Kinetics and mechanism of the dissociation of a sodium-calix [4] arene ester complex in nonaqueous solution

Article abstract of DOI:10.1039/a909414h

The kinetics and mechanism for the dissociation of sodium ion complexes of a calix[4]arene ester 1 were studied in nonaqueous solution by a dynamic ¹H NMR. Life times τ(c) of the Na⁺-1 complexes and activation parameters Δ(d)H((+)) and Δ(d)S((+)) for the dissociation process were determined in five organic solvents. In methanol, the life time (8.8 x 10^-3 s) of the sodium complex at 25°C was 440 times larger than that of crown ether (18C6) and was ca. 40 times less than that of cryptands (C211 and C222). The activation parameters for the dissociation process, Δ(d)H((+)) of 67, 64, 57, 57, and 46 kJ mol^-1, and Δ(d)S((+)) of -22, -29, -7.7, -13, and -33 J mol^-1 K^-1 were determined in deuteriated nitromethane, acetonitrile, acetone, methanol, and dimethylformamide, respectively. It was observed that the activation enthalpies tend to decrease with increasing the electron- donating ability of solvents as indicated by the Gutmann donor number, while the activation enthalpies do not correlate to the donor number and all the values are negative. These results suggest that in the activated state of the Na⁺-1 complex, additional solvent molecules bind to the sodium ion encapsulated by ethoxycarbonylmethoxy groups in 1, and the disruption of the bonding between a sodium ion and the oxygens in the OCH₂CO moieties plays a major contribution in the dissociation process. In acetonitrile, the life times of the Na⁺-1 complexes were not affected by the concentration of the free ligand of 1, suggesting that the dissociation proceeds via a unimolecular dissociation not a bimolecular exchange between free and complexed 1.

Full text of DOI:10.1039/a909414h

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Kinetics and mechanism of the dissociation of a sodium-calix[4]arene
ester complex in nonaqueous solution
Takashi Jin
Section of Intelligent Materials and Devices, Research Institute for Electronic Science, Hokkaido
University, Sapporo 060-0812, Japan. E-mail: jin=imd.es.hokudai.ac.jp
Received 29th November 1999, Accepted 1st February 2000
Published on the Web 9th March 2000
The kinetics and mechanism for the dissociation of sodium ion complexes of a calix[4]arene ester 1 were
studied in nonaqueous solution by a dynamic 1H NMR. Life times q of the Na`-1 complexes and activation
c
parameters * Ht and * St for the dissociation process were determined in Ðve organic solvents. In methanol,
d
d
the life time (8.8 ] 10~3 s) of the sodium complex at 25 ¡C was 440 times larger than that of crown ether
(18C6) and was ca. 40 times less than that of cryptands (C211 and C222). The activation parameters for the
dissociation process, * Ht of 67, 64, 57, 57, and 46 kJ mol~1, and * St of [22, [29, [7.7, [13, and [33 J
d
d
mol~1 K~1 were determined in deuteriated nitromethane, acetonitrile, acetone, methanol, and
dimethylformamide, respectively. It was observed that the activation enthalpies tend to decrease with
increasing the electron-donating ability of solvents as indicated by the Gutmann donor number, while the
activation enthalpies do not correlate to the donor number and all the values are negative. These results
suggest that in the activated state of the Na`-1 complex, additional solvent molecules bind to the sodium ion
encapsulated by ethoxycarbonylmethoxy groups in 1, and the disruption of the bonding between a sodium ion
and the oxygens in the OCH CO moieties plays a major contribution in the dissociation process. In
2
acetonitrile, the life times of the Na`-1 complexes were not a†ected by the concentration of the free ligand of 1,
suggesting that the dissociation proceeds via a unimolecular dissociation not a bimolecular exchange between
free and complexed 1.
