A Raman spectroscopic study of the XeOF 4/XeF 2 system and the X-ray crystal structure of α-XeOF 4·XeF 2

Abstract

The mixed oxidation state complexes, α-XeOF 4·XeF 2 and β-XeOF 4·XeF 2, result from the interaction of XeF 2 with excess XeOF 4. The X-ray crystal structure of the more stable α-phase shows that the XeF 2 molecules are symmetrically coordinated through their fluorine ligands to the Xe(VI) atoms of the XeOF 4 molecules which are, in turn, coordinated to four XeF 2 molecules. The high-temperature phase, β-XeOF 4·XeF 2, was identified by low-temperature Raman spectroscopy in admixture with α-XeOF 4·XeF 2; however, the instability of the β-phase precluded its isolation and characterization by single-crystal X-ray diffraction. The Raman spectrum of β-XeOF 4·XeF 2 indicates that the oxygen atom of XeOF 4 interacts less strongly with the XeF 2 molecules in its crystal lattice than in α-XeOF 4·XeF 2. The 19F and 129Xe NMR spectra of XeF 2 in liquid XeOF 4 at −35 °C indicate that any intermolecular interactions that exist between XeF 2 and XeOF 4 are weak and labile on the NMR time scale. Quantum-chemical calculations at the B3LYP and PBE1PBE levels of theory were used to obtain the gas-phase geometries and vibrational frequencies as well as the NBO bond orders, valencies, and NPA charges for the model compounds, 2XeOF 4·XeF 2, and XeOF 4·4XeF 2, which provide approximations of the local XeF 2 and XeOF 4 environments in the crystal structure of α-XeOF 4·XeF 2. The assignments of the Raman spectra (−150 °C) of α- and β-XeOF 4·XeF 2 have been aided by the calculated vibrational frequencies for the model compounds. The fluorine bridge interactions in α- and β-XeOF 4·XeF 2 are among the weakest for known compounds in which XeF 2 functions as a ligand, whereas such fluorine bridge interactions are considerably weaker in β-XeOF 4·XeF 2.