Über die Chemie der Pentafluoride des Broms und des Niobs und die Rolle des Antimonpentafluorids in der chemischen Fluorsynthese
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Philipps-Universität Marburg
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Abstract
This thesis is divided into three different topics of fluorine chemistry:
•The synthesis and characterization of bromine pentafluoride and fluoridobromates(V)
•The mechanistic elucidation of the chemical fluorine synthesis
•The synthesis and characterization of niobium pentafluoride and ternary and quasi-ternary niobium fluorides
Bromine pentafluoride was first synthesized by a photochemical reaction of fluorine with BrF3 at room temperature. It was investigated using spectroscopic and crystallographic methods, whereby its previously incorrectly determined crystal structure was corrected, and a further low-temperature modification was discovered.
The establishment of a practicable method for the preparation of BrF5 in the laboratory paved the way for further investigations into the subsequent chemistry of the compound. The reactions of BrF5 with KF, RbF, CsF and [NMe4]F lead to the formation of the corresponding fluoridobromates(V) with octahedral [BrF6]– anions. The compounds were characterized by vibrational spectroscopy and X-ray diffraction. By powder X-ray diffraction the previously unknown structure of anhydrous [NMe4]F was also elucidated.
From solutions of Cs[BrF6] or [NMe4][BrF6] in BrF5, the compounds Cs[Br3F16] or [NMe4][Br4F21]·BrF5 crystallize at low temperatures. These are the first two compounds with oligonuclear fluoridobromate(V) anions, namely the C3-symmetric, propeller-shaped [μ3-F(BrF5)3]– anion and the tetrahedrally shaped [μ4-F(BrF5)4]– anion. The [μ4-F(BrF5)4]– anion is the first example of a μ4-like coordination of an F– ion that is neither bonded to metal nor hydrogen atoms. A special feature of these ions is that the free electron pairs of the Br atoms, in contrast to the [BrF6]– anion, are stereochemically active, as could be shown based on the crystal structures and quantum chemical analyses of the electronic structure.
In the presence of hydrogen fluoride, cocrystals of fluoridobromates(V) and hydrogen fluoride were obtained. In the compounds [NMe4][(BrF5)6(HF2)] and Cs2[(BrF5)6(HF2)(H2F3)], [HF2]– and [H2F3]– anions occur whose F atoms are coordinated by BrF5 molecules. The propeller-like coordination of an F atom by three BrF5 molecules, as known from the [Br3F16]– anion, appears as a recurring structural motif in both compounds.
Since the discovery of the element fluorine in 1886, electrochemical fluorine synthesis
according to Henri Moissan was the only way to produce fluorine for a long time. Fluorine
synthesis by purely chemical means, on the other hand, was long considered impossible and
only succeeded 100 years later, in 1986, through the reaction of K2[MnF6] with the Lewis acid
SbF5. However, the originally postulated reaction mechanism, according to which unstable
MnF4 is formed, which spontaneously decomposes to MnF3 and elemental fluorine, contradicts
the known thermal stability of MnF4 and has not been proven experimentally.
Based on experimental and quantum chemical investigations, the actual mechanism of chemical
fluorine synthesis was elucidated in this work: While the spontaneous decomposition of MnF4
only occurs above 170 °C, the reductive elimination of F2 in the presence of SbF5 already takes place at room temperature. In addition to elemental fluorine, the reaction produces the Mn(II) compound Mn[Sb2F11]2, which was detected spectroscopically and by means of crystal structure analysis.
SbF5, which is present in the liquid phase at equilibrium in the form of the more Lewis-acidic dimer Sb2F10 or as an SbnF5·n oligomer (n ≥ 3), plays a decisive role in the reaction. Quantum chemical calculations on a molecular model system in the gas phase suggest that two fluoride ions are abstracted from MnF4, forming two [Sb2F11]– anions that coordinate the Mn(IV) atom as tridentate ligands. Through the formation of the second [Sb2F11]– ligand and its coordination to the Mn(IV) atom, two F atoms terminally bound to the Mn atom are displaced, which enables their reductive elimination as F2. In the crystal structure of the product Mn[Sb2F11]2, the Mn(II) atom is octahedrally coordinated by six fluoride ions of the surrounding [Sb2F11]– ligands.