alkali-metal ion complexes of calix[4]arenes in solution.11h14
Introduction
Previously, we have studied the dissociation kinetics of the
Na`-1 complex in a chloroformÈmethanol mixture by an
analysis of 23Na longitudinal magnetization recovery curve
and 1H NMR spectroscopy.11 We have found that the
calix[4]arene ester 1 forms a sodium complex with a 1 : 1 stoi-
chiometry and the dissociation of the Na`-1 complex pro-
ceeds via an unimolecular dissociation mechanism in the
chloroformÈmethanol mixture. More recently, Israeli and
Detellier have studied the dissociation kinetics of the Na`-1
complex in a chloroformÈacetonitrile mixture by an analysis
of the 23Na NMR line widths.13 They have reported a 2 : 1
(1/Na`) complex as an intermediate in the pathway of the
exchange of sodium ions between the 1 : 1 complex in the
mixture solution.13 However, there has been no systematic
data for the kinetics of sodium ion exchange on the Na`-1
complex in various organic solutions. In this paper, lifetimes
of the Na`-1 complex and activation parameters of the disso-
ciation reaction in deuteriated nitromethane, acetonitrile,
acetone, methanol, and dimethylformamide are presented. In
addition, it is demonstrated that the dissociation of the Na`-1
complexes in acetonitrile follows an unimolecular dissociation
mechanism not a bimolecular exchange mechanism. By com-
parison of the formation and the dissociation rates in acetoni-
trile, the kinetic factors which a†ect the sodium selectivity of 1
is discussed.
Calixarenes prepared by the condensation of p-alkylphenol
and formaldehyde are an interesting class of host molecules,
which are characteristic of cyclic oligomers made up by
phenol units.1 Much attention has been paid to the design of
calixarene derivatives with selective binding properties for
particular ions and neutral molecules in solution. Functional
modiÐcation of the lower rim of parent calix[n]arenes has led
to a variety of ionophoric compounds.2h7 It has been shown
that ester and amide derivatives of calix[n]arenes have
remarkable complexing abilities toward alkali-metal ions in a
similar manner as crown ethers and cryptands.7 Among a
large number of ionophoric calix[n]arenes, particularly inter-
esting is the calix[4]arene ester 1. This calix[4]arene ester is
known to have selective complexing ability toward sodium
ions and acts as a Na` carrier in bilayer membranes as well as
bulky liquid membranes.7h10 To the best of our knowledge,
the calix[4]arene ester 1 is the Ðrst example of a synthetic
Na` carrier which is active in phospholipid bilayer mem-
brane.8,10 From the dynamic 23Na NMR study in a vesicle
system, we have found that the transport rates of sodium ions
by 1 across phospholipid bilayers are comparable to the rates
by an antibiotic ionophore, monensin.8 The actual mechanism
of the transport of sodium ions by 1 involves the formation
and dissociation of the sodium complexes at
a water/
membrane interface. For a deeper understanding of the
mechanism of the selective Na` transport by 1 across the
bilayer membrane, it is of importance to characterize the
kinetics and mechanism of the ion exchange reaction on
Na`-1 complexes in solution.
Experimental
p-tert-Butylcalix[4]arene was purchased from Acros Organics.
The calix[4]arene ester 1 (Fig. 1) was synthesized by the liter-
ature method.7 Sodium thiocyanate, potassium thiocyanate
and lithium chloride were of analytical reagent grade quality
Up until now, only a few reports have addressed the
kinetics and mechanism of the formation and dissociation of
DOI: 10.1039/a909414h
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ting signals arising from bridging methylene protons observed
at 3.14 and 4.88 ppm show that 1 forms a cone structure in
chloroform. From the NOE experiment, the signals at 3.14
and 4.88 ppm were assigned to the equatorial and axial
protons of the bridging methylene, respectively. When micro-
liter aliquots of deuteriated chloroform solution of NaSCN
was added to the solution of 1, new signals immediately gener-
ated. The intensity of these signals increased with increasing
the amounts of NaSCN added, while the signals of the
uncomplexed 1 became small and completely diminished at
the concentration ratio of R \ [Na`]/[1] \ 1. It should be
noted that the spectra at R \ 1 becomes a simple pattern
again, where the spectrum is assigned to the cone conforma-
tion of complexed 1 with a C symmetry. When more
4v
NaSCN up to R \ 2 was added, no further spectral change
occurred. Thus, it is concluded that 1 forms a 1 : 1 complex
with sodium ions and exists as a free and a 1 : 1 complexed
species in chloroform. Except for the di†erence in tem-
perature, similar spectral behavior as shown Fig. 2 was
observed in the Ðve solvents studied.
