Chromium(VI) Oxyfluoride Dianions, [Cr2 O4 F6 ]2- and [CrO2 F4 ]2- ; Syntheses and Structures of [XeF5 ]2 [Cr2 O4 F6 ], [XeF5 ]2 [Cr2 O4 F6 ] ⋅ nX (X=HF, n=4; X=XeOF4 , n=2), and [XeF5 ][Xe2 F11 ][CrO2 F4

Abstract

The fluorobasic character of the strong oxidative fluorinator, XeF6 , and the oxidative resistance of the [XeF5 ]+ and [Xe2 F11 ]+ cations have been exploited for the syntheses of several novel Cr(VI) dianion salts. The reactions of XeF6 and CrO2 F2 in anhydrous HF and by direct fusion of the reactants in melts have yielded the first dinuclear Cr(VI) oxyfluoro-dianion salts, [XeF5 ]2 [Cr2 O4 F6 ], [XeF5 ]2 [Cr2 O4 F6 ] ⋅ 4HF, [XeF5 ]2 [Cr2 O4 F6 ] ⋅ 2XeOF4 , and mononuclear Cr(VI) oxyfluoro-dianion salt, [XeF5 ][Xe2 F11 ][CrO2 F4 ]. The salts were structurally characterized by low-temperature (LT) single-crystal X-ray diffraction (SCXRD) and LT Raman spectroscopy. The [CrO2 F4 ]2- and [Cr2 O4 F6 ]2- dianions have distorted octahedral cis-dioxo Cr(VI) coordination spheres in which two F-atoms are trans to one another and two F-atoms are trans to O-atoms, where the [Cr2 O4 F6 ]2- dianion is the fluorine-bridged dimer of the [CrO2 F3 ]- anion. Quantum-chemical calculations were used to obtain the energy-minimized, gas-phase geometries, and the calculated vibrational spectra of the gas-phase dianions and their ion-pairs, which were used to aid in the vibrational frequency assignments of the crystalline salts. NBO and MEPS analyses and SCXRD show these salts are comprised of intimate ion-pairs in which their cations and anions interact through primarily electrostatic Xe- - -F σ-hole bonds.