Perfluoro Alkyl Hypofluorites and Peroxides Revisited

Abstract A more convenient synthesis of the perfluoro alkyl hypofluorite (F3C)3COF as well as the hitherto unknown (C2F5)(F3C)2COF compound is reported. Both hypofluorites can be prepared by use of the corresponding tertiary alcohols RFOH and elemental fluorine in the presence of CsF. An appropriate access to these highly reactive hypofluorites is crucial. The hypofluorites are then transferred into their corresponding perfluoro bisalkyl peroxides RFOORF [RF=(F3C)3C, (C2F5)(F3C)2C] by treatment with partially fluorinated silver wool. NMR, gas‐phase infrared, and solid‐state Raman spectra of the perfluoro bisalkyl peroxides are presented and their chemical properties are discussed.


Results and Discussion
Perfluoroalkylh ypofluorites, R F OF (1), were obtained according to Equation(1) by treatment of the corresponding carbonyl compound with elemental fluorine in as tainless-steel reactor in the presence of anya lkali metal fluoride, MF.T he mixture was initially cooled to À196 8C, and then slowly heated to À78 8C. [17,28,29] Because these mixtures are highly hazardous and may explode spontaneously, [28] we first improved this synthesis to avoid any possible local heat formation and pressure increases during the reaction.
The number of perfluoroalkyl hypofluorites 1 obtainable by this procedure can be furtheri ncreased by using perfluoro alkylalcohols 3 insteado ft he perfluoro carbonyl compounds according to Equation (2). First, the perfluoro alkylalcohols 3b,c were added to an excesso fc esium fluoridei nastainless-steel reactor and the mixture was thoroughly shaken at room temperaturet od issipate the reactionh eat of the subsequent fluorination reaction. It is assumed that the alcohol reacts with CsF to give the corresponding cesium alcoholate and CsHF 2 . [29] The reactor was cooled to À78 8Ca gain and elemental fluorinew as added in small portionsv ia as tainless-steel line until no fluorine was consumeda nymore and the pressure remained constant. Excessf luorine was then removed from the reaction mixture at À196 8Ca nd the hypofluorites 1 thus formedw ere distilled out of the reactor and purified by trap-to-trap distillation. For the hypofluorites (F 3 C) 3 COF (1b)a nd (C 2 F 5 )(F 3 C) 2 COF (1c) the conversion is quantitative. This fluorination procedure was finally also used for the synthesis of the known hypofluorites F 3 COF (1a), CF 3 CF 2 OF (1d), and (F 3 C) 2 CFOF (1e)s tartingf rom the perfluoro carbonyl compounds F 2 CO, F 3 CC(O)F,a nd (F 3 C) 2 CO, respectively,a ccording to Equation (1). TheirI Ra nd NMR spectra match those previously reported. [2] Quantum-chemical calculations indicatet hat apart from 1a,a ll these perfluoro alkyl hypofluorites are thermodynamically unstable by up to À400 kJ mol À1 in terms of elimination of CF 4 and formation of the corresponding carbonyl compounds (see the Supporting Information, Ta ble S2.1). As expected, elimination of elemental fluorine is endothermic by 120-150kJmol À1 .N evertheless,t hese perfluoro alkyl hypofluorites are kinetically stable in the gas-phase up to 110 8C. [2] Therefore, we were ablet op rovide atmosphericp ressure chemicali onization (APCI) mass spectra of tert-butyl hypofluorite 1b (see the Supporting Information, Figure S1.2). The predominant speciesi nt he negative mode is the alkoxide ion [(F 3 C) 3 CO] À (m/z = 235), whereas the molecular ion peak at m/z = 254 is not observed. Interestingly,t he peak at m/z = 285 can be assigned to an adduct or insertiono faCF 2 fragment to the alkoxide ion [(F 3 C) 3 CO + CF 2 ] À ,w hich was previously observed for ap erfluorinated ether bearing the perfluoro-tert-butyl group. [30] Only perfluoro tert-pentyl hypofluorite (1c), which is stable in solution for