The Chemistry of Silicon Difluoride
Dale L. Perry and John L. Margrave Rice University Houston. Texas 77001
A reactive carbene analog
Although students are introduced quite early to the tetravalent state of Group IVA elements via freshman chemistry, little attention is usually given to the chemistryof the dipositi\.eoxidationstateofthelighter memhrrsofthisgroup.~he +2state beromes morestablecoina down thegroup,and there is a rather extensive chemistry G o w n for this state of the heavier congeners germanium, tin, and lead; in contrast, however, the types of dipostive compounds which exist for both carbon (compounds known as carhenes) and silicon (known as silylenes) are short lived and must be investigated in studies of using techniques which are transient species, techniques which are primarily spectro.mn;r ;" nrtn.o One such reactive moiety is silicon difluoride, S F 2 . While investigations have been carried out on other divalent silicon comnounds-amone them SiS and SiO-more intense research efforts have ieen focused on silicon difluoride for the past decade. The relative ease of preparation of the compound coupled with a considerably longer half-life than its carbon analog makes SiFz an interesting and convenient species to study in termsof both its basic chemistry and its properties. The present paper is a survey of the properties, reactions, and spectroscopic studies of SiF2. This review is intended to provide an introduction to a research area which has proved to be rich in basic chemistry and shows promise of even greater growth in the future. l . . ..""-I.
~
~
Synthesis of SlF2 Silicon difluoride is rather easily prepared ( I , 2 ) by the reaction of silicon and silicon tetrafluoride (Fig. 1) a t elevated temperatures -1100-120O0C a t low pressur& c:,.) r E : W 1\51 . I ~ , --. ",\B, I "LL 'OE:W.,~> 2\61 *U"
(\A, 11
This reaction results in a~proximatelya 50% yield of SiFz (wirh the remainder of the& mixture being Sic4)with some dis~ronortlonationoccurring if the pressure oiSiFz exceeds l-i to&. By continued pumping of the system, this problem is eliminated, and the SiFz that is formed can he removed for the study of its properties or reactions. The half-life of SiFz is approximately 150 sec, considerably more than its fluorinated carhene analop ~ ~ ~ ~ ~ - .(Table 1).Interestinelv. SiF? - shows no tendency to undergo gas-phase autopolymerization to form ~~
~~~
~
~
~
extended perfluorinated silanes such as Si2F6, Si3Fs, etc; however, condensation of the gaseous SiFz-SiFa mixture with liquid nitrogen results in a reddish yellow polymer6if~1,-which turns white upon warming to room remperature. The compound is pyrophoric and must be handled in anhydrous, inert environments. There is also no evidence for formation of gas-phase silylenes like SizF4, SisF6, etc. of Polymeric SiFz Although white (SiFz), ignites spontaneously in moist air, its reaction with water is not so complete. The principal products evolved are hydrogen and silanes. Hydrogen is also produced in the reaction of silicon difluoride polymer with concentrated alkali ( 2 ) .Thermal decomposition of @IF2), under vacuum in the 200-350°C range effects the formation of perfluorosilanes, SinF2n+2. and a silicon-rich polymeric solid
-
[ ( n + 2)lxl(SiFr), SinFz,+~+ 2(SiF)(polymer) (2) At higher temperatures on the order of 400°C, the decom~osition becomes even more rapid resulting in mostly silicon and SiF4. Mass spectrometric investigations reveal ~ e a k for s the series (Si,Fzm+~,)+, where n varies from 1to at least 14 and Y = 0-6. Perhaps the most useful reaction is the acid hydrolysis of polymeric SiFz using 20% aqueous hydrofluoric acid to yield a comhinntion of silanes
The predominant silwes formed are SizH6, Si3Hs. and nSidHln with other lonaer chain species also heing observed ( ~ a h i 2). e The silanes-may he separated by bothfractional condensation and gas-liquid chromatography. Polymeric SiFz also reacts with alcohols, ketones, amines, and ethers. Highly reactive hut complex adducts are formed with diethylamine and diethyl ether (2).Presumably, the Si-Si skeleton is left intact in the (SiFz),. n(CH3)zO adduct, since acid hydrolysis of the complex yields small amounts of silanes.
