Fluorine bridges

Mote stable catalysts ate obtained by using fluorinated graphite or fluorinated alumina as backbones, and Lewis acid halides, such as SbF, TaF, and NbF, which have a relatively low vapor pressure. These Lewis acids ate attached to the fluorinated soHd supports through fluorine bridging. They show high reactivity in Friedel-Crafts type reactions including the isomerization of straight-chain alkanes such as / -hexane. 

Catioas that are moaoauclear ia xeaoa and the FgXe—F------XeFg (Xe2F ) cation iateract with their fluoroanioas through oae or ore fluorine bridges. 

Details of the stmcture and fluorine bridging ia the F02Xe---F--Xe02F+ (Xe2 04F3 ) catioa are unknown, but the Xe-----F----Xe arrangement is assumed... 

The stmcture coasists of opea polymeric chaias, [Xe03F ] with two fluorine bridges to each xeaoa atom. 

Titanium Tetrafluoride. Titanium tetrafluoride [7783-63-3] is a white hygroscopic soHd, density 2798 kg/m, that sublimes at 284°C. The properties suggest that it is a fluorine-bridged polymer in which the titanium is six-coordinate. The preferred method of preparation is by direct fluorination of titanium sponge at 200°C in a flow system. At this temperature, the product is sufficiently volatile that it does not protect the unreacted sponge and the reaction proceeds to completion. The reaction of titanium tetrachloride with cooled, anhydrous, Hquid hydrogen fluoride may be used if pure hydrogen fluoride is available. 

It has a VF4 type puckered sheet structure with 6-coordinated ruthenium four fluorines bridge, two non-bridging ones are trans with the terminal distances shorter as expected (Table 1.1). It is paramagnetic (/xeff = 3.04/xB at room temperature). 

Martin observed an extremely facile thermal decomposition of bisperester 53, which gave 54 (Scheme 7).74 The decomposition proceeds via a transition state 55, with the simultaneous participation of the cr (0 O)—np(S)—a (0-0) type.74 The transition state can be draw by the 5c 6e interaction. Alvalez discussed the structure and the stability of Br42, together with neutral Br4.lla The dianion could be analyzed by the 4c 6e model, if the four atoms align linearly with suitable distances. Farnham investigated the fluorine-bridged linear C-I- -F- I C interaction based on 5c-6e.llb... 

The lack of secondary Te- -Cl interactions can be correlated with the presence of two aryl substituents with n-donating NMe2 groups. In an opposite way, electron-withdrawing fluorine atoms enhance the Lewis acidity of compounds (RF)2TeX2, leading to fluorine-bridged chains (with X=F) and related chain structures with X=C1 or Br.107... 

F and 119Sn NMR. Fluorine exchange occurs 165 between 4- and 5-coordinated tin complexes, possibly through chlorine and fluorine-bridged species. 

SbF5, BF3, NbFj, and TaF5 led likewise to the conclusion that, in the solid state, these compounds are best formulated as predominantly ionic, although the ions interact rather strongly by fluorine bridging. This bridging apparently persists in the molten state and to some extent in solution in nitrobenzene (25, 82). 

TeF4 and SbF5 form a 1 1 adduct (16). The Raman spectrum of this complex in the molten state has been recorded (65). It was not possible, however, to decide between the most plausible alternatives, i.e., ionization into the ions (TeF3)+ and (SbF ), or the formation of a fluorine-bridged structure F3TeFSbF5. 

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... 

No rational monomeric structure can be devised to fit these data. Actually, SbF6 is a very viscous liquid and it must be significantly associated. Cyclic or linear polymers could be formed by association through trans fluorine bridge bonds. However, this type of polymer would have as the recurring unit... 

Meinert and others [85] have stated that their concept for the electrochemical fluorination of organic compounds is based on the assumption that the first step is the anodic oxidation of the organic molecule. The electrochemical process is promoted by weakening of the C-H bonds due to hydrogen-fluorine bridges. After anodic withdrawal, the C-F bond is formed by insertion of a fluoride ion, present in the Helmholtz-double-layer at the electrode surface. 

Hence, it is clear that most highly fluorinated bridged ring compounds can only be prepared by fluorination of partially fluorinated precursors, otherwise rearrangements will probably predominate. 

---