severalm inutes, is found to decompose readily at room temperature within seconds to yield selectively (F 3 C) 2 CO and C 2 F 6 ,a sp roved by their gas-phase IR spectra [Eq. (3)]. [31,32] Probably as ar esult of the lack of stability,h ypofluorite 1c has not yetb een characterized. [10,33] We therefore presentl ow-tem-peratureN MR as well as gas-phase IR spectra of this poorly knownc ompound. These spectrac learly confirmt he presence of 1c.T he fully coupled 13 CNMR spectrum (see the Supporting Information, Figure S1.3) shows two quartets at d = 118.5 ppm (C-CF 3 )a nd d = 116.8 ppm (F 3 C-CF 2 ), where the lattero ne partly overlaps with the CF 2 triplet at d = 109.8 ppm. The chemical shift of the quaternary carbon atom appears at d = 87.7 ppm. The first-order 19 FNMR spectrum of hypofluorite 1c ( Figure 1) shows ad oublet of septets for the CF 2 nucleia td = À117.8 ppm, indicating a 4 J(F,F) coupling to OF and to the two CF 3 nuclei of 11.6 Hz. The F 3 CCF 2 signal at d = À81.9 ppm also showsadoublet of septets with similar coupling constants of The IR spectrumo fh ypofluorite 1c in the gas phasei scompared to ac omputed IR spectruma tt he DFT-B3LYP/aug-cc-pVTZ level of theoryi nF igure 2. The experimental bands in the mid-IR range from 1107 to 878 cm À1 are split into two components, probably owing to the presence of at least two rotational conformers in the gas phase. Indeed, the DFT calculations revealed slightly different IR spectra for the different trans and gauche rotational conformers of 1c (Figure 3, Ta ble 1, and the SupportingI nformation) and ag lobal minimum for the t-1 structure. This result agreesw ell with the experimental IR spectrum, which shows strong absorption for the CÀFs tretching modes in the regionf rom 1291 to 1232 cm À1 and the characteristic deformation bands of the CF 3 groups at 766, 743, and7 30 cm À1 .F urthermore, the bands at 1107 and 1078 cm À1 are tentatively assigned to CÀOs tretching modes of different rotamers of 1c.A ccording to the calculations, the OÀFs tretching modeh as ar elatively low intensity and its positionv aries by up to 43 cm À1 depending on the conformer of 1c (see Ta ble 1a nd the Supporting Information). It can tentatively be assigned in the experimental gas-phase IR spectrum to weak absorptions at 925 and 913 cm À1 ,b ut we cannote xclude that this band is superimposed by the asymmetric stretching mode of the C-(CF 3 ) 2 fragment located at 1003 cm À1 .T he bands at 895 and 878 cm À1 represent CÀC stretching modes of the pentafluoroethyl group of 1c.T he calculated positiono ft his band varies for the different conformers by 36 cm À1 .T he weak band at 613 cm À1 can be associated with the CC 3 deformation, whereas weaker deformation modes of the CF 3 group are found at 539 and 513 cm À1 .T he very weak absorption at 484 cm À1 fits well to ac omputed deformation mode of the C 2 F 5 group.
Chem  4)].T hisr eactiond oes not proceed at low temperatures of À78 8C, whereas at 0 8Cm ainly decomposition products of the hypofluorites are formed [1b:C F 4 ,( F 3 C) 2 CO; 1c:C 2 F 6 , (F 3 C) 2 CO]. [31,32,34] When the reactionv essel is held at temperatures of À50 to À45 8Cf or 48 to 72 h, peroxide 2b is obtained from 1b andc an be separated by trap-to-trap distillation in a À78 8Ct rap from the more volatile side products CF 4 and (F 3 C) 2 CO in an overall yield of > 70 %. Pure peroxide 2b is rather stable at ambient temperature and decomposesa tt emperatures above 100 8Ct oy ield (F 3 C) 2 CO and C 2 F 6 with an activation energy of 148.7 AE 4.4 kJ mol À1 . [35] Similarly, hypofluorite 1c reacts to give the peroxide 2c and this was purified from the volatile side products C 2 F 6 and (F 3 C) 2 CO by trap-to-trap distillation, where it remains in a À78 8Ct rap in ay ield of up to 66 %.
