J . Phys. Chem. 1986, 90, 14-16
14
Detection of Rb- in Crystalline Rubidldes by *'Rb Nuclear Magnetic Resonance Mary L. Tinkham, Ahmed Ellaboudy, James L. Dye,* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
and Patrick B. Smith Dow Chemical Company, Midland, Michigan 48640 (Received: September 25, 1985)
The rubidide anion, Rb-, was detected in crystalline alkalides by "Rb magic-angle-spinning (MAS) NMR spectroscopy. The compounds Rb+(15-crown-5),Rb-, Cs+(15-crown-5),Rb-, and Cs+(18-crown-6),Rb- were found to be pure rubidides with the same isotropic chemical shift of -186 f 2 ppm relative to Rb+(aq). The frequency dependence of the chemical shift and line width correspond to a quadrupole coupling constant of 1.2 k 0.2 MHz for Rb-. The systems RbK(15-crown-5),, RbK( 18-crown-6),and Rb( 18-crown-6) are mixtures. Samples of Rb2(cryptand2.2.2) and Rb2(18-crown-6) that were expected to contain Rb- showed no signal. Although the signal of Rb+ could be detected in the simple salts RbCl, RbI, and RbSCN, the complexed rubidium cation could not be detected in the solid state. Signals from both Rb+(15-crown-5), ( A U ~is:, 5000 ~ Hz) and Rb- ( A v I I 2= 150 Hz) were detected in solutions of Rb'(lS-cro~n-5)~Rb-in dimethyl ether in the presence of excess 15-crown-5. This work represents the first observation of the NMR spectrum of Rb- in solids and of complexed Rb+ in solution.
Introduction Alkali metal N M R spectroscopy has been used to study complexes of cations by macrocyclic ligands (M+C,) in a variety of solvent^.^-^ Although this technique was successful for Na+, K+, and Cs+, extreme line broadening prevented the study of rubidium complexes. In fact, the exchange-averaged s7Rb+signal disappeared at (1 8-crown-6)/(Rb+) mole ratios below those required for complete c~mplexation.~ Alkali metal anions have been observed by N M R methods in nonaqueous solutions for Na-, K-, Rb-, and C S - ~ - and ' ~ also for Na-, K-, and Cs- in crystalline salts (alkalides) by magic-angle-spinning (MAS) solid-state NMR.'"I7 The presence of Rb- in crystalline alkalides was suggested by the optical spectra of thin dry films's-20 and later confirmed by ru-
( I ) Lindman, B.; Forsen, S. In 'NMR and the Periodic Table", Harris, R. K., Mann, E. M., Eds.; Academic Press: New York, 1978, Chapter 6 and references therein. (2) Shamsipur, M.; Popov, A. I. J . Am. Chem. SOC.1979, 101, 4051. (3) Kauffman, E.; Dye, J. L.; Lehn, J.-M.; Popov, A. I. J. Am. Chem. SOC. 1980, 102, 2274. (4) Lin, J. D.; Popov, A . I. J . Am. Chem. SOC.1981, 103, 3773. (5) Khazaeli, S.;Dye, J. L.; Popov, A. I. Spectrochim. Arra, Part A 1983, 39A, 19. (6) Ceraso, J. M.; Dye, J. L. J. Chem. Phys. 1974, 61, 1585. (7) Dye, J. L.; Andrews, C. W.; Ceraso, J. M. J. Phys. Chem. 1975, 79, 3076. ( 8 ) Edwards, P. P.; Guy, S. C.; Holton, D. M.; McFarlane, W. J. Chem. SOC.,Chem. Commun. 1981, 1185. (9) Edwards, P. P.; Guy, S.C., Holton, D. M.; Johnson, D. C.; McFarlane, W.; Wocd, B. J. Phys. Chem. 1983, 87, 4362. (IO) Holton, D. M.; Edwards, P. P.; Johnson, D. C.; Page, C. J.; McFarlane, W.; Wood, B. J. Chem. SOC.,Chem. Commun. 1984, 740. ( 1 1 ) Edwards, P. P. J. Phys. Chem. 1984, 88, 3772. (12) Phillips, R. C.; Khazaeli, S.;Dye, J. L. J . Phys. Chem. 1985,89, 606. (1 3) Edwards, P. P.; Ellaboudy, A.; Holton, D. M. Nature (London) 1985, 31 7, 242. (14) Tinkham, M. L.; Dye, J. L. J . Am. Chem. SOC.1985, 107, 6129. (15) Ellaboudy, A.; Dye, J. L.; Smith, P. B. J. Am. Chem. SOC.1983, 105, 6490. (16) Dye, J. L.; Ellaboudy, A. Chem. Br. 1984, 20, 210. (17) Ellaboudy, A.; Tinkham, M. L.; Van Eck, B.; Dye, J. L.; Smith, P. B. J . Phys. Chem. 1984,88, 3852. (18) Dye, J. L.: Yemen, M. R.; DaGue, M. G.; Lehn, J.-M. J. Chem. Phys. 1978, 68, 1665. (19) Dye, J . L.; DaGue, M. G.; Yemen, M. R.; Landers, J. L.; Lewis, H. L. J . Phys. Chem. 1980, 84, 1096.
