WILLIAMC. SMITH AND VAUGHNA. ENGELHARDT
3835
infrared and mass spectrometric analyses were obtained they are indicated in the table and were in agreement with the structures of the products. The fluorine magnetic resonance spectra were obtained using a Varian high resolution n.m.r.
[CONTRIBUTION NO,
539
Vol. 82
spectrometer and electromagnet*za t a frequency of 30 Mc. and a field of 7,500 gauss. (12) Varian Associates, Palo Alto, California.
FROM CEXTRAL RESEARCH DEPARTMENT, E X P E R I M E N r A L STATION, A S D COMPASY, \vILMINGTON, DELAWARE]
E.I. DU
P O N T D E NEMOUKS
Chemistry of Sulfur Tetraflu0ride.l V. Preparation of Sulfur Oxytetrafluoride and Sulfur Hexafluoride by Oxidation of Sulfur Tetrafluoride BY WILLIAXC. SXITHAND VAUGHNA. ENGELHARDT RECEIVED SEPTEMBER 25, 1959 Sulfur oxytetrafluoride (SOF4) was synthesized by the reaction of sulfur tetrafluoride (SF4) with oxygen in the presence of catalytic amounts of NOz in high yield and conversion; this represents the first direct method for the preparation of SOF, not based on elemental fluorine. Oxidation of SF4 with such inorganic oxides as N o t , CrOr and CeOz gave sulfur oxytetrafluoride, sulfur hexafluoride and sulfuryl fluoride. Sulfur hexafluoride also was obtained by the microwave excitation of
SF,. Introduction Sulfur oxytetrafluoride (SOF4) has been prepared previously only by the reaction of thionyl fluoride (SOF2) with elemental fluorine.z-4 Sulfur hexafluoride has been prepared generally by oxidative fluorination reactions based on elemental fluorine. In an investigation of the chemistry of sulfur tetrafluoride (SF4), which can be prepared directly from sulfur dichloride and sodium fluoride in an acetonitrile medium,5 it has been found that this compound can be oxidized to SOFA and SFG by several chemical methods. Discussion Oxidation of SF4 with Nitrogen Oxides.-The nitrogen oxides and their salts are good reagents for effecting the oxidation of SF4. The best oxidizing agents in this class are those in which the nitrogen atom is in a tripositive or higher oxidation state. Nitrogen dioxide, sodium nitrite and sodium nitrate were effective a t 300°, but little or no oxidation occurred when SF, was heated with nitrous oxide or nitric oxide a t this temperature. The formation of SOF4is favored a t lower temperatures and lower SF4 concentrations; SF, is the primary oxidation product a t higher temperatures and with higher SF4 concentrations. The effect of the SF4 concentration can be seen from the idealized equations
+ BSF4 --+ 5SOF4 + SOFZ+ 2iYaF 4-Si (1) 2Sar\’03 + llSF4 + 5SFs + BSOI.2 -I- 2NrtF 4- KQ ( 2 ) 2SaS03
The use of NaN03 resulted in the formation of SOF: and SFs in yields of 57 and 20 mole % j . respectively. X volatile, reactive solid was obtained in all of the reactions in which nitrogen dioxide or nitrite or nitrate salts were employed for the oxidation of SF4. This solid was shown to have the empirical (1) Paper I V , A. L. Oppegard, W. C. Smith and V. A. Engelhardt TKISJOURNAL, 82, 3835 (1960). (2) W. Kwasoik, “Inorganic Chemistry,” Vol. I, 1948, pp. 192-193. (3) IT. Jonas. Z. o n o ~ g allgem. . Chem., 265, 273 (1951). (4) 1;. €3. Dudley, G. H. Cady and D. F. Eggers. lr., T I i 1 : j J O U K N A ~ , , 78, 200 (1956). (2) Paper I , C. W. T u l l u c k F S F‘awcerl \V C Smith dii(! 1 ) 11. Coffinan ibid.. 82, 53!1 ~ 1 9 h n I
formula N203SF6, but the structure has not been determined. The solid dissociated reversibly a t room temperature; one of the dissociation products was shown to be SOF4,and the others contained NO groups as determined by mass spectrometric analysis. One method of representing the structure of this product would be NzOz+SOFB-, which is similar to Nz02+HS04-.6 The formation of the SOFS- ion is not unreasonable, as cesium fluoride has been shown to combine with SOF4 to give a reactive solid believed to be CsSOF6. Such a simple formula probably is incorrect, however, as the presence of an N202+ ion having an unpaired electron would be expected to impart color to the product. It is suggested that the actual structure may involve the SOFB- ion together with a positive ion or ions containing only nitrogen and oxygen in which all of the electrons are paired. Catalytic Oxidation of SF, with Oxygen.