7232 were low. Therefore, a quantitative analysis was made, on a fresh reaction mixture, making use of Beer's law as applied to cerrain distinct bands in the ir spectra of the mixture and pure samples of the products.I4 The bands employed were 820 and 1690 cm-1 (mesitoin and mesitil) and 852 cm-l (all three compounds). The percentage yields thereby obtained were 45, 2, and 53% for mesitoin, mesitil, and the other compound, respectively. The other compound was shown to be identical with the dimeric compound reported by Fuson from hypochlorite oxidation of 2,2-dimesitylvinyl a l c ~ h o lIts . ~ ir spectrum showed a carbonyl band at 1720 cm-I, but no hydroxyl band. Its nmr spectrum and characterization as the dimer (XV) of radical XI1 were discussed earlier. When a solution of the dimer approached room temperature, it lost its color, but the color reappeared when the solution was warmed to 45' or higher. Ozonation of 2,2-dimesitylvinyl alcohol at - 7 8 O gave a faint pink color, but no dimer was observed. The major product was mesitoin (XIII).
Acknowledgment. This work was supported by grants from the Robert A. Welch Foundation (F-042) and the N a tional Science Foundation (GP-28187) for which the authors are grateful. They also thank Professor Allen J. Bard and associates for help and advice with the epr studies.
References and Notes
(1) P. S.Bailey, F. E. Potts. 111, and J. W. Ward, J. Amer. Cbem. Soc., 92, 230 (1970). (2)P. S.Bailey, J. W. Ward, R. E. Hornish. and F. E. Potts. 111, Advan. Cbem. Ser., No. 112,l(1972). (3) P. S. Bailey and A. G. Lane, J. Amer. Cbem. Soc., 89,4473 (1967). (4)R. C. Fuson, D. H. Chadwick, and M. L. Ward, J. Amer. Chem. SOC.,68, 389 1 1946). (5) P.-D.'Bartlett and T. Funahashi, J. Amer. Chem. Soc., 84,2596 (1962). 1 61 P. S.Bailev. J. E. Keller. and T. P. Carter.. Jr... J. Ora Cbem... 35., 2777 ~, (1970). (7)P. S. Bailey, J. W.Ward, T. P. Carter, Jr., E. Nieh, C. M. Fischer. and A. Y. Khashab, J. Amer. Cbem. Soc., 96,6136(1974). I .
I
(8) R. Trambarulo, S.N. Ghosh, C. A. Burrus, Jr.. and W.Gordy, J. Chem. Phvs.. 21.. 851 , ~ _ 11953). R . C. Fuson. P. L. Sodhwick, and S. P. Rowland. J. Amer. Chem. SOC., I
I
66,1109 (1944). R. C. Fuson, N. Rabjohn, and D. J. Byers, J. Amer. Cbem. Soc., 66,
1272 (1944). P. S.Bailey, T. P. Carter, Jr., C. M. Fischer, and J. A. Thompson, Can. J. Chem., 51, 1278 (1973). A. M. Reader, P. S. Bailey, and H. M. White, J. Org. Chem., 30, 784
(1965). The authors thank Professor Nathan L. Bauld for the use of this cell. R. T. Conley, "Infrared Spectroscopy," Allyn and Bacon, Boston, Mass., 1966,pp 203-207.
Electron Paramagnetic Resonance Studies of Ion Pairs in Solutions of Cation Radicals Gustavo Goez-Morales and Paul D. Sullivan* Contribution f r o m the Department of Chemistry, Ohio University, Athens, Ohio 45701. Received May 10, 1974
Abstract: Effects due to the formation of ion pairs between the 1,2,4,5-tetramethoxybenzene(TMB) cation radical and halide anions in solution have been observed by esr spectroscopy. In particular the g shift (gfree- gionpair) is much greater in iodine oxidized systems than in bromine oxidized systems. The g shift also increases as the solvent becomes less polar. A consideration of the oxidation mechanism suggests that the counteranion is 13- for iodine oxidized systems and is a mixture of Br3- and Br- for bromine oxidations. The effects of halogen concentration, addition of halide ions, and temperature are also indicative of the existence of equilibria, possibly of the contact ion pair F! solvent separated ion pair variety.
