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Spectrophotometric study of the structure and reactivity of organic salts of hexachloroantimonic acid
Structure and reactivity of organic s~lts of hoxachloroantimoaic acid
2537
4. M. L. WALL&CH, J. Polymer Sci. A-2, 5: 653, 1967 5. A. V. PAVLOV, A. G. CHERNOVA a n d N. K. PINAYEVA, Vysokomol. soyed. BI4: 415, 1972 6. M. L. HAGGINS, J. Amer. Chem. Soe. 64: 2716, 1942 7. W. J. ARCHIBALD, J. Appl. Phys. 18: 362, 1947 8. W. J. ARCHIBALD, J. Phys. Colloid Chem. 51: 1204, 1947 9. V. M. MEN'SHOV, V. V. KORSHAK, G. I. TIMOFEYEVA and S. A. PAVLOVA, Vysokomol, soyed. AI4: 1766, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 8, 1978, 1972) 10. G. C. BERRY, H. MOMURA and K. G. MAYHAN, J. Polymer Sci. A-2, 5: 1, 1967 11. P. $. FLORY, Principles of Polymer Chemistry, New York, 1953 12. L. V. DUBROVINA, S. A. PAVLOVA, V. A. VASNEV, S. V. VINOGRADOVA and V. V. KORSHAK, Vysokomol. soyed. A12: 1308, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 6, 1484, 1970) 13. V. V. KORSHAK and S. V. VINOGRADOVA, Vysokomol. soyed. A13: 367, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 2, 415, 1971) 14. L. V. SOKOLOV, Yu. V. SHARIKOV and R. P. KOTLOVA, Vysokomol. soyed. AI2: 1934, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 9, 2190, 1970) 15. I. K. NEKRASOV and S. Ya. FRENKEL', Dokl. Akad. Nauk SSSR 203: 1354, 1972
SPECTROPHOTOMETRIC STUDY OF THE STRUCTURE A N D REACTIVITY OF ORGANIC SALTS OF HEXACHLOROANTIMONIC ACID* V. t ). VOLKOV, E. F. OLEI~K, ¥C. N. SMm~OV, B. A. Ko~-~OV, A. I. YEFREM[OVA, B. A. I~OZENBERG a n d N. S. YENIKOLOPYA~ I n s t i t u t e of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 12 March 1973) I n a study of the electronic spectra of onium and carbonium salts of hexachloroantimonic acid it has been found that the absorption band with 2max-~ 272 n m is produced b y electronic transitions in the free SbCl~ anion and in the ion pair R+SbCI~, and is independent of the nature of the cation. The cation strongly affects the stability of the SbCl[ anion. I t is shown t h a t earboxonium salts, which are active centres for polymerization of cyclic acetals or are formed in initiation of polymerization of tetrahydrofuran b y trityl hexachloroantimonate, are unstable and decompose reversibly to the molecular forms.
ORGAniC salts of hexachloroantimonic acid are used extensively as catalysts for polymerization of vinyl monomers [1-3] and heterocyclic compounds [4-9]. These salts show characteristie absorption in the ultraviolet region of the spee* Vysokomol. soyed. A16 No. 10, 2190-2195, 1974.
2538
V . P . VOLKOV et al.
t r u m a n d this has been m a d e use of for s t u d y of t h e / m e c h a n i s m o f the r e a c t i o n o f t h e active, p r o p a g a t i n g centres with various c o m p o n e n t s of the reaction s y s t e m in p o l y m e r i z a t i o n of cyclic ethers a n d acetals. The observed spectral changes were a t t r i b u t e d to change in t h e n a t u r e of the p r o p a g a t i n g cations [7]. I n the w o r k r e p o r t e d here an e x p e r i m e n t a l s t u d y has been m a d e o f the electronic a b s o r p t i o n spectra of a n u m b e r of carbonium, o x o n i u m a n d a m m o n i u m salts o f h e x a c h l o r o a n t i m o n i c acid a n d an i n t e r p r e t a t i o n of the a b s o r p t i o n b a n d s f o u n d is given. Using this assignment of the bands a s p e c t r o p h o t o m e t r i c invest i g a t i o n of t h e r e a c t i o n of these salts with linear a n d cyclic ethers a n d acetals was carried out, a n d on the basis o f the i n f o r m a t i o n o b t a i n e d some conclusions were d r a w n a b o u t the m e c h a n i s m of the individual stages of cationic polymerization o f cyclic ethers a n d acetals u n d e r the influence of organic salts of h e x a c h l o r o a n t i m o n i c acid. EXPERIMENTAL