-Sulfur tetrafluoride does not react with oxygen a t 300°, but in the presence of NO2 as catalyst sulfur(V1) fluorides were obtained in high yields and conversions. This method proved particularly valuable as a route to SOFl; a t 200’ SOF4 was obtained routinely in 75-8070 yield and conversion. The amount of oxygen employed was a t least equivalent to the amount of SFa used but could be present in excess without adversely affecting the yield of SOF4. The reaction may be run a t temperatures as low as 140’, but a t this temperature a longer reaction period is required and the yield is less satisfactory as thionyl fluoride is formed in higher concentrations. The oxidation process is presumed to involve the oxidation of SF4 by X 0 2 to give SOP4 and KO. The NO is then assumed t o undergo oxidation by oxygen to regenerate NO?. This reaction mechanism is supported by the experimental observations : (1) SF4 does not react with oxygen alone a t temperatures up to 3 0 0 O . ( 2 ) The reaction of SF4 and NO2 a t 200’ gives KO as the principal reduction product. (3) The inoderately rapid, reversihle reaction of NO and oxygen to give N O z takes place in t.he (6) F. See1 607 (1453)
€3
J:idx
I_ l?irIil and B Volkl. Z .Tnltiriorsrh
,
8B,
Aug. 5 , 19(iO
PREPARATION O F SULFUR OXYTETRAFLUORIDE AND
gas phase a t the reaction temperatures employed and the equilibrium lies to the right7 Oxidation of SF4with Metal Oxides.-The oxidation of SF4 to sulfur(V1) fluorides also has been accomplished with a number of metal oxides. The oxides which effect the oxidation of SFd are those whose corresponding fluorides are either good oxidizing agents themselves or are unknown.s Both binary and ternary oxides were employed as oxidizing agents, and of these the most effective were chromium(V1) oxide and the alkali metal chromates. This method suffers from the disadvantage that the fluorine of the SF4 is inefficiently converted to SOF4 and SF6; a considerable amount of fluorine combines with the metal to form metal fluorides and/or oxyfluorides. Thionyl fluoride is a major constituent of the volatile product and cannot be removed completely by distillation. Pyrolysis of SF4.-The very high thermal stability of sulfur tetrafluoride has been demonstrated in pyrolysis s t ~ d i e s . At ~ 500-1000" and contact times of several hours less than 1% conversion of SF4 to SFfitakes place. Equilibria data for the reaction 3SF4
2SFe $- S
are consistent with these experimental results in that the calculated equilibrium constants in the temperature range 500-1000° are very small.'O The experimental details of this work have been described elsewhere.g In contrast to thermal pyrolysis i t has been found that microwave excitation of SF4 leads to SFs in about 59y0 conversion and 80% yield. The energies involved in this method are very large, and SF4 probably undergoes nearly complete dissociation into S and F. under these conditions. Recombination of these atomic species under nonequilibrium conditions then would lead to the preferential formation of SFe. Such a process is, a t least formally, analogous to the reaction of sulfur with elemental fluorine. It is also of interest to note that electrolysis of SF4-liquid hydrogen fluoride solutions has produced SFB with an over-all current efficiency of 7oy0.11 The reaction is represented by the equation SF4
electrolysis + 2HF + SF5 f HZ
It is therefore apparent that the conversion of SF4 to SF, in high yield and conversion can be accomplished only if sufficiently high energy sources are provided. Experimental Preparation of SOF,from Sulfur Tetrafluoride and Oxygen in the Presence of Nitrogen(1V) Oxide as Catalyst.-Reac( 7 ) W. F. Giauque and J. D. Kemp, J . Chem. Phys.. 6, 40 (1938); E. Briner, W. Pfeiffer and G.Malet, J . chim. p h y s . , 2 1 , 25 (1924). (8) Paper IV (ref. 1 ) ; metal oxides whose corresponding fluorides are not strong oxidizing agents are converted to these fluorides by treatment with SF,, and no oxidation takes place. (9) E. L. Muetterties. U. S . Patent 2,883,267, April 21, 1959. (IO) J . D. Vaughan and E. L. Muetterties, unpublished results. ( 1 1 ) E. H. Man, U. S. Patent 2,904,476, September 15, 1959.
