom pri temperaturah do 45 °C. V prisotnosti SbF5 nastane FXe+IrSbFn- . Pri reakciji med ksenonom in osmijevim heksafluoridom v raztopini opazimo modro obarvan kompleks, ki ga ni mogoče izolirati kot trdno snov.