Radiotracer Studies of the Fluorine Bond Lability of Some Binary Fluorides

Ghorab, Mohammed Fouzi (1988) Radiotracer Studies of the Fluorine Bond Lability of Some Binary Fluorides. PhD thesis, University of Glasgow.

Abstract

Fluorine bond lability with respect to exchange is an important chemical property of binary fluorides. This can be useful in predicting new synthetic routes for derivatives of fluorine compounds or elucidating the mechanisms of processes taking place at fluorinated surfaces. In the present study fluorine bond lability with respect to exchange of some fluoroanions namely, boron, phosphorus, arsenic, antimony, niobium and tantalum has been investigated during their interaction with [18F]-labelled hexafluorides of molybdenum, tungsten and uranium in acetonitrile. Two distinct types of exchange behaviour are identified among the fluoroanions. It is firmly established that the M-F bond (M = B,P, Nb and Ta) in BF4-, PF6-, NbF6 and TaF6- is labile with respect to exchange whereas the As-F and Sb-F bonds in AsF6- and SbF6- are inert although AsF6- is found to exchange in the presence of UF5 18F under the same conditions. The latter result is consistent with the higher fluoride ion affinity of UF6 as compared with that of MoF6 or WF6. An associative mechanism is used to account for the fluorine exchange of BF4-, PF6- NbF6-, TaF6- and AsF6- although a dissociative mechanism can not be ruled out. The difference in the fluoride bond lability among the fluoroanions under investigation is consistent with the difference in the Lewis acidity order of their parent fluorides. Evidence for the existence of the heptafluoromolybdate(VI) ion in solution has been obtained for the first time in this study by reaction between CsF and MoF5 18F in MeCN. The existence of the heptafluorotungstate(VI) ion in solution is confirmed by reaction between CsF and WF5 18F in MeCN. Both ions are assigned in the symmetry. The fluorine bond lability with respect to exchange of MoF7- and WF7- has been investigated both under homogeneous (MeCN) and heterogeneous (gas plus solid) conditions in the presence of MF5 18F (M = Mo, W or U). The WF7- ion has been used as its copper(II), thallium(I), nitrosonium and caesium salts whereas MoF7- ion has been used as its caesium salt only. All systems are found to undergo rapid and complete [18F]-fluorine exchange with MF518F (M = Mo or W) in MeCN at room temperature and below (253K). The nature of the cation does not have any effect on the exchange behaviour of the systems. However under heterogeneous conditions the same systems undergo partial exchange in the presence of MF5 18F (M = Mo, W or U). In MeCN the experimental observations are consistent with, but do not prove, a displacement mechanism for the exchange and an associative mechanism can not be ruled out. Under hetero- geneous conditions fluorine exchange is best accounted for by an associative mechanism. The partial exchange of [ 18F]-fluorine is ascribed to the anion-cation interaction. The reaction between activated caesium fluoride and MF5-18F (M = Mo, W and U) results in the formation of more than one species, MoF7- , WF7- and UF8 2- being the major ones. The formation of more than one species is ascribed to the lack of uniformity of the surface reactions. The amount of hexa- fluoroacetone used during the activation of CsF is found to have a direct effect on its reactivity and most probably its surface area. [ 18F]-Fluorine exchange is observed only when uptakes of MF6 are considerable. The exchange is shown to take place between the adsorbed species and free hexafluorides. An associative mechanism similar to that between MF7- (M = Mo or W) and MF518F (M= Mo,W or U)under heterogeneous conditions is used to account for the exchange between adsorbed species and free MF518F (M = Mo, W). The interaction of MF518F (M = Mo, W and U) with CuF2 and T1F under heterogeneous conditions results in a smaller uptake and [18F]-fluorine exchange in the case of CuF2 as compared with T1F. In neither case has it been possible to determine the nature of the adsorbed species. The difference in affinity for the hexafluorides between T1F and CuF2 is consistent with the difference in the Lewis acidity of T1+ and Cu2+. The reaction between tris(dimethylamino)sulphonium difluorotrimethylsilicate, TAS+F , and MF6 (M = Mo and W) in MeCN yields a brown viscous liquid thought to be due to either the formation of MF7- or MF5NMe2 (M = Mo and W) species. It is shown that the reaction between TAS+F- and BF3 or PF5 results in the formation of colourless crystalline TAS+BF4- or TAS+PF6- salts respectively. Complete [18F]-fluorine exchange between TAS +F- and [18F]-labelled Me3SiF is found to take place in MeCN most probably via an associative mechanism.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Inorganic chemistry
• Date of Award: 1988
• Depositing User: Enlighten Team
• Unique ID: glathesis:1988-77724
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77724