HEXAFI,ITORI~)E
3839
tions were carried out iii 400-ml. stainless steel pressure vessels. As SOF4was the product particularly desired, no attempt WRS made t o recover the more volatile SFa quantitatively. I n a typical experiment 108 g. (1.00 mole) of SF4, 16.0 g. (0.50 mole) of oxygen and 9.0 g. (0.21 mole) of NO2 were heated a t 200' for 71/3 hours. T h e bomb was bled to atmospheric pressure through a trap cooled in solid carbon dioxide-acetone. The liquid condensed in the trap (108 g.) was transferred t o a cylinder cooled in liquid nitrogen; the small amount of less volatile solid product was discarded. Mass spectrometric analysis of the product in the liquid phase showed t h a t it contained S6.070 SOFa, S.7% SOF2, 2.5Y0 S02F2, 1.47, NO and/or KOF, and trace amounts of SFej N20, SiF4, N2 and 0 2 ; 110 SF4 was present. The yield of SOF4 was SO% based on the mass spectrometric data and the SF, employed in the reaction.'? All of the major impurities in the SOF4 except S02Fs were removed from the crude product by selective absorption in dimethylformamide.13 I n a typical purification 85 g. of crude SOF4 was scrubbed twice with 50-1nl. portions of dimethylformamide; this was done by condensing the crude SOF4 in a cylinder containing the dimethylformamide and allowing the cylinder t o stand for one-half hour at room temp. In the first wash 11 g. of material was dissolved by the solvent and in the second a n additional 2 g. was removed. Mass spectrometric analysis of the purified product showed t h a t it contained only SOF4 (96.5 mole yo),S02F2 (3 mole %) together with minor amounts of nitrogen (0.3 mole %) and SiF4 (0.2 mole %). The composition of the scrubbed niaterial also was confirmed by infrared'4 and nuclear magnetic resonance's analyses. There mas no reaction between SF4 and oxygen alone under comparable conditions. h'itrogen(I1) oxide reacted with SF4t o only a slight extent even a t 300". At this temperature with a reaction period of 9 hours the yields of SOF, and SFg were 5 and l'%, respectively. Oxidation of SF4 with Nitrogen(1V) Oxide.-An 80-ml. stainless steel-lined bomb charged with 4.6 g. (0.10 mole) of and 44 g. (0.40 mole) of SF4 was nitrogen dioxide (NO?) heated a t 200' for 2 hours, a t 250" for 2 hours, and then a t 300' for 8 hours. A volatile product, weighing 38.0 g., was transferred to a cylinder cooled in liquid nitrogen. A yellow solid, weighing 2.4 g., which reacted vigorously with water and formed brown fumes when exposed to the atmosphere, also was obtained. Mass spectrometric analysis of the volatile product in the liquid phase showed t h a t it contained 7% SOF,, 5% SF6, 2% sulfuryl fluoride (SO~FZ), 21% SR, and 44% elemental nitrogen. Calculations based on these data showed t h a t the yields of SOP4 and SF6were 19 and 14%, respectively. The reactive solid product described above was obtained as a major product when the reaction was carried out at a lower temperature with equivalent amounts of the reactants. A bomb charged with 23 g. (0.50 mole) of NO2and 54 g. (0.50 mole) of SF4 was heated at 150' for 14 hours. The bomb was bled to atmospheric pressure through a trap cooled in acetone-solid carbon dioxide