J. Phys. Chem. 1982, 86, 1611-1614
1811
Electron Spin Resonance Studies of Divalent Cation-Anion Radical Ion Pairs. The 2,6-Di-tert-butylbenzoqulnone Anion Radical LUISEchegoyen, Ileana Nleves, Depertment of Chemistry, University of Puerto Rico, Rjo Piedras, Puerto Rlco 0093 1
and Gerald R. Stevenson Department of Chemistry, Illinois State University, Normal, Illinois 61761 (Received: April 24, 1961; In Flnai Form: November 6, 1981)
The anion radical of 2,6-di-tert-butylbenzoquinonewas generated in hexamethylphosphoramide, and the corresponding ion pairs with Mg2+,Ca2+,and Ba2+were produced by additions of the divalent cation perchlorate salts. The resulting free ion-ion pair equilibria are slow on the ESR time scale, thus allowing simultaneous observation of the spectra for all species present. An intermediate, diamagnetic species was observed upon addition of Mg2+and has been attributed to the formation of a tight ion pair between the cation and the benzoquinone dianion formed upon disproportionation. This species disappeared as more Mg2+was added to the solution, giving rise to a very strong ESR signal attributed to a tight, paramagnetic ion pair. Addition of Ca2+results in two distinct, time-resolved ESR signals assigned to two different ion pairs which probably arise from cation complexation with the nonequivalent oxygen atoms on the quinone anion radical. Additions of tripyrrolidinephosphoramideto the diamagnetic magnesium anion radical solution displaces the equilibrium toward the formation of the free anion radical.
Introduction Alkali metal reduction of a-electron acceptor molecules in hexamethylphosphoramide (HMFA) yields solutions of the anion radicals that are free from ionic association with the alkali metal cation.'J However, the addition of alkali metal salts often results in the formation of the ionically associated specie^.^^^ The free ion-ion pair equilibria in HMPA can be either slow or fast on the ESR time scale, resulting in either simultaneous observation of the two species (ion pair and free ion) or the observation of a weighted-averaged specie^.^ The determinant factor controlling whether the equilibrium is fast or slow on the ESR time scale is that of ion pair dissociation, which is controlled by the strength of the Coulombic attraction between the anion radical and the counterion.6 Indeed, the rate of ion pair dissociation and formation of the nitrobenzene anion radical-potassium ion pair is slow on the ESR time scale, but that for p-dinitrobenzene is fast because of the more delocalized charge density in the latter ~ystem.~ The sodium reduction of 2,6-di-tert-butylbenzoquinone (DBQ) in HMPA leads to the unassociated anion radical. However, addition of alkali metal perchlorates or iodides to the anion radical solution results in the formation of ion pairs that exist in rapid (on the ESR time scale) equilibrium with the free ions (eq 11, and only time-averaged
P
?
(1) Cserheayi, A.; Jam-Grodzinski, J.; Szwarc, M. J.Am. Chem. SOC. 1969,91, 1892; (2) (a) Levin, G.; Jagur-Grodzinski,J.; Szwarc, M. J . Am. Chem. SOC. 1970,92,2268. (b) Cserhegyi, A.; Chauduri, J.; Franta, E.; Jagur-Grodzinski. J.: Szwarc. M. Zbid. 1967.89. 7129. (3).St&enaon,'G. R.; Alegria,'A. J. Phys.Chem. 1974, 77, 3100. (4) Echegoyen, L.; Hidalgo, H.; Stevenson, G. R. J.Phys.Chem. 1973, 77, 2649. (5) Stevenson, G. R.; Alegria, A. J. Phys. Chem. 1975,80,69. (6) Stevenson, G. R.; Alegria, A.; McB. Block, A. J. Am. Chem. SOC. 1975, 97, 4859. (7) Stevenson, G. R.; Alegria, A. J. Phys.Chem. 1975, 79, 1042.