The literature concerned with the study of anion radicalmetal cation ion pairs (A-M+) by esr spectroscopy is considerab1e.l By contrast there is almost no work in the literature on cation radical-counteranion ion pair (C+X-) interactions. There has been one study in which direct effects on the esr spectrum of C+X- ion pairs have been postulated,* and there have been two studies in which ion pair formation has been suggested as effecting information derived from esr ~ p e c t r a .There ~ , ~ is also a report of the interaction of halide ions with metalloporphyrin cation radicals in which hyperfine splitting from the halide ions has been detected, although the species present in these studies are thought to be complexes rather than ion pairs.5 Due to this obvious lack of information on C+X- ion pairs and because of the current growing interest in the chemistry of cation radicals,6 as well as their importance as intermediates in many oxidation processes (see for example ref 7-10), it seemed important to explore the possibility, given the right set of conditions, of observing effects in the esr spectrum of a cation radical due to ion pair formation. Our first consideration was how to optimise conditions so that effects due to ion pairing would be most likely to occur. In general, cation radicals have been prepared by oxidation Journal of the American Chemical Society
/ 96:23 /
with compounds such as H2SO4, AICI3, SbC15, BF3, etc., in solvents of high dielectric constant (CH3N02, CH3CN, CF3COOH). Under these conditions not only are the solvents likely to strongly solvate the ions but the counteranion is probably a complicated species. Obviously one needs to carry out the oxidations in solvents of lower dielectric constant with an oxidant which will produce a relatively simple anionic species, if one is to observe any effects due to ionpair formation. The oxidants of choice are therefore molecular bromine or iodine, since they are known to act as oneelectron oxidants toward some easily oxidized compounds and the counteranions should be relatively simple species. The compound to be oxidized, 1,2,4,5-tetramethoxybenzene (TMB), was selected for several reasons. The oxidation potential of T M B is 0.82 V1I which indicates that oxidation by bromine, a t least, should be successful. In addition the cation radical of T M B had been previously investigated by electrochemical oxidation1' and by chemical oxidation in H2S04 or AlC13-CH3N0212 and had been shown to be a relatively stable species as compared to other cation radicals. The esr spectrum of TMB*+ is also relatively simple consisting of 13 groups of triplets (aH" = 2.20, a H C H = 0.85 G) and is easily recognizable (see Figure 1).
November 13. 1974
7233 Experimental Section 1,2,4,5-Tetramethoxybenzene(TMB) was prepared by treatment of p - benzoquinone with fused ZnClz in absolute methanol to obtain 2,5-dimethoxy-l,4-benzoquinonewhich was then converted to 2,5-dimethoxyhydroquinone as described by Baker.I3 The product was then treated with dimethyl sulfate as described by Benington, et a/., I 4 to give TMB (mp 102-103O, lit. 1O2.5OI3), which was further purified by sublimation. An alternative source of TMB was as an impurity in commercially available 1,2,4-trimethoxybenzene. The solvents used in this study were all the best available commercial samples which were dried over molecular sieves, distilled, and carefully degassed by repeated freeze-pumpthaw cycles before use. Samples were prepared in the usual way for aluminum chloride oxidations.Is When bromine was used as oxidant, solutions of TMB in the desired solvent were prepared and degassed in the esr cell. Bromine was also degassed in a separate container on the vacuum line; this was connected to an evacuated compartment whose volume (1.05 cm3) represented 1 unit of bromine in our study. Bromine vapor at room temperature was allowed to_ flow freely into this compartment and was then transferred 'through the vacuum line to the frozen TMB solution. Initially the minimum number of bromine units necessary to produce a detectable and resolved esr signal was transferred, and then, as required, excess bromine was added. Oxidation with iodine was achieved by preparing saturated solutions of iodine in the different solvents. To these or diluted solutions one or two drops of 1,2,4-trimethoxybenzene (containing TMB as impurity) were added. The esr spectra were recorded on a Varian E-15 x-band spectrometer. The g factors were measured in a dual sample cavity using the perylene radical anion (g = 2.002671 i 0.000003)'6 as a standard. The field difference between the two cavities was small (ca. 0.02 G), and although no attempt was made to obtain absolute g values, relative values being of primary interest, the numbers quoted should be within 50.00002 of the absolute values. The magnetic field was typically scanned at 0.3 G/min with chart speeds corresponding to 0.066 G/cm; these conditions gave a shift in g of 3.9 X per centimeter shift of the center of the spect r u m relative to the center of the perylene spectrum. For each sample at least four spectra were recorded and the g values reported are the average of the four measurements. The absorption spectra were measured at room temperature with a Cary-14 spectrophotometer, using a 1-cm path length cell. In some measurements, a special accessory was used so that the solution containing TMB.+ radicals could be transferred directly from the esr cell to a uv-visible air-free cell.
Figure 1. The esr spectrum of the 1,2,4,5-tetramethoxybenzenecation radical in AICI3-CH3NOz at -20'.
