J . Phys. Chem. 1984,88, 3852-3855
3852
Magic-Angle Spinning Sodium-23 Nuclear Magnetic Resonance Studies of Crystalline Sodides Ahmed Ellaboudy, Mary L. Tinkham, Bradley Van Eck, James L. Dye,* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
and Patrick B. Smith Dow Chemical Co., Midland, Michigan 48645 (Received: August 24, 1983; In Final Form: November 2, 1983)
Magic-angle spinning 23Naand 133CsNMR spectra were measured for eight crystalline sodide salts, in which the cations are complexed by cryptand [2.2.2], (C222), 18-crown-6 (18C6), or 15-crown-5 (15C5). The homonuclear sodide Na'C222-Nashows separate peaks for Na' (6 -23.7 ppm) and Na- (6 -61.3 ppm). Each of the heteronuclear compounds of stoichiometry ML,Na (M = K, Rb, or Cs; L = C222, 15C5, or 18C6; n = 1 or 2) had only a single peak of Na- and none corresponding to Na', thus proving that all of these salts are sodides. Only for K'18C6.Na- (6 -55.9 ppm) was the chemical shift substantially different from that of Na- in other crystalline sodides and of Na- in solution. The NMR spectrum of Cs'18C6.Nashowed a peak for Cs+ at a chemical shift of -61 ppm, similar to that of Cs' in the "sandwich" complex Cs+ ( 18C6)2. This suggests alternate stacking of Cs' and 18C6 in the crystal. Three new crystalline sodides were synthesized. Analysis and the NMR results showed them to be Rb'18C6*Na-, K+(15C5)z-Na-, and Rb+(15C5)rNa-.
Introduction Sodides are members of a group of compounds called alkalides which have been synthesized in our laboratories in recent years.'" A number of compounds with the stoichiometry MLNa, in which M is an alkali metal and L is a crown ether or ~ r y p t a n d have ,~ been synthesized and a n a l y ~ e d .Characterization ~ of alkalides includes pressed powder dc conductivity:*9 magnetic susceptibility and EPR spectroscopy,1°and the optical absorption spectra of thin films produced by rapid solvent e ~ a p o r a t i o n ~or ~ -by ' ~ vapor dep ~ s i t i o n . ' ~The optical spectrum has been used to identify the alkali metal anion in heteronuclear alkalides since it was shown" that the peak position in M'C222.M- corresponds closely to that observed for the corresponding anion in solution. Thus, Na-, K-, Rb-, and Cs- in M'C222.M- have absorption peaks at 15 400, 11 900, 11 600, and 10 500 cm-' for Na-, K-, Rb- and Cs-, respectively, compared with values of 15 400, 12 000, 11 200, and 9800 in ethylenediamine solution^.'^ The identification of heteronuclear films of stoichiometry MLNa as sodides was based upon the absorption spectra, which showed a single absorption maximum similar to that in NaeC222.Na-.5-'z However, because of the sensitivity of the peak position to the surroundings, the assignments were not completely unambiguous. For example, the compound of stoichiometry K18C6Na exhibited a broad peak centered a t 13 300 cm-'. If this were a peak of Na-, then K'18C6 caused a red shift of 2700 (1) Dye, J. L.; Ceraso, J. M.; Lok, M. T.; Barnett, B. L.; Tehan, F. J. J . A m . Chem. Soc. 1974, 96, 608. (2) Tehan, F. J.; Barnett, B. L.; Dye, J. L. J. A m . Chem. SOC.1974, 96, 7203. (3) Dye, J. L.; Andrews, C. W.; Mathews, S. E. J . Phys. Chem. 1975, 79, 3065. (4) Dye, J. L. Angew. Chem., Int. Ed. Engl. 1979, 18, 587. (5) Van Eck, B.; Le, L. D.; Issa, D.; Dye, J. L. Inorg. Chem. 1982, 21, 1966. (6) Dye, J. L. J . Phys. Chem. article in this issue. (7) The abbreviation C222 will be used in this article for macrocyclic polyether cryptand [2.2.2]; 18C6 and 15C5 represent the cyclic polyethers 18-crown-6 and 15-crown-5, respectively. (8) Dye, J. L. J . Phys. Chem. 1980, 84, 1084. (9) Dye, J. L.; Papaioannou, J. to be submitted for publication. (10) Issa, D.; Ellaboudy, A,; Janakiraman, R.; Dye, J. L. J . Phys. Chem. article in this issue. (1 1) Dye, J. L.; Yemen, M. R.; DaGue, M. G.; Lehn, J.-M. J . Chem. Phys. 1978, 68, 1665. (12) DaGue, M . G.; Landers, J. S.; Lewis, H. L.; Dye, J. L. Chem. Phys. Lett. 1979, 66, 169. ( 1 3 ) Dye, J. L.; DaGue, M. G.; Yemen, M. R.; Landers, J. S.;Lewis, H. L. J . Phys. Chem. 1980, 84, 1096. (14) Le, L. D.; Issa, D.; Van Eck, B.; Dye, J. L. J. Phys. Chem. 1982,86, 7. (15) Dewald, R. R.; Dye, J. L. J . Phys. Chem. 1964, 68, 121.
