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Lithium-7 NMR studies of macrobicyclic complexes in ambient temperature molten salts
Pdyhedron Vol. 7. No. 15, pp. 1341-1347.1988 Printed in Great Britain
0
LITHIUM-7 NMR STUDIES OF MACROBICYCLIC COMPLEXES IN AMBIENT TEMPERATURE MOLTEN RICK R. RHINEBARGER
and ALEXANDER
0277~5387188 S3.M)+.00 1988 Pergamon Press plc
SALTS
I. POPOV*
Department of Chemistry, Michigan State University, East Lansing, MI 48824, U.S.A. (Received 23 November 1987 ; accepted 22 February 1988) Abstrac&-Complexation of the Li+ ion by macrobicyclicdiazapolyoxa ligands (cryptands) was studied in a 45 : 45 mol% altinum(III) chloride-N-(n-butyl)pyridinium chloride mixture at 40°C. Lithium-7 NMR measurements at this temperature and a field strength of 42.28 kG, showed that the exchange of the Li+ ion between the “free” and complexed sites is slow, and separate resonance signals were observed for the free and complexed Li+ ion. Conditional formation constants were obtained from the integrated areas of the resonance lines ; the complex stabilities were found to decrease in the order C221. Li+ > C222 - Li+ > C222B * Li+. In the case of the C222 * Li+ complex, a decomplexation rate constant of N 5 x lo2 s- ’ was obtained at the coalescence temperature of 40°C. Solid state 7Li NMR spectra were obtained for the C211- LiA1C14and C222B * LiAlCl., complexes, in the static and magic angle spinning modes. These measurements indicate that the electric field gradient at the 7Li nucleus in these complexes is small, yielding narrow resonance lines in the MAS mode.
Molten salt systems, which are liquid at room temperature, are interesting non-aqueous solvents with some unusual properties. Some ten years ago Osteryoung et al.’ reported that the aluminum(III) chlorideN-(n-butyl)pyridinium chloride mixtures, containing 4466 mol% of AlC13, are liquid at, or near, room temperature. Since that time numerous studies of this and similar systems have been described in the literature. We reported previously 7Li NMR studies of lithium chloride solutions in these melts’ and, in particular, formation of lithium ion complexes with several crown ethers, 3 These studies indicated’ that in the basic mixtures (< 50 mol% AlC13) lithium chloride exists as the dichloride anion, LiCl; and that the stabilities of the crown ether complexes with the Li+ ion follow approximately the same trend as the one observed in many non-aqueous solvents. Unfortunately, crown ethers decompose rapidly in acidic mixtures (> 50 mol% AlC13) which, obviously, precludes complexation studies in these media. It seems likely that with two-dimensional crown *Author to whom correspondence should be addressed.
ethers the complexed lithium ion may still be bonded to a chloride ion or ions. On the other hand, macrobicyclic diazapolyoxa ligands (cryptands), originally synthesized by Lehn et aL4 form very stable three-dimensional complexes (cryptates) with a variety of metal ions, and with alkali cations in particular.’ When the size of the cation is equal to, or somewhat smaller than the cryptand cavity, the cation inside the cavity is completely insulated from the environment (“inclusive complex”6). It was of interest to us to investigate the complexation reactions of the lithium ion with several cryptands with different cavity sizes, and particularly to determine whether the cryptated lithium ion is still bonded to one or more chloride ions.
EXPERIMENTAL N-(n-butyl)pyridinium chloride (BPCl) and aluminum(II1) chloride were prepared as described previously. ’ Melt batches were prepared by mixing weighed amounts of AlC13 and of BPCl in Pyrex weighing bottles using Teflon-coated magnetic stirring bars. References to “basic melt” within the
1341
1342
R. R. RHINEBARGER
text refer specifically to the melt with 45.00 mol% AlCl,-55.00 mol% BPCl composition. All operations were carried out in a dry box under an inert atmosphere (est. H,O/O, c 10 ppm) at room temperature. Lithium chloride (Fisher) was dried at 100°C for three days. Lithium tetrachloroaluminate was prepared by fusing equimolar amounts of LiCl and AlC13 at 150°C in a vacuum-sealed Pyrex ampule. The melting point of LiAlCl,, in a sealed capillary, was 144+2”C (ref. 7, 143.o”C). Cryptands C211, C221, C222 and C222B (monobenzo C222) were obtained from MCB Manufacturing Chem. Inc. and were used as received. Stock solutions of lithium chloride in the basic melt (1.00 mol% LiCl in 45.00 mol% AlCl, melt) were obtained by adding appropriate amounts of LiCl to pure basic melt and stirring the solution until it became homogeneous (ca 24 h). Samples with different ligand/Li+ mole ratios for NMR measurements were prepared by adding weighed amounts of the cryptands to the stock solution, stirring for 30 min and removing 0.5 g samples at the desired cryptand/lithium ion mole ratios. These samples were sealed under vacuum. All ‘Li NMR measurements were obtained at 40 (+ 1)“C on a Bruker WH-180 NMR spectrometer at a resonance frequency of 69.951 MHz (field strength of 42.28 kG). Sample tubes were mounted co-axially within 10 mm o.d. NMR tubes containing the external reference solution (0.015 M LiCl in D20 for *H lock). In cases where sample and reference 7Li signals overlapped, a secondary external reference solution (0.015 M LiCl in pyridine) was used, and spectra were obtained without lock. Chemical shifts are reported vs the external aqueous reference, and are corrected for the magnetic susceptibility of basic melt.* Paramagnetic shifts from the reference are indicated to be positive. Deconvolution of the ‘Li NMR spectra to obtain integrated areas was performed by using the NTCCAP subroutine provided in the Nicolet 1180 software package. Lithium-7 solid state NMR spectra were obtained at 22°C in the static and magic angle spinning (MAS) modes by using a Doty Scientific multinuclear solid state probe. Samples were contained in Delrin plastic rotors with end caps. Chemical shifts are referenced to the primary external reference solution (static) by sample substitution. Elemental analyses for the C211- LiAlC14 and C222B - LiAlCl, complexes were performed by Galbraith Laboratories. These results are given in Table 1.