0022-3654/82/2086-1611$01.25/0
species are ~bserved."~The experimental data indicated that the DBQ-.-K+ ion pair was formed only through the nonsterically protected oxygen atom, while the ion pairs involving Li+ and Na+ could involve interactions through either of the two oxygen atoms as shown in eq Formation of diamagnetic "dimers" of ion radical systems has been reported for quinoline radical ions,1° ketyls of benzophenones," and phenanthrequinone anion radic a l ~ . ' ~The former two indicate the formation of a-bonded diamagnetic dimers between the monomeric anion radicals, while cation-mediated a dimers were suggested in the latter reference. These latter diamagnetic dimers were not formed in HMPA solution where the red monomeric free anion radical predominates. Besides the formation of a-bonded dimers and cation-mediated a dimers, anion radical solutions can become diamagnetic upon additions of metal cations if disproportionation is favored by Coulombic intera~ti0ns.l~This has been observed in the case of the cyclooctatetraene anion radical, where additions of alkali metal salts result in the gradual disappearance of the anion radical ESR signal because of the formation of tight ion pairs with the dianion. We wish to report here the formation of the ion pairs of DBQ-. with divalent cations (Mg2+,Ca2+,and Ba2+)in HMPA, which result in slow free ion-ion pair equilibria on the ESR time scale. Experimental Section ESR spectra were recorded by using the X band of a Varian E-9 ESR spectrometer. Samples of the anion radicals were prepared under vacuum as described prev i o ~ s l y .DBQ ~ and HMPA were carefully purified before use by recrystallization and double vacuum di~tillation,~ respectively. The divalent cation perchlorate salts were dried in a vacuum oven for 48 h before use. Stock solutions of these salts were prepared (ca. 0.1 M) under vacuum. Aliquots from these were added to the anion radical sol.839
(8) Stevenson, G. R.; Alegria, A. J. Phys.Chem. 1974, 78, 1771. (9) Alegria, A.; Concepcibn,R.; Stevenson, G. R. J.Phys.Chem. 1975, 79, 361. (10) Chaudhuri, J.; Kume, S.;Jagur-Grodzinski, J.; Szwarc, M. J. Am. Chem. SOC.1968,90, 6421. (11) Hirota, N.; Weissman, S. I. J. Am. Chem. SOC.1964, 86, 2537. (12) Staples, T. L.; Szwarc, M. J . Am. Chem. SOC.1970, 92, 5022. (13) Stevenson, G. R.; Ocasio, I. J.Phys.Chem. 1975, 79, 1387.
0 1982 American Chemical Society
1612
The Journal of phvsical Chemistry, Vol. 86, No. 9, 1982
Echegoyen et ai.
TABLE I: Enthalpies and Spectral Parameters for the Equilibria and Species of Cation Salts DBQ-. (free) coupling constant, G AHdiss, kcal/mol H central line (ion pair) - H central line (free ion), G
2.39
DBQ-.with Added Alkaline Earth
DBQ-.-
Mg
DBQ-.-Ca+
0.86
1.28,=1.46,b 1.12,b1.67,' 0.03Cd
-0
-0.9
0.58
0.33
t
0.5
DBQ-.-Ba+ 1.42 -1.1 * 0.2 0.41
a This coupling constant refers to the triplet of the ion pair observed at tempeatures below 4 5 "C;see text for further explanations. The two coupling constants refer to two distinct triplet ion pairs observed at temperatures between 45 and 7 5 C. ' The 1.67 value refers to coupling with two hydrogens. 0.03 refers to coupling to 18 equivalent hydrogens (from the tert-butyls).
lutions while keeping the systems under a positive, dry nitrogen atmosphere.
Results and Discussion OB@. with Added Mg(ClO4)z. The characteristic ESR triplet (aw = 2.39 G) was observed upon sodium reduction of DBQ in HMPA5 (Figure 1A). Addition of small amounts of magnesium perchlorate to the anion radical solution resulted in a gradual decrease of the ESR signal and concomitant disappearance of the yellow color of the solution. When the added Mg2+concentration was half that of the anion radical, an extremely weak ESR signal was detected (Figure 1B). Further salt additions resulted in the appearance of a very strong ESR signal consisting of a triplet with a coupling constant of 0.86 G (Figure lC, Table I). Note that the spectrum in Figure 1B corresponds to a weak superposition of the spectra on Figure 1, A and C. Spectrum 1C is slightly displaced upfield (0.58 G relative to that of the free anion radical). This shift, coupled to the smaller observed coupling constant, indicates that spectrum 1C corresponds to that of an ion pair. As opposed to the behavior observed upon addition of alkali metal salta, the equilibrium between the free ion and the ion pair is slow on the ESR time scale, thus allowing simultaneous observation of the two spectra. This result is not so surprising when considering the increased Coulombic interaction that results as a consequence of the higher charge on the cation. As has already been mentioned, the total spin concentration markedly decreased upon addition of the magnesium salt, and it was accompanied by fading of the yellow color of the free anion radical solution. This behavior can be explained by the reaction sequence presented in eq 2. BDBQ-. ==DBQ2-
C
+ DBQ=w2+ DBQ2-**.Mg2++ Mg*+
DBQ G2(DBQ-.-.Mg2+) (2) Benzoquinone anion radicals are known to disproportionate to form the corresponding diamagnetic dianions and the neutral m01ecule.l~ At high concentrations of the anion radical (relative to that of the added salt),interaction of the cation with the dianion is Coulombically favored over the interaction with the anion radical (1:l complex). As more magnesium is added, the equilibrium is displaced toward the complex with the anion radical because it becomes stoichiometrically favored. This explanation is similar to that offered previously in the case of hydrogen bonding between ammonia and the cyclooctatetraene anion radical and dianion.15 In this report, the disproportionation of cyclooctatetraene anion was favored at low ammonia concentrations because of the preferred interaction of ammonia with the dianion while the comproportionation (14)Sullivan, A. B.;Reynolds, G. F. J. Phys. Chem. 1976,80, 2671. (15)Stevenson, G. R.;Vassos, A. J. Phys. Chem. 1977,81, 1526. (16)Ozari, Y.;Jagur-Grodzinski, J. J. Chem. SOC.,Chem. Commun. 1974, 295.