The spectrophotometers used for the spectrophotometric investigation were SFD-2 Hitachi EPS-3 double beam instruments, using totally sealed, cylindrical quartz cells with an absorbing layer of thickness 0.1-1 cm. The cells were filled under vacuum ( < 10-a torr). Electrical conductivity was measured in special quartz cells (/= 1 cm) with platinum electrodes, using a constant current source. Tritflium salts with SbF~, SbCl~ and BF~ anions were prepared by the methods of references [9-11] respectively. Methoxymethyl hexachloroantimonate (MMHCA) was synthesized by the method of reference [12] and dioxoleninm hexaclOoroantimonate by the method of reference [5]. Triethyloxonium hexachloroantimonate (TOItCA) and tetrafluorobor~te were synthesized by Meerwein's method [13]. Tetraethylammonium and tetrabutylammonium hexachloroantimonate (TBAI-ICA) were obtained by the method of reference [14]. Tetrabutylammonium hexafluoroantimonate was prepared from tetrabutylammonium fluoride and hexafluoroantimonic acid in 40% h y d r o p h o s p h o r u s acid, m.p. 220-221 °. Antimony pentachloride (AP) was purified by fractionation i n vacuo over phosphorus pentoxide. Antimony methoxytetrachloride was prepared from AP and dimethyl sulphite [15]. Antimony diethoxytetrachloride was synthesized from AP and ethyl acetate [16]. Antimony trichloride (AT) was purified by three recrystallizations from dry carbon tetrachloride. The solvents, ethers and acetals were prepared by the methods of reference[17]. RESULTS AND DISCUSSION
E x a m i n a t i o n of the electronic spectra of the h e x a c h l o r o a n t i m o n a t e s w i t h t h e organic cations PhaC +, (C2Hs)aO + a n d (C4Hg)~N + showed t h a t all these salts give a n a b s o r p t i o n b a n d with the m a x i m u m a t 272 n m (Figs. 1 a n d 2). M o r e o v e r electronic spectra of HaO+SbCI~ a n d its inorganic salts are k n o w n [18-20] a n d these also show a b s o r p t i o n a t ),max=272 nm. F r o m these results it m a y be a s s u m e d t h a t t h e position of the b a n d of h e x a c h l o r o a n t i m o n a t e s in t h e region of 2max=272 n m is i n d e p e n d e n t of the n a t u r e of the cation. I t was f o u n d also t h a t o x o n i n m a n d a m m o n i u m salts with B F ~ a n d SbF~ anions do n o t a b s o r b in the u l t r a v i o l e t a n d visible regions of the spectrum. T h e e x t i n c t i o n coefficient of T O H C A , e=10,800=1=800, is i n d e p e n d e n t o f t h e
Structure and reactivity of organic salts of hexaehloroantimonic acid
2539
concentration of the salt in the range 2 x 10-3-2"24 x 10 -5 mole/1. (Fig. 2), which corresponds to change in the degree of dissociation over the interval 0.29-0.93 (K~)°=2"4 X 10 -~ mole/1.).
!\I
D
D
1"0 -1.1
0"5 -
b
;.8
V.\ 2 f ,,.,,
I
-
\
X~
1
3z
o.li
\\--
o.,
/ 0
260
300 ~'i(}. 1
A~ nm
3z~
I
1
I
1
21-/0
260
280
300
I
320 vT,nm
Fro. 2
(FIG. I. Absorption spectra of tritylium salts in CH2C12 at [Ph3C+A-] ~ 3 × l0 -~ mole/1., where A-=SbCI[ (1), SbF[ (2) and BF~ (3). FIG. 2. Absorption spectra (a) and the dependence of on the concentration of the salt co (b) of TOHCA (1) and TBAHCA (2) in CH2CI~ at [TOHCA]=I.8× 10-4 and [TBAHCA] = 2 × 10-4 mole/1. Similar results were obtained for TBAHCA (Fig. 2), for which e = 10,400~: 200 over the range of concentrations from 1.68×10 -3 to 2.1× 10 -5 mole/1, which corresponds to change in the degree of dissociation of the salt from 0.18 to 0.8 (K~°=0.5 × 10 -~ mole/1.)*. These results lead to the conclusion t h a t the formation of ion pairs or other ionic associations does not alter the intensity or even less the position of the band at ~.max~-~272 nm, i. e. the electronic transitions producing this band are not related to electrostatic interaction between the anionic and cationic components of the salts in ion pairs. Thus the absorption band of hexachloroantimonates with ~max=272 nm is produced by electronic.transitions in the SbCl~ anion itself (in both the free ion and in ion pairs). I t should be noted t h a t the spectral pattern of these salts is fairly substantially dependent on the nature of the cation. For example, whereas the spectra of TOHCA, t~--O+ C
SbCl~ and TBAHCA give only one, symmetrical absorption
band, the contour of which is satisfactorily described by the Gauss equation
[17], the spectra of tritylium hexachloroantimonate (THCA), (--~//--CHaSDCI~ O+ * The dissociation constants of the salts were determined by A. T. Ponomarenko.
2540
V.P. VoIa~ov et al.
and MMHCA are very complex. The spectrum of M ~ H C A has two well-defined bands at 272 and 235 nm, with e=72~--5700-~200 and e~35----2800:L400. There are two important, noteworthy facts, namely that e27~ is considerably lower t h a n for oxonium and ammonium salts and that the spectrum of this _D 1.0
0"5
!
