Effects of Ion Association on CyclooctatetraeneDianion
1387
Effects of Ion Association upon the Solubilities of the Cyclooctatetraene Dianion Gerald R. Stevenson* and lgnacio Ocaslo University of Puerto Rlco, Department of Chemistry, Rio Piedras, Puerto Rico 0093 1 (Received October 21, 1974; Revised Manuscrlpt Received April 4, 1975) Publication costs assisted by the University of Puerto Rico
The solubilities of disodium cyclooctatetraeneide and dipotassium cyclooctatetraeneide have been accurately determined in hexamethylphosphoramide (HMPA) and tetrahydrofuran (THF) as a function of temperature. For the T H F systems the dianion was considered to exist as a quadruple ion (associated with two cations) as previously reported, and the enthalpies of solution were found to be 1.09 and 5.13 kcal/mol for the potassium and sodium salts, respeptively. The fact that AHo of solution is positive indicates that the crystal lattice energies are larger than the solvation energies of the quadruple ions. In HMPA the dianion was considered to exist as a free ion. However, considerable curvature was observed in a plot of In K,, vs. 1/RT. This curvature was attributed to the formation of ion pairs between the cyclooctatetraene (COT) dianion and one potassium cation. Addition of KI to solutions of the COT dianion and anion radical in HMPA shift the observed reverse disproportionation (comproportionation) equilibrium constant to smaller values confirming the formation of ion pairs. The sodium salt did not exhibit curvature in the van’t Hoff plot, and the comproportionation equilibrium constant was invariant with the addition of NaC103 to solutions of the COT dianion and anion radical in HMPA.
The disproportionation of the cyclooctatetraene (COT) anion radical to form the COT dianion and neutral molecule is of particular chemical interest, since it represents one of the very few reversible chemical reactions in which a compound is interchanged between an aromatic and antiaromatic system. For this reason, the thermodynamic parameters controlling the reverse disproportionation reaction (eq 1) should yield a lot of information concerning the
resonance energy of the dianion and anion radical, the electron-electron repulsion energy in the dianion, and other intramolecular effects. Unfortunately much of this information is obscured by the fact that ion pairing strongly perturbes these thermodynamic parameters and even the stoichiometry of the reaction.1-6 Despite this, some information has been obtained concerning the relative electronelectron repulsion energies in some substituted COT’S in hexamethylphosphoramide (HMPA).7,s The nature of ion pairing of the anion radical of COT has been studied in some detail in the etheral solvents and in ammonia by electron spin resonance (ESR).1-4 Ion pairing of the dianion is, however, much more difficult to observe directly. These difficulties were overcome by Cox and coworkers: who described the NMR spectra of the COT dianion with several different alkali metal cations and solvents. They found that in tetrahydrofuran (THF) the dianion of COT exists as a quadruple ion, where the dianion is ion paired to two cationsg These ion pairing effects not only alter the disproportionation thermodynamics but considerably change the solubility of the dianion, as evidenced by the fact that the observation of precipitated dianion was found to be a function of both the solvent and the counterion.l Our intention in this report is in part to describe the thermodynamic parameters controlling the solubility of the COT dianion in
T H F and HMPA and use this information to gain insight into the nature of ion pairing of the dianion in HMPA. It has been previously observed that most anion radicals in HMPA are fully dissociated.lOJ1 Only a few anions of highly polar nature where a large charge density exists in a localized area form ion pairs in HMPA.12 This ion pairing can be increased by the addition of alkali metal salts to the anion radical solution.12J3 To date there are no known ion pairs of hydrocarbon anion radicals in HMPA. There is some evidence that the COT dianion is ion paired even in HMPA.6 This evidence comes from the fact that the K,, for the reverse disproportionation reaction (eq 1)varies with the cation.6 This observation, however, seems to be inconsistent with the fact that K,, was found to fit the stoichiometry of eq 1. If the anion radical was free of ion pairing and the dianion was associated with one cation, K,, would be expected to be given by
K,,
=
Here we wish to report the thermodynamic parameters controlling the solubility of the COT dianion as a function of the solvent and counterion and the role ion pairing plays in determining these solubilities.
Experimental Section The solubility studies were carried out with the use of the apparatus shown in Figure 1. About 10 ml of T H F was distilled from the solvated electron through the vacuum line into bulb C of the apparatus, which contained a weighed portion of alkali metal. A little less than 0.5 equiv of COT was then distilled into bulb C via a break seal. This mixture was stirred at room temperature until all of the COT was converted into the dianion. The ESR spectrum of The Journal of Physical Chemistry, Vol. 79, No. 14, 1975
1388
1 3
G.R. Stevenson and I. Ocasio
i i
Figure 1. Apparatus used for the solubility determinations.
