2649
EFFECT OF CATIONON THE NMRSPECTRUM OF FLUORENYL CARBANION
Effect of Cation on the Nuclear Magnetic Resonance Spectrum of Fluorenyl Carbanion
by Richard H. Cox Department of Chemistry, University of Georgia, Athens, Georgia 30601 (Received December $0, 1968)
The nmr spectra of fluorenyl lithium, sodium, potassium, and rubidium have been analyzed in terms of chemical shifts and coupling constants. Cation has no effect on the coupling constants. Chemical shifts of the carbanion appear to lower field with increasing charge/radius ratio of the cations with lithium being an exception. The results are discussed in terms of bonding properties of the cations and of the types of ion pairs formed with the cations.
That the chemical shift of a proton in an aromatic molecule reflects to some degree the n-electron density on the attached carbon atom has long been recognized as a fundamental concept in nuclear magnetic resonance (nmr) spectroscopy.’ Earlier work in this area suggested a linear correlation between proton chemical shifts and the electron density located on the carbon a t ~ m . ~ Schaefer -~ and S ~ h n e i d e r in , ~ an attempt to further define this relationship, gave several additional effects other than electron density which must be considered. These additional contributions6 to the chemical shift are: (1) the magnetic anisotropy of heteroatoms and of substituents, (2) ring current effects, (3) solvent effects, and (4)ion association effects of aromatic ions. Of these contributions, the effects of the first three can be adequately treated in favorable case~.6-~However, the effect of ion association or of counterion on the nmr spectra of aromatic ions has not been considered as extensively.lo Cation was reported to have no effect on the nmr spectrum of triphenylmethyl carbani0n.l‘ Recently, we reported the analysis of the nmr spectrum of fluorenyl potassium (l).l2 From a comparison of chemical shifts with those reported for fluorenyl sodium5 and fluorenyl lithium,’3 there appears to be a considerable effect of cation on the nmr spectrum of the fluorenyl
carbanion (1). Clearly, the effect of counterion should be taken into consideration in correlations of chemical shifts of aromatic ions. I n this paper we wish to report on the nature of the effect of cation and ion association on the nrnr spectra of carbanions. The nrnr spectra of fluorenyl lithium, sodium, potassium, and rubidium have been analyzed under identical conditions in terms of chemical shifts and coupling constants. Results are discussed in terms of bonding properties of the cations.
Experimental Section Materials. Fluorene, of commercial origin, was used without further purification. Samples were prepared under high vacuum in usual nmr tubes designed with an “onion-dome” bulb to accommodate the alkali metal mirror. Sodium and potassium mirrors were prepared by distilling the respective metals into the bulb. The rubidium metal was prepared by fusing rubidium chloride and calcium under vacuum. Freshly cut lithium metal, under an argon atmosphere, was placed in the bulb and the tube immediately placed under vacuum. I n this maner, a “reactive” metal surface was available for the reduction. Tetrahydrofuran was distilled into the nmr tube after being dried with sodium benzophenone ketyl. Tetramethylsilane (TMS) was added and the samples were sealed. The final concentration of fluorene was 0.45 M . Spectra. Proton nrnr spectra were obtained using a Varian Associates HA-100 spectrometer with a probe temperature of 29”. Calibration of the spectra was by the usual side-band method. Line positions were ob(1) J. W.Emsley, J. Feeney, and L. S. Sutcliffe, “High Resolution Nuclear Magnetic Resonance Spectroscopy,” Vol. 1, Pergamon Press, New York, N. Y., 1965, p 149. (2) H. Spiesecke and W. G. Schneider, Tetrahedron Lett., 468 (1961). (3) G. Fraenkel, R. E. Carter, A. McLachlan, and J. H. Richards, J. Amer. Chem. Soc., 82, 5846 (1860). (4) C. MacLean and W. G. Schneider, Mol. Phy8., 4, 241 (1961). (5) T.Schaefer and W. G. Schneider, Cun. J. Chem., 41, 966 (1963). (6) G. Fraenkel, D.G. Adams, and R. R. Dean, J. Phys. Chem., 72, 944 (1968). (7) J. A. Pople, J. Chem. Phys., 24, 1111 (1956). (8) C. E.Johnson, Jr., and F. A. Bovey, ibid., 29, 1012 (1958). (9) P. Laszlo in “Progress in Nuclear Magnetic Resonance Spectroscopy,” J. W. Emsley, J. Feeney, and L. H. Sutcliffe, Ed., Vol. 3, Pergamon Press, New York, N. Y., 1967,p 231. (10) G. Fraenkel, D. G. Adams, and J. Williams, Tetrahedron Lett., 767 (1963). (11) V. R. Sandel and H. H. Freedman, J. Amer. Chem. Soc., 85, 2328 (1963). (12) R. H. Cox, E. G. Janzen, and J, L. Gerlock, ibid., 90, 5906 (1968). (13) J. A. Dixon, P. A. Gwinner, and D. C. Lini, ibid., 87, 1379 (1965). Volume 79, Number 8
August 1969
RICHARD H. Cox
2650 Table I : Nmr Parameters of the Fluorenyl Carbanion Cation
Li Xa
K Rb a
Via
VZ
7.209 7.376 7.272 7.247
6.728 6.899 6.808 6.806
I n ppm downfield from TMS.
