A Selective Trap for Monitoring Atomic Fluorine
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Journal of Fluorine Chemistry 91 (1998) 75-79 The persistent tertiary F-4-ethyl-3,4-dimethyl-3-hexyl radical: a selective trap for monitoring atomic fluorine Udo Grolj *, Dietmar Pfeifer Instituijiir Chemie der Humboldt-Universiiiit zu Berlin, Hess&he StroJe 1-2, 10115 Berlin, Germqy Bundesanstalt ftir Materiulforxhung OE 10.11. Rudower Chaussee 5, 12489 Berlin, German? Received 12 July 1996; received in revised form 23 April 1998; accepted 24 April 199X Abstract The title radical 1 was obtained by elementalfluorination of perfluoro-3,4-dimethyl- ethyl-hex-cis-2-ene (tetrafiuoroethylene pentamer). Its high resolution EPR-spectrum was measured consisting of more than 960 lines. Based on semiempirical quantum chemicalcalculations, the simulationof the experimentalspectrum was performed. This way, structuralparameters such as torsion angles, coupling constants of hypertine splitting (HFS), charge and spin densities were obtained. On this basesthe geometricstructure of a sterically lockedradical is presented, which shows only a small deviation from planarity at the radical site. The chemical and thermal stability was also studied and the kineticsof the thermalpyrolysis of the radicalwas investigated. The thermaldecomposition obeys a first-orderkinetic law with an enthalpy of activation of 19.6 kcal/mol. Because the formation of the title radical depends on the existence of atomic fluorine, this fact is used for the detectionof intermediateradical fluorineatoms in fluorinationprocesses and the evaluationof their mechanisticpathway. Thus, the stable radicalis formed from reactionswith XeF,, CF,OF and by the electrochemicalfluorination process (ECF). 0 1998Elsevier Science S.A. All rightsreserved. Keywords: Stable radical; High resolution EPR; Spectra simulation; Kinetics of pyrolysis; Atomic fluorine monitor 1. Introduction Whereas cr-fluororadicals prefer to be pyramidal to mini- mize inductive repulsions, P-fluorination does not signifi- Steric and electronic requirementsfavour according to the cantly affect the radical geometry [ 1I. Thus, the F,C . radical subsequentequation the formation and stability of the title is tetrahedral with a high barrier of inversion of about 25 fluorocarbon radical and the corresponding pet-fluorinated kcal/mol. On the contrary, the perfluoro terr-butyl radical carbanion, which differ only by one electron at carbon atom (CF,),C . exhibit a more planar geometry. Scherer et al. [ 21 3. Concerning the structure at this site of the molecule the hasdiscovered a new type of persistenttertiary Buororadicals, question is raised, whether it is planar (sp2) or tetrahedral e.g., iso-C3F7)&F&. , by addition of elemental fluorine to ( sp3). Therefore, a comparisonof the geometric structure of branched fluoro-olefins. Functionalized radicals of this type the two, their spectroscopicdata and chemistry are of interest. are reported by Sterlin et al. [ 31. Structural peculiarities of the comparableand isolablecar- banion (see scheme) obtained by an extensive NMR inves- tigation will be reported elsewhere. Specifically, at low temperaturesall alkyl-group rotations are frozen and the rota- tional barrierswere calculated from the ’ ‘F-NMRparameters. In accordancewith the splitting pattern of signalsand from quantum chemical calculations the carbanion centre is essen- tially planar. These data contribute in elucidating the radical F structure to a great extent. 2. Experimental details The title radical was obtained by elemental fluorination of * Corresponding author. Tel.: + 49-30-20937305; Fax: + 49-30-20937277 the correspondingparent fluoro-olefin under definite condi- 0022-l 139/98/$ - see front matter 0 1998 Elsevier Science S.A. All rights reserved PIISOO22-1 139(98)00201-2 76 U. GroJ, D. Pfeifer/ Journal of Fluorine Chemistry 91 (I 998) 75-79 Table I Molecular parameters of the C,,F,, radical Group Site Sets of nuclei HFS (mT) Torsion angle (deg) F IGI 1 3.56 11.4 F IC,l 1 3.02 49.5 F I&l 1.83 56.4 F ICI1 2 1.83 59.4 CC, [Gl 3 1.69 3.12 CF,CFZ lb1 0.23 34.0 CSCF, lb1 0.23 26.2 CF, lC,l 3 0.08 89.4 F I 3’25 3’30 3’35 3’40 nT Fig. 1. High resolution EPR-spectrum of F-4-ethyl-3,4-dimethyl-3-hexyl radical. tions [ 41. EPR-spectra were measured from degassed and diluted samples in the X-band at 9.5 GHz microwave fre- quency. The kinetics of the thermal decay of the radical 1 was studied diluted in perfluorodecalin as solvent in a tem- perature range from SO-140°C. The formed radical 2 per- fluoro 3-methyl-3-pentyl was analyzed by spectra simulation. The solid fluorination agents xenon difluoride, cobalt( III) - fluoride or Bamette’s salt (up to 2 g) were placed in areaction tube with the F-olefin and if necessary, gently warmed up to Fig. 2. Scheme of the angular arrangement of substituents in relation to the about 50°C. The obtained persistent radical in admixture with p,-orbital plane level. excess