and A. I. POPOV Table
Element C H N 0 Li Al Cl
1. Elemental analysis for C211. LiAlC14 and C222B - LiAICl, complexes C211 -LiAlC14 % theor. % found 40.6 6.3 4.6 18.5 0.9 3.7 21.8
RESULTS
41.8 5.7 4.4 20.2 1.1 4.3 22.4
C222B - LiAlCI, % theor. % found 36.4 6.0 5.3 18.8 0.8 4.9 28.1
36.2 6.1 6.0 13.8 1.5 5.8 30.5
AND DISCUSSION
Lithium-7 NMR measurements in the melt Lithium-7 NMR spectra were obtained for samples with cryptandjithium ion mole ratios varying from zero to 4.37: 1 (C222), 3.44: 1 (C221) and 1.18 : 1 (C222B). For C222, only mole ratios from 0.757 : 1 to 2.80 : 1 could be studied. Typical spectra for the C211, C221 and C222B - Li+ systems are shown in Fig. 1. In each case, upfield signals are assigned to the cryptated Li+ ion. The chemical shifts for complexed Li+ were unaffected (within f 0.02 ppm) by changes in the cryptandjithium ion
8
(ppm)
Fig. 1. Lithium-7 NMR spectra at 40°C in basic 0.949 mol % C222B+ 1.00 mol % LiCl ; melt. ---0.988 mol % C221+0.992 mol % LiCl ; . . . . 0.888 mol % C211+0.985 mol % LiCl. In each case the diamagnetic signal is that of the complexed Li+ ion.
Lithium-7 NMR of macrobicyclic complexes Table 2. Lithium-7 chemical shifts for the lithium cryptates in basic AlCl,-(BP)Cl melt at 40°C Cryptand c211 c221 c222 C222B
(Ppm) -0.79 -1.49 - 1.01* -1.08
“Versus 0.015 M LiCl in DzO external reference ; corrected for the difference in magnetic susceptibility between basic melt and water; uncertainty = f0.02 ppm. *At a 1: 1 C222 : Li+ mole ratio.
s
-0.9
t
-0.7
Et -
1343
I * ****ttt t
t
1 f
i cn
(4
C2ll/Li+
MOLE
RATIO
-1.5
mole ratios. A summary of chemical shift values for these systems is given in Table 2. As shown in Fig. 2, the high frequency signals attributable to the lithium chloro complexes, were observed to shift paramagnetically with the increasing cryptand/lithium ion mole ratio. We reported previously2 that in the basic melt lithium chloride exists as the dichlorolithium ion LiCl;. Concentration dependence of the 7Li chemical shift as a function of LiCl concentration has been interpreted as being due to a dimerization equilibrium. *
c
t
‘tt
t*
I
I
I
I
I
2LiCl-2 Kd_ -Li2Cl:-
I
I
0.2
0.4
0.6
0.8
1.0
1.2
I.4
C22l /Li
-1.1 t g
t
t
1.8 -
(b)
where log& = 2.82f0.39. At an analytical LiCl concentration of 1.OOmol%, approximately 90% of the salt exists as the dilithium tetrachloro complex. The above results indicate that observed paramagnetic shift of the “free” Li+ ion results from the dissociation of the lithium chloro complexes when the cryptate is formed. The constancy of the chemical shifts for the cryptated lithium ion sites, as the total amount of the macrobicyclic ligand is varied, indicates that the lithium ion resides within the cryptand cavities, i.e. inclusive complexes are formed. Similar behaviour has been observed by Cahen et al.’ in a 7Li NMR study of the C21 llithium ion system in various non-aqueous solutions. It is apparent that in the basic melt, 40°C is well below the coalescence temperature for the C211, C221 and C222B * Li+ systems. Slow exchange is observed and the signals for the “free” and complexed lithium ion sites are well resolved. Lithium-7 NMR spectra obtained for the C222 * Li+ system in basic melt were found to be quite different from those of the other three cryptates (Fig. 3). In this case, the “free” and cryptated lithium ion signals overlapped, yielding broad (ca 100 Hz) lines. In addition, the upfield signal assigned to the C222 - Li+ cryptate exhibited a significant (- 0.5 ppm) diamagnetic shift with increas-
t
- 1.4 t I
+
MOLE RATIO
tt
tt
t
+
-1.0 I
(cl
0.2 C2228/
.
. Li+
MOLE
RATIO
Fig. 2. Chemical shifts of the “free” (a) and cryptated (*) lithium ion signals as a function of the cryptand/lithium ion mole ratio at 40°C in basic melt; (a) C211, (b) C221 and (c) C222B.
ing C222/Li+ mole ratio. It should be noted that the linewidths of the other three systems ranged from ca 8 (C221) to 20 Hz (C222B). The linewidths of the C222 - Li+ complex signals decreased with increasing mole ratio, tending to values which are
1344
R. R. RHINEBARGER
and A. I. POPOV
the signals due to “free” (Sr) and cryptated (6,) lithium ion populations in the C221, C222 and C222B * Li+ systems at 40( f 1)“C in the basic melt. Assuming that the dimer must dissociate in order for formation of the cryptate complex to occur,
c222 / Li + MOLE RATIO: 2.60
LiCl; +LK.‘ _
5
0 8
where L is the cryptand. We have previously noted’ that small changes in composition in basic melt solutions of LiCl have no effect on 7Li chemical shifts. Since the concentration of free chloride ions in basic melt is in a ten-fold excess of either the lithium ion or cryptand concentrations, small changes in [Cl-] have little effect on the equilibrium reaction. Therefore, in the derivation of the equilibrium expression, we can neglect the chloride ion concentration and write,
-5
(ppm)
Fig. 3. Lithium-7 NMR spectra for the C222 * Li+ system at 40°C in basic melt as a function of C222/Li+ ratio.
