Structural and vibrational characterization of [KrF][AuF 6] and α-[O 2][AuF 6] using single crystal X-ray diffraction, Raman spectroscopy and electron structure calculations

Abstract

The salt [KrF][AuF 6] has been prepared by the direct oxidation of gold powder in anhydrous HF at 20 °C using the potent oxidative fluorinating agent KrF 2. The KrF + salt readily oxidizes molecular oxygen at ambient temperature to yield [O 2][AuF 6]. Variable temperature Raman spectroscopy has been used to identify a reversible phase transition in [O 2][AuF 6], which occurs between −114 and −118 °C. Single crystal X-ray diffraction has been used to characterize the low-temperature, α-phase of [O 2][AuF 6]. The phase transition is attributed to ordering of the O 2 + cation in the crystal lattice, which is accompanied by minor distortions of the AuF 6 − anion. The α-phase of [O 2][AuF 6] crystallizes in the triclinic space group P 1 ̄ , with a=4.935(6) Å, b=4.980(6) Å, c=5.013(6) Å, α=101.18(1)°, β=90.75(2)°, γ=101.98(2)°, V=342.97 Å 3, Z=1, and R 1=0.0481 at −122 °C. The structure of the precursor, [KrF][AuF 6], has also been determined by single crystal X-ray diffraction and crystallizes in the monoclinic space group Cc with a=7.992(3) Å, b=7.084(3) Å, c=10.721(4) Å, β=105.58(1)°, V=584.8(4) Å 3, Z=4 and R 1=0.0389 at −125 °C. The KrF + and AuF 6 − ions interact by means of a FKr---FAu fluorine bridge that is bent by 125.3(7)° about the bridge fluorine. The KrF t and Kr---F b bond lengths in [KrF][AuF 6] were determined to be 1.76(1) and 2.15(1) Å, respectively. The energy minimized structures of the [KrF][AuF 6] ion-pair and the AuF 6 − anion have been determined at the Hartree–Fock (HF), MP2 and local density functional (LDF) levels of theory. These calculations have also been used to assign the vibrational spectrum of the [KrF][AuF 6] ion-pair in greater detail and to reassign the vibrational spectrum of the AuF 6 − anion.