I
220
260
I
Joo ~nm
Fm. 3. Absorpbion spectra of AP (1) and MM/-ICA (2) at concentrations of 6.2 × 10-* mole/1. salt is the same as the spectrum of A P at the same concentration (Fig. 3). The obvious conclusion from these observations is that in solution in methylene chloride MMHCA decomposes to the molecular forms according to the equation CHs--O+:CHzSbCI~ ~- CH3OCH=Cl~SbC16,
(1}
the equilibrium being almost completely displaced to the right. This conclusion is in complete agreement with the similar conclusion arrived at on the basis of kinetic data from polymerization of cyclic acetals in the presence of R+SbCI~ catalysts [21, 22]. Interpretation of the shortwave band ( ~ 235 nm), which occurs in the spectra of hexachioroantimonates that can dissociate to molecular forms, is much less certain. From this point of view study of the absorption spectra of AP and of the products of its chemical reactions with various oxygen containing compounds is of particular importance. The bands in the spectrum of AP can be explained alternatively either by absorption by the molecular (SbC15 with 2max--272 n m and SbC1s with ~max : 2 3 5 nm), or the ionic forms of the products of self-ionization (SbCl~)~ ~ SbCI~+SbCI~ (SbCl~ with 2raax:272 nra and SbC1$ with 2max----235nm)
(2)
The extinction coefficient of AP (e~7~5700~:600) is exactly half the extinction coefficient of SbCI~, which is in accord with complete ionization of A P according to eqn. (2). Furthermore the extinction coefficient of the 2max-~-235 n m
Structure and reactivity of organic salts of hexachloroantimonic acid
2541
band, calculated on AP, e----2800±400, is also half the extinction coefficient of the single band of the salt (C4I-[ ,)4N+SbCI~, with )~nax= 238 nm and ~= 5 7 ~)0~ 6 00, due to absorption b y the SbC1j anion. It should be noted that the position of the bands and the intensity of absorption of anions and cations of the same chemical type are the same as a rule [23]. Therefore this fact must a!so be taken as proof of the self-ionization of AP in solution. This reaction becomes especially probable in the presence of oxygen containing compounds, which b y solvation stabilize the rather unstable SbCl + cation. On the basis of the spectrophotometric study of organic salts of hexachloroantimonic acid conclusions can be drawn about the relative reaetivities of the cations. The extinction coefficients of the salts decrease in the following order: q-
R4N +
+
~-R30 + >Ph~C >>R--O--~CH2
(3)
It is evident that this order reflects the state of equilibrium of these salts in solution in their dissociation to the molecular forms. Whereas the first two slats are completely stable in solution the earboxonium salt is practically completely dissociated to the molecular forms, while the tritylium salt occupies an intermediate position. The ability to accept a hydride-ion b y the reaction R+q-RI--H -, R + + R - - H ,
(4)
where R + is any cation and 1~ a hydride-ion donor, increases in the same order, the first two members of the series not taking part in this reaction at all. Bearing in mind also the fact that the cation R4N + is not capable of alkylating oxygen containing compounds and that for the other members of the series this ability increases from left to right, it may be concluded that the reactivity of this series of salts, which is determined b y the electrophilie strength of th~ c-~tion, must increase in the following order: +
~-
+
+
R--O~--CH2 >Ph3C >R30 >>R4N
(5)
The results of the spectroscopic examination of the reaction of these salts with various oxygen containing compounds are in complete agreement with the information obtained in the study of the structure of the hexachloroantimonates. When TOHCA interacts with T H F the transalkylation reaction a3o÷ ÷ o / ~
~_ R _ O , \ / -
÷ R2o,
(6)
resulting in formation of an active centre, occurs, b u t there is no spectroscopic change. Similar results are obtained when this salt reacts with aeyclie ethers (diethyl ether). A different situation is found in the reaction o} T O H C h w t h cyclic a~d linear acetal~ (dioxolane, m':thylal) Here, in the presence of the acetals the strong band of TOHCA with )~max=272 nm weakens and a new band with 2max=235 nm appears (Fig. 4). These spectral changes are caused b y dis-
2542
V . P . VoLxov et al.
sociation of the carboxonium ion formed from the aeetal, to molecular forms and subsequent self-ionization of the AP R
R
(C,Hs)30+SbCh - + 0 / /
~_~C2H~--0(
\
SbCh- + (C2H5)20
\
CH~OR'
(7)
CH~OR'
,
II
H'0CthCI + SbC15~_ R --O+=CH2SbCI~- + BOC2Hs
(8)
The AP formed can be reduced to AT SbC15 -[-- 0 ~ 0
\
/
CH,
-.-,
0/'-~'X'0 q- H+SbCh \ / CH
(9)
HCI ~- S~b~CI,
&
Moreover in the presence of aeetals replacement of chlorine of the following type is possible [24]: SbCls-HROCHIOR ~- SbC14OR-~ROCH~C1
(10)
This reaction produces antimony compounds, the absorption bands of which move toward the shortwave region of the spectrum as the replacement of chlorine atoms by alkoxy groups increases [18, 20]. Similar spectral changes occur when l 2P 2"0
0.5
1.2
,.,2 /
llii5
O.q 0
zlo
I
80 T/me, m/n Fio. 4
I
120
280
280
300
320 v~ ; /7/77
FIG. 5
FIG. 4. Kinetics of change in the absorption spectrum of the system consisting of 6 × 10 -4 mole/], of TOHCA and 5 mole/], of 1,3-dioxolane in CH2C12 at 20 °, at 2-----235 nm (1) and 272 nm (2). FIG. 5. Absorption spectr~ of the THCA-MT system in CH2C12: 1--[THCA]----6.4X 10 -6 mole/].; 2--THCA-~CIT (0.1 mole/].); 3--spectrum of the system after decolourization; g - - [ T P M ] = 6 . 5 X 10 -5 mole/].; 5--residual spectrum after subtraction of the absorption of TPM from spectrum 3.