YB
6.353 6.546 6.442 6.441
I n Hz.
Y4
7.816 8.004 7.865 7.815
Y9
5.819 6.035 5.893 5.884
Jizb
Jia
Jl4
Jza
J24
Ja4
J4s
8.05 8.16 8.11 8.12
1.04 1.08 0.92 1.06
0.66 0.81 0.86 0.81
6.60 6.65 6.69 6.61
0.91 0.94 1.16 1.22
7.74 7.85 7.81 7.84
0.63 0.68 C
0.72
This coupling was not resolved and was removed by decoupling.
taincd by averaging the results of two upfield and two downfield scans. A scan width of 50 Hz was employed with a sweep time of 1000 sec. Frequency-sweep spindecoupling experiments were performed using a Hewlett-Packard 201CR audiooscillator monitored by a Varian V-4315 frequency counter. The variable temperature experiments were carried out on a Hitachi R-20 spectrometer operating at 60 MHz.
stants with cation are within experimental error. On JI4, forming the carbanion, an increase is found for J12, J24, and J 3 4 , whereas J I and ~ J 2 3 are found to decrease. These changes in the vicinal coupling constants are in accord with those predicted from correlations between bond order and coupling constants.20r21 The chemical shifts are found to reflect the nature of the cation (Table I). Of the four cations, the chemical shifts of fluorenyl lithium appear at highest field whereas Results and Discussion the shifts of fluorenyl sodium appear at lowest field. The reduction of fluorene with alkali metals first Similar results have been reported for the cyclopentadiene carbanion.6 Considering all four cations, one produces the radical anion which at room temperature might expect a correlation between the chemical shifts rapidly decomposes to give the ~ a r b a n i 0 n . l ~In this study, the reduction was followed by allowing the soluof the carbanion and some property of the cations relattion to come in contact with the alkali metal mirror ing their bonding properties. llIaking the reasonable assumption5 that the chemical shifts of the carbanion until the signals from the C-9 methylene protons of the are related to electron density, then an increase in elecstarting hydrocarbon could no longer be detected. At tron density between cation and carbanion should result this point, the spectrum was that of the fluorenyl in a decrease in the chemical shift values (higher parts carbanion. The reductions were carried out under per million) of the carbanion. identical conditions (solvent, concentration, and temExamination of the data in Table I shows that the perature) so that any differences in the spectrum of the chemical shifts of the carbanion appear to lower field in carbanion are due to the effect of cation. Spectra were the order R b < K < Na. Fluorenyl lithium is an exanalyzed in terms of chemical shifts and coupling constants with the aid of the computer program ~ ~ c 0 ~ 3 ception . l ~ and will bg discussed separately. There is a reasonable correlatih between chemical shifts of the carResults from the analysis are given in Table I. That banion and the following properties of the cations (Rb, proton 9 is coupled to protons 4 and 5 was confirmed by K, Na) : (1) ionic radius, (2) first ionization potential, a spin-decoupling experiment. The magnitude of this and (3) charge/radius ratio. These results may be coupling is similar to other five-bond couplings in rationalized by employing Fajans’ rules.22 Fajans aromatic systems.16 The chemical shifts of fluorenyl carbanion (Table I) were assigned by comparison with the shifts of methyl(14) J. J. Eisch and W. C. Kaslca, J . Org. Chem., 27, 3745 (1962). (15) S. Castellano and A. Bothner-By, Mellon Institute, Pittsburgh, fluorenyl ~arbani0na.l~Neither resonance consideraPa., 1966. tions nor Hiickel M O calculations alone account for the (16) M. W. Jarvis and A. G. Moritz, Aust. J . Chem., 21, 2445 order of appearance of the chemical shifts in the spectra. (1968), and references therein. Ring-current contributions7 to the shifts calculated (17) R. H. Cox, unpublished results. using the tables of Johnson and Bovey* also predict a (18) The bond orders and electron densities were calculated using a modified w ’ technique [A. Streitwieser, A. Heller, and M.F. Felddifferent order for the chemical shifts than is observed. man, J . Phys. Chem., 68, 1224 (1964)l. Standard bond lengths and bond angles were assumed. Starting parameters used in the calculaHowever, by using both the contribution due to ring tions were talcen from A. Streitwieser, “Molecular Orbital Theory for currents and the contribution due to excess charge Organic Chemists,” John Wiley & Sons, Inc., New York, N. Y., 1961, p 135. The contribution to the chemical shifts due to excess density, the correct order for the chemical shifts is precharge density was calculated using the equation 6 = 10.6 ppm/ dicted.‘* Chemical shifts calculated in this manner electron.5 agree to within 0.3 ppm with the experimental shifts of (19) K. D. Bartle and D. W. Jones, J . Mol. Struct., 1, 13 (1967). fluorenyl carbanion. (20) N. Jonathan, S. Gordon, and B. P. Dailey, J . Chem. Phus., 36, 2443 (1962). Cation has little effect uopn the coupling constants of (21) W. B. Smith, W. H. Watson, and S. Chiranjeevi, J. Amer. fluorenyl carbanion (Table I). Although the couplings Chem. Soc., 89, 1438 (1967). have changed considerably from their values in the (22) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemisstarting hydrocarbon, l 9 changes in the coupling contry,’’ Interscience Publishers, New York, N. Y., 1962, p 157. The Jozirnal of Physical Chemistry