0022-3654/86/2090-0014$01.50/0
bidium X-ray absorption spectroscopy (XANES and EXAFS).Z1 We report here two new observations in the area of 87RbN M R spectroscopy, the first N M R observation of Rb- in several crystalline rubidides and the first N M R observation of Rb'C,, in solution.
Experimental Section Crystalline rubidides were prepared as described e l ~ e w h e r e . * ~ * ~ ~ ~ ~ ~ Because of their reactivity and thermal instability, it was necessary to handle them in vacuo or under an inert atmosphere while cold (C-20 "C). Polycrystalline samples for 87Rbsolid-state N M R studies were loaded while cold into either axial Doty or Andrew-Beam rotors under a dry nitrogen atmosphere. The 87Rb MAS spectra were obtained at three field strengths, 4.227, 4.698, and 11.774 T with Larmor frequencies of 58.90, 65.44, and 163.6 MHz, respectively. The two lower field instruments were Bruker W H 180 and C X P 200 N M R spectrometers, respectively, while the highest-field system was a "home-built" FT-NMR spectrometer at the University of Illinois at Urbana.25 Temperatures ranged from -100 to -10 OC. All chemical shifts are relative to RbCl in H 2 0 at infinite dilution. Upfield (diamagnetic) shifts are negative. Static spectra and IH-decoupled spectra were obtained only at 65.44 MHz. Solutions of Rb+( 15-crown-S),.Rb-[Rb+( 15CS),Rb-] in dimethyl ether (Me20) were prepared by distilling purified Me202, into an evacuated 10-mm N M R tube that contained weighed amounts of Rb+(15C5)2Rb- and 15C5. The latter was present in excess to ensure complete complexation. The tube was then flame-sealed under vacuum. Spectra were recorded on a Bruker WH180 spectrometer at -40 OC. Results and Discussion The outstanding ability of alkali metal MAS-NMR to identify the species present in polycrystalline alkalides is illustrated in Figure 1, which shows only the peak characteristic of Cs+(18C6), in the 133CsN M R spectrum and only that of Rb- in the "Rb (20) Van Eck, B.; Le, L. D.; Issa, D.; Dye, J. L. Inorg. Chem. 1982, 21, 1966. .. .. (21) Fussi, 0.;Kauzlarich, S.M.; Dye, J. L.; Teo, B. K. J. Am. Chem. SOC.1985, 107, 3727. (22) Dye, J. L. Prog. Inorg. Chem. 1984, 32, 327. (23) Dye, J. L.; Andrews, C. W.; Mathews, S.E. J . Phys. Chem. 1975, .79 ., - - - -.