Fig. 1 Ligand structures.
and dried under vacuum at 80 ¡C for 24 h prior to use.
Dynamic 1H NMR spectra
Nitromethane-d ([99%), acetonitrile-d ([99%), acetone-d
3
3
6
As can be seen from Fig. 2(b), most well-resolved signals of the
free and complexed species of 1 are the signals in the aromatic
proton regions. Thus, we measured the temperature depen-
dence of the NMR signals of the aromatic protons of 1 in Ðve
solvents at ca. R \ 0.5. Fig. 3 shows typical data for experi-
mental and calculated 1H NMR spectra of 1 at R \ 0.6 in
acetone. Two signals at [10 ¡C show the signals of aromatic
protons resulting from free (higher Ðeld) and complexed
species (lower Ðeld) of 1. With increasing the temperature, two
signals became coalesced, indicating that the sodium ion
exchange on the Na`-1 complex takes place and the exchange
rate increases with increase in temperature. By the lineshape
analysis for two-site exchange15 between the free and com-
(99.8%), methanol-d (99.8%), dimethylformamide-d (99.5%),
4
7
and chloroform-d (99.8%) were purchased from Merck. All sol-
vents were stored over 4 A molecular sieves. 1H NMR spectra
were measured on Varian XL-200 at 200 MHz and Bruker
XL-400 at 400 MHz. The chemical shifts of 1H NMR spectra
were referred to 1% TMS which was added to the deuteriated
solvents. The temperature calibration was made with a thermo-
couple inserted in a nonspining tube containing deuteriated
solvents. The error on the temperature was estimated to be
0.5 K. The temperature-dependent 1H NMR spectra were
subjected to complete line shape analyses15 to obtain the tem-
perature variation of the life times, q , of alkali-metal ion com-
c
plexes.
plexed 1, the life times q of the sodium complexes were
obtained. In acetone, the life times of the sodium complex
c
Results and discussion
decreased from 96 to 0.58 ms in the range of [10 to 50 ¡C.
For other solvent systems, the lineshape analysis of
temperature-dependent 1H NMR spectra was also undertaken
and the life times of the sodium complexes in these solvents
were obtained.
Change in 1H NMR spectra in the presence of sodium ions
Fig. 2 shows the typical data for changes in the 1H NMR
spectra of 1 upon addition of NaSCN in chloroform at 25 ¡C.
In the absence of NaSCN, 1H NMR spectra of 1 a†orded a
Life times of the sodium complex and activation parameters for
the dissociation reaction
simple pattern characterizing the C symmetry. The AB split-
4v
Fig. 4 shows the temperature dependence of the life times q of
c
the sodium complexes in Ðve solvents. The life times are sig-
niÐcantly inÑuenced by the temperature and the nature of the
solvents. For the exchange of sodium ion, we assumed a two-
jump model, where two di†erent sites correspond to the
complex and (Na`-1) and the other is the solvated sodium
ions:
kd
Na`-1 E8F Na` ] 1
(1)
Eqn. 1 is meant to represent the general situation of the disso-
ciation reaction and not to imply any speciÐc mechanism of
the dissociation. The activation parameters * Ht and * St for
d
d
the dissociation of the complex were calculated by the Eyring
equation [eqn. (2)] in which all symbols have their usual
meaning.