Structure and reactivity of organic salts of hexachloroantimonic acid
2543
TOHCA reacts with vinyl butyl ether and acetaldehyde. Here, as in the case of acetals, intermediate carboxonium salts are formed, the subsequent reactions of which are similar to reactions (8)-(10). I t should be noted t h a t the presence of acetals in no way affects the contour of the absorption band of RdN+SbC1 ~ . This means that the primary, transalkylation reaction does not occur in this system. This fact also shows that the mechanism of reaction of the complex anion proposed by Sims [25] is erroneous. I n our opinion the initial stage of reaction of the complex anions must be decomposition of the ion pair to molecular forms according to eqn. (8). I t is known [6] the primary stage of initiation of polymerization of T H F by tritylium salts is hydride transfer. The subsequent route of conversion of the carboxonium salt to an active polymerization centre is however debatable. For spectroscopic investigation of this reaction, in order to prevent polymerization we used the unpolymerizable homologue of T H F 2-methyltetrahydrofuran (MT). As would be expected, when MT reacts with THCA formation of the unstable carboxonium salt ~ /~--CH3SbCI~ is accompanied by its decomposition to O+ the molecular forms CtI3 /---~--CHaSbCI~-. -~-/\' - - -/~\ / \ ~"
O+
0
+ SbCls
(11)
C1
and in turn by fall in the intensity of the band with ~max=272 n m (Fig. 5). In addition to increase in the intensity of absorption in the shorbwave region in the 300-320 n m region of the spectrum a point of inversion is clearly seen, indicating the reversibility of the latter process. We would point out t h a t it is not due to absorption by triphenylmethane (TPM), because it continues after complete decolourization of the solution, when all the PhaC + has been converted to TPM. This process is accompanied b y a sharp decrease in the electrical conductivity of the system. A possible sequence of reactions that would explain this is as follows r--~--CttaSbCI~-+_~ CH3C--(CII~.--)a--CI nu S];CL
(l+ o I!
li
O
/--~
SI,Cla -I- CI]a--C--(CI [z--/a--CI-~- ~
~ _
(CH~.--)3--CI i ÷/
?--CH:, ~_ ShCI.~O - C - - O \
o
[
& &
There is practically no dissociation of the zwitter-ion thus formed, because of strong electrostatic interaction of the charges [26]. Translated by E. O. P~rIT.LIPS
2544
V . P . V o ~ o v et al.
REFERENCES 1. A. LEDV~ITH, J. ~ e r .
2. 3. 4. 5. 6. 7.
8.
9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
22.
23.
24.
25. 26.
Chem. Soe. 91: 317, 1969
C. BAWN, C. FITZSIMMONS a n d A. LEDVITH, P o l y m e r 12: 119, 1971 M. SAMBHI, Macromolecules 3: 351, 1970 S. SLOMKOWSKY a n d S. PENCZEK, Chem. Commun., :No. 20, 1347, 1970 P. KUBISA and S. PENCZEK, Makromolek. Chem. 144: 169, 1971 M. DREYFUSS, L. W E S T F A H L and P. DREYFUSS, Maeromolecules 1: 437, 1968 Ye. B. LYUDVIG, Ye. L. BERMAN, Z. N. NYSENKO, V. A. PONOMARENK0 and S. S. MEDVEDEV, Vysokomol. soyed. A13: 1375, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 6, 1546, 1971) E. F. OLEINIK, O. A. PLECHOVA, V. M. NOVOTORTSEV, L. V. KO1KPANEYETS, V. P. VOLKOV, Ye. F. RAZVODOVSKII antt N. S. YENIKOLOPYAN, Dokl. Akad. N a u k SSSR 199: 388, 1971 Yu. N. S1KIRNOV, V. P. VOLKOV, E. F. OLEINIK, B. A. KOMAROV, B. A. ROZENBERG and N. S. YENIKOLOPYAN, Vysokomol. soyed. A16: 735, 1974 (Translated in P o l y m e r Sei. U.S.S.R. 16: 4, 846, 1974) P. BOWYER, A. L E D W I T H and D. SHERINGTON, J. Chem. Soc. B: 1511, 1971 H. DAUBEN, L. HONNF~ and K. HARMON, J. Orgaq. Chem. 25: 1442, 1960 B. A. KOMAROV, Dissertation, 1973 H. MEERWEIN, J. p r a k t . Chem. 147: 257, 1937 G. COWELL and A. LEDWITH, J. Chem. Soc. B: 227, 1970 A. 1KEUWSEN and H. M~GLING, Z. anorgan, und allgem. Chem. 285: 262, 1956 R. PAUL and K. MULHOTRA, Z. anorgan, u n d allgem. Chem. 325: 302, 1963 B. A. ROZENBERG, Dissertation, 1971 G. M. DOKAR and B. Z. IOFA, R a d i o k h i m i y a 7: 25, 1965 R. WALTON, R. 1KATTHEWS and C. JORGENSEN, Inorg. Chim. A c t a 1: 355, 1967 H. NEUMANN, J. Amer. Chem. Soc. 76: 2611, 1954 Yu. N. SMIRNOV, V. P. VOLKOV, B. A. R 0 Z E N B E R G and N. S. YENIKOLOPYAN, Vysokomol. soyed. A I 6 : 283, 1974 (Translated in P o l y m e r Sci. U.S.S,R. 16: 1, 327, 1974) V. V. IVANOV, R. D. SABIROVA, O. A. PLECItOVA, G. M. TARASOVA, I. S. MOROZOVA, V. P. VOLKOV, Yu. N. SMIRNOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. B14: 743, 1972 E. STREITWIESER, Teoriya m o l e k u l y a r n y k h orbit d l y a khimikov-organikov (Theory of Molecular Orbitals for Organic Chemists). p. 213, Izd. "Mir", 1965 (Russian translation) N. I. VASIL'EV, V. I. I R Z H A K , V. I. KARTSOV1VIK and N. S. YENIKOLOPYAN, Tezisy I I I konferentsii po voprosam khimii i fiziko-khimii p r i r o d n y k h poliatsetali Abstracts of the 3rd Conference on Problems in the Chemistry and Physical Chemistry of N a t u r a l Polyacetals). p. 32, Frunze, 1971 D. SIMS, Makromolek. Chem. 98: 235, 245, 1966 Ye. V. KOCHETOV, A. A. BERLIN and N. S. YENIKOLOPYAV, Vysokomol. soyed 8: 1022, 196~ (Translated in Polymer Sci. U.S.S.R. 8: 6, 1122, 1966)
