J. Phys. Chem. 1993, 97, 763-766
763
Relaxation Study of Potassium Anions and Cations in Tetrahydrofuran Containing 15-Crown-5 by 39KNMR Maria Sok64' Janusz Grobelny, Zbigniew Grobelny, and Andrzej Stolarzewicz Institute of Polymer Chemistry, Polish Academy of Sciences, 41 -800 Zabrze, Poland Received: October 7 , 1991; In Final Form: October 16, 1992
The 39Kresonance signals due to potassium anions and complexed potassium cations in blue potassium solution in THF containing 15-crown4 were observed within a broad temperature range. Spin-lattice and spin-spin relaxation times were measured for both types of potassium ions. The comparison of relaxation behavior in potassium solutions with various cation complexing agents was made and discussed in terms of interionic interactions.
Introduction Potassium anions (K-) in organic solvents containing various K+ complexing agents have recently been found to be useful in the reduction of some organic compounds,]J as well as in anionic polymerization processes.3~~ The investigations of such solutions by means of 39K NMR spectroscopy have yielded a direct proof of the existence of potassium anion^.^^^ Edwards et ale5reported the observations of the 39K NMR spectrum of K- from solutions of potassium metal and potassium/cesium alloy in the liquid crown ethers and in tetrahydrofuran containing 12-crown-4. The presence of potassium anions in addition to potassium complexed cations was also proved for the solution of K+(15-crown-5)2.K- alkalide in dimethyl ether.6 In our previous works we studied the relaxation behavior of potassium solutions in THF containing 18-crown-67 or cryptand[2.2.2]* by 39KNMR. However, severe broadening of the line due to the potassium cation within the whole temperature range studied confined the analysis of the relaxation to K-. The aim of the present work was to gain a better understanding of relaxation processes in potassium solutions containing a cation complexingagent. A solution in tetrahydrofuran with 15-crown-5 was selected. The system is known to give two distinct, wellresolved 39KNMR lines9due to both K- and K+, thus providing a promiseof a deeper insight into relaxation behavior and interionic interactions.
Experimental Section Sample Preparation. Tetrahydrofuran (THF) was purified by themethoddescribede1sewhere.l0The potassium bluesolution was obtained by the contact of a potassium mirror with the solvent containing 15-crown4 (1,4,7,10,13-pentaoxacyclopentadecane) at ambient temperatureduring 15 min, as in ref 9. The potassium solutions were then filtered through a glass frit into a IO-" NMR sample tube. The concentration of 15-crown4 was 0.10.4 M and that of K- (equal to K+) was approximately equal to half of the respective crown ether concentration. The potassium solutions of the concentrations from this range were found to be stable throughout the experiments9 At higher concentrations the solutions became less stable and considerable amounts of decomposition products were observed. Moreover, there is a reasonable fear of diverse contribution of paramagnetic species relative to potassium ions when the concentration of potassium is increased by addition of the crown ether, and the increase in the amount of 15-crown-5 does not necessarily result in an enhanced concentration of potassium anions. In the extreme case of a very unstable potassium solution in pure 15-crown-5, the maximum total concentration of potassium is 0.1 M and the * To whom correspondence should be addressed. 0022-3654/58/2097-0763$04.00/0
formation of electrons at the expense of K- is favored.ll Below 0.1 M no reasonable NMR data could be obtained due to low 39K NMR sen~itivity.~ Measuremts. A Varian VXR-300 multinuclearpulsed NMR spectrometer operating at the39Kresonance frequency of 14 MHz (300 MHz for 'H) was used to record the 39Kspectra. Chemical shifts were measured relative to the K+ signal from aqueous solutions of potassium fluoride. The measurements were carried out within the range 183-333 K with a 39Ka / 2 pulse width of 65 ps, acquisition time of 0.2 s, and 200 scans. Spin-lattice relaxation time values were obtained with a standard inversionrecovery sequence, and spin-spin relaxation times were calculated from line widths. A 7594s delay was used prior to acquisition to avoid base-line distortions. Electron spin resonance (ESR)measurements were carried out on an ESR 300 Bruker X-band spectrometer in the temperatre range 163-293 K, employing 100-kHz field modulation and a microwave frequency of ca. 9.4 GHz. In order to determine the concentration of paramagnetic species, the double rectangular cavity was used. Solutions of 2,2-bis-(4-tert-octylphenyl)-lpicrylhydrazyl (DPPH) served as a referencein these experiments.
