4707
J. Phys. Chem. 1994,98, 4707-4712
Conformational Effects in Crown Ether Complexation of Lithium Picrate in Solvents of Low Polarity Bruno Roland and Marcel Van Beylen' Loboratory of Macromolecular and Physical Organic Chemistry, University of Leuven, Celestijnenlaan 200 F, B-3001 Heverlee, Belgium
Johannes Smid Polymer Research Institute, Chemistry Department, College of Environmental Science and Forestry, State University of New York. Syracuse, New York 13210 Received: November 9, 1993; In Final Form: March I , 1994'
The binding of lithium picrate (LiPic) to isomers L1 and L2 of the crown ether 2,2,7,7,12,12,17,17-octamethyl21,22,23,24-tetraoxapntacyclo[ 16.2.1. 1396.18J1.113J6]tetracosane (I) was studied spectrophotometrically in chloroform, chlorobenzene, and four ethereal solvents. In the latter, binding of L1 to LiPic is weak, but L2 yielded binding constants, K,,of 60-7500 M-1, depending on the solvating power of the ether. In chloroform (with 1% ethanol), L2 gives an inclusion complex with a spectrum identical to that of a separated ion pair, and Kc= 1.2 X 106 M-l, while L2 gives an externally solvated LiPic ion pair complex. Complexations are remarkably slow due to the difficulty in rearranging L1 and L2 to conformations in which all four oxygen atoms have the proper orientation for effective complexation of the Li ion. In CHCl3, the complexation rate constant for L I is 7 X lo2M-l s-l and thedecomplexationrateconstant is 6 X 10-W. Inchlorobenzene thereisspectrophotometric evidence for the presence of 2:l and 4:l LiPic-crown ether complexes.
Introduction The binding of lithium picrate to 2,2,7,7,12,12,17,17+ctamethy1-21,22,23,24-tetraoxapentacyclo[ 16.2.1, 18J1.1 13J6]tetracosane (I) in chloroform has been reported earlier by Furakawa et ai.' This crown ether is known to bind to fluor-
TABLE 1: Absorption Maxima and Binding Constants (K) for the Complexation of Lithium Picrate with L1 and L2 in Ethereal Solvents after after uncomplcxed addition of L1 addition of L2 solvent tetrahydrofuran tctrahydropyran dioxane diethyl ether
A,
,A,
AIM,
(nm)
(nm)
(nm)
&(M-')
354 351 350 347
60 4 X lo3 2.5 X lo3 7.5 x 103
345 342 336 330
380 355,375
Experimental Section
Me
Me I
enyllithium in ethers and toluene.2 It also affects the tacticity in the polymerization of methyl methacrylate2 and the stereospecificityof polymerizationof isoprene3with LiC12as initiator. A solid complex with LiAlH4 has been isolated by Wiegers and Smith4 and appeared to be a 1:1 complex from elemental analysis. In all these studies, no attempt was made to separate the isomers of this crown ether and/or identify their configurations. From the melting points which were reported in some cases, we conclude that mostly a mixture of isomers was used. In this work we have studied the complexation of lithium picrate (LiPic) with two isomers of crown ether I (LI and L2) in solvents of low polarity. The complexationappearsto bequitecomplicated but is in good agreement with the structures of the two isomers, as determined by X-ray analysis and reported in an earlier paper.s Formation of thecomplexes was unexpectedly slow, which allowed rate constants for complexation and decomplexation in chloroform to be determined by conventional means.
* To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, April 1, 1994.
Crown ether I was synthesized by a method similar to that described by Chastrette6with the modifications that hydrogenation was carried out at atmospheric pressure and room temperaturea5 The separation and structural analysis of the isomers are described elsewhere.5 Lithium picrate was obtained by neutralizing picric acid, which was recrystallized twice from ethanol, with lithium hydroxide in methanol. Thesalt was recrystallized three times from methanol and dried under vacuum. Diethyl ether, tetrahydropyran, and tetrahydrofuran were refluxed over potassium-sodium alloy and freshly distilled before use. Chloroform was washed once with concentrated sulfuric acid, twice with 10% sodium hydroxide solution, and four times with distilled water. The chloroform was then dried over calcium chloride and fractionated. It was stored over calcium hydride and freshly distilled before use. Chlorobenzene was refluxed over phosphorus pentoxide and fractionated. The middle fraction was stored on calcium hydride and freshly distilled before use. Since lithium picrate is insoluble in the chlorinated hydrocarbons, small amounts of a strongly solvating agent (ETOH, THF) were added. Solutions were prepared by adding an appropriate amount (0.1-1%) of a concentrated lithium picrate solution in such a solvating agent to the pure solvent. The spectrophotometric measurements were carried out with a Cary 17D spectrophotometer by titration of the lithium picrate
0022-3654/94/2098-4707$04.50/0 0 1994 American Chemical Society
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Roland et al.
