Lithium-7 NMR Study of Lithium Cryptates
1289
Lithium-7 Nuclear Magnetic Resonance Study of Lithium Cryptates in Various Solvents Yves M. Cahen, James L. Dye, and Alexander 1. Popov* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received November 6, 1974) Publication costs assisted by Michigan State University
Lithium-7 NMR studies were performed on lithium ion complexes with cryptands C222, C221 and C211 in water and in several nonaqueous solvents. In the case of the first two cryptands the exchange between the free and complexed lithium ion was fast by the NMR time scale and only one population-average resonance was observed. Cryptand 211 forms much more stable lithium complexes and two 7Li resonances (corresponding to the free and the bound Li+) were observed for solutions containing excess of the Li+ ion. The limiting chemical shifts of the complex were found to be independent of the solvent indicating that the lithium ion is completely shielded by the cryptand. Formation constants of lithium-C222 complexes were determined in water and pyridine solutions. The values obtained were, log K H ~ = O 0.99 f 0.15 and log Kpy = 2.94 f 0.10.
Introduction In recent years a number of new macrocyclic ligands have been synthesized which form stable complexes with alkali metal ions. Crown ethers, developed by Pedersen,’ were the first such complexing agents to appear. Shortly thereafter Lehn2 introduced a new class of complexing agents of hexaoxadiamine macrobicyclic type called “crypt a n d ~ ”which ~ complex alkali metal cations to an even greater extent than the crown ethers. Cryptands form an inclusion type complex where the metal ion is trapped inside the cavity of the ligand. Lehn and coworkers have studied extensively alkali metal cryptates in solutions4 primarily by potentiometric technique and by proton NMR. For example they were able to study potassium and barium cryptates in deuteriochloroform by proton NMR.5 We wish to report the effects of the formation of lithium cryptates on the 7Li NMR spectra and the determination of the formation constant of lithium-C222 complexes in water and pyridine by this technique. A preliminary communication of some of our results was published earlier.6 Experimental Section The 211 and 221 cryptands (I) were obtained from E. M.
2,2,1, a = 0, b = c = 1
2,1,1,
u = b = 0, c = 1
Laboratories Inc. and were used as received. Lithium-7 NMR measurements indicate that their purity is greater than 98%. Cryptand 222 (I) was prepared by a modification7 of the method of Dietrich et a1.* These compounds will be identified in the following text by C211, (2221, and C222, respectively. Nitromethane, dimethyl sulfoxide, tetrahydrofuran, pyridine, propylene carbonate, dimethylformamide, formamide, and chloroform were purified and
dried by previously described techniques.8 Lithium perchlorate (Fisher) was dried at 190’ for several days (water content 0.2% as determined by Karl Fischer titration). Lithium iodide (K & K Laboratories) was purified by recrystallization from acetone and dried under vacuum over PzOb by elevating the temperature progressively from 28 to 82’ (water content, 0.1%). Solutions of cryptands and lithium salt were prepared at various ligand/Li* mole ratios in all of the above solvents except chloroform where solutions with excess lithium salt could not be prepared due to the lack of solubility. In this case, a t least an equimolar amount of a cryptand was necessary to solubilize the lithium salt. Nuclear magnetic resonance experiments were carried out on a Varian DA-60 spectrometer in a V-4333 probe at 23.3 MHz and 1.4092 T. Spinning 5 mm sample tubes were used. A 1-mm 0.d. melting point capillary inserted coaxially in the sample tube and filled with an appropriate solution (usually a 4 M aqueous LiC104 solution) was used as an external standard. The observed chemical shifts were corrected for the bulk diamagnetic susceptibility of the solvents. The measurements were made at probe temperature of 30 f 1’.