Results and Discussion 39KNMR spectra of the 0.2 M potassium solution in THF containing 15-crown-5 (1 5C5) recorded in a broad temperature range are presented in Figure 1. At lower temperatures the spectra exhibit two, well-resolved resonance lines of nearly identical integral intensities. A line at -104 ppm can be unambiguously assigned to potassium anions, and a broad signal is evidently due to complexedpotassium cati0ns.5~Both line widths and chemical shifts of the signals are observed to change with temperature whereas the integral intensity of the K+(1SC5)z peak equals that of the K- line within experimental error. The K- line narrows from 6.3 Hz at 168 K to 1.7 Hz at 183 K and then broadens, attaining 290 Hz at 303 K. The width of the line due to K+ decreases from about 1500 Hz at lowest temperature to 98 Hz at 253 K, which is followed by an increase to 770 Hz at 303 K. The changes in the K+ line width are accompanied by an upfield shift of the line to -18.8 ppm at 293 K. The position of the Kline remains constant up to 293 K. Above 293 K a downfield shift of the K- signal is observed. At still higher temperatures the shifting of both signals toward each other results finally in only one, very broad, nonsymmetrical line centered at about -88 ppm at 333 K. The widths of the K+ and K- lines vs temperature for K+(I 5C5)2,K-solution as well as the corresponding values of the potassium anion line widths for the previously studied solutions containing 18-crown-6 (18C6) or cryptand[2.2.2] (C222) are presented in Table I. As for the other systems the widths of the K+ line are markedly greater than those of the K- line. The difference in the widths of the lines attributed to K+(15C5)~ and Q 1993 American Chemical Society
Sok& et al.
764 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
-
233 K
I
253K
Figure 1. j9K NMR spectra of K+(15C5)2,K- 0.2 M solution in THF recorded within the temperature range between 168 and 333 K.
TABLE I: Com II of the 3% LLse Widths, AYI/Z,for 0.2 M Potassium SoPU ~ OOMin THF Containing 18-Crown-6, Cryptpnd[2.2,21, and 15-Crown-5 Avipr Hz T,K 183 193 203 213 223 233 243 253
263 273 283 293
18C6 K-
c222 K-
5.3 4.5 4.6 4.9 6.3 10.6 16.8 28.9 49.0 83.8 132.7 199.0
4.2 3.2 3.2 3.5 3.5 5 .o
15cs K-
K+
1.7 1.9 2.2 2.7 3.2 4.0 6.4 6.4 10.6 31.8 50.5 160.0
637 455 277 199 168 145 110 98 114 163 187 366
K-can be explained by lower spherical symmetry of the electric field sensed by the potassium cation and/or an increased rotational correlation time, 7c, characterizing the fluctuations of the K+ environment. The lines due to K+and K-may also be influenced by the presence of paramagnetic species via a paramagnetic interaction and/or a modification of the 39Knuclear quadrupole coupling mnstant by electrons. In order to identify the paramagnetic species, the ESR measurements were employed. Figure 2 displays the ESR spectrum recorded at 163 K which may result from the
10 G
Figure2. ESRspcctrumofK+(15C5)~,K-0.2MsolutioninTHFr~rdcd at 163 K within a sweep width of 10 G and with a microwave power of 20 p w .
superposition of a narrow resonance at g = 2.0028 close to the free-spin valueii and a broader resonance due to K+( 15C5)2,4pairs.9 The total concentration of electrons was found to be of the order of 10-3 M in the temperature range between 163 and 293 K. It is worth noting that the paramagnetic electron-cation aggregates M+e,- in some liquid-metal solutionsl2-14 and M+(crownether)e,- for frozen-alkali-metal solutions' 1 have been
Relaxation Study of Potassium Anions and Cations
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 765
30 20 10 0 -10 -20 .)O -40 -50 -60 -?O
U
t
u
(a I (b) (C) Figure 3. Schematic representation (not to scale) of theeffect of increasing
K+isolation from interactions with K-due to the complex structure: (a) K+complexed by an 18-crown-6 molecule; (b) K+trapped by a cryptand(2.2.21 molecule; (c) K+ located between two 15-crown-5 molecules in a sandwich fashion."