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
L
1
Figure 1. (a, Left) Molecular structure of isomer
L1.
1
(b, Right) Molecular structure of isomer Lz.
0 TETRAHYDROFURAN
A
DIOXANE
6 TETRAHYDRWYRAN
n
DIETHYLETHER
” 0.00
0.00
I
0
1.00
I
2.00
(E]
xi03
3.00
Figure 2. R as a function of [E] for the complexation of lithium picrate with isomer Lz in different solvents.
with the same solution containing a given concentration of crown ether. When high crown concentrations were required, weighed amounts of the solid crown were added.
Results and Discussion Complexation in Ethers. From space-filling models (Figure la,b), it is seen that both isomers of this crown ether are strongly sterically hindered. Therefore, it is difficult to bring the oxygen atoms into the right conformation for complexation with the lithium ion, resulting in a less stable complex. The results of the binding study are summarized in Table 1. Isomer L1 seems to bind much more weakly than Lz. Although the solubility of L1 in these solvents is too low to bring the complexation to completeness, we could derive from the shape of the spectra that in T H F a shift to about 380 nm occurred with an isosbestic point. In tetrahydrofuran and dioxane thesolubility and binding of L1 were insufficient to see a significant spectral
change. In diethyl ether a broad band at 355 nm appeared with a shoulder near 375 m, which indicated the formation of two different kinds of complexes. Since the binding of Lz was much stronger and could be brought to completeness, it was possible to determine the binding constant K,= [ELiPic]/[LiPic] [E], where [E] is theconcentration of free crown and [ELiPic] and [LiPic] are the concentrations of complexed and uncomplexed lithium picrate, respectively. The binding constants were obtained from the slopes of plots of R = [ELiPic]/[LiPic] vs [E] (Figure 2). The KEvalues (Table 1) indicate a stronger binding in the poorer solvating ethers, probably because it is easier for the crown to remove these solvent molecules from the cation on complexation. Although Lz binds much more strongly to the lithium picrate than L1, the bathwhromic shifts are smaller (see Table 1). Apparently, an external complex with a shorter interionicdistance is formed by Lz while L1 tends to form a kind of inclusion complex in which the lithium ion is further separated from the picrate anion. This will be further discussed later.
The Journal of Physical Chemistry, Vol. 98, No. 17, I994 4709
Crown Ether Complexation of Lithium Picrate For both isomers the complexation proceeds very slowly. This is probably due to slow conformational changes of the very sterically hindered crown ethers, rather than to a slow solubilization effect as suggested by Pascault and co-workers,2 since complexation is also slow when the dissolved crown ether is added to a lithium picrate solution. Complexation in Chloroform. To solubilize lithium picrate in chloroform, small amounts of ethanol or tetrahydrofuran were added. Addition of isomer L1 to lithium picrate in chloroform containing 1% ethanol results in a bathochromic shift from 338 to 378 nm with an isosbestic point a t 350 nm. From a plot of R vs [E], Kc = 1.2 X lo6 M-l was obtained. The isosbestic point disappeared when tetrahydrofuran was utilized instead of ethanol as the solubilizing agent. Addition plots of R vs [E] show significant upward curvature, especially at the lower tetrahydrofuran concentrations. Possibly an intermediate 2:l lithium picrate-crown ether complex is formed. Again the complexation occurred so slowly that it was possible to monitor the reaction spectrophotometrically as a function of time. The following rate equation was derived:
[Li EPicl,([Pic],[E~,-[Li
EPic][Li EPic1,I
[Pic],[E],([LiEPic],-[LiEPic~~
2.00
ki
Li'Pic-
+ E ki Li'EPic-
(LiEPic) = k,([Pic], - [LiEPic]) ([E], - [LiEPic]) dt k,[LiEPic] (1) [Pic], and [E], being the total concentrations of lithium picrate and the crown ether, respectively. At equilibrium, k, = k,
([Pic], - [LiEPic],) ([E], - [LiEPic],) [LiEPic] e
(2)
where [LiEPic], is the concentration of complexed ion pairs, Combining eqs 1 and 2 gives
Figure 3. Kinetics of the complexation of lithium picrate with isomer LI in chloroform + 1% ethanol (monitored at 378 nm).