EFFECT OF CATIONON THE NMRSPECTRUM OF FLUORENYL CARBANION stated that when two ions, A+ and B-, are placed in close proximity, there will be an interaction over and above their coulombic attraction, mutal polarization. The smaller and more highly charged the cation, the more distortion of the charge distribution in the neighboring anion. Stated another way, within a given series of cations bearing identical charges, the cation with the largest ionic radius will form the more ionic bond to a given anion. Therefore, there will be less distortion of the charge with rubidium than with sodium and, hence, the protons of fluorenyl rubidium should be more shielded than those of fluorenyl sodium. This interpretation is in agreement with the experimental results. However, this interpretation would predict that fluorenyl lithium should be deshielded relative to fluorenyl sodium. Several studies of the absorption spectra of carbani o n and ~ of~ the~ esr~spectra ~ ~ of radical anion^*^*^^ have recently provided conclusive evidence for the existence of two types of ion pairs in solution (solvent-separated and contact ion pairs). The equilibrium between the two types of ion pairs has been shown to depend on temperature, cation, and solvent. 23--26 From their studies
R-M+
+ solvent
R-llM+
(1)
of the absorption spectra of fluorenyl carbanion with various cations, Smid and c o w ~ r k e r found s ~ ~ ~that, ~ ~ in T H F at room temperature, fluorenyl lithium exists predominantly as solvent-separated ion pairs whereas fluorenyl sodium and potassium are mainly in the form of contact ion pairs. Furthermore, theyZ3vz4 found that a t -70" the equilibrium changes such that fluorenyl sodium exists predominantly as solvent-separated ion pairs and that fluorenyl potassium was unaffected. The important factor in determining the fraction of solvent-separated ion pairs appears to be the solvation state of the cation.24 At 25", the solvation energy of the Li cation is large enough to overcome the coulumbic attraction between cation and anion whereas for fluorenyl sodium, a temperature of -70" is needed. Changing to more polar solvents also increases the fraction of solvent-separated ion pairs.23
2651
The nmr spectrum of fluorenyl lithium is unaffected by lowering the temperature to -70". However, at -70" the spectrum of fluorenyl sodium has shifted upfield such that the chemical shifts are identical with those of fluorenyl lithium. The nmr spectrum of fluorenyl potassium is not affected by lowering the temperature. Therefore, the deviations of the chemical shifts of fluorenyl lithium may be rationalized by the difference in the type of ion pair formed. With solvent separating fluorenyl carbanion and the lithium cation, more of the charge will reside within the carbanion and, hence, the protons will be more shielded than expected from correlations of the charge/radius ratio of the cation. For fluorenyl carbanion, the shifts of the lithium salt appear to be limiting. It is doubtful, however, that the shifts of fluorenyl lithium are those of the "free" carbanion. Conductance studies show that, at the concentrations used in this study, only a few per cent of "free" ions are present.21 I n conclusion, we have shown that the chemical shifts of a given anion are dependent on the nature of the cation. Furthermore, it appears that nmr spectroscopy may be used to distinguish between different types of ion pairs. If contact ion pairs are formed, then on the basis of the correlation between chemical shifts of the carbanion and the charge/radius ratio of the cations, the chemical shifts will appear to lower field in the order CS < R b < K < Na < Li. When solvent-separated ion pairs are formed, deviations to higher field from the above order will be observed or if the ions are completely dissociated, cation will not have any effect on the chemical shifts of the anion.'l
Acknowledgment. This work was partially supported by funds provided through the Director of General Research of the University of Georgia. (23) T.E. Hogen-Esch and J. Smid, J. Amer. Chem. SOC.,88, 307, 318 (1966). (24) L. I. Chan and J. Smid, ibid., 90, 4654 (1968). (25) N. Hirota, ibid., 90, 3603 (1968). (26) N.Hirota, R. Carraway, and W. Schook, ibid., 90, 361 1 (1968)
Volume 73, Number 8 August 1969