similar to those observed in the other systems ( W1,2= 13 Hz at a mole ratio of 2.80 : 1). Thus, it appears that for the C222 * Li+ system, 40°C is near, but slightly below the coalescence temperature. The exchange is slow and the signals for the two lithium ion sites are broadened by the exchange to the point where they overlap. At the coalescence temperature, the rate of a firstorder decomplexation process (k_ i) is given by eq. (l)‘O k_ 1 = nAr/,f2
LiL+ + 2Cl-
(1)
where Av = VA- vi,, and A and B refer to the “free” and complex sites in the absence of exchange. We assume that eq. (1) applies to the C222/Li+ system at 40°C and that the chemical shift observed for the complex at a C222/Li+ mole ratio of 2.80 : 1 (6 = BB= - 1.49 ppm) is a fair approximation of the limiting chemical shift of the complex site. The chemical shift of the “free” lithium ion (site A, no C222 present) is dA = + 1.55 ppm. Thus, A6 = 3.04 ppm or 213 Hz at the stated resonance frequency. Substituting this value into eq. (1) yields k-, - 5x10*s-‘.C0~etal.“haveestimatedak_, value of > 3 x lo* s- ’ for the C222/Li+ system in methanol at 25°C. It should be noted that in our case eq. (1) does not yield the true value of the firstorder decomplexation rate constant. The transfer of the lithium ion from C222 * Li+ to Li2C1iinvolves more then one step, therefore, the value we obtained is a “conditional” rate constant. The NTCCAP subroutine in the Nicolet software package was used to calculate integrated areas for
[LiL+] ’ = [LiCl;][L] *
(2)
We define : D 3 [Li2Cl:-] ; A4 z [LiCl:] ; C = [LiL+] ; [L] = concentration of the free ligand ; N = total concentration of the ligand ; P z total concentration of the Li+ ion and f = fraction of the total area due to the cryptated Li+ ion. Equilibrium relationships and mass balance yield,
&=y$ C KS = MXL
(4)
N=L+C
(5)
P = M+2D+C
(6)
c=
fP.
(7)
From the above equation we easily obtain an expression for K, in terms of known or measurable quantities, Kd, P, N and $ JK,f P Ks = -1+[1+8K,P(l-f)]“*[N-Pf]’
(8)
A similar procedure was not feasible for the C211-Li+ system due to the formation of a precipitate at mole ratios between zero and one. Samples with mole ratios of 1: 1, or slightly above this value, remained homogeneous, but the signal for the “free” lithium ion site was too low in intensity to be detected. The results of these calculations are given in Table 3 along with values previously reported for lithium cryptates in aqueous and some non-aqueous media.g*‘2*‘3In general, the stability constants for the cryptates in basic melt are smaller than those
Lithium-7 NMR of macrobicyclic complexes
1345
Table 3. Comparison of lithium cryptate stability constants (log&) in the basic melt, water and some non-aqueous solvents”
Solvent
c211
Cryptand c221 c222
Water Methanol Ethanol Acetonitrile Propylene carbonate n-Methylpropionamide Dimethylformamide Dimethylsulphoxide Pyridine
5.5 8.0, 8.4, 10 12.4, 6.43 6.85 5.g4 -
2.50 5.3* 5.38 10.3 9.60 3.4, 3.58 2.7, -
Basic melt
-
(kO.15) 2.51
C222B
0.99 2.6 2.3 7.0 6.94 2.9, 1.0 2.94’ (kO.12) 2.06
2.19’ &Z)
o Ref. 11 except as noted. bRef. 12. ‘Ref. 8.
obtained in non-aqueous solutions. It is interesting to note that in basic melt, the change in complex
stabilities in going from C222 to C221 is either equal to, or less (by 2-3 log& units) than that found in other solvents. This apparently reduced complexation selectivity with respect to cryptand cavity size, and overall reduced stabilities, as compared to other media, is a reflection of the stability of the dilithium chloro complex in the basic melt. As mentioned below, in the solid C222B * Li+ complex, the cation is bonded only to the donor atoms of the ligand, i.e. lithium chloro complexes must dissociate before the above lithium cryptate can be formed. It seems quite likely that the structure of the C222B * Li+ cryptate in the melt is very similar to that in the solid state (especially in view of the slow ambient temperature exchange of the Li+ ion). On the other hand, it also seems likely that in the Li+ complexes with the two-dimensional crown ethers, the cation remains bonded to the chloride ion or ions. In such cases, the “conditional” formation constant of the crown complexes would be roughly of the same magnitude as that for the formation of the cryptate. Unfortunately, thus far, attempts to isolate a crystalline Li+ complex with a crown ether have been unsuccessful. In comparing the C222B * Li+ and C222 - Li+ cryptates, it is apparent that the presence of the benzene ring in the former case reduces the decomplexation rate. The steric influence of the benzene ring may also be responsible for the slightly smaller stability constant for the C222B * Li+ cryptate versus C222 * Li+ .
Cryptate precipitates and 7Li MAS NMR Precipitates were observed to form in C211* Li+ samples in the basic melt with mole ratios between zero and 1: I. This material was isolated by vacuum-filtration of the melt solution containing a ca 1: 1 C211: Li+ mole ratio, washing obtained crystals with benzene, and drying them under vacuum for several days. Elemental analysis of the product (Table 1) showed it to be the C211 *LiA1C14 complex. A solid, analysed as C222B * LiAlCl,, was obtained with the same procedure as C222B : Li+ at mole ratios greater than 1: 1. The crystal and molecular structure of this latter complex has been recently published. l4 In the basic melt the C221. Li+ system exhibits no tendency towards precipitate formation ; at 40°C samples remained homogeneous at all mole ratios studied. Lithium-7 solid state NMR spectra (static and MAS modes) were obtained for polycrystalline samples of LiCl, LiAlC14, C211. LiA1C14 and C222B * LiAlC14. The spectra obtained from LiA1C14 and the C222B * LiAlC14 complex are shown in Figs 4 and 5. All NMR data are summarized in Table 4. The classic Pake doublet (m = l/2 + m = - l/2 transition) was observed for LiCl in the static mode. According to Fyfe,” for a nucleus with a spin of 312 in a glass or polycrystalline powder, experiencing large quadrupolar interactions, the observed spacing of the doublet equals (25/9)A,, with Az defined by, A2
=
(3/W + x2/vo
(9)
1346
R. R. RHINEBARGER
and A. I. POPOV where x is the nuclear quadrupole coupling constant (e*qQ/h). The observed splitting (5.5 kHz) yields A2 = 2000 Hz and x = 1.7 MHz. This result is within an order of magnitude of the value for ‘Li in lithium silicate glasses (910 kHz) obtained by Tokuhiro et al. ’6 No splitting was observed in the static spectrum of LiAlC14 (Fig. 4), which indicates a smaller quadrupolar interaction for the Li nucleus in this compound. In this case,15 Av,,, = A, = x/4.