successfully detected by ESR. The ESR results along with the NMR data seem to indicate that the shifting of the K+ resonance vs temperature accompanied by the decrease in the cation line width up to 253 K may be due to the existence of a rapid equilibriumbetween "free" complexed cations and K+(15c5)2,%ion pairs. The observed shift of the K+ NMR line for the K+(lsC5)2,K- system can thus be considered as resulting from the Fermi contact interactions of the complexed potassium cation with the unpaired electron. The values of the hyperfine splitting constant (hfsc) were evaluated from the following general formula15J6
where A denotes the NMR hfsc, Au stands for chemical shielding, and other symbols have their usual meaning. At very low temperature Au should determine the experimental shift (&xp) due to an increased average solution of the ions. At 168 K the position of the K+ line is about 0 ppm and thus it can be accepted that Au = 0. At higher temperatures the Fermi contact term in eq 1 is expected to dominate bcxp The NMR hfsc values obtained in such an approach are small and vary linearly from 0 to -1.12 X 10-3 mT between 168 and 293 K. As seen from Table I, the narrowest K- lines are obtained for the K+(1Sc5)2,K- system. Also the width of the line due to K+ is smallest in this case (the systems with other complexing agents give rise to very broad K+ signals disappearing into the base line7+). In our previous paper* the width of the K- line was suggested to be closely related to a kind of potassium cation complexing agent through its ability to separate both ions. The width of the K- signal was considered in terms of K+ availability for interactions with K-. By comparison of the K- line widths in various systems, it seems that the greatest efficiency in mutual isolation between K- and its counterion may be ascribed to 1 SC5 (Figure 3). Moreover in the case of 15C5 the spherical symmetry of the electric field sensed by K+ seems to be markedly higher than that due to 18C6 or C222, as results from the comparison of K+ line widths for different systems. It seems that the paramagnetic interactions with solvated electrons present in the solutions can also affect the observed differences in both K+ and K- line widths from the potassium solutions with various complexants. The presence of two well-defined resonance signals due to K+(15C5)2 and K- within the broad temperature range rendered it possible to describe the relaxation behavior of both type of ions, contrary tothesystems with 18C67or C2228forwhich the analysis was limited to potassium anions. The semilog plots of spinlattice, TI, and spin-spin, T2, relaxation times vs reciprocal temperature for K+( 15C5)2 and K- are presented in Figure 4, and the experimental data are given in Table 11. The overall relaxation behavior of potassium solutionswith cation complexing agents is governed by quadrupolar, paramagnetic, and exchange effects.5-8 The initial increase in TI for K- between 183 and 2 13 K can be referred mainly to a decrease in rotational correlation time of motion causing quadrupolar relaxation. Above 213 K some changes of both the direction and magnitude of the electric
-80
-90
-105
f
h
v VI
kN I-
lo-+'
3.2
36
40
4 L
48
5.2
56
6.0
1 0 ) / ~( K-')
Figure 4. Temperature dependences of relaxation times of potassium ions for 0.2 M potassium solution in THF containing 15-crown-5: ( 0 ) TI, K-;( 0 ) T2. K-;(A) TI, K+;(A) T2; K+.
TABLE Ik Longitudinal, TI, and Transverse, T2,Relaxation Tiares of K- and K+ vs Temperature for 0.2 Ma Potassium Solution in THF Containing 1 x5 TI,ms T2, ms T, K K+ KK+ K183 193 203 213 223 233 243 253 263 273 283 293 303
0.5 0.72 1.25 1.65 2.0 2.3 3.1 3.4 3.5 4.1 3.6 3.4 2.2
430 560 5 50 560 440 260 120 60 34 29 12 4.8 2
0.5 0.7 1.2 1.6 1.95 2.2 2.9 3.3 2.8 1.95 1.7 0.88 0.5
190 170 150 120 100 80 50 50 30
IO 6.2 2 1.1
The results were found to be invariant with concentration in the range 0.1-0.4 M.