and both liquid and solid products were collected in the trap. The condensed liquid was transferred t o a cylinder cooled in liquid nitrogen; this product, which weighed 38 g., was shown by mass spectrometric analysis to contain in mole 7050, SOFz; 21, S02F2; 7 , unreacted SF4,about 1 mole 7, each of SFg, SOF4, nitrogen and about 15 mole 7, of a species t h a t yielded the N O fragment in the spectrometer. The solid product was allowed to warm up until it apparently was free of liquid and was colorless; it weighed 36.2 g. This product volatilized on warming but could be stored indefinitely a t acetone-solid carbon dioxide temperatures without change. This oxygenand moisture-sensitive solid was shown by elemental analysis to have the empirical formula FsN203S; i t was identical with the by-product obtained from the reaction of N a N O s with SF,.
(12) Preliminary work dealing with this synthesis has appeared elsewhere (W. C. Smith, U. S. Patent 2,907,636, October 6, 1959). (13) The authors are indebted to Dr. R . D. Cramer for the purification procedure. (14) F. B. Dudley, G . H. Cady and D. F. Eggers, Jr., THISJOURNAL, 7 8 , 1553 (1956). (15) F. B . Dudley, J. N. Shoolery and G . H. Cady, i b i d . . 78, 568 (1956).
WILLIAMC. SMITH AND VAUGHNA. ENGELHARDT
3840
OXIDATION O F
---Solid Oxide
reactant--Mole
SF4. Mole
0.20
0.60
T;mp., C.
Solid,
Time,
hr.
CrOs
.20
.81
100 2 200 4 300 6 As above
SanCrOl
.10
.37
As abovc
KnCrnO:
.10
.71
0
hInO2
.15
.6
CeOz
.20
.61
PbOz
.l5
.42
KaNOz
.20
.76
Sah'Oa
.10
.48
CrOs
TABLE I SF4 JVITH S E L E C T E D OXIDES ----
300 2 360' 12 100 2 135' (flash) 100 2 200 4 300 6 200 2 2 50 2 300 8 As above
g.
products
Gas, total wt.
Vol. 82
( s1
~ _ _ _
-_-__---
----Oxidation Compound
prr,ducts"------Yield(';) b
23 ( t a n ) SOFa SFsC SOFa SF6' Moderate anits. SOF4, SO,F,' Moderate anits. SF6, S02Fl Small anits. SOFI, SFsk Sniall atnts. SOlF2, trace SFe" Traces SOF4, SF6, considerable oxygen" SOF, SFs'
59.6
23 (tan)d
76.0
18.8 (pale green) 33.0
31.0
12'
86.0
34"
54.3
31.5'
43.5
9 . 5 (colorless
72.0
85.0
46 16 25 40
8 42
3 . 9 (color57 less) 20 a Yields calculated from mass spectrometric data and based on idealized equations. Based on metal oxide. c Product Solid product contained 37.12%, chromium, contained: 22% SOF4; 5% SFe; 1074 SOgF?; 575h SOP2 and 55% SF,. 60.50 fluorine, and 3.89% sulfur; X-ray analysis indicated it was amorphous. e Product contained (/): 10% SOF,; 127; SF6; 955 S02F2; 67% SOF? and 2% SF,. Product contained: 774 SOF,; 0,3% SF6; 79; SOrF2; 52% SOF2 and 207; SFa. On rising temperature, an exothermic reaction occurred a t 290' and temperature rose t o over 400". No further heating employed. *Product contained 13.5% SFB; 18% SOzFz; 59% SOFz and 5.97, SFa; only a trace of SOF,. i N o reaction a t 300" after 6 hours. j X-ray showed product was primarily ILInF?, but contained small amount of MnOn. k Product contained: 3% SOFa; 5% SFs; 1% SOzFz; 50% SOFn; 40% SF4. On heating reactants above IOO", flash from 135-180' recorded; no further heating employed. '" X-Ray pattern in good agreement with t h a t of CeF3. Product contained: 0.55'0 SF6; 470 S O 8 z ; 42% SOFz; 54% SF4; oxygen observed on upright sampling. p X-Ray pattern t h a t containing small amount of PbOn. Calcd. for PbFn: Pb. 84.60: F. 15.50. Found: P b . 