It was also shown that no reductive elimination of F2 occurs in the reaction of K2[MnIVF6] with a large excess of SbF5 when anhydrous hydrogen fluoride (aHF) is used as a solvent. In aHF, SbF5 is present in the form of the anionic species [SbF6]–, [Sb2F11]– or [SbnF5n+1]–, depending on the concentration. This confirms the crucial role of neutral SbF5 and its oligomers in the chemical fluorine synthesis, without which the reduction of the Mn atom and the release of F2 would not occur. Instead, the previously unknown Mn(IV) compound K3[(MnIVF)(SbF6)5]F was obtained and characterized by X-ray diffraction, Raman spectroscopy and quantum chemically.
Various methods for the preparation of niobium pentafluoride, NbF5, were investigated. These included fluorination reactions of niobium metal, Nb2O5 and NbCl5 with elemental fluorine and aHF as well as the synthesis and thermal decomposition of various fluoridoniobates(V). The reactions of niobium metal, NbCl5 or Nb2O5 with elemental fluorine and the reaction of NbCl5 with aHF proved to be suitable for producing NbF5 in high purity and in good yield on a laboratory scale. For industrial applications, direct fluorination of niobium metal ingots is preferred, as no chlorine- or oxygen-containing impurities or by-products are produced. With suitable reaction setup, the unused F2 can be reused, so that a quantitative reaction in terms of Nb and F2 is possible. An alternative is the reaction of NbCl5 with aHF.
The preparation of NbF5 by the thermal decomposition of Sr[NbF7], Ba[NbF7] and other fluoridoniobates(V) is possible in principle, but only yields small amounts of NbF5 and requires further optimization. Sr[NbF7] and Ba[NbF7] were obtained by reacting the corresponding alkaline earth metal fluorides with Nb2O5 in aHF. The compounds were analyzed by vibrational spectroscopy and powder X-ray diffraction and their crystal structures were determined. From solutions of Sr[NbF7] in aHF, single crystals of the previously unknown compound Sr[NbF6(HF)][H3F4] were obtained, which crystallizes isotypic to the literature known compound Sr[AuF6(HF)][H3F4].
The compound [H3O][NbF6] could be identified as a product of the partial hydrolysis of NbF5, selectively synthesized, and characterized. A special feature of the compound is its pronounced polymorphism: At room temperature, [H3O][NbF6] crystallizes in the polar, orthorhombic space group Iba2 (no. 45), with the dipole moments of the [H3O]+ cations aligned along the polar axis. When the compound is cooled, the orientation of the [H3O]+ cations changes so that their dipole moments cancel each other out in the non-centrosymmetric low-temperature modification (space group I213, no. 199) below 137 K. Above 363 K, [H3O][NbF6] is present in its high-temperature modification. This exhibits a plastic disorder of the anions and cations and crystallizes in the cubic, centrosymmetric space group Pm3̅m (no. 221). Since the polar orientation of the [H3O]+ ions is thus reversible at high and low temperatures due to the observed phase transitions, the compound fulfills all the structural requirements for ferroelectric properties.
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Dates
Created: 2025Issued: 2025-05-15Updated: 2025-05-15
Faculty
Fachbereich Chemie
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
chemische FluorsyntheseNiobium pentafluorideBromine pentafluorideNiobpentafluoridchemical synthesis of fluorine
DFG-subjects
Fluoridobromate(V)FluorchemieHalogenfluoridePolyinterhalogenanionenpräparative ChemieBrompentafluorid
DDC-Numbers
500
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Möbs, Martin (M. Sc.) (0009-0009-1362-8212): Über die Chemie der Pentafluoride des Broms und des Niobs und die Rolle des Antimonpentafluorids in der chemischen Fluorsynthese. : Philipps-Universität Marburg 2025-05-15. DOI: https://doi.org/10.17192/z2025.0111.
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This item has been published with the following license: In Copyright