However,a ttempts to convertt he hypofluorites CF 3 CF 2 OF (1d) and (F 3 C) 2 CFOF (1e)w ith fluorinated silver wool under similar conditions to the corresponding perfluoro bisalkyl peroxides failed andl ed solely to decomposition products (1d:C F 4 , [34] F 2 CO, [36] 1e:C F 4 , [34] F 3 CC(O)F), [37] which were identified by IR spectroscopy.T he composition of the fluorinated silver wool used in the synthesis of the peroxides 2b and 2c [Eq. (4)] was prepared as described in the Experimental Sectiona nd was investigated by powder X-ray diffraction analysis. It consists mainly of silver(I)f luoride, AgF,b ut also contains some silver subfluoride, Ag 2 F, silver(II) fluoride, AgF 2 ,a nd elemental silver, Ag (see Figure S1.1 in the Supporting Information). However, attempts to reproduce the above described conversion of hypofluorites 1 to peroxides 2 by using either elemental Ag or commercialA gF instead of the fluorinated silver wool failed and only decomposition products of the hypofluorites were obtained. The mechanism of this solid-gas reaction for the formation of peroxides from hypofluorites is still unknown and further studies are necessary to explore this reaction. Powder diffraction measurements of the fluorinated silver wool prior and after several batches ( Figure S1.1 in the Supporting Information) indicates an increase in silver(I) fluoride at the expense of silver(0) or silver subfluoridesw ithin several reactions. From this result,i tc an be assumed that the active site of the partially fluorinated silver wool consists of an incompletely coordinated silver subfluoride or silver(0) species, whicha cts as af luorine-atoma cceptor. We noticed that the fluorine-atoma cceptor ability of this speciesd epletes, and thus, the yield of peroxide formation decreases after severals uccessful batches, very likely owing to fluorination of the active silver species andf ormation of inactives ilver(I) fluoride.B ased on this assumption, the following reaction mechanism can thus be postulated for the solid-gas reaction. First, the active silver site may undergo an oxidative addition of R F OF to form an alkoxide silver(II) intermediate, which then decomposesi nto alkoxyl radicals, R F OC, and silver(I)f luoride [Eq. (5)].T he free or loosely bound R F OC radicals mayt hen combine to form the peroxides 2.
There are precedents for the formation of silver(II) alkoxides and their decomposition into alkoxyl radicals. Wechsberg and Cady previously described the reactiono fA gF 2 with F 2 CO and fluorine and assumedt he formation of an Ag II (OR F ) 2 intermediate. [38] Owing to the high oxidationp otentialo fA g II (electron affinity:2 1.45 eV), [39] the proposed Ag II alkoxide intermediates are prone to al igand-to-metal electron-transfer (LMCT) and even to the formation of alkoxyl radical intermediates such as Ag I (OCR F )(OR F ). [39] As imilarr adical mechanism hasb een proposed for the AgF 2 -catalyzed low-temperaturer eaction of F 2 with SO 3 to yield peroxyd isulfuryl difluoride, (FSO 2 O) 2 . [40] The 13 C{ 19 F} DEPTQ NMR spectrum [41] of neat[ (F 3 C) 3 CO] 2 (2b)s hows two signals at d = 118.8 and 84.3 ppm associated with the CF 3 and the quaternary carbon nuclei. The 19 FNMR spectrum shows as inglet at d = À69.6 ppm (lit.: [42] À70.0 ppm) whereas the resonance in the 17 ONMR spectrumo ccurred in the characteristic region of ap eroxide compound [43] at d = 246 ppm, see Figure S1.4 (in the Supporting Information). The oxygen atoms of (F 3 CO) 2 (2a)r esonate at 262 ppm in the 17 ONMR spectrum ( Figure S1.5 in the Supporting Information). The base peak in the APCI mass spectrum of 2b ( Figure S1.6 in the Supporting Information) at m/z = 235 represents the [(F 3 C) 3 CO] À fragment. Also, the [(F 3 C) 3 C] À fragment can be assigned to the m/z = 219 peak, whereas the molecular ion peak at m/z = 470 is not present. The 13 C{ 19 F} DEPTQ NMR spectra of [(C 2 F 5 )(F 3 C) 2 CO] 2 (2c;F igure S1.7 in the SupportingI nformation) with optimized 1 J coupling constantso f2 90 Hz and 35 Hz, respectively,s how the expected signals for the fluorine substituted carbon atoms at 118.5, 116.7, and 115.2 ppm and the resonance of the quaternary carbon atom at 85.6 ppm. They are slightly shiftedt ol ower field comparedw ith the corresponding hypofluorite 1c.T hree signals observed in the 19 FNMR spectrum ( Figure S1.8 in the SupportingI nformation) are also shifted by Dd = 2ppm to lower field compared with the spectrum of the reactant 1c.T his consistent lowf ield shift of the NMR signals underlines the strong electron-withdrawing effect of the perfluorinated tert-pentyl group of peroxide 2c.
