Die Chemie der Fluoride und Oxidfluoride der Metalle Molybdän, Wolfram und Uran sowie Versuche zur Synthese von Metallnitridfluoriden
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Abstract
The paper reports on the syntheses and characterization of new fluorides and oxide fluorides of the metals molybdenum, tungsten, and uranium. In addition, experiments on the preparation of metal nitride fluorides are reported.
Abstract
This work reports on the chemistry of fluorides, oxidofluorides and attempts to synthesize
nitride fluorides of the metals molybdenum, tungsten and uranium. The results are divided into
three parts in the chapters 4 to 6.
Oxidofluoridometallates of Molybdenum, Tungsten and Uranium
During the studies of the reaction behavior of MoOF4 its crystal structure was redetermined by
X-ray diffraction measurements on single crystals. On this basis periodic quantum-chemical
calculations with the density functional theory method utilizing a hybrid functional (DFT-
PBE0) for the solid state were carried out to assign the observed IR- and Raman-vibrations.
Furthermore, a new synthetic route for MoOF4 is presented, based on the reaction of
molybdenum oxides with bromine trifluoride. Analogous reactions of the oxides of tungsten
and uranium with bromine trifluoride produced volatile hexafluorides rather than their oxide
tetrafluorides. Attempts to synthesize the compounds MoOF2, MoOF3 and MoO2F2 using
anhydrous hydrogen fluoride or hydro fluoric acid were unsuccessful.
The reactions of CaCl2, SrF2 and BaF2 with a fourfold excess of MoOF4 in anhydrous hydrogen
fluoride yielded colorless single crystals of the oxidofluoridomolybdate salts Ca[Mo2O2F9]2·HF
(P21/c), Sr[Mo2O2F9]2·HF (P1̅) and Ba[Mo2O2F9]2·2HF (R3̅). Their crystal structures were
determined by single crystal X-ray diffraction. They are the first representatives of salts to
contain both alkali earth metal cations and a dinuclear [Mo2O2F9]− anion. Based on their crystal
structures periodic quantum-chemical calculations with the density functional theory method
utilizing a hybrid functional (DFT-PBE0) for the solid state were carried out to assign the
observed IR- and Raman-vibrations.
Attempts to synthesize oxidofluoridotungstate salts which contain the trinuclear
[W3O3F13]− anion in analogy to the known molybdate salts failed. The reactions of tungsten
oxides with chlorine trifluoride probably yielded volatile tungsten hexafluoride.
Light blue crystals of Barium pentafluoridooxidomolybdate(V), Ba[MoOF5], were synthesized
directly for the first time by reacting BaCl2 with MoCl5 under hydrofluorothermal conditions.
Its crystal structure was redetermined. Based on the improved crystal structure periodic
quantum-chemical calculations with the density functional theory method utilizing a hybrid
functional (DFT-PBE0) for the solid state were carried out to assign the observed IR- and
Raman-vibrations.
Abstract
XIII
In Figure 1 a schematic overview of the compounds synthesized in this section is shown.
Figure 1: Overview of the oxidofluoridomolybdate salts synthesized in this section.
Attempted syntheses of nitride fluorides
The syntheses of nitride fluorides of the metals molybdenum, tungsten and uranium were
investigated to determine whether controlled ammonolysis is possible through the reaction of
nitrides with anhydrous hydrogen fluoride. An initial attempt was made to dissolve the nitrides
Mg3N2, Ca3N2, BN, Si3N4, and Mo2N in anhydrous hydrogen fluoride.
No reaction or dissolution was observed for BN, Si3N4, and Mo2N in anhydrous hydrogen
fluoride. Mg3N2 and Ca3N2 were dissolved in anhydrous hydrogen fluoride at 80 °C, remaining
dissolved even after cooling to room temperature. After the removal of the solvent, colorless
powders were obtained. CaF2 and MgF2 were identified as reaction products by powder X-ray
diffraction. In addition, other unknown compounds were also formed that have not yet been
characterized.