2537
4. M. L. WALL&CH, J. Polymer Sci. A-2, 5: 653, 1967 5. A. V. PAVLOV, A. G. CHERNOVA a n d N. K. PINAYEVA, Vysokomol. soyed. BI4: 415, 1972 6. M. L. HAGGINS, J. Amer. Chem. Soe. 64: 2716, 1942 7. W. J. ARCHIBALD, J. Appl. Phys. 18: 362, 1947 8. W. J. ARCHIBALD, J. Phys. Colloid Chem. 51: 1204, 1947 9. V. M. MEN'SHOV, V. V. KORSHAK, G. I. TIMOFEYEVA and S. A. PAVLOVA, Vysokomol, soyed. AI4: 1766, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 8, 1978, 1972) 10. G. C. BERRY, H. MOMURA and K. G. MAYHAN, J. Polymer Sci. A-2, 5: 1, 1967 11. P. $. FLORY, Principles of Polymer Chemistry, New York, 1953 12. L. V. DUBROVINA, S. A. PAVLOVA, V. A. VASNEV, S. V. VINOGRADOVA and V. V. KORSHAK, Vysokomol. soyed. A12: 1308, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 6, 1484, 1970) 13. V. V. KORSHAK and S. V. VINOGRADOVA, Vysokomol. soyed. A13: 367, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 2, 415, 1971) 14. L. V. SOKOLOV, Yu. V. SHARIKOV and R. P. KOTLOVA, Vysokomol. soyed. AI2: 1934, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 9, 2190, 1970) 15. I. K. NEKRASOV and S. Ya. FRENKEL', Dokl. Akad. Nauk SSSR 203: 1354, 1972
SPECTROPHOTOMETRIC STUDY OF THE STRUCTURE A N D REACTIVITY OF ORGANIC SALTS OF HEXACHLOROANTIMONIC ACID* V. t ). VOLKOV, E. F. OLEI~K, ¥C. N. SMm~OV, B. A. Ko~-~OV, A. I. YEFREM[OVA, B. A. I~OZENBERG a n d N. S. YENIKOLOPYA~ I n s t i t u t e of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 12 March 1973) I n a study of the electronic spectra of onium and carbonium salts of hexachloroantimonic acid it has been found that the absorption band with 2max-~ 272 n m is produced b y electronic transitions in the free SbCl~ anion and in the ion pair R+SbCI~, and is independent of the nature of the cation. The cation strongly affects the stability of the SbCl[ anion. I t is shown t h a t earboxonium salts, which are active centres for polymerization of cyclic acetals or are formed in initiation of polymerization of tetrahydrofuran b y trityl hexachloroantimonate, are unstable and decompose reversibly to the molecular forms.
ORGAniC salts of hexachloroantimonic acid are used extensively as catalysts for polymerization of vinyl monomers [1-3] and heterocyclic compounds [4-9]. These salts show characteristie absorption in the ultraviolet region of the spee* Vysokomol. soyed. A16 No. 10, 2190-2195, 1974.
2538
V . P . VOLKOV et al.
t r u m a n d this has been m a d e use of for s t u d y of t h e / m e c h a n i s m o f the r e a c t i o n o f t h e active, p r o p a g a t i n g centres with various c o m p o n e n t s of the reaction s y s t e m in p o l y m e r i z a t i o n of cyclic ethers a n d acetals. The observed spectral changes were a t t r i b u t e d to change in t h e n a t u r e of the p r o p a g a t i n g cations [7]. I n the w o r k r e p o r t e d here an e x p e r i m e n t a l s t u d y has been m a d e o f the electronic a b s o r p t i o n spectra of a n u m b e r of carbonium, o x o n i u m a n d a m m o n i u m salts o f h e x a c h l o r o a n t i m o n i c acid a n d an i n t e r p r e t a t i o n of the a b s o r p t i o n b a n d s f o u n d is given. Using this assignment of the bands a s p e c t r o p h o t o m e t r i c invest i g a t i o n of t h e r e a c t i o n of these salts with linear a n d cyclic ethers a n d acetals was carried out, a n d on the basis o f the i n f o r m a t i o n o b t a i n e d some conclusions were d r a w n a b o u t the m e c h a n i s m of the individual stages of cationic polymerization o f cyclic ethers a n d acetals u n d e r the influence of organic salts of h e x a c h l o r o a n t i m o n i c acid. EXPERIMENTAL