81.72: F . 12.16. '2 Product for PbF,, contained: 0.3LJ-SOF4; 0 . 1% SF6; 29% SOFn; 67'6 SF4; oxygen (1:; in liquid phase, 16% in vapor phase). Product contained: 3.576 SOFa; IOT% SFs; 497i SOFr; 31?; SF4. ' Product contained: 2755 SOP(; 5.2%>SFs; 1% SOyFz; Na;1% 10, .Us0 isolated sinall amount of colorless volatile solid siniilar to t h a t obtained in S02-SF4 SOF?; reaction
'
Q
~
~
~
~~~
I~
~
I~~
~
Attempts t o characterize the solid material further met with only limited success. The solid decomposed in efforts t o effect solution in a variety of organic solvents. Contact with dry air resulted in the rapid formation of brown fumes. Mass spectrometric studies showed t h a t t h e solid dissociated on warming t o give SOFa as the only sulfur-containing species together with one or more compounds having an NO+ fragmelit ion. Calcd. for F&&S: F, 46.78; N, 13.75; S, 15.79. Found: F, 46.39; N, 13.47, 13.95; S,16.12. Oxidation of SF, with Selected Oxides and Salts.-Reactions were carried out in 145-1111. "Hastel1oy"-lined pressure vessels t h a t were agitated during t h e reaction period. The oxide or salt was charged into the bomb, the latter was cooled in a n acetone-solid carbon dioxide bath and t h e desired amount of SFawas weighed in. Volatile products were collected in stainless-steel cylinders cooled in liquid nitrogen and analyzed b y mass spectrometric analysis in t h e liquid phase. Solid products were removed from the reactor and transferred under a blanket of nitrogen t o polythene bottles.Ib The experimental results are summarized in Table I. T h e
effect of the reactant ratio on the ratio of SOF4 and SF,iii the volatile product is seen from a study of the Cr03-SF4 reaction. The nitrite and nitrate salts and the chromium ( V I ) oxides are effective reagents for the oxidation of SF., but the other oxides investigated were poor in this respect. Microwave Excitation of SF4.'7-The apparatus and procedure were similar t o those described in the literature,'* the only difference being the use of an alumina liner in the reactor. In one experiment SFI (ca. 0.1 g./min.) and N:! ( c u . 0.2 g./min.) were passed through the reaction zone a t a total pressure of 4 mm. for thirty minutes. The condensable products were trapped a t the exit of the reactor on a liquid nitrogen-cooled cold finger. T h e product (2.3 g.) collected ou the cold finger was transferred to a stainless-steel cylinder. Mass spectrometric analysis of this material a t 25" showed that it contained 53y0 SFE; 28% SF4; 13% SOFp; 2%, SiF4; 1% SO?; 0.2% SOF4. T h e yield of SFs based on this analysis was 80% and the conversion 59Yc.
(10) Several early experiments dealing with this study have been described prerjoi>sly (W. C. Smith, U. S. P a t e n t 2,904,398, September 15, 1959)
(17) T h e authors are indebted t o Dr. C. S . Cleaver f o r this experiment. (18) R. L. M c C a r t h y , J . Chem. P h y s . , 22, 1360 (1954).
Acknowledgments.-The authors are indebted to Drs. E. H. LTan, E. L. Muetterties and V'. D . Phillips for their contributions to this work.