TABLE 2 Rate Constants for the Complexation of Lithium Picrate by Isomer L1 in Chloroform Containing 1% Ethanol, Measured at 338 nm (Disappearance of Uncomplexed Ion Pair) and at 378 nm (Formation of Complexed Ion Pairs) at 338 nm
d[LiEPic] ([LiEPic], - [LiEPic]) ([Pic],[E], - [LiEPic].[LiEPic],)
-
[Piclo (M) 4.0 X
k1
[LiEPic],
d t (3)
Assuming that the initial complex concentration is zero, integration gives
In
[LiEPic],( [Pic],[E], - [LiEPic] [LiEPic],)
-
[Pic],[E],( [LiEPic], - [LiEPic])
( k , + k,([E],
+ [Pic], - 2[LiPic],))t
(4)
In an analogous way, if the disappearance of uncomplexed ion pairs is followed, [LiPic], - [Pic], [LiPic], - [LiPic]
k, [LiPic],
[Piclo + k,-[LiPic] t e
(5)
where [LiPic], is the concentration of uncomplexed ion pairs a t equilibrium.
le5
[El0 (M)
6.0 X 4.0X 3.0X 2.0 X 1.OX 2.0 X 10-' 4.0 X 3.0X 2.0X 1.5 X 1.OX 5.0 X 1.0 X lt5 1.5 X 1.OX 7.5 X 5.0X 2.5 X
le5 1C5
le5 1c5
lC5 10-5
1t5 10-6 10-6 10-6 10-6
at 378 nm
kl s-l M-I)
k2 (104 s-1)
kl (10-2 s-l M-1)
k2 (104 s-1)
6.37 6.16 6.38 7.45 6.41 8.65 7.95 6.79 7.13 6.30 7.11 6.71 6.59 6.76 6.19 7.69
5.18 5.58 6.01 7.69 10.9 7.55 6.21 6.02 6.51 7.48 10.6 5.57 5.88 6.07 5.83 10.2
6.25 7.02 7.38 7.49 7.68 7.53 7.23 6.55 7.02 7.85 7.28 7.22 6.15 6.24 6.84 8.09
5.02 6.25 6.67 6.95 9.00 5.93 6.51 5.50 6.12 6.33 6.16 5.82 5.36 5.47 5.36 7.68
Kinetic measurements were conducted a t different crown ether and lithium picrate concentrations. Plots of the expressions shown in eqs 4 and 5 always gave straight lines (e.g., Figure 3). The results are summarized in Table 2 and give kl = (8.0 f OS) X lo2 M-l s-1 and k2 = (6.7 f 1) X 10-4 s-I. The complexation with La in chloroform always occurred with an isosbestic point, and the spectrum of lithium picrate was shifted to 357 nm. This bathochromic shift is again smaller than for L1, probably because the structure of the isomer is such that it can position its oxygen atoms better for an external complexation of the tight lithium picrate ion pair. Although the binding with L2
Roland et al.
4710 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 A4
1
0.4
0.
0.8
0.
0.4
0.
0.2 0.
0.C
0.
^'1
"'I
0.80.
0.80
0.60.
0.00
0.40
0.40
A
0.20
I
I
350
400
1 450
I
0.001
I
I
I
9w
400
I 450
Figure 4. (a, Top left) Absorption spectra for the complexation of lithium picrate with L1 in chlorobenzene + 0.2% THF. (b, Top right) absorption spectra for the complexation of lithium picrate with LI in chlorobenzene + 0.5% THF. (c, Bottom left) Absorption spectra for the complexation of lithiumpicrate with Ll inchlorobenzene 2%THF. (d, Bottom right) Absorption spectra for thecomplexation of iithiumpicrate with LIinchlorobenzene 5% THF.
