The Tris(pentafluorophenyl)methylium Cation: Isolation and Reactivity

Abstract Herein, we present two different routes for the synthesis of the perfluorinated trityl cation, which allowed the handling of the free, uncoordinated species in organic solvents for the first time. The usage of the weakly coordinating anion [Al(OTeF5)4]− and its derivatives allows the characterization of this species by NMR spectroscopy and most importantly by single‐crystal X‐ray diffraction. The high hydride ion affinity of the cation is shown by hydrogen abstraction from isobutane. Furthermore, cyclic voltammetry reveals its oxidative potential which is supported by the reaction with tris(4‐bromophenyl)amine, giving rise to the formation of the ammoniumyl radical cation, also known as “magic blue”.


Experimental Procedures
All preparative work was carried out using standard Schlenk techniques. Glassware was greased with Triboflon III. The pentafluoroorthotelluric acid HOTeF5 [1] and [Al(OTeF5)3]2 [2] were prepared as described elsewhere. All solid materials were handled inside a glove box with an atmosphere of dry argon (O2 < 0.5 ppm, H2O < 0.5 ppm). All solvents were freshly dried with CaH2 before use and stored on molecular sieve. NMR spectra were recorded on a JEOL 400 MHz ECS or ECZ spectrometer. Chemical shifts and couplings constants of strongly coupled spin systems are given as simulated by gNMR. [3] Crystal data was collected with MoKα radiation on a Bruker D8 Venture diffractometer with a CMOS area detector. Single crystals were picked at −40 °C under nitrogen atmosphere and mounted on a 0.15 mm Micromount using perfluoroether oil. The structure was solved with the ShelXT [4] structure solution program using intrinsic phasing and refined with the ShelXL [5] refinement package using least squares on weighted F2 values for all reflections using OLEX2. [6] CCDC 2154970 and CCDC 2153771 contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre. Cyclic voltammetry was performed on an Interface 1010 B Potentiostat/Galvanostat/ZRA from Gamry Instruments. The investigations were carried out starting from 0 V going to the reduction first and then to the oxidation. The measurements were performed at a scan rate of 100 mV/s in anhydrous solvents under argon atmosphere without extra supporting and platinum wires as working-, counter-, and quasi-reference electrodes. The voltammograms were internally referenced against Fc 0/+ . The compound was freshly prepared in the cyclic voltammetry cell with a concentration of 0.078 M in ortho-difluorobenzene. The Turbomole program [7] was used to perform calculations at the unrestricted Kohn-Sham DFT level, using the B3LYP hybrid functional [8] (with RI [9] ) in conjunction with the valence triple-ζ basis set with two sets of polarization functions (def2-TZVPP) [10] . Minima on potential energy surfaces were characterized by normal mode analysis. Thermochemical data is provided without counterpoise correction but including zero-point energy correction as obtained from harmonic vibrational frequencies.

C(C6F5)3OH
In a Schlenk flask fine magnesium powder (2.43 g, 100 mmol, 3 eq.) was suspended in diethylether (150 ml) and cooled to 0 °C. Bromopentafluorobenzene (24.7, 100 mmol, 3 eq.) was added dropwise and the mixture was allowed to warm to room temperature. After 4 hours, methyl chloroformate (3.15 g, 33 mmol, 1 eq.) was added in small portions and the reaction mixture was consecutively stirred for 36 hours at room temperature, finally followed by 4 hours stirring under reflux conditions. Afterwards, the mixture was treated with diluted HCl solution (10 %, 20 ml) and then extracted with diethyl ether (3x 30 ml). The collected organic phases were w ashed with dist. water (3x 30 ml) and Brine solution (3x 30 ml). After drying with MgSO4, all volatiles were removed under reduced pressure. The resulting crude oil was then refined via fractionated sublimation. The first fraction (50 °C, 1*10 −3 mbar) belongs to the sideproduct decafluorobenzophenone. The desired product was collected at 100 °C and 1*10 −3 mbar as yellow crystals (6.73 g, 35 %). 1  These data are in agreement with literature values of C(C6F5)3OH. [11]

C(C6F5)3Cl
This synthesis is a modified version of an already reported procedure. [11] Tris(pentafluorophenyl)methanol C(C6F5)3OH (3.53 g, 7 mmol) was dissolved in thionyl chloride (25.00 g) resulting in a yellow solution. Pyridine (0.49 g, 7 mmol, 1 eq.) and dimethylformamide (0.52 g, 7 mmol, 1 eq.) were added and the mixture was brought to reflux at 80 °C for 48 hours under constant stirring. Afterwards, the mixture was cooled to room temperature and decanted on ice water. It was treated with small portions of saturated NaHCO3 solution until the formation of gas stopped. The aqueous phase was extracted with dichloromethane (3x 30 ml). The collected organic phases were subsequently washed with saturated NaHCO3 and Brine solution (each 3x 30 ml), dried with MgSO4 and finally all volatiles were removed under reduced pressure. After recrystallization in n-pentane at -16 °C the product was obtained as yellow powder (0.95 g, 22 %). 19  These data are in agreement with literature values of C(C6F5)3Cl. [11] [
Finally, isobutane (7 mg, 0.12 mmol, 4 eq.) was condensed on the mixture at −196 °C. The reaction mixture was then brought to -60 °C and further warming was monitored and analyzed with low-temperature NMR spectroscopy

Reaction of [C(C6F5)3][Al(OTeF5)4] with ferrocene
A stock-solution of [C(C6F5)3][Al(OTeF5)4] in ortho-difluorobenzene (0.05 mmol/ml) at -30 °C was treated with solid ferrocene (9 mg, 0.05 mmol). Upon contact the solution immediately changed its color to blue and further reacted to a green solution, indicating the successful formation of the ferrocenium cation. The mixture was analyzed by EPR spectroscopy

Variable-Temperature NMR Spectra
After the sample has been warmed for several hours to 0 °C, the sample was measured again at -60 °C. The 19 F NMR spectrum reveals the complete conversion of cation [C(C6F5)3] + to perfluorotrityl methane. Furthermore, the formation of HOTeF5 was observed in the 19 F and 1 H NMR spectra, probably occurring due to partial decomposition of [Al(OTeF5)4-xClx]via the protonation of the Brønsted acidic tert-butyl cation. Similar decomposition reactions have been reported by Krossing et al. when they treated AlBr3 with tertbutylbromide. [12] The broadened signals of the isobutane in the 1 H and 13 C NMR spectra are resolved again, suggesting that the dynamic exchange ceased. Additionally, several signals with low intensity in the range of 1 to 3 ppm in the 1 H NMR and 0 to 50 ppm in the 13 C NMR are observed, which correspond to isobutene oligomerization products. Still, no clearly assignable signals of the tert-butyl cation are found, which most likely reacted with the excess isobutane of the reaction mixture. Additional experiments were performed in which a slight excess of HOTeF5 is added to the reaction of the Lewis acid Al(OTeF5)3(SO2ClF) and C(C6F5)3Cl in SO2ClF in order to form HCl and therefore remove the chloride source before the isobutane is added. Furthermore, an equimolar amount of isobutane was used. These attempts also resulted in the rapid oligomerization of the formed tert-butyl cation at 0 °C and finally yielded the polymerization of the sample at room temperature.

Crystal data
Summary of crystal data and refinement results