1 GAUSS c .
Figure 1. (A) ESR spectrum of the DBQ-. generated by sodium reduction in HMPA; this spectrum corresponds to the free anion radlcal. (8) ESR sped" after addltkn of 0.5 equiv of magnesium perchlarate per anbn radical equivalent. Notice from the observed signal-to-ndse level how weak these signals are. (C) ESR spectrum after addition of a large excess of magnesium perchlorate to the anion radical solution.
equilibrium was favored stoichiometrically at high ammonia concentrations. It is interesting to note that of the three alkaline earth cations studied Mg2+ is the only one favoring the formation
Divalent Cation-Anion Radical Ion Pairs of this diamagnetic ion pair with the quinone dianion. In order to confirm the reversibility of the sequence spectrum 1A lB, we added tripyrrolidinephosphoramide (TPPA) to a solution exhibiting the spectrum shown in Figure 1B. TPPA has been shown to be about 30% better for solvating alkali metal cations than HMPAle and thus should favor the formation of the DBQ free anion radical. When 5% (v/v) TPPA was added to solutions exhibiting the weak spectrum shown in Figure lB, the ESR signal changed to that of the free anion radical (Figure 1A). After TPPA addition it was not possible to restore the diamagnetic or the ion pair spectrum even after several successive additions of the magnesium salt to the solution. Temperature variations of all samples (Figure 1A-C) failed to show any line-width effects, coupling-constant changes, or any significant relative concentration change of the two paramagnetic species. This suggests that the ion pairs are not exchanging appreciably and can thus be represented as quasi-static structures within the temperature range studied. Although it was not possible to determine the free Mg2+concentration, especially when the solution was essentially diamagnetic (low Mg2+concentration), the insensitivity of the ESR signals to temperature variations suggests a value of AHdiaa that is very close to zero (Table I). This means that Mg2+ has comparable affinity for the anion radical and for HMPA. Solvation competes effectively with ion pair formation. DBQ-. with Added Ca(C10J2. Additions of calcium perchlorate to the free DBQ--HMPA solution resulted in the gradual disappearance of the original spectrum and the simultaneous appearance of a new triplet signal with a coupling constant of 1.28 G, the latter being assigned to an ion pair, analogous to the previous case. However, Ca2+ additions do not seem to generate diamagnetic solutions as evidenced by the color persistence of the solution and the strong signal observed throughout the salt-addition experiments. At high Ca2+concentration (21, Ca2+:DBQ--),where the triplet assigned to the ion pair is almost exclusively observed, temperature variations resulted in unusual and unexpected behavior (see Figure 2). At temperatures above 45 "C, an additional splitting of the first and last lines of the ion pair triplet was observed (Figure 2B). These two lines completely disappeared and gave rise to two sets of two lines each, both pairs arranged equidistantly from the original lines of the ion pair. Considerable asymmetry can be observed in this new spectrum, suggesting the presence of two or more spectra with different coupling constants and slightly different g factors. Based on this assumption, the spectrum can be interpreted in terms of two distinct triplets, one with a coupling of 1.46 G and another one with a coupling constant of 1.12 G, the average being 1.29 G, very close to the observed splitting of the ion pair. These two triplets have very similar g values, so close in value that they are experimetnally indistinguishable (Figure 2B). It should be noted that all coupling constants are smaller than that of the original free DBQ-e. Furthermore, the larger splitting in Figure 2B is very similar to that observed for the Ba2+-DBQ-. ion pair (1.42 G , see section below) while the smaller splitting is somewhat close to that of the Mg2+-DBQ-. ion pair previously mentioned. If the temperature of the sample exhibiting spectrum 2B is lowered, the signal of the 1.2842 triplet is reversibly restored. On the other hand, further temperature increases (higher than 75 "C) result in the appearance of still another ESR signal (see Figure 2C). This spectrum consists of a triplet splitting of 1.67 G and a smaller coupling with 18 equivalent hydrogens of 0.03
The Journal of physical Chemistry, Voi. 86, No. 9, 1982 1813
-
Figure 2. (A) ESR spectrum of free DBQ--. (B) Spectrum after addition of approximately 1 equiv of calcium perchlorate per anion radical equivalent. (C) ESR spectrum corresponding to the same solution as in part B but after increasing the ESR cavity temperature to 80 "C.