+
+
is much stronger, the optical shift is smaller since the increase in the interionic distance of the complexed ion pair is less. From the structures in Figure 1 it is easily seen that both isomers would require important conformational changes to bring
the four oxygen atoms into the right orientation to complex with the lithium ion. These conformational changes are expected to be rather difficult, resulting in a very slow complexation. In the uncomplexed Ll,the oxygen atoms are positioned two by two
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4711
Crown Ether Complexation of Lithium Picrate
TABLE 3 Solubility of Lithium Picrate in Solutions of Isomer L1 in Chlorobenzene [Elo (MI P c l o (MI [PicIo/[Elo 5.00 X 1 P 1.25 X lW5 5.00 x lo-’ 2.50 X 10-4
1.75 X 4.30 X 10-’ 1.69 X 10-4 7.75 x 10-4
3.50 3.43 3.31 3.10
(both separated by 4.78 A) with the pairs being connected by a twofold axis.5 This might explain the formation of a 2:l lithium picratecrown ether complex by this isomer, one ion pair being complexed by 0 2 2 and 0 2 4 at one side of the crown ether molecule and another by 0 2 1 and 0 2 3 at the other side. When more crown is added, a change in conformation might occur so that the four oxygen atoms bind to one single lithium ion, resulting in a 1:l complex. In isomer L 2 , 0 2 2 and 0 2 4 are 4.24 8, apart while the distance between 0 2 1 and 0 2 3 is 5.77 A, which is too large to complex with the small lithium ion. This is possibly the reason why isomer LZ does not form a 2:l complex. Probably an ion pair is attracted by 0 2 2 and OZ4,followed immediately by the conformational change needed to coordinate the four oxygen atoms with the lithium ion. The X-ray structure of this complex5 shows that all four oxygen atoms are about equally involved in this complexation. The various observations on the structure and behavior of the lithium picrate complexes with this crown ether are consistent with data on lithium salt complexes with the parent macrocycle 16-crown-4 and its octamethyl derivative in solution and in the solid Although the exact spectrum of the 2:l complex with L1 in chloroform is not known, we estimated from the spectra that its absorption maximum is situated somewhere between 340 and 350 nm. In chlorobenzene, the intermediate complex is much more pronounced and absorbs at 345 nm. This means that in the 2:l complex with L1 (A,, = 345 nm) and the 1:l complex with Lz (A,, = 357 nm) only a stretched contact ion pair is formed, after external complexation by the crown ether. However, in the = 378 nm), the two ions are much 1:l complex with L I (A,, farther separated, like in a solvent-separated ion pair. We visualize this ion pair to be a kind of inclusion complex, with the lithium ion somewhere inside thecrownether, comparable to thecryptates. Further evidence for such an inclusion complex is found in the very slow decomplexation of this complex, which was achieved by adding a large excess of THF (2: 1 THF-CHCl3). As assessed by the decrease in the 378-nm peak, it can be shown that the displacement of the crown ether by THF takes more than 30 min, indicating the need for an important change in conformation for decomplexation. Complexationin Chlorobenzene. The complexation of lithium picrate by isomer L1 was studied in chlorobenzene containing different amounts of THF (0.2-5%). While in chloroform only a deviation from the isosbestic point is found, a new intermediate absorption maximum at 345 nm is observed in chlorobenzene containing THF (Figure 4). When the THF concentration is increased, this absorption becomes weaker and above 5% THF the isosbesticpoint isrestored. At thelowest THFconcentrations, two partial isosbestic points areobsexved: one at 33 1 nm resulting from the conversion of the contact ion pair into the intermediate complex and one at 355 nm from the further conversion into the complex which absorbs at 380 nm. Quantitative analysis of such a system requires a knowledge of the extinction coefficients of the three ion pairs at the three absorption maxima. The spectra of two of the ion pairs, the uncomplexed ion pair 1(Amx i= 326 nm) and the inclusion complex 3 (Amx = 380 nm) are known and, therefore, so are their extinction coefficients. However, the exact spectrum of the intermediate complex 2 (A, = 345 nm) could not be obtained, and it was impossible to determine its extinction coefficients directly. They
1
Figure 5. Absorption spectra for the complexation of lithium picrate with L2 in chlorobenzene 0.5% THF ([Piclo = 5 X lo-’ M).