Table I. g Shifts and Line Widths for TMB. + Oxidized with Aluminum Trichloride in the Appropriate Solvent (T = - 20") Solvent
g"
Chloroform 1-Chloropropane Dichloromethane 1,2-Dichloroethane 1,l-Dichloroethane n-Heptyl cyanide Hexanenitrile Valeronitrile Butyronitrile Proprionitrile Acetonitrile 1-Nitrohexane 1-Nitropentane 1-Nitrobutane 1-Nitropropane Nitroethane Nitromethane
2.004030 2.003944 2.003940 2.004045 2.004039 2.003979 2.003977 2,003984 2.003960 2.003970 2.003980 2.003956 2.003952 2.004010 2.003954 2.003998 2,003959
~ g (b~ 1 0 5 )
7.1 + 0.5 -1.5 i 0 . 5 - 1 .9 i 0 . 2 8.6 f 0 . 2 8 , O f 0.2 2.0 i0 . 2 1.8 i0.2 2.5 f 0.3 0.2 f 0.2 1 . 1 5 0.2 2.1 i 0 . 2 - 0 . 3 1 0.2 -0.7 i 0 . 6 5.1 i. 0 . 2 -0.5 f 0 . 2 1.2 f0.3 0
Line widthc (G) 0.080 0.117 0.141 0.051 0.041 0.069 0.049 0.059 0.044 0.048 0.045 d d d d 0.048 0.040
ORelative to perylene radical anion g = 2.002671. b A g = g(AIC13-nitromethane) - g(A1C13-solvent), error limits represent the errors involved in the determination of A g from four measurements. Peak to peak line widths. d Not measured.
i and the y's are factors which depend upon both the spinorbit coupling parameters and the orbital energy of the unpaired electron. One can illustrate the validity of this expression by calculating g for TMB.+ from the known g value of the related compound p - dimethoxybenzene18 3.45 3 G, where( D M B ) . g ( D M B ) = 2.00368 and a H o ~= ~ as a H ~ = 2.21 ~ G~ for TMB. , T h e expressions for A g F are
Results and Discussion Initial Observations. Our initial experiments were concerned with a comparison of the TMB.+ esr spectra when oxidized with aluminum trichloride, bromine, or iodine in a variety of solvents of different polarity. The results a r e presented in Tables I and 11. The major difference found in these experiments was the change in g factor measured for TMB-+. T h e hyperfine splittings were relatively invariant to the conditions employed (to within a few per cent), and no attempt was made to accurately measure them. In Tables I and I1 the A g values listed represent the shift in g between the value for TMB-+ in the solvent-oxidant system of interest minus the value for TMB.+ in AIC13-CH3N02. This latter value wa- chosen as a reference since the conditions were thought to be most favorable for the existence of solvated "free" ions in solution. This g value of 2.003959 f
AgF(TMB) - 4 ( 2 . 2 1 ) ~ , AgF(DMB) z(3.45)~~ which if the yo's are taken to be equal gives AgF(TMB) = 17.7 or g ( T M B ) = 2.00409, in reasonable agreement with the experimental value of 2.00396, considering the approxi-
0.000003 is still quite different from the free spin value and this residual difference can be attributed to contributions from spin density on the oxygen atoms. As shown by Stone17 the deviation of the g factor from the free spin value in a series of related radicals can be written a s the summation of contributions from the different atoms in the molecule. Thus for TMB-+, AgF = Zopoyo Zcpcyc,where AgF = gfree,on gelectron and pi is the spin density on a t o m
mations made. Returning to Table I it is seen that Ag for AIC13 oxidized systems is quite small. Since it is believed that the effect of the counteranion in these systems is negligible this implies that the effects observed a r e attributed to the solvents, which have only a small effect on the g factor of TMB-+. On oxidation with bromine the observed g shifts a r e much larger than t h e shifts in AIC13 oxidized systems, espe-
-
+
Goez-Morales, Sullivan
given as AgF(DMB) = 2PoYo i-ZPcYc AgF(TMB) = 4P0Y0 + Z P J C Since the contributions of the oxygen atoms a r e much greater than those of the carbon atoms (spin-orbit coupling = 151, tC= 28 cm-I), one can neglect the conconstants tributions from the carbon atoms. If one further assumes that pori is proportional to a " O C H ~ ,one obtains
to
/
Esr Studies of Ion Pairs in Solutions of Cation Radicals
7234 Table 11. g Shifts and Line Widths for TMB. Oxidized with Bromine and Iodine in the Appropriate Solvent ( T = -20”) +
-Dielectric Solvent Chloroform 1-Chloropentane 1-Chloropropane Dichloromethane 1,2-Dichloroethane 1.I-Dichloroethane u-Heptyl cyanide Hexanenitrile Valeronitrile Butyronitrile Propionitrile Acetonitrile 1-Nitrohexane 1-Nitropentane 1-Nitrobutane 1-Nitropropane Nitroethane Nitromethane
-20” 5.58 b b 10.85 13.25 13.72 b b 24.2 b b 43.55 h b b b b 45.70
constant5-+20” (or as indicated) 4.80 6 . 6 (11) 7.7 9.1 10.6 10.45 13.89(25) 17.26(25) 19.71 ( 2 5 ) 20.3 (21) 27.2 37.5 b b b 23.2(30) 30.0 (18) 39.40
-g
Bromine oxidations----Line widthc Ag(X105) (GI