(24) Tinkham, M. L.; Tientega, F. N.; Dye, J. L., unpublished results. (25) Schramm, S.;Oldfield, E. J. Am. Chem. SOC.1984, 106, 2502.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 1, 1986 15
Letters
TABLE I: 87Rb NMR Chemical Shifts“ and Line Widths of Polycrystalline Rubidides and Simple Rubidium Saltsb
65.443 MHz 58.90 M H z 6
A v i p Hz
6
IH coupled A v i p Hz
RbC1, M A S RbI, MAS RbSCN, M A S
+125 (0.5) +179 (0.5)
150 (10) 820 (30)
+I27 (0.5) +I78 (0.5)
170 (10) 1400 (30)
K,Rb(15C5)2, M A S K,Rb(15C5)2, static Rb+(15CS)yRb-, M A S Rb+(15C5)2.Rb-, static Cs+(lSCS)yRb-, MAS Cs+(15C5),.Rb-, static CS+(18C6)yRb-, M A S C~+(l8C6)~-Rb-, static Rb18C6 K,Rb18C6
-198 (1)
855 (30)
-191 (1)
-199 (1)
850 (30)
-191 (1)
-196 (2)
800 (30)
-189 (2)
compd
-194 (1) -197 (2)
163.61 MHz
‘H decoupled Avip Hz
6 + I 2 8 (0.5)
370 (30) 1400 (30) 460 (30) 1830 (30) 490 (30) 1830 (30) 650 (30) 2450 (30)
350 (30) 1040 (30) 480 (30) 1400 (30) 490 (30) 1160 (30) 680 (30) 2210 (30)
-20.5 (0.5) -29.8 (0.5) -187 (1)
550 (10)
-188 11)
300 (10)
-187 ( I )
560 (10)
-187 (1)
420 (IO)
d
d 520 (10) 400 (10)
920 (30) d
d
-185 (1) -193 (1)
d
AVip Hz 240 (10) C
“Chemicalshift from Rb+(aq) at infinite dilution. *Uncertainty . aiven - in parentheses. CPeakoverlap prevents measure of full width at half-height. dNo signal. dependent. Although XANES data21!27indicated that these alkalides contain both Rb+( 18C6) and Rb-, not all samples yielded 87Rb MAS-NMR spectra. For example, one preparation of stoichiometry KRb18C6 gave two Rb- signals at 163.6 M H z indicating, perhaps, the presence of mixtures of K+18C6Rb-, Rb+lSC6Rb-, and probably the corresponding potassides. However, no Rb- signal was observed with subsequent preparation, nor was any 39K signal observed. Likewise, only one preparation of nominal stoichiometry Rb18C6 showed the 87Rb peak of Rb- while crystals of stoichiometry Rb218C6 showed no 87Rbpeaks. Yet every sample prepared from rubidium and 18C6 showed evidence27 in the XANES studies for the presence of Rb-. Thus, the absence of the N M R peak of Rb- cannot be used as evidence that the rubidide ion is not present. The variability from one preparation to the next in these systems (18C6 with R b and/or K) indicates that 200 100 0 I -100 -200 -300 -400 the factors that determine which crystalline phases precipitate from solution were not under control in these cases. In contrast, the compounds Rb+(15C5)2Rb-, Cs+( 15C5)2Rb-, and Cs+400 200 0 -200 -400 -600 (18C6)2Rb- yield no such ambiguities. In addition to species identification, 87Rb N M R studies of Chemical Shift from M*(aq), ppm crystalline rubidides yield information about the isotropic chemical Figure 1. (at 65.61 MHz) and *’Rb (at 163.6 MHz) MAS-NMR shift and quadrupolar coupling constant of Rb-. Only the central of polycrystalline Cs+(18-~rown-6)~.Rb-. m = -l 12,can be observed. The secondtransition, m = 1 / 2 order quadrupolar shift of this transition affects both the line width spectrum. The chemical shifts are characteristic of both the and the chemical shift observed by MAS-NMR. For the axially ‘sandwich” cation Cs+( 18C6)215-16 at -57 ppm and the Rb- anion symmetric case, the frequency difference between the singularities at -187 ppm (solution values range from -185 to -197 p ~ m ) . ~ . ’ ~ is given by28 A small amount of Cs- would be easily detectable as a narrow
I
Rb-
-
Thus the salt of stoichiometry peak at -228 to -292 ppm.7*15s16 C ~ R b ( 1 8 C 6 is ) ~the “clean” rubidide rather than the ceside or a mixture. This conclusion was also reached on the basis of X-ray absorption studies.21 Table I gives the chemical shifts and full widths at half-height for the 87RbMAS-NMR spectra of three simple rubidium salts and six alkalides that contain Rb-. Only for the simple salts could the signal of Rb+ be detected. The extreme quadrupolar broadening of complexed Rb+ prevented its detection in model salts such as Rb’(lSC6)SCN- and in the compound Rb+(1 5C5)2Na-. The latter is known from the crystal structurez6 to contain the sandwich cation Rb+(15C5)2. In some cases, the expected signal of Rb- was not detected. For example, we expect on the basis of optical spectrals and X-ray absorption studies2’ that the compounds of stoichiometry Rb2(cryptand 2.2.2) and KRb(cryptand 2.2.2) contain Rb-; yet no 87Rbsignal could be detected in the MAS studies of these systems. As expected, the electride Rb+( 15C5),e- showed no signal of either Rb+ or Rb-. Salts that contain rubidium and 18-crown-6( 18C6) with and without potassium appear to be mixed systems that are preparation (26) Fussl, 0.; Ward, D. L.; Dye, J. L., unpublished results.