~
Table 1
Praosrtieg of GareourCF..
CF,
pro pert^ Bond Length.
A
1.591
looe 59'
104' 56
0.46 -39.3
Dipole moment. D k y mol-'
AHl"
D. cal mol T I L
SiF,
1.300,
Bond Angle
1.23 -139 143
125
150
1
wc
SiF..GeF.' GeF,
1.7321 9 7 O 10' 2.68
-136.9 131
stable
a All values taken from Margrave. John L.. and Wilson. Paul W..
Acc. Chem. R e r . 4, 145. (1971).
Table 2. Soecler
-,s. a Figure 1. Apparatus far the preparation and reaction of SF2. (Reprinted horn Margrave. J. L.. and Wilson. P. W.. A c c Chem. Res.. 4, 145 (1971).)
696 / Journal of ChemicalEducation
n-Si,H,, n-Si,HI2
Products Farmed
% o f total
-. .-
15.0 8.6
from ISIF,), and 20% H F SDBC~~E
. ..,
% of total
2'
i-SI6H3* (SiH,12S~4H,
214 "0.1
Readlons of SIF2 Monomeric silicon difluoride undergoes many reactions with a variety of compounds, both organic and inorganic. The reaction of SiFz with water (3)yields ( H F z s i ) ~ as 0 the princinal nroduct. i.e., l.l.l'.ll-tetrafluorodisiloxane. Lesser athunk uf trifluorosilabe, hexafluorodisiluxan~,and perfluorotrisiloxane are also produced in the reartion. Since the predominant product is t&rafluorodisiloxane, the most likely reaction route is
Ppiymer (7WYidd)
rSiFrC;iFi--SiF,-
4~.JIw.,
~
i
~
~
S5 i .Q--s~F~-*~~-s~F,. ~ i c
I
~IBPI"
Presumably, the diradical mechanism (discussed below) involved in many reactions of SiFz is not operable here, since such a reaction route would dictate the formation of HFzSiSiFz-0-SiF2SiF2H: none of this compound is detected in the mixture of products. When hydrogen sulfide and SiFz are co-condensed a t -196°C and then allowed to warm to room temperature (41, a reaction occurs which results in the formation of compounds identified by mass spectrometric and nmr studies as the following:
Compounds 1-111 are the predominant products, while lesser amounts of N and V are formed. SiFzHSH (I) reacts with HCI to form SiFzHCl and HnS. Silicon difluoride reacts with methanol (5) to form SiF3H and CH30SiF3 in approximately equal amounts with (CH30)zSiFz heing obtained as aminor product. The formation of the first two products can he attributed to the following: CHBOH+ SiF4
-
CHJOS~FJ + HF
--
(8)
SiFz + HF SiFsH (9) Production of (CH30)2SiF2can occur through either reaction of the first compound with methanol CHJOH + CHJOS~FJ (CH30)2SiF2+ HF or by the following path
(10)
CHJOH + SiF2 CHsOSiFzH (11) CHsOSiFzH + CH30H (CH301zSiF~ + Hz (12) Reaction of SIF2 with Aromatic Compounds The reaction of silicon difluoride with benzene (6) is a unique reaction for which there is no exact parallel in carhene chemistry. When these two reactants are co-condensed a t -196"C, a hrown polymer is formed. Upon heating to room temperature, the polymer becomes yellow, and benzene, oerfluorotetrasilane. and CeHdSiF?). .. .. - . .. adducts are released. iXatillation of the polymer yields a mixture uf solids and liquids. Mass snrctral darn f i r the distillate indiratp the Dresence of compoun& of the type C6H6(SiF2),(where n = 2 at least 81, and the predominant species is C6HsSisFs. Additional infrared and ultraviolet spectra coupled with results from the hydrolysis of the compound suggest the structure
SiF.
iSiF1l. Unreanive with Benzene
/
Figure 2. A possible mechanism for the
\
w SiF,
SiF,
(%YW
reaction of SiFzwith benzene.