field gradients seem to induce the rapid decrease in TI.Spinspin relaxation time values, T2, are lower than TIover the entire temperature range and decrease with temperature, exhibiting a particular relaxation enhancement above 253 K. The inequality of TIand T2 suggeststhe presence of exchange processes affecting mainly or exclusively T2. For the potassium cation the increase in TI and T2 (up to 273 and 253 K, respectively)and the equality of both relaxation times between 183 and 253 K are observed. Such behavior could indicate that up to 253 K the pure quadrupolar mechanism, determined mainly by T ~operates , in the system. Nevertheless, in view of the ESR and NMR data discussed earlier, we cannot exclude that along with thequadrupolar relaxation there is another process contributing to the overall relaxation. This may be a fast exchange involving electrons and complexed cations: *K+(15C5)2,e-+ K+(15C5), e K+( 15C5),,e-
+ *K+(15C5),
(2)
Above 253 K both TIand T2 start to split and then decreased (Figure 4). Taking into account that up to 253 K for K+ TI = T2, the difference in anion relaxation times can be presumed to result from a transfer of K- nuclei between various environments and not any exchange process involving both K+ and K-. Such an explanation was already suggested for K+(18C6),K- and K+(C222),K- although no relaxation data for potassium cation could be obtainedm7s8
766 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993 t (“c) 10 20 10
-
0
-20 -30
-10
-40
h
‘
100 130
1 3 57
3 85
4 13
4 40
4 67
495
522
550
103/T ( K - i ) Figure 5. Temperature dependence of the 39K( 0 )and 39K+ (0) excess
relaxation rates for 0.2 crown-5.
M potassium solution in T H F containing
15-
When it is assumed that the overall relaxation derives from the quadrupolar, paramagnetic, and exchange effects and the latter mechanism influences mainly spin-spin relaxation, the (1/ T z )(1/ T I )expressioncan be related to the efficiency of the relaxation due to exchange pr~cesses.~JThe results of such an approach are presented in Figure 5 . The semilogplot of the excess relaxation rate of K- up to 253 K presumably corresponds to the above mentioned transfer of potassium anions between various environments. Above 253 K the excess relaxation rate of K- is enhanced markedly, being accompanied by a considerable increase in K+ excess relaxation rate. The observations suggest that at these temperatures, in the K+(1SC5)2,K- system, an exchange process involves both types of ions. The changes in the excess broadening of the K+, and K- lines (Figure 5 ) are accompanied by a shifting of the signals’ maxima toward each other with a tendency to coalesce (Figure 2). The coalescence pattern found in the studied system suggests (that the net effect could be explained by a cation-anion exchange:
However, the exchange processes in the K+( 15C5)2,K- solution seem to be more complex because various pathways are expected to contribute to the overall mechanism of exchange. This may be reflected by the difference in the excess relaxation rates of K+ and K-. Otherwise both peaks would be broadened by the same amout; Le. the same excess relaxation rates would be observed. The observations concerning the relaxation behavior of the potassium solution containing 15CS (Figure 4) suggest that the equilibrium of the reaction
K+(ISCS),,K-
K+(~scs)~~~K-
*
solvent separated ion pairs K+( 1X S ) , K- (4) solvated ions is shifted to the right below 253 K. At higher temperatures contact ion pairs prevail due to desolvation and consequently cationanion exchange processes may occur. The situation becomes even more complex if one takes into account that processes such as contact ion pairs
K- a K + e,-
+
and/or
K- a K+ + 2e,-
(5)
may also play an important role’* at high temperatures. Up to 293 K the areas under both K+ and K- peaks are equal and the total concentration of electrons was found by ESR not to increase significantly, indicating the equilibria of the reactions ( 5 ) to be strongly shifted to the left. Nevertheless, it is not necessarily valid at higher temperatures, as the integral intensities of the signals are out of control. The information gained on the changes of the interactions between the ions in the K+(15C5)2,K- solution with temperature
Sokd et al. will be useful in our further studies of reaction mechanisms with potassium anions, particularly in the case of anionic polymerization processes with these anions as an initiator. It is worth mentioning that all the spectral characteristics were the same for other solutions from potassium concentrations rangingbetween0.1 and0.4M. Thismayimply thattheexchange involving separated ions can be neglected. On the other hand the observed invariance of the NMR parameters with concentration may be a result of the compensating effects of concentration changes on various relaxation mechanisms.19 However, it cannot be excluded that the concentration range available for study may be too narrow for the concentration influence on the relaxation behavior to be observed. Consequently, no attempt could be made to determine the contributions to the relaxation processes from the various mechanisms.