*v=
L 224 V Q 2 V L [Z(Z+
1p;]
in which vQ, the nuclear quadrupole frequency, is related to the nuclear quadrupole moment Q and the electric field gradient eq by VQ
=
3e2qQ
21(2Z- 1)h
=
I( e
W )
2
Equation 1 may be used with the line width to estimate the maximum value of vQ. Of course, incomplete removal of proton dipolar coupling and chemical-shift anisotropy by spinning at the magic angle will also contribute to the line width. The absence of well-defined singularities (Figure 2) suggests a distribution of quadrupolar coupling constants. The line widths of the 87Rb MAS-NMR peaks of Rb- in the various rubidides given in Table I yield a maximum quadrupole coupling constant, 2vQ, of 1.3 MHz, (27) Fussl, 0.; Teo, B. K.; Dye, J. L., unpublished results. (28) Kundla, E.; Samoson,A,; Lippmaa, E. Chem. Phys. Left. 1981, 83, 229.
16 The Journal of Physical Chemistry, Vol. 90, No. 1, 1986
Letters I
IO0
0
-100
- 200
- 300
Chemical Shift from Rb+(aq). ppm
I
0
1
- 100 Chemical Shift
I
- 200
I
Figure 3. *'Rb NMR spectrum of Rbt(15-crown-5)2+Rbin dimethyl ether (0.068 M) at 235 K with 24000 acquisitions. A has 40-Hz exponential broadening while B has 500-Hz exponential broadening.
I
-300
from Rbt (aq), p p m
Figure 2. *'Rb (at 65.4 MHz) NMR spectra of polycrystalline Rbt-
(15-cro~n-5)~.Rbat 225 K top, static-coupledspectrum; bottom, MAS coupled spectrum. Both spectra were obtained with 2400 acquisitions and have 50-Hz exponential broadening. with a standard deviation of f 0 . 3 MHz. The chemical shifts of Rb- decrease with increasing Larmor frequency, as expected from the quadrupolar shift of the central transition. For 87Rb with nuclear spin 3 / 2 the shift in ppm is related to the Larmor frequency byZs 6 = 6, -
(3)
in which 6,, is the isotropic chemical shift. The data in Table I (excluding RbK18C6) yield 6, = -186 f 2 ppm and 2vQ = 1.2 f 0.2 MHz. A quadrupolar coupling constant ( 2 q Q / h ) of 1.2 MHz for Rbis consistent with the estimatez9 of 1 0 . 2 3 M H z for Na- in Na+C222Na-. If we assume the same external field gradient, then correction for the differences in quadrupole moment and the Stemheimer antishielding factor (of the cations) yields an estimate of 1 2 . 2 M H z for Rb-. The absence of a signal for complexed Rb+ is not surprising in view of the large quadrupolar broadening. Based upon the quadrupole-broadened MAS-NMR signal29 of N a + in Na' (18C6)SCN-, we estimate the MAS line width of Rb'(18C6) to be -60000 Hz, which is too broad to detect with our instrumentation. Static spectra of Rb-, with and without proton decoupling, were measured at 65.4 MHz (Table I). The quadrupolar contribution is expected to increase the square root of the second moment, which is approximately proportional to the line width, by a factor of 2.7 to 3.6, depending upon the asymmetry parameter, 7, from MAS to static3, Since the line widths observed by MAS-NMR were essentially unchanged upon proton decoupling, we assume that the dipolar contribution from coupling to protons has been removed by spinning. The increase in line width from spinning to static-decoupled spectra falls within the range expected from the quadrupolar contribution. This indicates that the contribution from chemical shift anisotropy is small. Reduction of the static line width of Rb- upon proton decoupling ranged from -250 H z for C ~ ' ( l 8 C 6 ) ~ R b -to -650 Hz for Cs+( 15C5)zRb-. Calculation of the expected dipolar contribution by the method of Van Vleck3' would require knowledge of the crystal structures, which are not available for these compounds. However, an estimate can be made by increasing the known Nato H distances from the crystal structure3zof Cs'( 18C6),Na- to (29) Ellaboudy, A.; Dye, J. L. J . Mugn. Reson. in press. (30) Behrens, H.-J.; Schnabel, B. Physicu, 1982, 1148, 185. (31) Van Vleck, J. H. Phys. Rev. 1948, 74, 1168. (32) Dawes, S . B.; Ward, D. L.; Dye, J. L., unpublished results