1
k T
h
A
[* Ht * St
B
B
d
d
k \
\
exp
]
(2)
d
q
RT
R
c
The life times of the complex and the activation parameters
for the dissociation are summarized in Table 1, together with
Fig. 2 Change in 1H NMR spectra of 1 (5 mM) upon additions of
the data earlier reported for Na`-18C6,16,17 Na`-C21C ,18
NaSCN in CDCl : (a) R \ [Na`]/[1] \ 0, (b) R \ 0.5 and (c) R \ 1.
5
3
Na`-C211,19,20 Na`-C222,19,20 and Na-monensin.21 The
Open and closed circles indicate the aromatic protons in free and
complexed 1, respectively.
values of q at 25 ¡C exhibit 3300-fold variation among the Ðve
c
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Fig. 3 Typical experimental and calculated data for the temperature-dependent 1H NMR spectra of the aromatic protons of 1 (5 mM) at
R \ 0.6 in acetone.
solvents. In the case of nitromethane and acetonitrile, the life
times of the complexes at 25 ¡C are determined by the
extrapolation of the linear q vs. 1/T plot to the temperature.
c
It is seen that the life times tend to decrease with increasing
the electron-donating ability of solvents. In methanol, the life
time (8.8 ] 10~3 s) of the sodium complexes at 25 ¡C was 440
times larger than that of crown ether (18C6) and was ca. 40
times less than that of cryptands (C211 and C222). It is of
interest to note that the life time of the Na`-1 complex is of
similar order to that of the sodium complex of an antibiotic
ionophore, monensin.21
Fig. 5 shows a plot of * Ht and * St vs. the Gutmann
d
d
donor number22 of the solvents. It is seen that the activation
enthalpies tend to decrease with increasing the donor number,
while the activation entropies hardly depend on solvents and
all the values are negative. These Ðndings suggest that in the
activated state of the Na`-1 complex, additional solvent mol-
ecules coordinate to the sodium ion encapsulated by eth-
oxycarbonylmethoxy groups in 1, and the activated complex
Fig. 4 Plots of the life times q of the Na`-1 complexes vs. the
c
inverse of the temperature in Ðve solvents: nitromethane (È), acetoni-
trile (|), methanol (K), acetone (L) and DMF ()).
Table 1 Life times of sodium complexes and activation parametersa for the dissociation of sodium complexes in nonaqueous solution
Counter
anion
q /s
* Gt/kJ mol~1
c
d
Complex
Solvent
D b
eb
(298 K)
* Ht/kJ mol~1
* St/J mol~1 K~1
(298 K)
N
d
d
Na`-1
Nitromethane
Acetonitrile
Acetone
Methanol
DMF
2.7
14.1
17.0
19.0
26.6
35.9
37.5
20.7
32.6
36.7
NCS~
NCS~
NCS~
NCS~
NCS~
4.0
0.71
3.8 ] 10~3
8.8 ] 10~3
1.2 ] 10~3
67 ^ 14
64 ^ 1
57 ^ 3
57 ^ 4
46 ^ 3
[22 ^ 39
[29 ^ 4
[7.7 ^ 9
[13 ^ 14
[33 ^ 10
77 ^ 26
73 ^ 2
59 ^ 6
61 ^ 8
56 ^ 6
Na`-18C6
Methanolc
Methanolc
19.0
19.0
32.6
32.6
NCS~
NCS~
1.3 ] 10~5
2.7 ] 10~5
53.6
38.1
27.8
[30.1
45.3
47.1
Na`-C21C e
5
Methanol
Methanol
Methanol
Methanol
19.0
19.0
19.0
19.0
32.6
32.6
32.6
32.6
ClO
5.5 ] 10~4
0.40
44.9
[31.9
54.4
4
Na`-C211f
Na`-C222f
Na-monensing
Cl~
Cl~
0.35
Monensin~
1.5 ] 10~2
43.1
[66.0
62.7
a Quoted errors represent one standard deviation obtained from a linear regression analysis of the temperature dependence of experimental q
data through eqn. (2). b Ref. 22. c Ref. 16. d Ref. 17. e Ref. 18. f Ref. 19, 20. g Ref. 21.