0022-3654/84/2088-3852$01.50/0
cm-I from the position of Na- in Na+C222*Na-. If, on the other hand, the absorption were due to K-, then a blue shift of 2100 cm-' from the absorption of K- in KT222.K- or 1100 cm-' from that in K'18C6.K- had occurred. It was presumed that close cation-anion interactions occur in the crown-ether salts because of the open axial positions in the M'18C6 complex. In any event, the optical spectrum did not permit us to determine whether K18C6Na is a sodide, a potasside, or a mixed system. Alkali metal N M R spectra have been widely studied in both aqueous and nonaqueous solvents.16 The existence of Na- and Na'C222 in solutions of sodium and cryptand[2.2.2] in ethylamine, methylamine, and t e t r a h y d r ~ f u r a n ' ~and J ~ of Na- and Na'18C6 in solutions of sodium and 18-crown-6in methylaminels was confirmed by the presence of two peaks in the 23NaN M R spectrum. The unusual chemical shift of Na- [62 ppm upfield from Na'(aq) but only 2 ppm upfield from Na(g)] clearly showed that the 2p electrons of Na- are shielded from interaction with the solvent by the two 3s electrons. The substantial paramagnetic shifts of Na+ in solutions and in crystalline salts from that expected for Na+(g) is caused by interaction by solvent or anion electron density with the p orbitals of Na+.19320 Thus, 23Na N M R spectroscopy would be diagnostic for Na- in crystalline sodides; only the extreme line broadening observed for solids in conventional measurements causes difficulties. Andrew and c o - ~ o r k e r s *observed ~ - ~ ~ that rapid rotation of a single crystal of NaCl about an axis which makes an angle 0 with the field direction resulted in a narrowing of the 23Napeak depending upon the angle. Since that time, a number of studies of high-resolution solid-state 23NaN M R spectra have been made by spinning the samples at or near the "magic angle", 0 = 54.74°.24-27 In this paper we report magic-angle spinning (16) Lindman, B.; Forsen, S.'NMR and the Periodic Table"; Harris, R., Mann, B., Ed.; Academic Press: New York, 1978. (17) Ceraso, J. M.; Dye, J. L. J. Chem. Phys. 1974, 61, 1585. (18) Dye, J. L.; Andrews, C. W.; Ceraso, J. M. J . Phys. Chem. 1975, 79, 3076. (19) Ikenberry, D.; Das, T. P. Phys. Reo. A 1965, 138, 822. (20) Deverell, C. Progr. Nucl. Magn. Reson. Spectrosc. 1969, 4, 278. (21) Andrew, E. R. Arch. Sci. (Geneua) 1958, 11, 223. (22) Andrew, E. R.; Bradbury, A.; Eades, R. G. Nature (London) 1958, 182. 1659. (23) Andrew, E. R.; Bradbury, A.; Eades, R. G. Nature (London) 1959, 183, 1802. (24) Andrew, E. R. Prog. Nucl. Magn. Reson. Spectrosc., 1971, 8, 1. (25) Kundla, E.; Samoson, A,; Lippmaa, E. Chem. Phys. Lett. 1981,83, 229. (26) Oldfield, E.; Schramm, S.;Meadows, M. D.; Smith, K. A.; Kinsey, R. A.; Ackerman, J. J . Am. Chem. SOC.1982, 104, 919. (27) Ganapathy, S.; Schramm, S.;Oldfield, E. J . Chem. Phys. 1982,77, 4360.