accommodate the larger size of Rb-. These calculations yield a dipolar contribution of -900 Hz, which is much larger than that observed for this rubidide. This suggests, as with s o d i d e ~rapid ,~~ motion of the CHI protons of the 18C6 macrocycle at the temperatures at which the N M R measurements were made. The large quadrupolar broadening of the 87RbN M R signal of complexed Rb' has hitherto prevented detection of this species even in solution.5 By using the presumably more symmetric complex Rb+( 15C5)*, we hve succeeded in detecting both the complexed cation and Rb- in dimethyl ether solutions. The spectrum of 0.068 M Rb+( 15C5)*Rb- in MezO in the presence of excess 15C5 (mole ratio 15C5/Rb+ = 14) is given in Figure 3. The peaks at +33 and -192 ppm are assigned to Rb+(15C5)* and Rb-, respectively. The Rb- peak position agrees with the chemical shift range of -185 to -197 ppm observed p r e v i o ~ s l y . ' ~ ~ ~ The close agreement with the value of -186 ppm found for Rbin crystalline samples demonstrates how little this species is perturbed by its surroundings. The line width of Rb- (1 50 Hz), together with a nuclear quadrupole coupling constant of 1.2 MHz estimated from the solid-state spectra, yields an approximate s for this species. This is, of course, correlation time of only a rough estimate since the field gradient in solution may differ appreciably from that in the solid. The much broader line of Rb+(15C5)z (- 5000 Hz) demonstrates the much lower symmetry of the cation environment relative to the anion. The complexed cation, being larger, probably has a longer correlation time than the anion. If, however, one uses the same correlation time for the cation as for Rb-, the line width yields a quadrupole coupling constant of -7 M H z for Rbf(15C5)z. This, in turn, would give an MAS line width in the solid of -24 kHz, which is too broad to measure.
-
Summary The use of 87RbMAS-NMR provides an excellent confirmation of the presence of Rb- in crystalline alkalides. However, the absence of a signal does not necessarily mean that Rb- is absent. The signal of complexed Rb' in the solid is too broad to detect but both Rb'( 15C5)z and Rb- were detected in MezO solutions. The quadrupole coupling constant of Rb- in crystals is 1.2 f 0.2 MHz, while that of complexed Rb+ is probably at least an order of magnitude larger. Comparison of proton-coupled and proton-decoupled static line widths indicated the presence of -CHz motion in the complexant, even in the solid state. Extrapolation of the chemical shift to infinite frequency gives an isotropic chemical shift of -186 f 2 ppm for Rb- in crystalline rubidides, which falls within the range of Rb- chemical shifts in solution.
Acknowledgment. This work was supported by National Science Foundation Solid State Chemistry Grants DMR-79-21979 and DMR-84-14154. We thank K. M. Johnson for technical assistance and Professor E. Oldfield and Mr. B. Montez of the University of Illinois a t Urbana for assistance with the studies at 163 MHz.