Resonance
With Av 1,2= 420 Hz, a x value of 1.7 kHz is obtained. This very small value for x is reasonable in view of the known crystal structure of LiA1C14 (SpZiCe group p2,/c), which is made up of LiCl6 octahedral layers linked together by AlCl, tetrahedra. ’ 7 The narrowing effect of magic angle sample spinning is seen in the spectra for C222B * LiAlCl, (Fig. 5). In the spinning mode, the central transition is partially resolved from the broad background. This broadening, due to dipolar interactions, is not completely removed at the spinning rate of 2.1 kHz. By using eq. (10) and since Av,,~ = 1600 Hz (static), a coupling constant of 6.4 kHz is obtained for this complex. For the C211- LiA1C14 sample, stable spinning rates of > 1 kHz could not be obtained. Thus, very little reduction in the dipolar broadening was observed. In the MAS mode, the central transition was barely discernable at this spinning rate. A coupling constant of ca 8 kHz is estimated from Av ,,* of approximately 2000 Hz in the static mode. The coupling constants for the ‘Li nucleus in the two cryptate complexes are quite small, indicating highly symmetric environments for the lithium ion. Evaluation of the differences in chemical shifts among the four compounds (Table 4) is difficult due to the lack of literature values for these or other lithium compounds in the solid state. Solid state 7Li NMR measurements by Dye and Ellaboudy18 show
Frequency
Fig. 4. Solid state ‘Li NMR spectra of LiAlC&. Upper spectrum--static mode ; lower spectrum obtained with “magic angle” spinning.
I
Resonance Frequency
Fig. 5. Solid state ‘Li NMR spectra of the complex, C222B- LiAlC14.Upper spectrum-static mode ; lower spectrum obtained with “magic angle” spinning.
Table 4. Lithium-7 solid state NMR results for various lithium compounds
Sample
(Ppm)
MAS mode
Linewidth (Hz) Static mode
x (kHz)
0.015 M
LiCl/D*O LiCl LiAlCl., C222B - LiAlC14 C211. LiAlCl,
0.0 -3.2 -0.8 +1.0 +1.0
(10)
450 @ 2.7 kHz 58 @ 1.8 kHz 2OOa2.1 kHz cu 2000 @ 1 kHz
19 Splitting : 5.5 kHz 420 1600 ca 2000
1700 1.7 6.4 8
Lithium-7 NMR of macrobicyclic complexes shifts of -2.8 ppm for LiCl, - 1.1 ppm for LiI, -0.8 ppm for C211. LiI, and -3.5 ppm for Li+ * C211- Naa. Data given in Table 4 show that the cryptands effectively remove the shielding influence of the chlorine atoms in the AlCl; ion on the ‘Li nucleus ; the shifts for LiCl and LiAlCl, are more negative (by about 2-3 ppm) than those of the cryptates. This lack of Li-Cl interactions is understandable since the lithium ion is expected to reside within the ligand cavities, a fact which has been confirmed for the C222B - LiAlCl, complex. I4 chemical
Acknowledgement-We gratefully acknowledge the support of this study by a National Science Foundation research grant (CHE-85 15474).
5.
6. 7. 8. 9.
10. 11. 12. 13.
REFERENCES 14. 1. R. J. Gale, B. Gilbert and R. A. Osteryoung, Znorg. Chem. 1978, 17, 2728 ; R. J. Gale and R. A. Osteryoung, Znorg. Chem. 1979,18, 1603. 2. F. Taulelle and A. I. Popov, Polyhedron 1983,2,889. 3. R. R. Rhinebarger, J. W. Rovang and A. I. Popov, Znorg. Chem. 1984, 23, 2557. 4. B. Dietrich, J.-M. Lehn and J.-P. Sauvage, Tetruhedron Lett. 1969,2882, 2889.
15. 16. 17. 18.
1347
A. I. Popov and J.-M. Lehn, in Coordination Chemistry of Macrocyclic Compounds (Edited by G. A. Melson), Chap. 8. Plenum Press, New York (1979). E. Mei, A. I. Popov and J. L. Dye, J. Am. Chem. Sot. 1977,99,6532. J. Kendall, E. D. Crittenden and H. K. Miller, J. Am. Chem. Sot. 1923,45,976. R. R. Rhinebarger, J. W. Rovang and A. I. Popov, Znorg. Chem. 1986, 25,443O. Y. M. Cahen, J. L. Dye and A. I. Popov, J. Phys. Chem. 197579, 1289. J. Sandstrom, Dynamic NMR Spectroscopy. Academic Press, New York (1982). B. G. Cox, H. Schneider and J. Stroka, J. Am. Chem. Sot. 1978,100,4746. M. K. Chantooni and I. M. Kolthoff, J. Solution Chem. 1985,14, 1. B. G. Cox, D. Knop and H. Schneider, J. Phys. Chem. 1980,84,320. D. L. Ward, R. R. Rhinebarger and A. I. Popov, Znorg. Chem. 1986,25,2325. C. A. Fyfe, Solid State NMR for Chemists. C.F.C. Press, Guelph, Canada (1983). T. Tokuhiro, L. E. Iton and E. M. Peterson, J. Chem. Phys. 1983,78,7473. G. Mairesse, P. Barbier and J.-P. Wignacourt, Acta Cryst. 1979, B35, 1573. J. L. Dye and A. S. Ellaboudy, unpublished results.