The spectrophotometers used for the spectrophotometric investigation were SFD-2 Hitachi EPS-3 double beam instruments, using totally sealed, cylindrical quartz cells with an absorbing layer of thickness 0.1-1 cm. The cells were filled under vacuum ( < 10-a torr). Electrical conductivity was measured in special quartz cells (/= 1 cm) with platinum electrodes, using a constant current source. Tritflium salts with SbF~, SbCl~ and BF~ anions were prepared by the methods of references [9-11] respectively. Methoxymethyl hexachloroantimonate (MMHCA) was synthesized by the method of reference [12] and dioxoleninm hexaclOoroantimonate by the method of reference [5]. Triethyloxonium hexachloroantimonate (TOItCA) and tetrafluorobor~te were synthesized by Meerwein's method [13]. Tetraethylammonium and tetrabutylammonium hexachloroantimonate (TBAI-ICA) were obtained by the method of reference [14]. Tetrabutylammonium hexafluoroantimonate was prepared from tetrabutylammonium fluoride and hexafluoroantimonic acid in 40% h y d r o p h o s p h o r u s acid, m.p. 220-221 °. Antimony pentachloride (AP) was purified by fractionation i n vacuo over phosphorus pentoxide. Antimony methoxytetrachloride was prepared from AP and dimethyl sulphite [15]. Antimony diethoxytetrachloride was synthesized from AP and ethyl acetate [16]. Antimony trichloride (AT) was purified by three recrystallizations from dry carbon tetrachloride. The solvents, ethers and acetals were prepared by the methods of reference[17]. RESULTS AND DISCUSSION
E x a m i n a t i o n of the electronic spectra of the h e x a c h l o r o a n t i m o n a t e s w i t h t h e organic cations PhaC +, (C2Hs)aO + a n d (C4Hg)~N + showed t h a t all these salts give a n a b s o r p t i o n b a n d with the m a x i m u m a t 272 n m (Figs. 1 a n d 2). M o r e o v e r electronic spectra of HaO+SbCI~ a n d its inorganic salts are k n o w n [18-20] a n d these also show a b s o r p t i o n a t ),max=272 nm. F r o m these results it m a y be a s s u m e d t h a t t h e position of the b a n d of h e x a c h l o r o a n t i m o n a t e s in t h e region of 2max=272 n m is i n d e p e n d e n t of the n a t u r e of the cation. I t was f o u n d also t h a t o x o n i n m a n d a m m o n i u m salts with B F ~ a n d SbF~ anions do n o t a b s o r b in the u l t r a v i o l e t a n d visible regions of the spectrum. T h e e x t i n c t i o n coefficient of T O H C A , e=10,800=1=800, is i n d e p e n d e n t o f t h e
Structure and reactivity of organic salts of hexaehloroantimonic acid
2539
concentration of the salt in the range 2 x 10-3-2"24 x 10 -5 mole/1. (Fig. 2), which corresponds to change in the degree of dissociation over the interval 0.29-0.93 (K~)°=2"4 X 10 -~ mole/1.).
!\I
D
D
1"0 -1.1
0"5 -
b
;.8
V.\ 2 f ,,.,,
I
-
\
X~
1
3z
o.li
\\--
o.,
/ 0
260
300 ~'i(}. 1
A~ nm
3z~
I
1
I
1
21-/0
260
280
300
I
320 vT,nm
Fro. 2
(FIG. I. Absorption spectra of tritylium salts in CH2C12 at [Ph3C+A-] ~ 3 × l0 -~ mole/1., where A-=SbCI[ (1), SbF[ (2) and BF~ (3). FIG. 2. Absorption spectra (a) and the dependence of on the concentration of the salt co (b) of TOHCA (1) and TBAHCA (2) in CH2CI~ at [TOHCA]=I.8× 10-4 and [TBAHCA] = 2 × 10-4 mole/1. Similar results were obtained for TBAHCA (Fig. 2), for which e = 10,400~: 200 over the range of concentrations from 1.68×10 -3 to 2.1× 10 -5 mole/1, which corresponds to change in the degree of dissociation of the salt from 0.18 to 0.8 (K~°=0.5 × 10 -~ mole/1.)*. These results lead to the conclusion t h a t the formation of ion pairs or other ionic associations does not alter the intensity or even less the position of the band at ~.max~-~272 nm, i. e. the electronic transitions producing this band are not related to electrostatic interaction between the anionic and cationic components of the salts in ion pairs. Thus the absorption band of hexachloroantimonates with ~max=272 nm is produced by electronic.transitions in the SbCl~ anion itself (in both the free ion and in ion pairs). I t should be noted t h a t the spectral pattern of these salts is fairly substantially dependent on the nature of the cation. For example, whereas the spectra of TOHCA, t~--O+ C
SbCl~ and TBAHCA give only one, symmetrical absorption
band, the contour of which is satisfactorily described by the Gauss equation
[17], the spectra of tritylium hexachloroantimonate (THCA), (--~//--CHaSDCI~ O+ * The dissociation constants of the salts were determined by A. T. Ponomarenko.
2540
V.P. VoIa~ov et al.
and MMHCA are very complex. The spectrum of M ~ H C A has two well-defined bands at 272 and 235 nm, with e=72~--5700-~200 and e~35----2800:L400. There are two important, noteworthy facts, namely that e27~ is considerably lower t h a n for oxonium and ammonium salts and that the spectrum of this _D 1.0
0"5
!
