Lewis-Acid Properties of Technetium(VII) Dioxide Trifluoride, TcO2F3: Characterization by 19F, 17O, and 99Tc NMR Spectroscopy and Raman Spectroscopy, Density Functional Theory Calculations of TcO2F3, M+TcO2F4- [M = Li, Cs, N(CH3)4], and TcO2F3·CH3CN, and X-ray Crystal Structure of Li+TcO2F4-

Abstract

Technetium(VII) dioxide trifluoride, TcO2F3, behaves as a Lewis acid toward the fluoride ion and acetonitrile forming M+TcO2F4- [M = Li, Cs, N(CH3)4] salts and TcO2F3·CH3CN. Fluorine-19 NMR spectroscopy established that the TcO2F4- anion has a cis-dioxo geometry in CH3CN solution. Variable-temperature 19F NMR studies of TcO2F4- and TcO2F3·CH3CN in CH3CN revealed that the one-bond couplings between 99Tc and the two fluorine environments exhibit widely different degrees of quadrupolar collapse. The 17O NMR spectra of 17O-enriched TcO2F4- and TcO2F3·CH3CN and the 1H and 13C NMR spectra of TcO2F3·CH3CN indicated that chemical exchange occurs between TcO2F3·CH3CN and CH3CN solvent. The TcO2F4- anion was characterized by X-ray crystallography as its lithium salt crystallizing in the tetragonal system, space group P4̄21m, with a = 4.706(1) Å, c = 8.797(2) Å, V = 194.8(1) Å3, and Z = 2, at 20 °C. Refinement converged with R = 0.0339 (Rw = 0.0320). The anion geometry is a distorted octahedron with the two oxygen ligands cis to each other and is closely related to the [TcO2F4] units in polymeric TcO2F3. The Raman spectra of M+TcO2F4- [M = Li, Cs, N(CH3)4] and TcO2F3·CH3CN were assigned under C2v and Cs point symmetries, respectively. Density functional theory calculations at the local and nonlocal levels predict that monomeric TcO2F3 has a trigonal bipyramidal geometry (C2v point symmetry) and confirm that the cis-dioxo isomers of TcO2F4- and TcO2F3·CH3CN (CH3CN bonded trans to an oxygen) are the energy-minimized structures.