0
LITHIUM-7 NMR STUDIES OF MACROBICYCLIC COMPLEXES IN AMBIENT TEMPERATURE MOLTEN RICK R. RHINEBARGER
and ALEXANDER
0277~5387188 S3.M)+.00 1988 Pergamon Press plc
SALTS
I. POPOV*
Department of Chemistry, Michigan State University, East Lansing, MI 48824, U.S.A. (Received 23 November 1987 ; accepted 22 February 1988) Abstrac&-Complexation of the Li+ ion by macrobicyclicdiazapolyoxa ligands (cryptands) was studied in a 45 : 45 mol% altinum(III) chloride-N-(n-butyl)pyridinium chloride mixture at 40°C. Lithium-7 NMR measurements at this temperature and a field strength of 42.28 kG, showed that the exchange of the Li+ ion between the “free” and complexed sites is slow, and separate resonance signals were observed for the free and complexed Li+ ion. Conditional formation constants were obtained from the integrated areas of the resonance lines ; the complex stabilities were found to decrease in the order C221. Li+ > C222 - Li+ > C222B * Li+. In the case of the C222 * Li+ complex, a decomplexation rate constant of N 5 x lo2 s- ’ was obtained at the coalescence temperature of 40°C. Solid state 7Li NMR spectra were obtained for the C211- LiA1C14and C222B * LiAlCl., complexes, in the static and magic angle spinning modes. These measurements indicate that the electric field gradient at the 7Li nucleus in these complexes is small, yielding narrow resonance lines in the MAS mode.
Molten salt systems, which are liquid at room temperature, are interesting non-aqueous solvents with some unusual properties. Some ten years ago Osteryoung et al.’ reported that the aluminum(III) chlorideN-(n-butyl)pyridinium chloride mixtures, containing 4466 mol% of AlC13, are liquid at, or near, room temperature. Since that time numerous studies of this and similar systems have been described in the literature. We reported previously 7Li NMR studies of lithium chloride solutions in these melts’ and, in particular, formation of lithium ion complexes with several crown ethers, 3 These studies indicated’ that in the basic mixtures (< 50 mol% AlC13) lithium chloride exists as the dichloride anion, LiCl; and that the stabilities of the crown ether complexes with the Li+ ion follow approximately the same trend as the one observed in many non-aqueous solvents. Unfortunately, crown ethers decompose rapidly in acidic mixtures (> 50 mol% AlC13) which, obviously, precludes complexation studies in these media. It seems likely that with two-dimensional crown *Author to whom correspondence should be addressed.
ethers the complexed lithium ion may still be bonded to a chloride ion or ions. On the other hand, macrobicyclic diazapolyoxa ligands (cryptands), originally synthesized by Lehn et aL4 form very stable three-dimensional complexes (cryptates) with a variety of metal ions, and with alkali cations in particular.’ When the size of the cation is equal to, or somewhat smaller than the cryptand cavity, the cation inside the cavity is completely insulated from the environment (“inclusive complex”6). It was of interest to us to investigate the complexation reactions of the lithium ion with several cryptands with different cavity sizes, and particularly to determine whether the cryptated lithium ion is still bonded to one or more chloride ions.
EXPERIMENTAL N-(n-butyl)pyridinium chloride (BPCl) and aluminum(II1) chloride were prepared as described previously. ’ Melt batches were prepared by mixing weighed amounts of AlC13 and of BPCl in Pyrex weighing bottles using Teflon-coated magnetic stirring bars. References to “basic melt” within the
1341
1342
R. R. RHINEBARGER
text refer specifically to the melt with 45.00 mol% AlCl,-55.00 mol% BPCl composition. All operations were carried out in a dry box under an inert atmosphere (est. H,O/O, c 10 ppm) at room temperature. Lithium chloride (Fisher) was dried at 100°C for three days. Lithium tetrachloroaluminate was prepared by fusing equimolar amounts of LiCl and AlC13 at 150°C in a vacuum-sealed Pyrex ampule. The melting point of LiAlCl,, in a sealed capillary, was 144+2”C (ref. 7, 143.o”C). Cryptands C211, C221, C222 and C222B (monobenzo C222) were obtained from MCB Manufacturing Chem. Inc. and were used as received. Stock solutions of lithium chloride in the basic melt (1.00 mol% LiCl in 45.00 mol% AlCl, melt) were obtained by adding appropriate amounts of LiCl to pure basic melt and stirring the solution until it became homogeneous (ca 24 h). Samples with different ligand/Li+ mole ratios for NMR measurements were prepared by adding weighed amounts of the cryptands to the stock solution, stirring for 30 min and removing 0.5 g samples at the desired cryptand/lithium ion mole ratios. These samples were sealed under vacuum. All ‘Li NMR measurements were obtained at 40 (+ 1)“C on a Bruker WH-180 NMR spectrometer at a resonance frequency of 69.951 MHz (field strength of 42.28 kG). Sample tubes were mounted co-axially within 10 mm o.d. NMR tubes containing the external reference solution (0.015 M LiCl in D20 for *H lock). In cases where sample and reference 7Li signals overlapped, a secondary external reference solution (0.015 M LiCl in pyridine) was used, and spectra were obtained without lock. Chemical shifts are reported vs the external aqueous reference, and are corrected for the magnetic susceptibility of basic melt.* Paramagnetic shifts from the reference are indicated to be positive. Deconvolution of the ‘Li NMR spectra to obtain integrated areas was performed by using the NTCCAP subroutine provided in the Nicolet 1180 software package. Lithium-7 solid state NMR spectra were obtained at 22°C in the static and magic angle spinning (MAS) modes by using a Doty Scientific multinuclear solid state probe. Samples were contained in Delrin plastic rotors with end caps. Chemical shifts are referenced to the primary external reference solution (static) by sample substitution. Elemental analyses for the C211- LiAlC14 and C222B - LiAlCl, complexes were performed by Galbraith Laboratories. These results are given in Table 1.
