Synthesis and Lewis Acid Properties of (ReO<sub>3</sub>F)<sub>∞</sub> and the X‑ray Crystal Structures of (HF)<sub>2</sub>ReO<sub>3</sub>F·HF and [N(CH<sub>3</sub>)<sub>4</sub>]<sub>2</sub>-[{ReO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>‑O)]·CH<sub>3</sub>CN

A high-yield, high-purity synthesis of (ReO<sub>3</sub>F)<sub>∞</sub> has been achieved by solvolysis of Re<sub>2</sub>O<sub>7</sub> in anhydrous HF (aHF) followed by reaction of the water formed with dissolved F<sub>2</sub> at room temperature. The improved synthesis has allowed the Lewis acid and fluoride ion acceptor properties of (ReO<sub>3</sub>F)<sub>∞</sub> to be further investigated. The complex, (HF)<sub>2</sub>ReO<sub>3</sub>F·HF, was obtained by dissolution of (ReO<sub>3</sub>F)<sub>∞</sub> in aHF at room temperature and was characterized by vibrational spectroscopy and single-crystal X-ray diffraction at −173 °C. The HF molecules are F-coordinated to rhenium, representing the only known example of a HF complex with rhenium. The trirhenium dianion, [{ReO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>-O)]<sup>2–</sup>, was obtained as the [N­(CH<sub>3</sub>)<sub>4</sub>]<sup>+</sup> salt by the reaction of stoichiometric amounts of (ReO<sub>3</sub>F)<sub>∞</sub> and [N­(CH<sub>3</sub>)<sub>4</sub>]F in CH<sub>3</sub>CN solvent at −40 to −20 °C. The anion was structurally characterized in CH<sub>3</sub>CN solution by <sup>19</sup>F NMR spectroscopy and in the solid state by Raman spectroscopy and a single-crystal X-ray structure determination of [N(CH<sub>3</sub>)<sub>4</sub>]<sub>2</sub>[{ReO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>-O)]·CH<sub>3</sub>CN at −173 °C. The structural parameters and vibrational frequencies of the [{MO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>-O)]<sup>2–</sup> and [{MO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>-F)]<sup>−</sup> anions (M = Re, Tc) were calculated using density functional theory. The calculated geometries of [{ReO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>-O)]<sup>2–</sup> and [{TcO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>-F)]<sup>−</sup>, are in very good agreement with their experimental geometries. Calculated vibrational frequencies and Raman intensities have been used to assign the Raman spectra of (HF)<sub>2</sub>ReO<sub>3</sub>F·HF and [N­(CH<sub>3</sub>)<sub>4</sub>]<sub>2</sub>[{ReO<sub>3</sub>(μ-F)}<sub>3</sub>(μ<sub>3</sub>-O)]·CH<sub>3</sub>CN. The X-ray crystal structures of the byproducts, [N­(CH<sub>3</sub>)<sub>4</sub>]­[ReO<sub>4</sub>] and KF·4HF, were also determined in the course of this work.