The IR spectrumo f[ (F 3 C) 3 CO] 2 (2b)i nt he gas phase is showni nF igure 4t ogetherw ith the computed spectrum at the DFT-B3LYP/aug-cc-pVTZ level of theory.T he strongest absorptions are associated with the CF 3 stretching bands in the regiona round 1300 cm À1 (Table S1.1 in the Supporting Information). The sharp IR band at 1110cm À1 is assigned to aC ÀO stretching mode whereas the second CÀOs tretch appears at 1129 cm À1 in the low-temperature Ramans pectrum (Figure S1.9 in the Supporting Information). The characteristic CÀ C 3 stretching bandso ft he tert-butyl group are found at 1002 T he corresponding Raman band shows as trong absorption at 1027 cm À1 .T he calculated OÀOs tretching mode at 902 cm À1 can clearly be assigned to aR amanb and at 865 cm À1 .V ery strong Raman bands are also found for the symmetric CF 3 deformation modes at 783 and 749 cm À1 .T heir counterparts in the gasphase IR spectrum of peroxide 2b appear at 739 and 731 cm À1 .T he asymmetric CF 3 deformation modes are located at 541 and 496 cm À1 in the IR spectrum and at 569, 541, and 523 cm À1 in the Ramans pectrum.Acharacteristicd eformation of the CÀOÀOÀCp eroxidem oiety is assigned to aR aman band at 356 cm À1 ,c lose to the CF 3 rocking modes in the region from 339 to 296 cm À1 .T wo strong Raman bands at 241 and 123 cm À1 represent the CC 3 deformation modes of [(F 3 C) 3 CO] 2 (2b). Figure 5s hows the IR spectrum of [(C 2 F 5 )(F 3 C) 2 CO] 2 (2c)t ogether with the computed spectruma tt he DFT-B3LYP/aug-cc-pVTZ level of theory.A se xpected, it is very similart ot he spectrum of 2b and to that of its precursor 1c.T he IR spectrum shows,i na ddition to the strong CF 3 stretching modes in the region from 1277 to 1229 cm À1 ,t he characteristic CÀCs tretching mode of the C 2 F 5 group at 1340 cm À1 .W eak and broad bands at 1187 and 1177 cm À1 in the IR and the Raman spectra ( Figure S1.10 in the Supporting Information), respectively,a re assigned to stretching modes of the CF 2 group, and as trong IR absorption at 1105 cm À1 to the out-of-phase CÀOs tretching mode. The in-phase CÀOa nd C-CF 2 stretching modes of 2c are found in the Raman spectruma t1 132 and 1082 cm À1 ,r espectively.T he corresponding out-of-phase C-CF 2 stretching mode appeared in the IR spectrum at 1086 cm À1 .W eak to medium intensity bands around 1000 cm À1 in both the IR and Raman spectra are due to CC 3 stretching modes and as trong antisym-metricC F 2 stretching mode is found in the IR spectrum at 898 cm À1 (calcd:9 29 cm À1 ). The Ramana ctive OÀOs tretching mode appearsa t8 53 cm À1 ,i ne xcellent agreementw ith the calculation at 852 cm À1 ,a nd also the CÀOÀOÀCd eformation of the peroxide, located at 352 cm À1 in the Ramans pectrum, is very closetothat of 2b (356 cm À1 ).
Af ull list of all experimental and computed wavenumbers together with tentativea ssignment is given in the Supporting Information, Ta ble S1.2.
In previouss tudies, the synthetically valuablef luorinated radicals F 3 COC or (F 3 C) 3 COC were generated by photolysis of the corresponding peroxide (F 3 CO) 2 (2a)o r[ (F 3 C) 3 CO] 2 (2b), respectively. [19] The recently recorded gas-phase UV/Vis spectra of the perfluorinated bisalkyl peroxides (R F O) 2 [R F = F 3 C 2a, (F 3 C) 3 C 2b,a nd (C 2 F 5 )(CF 3 ) 2 C 2c]s how that for 2a the lowest UV transition is below 200 nm, whereas the bulkiers ubstituted peroxides 2b,c exhibit weaker redshifted transitions at 253 and 250 nm, respectively. [18] As describedp reviously,f errocene, Fe II Cp 2 ,i so xidized to ferrocenium,[Fe III Cp 2 ] + ,bya ddition of peroxide 2b [Eq. (6)]. [18] An immediate color change of the solid from orange to dark green is observed during the reaction, which is typical for the formation of af errocenium cation. Indeed, the IR spectrum of the solid ( Figure S1.11i nt he Supporting Information) shows the characteristic vibration modes of the ferrocenium cation. [44] For example, the weak n(CH)m ode is blueshifted by 36 cm À1 to 3121 cm À1 with respectt of errocene and the n(CC) mode at 1421 cm À1 is also hypsochromically shiftedb y1 4cm À1 in comparisont ot he reactant ferrocene. The bands at 965, 724, and 536 cm À1 as well as strong absorption bands in the region from 1300 to 1100 cm À1 are characteristicf or the anion, [OC(CF 3 ) 3 ] À . [45] Additionally,t he APCI mass spectra ( Figure S1.12 in the Supporting Information) show ap eak at m/z = 235 in the negative mode, characteristic for the [OC(CF 3 ) 3 ] À alkoxide anion. In the positive mode, an analogous decomposition pathway,c ompared to that of ferrocene, is observed. These spectra confirm the formation of [FeCp 2 ][OC(CF 3 ) 3 ].