2.004656 2.004755 2.004520 2.004316 2.004108 2.004371 2.004320 2.004285 2.004156 2.004213 2.004086 2.004001 2.004022 2.004020 2.004004 2.004009 2.003983 2.003978
77.8 =k 0 . 2 79.610.2 56.1 1 . 0 . 2 3 5 . 4 10.2 14.9 1 0 . 2 41.2f0.3 36.1 + 0 . 1 32.6 1 0 . 4 19.7 i 0 . 6 15.4 4Z 0 . 2 12.7 f 0 . 2 4 . 2 i0 . 2 6 . 3 4Z0.2 6.1i O . 1 4.5 1 0 . 2 5.010.2 2.4 i 0 . 2 1.910.2
0.252 0.302 0,178 0,326 0.134 0,216 0.153 0.091 0.280 0.234 0.047 0.076 0.249 0.075 0,064 0,093 0.064 0.047
--g
2.006631 2.005791 2.005570 2.005473 2.005203 2.005311 2.004511 2.004440 2.004608 2.004501 2.004711 2.004361 2.004500 2.004490 2.004469 2.004299 2.004238 2.004128
Iodine oxidations----Ag ( X 105)
267.2i0.4 183.2 i 1 .O 161.1 1 1 . 9 151.4i0.5 1 2 4 . 4 10 . 3 135.2 i 0 . 3 5 5 . 2 1 0.2 48.110.3 64.9 1 0 . 3 54.21.0.2 75.21.0.2 40.2 i 0 . 2 54.1 i 0 . 2 53.14Z0.2 51.0iO.2 34.0~k0.2 27.9 4Z 0 . 2 16.9 1 0 . 2
Line widthc 0.502 0.441 0.438 0.646 0.387 0.486 0.381 0.224 0.200 0.247 0.237 0.127 0.336 0.188 0,288 0.384 0.154 0.088
a From Landolt-Bornstein, Band 11, Teil 6; Riddick and Bunger, “Organic Solvents,” Wiley-Interscience, New York, N.Y.. 1970. Not available. c 3: 10
cially in the solvents of lower dielectric constant. The results listed in Table I1 represent the g shift observed when a minimum amount of bromine has been added to the solventTMB mixture so t h a t a signal of reasonable intensity and resolution is obtained. Oxidation with iodine gave even larger g shifts as shown in Table 11. The actual oxidation was in itself initially somewhat surprising since the standard reduction potential of 1211- is 0.535 V which would suggest that iodine should not be capable of oxidizing TMB to any extent. However, when saturated solutions of iodine in the appropriate solvents a r e used, esr signals corresponding to TMB.+ a r e detected. The results in Table I1 correspond to the oxidation of TMB with saturated solutions of iodine in the appropriate solvent. T h e actual mechanism of this oxidation will be considered later. Several general observations can be made about the results in Tables I and 11. The observed g shift in a given solvent always follows the order 12 oxidation > Br2 oxidation > AlC13 oxidation. T h e g shifts are approximately the same for all solvents in AlC13 oxidations; in Br2 and 12 oxidations the g shifts generally increase as the dielectric constant of the solvent decreases. Plots of Ag vs. e for Br2 and I2 systems are, however, fairly scattered and a r e not reproduced. (Plots of Ag vs. Z or Ag vs. E T where the appropriate Z and E T values are available are also somewhat scattered.) T h e line widths show no apparent trends in the AlC13 oxidized systems; however, for Br2 and 12 systems the line widths tend to increase as e decreases and the line widths for 1 2 systems are generally larger than those of Br2 systems. All of these results a r e consistent with the existence of ion-pair interactions between TMB.+ and a halide anion, these interactions becoming stronger in solvents of lower solvating power. g Shifts in Ion Pairs. T h e large positive g shifts observed above are, a s first suggested by S y m ~ n s , ~precisely ,’~ what one would expect t o observe on ion-pair formation between cation radicals and halide anions. It is perhaps useful a t this point to compare the effects of ion-pair formation for anion radicals and alkali metal cations ( A - M + ) t o t h a t of cation radicals and halide anions (C+X-). It is well known t h a t in the case of A-M+ ion pairs the most obvious possible effect on the esr spectrum is the appearance of additional splittings due to a hyperfine interaction of the metal ion.’ This splitting arises from the presence of spin density in a n alkali
Journal of the American Chemical Society
/
96:23
metal s orbital and is most simply visualized a s arising from an electron transfer from A- to M+. In the case of C+Xion pairs one requires the transfer of spin density from Xto C+ leaving a hole in the p shell of the halide. Before any hyperfine interaction could occur t h e inner s electrons would have to be polarized. It is therefore not likely t h a t hyperfine splittings from X- would be detected in C+X- ion pairs except under unusual conditions. It is also known that the formation of A-M+ ion pairs can result in g shifts from the “free ion” g value. It should be noted, however, that electron transfer into the valence s orbital of the alkali metal will not itself affect the g value. If the molecular orbital of t h e unpaired electron does have some small contribution from the metal pz orbital then coupling to the metal p, and py orbitals will result in a shift of the g value. The shifts of t h e diagonal elements of t h e g tensor a r e calculated to be20