and A. I. POPOV Table
Element C H N 0 Li Al Cl
1. Elemental analysis for C211. LiAlC14 and C222B - LiAICl, complexes C211 -LiAlC14 % theor. % found 40.6 6.3 4.6 18.5 0.9 3.7 21.8
RESULTS
41.8 5.7 4.4 20.2 1.1 4.3 22.4
C222B - LiAlCI, % theor. % found 36.4 6.0 5.3 18.8 0.8 4.9 28.1
36.2 6.1 6.0 13.8 1.5 5.8 30.5
AND DISCUSSION
Lithium-7 NMR measurements in the melt Lithium-7 NMR spectra were obtained for samples with cryptandjithium ion mole ratios varying from zero to 4.37: 1 (C222), 3.44: 1 (C221) and 1.18 : 1 (C222B). For C222, only mole ratios from 0.757 : 1 to 2.80 : 1 could be studied. Typical spectra for the C211, C221 and C222B - Li+ systems are shown in Fig. 1. In each case, upfield signals are assigned to the cryptated Li+ ion. The chemical shifts for complexed Li+ were unaffected (within f 0.02 ppm) by changes in the cryptandjithium ion
8
(ppm)
Fig. 1. Lithium-7 NMR spectra at 40°C in basic 0.949 mol % C222B+ 1.00 mol % LiCl ; melt. ---0.988 mol % C221+0.992 mol % LiCl ; . . . . 0.888 mol % C211+0.985 mol % LiCl. In each case the diamagnetic signal is that of the complexed Li+ ion.
Lithium-7 NMR of macrobicyclic complexes Table 2. Lithium-7 chemical shifts for the lithium cryptates in basic AlCl,-(BP)Cl melt at 40°C Cryptand c211 c221 c222 C222B
(Ppm) -0.79 -1.49 - 1.01* -1.08
“Versus 0.015 M LiCl in DzO external reference ; corrected for the difference in magnetic susceptibility between basic melt and water; uncertainty = f0.02 ppm. *At a 1: 1 C222 : Li+ mole ratio.
s
-0.9
t
-0.7
Et -
1343
I * ****ttt t
t
1 f
i cn
(4
C2ll/Li+
MOLE
RATIO
-1.5
mole ratios. A summary of chemical shift values for these systems is given in Table 2. As shown in Fig. 2, the high frequency signals attributable to the lithium chloro complexes, were observed to shift paramagnetically with the increasing cryptand/lithium ion mole ratio. We reported previously2 that in the basic melt lithium chloride exists as the dichlorolithium ion LiCl;. Concentration dependence of the 7Li chemical shift as a function of LiCl concentration has been interpreted as being due to a dimerization equilibrium. *
c
t
‘tt
t*
I
I
I
I
I
2LiCl-2 Kd_ -Li2Cl:-
I
I
0.2
0.4
0.6
0.8
1.0
1.2
I.4
C22l /Li
-1.1 t g
t
t
1.8 -
(b)
where log& = 2.82f0.39. At an analytical LiCl concentration of 1.OOmol%, approximately 90% of the salt exists as the dilithium tetrachloro complex. The above results indicate that observed paramagnetic shift of the “free” Li+ ion results from the dissociation of the lithium chloro complexes when the cryptate is formed. The constancy of the chemical shifts for the cryptated lithium ion sites, as the total amount of the macrobicyclic ligand is varied, indicates that the lithium ion resides within the cryptand cavities, i.e. inclusive complexes are formed. Similar behaviour has been observed by Cahen et al.’ in a 7Li NMR study of the C21 llithium ion system in various non-aqueous solutions. It is apparent that in the basic melt, 40°C is well below the coalescence temperature for the C211, C221 and C222B * Li+ systems. Slow exchange is observed and the signals for the “free” and complexed lithium ion sites are well resolved. Lithium-7 NMR spectra obtained for the C222 * Li+ system in basic melt were found to be quite different from those of the other three cryptates (Fig. 3). In this case, the “free” and cryptated lithium ion signals overlapped, yielding broad (ca 100 Hz) lines. In addition, the upfield signal assigned to the C222 - Li+ cryptate exhibited a significant (- 0.5 ppm) diamagnetic shift with increas-
t
- 1.4 t I
+
MOLE RATIO
tt
tt
t
+
-1.0 I
(cl
0.2 C2228/
.
. Li+
MOLE
RATIO
Fig. 2. Chemical shifts of the “free” (a) and cryptated (*) lithium ion signals as a function of the cryptand/lithium ion mole ratio at 40°C in basic melt; (a) C211, (b) C221 and (c) C222B.
ing C222/Li+ mole ratio. It should be noted that the linewidths of the other three systems ranged from ca 8 (C221) to 20 Hz (C222B). The linewidths of the C222 - Li+ complex signals decreased with increasing mole ratio, tending to values which are
1344
R. R. RHINEBARGER
and A. I. POPOV
the signals due to “free” (Sr) and cryptated (6,) lithium ion populations in the C221, C222 and C222B * Li+ systems at 40( f 1)“C in the basic melt. Assuming that the dimer must dissociate in order for formation of the cryptate complex to occur,
c222 / Li + MOLE RATIO: 2.60
LiCl; +LK.‘ _
5
0 8
where L is the cryptand. We have previously noted’ that small changes in composition in basic melt solutions of LiCl have no effect on 7Li chemical shifts. Since the concentration of free chloride ions in basic melt is in a ten-fold excess of either the lithium ion or cryptand concentrations, small changes in [Cl-] have little effect on the equilibrium reaction. Therefore, in the derivation of the equilibrium expression, we can neglect the chloride ion concentration and write,
-5
(ppm)
Fig. 3. Lithium-7 NMR spectra for the C222 * Li+ system at 40°C in basic melt as a function of C222/Li+ ratio.