+
were calculated from the following equation:’O
with e; = extinction coefficient of the intermediate ion pair complex 2 at wavelength A, = extinction coefficient of the ion pair complex 3 a t the first isosbestic point (331 nm), and €355 = extinction coefficient of the noncomplexed ion pair 1 at the second isosbestic point (355 nm). Once these extinction coefficients are known, the concentrations C1, Cz,and C3 of the three different ion pairs can be calculated by solving the following set of equations:
A’ = c:Cl + E&
+ 4C3
A’ = E ~ C+,
+ e$,
A3 = ciCl
+ E;C*+ e:C3
where Ai is the absorbance a t the A, of ion pair i and e; is the extinction coefficient of ion pair j a t the ,A, of ion pair i. Details of the calculations and values of C1, C 2 , and C 3 for all mixtures shown in Figure 4 can be found elsewhere.10 The data show that the ratio (Cz+ C 3 ) / [El0 (Le., the fraction of LIbound in the two ion pair complexes) a t the lowest THF concentration (0.2%) decreases from 3.3 to 0.2 as [Eloincreases from 2.5 X 10-6 M to 2.5 X 10-4 M, the total lithium picrate concentration being 4.9 X 10-5 M. If the stoichiometry of both complexes is 1:1, the ratio (Cz+ C3)/[E]o cannot exceed the value 1. Ratios such as
4712 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
3.3 mean that the intermediate ion pair complex 2 has not only 2:l but also partially 4:l stoichiometry. At low [El0 and excess [LiPic]~each oxygen atom in LI can bind a LiPic. As more crown is added, a redistribution takes place that favors 2:l and 1:l complexes. At higher T H F content (e.g., 5% THF), theratio (C2 + C3)/[E]o never exceeds 0.9 even at [El0 = 3.8 X 1od M. Competition with T H F molecules reduces the fraction of 4: 1 and 2:l ion pair complexes. Analogous results were obtained from solubility experiments. Although lithium picrate is insoluble in pure chlorobenzene, some can be solubilized by adding crown ether Ll. The results of these solubilization experiments are listed in Table 3. Again, the crown seems to be able to bind more than three lithium ions. We do not exclude the possibility of a 2: 1 dimer-crown complex (two dimers bound to one L J instead of a 4:l complex. Conductance measurements'l have shown that lithium picrate forms a certain percentage of associated ion pair dimers in these solvents, which increases when the THF concentration is lowered. Thecomplexation with L2 occurs via an isosbesticpoint (Figure 5 ) . When theconcentrations were calculated from these spectra, the presence of a small fraction of a 2:l complex or complexed dimer was found again, although it was not evident in the spectra. As in the other solvents, the binding of this isomer is again much
Roland et al. stronger, but only an external complex is formed, resulting in a smaller spectral shift. This was also shown by the X-ray analysis of this complex, which is described elsewhereas
Acknowledgment. The authors gratefully acknowledge the financial support of this research by the National Fund for Scientific Research of Belgium and by a grant from IWONL to B.R. They also thank NATO for a travel grant. References and Notes (1) Kobuke, Y.;Hanji, K.; Horiguchi, K.; Assada, M.;Nakayama, Y.; Furukawa,I. J . Am. Chem. Soc. 1976,98, 7414. (2) Pascault, J. P.;Chastrette, F.;Pham, Q.T. Eur. Polym. J. 1976,12, 273. (3) Jmia,E.;Gole,J.;Pascault,J.P.;Pham,Q.T.;Rashkov,I. B.Polymer 1981,22, 1419. (4) Wiegers, K. E.;Smith, S. L. J. Org. Chem. 1978,43, 1126. ( 5 ) Van Beylen, M.;Roland, B.; King, G.; Aerts, J. J. Chem. Res. 1985, S,338;J . Chem. Res. 1985,M,4021. (6) Chastrette, M.;Chastrette, F.; Sabadie, I. Org. Synrh. 1977,57,74. (7) Aalmo, K. M.;Krane, J. Acru Chem. Scund. 1982,A36.227. (8) Groth, P.Acru Chem. Scund. 1981,A35, 460. (9) Dale, J.; Krane, J. J. Chem. Soc. Chem. Commun. 1972, 1012. (10)Roland, B. Ph. D. Thesis, K.U. Leuven, 1982. (1 1) Van Beylen, M.;Roland, B. Unpublished results.