in which the predominant feature is the bridging -SiF2species. Presumably, the reaction occurs via formation of free radicals (Fig. 2) which will be discussed below in greater detail. Silicon difluoride reacts in an analogous fashion with toluene to form complexes with the proposed structures
The major product is I with lesser amounts of compounds I1 and 111also heing formed. Again, the proposed structures are consistent with proton and fluorine nmr and mass spectrometric studies. Mouofluorobenzene and p-difluorobenzene reactions with SiFz yield similar products, also, but in addition yield some aromatic species in which SiFz insertion into the +C-F bond occurs. Hexafluorobenzene (CsFs) yields mono-, his-, and tris-SiFs substituted aromatic molecules (6). Reactions of S F 2 with Oxygen, Sulfur, and Selenium Silicon difluoride reacts with oxygen, sulfur, and selenium to afford several products, including species of the types Si,0,F2,, Si,S,F2,, and Si,Se,Fz, (7). The most volatile. sulfur complex obtained is Si2SF6, while Si,S,F2, can he readily pyrolyzed from the initial polymeric solid; the volatile complex derived from the SiFz 0 2 reaction is hexafluorodisiloxane (SiFsOSiFa) along with a smaller amount of perfluorotrisiloxane (SiFsOSiFzOSiFs). The Si,O,Fz, compounds are probably cyclic with alternating oxygen and silicon bonds, such structures heing analogous to the silicon oxychlorides (8). Interestingly, the matrix isolated infrared spectra of the SiFz 0 2 reaction products indicate a structure consistent with that of siliconyl difluoride, the silicon analog of carhonyl fluoride. If such a formulation is found to be correct, this reaction technique may be useful in forming the much discussed multiple bonded silicon species (9,10). Species such as Siz0F6, Si30Fs. Si30zF8, Si202F4. and Si103F~are obtained from the reaction of SiFz with thionyl 11;oride ( 1 1 , separated by trap-to-rrap and low-temperatuie distillation: the most unique nroduct, however, to be ohtained from the reaction of sip2 with a fluorinated species is the compound produced by the SiFzICF3COC1 (trifluoroacetyl chloride) reaction which is postulated as
+
+
CF,,
,0-SiF,-SF,
C CI/
\
\siF,-s,F,-o/C\cI
The structure is corroborated by infrared, nmr, and mass spectrometric studies as well as elemental analyses. Volume 53,Number 11, November 1976 / 697
Reactions of SiF2 with Alkenes and Alkynes The SiF2 reactions with alkenes and alkynes yield a variety of products. For example, the volatiles C2H4Si2F4,C4HsSi2F4, and small amounts of C2H4SiF2(12) are formed in the S i F d CzH4reaction. Nmr spectra of the C2HaSizF4 and C4H8Si2F4 moieties are consistent with structures I and 11, respectively.
I
H, I1
111
N
V
If trifluoroethylene is reacted with silicon difluoride, complexes with the structures 111, IV, and V (all in good agreement with proton nmr studies) are isolated, with isomer I11 being the predominant species. Three products are obtained from the low-temperature condensation of SiF2 with acetylene (13): tetrafluorodisilacyclobutene, 3,4-tetrafluorodisilahex-1-yne-5-ene,and 5,6,7-hexafluorotrisilanorborn-2-ene (structures I, 11, and 111, respectively).