similar to those observed in the other systems ( W1,2= 13 Hz at a mole ratio of 2.80 : 1). Thus, it appears that for the C222 * Li+ system, 40°C is near, but slightly below the coalescence temperature. The exchange is slow and the signals for the two lithium ion sites are broadened by the exchange to the point where they overlap. At the coalescence temperature, the rate of a firstorder decomplexation process (k_ i) is given by eq. (l)‘O k_ 1 = nAr/,f2
LiL+ + 2Cl-
(1)
where Av = VA- vi,, and A and B refer to the “free” and complex sites in the absence of exchange. We assume that eq. (1) applies to the C222/Li+ system at 40°C and that the chemical shift observed for the complex at a C222/Li+ mole ratio of 2.80 : 1 (6 = BB= - 1.49 ppm) is a fair approximation of the limiting chemical shift of the complex site. The chemical shift of the “free” lithium ion (site A, no C222 present) is dA = + 1.55 ppm. Thus, A6 = 3.04 ppm or 213 Hz at the stated resonance frequency. Substituting this value into eq. (1) yields k-, - 5x10*s-‘.C0~etal.“haveestimatedak_, value of > 3 x lo* s- ’ for the C222/Li+ system in methanol at 25°C. It should be noted that in our case eq. (1) does not yield the true value of the firstorder decomplexation rate constant. The transfer of the lithium ion from C222 * Li+ to Li2C1iinvolves more then one step, therefore, the value we obtained is a “conditional” rate constant. The NTCCAP subroutine in the Nicolet software package was used to calculate integrated areas for
[LiL+] ’ = [LiCl;][L] *
(2)
We define : D 3 [Li2Cl:-] ; A4 z [LiCl:] ; C = [LiL+] ; [L] = concentration of the free ligand ; N = total concentration of the ligand ; P z total concentration of the Li+ ion and f = fraction of the total area due to the cryptated Li+ ion. Equilibrium relationships and mass balance yield,
&=y$ C KS = MXL
(4)
N=L+C
(5)
P = M+2D+C
(6)
c=
fP.
(7)
From the above equation we easily obtain an expression for K, in terms of known or measurable quantities, Kd, P, N and $ JK,f P Ks = -1+[1+8K,P(l-f)]“*[N-Pf]’
(8)
A similar procedure was not feasible for the C211-Li+ system due to the formation of a precipitate at mole ratios between zero and one. Samples with mole ratios of 1: 1, or slightly above this value, remained homogeneous, but the signal for the “free” lithium ion site was too low in intensity to be detected. The results of these calculations are given in Table 3 along with values previously reported for lithium cryptates in aqueous and some non-aqueous media.g*‘2*‘3In general, the stability constants for the cryptates in basic melt are smaller than those
Lithium-7 NMR of macrobicyclic complexes
1345
Table 3. Comparison of lithium cryptate stability constants (log&) in the basic melt, water and some non-aqueous solvents”
Solvent
c211
Cryptand c221 c222
Water Methanol Ethanol Acetonitrile Propylene carbonate n-Methylpropionamide Dimethylformamide Dimethylsulphoxide Pyridine
5.5 8.0, 8.4, 10 12.4, 6.43 6.85 5.g4 -
2.50 5.3* 5.38 10.3 9.60 3.4, 3.58 2.7, -
Basic melt
-
(kO.15) 2.51
C222B
0.99 2.6 2.3 7.0 6.94 2.9, 1.0 2.94’ (kO.12) 2.06
2.19’ &Z)
o Ref. 11 except as noted. bRef. 12. ‘Ref. 8.
obtained in non-aqueous solutions. It is interesting to note that in basic melt, the change in complex
stabilities in going from C222 to C221 is either equal to, or less (by 2-3 log& units) than that found in other solvents. This apparently reduced complexation selectivity with respect to cryptand cavity size, and overall reduced stabilities, as compared to other media, is a reflection of the stability of the dilithium chloro complex in the basic melt. As mentioned below, in the solid C222B * Li+ complex, the cation is bonded only to the donor atoms of the ligand, i.e. lithium chloro complexes must dissociate before the above lithium cryptate can be formed. It seems quite likely that the structure of the C222B * Li+ cryptate in the melt is very similar to that in the solid state (especially in view of the slow ambient temperature exchange of the Li+ ion). On the other hand, it also seems likely that in the Li+ complexes with the two-dimensional crown ethers, the cation remains bonded to the chloride ion or ions. In such cases, the “conditional” formation constant of the crown complexes would be roughly of the same magnitude as that for the formation of the cryptate. Unfortunately, thus far, attempts to isolate a crystalline Li+ complex with a crown ether have been unsuccessful. In comparing the C222B * Li+ and C222 - Li+ cryptates, it is apparent that the presence of the benzene ring in the former case reduces the decomplexation rate. The steric influence of the benzene ring may also be responsible for the slightly smaller stability constant for the C222B * Li+ cryptate versus C222 * Li+ .
Cryptate precipitates and 7Li MAS NMR Precipitates were observed to form in C211* Li+ samples in the basic melt with mole ratios between zero and 1: I. This material was isolated by vacuum-filtration of the melt solution containing a ca 1: 1 C211: Li+ mole ratio, washing obtained crystals with benzene, and drying them under vacuum for several days. Elemental analysis of the product (Table 1) showed it to be the C211 *LiA1C14 complex. A solid, analysed as C222B * LiAlCl,, was obtained with the same procedure as C222B : Li+ at mole ratios greater than 1: 1. The crystal and molecular structure of this latter complex has been recently published. l4 In the basic melt the C221. Li+ system exhibits no tendency towards precipitate formation ; at 40°C samples remained homogeneous at all mole ratios studied. Lithium-7 solid state NMR spectra (static and MAS modes) were obtained for polycrystalline samples of LiCl, LiAlC14, C211. LiA1C14 and C222B * LiAlC14. The spectra obtained from LiA1C14 and the C222B * LiAlC14 complex are shown in Figs 4 and 5. All NMR data are summarized in Table 4. The classic Pake doublet (m = l/2 + m = - l/2 transition) was observed for LiCl in the static mode. According to Fyfe,” for a nucleus with a spin of 312 in a glass or polycrystalline powder, experiencing large quadrupolar interactions, the observed spacing of the doublet equals (25/9)A,, with Az defined by, A2
=
(3/W + x2/vo
(9)
1346
R. R. RHINEBARGER
and A. I. POPOV where x is the nuclear quadrupole coupling constant (e*qQ/h). The observed splitting (5.5 kHz) yields A2 = 2000 Hz and x = 1.7 MHz. This result is within an order of magnitude of the value for ‘Li in lithium silicate glasses (910 kHz) obtained by Tokuhiro et al. ’6 No splitting was observed in the static spectrum of LiAlC14 (Fig. 4), which indicates a smaller quadrupolar interaction for the Li nucleus in this compound. In this case,15 Av,,, = A, = x/4.