Attempts to perform an ammonolysis reaction of MoF6 with Ca3N2 in anhydrous hydrogen
fluoride yielded crystals of the compound Ca[Mo2O2F9]2·HF, which probably crystallized due
to unintended contamination of the reaction vessel by traces of water. The compound was also
obtained by reacting CaCl2 with MoOF4 in anhydrous hydrogen fluoride, as described above.
Ca[Mo2O2F9]2·HF crystallizes in the monoclinic space group P21/c. It is the only calcium salt
known to date that consists of discrete oxidofluoridomolybdate anions and represents a novel
structure type. Periodic quantum-chemical calculations with the density functional theory
method utilizing a hybrid functional (DFT-PBE0) for the solid state were carried out to assign
the observed IR- and Raman-vibrations.
Abstract
XIV
Further attempts to synthesize molybdenum nitride fluoride by reacting molybdenum nitride
chloride with hydrogen fluoride or elemental fluorine probably resulted in the formation of
volatile molybdenum hexafluoride.
Binary and ternary Fluorides of Molybdenum, Tungsten and Uranium
As a part of an industrial cooperation project, various reduction methods for uranium
hexafluoride known from the literature were tested. The reduction of uranium hexafluoride with
hydrogen at room temperature using heterogeneous platinum/hydrogen fluoride catalysis could
not be reproduced.
When reducing uranium hexafluoride with sulfur in anhydrous hydrogen fluoride, it was shown
that, depending on the reaction time, either U2F9 or UF4 is obtained. Short reaction times of a
few minutes to one day, favored the formation of U2F9, while longer reaction times of three
days yielded UF4. It seems that U2F9 acts as a kinetic intermediate product.
The influence of the uranium hexafluoride to sulfur ratio was investigated. With a twofold
excess of UF6, no significant reduction was observed. When UF6 and S8 were reacted in a molar
ratio of 1:1, with a slight excess of UF6, U2F9 formed immediately. Within three days, UF4 was
obtained out of the reaction mixture. When a twofold excess of sulfur was used, UF4 was
obtained directly. No initial formation of U2F9 was observed in this case.
As this was the first time that a simple synthetic route for the binary uranium fluoride U2F9 had
been found, first insights into its chemical properties could be presented. While attempting to
record a Raman spectrum of U2F9, the sample decomposed. Based on the greenish compound
that formed, it is likely that UF4 was formed. U2F9 decomposes in the presence of water within
one hour and yields an UF4 hydrate. When exposed to air, U2F9 turns green within three days
and yields UF4.
When investigating the Lewis acidity of U2F9 in anhydrous hydrogen fluoride, no reaction was
observed with CsF. In contrast a reaction took place if CsF is present during the reduction of
UF6 with sulfur in anhydrous hydrogen fluoride. One or more possible Lewis adducts were
obtained as indicated by the powder X-ray diffractogram of the isolated product.
Analogous attempts to reduce molybdenum and tungsten hexafluorides using sulfur produced
a red oil when reacting with molybdenum hexafluoride. So far, this red oil has not been
identified. No reaction was observed when tungsten hexafluoride was treated with sulfur in
anhydrous hydrogen fluoride.
Abstract
XV
By reducing uranium tetrafluoride with silicon at 1000 °C greenish single crystals of uranium
trifluoride were obtained. With those the first crystal structure determination of uranium
trifluoride was carried out by single crystal X-ray diffraction. Uranium trifluoride crystallizes
in the centrosymmetric space group P3̅ c1. All crystals examined were twinned by merohedry,
simulating a Laue symmetry of 6/mmm. Due to the absence of a polar axis in the determined
space group P3̅c1, UF3 does not have ferroelectric properties, which were predicted based on
the previously published space group P63cm.
Yellow needle like crystals of Dicarbonylgold(I)-hexafluoridouranate(V), [Au(CO)2][UF6],
was obtained by oxidizing gold powder with uranium hexafluoride in anhydrous hydrogen
fluoride in a carbon monoxide atmosphere. The crystal structure of the non-classical carbonyl
compound was determined by X-ray diffraction on single crystals in the chiral space group
P3221 and confirmed the predicted composition and molecular structure of the complex ions.