pounds as well as monitoring its reactions with a variety of reactants. One early report (20) centered on the spectrum of SiF2, its uolvmerization. , ~ , ~.and its reactions with small molecules in rare gas matrices. The band multipl&s for buth SiF, and SiF, in the SiF.*SiFl mixture are readdv assianable: moreover, two new bands at892 and 830 cm-I fbllowed by peaks at 930 and in the krmton 970 cm-I awDear . . when the warm-UD . process . and argon matrices is effeaed, accompanied by the decrease in intensitv uf the S F ? monomer band at 255 cm-I. The appearance i f the two band sets is indicative of the formation of polymeric SiF2. The same study also reports evidence for the reaction of silicon difluoride with BFa,Oz, CO and NO. A more recent study involving infrared measurements (21) describes the use of matrix isolation techniques to establish molecular parameters. Silicon difluoride, a V-shape molecule, belongs to symmetry point group C1,and should thus exhibit three fundamental vibrational modes. The infrared spectrum of matrix-isolatc.d SiF. indicates the v , . uv. and u., fundamental bands at 851, 343, a i d 865 cm-1, r&pectiveli, and isotope splitting can also be detected in the matrices. By utilizing isotopic frequency assignments, the F S i - F bond angle can be calculated as 97'-102', a range which is in excellent agreement with gas phase studies. Figure 4 shows part of the ~
~
Presumably, each of the compounds is formed uia attack of SizF4 radicals on the acetylenic triple bond with the initial reaction being HC CH + .SizFl. HC = CH-SiF2-SiFz (13) The reaction product in eqn. (13) may then (see Fig. 3) either undergo ring closure to form I or read further with SiF2 units to form polymers or react with acetylene to form the second possible intermediate
-
-
HC = CH-SiF2-SiFz-CH = CH The single crystal X-ray study of 111 has been reported (14). Other Reactions Investigations have also been carried out concerning the reaction of silicon difluoride with phosphine (15), germane (16), iodotrifluoromethane (17), boron trifluoride (18), and 3,3,3-trifluoropropyne (19). Spectroscopic Studies Silicon difluoride lends itself to study by a variety of spectroscopic techniques, both in its pure form and in its reactions with other compounds; infrared spectroscopy is especially effective in determining molecular parameters of the com-
Figure 3. A possible mechanism for the reaction of SiF2with acetylene 698 / Journal of Chemical Education
Figure 4. The inhared spectra of silicon difiuaride ( v , and us) in a neon matrix. Spectrum A was obtained at S'K, and spectrum B was the result of a warmup-recooling cycle. Asterisks indicate matrix peaks, while P, and P2 indicate polymer peaks. (Reprinted from Hastie, J. W.. Hauge, R. H.. and Margrave. J. L., J. Amer. Chem. Soc., 91, 2536 (1969).)
infrared spectrum of SiFz in a neon matrix hoth a t 5OK and after an annealing (warm-up)-recooling cycle. The microwave spectrum (22) of silicon difluoride reveals a hond angle of 100° 59' with a Si-F bond length of 1.591A. The magnitude of these two parameters possibly indicates p 2 hvhridization of the silicon orhitals and the P orbitals of the fluorine atoms. Presumably, the two non-bonding electrons have either s or s o character and are aired irr the sinelet " ground state. The ultraviolet absorption spectrum (23) in the 2130-2325 A region shows that the SiFz hond angle in the upper electronic state undergoes a rather great variation during transition. In all, 28 hands are assigned in the ultraviolet spectrum. Electron spin resonance studies (24) provide even more evidence for the mechanism of the S i F p C s H s and SiF? C2H2 reactions shown in Figures 2 and 3. The SiFz/SiFd mixture, when condensed at low temperatures, yields a stronger esr spectrum when a concentrated matrix is utilized in which the SiF2 molecules can interact. This indicates the existence of some type of radical polymeric species, thus explaining the observed chemical reactivity. The most likely formulation of this species would be a diradical of the type
chemistry involving SiFz has received only minimal attention. Divalent silicon compounds like SiF2 will provide an opportunity for more extensive research in the future, and should lead to the development of useful and unique chemical compounds. Acknowledgment The authors wish to thank the Armv Research Office. Durham, North Carolina, the National ~EienceFoundation. and the Robert A. Welch Foundation for financial support of research in fluorine chemistry at Rice University. O& of us (DLP) wishes to express his gratitude for a Robert A. Welch Postdoctoral Fellowship and a National Science Foundation Postdoctoral Fellowship. 11) Pease, D. C.. U.S.,Pstsnt 2.84W88 (June24.19581. 121 Timms. P. L.. KenLR. A,. Ehleit.T.C..and Msrrrave.J. L.. J Amrr Chem. S o r . 87. (3) Margrave, J. L., Sharp, K. G., and Wilson. P. W.,"J. Amer Chem Suc.. 92. IS30 ,14"", ,. ,. (4) Sharp, K. G., and Margrave, J. L.,lnorg. Chem.. 8,2655 (19691. 15) Margrave. J. L . . S h w , K.G..snd Wilson. P,W.,lnorp. Nurl. Chpm Lettcra. 5.995
...