Resonance
With Av 1,2= 420 Hz, a x value of 1.7 kHz is obtained. This very small value for x is reasonable in view of the known crystal structure of LiA1C14 (SpZiCe group p2,/c), which is made up of LiCl6 octahedral layers linked together by AlCl, tetrahedra. ’ 7 The narrowing effect of magic angle sample spinning is seen in the spectra for C222B * LiAlCl, (Fig. 5). In the spinning mode, the central transition is partially resolved from the broad background. This broadening, due to dipolar interactions, is not completely removed at the spinning rate of 2.1 kHz. By using eq. (10) and since Av,,~ = 1600 Hz (static), a coupling constant of 6.4 kHz is obtained for this complex. For the C211- LiA1C14 sample, stable spinning rates of > 1 kHz could not be obtained. Thus, very little reduction in the dipolar broadening was observed. In the MAS mode, the central transition was barely discernable at this spinning rate. A coupling constant of ca 8 kHz is estimated from Av ,,* of approximately 2000 Hz in the static mode. The coupling constants for the ‘Li nucleus in the two cryptate complexes are quite small, indicating highly symmetric environments for the lithium ion. Evaluation of the differences in chemical shifts among the four compounds (Table 4) is difficult due to the lack of literature values for these or other lithium compounds in the solid state. Solid state 7Li NMR measurements by Dye and Ellaboudy18 show
Frequency
Fig. 4. Solid state ‘Li NMR spectra of LiAlC&. Upper spectrum--static mode ; lower spectrum obtained with “magic angle” spinning.
I
Resonance Frequency
Fig. 5. Solid state ‘Li NMR spectra of the complex, C222B- LiAlC14.Upper spectrum-static mode ; lower spectrum obtained with “magic angle” spinning.
Table 4. Lithium-7 solid state NMR results for various lithium compounds
Sample
(Ppm)
MAS mode
Linewidth (Hz) Static mode
x (kHz)
0.015 M
LiCl/D*O LiCl LiAlCl., C222B - LiAlC14 C211. LiAlCl,
0.0 -3.2 -0.8 +1.0 +1.0
(10)
450 @ 2.7 kHz 58 @ 1.8 kHz 2OOa2.1 kHz cu 2000 @ 1 kHz
19 Splitting : 5.5 kHz 420 1600 ca 2000
1700 1.7 6.4 8
Lithium-7 NMR of macrobicyclic complexes shifts of -2.8 ppm for LiCl, - 1.1 ppm for LiI, -0.8 ppm for C211. LiI, and -3.5 ppm for Li+ * C211- Naa. Data given in Table 4 show that the cryptands effectively remove the shielding influence of the chlorine atoms in the AlCl; ion on the ‘Li nucleus ; the shifts for LiCl and LiAlCl, are more negative (by about 2-3 ppm) than those of the cryptates. This lack of Li-Cl interactions is understandable since the lithium ion is expected to reside within the ligand cavities, a fact which has been confirmed for the C222B - LiAlCl, complex. I4 chemical
Acknowledgement-We gratefully acknowledge the support of this study by a National Science Foundation research grant (CHE-85 15474).
5.
6. 7. 8. 9.
10. 11. 12. 13.
REFERENCES 14. 1. R. J. Gale, B. Gilbert and R. A. Osteryoung, Znorg. Chem. 1978, 17, 2728 ; R. J. Gale and R. A. Osteryoung, Znorg. Chem. 1979,18, 1603. 2. F. Taulelle and A. I. Popov, Polyhedron 1983,2,889. 3. R. R. Rhinebarger, J. W. Rovang and A. I. Popov, Znorg. Chem. 1984, 23, 2557. 4. B. Dietrich, J.-M. Lehn and J.-P. Sauvage, Tetruhedron Lett. 1969,2882, 2889.
15. 16. 17. 18.
1347
A. I. Popov and J.-M. Lehn, in Coordination Chemistry of Macrocyclic Compounds (Edited by G. A. Melson), Chap. 8. Plenum Press, New York (1979). E. Mei, A. I. Popov and J. L. Dye, J. Am. Chem. Sot. 1977,99,6532. J. Kendall, E. D. Crittenden and H. K. Miller, J. Am. Chem. Sot. 1923,45,976. R. R. Rhinebarger, J. W. Rovang and A. I. Popov, Znorg. Chem. 1986, 25,443O. Y. M. Cahen, J. L. Dye and A. I. Popov, J. Phys. Chem. 197579, 1289. J. Sandstrom, Dynamic NMR Spectroscopy. Academic Press, New York (1982). B. G. Cox, H. Schneider and J. Stroka, J. Am. Chem. Sot. 1978,100,4746. M. K. Chantooni and I. M. Kolthoff, J. Solution Chem. 1985,14, 1. B. G. Cox, D. Knop and H. Schneider, J. Phys. Chem. 1980,84,320. D. L. Ward, R. R. Rhinebarger and A. I. Popov, Znorg. Chem. 1986,25,2325. C. A. Fyfe, Solid State NMR for Chemists. C.F.C. Press, Guelph, Canada (1983). T. Tokuhiro, L. E. Iton and E. M. Peterson, J. Chem. Phys. 1983,78,7473. G. Mairesse, P. Barbier and J.-P. Wignacourt, Acta Cryst. 1979, B35, 1573. J. L. Dye and A. S. Ellaboudy, unpublished results.