Lewis acid fluorides

These studies at the same time aroused my interest in the mechanistic aspects of the reaetions, including the complexes of RCOF and RF with BF3 (and eventually with other Lewis acid fluorides) as well as the complexes they formed with aromatics. 1 isolated for the first time at low temperatures arenium tetrafluoroborates (the elusive (T-complexes of aromatic substitutions), although I had no means to pursue their structural study. Thus my long fascination with the chemistry of car-bocationic complexes began. 

Nitronium tetrafluoroborate was first prepared by adding a mixture of anhydrous hydrofluoric acid and boron trifluoride to a solution of dinitrogen pentoxide in nitromethane. Nitric acid can be used in place of dinitrogen pentoxide, and by replacing boron trifluoride by other Lewis-acid fluorides Olah and his co-workers prepared an extensive series of stable nitronium salts. ... 

Lewis acid (fluoride-ion acceptor) behaviour is exemplified by reactions with NOF and MF to give [N0]+[C1F2] and M+[C1F2] respectively (M = alkali metal or NH4). Lewis base (fluoride ion donor) activity includes reactions with BF3 and ASF5 ... 

This last reaction is typical of many in which F3CIO can act as a Lewis base by fluoride ion donation to acceptors such as MF5 (M = P, As, Sb, Bi, V, Nb, Ta, Pt, U), M0F4O, Sip4, BF3, etc. These products are all white, stable, crystalline solids (except the canary yellow PtFe ) and contain the [F2CIO] cation (see Fig. 17.26h) which is isostructural with the isoelectronic F2SO. Chlorine trifluoride oxide can also act as a Lewis acid (fluoride ion acceptor) and is therefore to be considered as amphoteric (p. 225). For example KF, RbF and CsF yield M [F4C10] as white solids whose stabilities increase with increasing size of M+. Vibration spectroscopy establishes the C4 structure of the anion (Fig. 17.29g). 

It was indeed found necessary to have excess antimony pentafluoride present in order to obtain stable alkylcarbonium hexafluoroantimonate complexes. Antimony pentafluoride is a liquid Lewis acid fluoride (b.p. 148-150°) of low dielectric constant (e 3), which has been shown by fluorine N.M.R. studies in the pure liquid state and in solution to exist in both cyclic and acyclic polymeric forms involving fluorine bridges. The antimony is in approximately octahedral co-ordination with predominant bridging by coordinating fluorines (Gillespie and Rothenbury, 1963). As fluorine generally does not show bridging properties, the structure of antimony pentafluoride itself indicates the very... 

Group-transfer polymerizations make use of a silicon-mediated Michael addition reaction. They allow the synthesis of isolatable, well-characterized living polymers whose reactive end groups can be converted into other functional groups. It allows the polymerization of alpha, beta-unsaturated esters, ketones, amides, or nitriles through the use of silyl ketenes in the presence of suitable nucleophilic catalysts such as soluble Lewis acids, fluorides, cyanides, azides, and bifluorides, HF. ... 

The acidity of perfluorinated sulfonic acids can be increased further by complexa-tion with Lewis acid fluorides, such as SbF5, TaF5, and NbF5.183 They have been found to be effective catalysts for n-hexane, n-heptane isomerization, alkylation of benzene, and transalkylation of alkylbenzenes (see Chapter 5). 

The pentafluorides, AsFs and SbFs, are prepared by direct fluorination of the elements or their oxides. Both AsFs and SbFs are used as Lewis acid fluoride acceptors in the preparation of many MFg salts. Anhydrous HF has greatly increased acidity in the presence of SbFs because of the creation of the H2F+ cation by removal of fluoride. The hexafluoroantimonate anion stabilizes many energetic cationic species, for example, [NF4+][SbF6 ]. 

The spontaneous oxidation of xenon by fluorine in the presence of a Lewis acid fluoride. A, was observed to occur only in the liquid phase. There was no interaction between Xe, F2 and ASF5 (strongest A) in the gas phase, in the dark, at 20°C at a total pressure of 10 atmospheres, over one day. When this mixture was cooled to -60°C... 

M. Lerner, R. Hagiwara and N. Bartlett, Synthesis of Main-Group Graphite Fluoroanion Salts with Chlorine-Assisted Oxidation by Lewis-Acid Fluorides, J. Fluorine Chem. 57 (1992) 1-13. 

Nitronium salts are colorless, crystalline, very stable compounds N02 BFlt decomposes at atmospheric pressure only above 170°, without subliming, into its components NO2F + BF3. The hexafluoroantimonate salts are even more stable. The higher thermal stability may be partially also a consequence of the higher boiling points of the corresponding Lewis acid fluorides compared to boron trifluoride. 

The nitrations are carried out either with an excess of the aronatic as diluent and introducing nitryl fluoride and the Lewis acid fluoride catalyst simultaneously at low temperature into the well stirred reaction mixture or in a suitable solvent such as tetramethylene sulfone which can be used as solvent advantageously if the catalyst fluoride does not interact with it (SbF being a strong fluorinating agent, attacks the solvent and cannot be used). 

Most of the solid compounds containing the CIF2 cation were prepared by direct reaction between the parent compound ClFa and the proper Lewis acids, fluoride acceptors such as BFa and pentafluorides of P, As and Sb, forming 1 1 compounds. In reaction with SnF4, a compound 2 1 is formed with an SnF anion. 

Instead of BF3, other Lewis acid fluorides such as PF5, TaFs, NbFs, SbFj can also be used effectively in related nitrations. 

Consequently, its use is not generally recommended and extreme caution is called for. In contrast, complex fluoride salts such as the tetrafluoroborate and hexafluorophosphate are very stable. Only on heating to higher temperatures (> 180-200°C) do they decompose into NO2F and the corresponding Lewis acid fluoride. 

Lewis acid fluorides to form KrF+ and Kt2F salts. The KrF+ cation is the strongest oxidative fluorinating agent known. 

Radon, Rn At. no. 86, at. wt 222, mp—71°C, bp -61.8°C. Radon is an intermediate radioactive decay product of Ra. Rn, the most stable isotope of radon, is obtained as a gas from aqueous solutions of RaCl2 and has been used as a radiation source and as a gaseous tracer. It is a considerable hazard in uranium mines. In some areas, radon in basements and in ground water is a potential health hazard because of its radioactivity. The ground state electronic configuration of radon is [Xe]4f " 5d °6s 6p. Because radon is intensely radioactive, the chemistry of radon has only been investigated on the tracer scale. Radon forms compounds, particularly a fluoride (likely RnF2), and solid adducts between the fluoride and Lewis acid fluorides. 

Using Super-Acids as Catalysts. Super-acid catalysts can directly condense CH4 into CJ hydrocarbons at relatively mild temperatures. The work in this area were mainly carried out by Olah and his co-workers (see review by Kuo (1987)). The super-acids are generally higher valency Lewis acid fluorides of metals of Groups IV, V and VI of the periodic table which were used at 50-200 C and 1 to >150 atm. Research in this area is exploratory. Also, handling of strong acids can be quite expensive. No thermochemical calculations were carried out because its reaction products were not sufficiently defined. 

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