Technetium(VII) and Rhenium(VII) Oxofluorides and the Role of Noble-Gas Fluorides in Their Syntheses

Abstract

<p>Technetium, a β-particle emitter with a half-life of 2.12 x 10⁵ years, represents 6% of the fission products in commercial nuclear reactors. Although ⁹⁹Tc is not released during normal operation of nuclear power reactors, technetium may be introduced into the environment at other stages of the nuclear fuel cycle and is particularly prevalent as a contaminant near uranium enrichment facilities using recycled UF₆. Until recently, the only known fluorine derivatives of Tc were the two binary fluorides, TcF₅ and TcF₆, two oxofluorides, TcOF₄ and TcO₃F and the four anions TcF₆²⁻, TcF₆⁻, TcF₇⁻ and TcF₈²⁻. The fluorine chemistry of technetium was recently extended with the preparation of the fluorine-bridged infinite-chain polymer TcO₂F₃ by fluorination of TcO₃F by XeF₆ in anhydrous HF. The purpose of this work has to study the fluoride-ion donor and acceptor properties of TcO₂F₃ and its rhenium analog, ReO₂F₃, as well as the synthesis and characterization of TcOF₅ and possibly TcF₇. The significant Lewis acidity of TcO₂F₃ toward the fluoride ion and the CH₃CN led to the prepation of the TcO₂F₄⁻ anion as its Li⁺, Cs⁺ and N(CH₃)₄⁺ salts, and the TcO₂F₃(CH₃CN) adduct. Characterization by ¹⁷O, ¹⁹F, ⁹⁹Tc NMR and Raman spectroscopy established that the TcO₂F₄⁻ anion has a cis-dioxo geometry, which was confirmed by a single-crystal X-ray structure determination of Li⁺TcO₂F₄⁻. By analogy, ReO₂F₃ is also a Lewis acid forming M⁺ReO₂F₄⁻ [M = Li, Na, K, Cs, N(CH₃)₄] and ReO₂F₃(CH₃CN). In addition, the oligomeric anions Re₂O₄F₇⁻ and Re₃O₆F₁₀⁻ were isolated and their structures were determined by X-ray diffraction. These anions consist of two and three fluorine-bridged ReO₂F₃-units in which the oxygen atoms are cis to each other and trans to the bridging fluorines. The X-ray structure of ReO₂F₃ was also determined for the first time and is similar to that of TcO₂F₃. Both TcO₂F₃ and ReO₂F₃ behave as fluoride-ion donors, dissolving readily in hydrogen fluoride solutions acidified with the strong Lewis acids AsF₅ and SbF₅, and forming MO₂F₃∙PnF₅ adducts (M = Tc, Re; Pn = As, Sb). The structures of TcO₂F₃∙SbF₅ and ReO₂F₃∙SbF₅ consist of infinite chains of alternating fluorine-bridged MO₂F₄⁻ and SbF₆-units with the oxygen atoms cis to each other and trans to the bridging fluorines. The crystal structure of TcO₂F₃∙XeO₂F₂, a byproduct of the synthesis of TcO₂F₃, was also determined by X-ray crystallography and consists of TcO₂F₃ infinite chains linked together by a layer of XeO₂F₂ through xenon-fluorine long contacts. The structures of TcO₂F₃ and ReO₂F₃ in SO₂CIF solution were elucidated by ¹⁹F NMR spectroscopy and consist of cyclic fluorine-bridged species, (MO₂F₃)n, where n= 3 for M= Tc and n=3, 4 and 5 for M=Re. A new technetium(VII) oxofluoride, TcOF₅, was prepared from the reaction of TcO₂F₃ with an excess of KrF₂ in anhydrous HF. This volatile orange solid was characterized by NMR and Raman spectroscopy which confirmed its pseudoctahedral geometry. The fluoride-ion donor behaviour of TcOF₅ was investigated resulting in the preparation of the dinuclear cation Tc₂O₂F₉⁺ as its Sb₂F₁₁⁻ and AsF₆⁻ salts, and the structure of Tc₂O₂F₉⁺Sb₂F₁₁⁻ was determined by X-ray crystallography. Attempts to prepare the heptacoordinated species TcOF₆⁻ and TcF₇ were unsuccessful.</p>