where K is a factor depending upon the amount of mixing, 5 is the spin-orbit coupling constant for t h e metal atom, 1c, is the ground state wave function and 1c,,,’s a r e the excited state wave functions, and E , and Eo a r e the energies of t h e excited and ground state orbitals. Considering only the p,, pp and pz orbitals one finds Ag = K K { / ( E , - E . x ) .Since { and K a r e positive quantities and since one considers the p; orbital to be perturbed by ion pairing so that E , < E , = E,”, the g shift is therefore predicted t o be negative and should be roughly proportional t o the spin-orbit coupling constant. This was shown to be the case for the alkali metal naphthalene anion pairs2’ a s indicated in Table 111. Considering the g shifts of C+X- ion pairs one notes that in this case electron transfer occurs directly out of the filled p shell of the halide ion resulting in a much stronger coupling of the p, and py orbitals. Additionally the energy term E , E , must now become positive since we a r e dealing with a hole in the p shell. The g shift is therefore expected to be positive and much larger than that of the A-M+ ion pairs since not only does the electron transfer take place directly out of the p shell but also t h e spin-orbit coupling constant of a halide ion such a s bromine or iodine is much larger than that of an alkali metal ({B~ = 2400, €1 = 5050
1 November
-
13, 1974
7235 Table In. g Shifts for Naphthalene Anion Radicals with Various Alkali Metals in Dimethoxyethanea
Spin-orbit coupling constant of cation (cm-l)
~ g (x b 105)
Cation Li-
0.29
-0.07 -1.13 -1.19 -4.84 -18.66
Na+ K+ Rb+ Cs+
11.5 38.5 158.4 369.4 ~~
~~~
Taken from ref 21. * A g = g(free ion) - g(ion pair). cm-l). The results in Table I1 are entirely consistent with this reasoning, showing large positive g shifts, with the TMB-12 shifts being much larger than the TMB-Br2 shifts. The structure of the TMB.+X- ion pair is intuitively expected to be similar to the structure of benzene-halogen charge transfer complexes,22that is with halogen anion situated above the plane of the benzene ring and able to overlap with the p-T orbitals on the carbon and oxygen atoms. This presumes that the TMB-+ molecule is a planar species; this is substantiated by the magnitude and temperature independence of the methoxyl group proton splitting constant since it is known that nonplanar methoxyl groups have greatly reduced splitting constants and are generally quite temperature dependent. l 8 The actual structure of TMB-+ is unknown but is probably in a trans-trans configuration. Oxidation Mechanism and Nature of the Halide Anions. In view of the somewhat unexpected oxidation of TMB by iodine, it became apparent that some consideration should be given to the mechanism of the oxidation and the nature of the halide ion in solution. The half-wave and standard oxidation potentials have been reported by several workers' for T M B and for the halogen-halide or halidehalide couples that are presumably involved in the oxidation of TMB. Thus for example the standard potentials vs. a saturated calomel electrode have been reported in acetonitrile23 as Br21Br- = 0.7 V, Brz(Br3- = f l . 1 V, 1211- = +0.26 V, and 12113- = +0.65 V. The actual half-wave potentials in a given solution may of course change considerably with the concentration of the ions in the solution. Thus Popov and D e ~ k i nhave ~ ~ shown that iodine forms 1:l complexes with acetonitrile with a formation constant of 0.57; the complexes then dissociate in solution according to eq 2 1923-24
CHsCN CH3CN.1,
+
1, f
CH3CN.1,
* (CHYCNI)'
I- + I,
-13
+
(1) I-
(2)
(3)
and 3. Reaction 1 is slow and steps 2 and 3 are rapid. Further studies24 showed that in a typical solution of 12 in CH3CN only 0.9% of the total iodine is present as 13- immediately after preparation. From the formation constant for 13- in acetonitrile, 106,6,23the concentration of I- in a 0.1 M solution is ca. 2.25 X M . These ionic concentrations give a half-have potential of 0.89 V for the 12-13couple. This compares with a half-wave oxidation potential of 0.81 V for TMB in CH3CN." One can therefore justify the oxidation of TMB by iodine in acetonitrile if the anionic species produced in solution is 13-. In the case of the bromine oxidations it is reasonable to assume from the potentials reported that both Br- and Br3- ions may be present in solution, the relative amounts of each being dependent on the initial concentration of molecular bromine. One presumes that similar considerations apply to the other solvents studied. A plausible oxidation mechanism of TMB by iodine may then be proposed as follows. The first step (eq 4) represents Goez-Morales, Sullivan