The powder X-ray diffractogram of the isolated product indicates the decomposition of
[Au(CO)2][UF6] into Au, CO, and UF6 in vacuum.
The ν(CO) band was observed in the Raman spectrum at 2250 cm−1. As expected for non-
classical carbonyls, this is greater than for the free CO molecule (2143 cm−1). Quantum-
chemical calculations using the PBE0 hybrid density functional theory method were performed
on the solid state of [Au(CO)2][UF6] and for the [Au(CO)2]+ cation in the gas phase to assign
the observed Raman bands.
Attempts to synthesize the analogous silver carbonyl [Ag(CO)2][UF6] resulted in colorless
needle like single crystals of the compound silver(I) hexafluoridouranate(V), Ag[UF6]. X-ray
diffraction measurements on single crystals showed that Ag[UF6] crystallizes in the Ca[TbF6]
structure type in the tetragonal space group P42/m.
The reaction of barium fluoride with uranium pentafluoride in anhydrous hydrogen fluoride
yielded greenish crystals of the compound Ba[U2F12]·1.36HF, which crystallizes in the
orthorhombic space group Pnma. It is likely that the compound Ba[U2F12]·2HF exists. The
removal of the solvent in vacuum might have caused a loss of a part of the HF molecules of
crystallization.
The syntheses of hepta- and octafluoridouranates(VI) were investigated in hydrogen fluoride,
chlorine trifluoride, and liquid uranium hexafluoride. In none of the experiments conducted
suitable single crystals were obtained for a structure determination.
Abstract
XVI
However, based on the cell parameters obtained from the indexed reflexes of the powder X-ray
diffractograms, the successful syntheses of some hepta- and octafluoridouranates(VI) can be
assumed.
In the reaction of rubidium fluoride with uranium hexafluoride in anhydrous hydrogen fluoride,
the reflexes were indexed with an orthorhombic cell with lattice parameters a = 12.408(19),
b = 8.658(5), c = 8.222(7) Å, V = 883.2(1) Å3, which corresponds to a volume of
4·[2V(RbF) + V(UF6)]. It is likely that Rb2[UF8] was obtained successfully.
The reaction of caesium fluoride with uranium pentafluoride in chlorine trifluoride yielded a
yellow, microcrystalline powder. A cubic unit cell with a lattice parameter of a = 11.0916(5) Å
and a volume of V = 1364,52(11) ų, which corresponds to the volume of 8·[V(CsF) + V(UF6)],
was obtained by the indexing of the observed reflexes in the powder X-ray diffractogram. The
same reflexes were observed in the powder X-ray diffractogram of the isolated product of the
reaction of caesium fluoride with liquid uranium hexafluoride at 120 °C, which was recorded
at room temperature.
The same reaction yielded at 80 °C a yellow microcrystalline powder. The recorded powder X-
ray diffractogram, which was recorded at room temperature as well, could be indexed with an
orthorhombic unit cell with the lattice parameters a = 14.039(4), b = 9.4799(19),
c = 6.4537(15) Å, V = 858.9(3) ų, which corresponds to the volume of 4·[2V(CsF) + V(UF6)].
The successful formation of Cs2[UF8] was therefore assumed.
A schematic overview of the reactions investigated in this section is shown in Figure 2.
Figure 2: Overview of the reactions investigated in this section. The crystal structures of the compound which are
shown with dotted lines were not determined.
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Issued: 2025-12-03
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Is based on: 10.1021/acs.inorgchem.3c01792Is based on: 10.1107/S2056989024005759Is based on: 10.1002/ejic.202400748Is based on: 10.1021/acs.inorgchem.5c00450Is based on: 10.1002/ejic.202500340Is based on: 10.1002/ejic.202500316Is based on: 10.1021/acs.inorgchem.5c03005
Faculty
FB15:Chemie
Language
de
Keywords
FluorideMolybdänWolframUran
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Wassermann, Tobias Burghardt: Die Chemie der Fluoride und Oxidfluoride der Metalle Molybdän, Wolfram und Uran sowie Versuche zur Synthese von Metallnitridfluoriden. : 2025-12-03.
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