G, the latter arising from the two equivalent tert-butyl groups. On the basis of these data, the two triplets observed in Figure 2B are assigned to the two possible ion pairs which can be formed through the interaction of the calcium cation with the two nonequivalent oxygen atoms of DBQ-a. These two ion pairs also interconvert slowly on the ESR time scale. Because of the narrow temperature range where spectrum 2B is observed, no quantitative data could be obtained that would enable the determination of the enthalpy for the exchange process between the two ion pairs. The high-temperature spectrum (Figure 2C) indicates that the triplet spectrum with the larger coupling constant is favored over that of the smaller triplet. This could perhaps be due to inhibition of interaction with the sterically hindered oxygen due to the increased rotational motion of the tert-butyl groups at high temperatures. Experiments were conduced at low Ca2+concentrations and relatively low temperatures (between 5 and 40 "C) in order to determine the enthalpy of dissociation of the 1.28-G ion pair to form the free anion radical. Relative concentrations of both species were determined by double integrations of the lower field lines of both triplet signals. These concentration ratios ((free ion)/ (ion pair)) were measured as a function of temperature between 5 and 40 "C. These ratios were found to be relatively insensitive to temperature, ranging from 2.1 to 1.8 over the entire temperature interval studied. Nevertheless, plots of the logarithm of these ratios vs: 1/T yielded straight lines from which the enthalpy of ion pair dissociation was obtained.
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Echegoyen et al.
The Journal of Physical Chemistry, Vol. 86, No. 9, 7982
TABLE 11: Coupling Constants for the 2,B-Di-tert-
0
ii
butylbenzoquinone aH, G
ion pair
k
.Po' K+ 2.10
I
0.86
1.12
c a 2 + . oo*
W
ca2+
V
'
1.46
GAUSS
Figure 3. (A) ESR spectrum of free DBQ-e. (B) ESR spectrum after additkn of 0.5 equiv of barium perchlorate per anion radical equivalent. (C) ESR spectrum atter addltlon of a large excess of the salt to the anion radical solution.
As expected from the temperature insensitivity of the equilibrium, the enthalpy value calculated was very small, only -0.9 f 0.5 kcal/mol (Table I). This value indicates slightly preferred solvation over ion pair formation. DBQ-. with Added Ba(C104)2. Additions of barium perchlorate to the free DBQ-. solution resulted in the gradual decrease of the original triplet while a new triplet increased. Both triplets are observed simultaneously as a function of the added concentration of the salt. The new triplet that arises has a coupling constant of 1.42 G (Figure 3A-C, Table I). There is no evidence of diamagnetic behavior of the solution after salt additions or evidence of formation of more than one type of ion pair. This behavior indicates that the Ba2+can only interact with the anion radical through one of the oxygens, as was the case with Mg2+. No line-width effects were noted upon temperature variations of samples exhiting spectra 3A-C. The ratios (free ion)/(ion pair) were determined as described in the previous section by using double-integration techniques on selected spectral lines. As was found for both the Mg2+and the Ca2+ion pair-free ion equilibria, the relative concentrations of the species did not change appreciably over the temperature range studied, 5-60 OC in this case. PloB of In [(free ion)/(ion pair)] vs. 1/T were linear, and the calculated AHHdh, was found to be -1.1 f 0.02 kcal/mol (Table I). This small value again reflects effective competition between solvation and ion pair formation.
Conclusions It has been previously shown that the potassium cation can interact only with the oxygen atom of DBQ-. that is not hindered by the bulky tert-butyl groups.s However, because of their smaller size, both Li+ and Na+ can associate with either of the two oxygen atoms. Comparing the crystalline ionic radii of the alkali metal cations with those of the alkaline earth cations, one expects magnesium to behave similarly to Li+ (0.66 vs. 0.68 A), calcium to behave similarly to Na+ (0.99 vs. 0.97 A), and barium to behave similarly to K+ (1.34 vs. 1.33 A). All of these predictions have been borne out by experiment. For all of the doubly charged cations the rate of ion pair formation and dissociation was found to be slow on the ESR time scale, This behavior is in contrast to the results with the alkali metals but may be in part due to the fact that the spectral changes are greater between the free ion and the alkali earth cation ion pairs than they are for the alkali metal cation-anion radical ion pairs (Table 11). The larger perturbation upon the electron distribution of the anion caused by ion association with the divalent metals relative to those caused by the alkali metal cations reflects the greater Coulombic attraction of the former. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work, and to the National Science Foundation (Grant CHE-7915201),the National Institutes of Health (Grant RR-8102-07), and the Research Center of the University of Puerto Rico (OCEGI office) for additional support.