Results and Discussion The 7Li chemical shifts were determined as a function of cryptand/Li+ mole ratios with the results shown in Table I. Typical spectra obtained with C211 are shown in Figure 1. The stability of a cryptate complex is largely determined by the size of the crypt cavity and by the solvating ability of the solvent. If the rate of exchange of the lithium ion between the two sites, free ion in the bulk solution and the complex, is greater than f l / r A v , where Au is the difference between the characteristic resonance (in Hz) in each site, only one population-average resonance is observed. This is the case with C222 which has a much larger cavity (2.8 A) than the bare lithium ion (1.56 A). In nitromethane, dimethyl sulfoxide, pyridine, and water only one 7Li resonance is observed. In dimethyl sulfoxide and water, the solvent molecules have a strong solvating ability and compete quite successfully with the ligand. Consequently, only a weak lithium cryptate complex is formed and a large excess of ligand is necessary to produce a variation of the observed 7Li chemiThe Journal of Physical Chemistry, Voi. 79, No. 73, 1975
1290
Y. M. Cahen, J. L. Dye, and A. i. Popov
TABLE I: 7Li NMR Study of 222-, 221-, 21 1-Lithium Complexes in Various Solvents at 30 &-lo [Li'], Salt
M
LiClO,
0.025
Solvent
Crypt 222 CH3N02
[Crypt- 'Li chemical andV[Li'] shift, ppm" Solvent 0.0 0.5 1.o 2 0.0 0.5 1.o 2 .o 0.0 0.7 1 2.5
.o
DMSO
LiC10,
Pyridine
0.025
LiC10,
0.025
.o
cob
LiI
HZO
0.010
0.0 1.o 10 .o 20 .o cab
Crypt 221 CH3N02
LiC10,
0.05
DMSO
LiCIO,
0.05
Pyridine
LiClO,,
0.05
a
0.0 0.5 1.0 2.0 0.0 0.5 1.0 2.0 0.0 0.5
0.35 0.75 1.02 1.03 0.97 0.97 0.96 0.96 -1.52 0.30 1.04 1.61 1.73 0 .oo 0.00, 0.09, 0.11 0.18 0.38 0.81 1.04 1.03 0.94 0.96 0.97 0.98 -2.16 -2.21, 1.87
Crypt 211 CHSN02
DMSO
LLi'], Salt LiC10,
0.15
LiI
0 -14
LiI,
0.14
0.13 0.20
LiI
0.14
LiCl
0.11
LiBPh,
0.10
THF
LiI,
PC
LiC10,
CHC1, HZO
LiI, LiI
DMF
LiC10,
Formamide
LiC10,
I
1
,"iJ! L
D MSO
DM F -0'5
00
0'5
1'0
2'0
A ppm
Figure 1. Lithium-7 NMR spectra of lithium-C211 cryptate in various solvents: [C211] = 0.25 M, [Li+] = 0.50 M. Chemical shift of LiC211 is at 0.41 ppm vs. aqueous LiCiOI solution at infinite dilution. cal shift (see Table I) from the position characteristic of the solvated Li+ ion in the given solvent. In nitromethane The Journal of Physical Chemistry, Vol. 79, No. 13, 1975
(Crypt- 'Li chemical a n d v [ L i + ] shift, ppm" 0 .o 1.o 0 .o
1.o
LiCl LiC104
Vs. aqueous LiC104 at infinite dilution (corrected for magnetic susceptibility). Calculated.
h
M
0 .o 0.4 0.9
1.o 0 .o 0.8 1.o 0 .o 1.o
0 .o 1.o 0 .o 1.o
0.61 0.41' 0.49 0.42
0.11 0.01 and 0.37 0.37 0.41
0.97 0.95 and 0.39 0.3.9 0.95 0.39 0 -88 0.3.9 0.97 0.42 0 -48 0.44 and 0.36 0.36 0.45 0.52 and 0.38 0.38 0.38
0 .o 0.5 1.o 0.25 0 .o 0.5 1.o 0.15 1.o 0.25 0 .o 0 .oo 0.00 and0.38 0.5 0.25 -0.50 0 .o -0.40 and 0.42 0.5 0 .o 0.25 4.39 -0.43 and 0.38 0.5 Chemical shifts of C-211 cryptates.