F-olefin were separated from solid or gaseous mate- rials and then transferred to 4 mm sealed quartz tubes for The radical bearing carbon atom is in the center of this monitoring the EPR-spectrum (see Fig. 1) . representation and is numbered accordingly. The plane level through the perpendicular p,-orbital gives the relative angular positions of the locked neighbouring groups. The assumption made is consistent with a conformationally 3. Results and discussion locked molecular structure, where two B-fluorines of the tri- fluoromethyl group at carbon atom 1 are eclipsed and those The complex hyperfine splitting (HFS)-spectrum was two at carbon 4 are in staggered position related to the half- measured in high resolution at 12O’C. Based on calculations, filled p,-orbital at the paramagnetic centre. While the first the hyperfine splittings of more than 960 lines were analyzed couple with 1.83 mT, the latterpresent the strongestcouplings and an assignment was undertaken. Simulation of the in the molecule with 3.56 mT and 3.02 mT, respectively. observed EPR-spectrum of 1 is based on semiempirical quan- However, the terminal y-CF,-group rotates with one coupling tum chemical calculations. Using the AM1 method and the constant of 1.69 mT. The long-range interaction of the CF,- UHF-approximation, geometry optimization was achieved group in R2 and R, with the unpaired electron is only 0.23 from a graphically designed molecular model of the radical. mT, that of CF, at carbon 3 is 0.08 mT. The inspection of Together with INDO-UHF calculations torsion angles of this structure implies that the arrangement at the paramagnetic atoms in B- and y-positions to the radical site, spin and charge carbon is nearly planar with only small distortion from sp’, densities and HFS coupling constants were calculated. With According to AM 1 calculations, the deviation from planarity the help of these parameters the fit of the measured and sim- at this molecular site is about 8”. ulated spectrum was performed. The coincidence of the spec- The comparable carbanion is planar as well due to NMR- tra is so high, that a graphical comparison of the two is measurements and in agreement with calculations. Expect- unnecessary. edly, calculated charge densities are decisively different at Six sets of coupling nuclei were included in the spectra carbon 3 (radical-O.200; carbanion-O.7 12). simulation. Additionally, torsion angles and coupling con- The unprecedented stability of the radical results from the stants are reported (Table 1) . sterical hindrance as well as from charge delocalization. The scheme of Fig. 2 presents the geometric feature of the Obviously, its persistence is not a thermodynamic but a radical, reflecting the structural properties given in Table 1. kinetic effect which arises from shielding by the sheer bulk U. GroJ, D. Pfeifer/Jound of Fluorine Chemistry 91 (1998) 75-79 77 reducing agents: e.g., secondary amine destroys the radical in a vigorous reaction. The reaction with NO was observed in the EPR-spectrometer,The signals of the radical rapidly disappeared,probably the oxidation of NO to NOF has taken place. 3.1. Kinetic investigations of the thermal decompositionof radical 1 When the diluted radical is heated to 12O”C,its spectrum disappearsover hours, superimposedin a complex way and Fig. 3. Front view of the space filling model of the radical 1. finally replaced by a new radical 2. The symmetric spectrum of the perfluoroalkyl group. On the contrary, the radical is consistsof eight major signals. According to spectra simu- thermally relatively unstable, becauseof considerablesteric lations this radical and its HFS-couplings of P-fluorines (CF, strain. 1.66 mT; CF, 1.84 mT) is consistent with the perfluoro-3- The long-term stability of radical 1 was measuredby EPR methyl-3-pentyl radical [ 51. Obviously, the weakestbond at over a period of several month at a storage temperature of the quatemary carbon is broken to give a relatively stable 8°C. The lossof radical intensity followed a first-order kinet- tertiary petiuoro-3-methyl-3-pentyl radical, which was ics with a half-live T,,~ of 89 days and a rate constant of obtained previously by Scherer et al. [ 61 and Krusic et al. 8.9. 10-8. [ 7 ] from photochemical and thermal decomposition. The spacefilling drawing of Fig. 3 gives a front view of The thermal decomposition of radical 1 dissolved in the the radical centre and its surrounding. Steric hindrance as inert, nonpolar solvent perfluorodecalin was carried out in a representedin Fig. 3 is undoubtedly responsiblefor the slug- temperature range from 100” to 130°C obeying first-order gish reaction of the radical with fluorine and for its stability kinetics (see Fig. 4). The radical concentrationsare not abso- towards oxygen, chlorine and to dimerization. On the other lute values, but a spectroscopic measure from which in hand, as a strong oxidant the radical is less stable to mild dependenceon time the kinetic law is obtained.