TMB
+
12
(4 1
a TMB.1,
TMB.1, + 12 TMBs'I3TMB.7; + S e TMB*'(s)
+ I. + 13-(s)
(5) (6 1
the formation of a charge transfer complex (an outer complex in Mulliken's terms26). This complex reacts with another molecule of iodine which acts as an auxilary acceptor to produce the TMB radical cation and the 13- ion.27 Equation 6 then represents the solvation of these species in solution. In support of these ideas some absorption spectra of TMB-12 systems showed the presence of bands from 13(A,, 360 nm) and from TMB.+ (A,, 450 nm)) in the more polar solvents. In less'polar solvents bands were seen which might be attributed to the charge-transfer complex. More quantitative studies of the absorption spectra are in progress. Additional support is also provided by the fact that several compounds are known to form solid paramagnetic semiconducting complexes with iodine having stoichiometries of 2 mol of compound to 3 mol of iodine (e.g., perylene,28 p h e n ~ t h i a z i n e and , ~ ~ carotene.30 It seems likely that the complexes also involve the 13- ion (eq 7). 2M
+
312 G ZM*'I,-
(71
It might also be noted that the mechanism outlined is similar in principle to the mechanisms proposed for the oxidation reactions of cobalt(II1) trifluoroacetateiO and A1C1331 and seems therefore to suggest a general mechanism for one-electron oxidations leading to cation radicals. Effect of XZ Concentration. Increasing amounts of Brz beyond the minimum resulted in larger g shifts, being particularly pronounced for the solvents of higher dielectric constant. For example doubling the amount of Br2 in butyronitrile resulted in an increase of Ag from 15.4 to 26.9 X while doubling the bromine concentration in chloroform resulted in a negligible change in Ag. The addition of excess bromine tended to decrease the line widths in solvents of higher polarity while in solvents of lower polarity the line widths remained about the same. An interesting observation was the presence of additional splittings in the esr spectra (see Figure 2) on the addition of excess bromine to TMB solutions in methylene chloride, n- heptyl cyanide, hexanenitrile, valeronitrile, butyronitrile, nitrobutane, nitropropane, and nitromethane. The origin of these splittings has not been definitely attributed to the halide ions, although this is certainly an attractive p ~ s s i b i l i t y However, .~~ the splittings may also arise from secondary radicals. Further work is required to clarify this point. The effect of 1 2 concentration on the spectra is similar to Br2. Thus increased I2 concentrations in low polarity solvents resulted in little change in Ag or in the line widths. In high polarity solvents, increased concentrations of I 2 caused increases in Ag (i.e., doubling the 12 concentration in butyronitrile changes Ag from 35.1 to 54.2 X and also caused increased line widths. The above results are certainly indicative of equilibrium shifts possibly represented by eq 5 or 6 or by shifts between ion pairs of the type (eq 8)
*
TMB**XTMB*'(s)X' e TMB*'(s) + X-(s) (8) "contact" "solvent-separated'' "free ions" where X- may represent I-, Br-, 13-, or Br3-. This type of equilibria has of course been observed previously for anion radical ion pairs.] Effect of Added X-. If trihalide anions X3- are indeed the counterions in solution, then addition of X- ions to the TMB-X2 solutions should increase the concentration of X3- (eq 9). This in turn should cause the equilibrium repre-
x, + x-
= x3-
(9)
/ Esr Studies of Ion Pairs in Solutions of Cation Radicals
7236
-----------?%'.'-------?,?,'\
-
x Figure 2. The esr spectrum (lower) o f the 1,2,4,5-tetramethoxybenzenie-bromine system in methylene chloride at -20°
varies smoothly from 80.1 X lo-' at +20° to 74.2 X lo-' at -60'. The shift is well represented as a linear function of the dielectric constant as shown in Figure 3. This strongly suggests that there is a rapid equilibrium in this solution between contact and some sort of solvent separated ion pair. Interestingly the line width seems to pass through a maximum at about -20', decreasing somewhat on either increasing or decreasing the temperature from this value. However, Ag does not always decrease with decreasing temperature; for example, in TMB-Br2-hexanenitrile Ag is found to increase from 32.9 X at +20° to 37.1 X 10-j at -40'. Obviously the effects occurring in different solvents must be studied independently.