0.10
and pyridine, which are poor solvating solvents, the respective complexes are readily formed as evidenced by the chemical shift of the 7Li resonance upon addition of the ligand. With C221 the exchange is slower because of the smaller cavity size of this ligand (2.2 8). In dimethyl sulfoxide C221 forms a weak complex with Li+ ion while in nitromethane the complex is more stable and the limiting chemical shift for Li+-C221 complex is reached a t 1:l ligand/Li+ mole ratio. In both cases only one population-averaged resonance line is observed because the exchange is fast on the NMR time scale. In pyridine at room temperature the exchange is slow enough and the difference between the chemical shift of both sites is large enough (97 Hz) so that two 7Li resonances are obtained, one for the complexed lithium at a t 1.87 ppm and one for the free lithium a t -2.02 ppm vs aqueous LiC104 solution a t infinite dilution (Figure 2). Cryptand 211 has a cavity radius nearly equal to that of the unsolvated lithium ion. It is expected, therefore, that very stable lithium complexes will be formed and that the exchange will be slow. The Li+-C211 system was investigated in nitromethane, dimethyl sulfoxide, tetrahydrofuran, propylene carbonate, chloroform, dimethylformamide, formamide, and water solutions. The data shown in Table I indicated that in all solvents the addition of the ligand in less than stoichiometric amounts results in two resonances
Lithium-7 NMR Study of Lithium Cryptates
1291 tjobsd=
[ ( K C t M- KCtL - 1) f ( K 2 C t L2 -t K Z C Y 22K2CtLCtM+ 2KCtL
+ 2KCtM -?1)”2]
, L
-30 7
-20
-10
--00
- -
-.
I O
20
30
A PPM
Figure 2. Lithium ion and lithium-C221 resonances in pyridine solutions: [C221] = 0.25 M, [Li’] = 0.50 M v s . aqueous LiC104 solution at infinite dilution.
corresponding to the free and the complexed lithium ion. Similar results were found in a 23Na NMR study of sodium-C222 complexes in ethylenediamines and in other so1vents.Q Not surprisingly, the chemical shift of the lithium ion complexed by C211 is essentially independent of the solvent and of the counterion used (Figure 1).In all cases it is found a t 0.40 f 0.03 ppm. In the complex, the lithium ion is completely encased by the ligand and, since the 7Li chemical shifts are dependent almost exclusively on the nearest neighbors of the lithium ion, they are insensitive to either the solvent molecules surrounding the cryptate or to counterion in cases where a low dielectric constant of the solvent (such as T H F or chloroform) would lead to ion pair formation. On the other hand, the limiting chemical shifts of Li+C222 and especially Li+-C221 complexes are definitely solvent dependent indicating that the looser structure of the complex permits the solvent molecules to approach sufficiently close to the metal ion so as to affect its resonance frequency. Lithium NMR has been shown to be a useful technique for the determination of the formation constants of weak and medium strength complexes.lOJ1 This technique was described in detail in an earlier publication which dealt with the determination of the formation constants of Li+glutarimides and Li+-tetrazole complexes in nitromethane. This approach was used in this work to determine the formation constants of Li+-C222 complexes in water and in pyridine. The technique involves the measurement of 7Li chemical shifts as a function of ligand/Li+ mole ratio followed by a computer fit of the data with the equation
[U] 2KCtM + 6 ,
which has two adjustable parameters, the formation constant K and the limiting chemical shift of the complex 6,. The other factors are the total concentration of the ligand, CtL, total concentration of the Li+ ion, CtM,chemical shift of the uncomplexed Li+, 6f, and the observed chemical shift, hobsd. The values obtained were log K = 0.99 f 0.15 and log K = 2.94 f 0.10, respectively. The previously reported value in methanol12 is log K = 2.65 which reflects its intermediate solvating ability for the lithium ion between that of water and of pyridine. Previous estimates of the log K values in water were -02 and