c
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k2
Na`-1* ] 1 E8F 1* ] Na`-1
(4)
The Ðrst is refereed to as the unimolecular dissociation mecha-
nism. The second is the bimolecular cation exchange by the
free ligand, which is known as the associative exchange
mechanism. From these equations, the life time of the Na`-1
complex is expressed as follows:
1
\ k ] k [1]
free
(5)
1
2
q
c
Therefore, we can determine the relative contribution of the
two dissociation mechanisms by the measurements of [1]
free
dependence on the life times of the Na`-1 complex. Fig. 6
shows the concentration dependence of [1]
on the 1H
free
NMR spectra in the aromatic protons of 1 in acetonitrile. The
signals at 7.08 and 7.42 ppm are ascribed to the free and com-
plexed species of 1, respectively. It should be noted that the
chemical shift and the linewidth of the complexed signal is less
Fig. 5 Plots of * Ht and * St for the dissociation reaction of Na`-1
d
d
a†ected by the increase in the concentration of [1] . Over
vs. the Gutmann donor number of solvents.
free
the range of the concentration ratio ([1] /[1]
) from 0.1
free
complex
to 4, the life times of the Na`-1 complexes were almost con-
stant (1/q \ 1.4 ^ 0.3 s~1) within the experimental error. This
result indicates that the unimolecular dissociation is dominant
is stabilized by the electron-donating nature of the solvent.
This indicates that the enthalpy term (the disruption of the
bonding between Na` and 1) plays a major contribution on
the dissociation in comparison with the entropy term. The
rearrangement of the binding groups prior to dissociation
should not be a major barrier to the dissociation. In this
system, the enthalpyÈentropy compensation e†ects were not
observed in contrast to the case of (dibenzo18C6-Na`)23 and
c
in the dissociation of the Na`-1 complexes in acetonitrile.
Recently, Israeli and Detellier have reported the formation
of a 2 : 1 complexes (1/Na`) in a chloroformÈacetonitrile
mixture based on the 23Na NMR linewidth analysis.13 They
have proposed that the outer-sphere 2 : 1 complex is an inter-
mediate in the pathway of the exchange of sodium ions
between 1 : 1 complexes in solution. If the formation of the
2 : 1 complexes takes place before the formation of the 1 : 1
complexes, it is anticipated that the life time of the 1 : 1
(18C6-Na`)24 systems: the * Gt tends to decrease with
d
increasing the donor number of solvents. This may have
resulted from the small Ñexibility of the binding sites (ester
moieties) of the complex, in which the molecular motion is
suppressed by the template e†ect by sodium ion.25 Since there
is no correlation between * St and the donor number, it is
d
suggested that the conformation of the Na`-1 complex in the
activated states is less a†ected by solvents.
It should be noted that there is no correlation between the
dielectric constants of the solvent and the life times of the
sodium complexes. Shinkai et al. have reported that 1 com-
plexes with sodium picrate to form solvent-separated ion pairs
in THF.26 Considering the small dielectric constant (7.6) of
THF, in the other solvents studied ([20.7), it is
not likely that the SCN anion forms an ion pair with a
sodium cation. From the X-ray analysis data of a tetraester
analogue (a potassium complex of p-tert-butylcalix[4]arene
tetraamide),27 it has been shown that the potassium ion is
completely encapsulated by the cavity constructed by the four
phenolic and the four carbonyl oxygen atoms of the
calix[4]arene. In addition, it has been shown that the values
of the 23Na spin-lattice relaxation rates of the Na`-1 com-
plexes in chloroformÈmethanol mixture were not a†ected by
counter anions, such as Br~ and SCN~.11 On the basis of
these Ðndings together with the dielectric constant data, it is
suggested that the ion-paring e†ect is negligible on the stabil-
ity of the Na`-1 complex in the solvent systems studied.