I
I
72
'
50
52
54
56
58
60
62
64
66
Figure 3. A plot of Ag vs dielectric constant o f chloroform at various temperatures for the TMB-Brz-chloroform system
sented by eq 8 to move toward the contact ion pairs thus resulting in an increase in Ag. Attempts to add sodium bromide to TMB-+-Br2 solutions were unsuccessful due to the low solubility of the bromide. However, the addition of sodium iodide to the TMB-Br2 system in nitropropane was successful. The spectra showed increasing Ag values as the amount of added N a I increased; thus for TMB-Br2 Ag was 15.3 X and this value increased with added N a I to 63.9 X before the signal finally disappeared. One presumes that the polyhalide ion Br2I- is formed under these conditions ( K ffor Br2I- 37033),and therefore in addition to moving the equilibrium more toward the contact ion pairs the presence of the iodine atom also causes a larger g shift. On adding sufficient I- the TMB.+ is finally reduced and the esr signal disappears. The addition of NaI to TMB-12 solutions always resulted in an increase in Ag, of about 20-80%, the percentage increase being smaller for the lower polarity solvents. The effect of the added I- on the line width depended on the solvent being used; for dichloromethane, 1,l-dichloroethane, and 1,2-dichloroethane the line width was reduced (i.e., in I,]-dichloroethane from 0.60 to 0.40 G), whereas for solvents of higher polarity the line width either increased or remained the same on addition of NaI. Effect of Temperature on Ag. A change in temperature is expected to shift any equilibria occurring in the solution; it may also cause a drastic change in the structure of the ion pair. The former situation is likely to result in a small but smooth change in Ag, whereas the latter might cause a large and perhaps discontinuous change. Initial experiments on the TMB-Br2 system in chloroform indicate that the former situation prevails. Thus Ag Journal of the American Chemical Society
Conclusions The above results strongly support the existence of ion pairs in solutions of the T M B cation radical with halide anions. The nature of the counterion is thought to be 13- in the case of iodine and possibly a mixture of Br3- and Br- in the case of bromine. The experiments describing the effects of added halogen, halide ion, or temperature, while qualitative, are indicative bf the occurrence of various possible types of equilibria. No attempt has been made to rigorously define these equilibria at this stage although possibilities have been mentioned. It is anticipated that more quantitative measurements in the future will address themselves to this problem in detail. Acknowledgments. Acknowledgment is made to the Research Corporation and to the Ohio University Research Institute for partial support of this research. References and Notes (1) See for example the recent reviews by M. C. R. Symons and by J. L. Sommerdijk and E. DeBoer in "Ions and Ion Pairs in Organic Reactions," Vol. 1, M. Szwarc, Ed., Wiley-lnterscience, New York, N.Y., 1972. (2) D. Romans, W. H. Bruning, and C. J. Michejda, J. Amer. Chem. SOC., 91, 3859 (1969). (3) M. K. Ahn and C. S. Johnson, J. Chem. Phys., 50, 632 (1969). (4) S. P. Sorensen and W. H. Bruning, J. Amer. Chem. SOC., 95, 2445 (1973). (5) A. Forman, D. C. Borg, R. H. Felton. and J. Fajer. J. Amer. Chem. SOC., 93, 2790 (1971). (6) A. Ledwith, Accounts Chem. Res., 5, !33 (1972). (7) P. J. Andrulis, M. J. S. Dewar, R. Dietz, and R. L. Hunt, J Amer. Chem. Soc.. 88, 5473 (1966). 18) . . E. I. Heiba. R. M. Dessau. and W. J. Koehl.. J. Amer. Chem. Soc., 91, 6830 (1969). (9) J. Fried and D. E. Schum, J. Amer. Chem. SOC., 89, 5508 (1967). (10) J. K. Kochi, R. T. Tang, and T. Bernath., J. Amer. Chem. Soc., 95, 7114 11973). (11) , . A. Zweia. W. G. Hodason. and W. H. Jura, J. Amer. Chem. SOC., 86. 4124 (1664). (12) W. F. Forbes and P. D. Sullivan, J. Chem. Phys.. 48, 1411 (1968). (13) W. Baker, J. Chem. SOC., 662 (1941). (14) F. Benington, R. D.Morin, and L. C. Clark, Jr., J. Org. Chem., 20, 103 119551. (15) W.F. 'Forbes and P. D. Sullivan, J. Amer. Chem. SOC., 88, 2862 (1966). (16) B. G. Segal and G. K. Fraenkel, J. Chem. Phys., 43, 4 19 1 (1965). (17) A. J. Stone, Mol. Phys., 6, 509 (1963); 7, 311 (1964). (18) P. D. Sullivan, J. Phys. Chem., 74, 2563 (1970).
/ 9 6 2 3 / November 13, 1974
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7237 (19) M. C. R. Symons, J. Phys. Chem., 71, 172 (1967). (20) A. Carrington and' A. D. McLachlan, "Introduction to Magnetic Resonance," Harper and Row, New York, N.Y., 1967. (21) w. G. Williams, R. J. Pritchett, and G. K. Fraenkel. J. Chem. Phys., 52, 5584 (1970). (22) 0. Hassel and C. Romming, Quart. Rev., Chem. SOC., 16, 1 (1972). (23) I. V. Nelson and R. T. Iwamoto, J. Electroanal. Chem.. 7, 218 (1964). (24) A. I. Popov and D. H. Geske, J. Amer. Chem. SOC., 80, 1340 (1958). (25) A. i. Popov and W. A. Deskin, J. Amer. Chem. SOC.,80,2976 (1958). (26) R. S. Mulliken and W. B. Person, "Molecular Complexes," Wiley-Interscience, New York, N.Y.. 1969.
for the (27) This type of mechanism was also suggested by Romans, et oxidation of tetramethylhydrazinewith iodine. (28) J. Kommandeur and F. R. Hall, J. Chem. Phys., 34, 129 (1961). (29) F. Gutmann and H. Keyzer, J. Chem. Phys., 46, 1969 (1967). (30) J. H. Lupinski, J. Phys. Chem., 67, 2725 (1963). (31) V. D. Pokhodenko, V. A. Khizhnyi, V. G. Koshechko, and 0. I. Shkrebtii, Lhkl. Akad. Nauk SSSR,210, 640 (1973); Dokl. Phys. Chem.. 210,437 (1973). (32) The possibility of a triple ion species Br-TMB+.Br- is not unlikely. (33) A. I. Popov, "Halogen Chemistry," Vol. 1, V. Gutman. Ed., Academic Press, New York, N.Y.. 1967.