Conclusions The relaxation behavior of the potassium cation in the K+(15C5)2,K- system is governed by quadrupolar (determined mainly by T ~and ) paramagnetic mechanisms with a contribution due to cation-anion exchange effects at higher temperatures. For the potassium anion, quadrupolar relaxation is affected by exchange in the whole temperature range of study. The exchange processes are assumed to take place through a transfer of Knuclei between various environments and through the cationanion exchange reactions at low and high temperatures, respectively. Aside from paramagnetic effects the presence of electrons is expected to modify the external electric field gradient at potassium nuclei, particularly at higher temperatures. The overall mechanism of cation-anion exchange seems to consist of many different pathways, the net effect being the twosite exchange of both ions. Spin-lattice and spin-spin relaxation times were evidenced to the closely related to the type of complexing agent used through the ability of the latter to separate the potassium anion from the interaction with its counterion. The availability of both ions for mutual interactions decreases in the following order: l8C6 > c222 > 15c5.
Acknowledgment. Theauthors areindepted to Dr. H. Janeczek for performing the ESR measurements of the potassium solution. References and Notes (1) Mathre, D. J.; Guida, W. C. Tefrahedron .Left. 1980, 21, 4773. (2) Dewald, R. R.; Jones, S.R.; Schwartz, B. S.J . Chem. Soc., Chem.
Commun. 1980, 272. (3) Jedlihski, Z . ; Kowalczuk, M.; Grobelny, Z.; Stolarzewicz, A. Makromol. Chem., Rapid Commun. 1983, 4, 355. (4) Jedlifiski, Z.; Kowalczuk, M.; Misidek. A. J . Chem. Soc., Chem. Commun. 1988, 1261. (5) Edwards, P. P.; Ellaboudy, A. S.;Holton, D. M. Nature (London) 1985, 317, 242. (6) Tinkham, M. L.; Dye, J. L. J . Am. Chem. Soc. 1985, 107,6129. (7) Sok61, M.; Grobelny, J.; Jedlihski, Z . Magn. Reson. Chem. 1990,28, 934. ( 8 ) SokM, M.;Grobelny, J.; Grobelny, 2.;JcdliAski, 2. Spectrochim. Acta 1991, 47A (1 I), 1547. (9) Grobelny, Z . ; Stolarzewicz, A.; Sok61, M.; Grobelny, J; Janeczek, H.J . Phys. Chem. 1992, 96, 5193. (IO) Grobelny, Z . ; Stolarzewicz, A. Pol. J. Chem. 1981, 55, 1933. (11) Holton, D. M.; Ellaboudy, A. S.;Edmonds, R. N.; Edwards, P. P. Proc. R . SOC.London, A 1988, 415, 121. (12) Dye, J. L. In Electronic Fluids; Jortner, J., Kestner, N. R., Eds.; Springer-Verlag: Berlin, 1973; p 77. (13) Edwards, P. P.; Catteral, R. Philos. Mag. 1979, 839, 371. (14) Edwards, P. P. Adv. Inorg. Chem. Radiochem. 1982, 25, 135. (15) Canters, G. W.;de Boer, E. Mol. Phys. 1973, 26, 1185. (1 6 ) Annual Reportson NMRSpectroscopy; Webb, G. A., Ed.; Academic Press: London, New York, Toronto, Sydney, and San Francisco, 1979. (17) Dye, J. L. Prog. Inorg. Chem. 1984, 32, 321, (18) Dye, J. L. In Progress in Macrocyclic Chemistry; Izatt, R. M., Christensen, J. J., Eds.; Wiley-Interscience: New York, 1979; Vol. 1, Chapter 2, pp 105-106. (19) Takeshita, T.; Hirota, N. J . Chem. Phys. 1973, 58, 3745.