0 1984 American Chemical Society
23Na N M R of Crystalline Sodides (MASS) 23NaN M R studies of a number of sodides as well as the 133CsMASS-NMR spectrum of Cs+18C6*Na-. In addition, the preparation and analysis of three new crystalline sodides, Rb+l8C6*Na-, K+( 15C5)2.Na-, and Rb+( 1SC5),-Na-, are reported.
.
Experimental Section Sample Preparation. The crystalline samples used in this investigation were precipitated either from 2-aminopropane-diethyl ether mixtures as previously described5 or by using dimethyl ether-trimethylamine or dimethyl ether-diethyl ether mixtures as described in a companion paper.6 Freshly prepared crystalline samples of the new compounds and of K18C6Na were analyzed for reducing power, metals, and complexant by the analysis scheme previously de~cribed,~ except that the 'H N M R procedure for the analysis of the complexant was slightly modified. Sodium acetate was used as an internal standard rather than potassium hydrogen phthalate, and a line-fitting program2* was used to fit the spectra to Lorentzian or Gaussian functions rather than relying upon machine integration of the spectra. The program gave the amplitude, full width at half-height, and standard deviation of the curve. This method gave less than 3% error with standard solution~.~~ Model Salts. Conventional salts in which Na+ or Cs' are complexed by crown ethers or cryptands were prepared by procedures similar to those of Peder~en.~O-~l The salts, together with stoichiometric amounts of complexant (mole ratio 1:l or 1:2), were dissolved in either hot methanol or 2-propanol, the solutions were filtered, concentrated by evaporation, and allowed to cool until crystals formed. Conventional salts without complexants were reagent grade and were used without further purification. Films for Optical Spectra. Thin films for optical studies were prepared by evaporating methylamine from freshly prepared solutions of the crystals as previously described.I3 The crystals were introduced either through the Teflon valve in an inert atmosphere or by using an additional side arm on the apparatus. In the latter case, heat-shrinkable Teflon tubing was used to introduce the crystals from a sealed tube, similar to the procedures previously described for metal introduction.* This method has the advantage that the sample is always under vacuum and can be kept cold if desired. Instrumentation. Preliminary nonspinning 23NaN M R spectra were obtained with a Bruker WH180 superconducting multinuclear spectrometer equipped with a Nicolet 1180 computer system. High-resolution solid-state 23Na MASS-NMR spectra were measured on a Bruker CXP-200 wide-bore spectrometer equipped with an Aspect 2000 computer system. A Bruker 360 magnet was used to obtain the 133CsMASS-NMR spectra. The available instrumentation did not permit measurement at more than one frequency for a given nucleus. A standard multinuclear MASS probe was used and the spinner gas was cooled to prevent decomposition of the sodide salts. Andrew-type rotors, made of either Delrin or poly(methy1-d3methacrylate) were used and were spun at the magic angle at rotation rates of 2-3 kHz. The rotor volume of about 0.4 cm3 was loosely filled with crystals. Single 90° pulses, spaced 5.0 s apart, were used at 52.94 M H z for sodium (4.7 T field strength) and at 47.24 MHz for cesium (8.4 T). Chemical shifts, corrected to Na+(aq) or Cs+(aq) at infinite dilution, were determined directly from the peak positions by comparison with NaCl(s), NaBr(aq), or CsI(aq) and were not corrected for second-order quadrupole effects. Upfield (diamagnetic) shifts are negative. Results New Sodides. Three new crystalline sodides were prepared for this study: Rb+18C6-Na-, K+( 15C5)2.Na-, and Rb+(lSCS),-Na-. (28) DISNMR, Disk Interactive Spectroscopy on the Aspect 2000 minicomputer version 81091, Bruker Instrument, Inc. 1981. (29) Tinkham, M. L. M.S. Thesis, Michigan State University, 1982. (30) Pedersen, C. J. J . Am. Chem. SOC.,1967,89, 7017. (31) Pedersen, C. J. J . Am. Chem. SOC.,1970, 92, 386.