Aliphatic Semidiones. XXV. Bicyclo[n. 2.llalkane-2,3-semidionesl Glen A. Russell,* George W. Holland,* Kuo-Yuan Chang, Robert G. K e ~ k eJohn ,~ Mattox, C. S. C. Chung, Kerry Stanley, Kirk Schmitt, R. Blankespoor, and Y. Kosugi Contribution from the Department of Chemistry, Iowa State University, Ames, Iowa 50010. Received May 16. 1974
Abstract: The esr spectra of a number of bicyclo[2.2.l]heptane-2,3-semidionesand bicyclo[3.2.l]octane-2,3-semidionesare reported, including tri- and tetracyclic derivatives. Assignments of hyperfine splitting constants have been made by studies of deuterium and alkyl derivatives. A number of long range transoid, coplanar interactions between protons and the electron spin are reported, discussed, and analyzed in terms of valence bond resonance structures. Extended Hiickel and/or INDO calculations are reported for bicyclo[2.2.1] heptane-2,3-semidione and several alkyl derivatives, the corresponding semifuraquinone, bicyclo[2.2. I]hept-5-ene-2,3-semidione,and the semifuraquinone derivative and exo- and endo- tricyclo[ 3.2.1 .02-4]octane-6,7-semidiones.
Esr hyperfine splittings (hfs) by hydrogen atoms separated from a center of spin density by three or four bonds have been reported for a few monocyclic species such as cyR-5exo clobutyl, cyclopentyl, and cyclohexyl radical^,^ piperidine n i t r ~ x i d e , ~and , ~ cycl~heptanesemidione.~ Resolution of these long range hfs requires a frozen conformation for the monocyclic system. Rigid bicyclic structures are often rich la. unsubstituted in such long range h f ~ . Since ~ , ~ our original report of such (R unspecified = H) long range hfs in bicyclic 1,2-~emidiones,~.~ similar observab, R-7a =D tions have been reported for other spin centers including C, R-7s = D alkyl radica1s,l0-l2 n i t r o x i d e ~ , s~e~m. i~q~~ i n o n e s , ' ~semi,'~ d, R-5,6exo = D furaquinones,I7 dicyanoethylene radical anions,ls dimethyle, R-7 = CH,O dialkylhydrazine radical cations,I9 and 1 , 4 - ~ e m i d i o n e s . I ~ - ~ ~ f, R-1,4 = D;R-7 = CH,O The present results complement previous reports on long 8. R-1 = CH, range splittings in 2,3-semidiones derived from bicycloh. R-7a = CH, [n.1 .O]alkane~,~ I bicyclo [ n. 1.1 ]alkanes,22 bicycle[ 2.2.21alScheme I k a n e ~ , ~bicycle[ ~ . ~ ~3.2.01 octane^,'.^,^^ bicyclo [ 3.2.21non a n e ~ , ~ bicyclo .~' [ 3.3.1 ] n o n a n e ~ ,bicyclo[ ~ 3.3.21decanes,25 and a variety of polycyclic derivatives of cyclobutanesemidior~e.~~ Synthesis of Bicyclo[2.2.l]heptanesemidiones. Treatment of bicyclo[2.2.l]heptan-2-onewith traces of oxygen in basic DMSO solution failed to yield the esr signal of the semidione although this technique can be applied to a wide variety of cyclohexanone ~ I e r i v a t i v e s . Reaction ~ ~ , ~ ~ of this ketone, and most other bicyclo[2.2.l]heptan-2-ones not containing a syn- 7 substituent with selenium dioxide, failed to yield significant amounts of the a-diketone although the crude product from norcamphor did yield the esr signal of la when treated with the propiophenone enolate ion in An in situ preparation of 3-hydroxy-2-norbornanone from norbornene oxide in DMSO containing a trace of boron trifluoride etherate (80°, 24 hr)29 yielded la when the DMSO solution was treated with potassium tert- butoxide (Scheme I), but this method gave mixtures of radicals
K 0
0'
0i, R-7a = CD, i, R-7s = CH, k. R-1,7~= CH, 1. R-7 = CH, m. R-1,7,7 = CH, n, R-l,7s = CH,; R-7a = CD, p. R-5,6exo = CH, q. R-B,6endo = CH,
B-, DMSO,
0 crude
t
0
4
when applied to other substituted norbornenes. 3-Bromonorcamphor in basic DMSO also yielded l a but this method30 failed for 3-bromocamphor itself. Russell, et al.
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Aliphatic Semidiones