The dissociation mechanism of the sodium complexes in
acetonitrile
At the condition of the ratio R \ [Na`]/[1] \ 1, the concen-
tration of free sodium ions should be very small compared to
the concentration of complexed sodium ions because the sta-
bility constant of the Na`-1 complex in acetonitrile is very
large ([106).7,28 Thus, it may be taken into account the
possibility of two dissociation mechanisms for the Na`-1
complexes:
Fig. 6 1H NMR spectra (25 ¡C) of the aromatic protons of 1 at dif-
k1
ferent ratios of [1] /[1]
in acetonitrile: (a) 0.1, (b) 0.4, (c) 1.1
Na`-1 E8F Na` ] 1
k~1
(3)
free
complex
and (d) 4.0. [1] \ 5 mM.
total
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complex should decrease with increasing [1] , as a result of
If the complexation proceeds via a simple di†usion control
reaction,21,29,30 the formation rates should be increased in the
order of K`-1 [ Na`-1 [ Li`-1 because the desolvation
energy decreases with the increase of ion radius.31 However,
free
the increase in contribution of the bimolecular dissociation
[see eqn. (5)]. However, the decrease in the life time of the
Na`-1 complex was not observed in the range of
[1] /[1]
from 0.1 to 4.
inverse behavior was observed in
k
:
k
(Li`-1) [
free
complex
~1
~1
k
(Na`-1) [ k (K`-1). In the formation process, the rate
~1
~1
determining step should be the rearrangement of the binding
groups to accommodate metal ions. The smaller ion may have
access more easily to the binding cavity created by the eth-
oxycarbonylmethoxy groups in 1. In the dissociation process,
Kinetic factors a†ecting the ion selectivity of 1 in acetonitrile
Fig. 7 shows the temperature dependence on the life times of
Li`, Na`, and K` complexes in acetonitrile. The life times of
the complexes were obtained by the line shape analysis of the
temperature-dependent 1H NMR spectra in 1-LiCl, 1-
NaSCN, and 1-KSCN systems. The order of the life times is
Na`-1 (0.71 s) [ Li`-1 (1.0 ] 10~2 s) [ K`-1 (3.3 ] 10~4 s)
at 25 ¡C, where except for the case of Li`-1, the values are
* Ht plays a major contribution in the energy barrier for the
d
dissociation of M`-1. The large variation (2100-fold) in k
1
could be explained by the di†erence in the electrostatic stabil-
ity of the metal ion complex.
It is of interest to compare the rate constants in the Na`-1
obtained by the extrapolation of the linear q vs. 1/T plot to
system with the data reported for Na`-18C624,32 and Na`-
c
the temperature.
C21C .18 The formation rate constant of the Na`-1 is in the
5
Table 2 summarized the rate constants of the dissociation
and formation reaction including activation parameters for
the dissociation process, together with the data earlier report-
ed for the Na`-18C616 and Na`-C21C 18 complexes. The
order of magnitude of the crown ether and the cryptand. The
dissociation rate of the Na`-1 is rather close to that of the
cryptand, and the rate is slower than that of the crown ether
by a factor of 2700. Judging from these observations, the
calix[4]arene ester 1 has the ““openÏÏ system for incoming
metal ions such as a crown ether, while the complexed 1 has
the ““closedÏÏ system for outgoing metal ions such as a crypt-
and.
5
dissociation rate constants (k \ 1/q ) are 100, 1.4 and 3030
1
c
s~1 for the Li`, Na`, and K` complex, respectively. Since the
unimolecular dissociation is dominant in acetonitrile, the for-
mation rate constants k can be calculated by k \ k ] K.
~1
~1
1
Recently, the accurate values of stability constants K in ace-
tonitrile have been reported by Daniel de Namor et al.28 by
potentiometric titrations: 106.20 (Li` complex), 107.68 (Na`
complex), and 10.4.04 (K` complex). By using these data, the
Conclusion
The kinetics and mechanism of the dissociation of the sodium
ion complexes of 1 in nonaqueous solution are summarized as
follows.
