Complexes containing Re(IV) are relatively rare. The most common and convenient Re(IV) starting materials are the A2[ReX6] (X = Cl, Br, I; A = NH4, K, Cs, Rb) which have been well-studied both spectroscopically and structurally. These [ReX6]2- (X = Cl, Br, I) salts can be prepared in high yield by the reduction of perrhenate with hypophosphorous acid in the corresponding concentrated acid. The corresponding fluoro analogues have been known since 80 years but their chemistries are less explored than the heavier halides. The K2ReF6 salt was first reported in 1934. Up to now, there is no well-documented study on its synthesis. The present work reinvestigates a method for preparing K2[ReF6] from the reaction of K2[ReBr6] with molten KHF2. Using this method, A2[ReF6] (A = NH4, Rb, Cs) salts were prepared and characterized by single crystal X-ray diffraction and Raman spectroscopy. The A2[ReF6] salts ( A= NH4, K, Rb, Cs) crystallize in the trigonal space group P-3m1 and form a slightly distorted octahedral system. Their structures consist of [ReF6]2- anions and cations (NH4+, K+, Rb+, Cs+) which are held together by electrostatic force, thus stabilizing the overall structure. The Re—F bond distances range from 1.948(3) Å to 1.961(2) Å. The Raman spectra of A2[ReF6] (A = K, Rb, Cs) further reveal the lowering of the symmetry of hexafluororhenate salts from Oh to D3d symmetry. For Oh symmetry, the Raman spectrum displays three bands for the A1g, Eg, and F2g symmetry species. However, due to the different degree of symmetry lowering from K2[ReF6] to Cs2[ReF6], the F2g band is split into two Raman-active bands (F2g → A1g and Eg). The Raman spectra of A2[ReF6] (A = K, Rb) salts exhibit four bands (624, 539, 244 and 224 cm-1 and 616, 535, 232, and 222 cm-1) respectively. Whereas the Raman spectrum of Cs2[ReF6] exhibits three bands (609, 529, and 218 cm-1).

Structural and vibrational spectroscopic studies of hexafluororhenate (IV) salts

255th National Meeting (2018)

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