A Selective Trap for Monitoring Atomic Fluorine

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 formedfrom 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 elementalfluorine 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 quantumchemical calculationsthe carbanion centre is essentially planar. These data contribute in elucidating the radical F structure to a great extent.

2. Experimental details The title radical was obtained by elementalfluorination of * Corresponding author. Tel.: + 49-30-20937305; Fax: + 49-30-20937277 the correspondingparent fluoro-olefin under definite condi-

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

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 temperature range from SO-140°C. The formed radical 2 perfluoro 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 signalsof the radical rapidly disappeared,probably the oxidation of NO to NOF has taken place.

3.1. Kinetic investigationsof 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 replacedby a new radical 2. The symmetric spectrum of the perfluoroalkyl group. On the contrary, the radical is consistsof eight major signals. According to spectrasimu- 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 temperatureof 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 (seeFig. 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.

-A.. -*A.. --.. A 1OO'C “'A

-2,5

500 1000 1500 2000 2500 3000 3500 4000 4500 time [set] Fig. 4. First-order kinetics of thermal pyrolysis of radical 1 at different temperatures. 78 U. Groj3, D. Pfeifer/ Joumal of Fluorine Chemistry 91 (1998) 75-79

0.00245 0,0025 0.00265 0,0020 0,002M 0,0027 1mtIcl Fig. 5. Arrhenius plot of the thermal decomposition of radical 1: estimation of EA.

Table 2 3.2. Radical I as a monitor.for atomicjuorine in Kinetic parameter of the thermal decomposition of radical 1 jluorination reactions

Sample t(K) k (SK’) 7 (min) E, (kcal/mol) The fact that the title radical is formed only in the presence of radical fluorine makes the F-olefin (CF,CF,)&F&- I 281 8.9. IO-” 128 160 19.6 C( CF3) =C( CF,)F a selective trap for fluorine radicals and 2 373 3.0. 1o-4 38.3 3 383 8.0.10-4 14.4 the resulting organic radical a monitor in EPR-spectroscopy. 4 393 1.o.1o-x 11.5 Using this method it was found that the electrochemical fluor- 5 403 2.4.10.-? 4.8 ination process in anhydrous hydrogen fluoride generates radical fluorine, because the expected radical 1 was formed from the F-olefin [ 41, The half time life r at 100°C is in the order of 39 min and Applying this methodology to fluorinations with XeF, and is thus comparable with that of 60 min for the Scherer radical CF,OF the radical was observed as well. In contrast to expec- [ 21. From the Arrhenius plot of the rate constants in Fig. 5 tations, these electrophilic fluorinating agents [ 9, lo] do not the activation entbalpy of decomposition is obtained with fluorinate the electron rich but the charge deficient site of the 19.6 kcal/mol. The tabulated k-value at 28 1 K (sample 1) double bond, thus underlining the pure radical nature of these fits the straight line in Fig. 5 as well. two processes. Obviously, the direction of fluorine addition The activation enthalpy of decomposition of 19.6 kcal/ is essentially determined by steric reasons of the strongly mol corresponds to a very low bond energy between the branched F-olefin. tertiary and quatemary carbon atoms. Tortelli et al. [ 51 have shown in a recent pyrolysis study of branched fluorocarbons that homolytic cleavage takes place at the most substituted C-C bonds, whereby the ethyl-group weakens the neigh- +CF30.+CF3-C(C*F,),-C’(CF,)-CF,-CF, bouring bond stronger than methyl ( see Table 2 for the kinetic It is interesting to note that in the above reaction, unlike parameter of the thermal decomposition of radical 1). the work of Corvaja et al. [ 111, fluorine but not the CF,O . - Therefore, these fluorocarbons are thermolabile. The low- moiety is added to the double bond. est bond energy reported in this respect is 36 kcal/mol in the Attempts to generate the radical 1 by reaction with the sterically stressed ( ~so-C~F,) *C( CF,) C2F, [ 81. highly reactive interhalogens ClF and ClF, as fluorinating lJ. GroJ3, D. Pfeifer/Joumal of Fluorine Chemistry 91 (1998) 75-79 79 agents failed. Certainly, their much higher dissociation [21 K.V. Scherer, T. Ono. K. Yamanouchi, R. Femandez, P. Henderson, enthalpy (60.3 kcal/mol and 41.6 kcaUmo1, respectively) H. Goldwhite, J. Am. Chem. Sot. 107 ( 198.5) 718. as compared to that of fluorine ( AhHDiss= 37.8 kcal/mol) is [31 S.R. Sterlin, V.F. Cherstkov, B.L. Tumanskii, E.A. Avetisyan, I. Flu- one reasonable explanation. orine Chem. 80 ( 1996) 77. 141 U. GroB, St. Riidiger, A. Dimitrov, J. Fluorine Chem. 76 (1996) 139. As expected, elemental chlorine ( AHDiss = 58.1 kcal/mol) [S 1 V. Tottelli, C. Tonelli. C. Corvaja, J. Fluorine Chem. 60 (1993) 165. did not give a comparable radical and also the solid fluori- I61 K.V. Scherer, R.E. Femande2,P.B. Henderson,P.J. Krusic, J. Fluorine nating agents CoF, and CH3-C6H4S02N(CH&F (Bar- Chem. 35 ( 1987) 7. nette’s salt) are not able to release fluorine in the temperature I71 P.J. Krusic, K.V. Scherer, J. Fluorine Chem. 35 ( 1987) 44. range up to 50°C. [81 B.E. Smart, Characteristics of C-F Systems, in: R.E. Banks, B.E. Smart, J.C. Tatlow (Eds.), Organofluorine Chemistry: Principles and Commercial Applications, Plenum, New York, 1994. [91 J.A. Wilkinson, Chem. Rev. 92 ( 1992) 505. References I101 S. Rozen, Chem. Rev. 96 (1996) 1717. [ 111 C. Corvaja, F. Cremonese, W. Navarrini, C. Tonelli, V. Tortelli, Tet- [ I] W.R. Dolbier Jr., Topics in Current Chemistry 192 ( 1997) 97. rahedron L&t. 36 (1995) 3543.