(19691. 161 Timms, P. L..Sfump.
D. D.. Kent. R. A..and Marerave,J. L.. J. A m m Chem Sor., 88.
w
P
.-" ,.""",. (7) Is1 Sham. K. G.. Ph.D. Dissertation. Rice University, 1969. lb) Besenhrueh G.. and
F
F
~ ~ ~J. L., ~ unpublished ~ s "re.u1ts. ~ , 18) Chamhar8.D. W . S . d 01.. J . Chem. Soc., 5088l19M1l. (91 Bush, R. D.. Golino. C. M.. and Sommer. L. H.. J. Amer Chem. Sac. 96. 7105
a&" ,,me,
In a very dilute matrix, however, no paramagnetic species is shown, thus indicating the SiF2 to be in the singlet state. Analogous studies involving irradiated fluorinated polymers such asTeflon yield similar results. Theg factor for the SiF2 spectrum is g 2.003 0.002, very similar to the value for a free electron.
(101 (11) (121 (13)
Summarv The chemical and physical properties of silicon difluoride are quite unique and indicate a wide range of possibilities for further studies. Reactions of SiF2 with other fluorides, mixed halofluorides (such as PF2I), and other compounds such as mercury alkyls (a possible route to the elusive perfluoro carbon-silicon com~ounds)afford quite interestina new svstems for study, especially asanalytical and separation techniques become more sophisticated. Investigations concerning the possibility of dimerized bridging of SiFz-analogous to the GeF2IGeF4 system-are needed; also, high temperature
(151 Langford, G.R.. Moody, D. C., and Odom, J. D., Inore. C h m . , 14.134 119751. (16) Soisn,O., andTimms,P.L.,Inorg. Chem.. 7,2157 119681. 1171 Margrave, J. L.. Sharp, K. G.,and Wilson, P. W., J. lnorp. N u r l Chpm.. 32, 1817 ,,,s,ni-, I181 Timms. P. L.. Ehlen,T.C.,Ms~rauc,J. L.. Brinekman.F. E., Fanar,T. C..andCwle. T. D., J. Amer Chem. Soc.. 87.3819 11965). (19) Liu, C. S..sndThompsm. J. C..lnorp Chem., 10.11W 119711. . 5,729 119661. (20) Bass1er.J. M..Timms, P. L.,andMargrav~.J . L . , l n o r ~Chem.. (21) Haptie, J. W., Hauge, R. H., and Margrave, J. L., J. A m m Chem Sr~r..91, 2516 (19691. (22) Rso, V. M.. Curl, R. F., Timm3.P. L..and Margrave, J. L.. J . Chem. P h y . 43,2557 i19651. (23) Khanna, V. M.. Berenbruch. G.: and Margrave. J. L.. J. Chem. P h p . 46. 2110 (19671. (24) Hopkins. H. P..Thompson, J. C..snd Margrave, .J. L.. J. Amer. Chem. Snc.. 90, 901 (19681.
= *
\.".-,. ,,O,",
Golino, C. M.,Bush,R. D.,endSommer, L. H.. J.Amer Chem. Soc., 98.614 (19741. 8harp.K. G.,and Margrave,J.L.,J.lnorg. Nucl Chem.. 33,2813 (1971). Thompson, J.C.,Margrave,J. L.,sndTimm%P.L., Chem. Cnmmun.. 5661196fil. Liu,C. S.,Msrgrave. J. L..Thompsan. J. C., and Timmr,P. L., Con. J. Chem.. 59.459 (19721. (141 L5u.C. S.. Nyhurg,S C..Sy%mmki.J.T..and Thompson. J. C.. J. Chem Snc. IDaltonl.
,.,*o ,,o,m A . * "
... .
Volume 53.Number 11, November 1976 / 699