(1) The life times of the Na`-1 complex tend to decrease
with increasing the electron-donating ability of the solvent as
indicated by Gutmann donor number.
(2) The enthalpy term (the disruption of the bonding
between Na` and 1) plays a major contribution on the disso-
ciation process compared to the entropy term.
values of k were calculated as follows: 1.5 ] 108, 6.7 ] 107
and 3.3 ] 107 dm3 mol~1 s~1 for the Li`, Na` and K` com-
~1
plexes, respectively. The variation in k
is within one order,
~1
while the variation in k is much larger than that in k by a
1
~1
factor of ca. 2100. This suggests that the sodium selectivity of
1 is mainly responsible for the di†erence in the dissociation
rate constants, not for the di†erence in the formation rate con-
stants.
(3) It is suggested that in the activation state of the Na`-1
complex, additional solvent molecules bind to the sodium ion
which is encapsulated by four ethoxycarbonylmethoxy groups
in 1.
(4) The dissociation of the Na`-1 complexes in acetonitrile
proceeds via an unimolecular dissociation mechanism.
(5) The sodium selectivity of 1 is mainly responsible for the
di†erence in the dissociation rate constants, not for the di†er-
ence in the formation rate constants.
The calix[4]arene ester 1 can be characterized as the host
molecule where the stability of the sodium complex in solu-
tion is intermediate between that of the sodium complexes of
crown ether (18C6) and cryptands (C211 and C222). In addi-
tion, the calix[4]arene ester 1 has an intermediate behavior of
kinetics between crown ether (18C6) and cryptand (C21C ),
5
formation of the complex occurs as for the crown ether, but
dissociation of the complex occurs as for the cryptand.
Fig. 7 Plots of the life times q of the alkali-metal ion complexes of 1
Acknowledgements
The author thanks Mr E. Yamada for the measurement of 1H
c
vs. the inverse of the temperature in acetonitrile: Li` complex (K),
Na` complex (L) and K` complex (|).
Table 2 Rate constants and activation parametersa for the dissociation of alkali-metal ion complexes in acetonitrile
k /s~1
1
k
/dm~3 mol~1 s~1
* Gt/kJ mol~1
~1
d
Complex
(298 K)
(298 K)
* Ht/kJ mol~1
* St/J mol~1 K~1
(298 K)
d
d
Li`-1
1.0 ] 102
1.4
3.0 ] 103
3.8 ] 103b
84.8
1.5 ] 108
6.7 ] 107
3.3 ] 107
2.3 ] 107c
1.0 ] 107
40 ^ 5
64 ^ 1
46 ^ 3
32 ^ 2b
58 ^ 1
[73 ^ 14
[29 ^ 4
[29 ^ 11
[65 ^ 8b
[14 ^ 2
62 ^ 9
73 ^ 2
55 ^ 6
53 ^ 0.4b
62 ^ 1
Na`-1
K`-1
Na`-18C6
Na`-C21C d
5
a Quoted errors represent one standard deviation obtained from a linear regression analysis of the temperature dependence of experimental q
data through eqn. (2). b Ref. 24. c Calculated by using the stability constant in ref. 32. d Ref. 18.
c
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16 S. F. Lincoln, A. White and A. M. Hounslow, J. Chem. Soc.,
Faraday T rans. 1, 1987, 83, 2459.
NMR spectra. The author also thanks Mr N. Kuroda and Mr
T. Ohta for technical support.
17 B. O. Strasser and A. I. Popov, J. Am. Chem. Soc., 1985, 107,
7921.
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18 S. F. Lincoln, I. M. Brereton and T. M. Spotswood, J. Am. Chem.
Soc., 1986, 108, 8134.
1
C. D. Gutshe, Acc. Chem. Res., 1983, 16, 161; C. D. Gutshe,
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