NMR Spectroscopic Evidence for the Intermediacy of XeF3– in XeF2/F– Exchange, Attempted Syntheses and Thermochemistry of XeF3– Salts, and Theoretical Studies of the XeF3– Anion

Abstract

The existence of the trifluoroxenate(II) anion, XeF3−, had been postulated in a prior NMR study of the exchange between fluoride ion and XeF2 in CH3CN solution. The enthalpy of activation for this exchange, ΔH⧧, has now been determined by use of single selective inversion 19F NMR spectroscopy to be 74.1 ± 5.0 kJ mol−1 (0.18 M) and 56.9 ± 6.7 kJ mol−1 (0.36 M) for equimolar amounts of [N(CH3)4][F] and XeF2 in CH3CN solvent. Although the XeF3− anion has been observed in the gas phase, attempts to prepare the Cs+ and N(CH3)4+ salts of XeF3− have been unsuccessful, and are attributed to the low fluoride ion affinity of XeF2 and fluoride ion solvation in CH3CN solution. The XeF3− anion would represent the first example of an AX3E3 valence shell electron pair repulsion (VSEPR) arrangement of electron lone pair and bond pair domains. Fluorine-19 exchange between XeF2 and the F− anion has also been probed computationally using coupled-cluster singles and doubles (CCSD) and density functional theory (DFT; PBE1PBE) methods. The energy-minimized geometry of the ground state shows that the F− anion is only weakly coordinated to XeF2 (F2Xe---F−; a distorted Y-shape possessing Cs symmetry), while the XeF3− anion exists as a first-order transition state in the fluoride ion exchange mechanism, and is planar and Y-shaped (C2v symmetry). The molecular geometry and bonding of the XeF3− anion has been described and rationalized in terms of electron localization function (ELF) calculations, as well as the VSEPR model of molecular geometry. Quantum-chemical calculations, using the CCSD method and a continuum solvent model for CH3CN, accurately reproduced the transition-state enthalpy observed by 19F NMR spectroscopy, and showed a negative but negligible enthalpy for the formation of the F2Xe---F− adduct in this medium.