I
220
260
I
Joo ~nm
Fm. 3. Absorpbion spectra of AP (1) and MM/-ICA (2) at concentrations of 6.2 × 10-* mole/1. salt is the same as the spectrum of A P at the same concentration (Fig. 3). The obvious conclusion from these observations is that in solution in methylene chloride MMHCA decomposes to the molecular forms according to the equation CHs--O+:CHzSbCI~ ~- CH3OCH=Cl~SbC16,
(1}
the equilibrium being almost completely displaced to the right. This conclusion is in complete agreement with the similar conclusion arrived at on the basis of kinetic data from polymerization of cyclic acetals in the presence of R+SbCI~ catalysts [21, 22]. Interpretation of the shortwave band ( ~ 235 nm), which occurs in the spectra of hexachioroantimonates that can dissociate to molecular forms, is much less certain. From this point of view study of the absorption spectra of AP and of the products of its chemical reactions with various oxygen containing compounds is of particular importance. The bands in the spectrum of AP can be explained alternatively either by absorption by the molecular (SbC15 with 2max--272 n m and SbC1s with ~max : 2 3 5 nm), or the ionic forms of the products of self-ionization (SbCl~)~ ~ SbCI~+SbCI~ (SbCl~ with 2raax:272 nra and SbC1$ with 2max----235nm)
(2)
The extinction coefficient of AP (e~7~5700~:600) is exactly half the extinction coefficient of SbCI~, which is in accord with complete ionization of A P according to eqn. (2). Furthermore the extinction coefficient of the 2max-~-235 n m
Structure and reactivity of organic salts of hexachloroantimonic acid
2541
band, calculated on AP, e----2800±400, is also half the extinction coefficient of the single band of the salt (C4I-[ ,)4N+SbCI~, with )~nax= 238 nm and ~= 5 7 ~)0~ 6 00, due to absorption b y the SbC1j anion. It should be noted that the position of the bands and the intensity of absorption of anions and cations of the same chemical type are the same as a rule [23]. Therefore this fact must a!so be taken as proof of the self-ionization of AP in solution. This reaction becomes especially probable in the presence of oxygen containing compounds, which b y solvation stabilize the rather unstable SbCl + cation. On the basis of the spectrophotometric study of organic salts of hexachloroantimonic acid conclusions can be drawn about the relative reaetivities of the cations. The extinction coefficients of the salts decrease in the following order: q-
R4N +
+
~-R30 + >Ph~C >>R--O--~CH2
(3)
It is evident that this order reflects the state of equilibrium of these salts in solution in their dissociation to the molecular forms. Whereas the first two slats are completely stable in solution the earboxonium salt is practically completely dissociated to the molecular forms, while the tritylium salt occupies an intermediate position. The ability to accept a hydride-ion b y the reaction R+q-RI--H -, R + + R - - H ,
(4)
where R + is any cation and 1~ a hydride-ion donor, increases in the same order, the first two members of the series not taking part in this reaction at all. Bearing in mind also the fact that the cation R4N + is not capable of alkylating oxygen containing compounds and that for the other members of the series this ability increases from left to right, it may be concluded that the reactivity of this series of salts, which is determined b y the electrophilie strength of th~ c-~tion, must increase in the following order: +
~-
+
+
R--O~--CH2 >Ph3C >R30 >>R4N
(5)
The results of the spectroscopic examination of the reaction of these salts with various oxygen containing compounds are in complete agreement with the information obtained in the study of the structure of the hexachloroantimonates. When TOHCA interacts with T H F the transalkylation reaction a3o÷ ÷ o / ~
~_ R _ O , \ / -
÷ R2o,
(6)
resulting in formation of an active centre, occurs, b u t there is no spectroscopic change. Similar results are obtained when this salt reacts with aeyclie ethers (diethyl ether). A different situation is found in the reaction o} T O H C h w t h cyclic a~d linear acetal~ (dioxolane, m':thylal) Here, in the presence of the acetals the strong band of TOHCA with )~max=272 nm weakens and a new band with 2max=235 nm appears (Fig. 4). These spectral changes are caused b y dis-
2542
V . P . VoLxov et al.
sociation of the carboxonium ion formed from the aeetal, to molecular forms and subsequent self-ionization of the AP R
R
(C,Hs)30+SbCh - + 0 / /
~_~C2H~--0(
\
SbCh- + (C2H5)20
\
CH~OR'
(7)
CH~OR'
,
II
H'0CthCI + SbC15~_ R --O+=CH2SbCI~- + BOC2Hs
(8)
The AP formed can be reduced to AT SbC15 -[-- 0 ~ 0
\
/
CH,
-.-,
0/'-~'X'0 q- H+SbCh \ / CH
(9)
HCI ~- S~b~CI,
&
Moreover in the presence of aeetals replacement of chlorine of the following type is possible [24]: SbCls-HROCHIOR ~- SbC14OR-~ROCH~C1
(10)
This reaction produces antimony compounds, the absorption bands of which move toward the shortwave region of the spectrum as the replacement of chlorine atoms by alkoxy groups increases [18, 20]. Similar spectral changes occur when l 2P 2"0
0.5
1.2
,.,2 /
llii5
O.q 0
zlo
I
80 T/me, m/n Fio. 4
I
120
280
280
300
320 v~ ; /7/77
FIG. 5
FIG. 4. Kinetics of change in the absorption spectrum of the system consisting of 6 × 10 -4 mole/], of TOHCA and 5 mole/], of 1,3-dioxolane in CH2C12 at 20 °, at 2-----235 nm (1) and 272 nm (2). FIG. 5. Absorption spectr~ of the THCA-MT system in CH2C12: 1--[THCA]----6.4X 10 -6 mole/].; 2--THCA-~CIT (0.1 mole/].); 3--spectrum of the system after decolourization; g - - [ T P M ] = 6 . 5 X 10 -5 mole/].; 5--residual spectrum after subtraction of the absorption of TPM from spectrum 3.