Elemental fluorine was added to as ample of [(F 3 C) 3 CO] 2 (2b) to at otal pressure of about 1bar at room temperature in a PFAt ube. The tube was then flame-sealed at liquid nitrogen  temperatures and after reaching room temperature, NMR spectra were recorded (see the Supporting Information, Figure S1.13). The elemental fluorine is detected at ac hemical shift of d = 425 ppm (liquid:4 22 AE 1ppm, gaseous:4 19 AE 1ppm). [46] Peroxide 2b resists elemental fluorine, and its solubility in 2b is consistent with the low dipole moment as indicated by the dihedrala ngle V of the peroxide unit of 1808 in the solid state and the perfluorinated nature of this compound. [18] The longitudinal relaxationt ime T 1 of the dissolved F 2 wasd etermined by an inversionr ecovery experiment to be T 1 = 13.5 ms ( Figure S1.13 in the SupportingI nformation). This is approximately 300 times larger than T 1 for gaseous fluorine of 0.045 ms in the pressure range from 1t o2bar. [47] This indicates that the fast relaxation owing to the spin-rotation mechanism observed for gaseous fluorine is hindered and proves that the fluorine is indeed dissolved in peroxide 2b.

Conclusion
We present ac onvenients ynthesis to highly reactive perfluoro alkyl hypofluorite compounds R F OF from the corresponding alcohol and fluorine with excess CsF.S pectroscopic analysiso f the hitherto undescribed (C 2 F 5 )(F 3 C) 2 COF with support of quantum-chemical calculations are reported. We provide an ew synthetic approach for perfluoro bisalkyl peroxides R F OOR F by the reactiono fh ypofluorites with fluorinated silver wool. Furthermore, we also show the inertness of [(F 3 C) 3 CO] 2 towards strong oxidizers such as elemental fluorine. The liquid temperature range from 16 to 99 8Cf or the nonpolar peroxide 2b together with its inertness towards strongly oxidizing halogensd emonstrates its potential as asolventfor oxidation and halogenation reactions. By irradiation with UV light,p erfluoro alkyl peroxides 2 can be activated to generate valuable R F OC radicals for synthetic applicationss uch as perfluoro alkoxy group transfer reagents.

Experimental Section
Experiments were carried out under strictly dry and oxygen-free conditions in glass tubes with Te flon valves or in stainless-steel vessels. Purchased starting materials were used without further purification. NMR spectra of neat liquid substances were recorded with aJ EOL 400 MHz ECS or ECZ spectrometer by using ac apillary filled with [D 6 ]acetone ( 1 HNMR:2 .05 ppm, 400.53 MHz; 13 CNMR: 29.8 ppm, 100.51 MHz) and CFCl 3 ( 19 FNMR:0ppm, 376.13 MHz) as external standards. The chemical shift and scalar coupling constants were obtained by the program Mestrenova 10.0. [48] Raman spectra were measured at liquid nitrogen temperature with a Bruker MultiRAM II spectrometer equipped with a1 064 nm CW DPSS laser and aL N 2 cooled germanium detector at ar esolution of 4cm À1 .G as-phase infrared spectra were recorded by using a Bruker Vector 22 spectrometer at ar esolution of 2cm À1 .U V/Vis spectra of gaseous samples were obtained by using aP erkinElmer Lambda-900 spectrophotometer.M ass spectra were measured with an Advion expression L compact mass spectrometer.T he m/z values of the monoisotopic peaks are given. The NMR relaxation time T 1 was determined by the 1808-t-908 pulse sequence technique. Powder diffraction data were collected with aSTOE IPDS II/T instrument at 290 Kw ith Mo Ka radiation (l = 0.71073 )b yu sing a graphite monochromator.I ntegration was performed with STOEX -Area V1.56, data analysis and Rietveld refinement were performed with X'Pert HighScore Plus V2.2c.

Safetyn ote
Caution! Extreme caution should be exercised when working with elemental fluorine and hypofluorites. Explosions have been reported [2,49] during handling of these extremely hazardous compounds. Although the described perfluoroalkyl peroxides were found to be insensitive to shock and friction [18] according to the U.N. Recommendations on the Transport of Dangerous Goods, [50] we cannot exclude explosive reactions in mixtures with other substances.