this solution could be monitored during the reaction with the extending ESR tube. When the reaction mixture failed to yield an ESR signal the reaction was considered to be complete. This normally took about 6 hr. After completion of the reaction the mixture was checked for dianion precipitate. If a precipitate could not be observed, some of the T H F was distilled from the mixture. Once the dianion precipitate was observed, the entire apparatus was taken from the vacuum line and immersed into a constant temperature bath and allowed to come to equilibrium for a period of 1 hr. Stopcock E was then opened and a portion of the saturated solution was allowed to pass through the frit and into the graduated tube F. The apparatus was then removed from the bath and the graduated tube separated from the rest of the apparatus. The solution in bulb F was rinsed into a volumetric flask and diluted to 100 ml with distilled water. This solution was then divided into three parts, which were subsequently titrated to a phenolphthalein end point with standardized HCl solution. The number of moles of COT dianion passed into bulb F was considered to be one-half of the number of moles of hydroxide in the THF-water solution. For the determination in HMPA, the dianion was formed in T H F in bulb A. After completion of the reaction, the T H F solution was separated from the excess alkali metal by passing this solution into bulb C. The T H F was then distilled off and the dry salt was kept under vacuum for a period of 1 hr. HMPA was then dried in bulb A with potassium metal and was distilled into bulb C, which was kept in liquid nitrogen. The saturated HMPA solutions were then treated in a manner identical with that for the T H F solutions. The effect of dissolved salt upon the disproportionation equilibrium was studied with the use of the apparatus shown in Figure 1. The dianion anion radical solution was formed in HMPA bulb A, and the apparatus was sealed off from the vacuum system at point C. An ESR sample of this solution was taken and the remainder of the solution was allowed to pass into bulb D, which contained a known portion of either potassium iodide or sodium chlorate. The apparatus was sealed at point E, and the solution was stirred until all of the salt had dissolved. An ESR sample was taken in the remaining ESR tube. X-Band ESR spectra were recorded on a Varian E-9 spectrometer equipped with a dual cavity. The temperature was controlled using a Varian V-4557 variable-temperature controller, which was calibrated with a copper-.constantan thermocouple. Spin concentrations were compared using the dual cavity technique. The HMPA was distilled from calcium hydride under reThe Journal of Physical Chemistry, Vol. 79, No. 14, 1975
duced pressure before use. The inorganic salts were dried in a vacuum oven a t 100' for 24 hr before use.
Results and Discussion In T H F the COT dianion is known to be ion paired with two alkali metal cations? thus AGo(soln) = -RT In (a2-,M+2) where (n2-,M+2) represents the concentration of the quadruple ion in the saturated solution. The solubility results a t the temperatures investigated are given in Table I. As the solubility data were obtained over a range of temperatures the standard enthalpy of solution was obtained from a plot of In (7r2-,M 3) vs. 1/RT, Figure 2. Since the values for the enthalpy determined in this manner depend upon the fact that only quadruple ions remain in solution, there could be considerable error in the enthalpies if the quadruple ion dissociates a t the lower temperatures. This dissociation, however, would result in curvature of the van't Hoff plot. Since no curvature is noted in Figure 2, it is safe to assume that the dianion exists in the form of the quadruple ion at all of the temperatures investigated. It is interesting to note that the solubility of the potassium salt is about two orders of magnitude greater than that for the sodium ion, and the enthalpy of solution of the potassium salt is less endothermic, Table 11. Both A S o and AHo represent differences in the solution state and the crystal. Thus, crystal lattice energies and entropies are very important in the enthalpy and entropy of solution. It would be difficult to try and distinguish between the importance of solvation and that of crystal lattice effects a t this point; but if we consider the third law entropies for the sodium and potassium salts to be about the same, it is clear that there is less solvent ordering for the potassium system. The theoretical entropy and enthalpy of solution of either salt from the gas phase are necessarily negative numbers due to the interactions between the solvent and the salts. The fact that both AHo and A S o are positive indicates that both of these salts have rather large crystal lattice energies. The situation for the COT dianion in HMPA is more complicated since the extent of ion pairing of the dianion is unknown in this solvent. If the COT dianion is free of ion pairing in HMPA, the standard free energy of solution is equal to where -yrz- and -yMt are the mean molal activity coefficients for the dianion and cation, respectively, and m,z- and mMt are the molalities of these ions in the saturated solution. For the experiments described here the concentrations are too large for the use of Debye-Heckel theory for the estimation of the activity coefficients. Further, the possibility of ion pairing exists and eq 3 should include another parameter representing the degree of dissociation of the ion pair.14 Since the concentrations were not sufficiently low for the use of Debye-Huckel theory and the ion association constants are unknown, AGO for the dissolution of the salts could only be roughly estimated from AGO = -RT In K,,
(4 )
where K,, was assumed to be equal to the product of the dianion and the square of the total solution cation concentration. The solubilities could be accurately determined, Table 111. The enthalpies of solution were estimated from plots of
Effects of Ion Association on Cyclooctatetraene Dianion
1389
TABLE I: Solubility ( M )of the COT Dianion (Quadruple Ion) in T H F at Various Temperatures Na,COT
K,COT
Solubility x io3
T , "C
3.00 2.34 5.48 8.04 8.15 9.38 9.57 12.2 19.2
-23.20 -22.0 0.5 7.8 8.0 11.8 16.1 24.9 35.0
Solubility x 10'
T , "C
3.92 4.26 4.62 4.80 5.14 5.30 5.59
-21.0 -15.7 4.5 1.0 11.2 18.7 26.1
K,COT Na,COT
AGO,
AH",
kcal/mol
AS", eu
0.35 2.59
1.09 * 0.05 5.13 + 0.3
2.5 8.5
i
0.02 0.02
'
o \ 1 8
! C
l(i/l
