Synthesis and properties of the tetrakis(trifluoromethyl)borate anion, [B(CF3)(4)](-): Structure determination of Cs[B(CF3)(4)] by single-crystal X-ray diffraction

Salts of the tetrakis(trifluoromethyl)borate anion, M[B(CF3)(4)], M = Li, K , Cs, Ag, have been prepared by two different routes for the first time. Th e colorless compounds are thermally stable up to 425 degreesC (Cs salt) and soluble in anhydrous HF, water, and most organic solvents. Single crystals of Cs[B(CF3)(4)] were grown from diethyl ether by diffusion of CH2Cl2 vapo r into the solution. The molecular structure was obtained by single-crystal X-ray diffraction. Crystal data: rhombohedral space group R3m (no. 160); a =7.883(1), c=13.847(4) Angstrom; V=748.2 Angstrom (3); Z=3; T=150 K; R1=0. 0118, wR2=0.0290. The internal bond parameters of the [B(CF3)(4)](-) ion we re compared to those of the C(CF3)(4) molecule. Due to a disorder of the an ions in the cesium salt, it is not possible to distinguish between T and T- d symmetry by X-ray diffraction experiments alone. However, a comprehensive IR and Raman study demonstrated that in the potassium and cesium salt as w ell as in aqueous solution, the anion exhibits T symmetry with all CF3 grou ps rotated off the staggered position required for T-d symmetry. The vibrat ional study is supported by DFT calculations, which provide, in addition to the equilibrium structure, and vibrational wavenumbers, estimates of IR an d Raman band intensities. The anion is resistant against strong oxidizing ( e.g., F-2) as well as reducing agents (e.g., Na) and is not affected by nuc leophiles like C2H5O- or electrophiles such as H3O+. It is very weakly coor dinating, as demonstrated by the low-equilibrium CO pressure over the [Ag(C O),][B(CF,),] (x=1, 2) co-adducts and the formation of [Ag(CO)(x)] [B(CF3)( 4)] (x = 3,4) at higher CO pressure. The B-11, C-13, and F-19 NMR data as w ell as the structural parameters of the anion are compared with those for o ther borates containing F, CN, and CF3 ligands.