Synthesis and Characterization of Salts Containing the BrO₃F₂^- Anion; A Rare Example of a Bromine(VII) Species

The BrO3F2- anion has been prepared by reaction of BrO3F with the fluoride ion donors KF, RbF, CsF, [N(CH3)4][F], and NOF. The BrO3F2- anion is only the fourth Br(VII) species to have been isolated in macroscopic quantities, and it is one of only three oxide fluorides that possess D3h symmetry, the others being XeO3F2 and OsO3F2. The fluoride ion acceptor properties of BrO3F contrast with those of ClO3F, which does not react with the strong fluoride ion donor [N(CH3)4][F] to form the analogous ClO3F2- salt. The single-crystal X-ray structures of [NO]2[BrO3F2][F] and [N(CH3)4][BrO3F2] confirm the D3h symmetry of the BrO3F2- anion and provide accurate Br−O (1.593(3)−1.610(6) Å) and Br−F (1.849(5)−1.827(4) Å) bond lengths. The salt, [NO]2[BrO3F2][F], is fully ordered, crystallizing in the monoclinic space group, C2/c, with a = 9.892(3) Å, b = 12.862(4) Å, c = 10.141(4) Å, β = 90.75(2)°, V = 12460(7) Å3, Z = 4, and R1 = 0.0671 at −173 °C, whereas [N(CH3)4)][BrO3F2] exhibits a 2-fold disorder of the anion, crystallizing in the tetragonal space group, P4/nmm, with a = 8.5718(7) Å, c = 5.8117(6) Å, V = 427.02(7) Å3, Z = 2, and R1 = 0.0314 at −173 °C. The 19F chemical shift of [N(CH3)4)][BrO3F2] in CH3CN is 237.0 ppm and is more deshielded than those of the previously investigated Br(VII) species, BrO3F and BrF6+. The vibrational frequencies of the BrO3F2- anion were determined by use of Raman and infrared spectroscopy and were assigned with the aid of electronic structure calculations and by analogy with the vibrational assignments reported for XeO3F2 and OsO3F2. The internal and symmetry force constants of BrO3F2- were determined by use of general valence force field and B-matrix methods, respectively, and are compared with those of XeO3F2, OsO3F2, and the unknown ClO3F2- anion. The instability of ClO3F2- relative to BrO3F2- has been investigated by electronic structure calculations and rationalized in terms of atomic charges, Mayer bond orders, and Mayer valencies, and the enthalpies of fluoride ion attachment to BrO3F and ClO3F.