Structure and reactivity of organic salts of hexachloroantimonic acid
2543
TOHCA reacts with vinyl butyl ether and acetaldehyde. Here, as in the case of acetals, intermediate carboxonium salts are formed, the subsequent reactions of which are similar to reactions (8)-(10). I t should be noted t h a t the presence of acetals in no way affects the contour of the absorption band of RdN+SbC1 ~ . This means that the primary, transalkylation reaction does not occur in this system. This fact also shows that the mechanism of reaction of the complex anion proposed by Sims [25] is erroneous. I n our opinion the initial stage of reaction of the complex anions must be decomposition of the ion pair to molecular forms according to eqn. (8). I t is known [6] the primary stage of initiation of polymerization of T H F by tritylium salts is hydride transfer. The subsequent route of conversion of the carboxonium salt to an active polymerization centre is however debatable. For spectroscopic investigation of this reaction, in order to prevent polymerization we used the unpolymerizable homologue of T H F 2-methyltetrahydrofuran (MT). As would be expected, when MT reacts with THCA formation of the unstable carboxonium salt ~ /~--CH3SbCI~ is accompanied by its decomposition to O+ the molecular forms CtI3 /---~--CHaSbCI~-. -~-/\' - - -/~\ / \ ~"
O+
0
+ SbCls
(11)
C1
and in turn by fall in the intensity of the band with ~max=272 n m (Fig. 5). In addition to increase in the intensity of absorption in the shorbwave region in the 300-320 n m region of the spectrum a point of inversion is clearly seen, indicating the reversibility of the latter process. We would point out t h a t it is not due to absorption by triphenylmethane (TPM), because it continues after complete decolourization of the solution, when all the PhaC + has been converted to TPM. This process is accompanied b y a sharp decrease in the electrical conductivity of the system. A possible sequence of reactions that would explain this is as follows r--~--CttaSbCI~-+_~ CH3C--(CII~.--)a--CI nu S];CL
(l+ o I!
li
O
/--~
SI,Cla -I- CI]a--C--(CI [z--/a--CI-~- ~
~ _
(CH~.--)3--CI i ÷/
?--CH:, ~_ ShCI.~O - C - - O \
o
[
& &
There is practically no dissociation of the zwitter-ion thus formed, because of strong electrostatic interaction of the charges [26]. Translated by E. O. P~rIT.LIPS
2544
V . P . V o ~ o v et al.
REFERENCES 1. A. LEDV~ITH, J. ~ e r .
2. 3. 4. 5. 6. 7.
8.
9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
22.
23.
24.
25. 26.
Chem. Soe. 91: 317, 1969
C. BAWN, C. FITZSIMMONS a n d A. LEDVITH, P o l y m e r 12: 119, 1971 M. SAMBHI, Macromolecules 3: 351, 1970 S. SLOMKOWSKY a n d S. PENCZEK, Chem. Commun., :No. 20, 1347, 1970 P. KUBISA and S. PENCZEK, Makromolek. Chem. 144: 169, 1971 M. DREYFUSS, L. W E S T F A H L and P. DREYFUSS, Maeromolecules 1: 437, 1968 Ye. B. LYUDVIG, Ye. L. BERMAN, Z. N. NYSENKO, V. A. PONOMARENK0 and S. S. MEDVEDEV, Vysokomol. soyed. A13: 1375, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 6, 1546, 1971) E. F. OLEINIK, O. A. PLECHOVA, V. M. NOVOTORTSEV, L. V. KO1KPANEYETS, V. P. VOLKOV, Ye. F. RAZVODOVSKII antt N. S. YENIKOLOPYAN, Dokl. Akad. N a u k SSSR 199: 388, 1971 Yu. N. S1KIRNOV, V. P. VOLKOV, E. F. OLEINIK, B. A. KOMAROV, B. A. ROZENBERG and N. S. YENIKOLOPYAN, Vysokomol. soyed. A16: 735, 1974 (Translated in P o l y m e r Sei. U.S.S.R. 16: 4, 846, 1974) P. BOWYER, A. L E D W I T H and D. SHERINGTON, J. Chem. Soc. B: 1511, 1971 H. DAUBEN, L. HONNF~ and K. HARMON, J. Orgaq. Chem. 25: 1442, 1960 B. A. KOMAROV, Dissertation, 1973 H. MEERWEIN, J. p r a k t . Chem. 147: 257, 1937 G. COWELL and A. LEDWITH, J. Chem. Soc. B: 227, 1970 A. 1KEUWSEN and H. M~GLING, Z. anorgan, und allgem. Chem. 285: 262, 1956 R. PAUL and K. MULHOTRA, Z. anorgan, u n d allgem. Chem. 325: 302, 1963 B. A. ROZENBERG, Dissertation, 1971 G. M. DOKAR and B. Z. IOFA, R a d i o k h i m i y a 7: 25, 1965 R. WALTON, R. 1KATTHEWS and C. JORGENSEN, Inorg. Chim. A c t a 1: 355, 1967 H. NEUMANN, J. Amer. Chem. Soc. 76: 2611, 1954 Yu. N. SMIRNOV, V. P. VOLKOV, B. A. R 0 Z E N B E R G and N. S. YENIKOLOPYAN, Vysokomol. soyed. A I 6 : 283, 1974 (Translated in P o l y m e r Sci. U.S.S,R. 16: 1, 327, 1974) V. V. IVANOV, R. D. SABIROVA, O. A. PLECItOVA, G. M. TARASOVA, I. S. MOROZOVA, V. P. VOLKOV, Yu. N. SMIRNOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. B14: 743, 1972 E. STREITWIESER, Teoriya m o l e k u l y a r n y k h orbit d l y a khimikov-organikov (Theory of Molecular Orbitals for Organic Chemists). p. 213, Izd. "Mir", 1965 (Russian translation) N. I. VASIL'EV, V. I. I R Z H A K , V. I. KARTSOV1VIK and N. S. YENIKOLOPYAN, Tezisy I I I konferentsii po voprosam khimii i fiziko-khimii p r i r o d n y k h poliatsetali Abstracts of the 3rd Conference on Problems in the Chemistry and Physical Chemistry of N a t u r a l Polyacetals). p. 32, Frunze, 1971 D. SIMS, Makromolek. Chem. 98: 235, 245, 1966 Ye. V. KOCHETOV, A. A. BERLIN and N. S. YENIKOLOPYAV, Vysokomol. soyed 8: 1022, 196~ (Translated in Polymer Sci. U.S.S.R. 8: 6, 1122, 1966)