The Journal of Physical Chemistry, Vol, 88, No. 17, 1984 3853 TABLE I: Analysis of Crystalline Sodidesa % deviation from presumed stoichiometry H, OHcollec- titration tion
compd
sample size,b mmol
K+(l5CS),.NaK+(15C5),.Na-C Rbi(15C5),.NaRbf(15C5),.NaK+18C6.Na' Kf 18C6.NaRbf18C6.Na'
0.180 0.180 0.093gd 0.0354e 0.346 0.131d 0.183
0
5.6
+1.3
-5.2
+1.2
+3.3
-4.2
+3.1
flame
emission M
Na
'IoWn
ether by 'H NMR
5.6 +0.6 -2.0
5.6 -9.2
-4.9 t1.5 +2.7
-0.1 +9.2 +0.8 +1.5 ~4.9 0
-8.3 C3.8
+
Based upon the reaction M+L,Na' 2H,O -+ M+L, + Na+ + 20H- H,. Sample size determined by mass unless indicated otherwise. Separate sample dissolved directly in D,O to determine crown ether content. Mass unavailable. Sample size determined from the amount of H, evolved. e Mass unavailable. Sample size determined by M flame emission.
+
A
' I I I . 1 1 , 11 I . I
100 50
0 -50 -100 -150 8(ppm)
Figure 1. Nonspinning 23NaNMR spectra of Na+C222,Na- and Na+C222.1-. Data were collected with 47 776 sweeps.
In addition, a previously prepared sodide, K+18C6.Na-, was resynthesized and analyzed to provide cleaner, solvent-free crystals. Table I gives the analytical results for these compounds expressed as percent deviation of each analysis method from the predicted stoichiometry. In all cases, the samples contained no apparent decomposition product and were stable at room temperatures, at least for short periods of time. Thermal decomposition during analysis or other sample manipulation therefore was not a problem. A more complete description of the thermal properties of these and other alkalides is given in a companion paper.6 Optical Spectra. Absorption spectra of thin solvent-free films of all the sodides listed in Table I were measured. In each case only a single absorption maximum was observed. As shown in Table 111, the peak position depends on the counterion and complexant and ranges from 13 800 cm-I for Rb+18C6*Na-to 15 400 cm-' for Na+C222-Na-. The sensitivity of the absorption maximum of Na- to environment is not unexpected since even larger solvent-dependent shifts have been observed for Na- in solution.32 However, such large variations in the position of the optical absorption peak make it difficult to unambiguously assign the absorptions to Na- rather than to K-, Rb-, or Cs- on the basis of optical spectra only. Nonspinning 23NaNMR Spectra. The 23NaN M R spectrum of Na+C222*Na-,obtained without spinning, is shown in Figure 1. It consists of a broad peak of width -2600 Hz centered at (32) Lok, M. T.; Tehan, F. J.; Dye, J. L. J . Phys. Chem. 1972, 76,2975.
~
'-
~~
3854 The Journal of Physical Chemistry, Vol. 88, No. 17, 1984
Ellaboudy et al. TABLE 111: Results of 23NaMASS-NMR Studies compd Na' e NaCl NaBr NaI Na+lSC5BrK+( 15C5),.NaRb+(lSCS);Na-
l
l
l
I
0
-100
l
+IO0
I
1
-200
6 [ppm FROM ~a+(aq.)]
Figure 2. Magic-angle sample spinning 23Na N M R spectrum of Na+C222.Na-. Also shown is the nonspinning spectrum. Data were collected with 276 sweeps for spinning. TABLE 11: Some 23NaNMR Results
0.7
NaCl (as) NaCl(s) NaT222.INa'C222.Na
18-69 -3200 -4700 2400-2800
-+12a -13' -64'
Av,,,, Hz
6a
0 +7.1 t5.6 -2.9 1-3.5 -61.3 -61.3
140 200 160 5 80 90 90
-21.9
500
-55.9
Na+I 8C6,SCNK+18C6.NaRb+l8C6*NaCs+18C6.Na-
-59.6 -62.9
120 90 75
Na'C222.SCNNaT222.INa"C222.NaNaT222.NaK T 2 2 2.NaR b T 2 22. Na'
-21 .o -21.2 -23.7' -61.3d -6 1.3 -61.3
1120 1300 1200 290 400 360
optical peak,b cm-'
14300 13 900 14000 13 800 14600
154OOi1 151OOi3 14000
Chemical shifts of separate samples were generally reproducible to t 1 ppm. Position of the optical absorption band of a thin Attributed to Na+C222. Attributed film from methylamine. to Na-. e Aqueous, infinite dilution.
'
23NaNMR for Solutionsb 6
Aui,,, Hz
2:
0 -10.7
THF
-10.1
MA EA THF
-6 1.9 -62.1 -62.8
5.2 31 120-1 70 51 11 6-9
