Effect of pressure on [2]pseudorotaxane formation and decomplexation and their corresponding activation volumes

Article abstract of DOI:10.1021/jo1003955

(Figure presented) In this study, we investigated the effect of pressure on the formation and decomplexation of [2]pseudorotaxanes. High pressure accelerated the formation of [2]pseudorotaxanes in an aprotic nonpolar solvent (CDCl₃/CD₃CN) via the slipping approach when using two crown ether/secondary ammonium salt systems: dibenzo[24]crown-8/ bis(cyclohexylmethyl)ammonium salt (1a/2a) and tetrabenzo[24]crown-8/ dibenzylammonium salt (1b/2b). The influence of pressure on the rate constants for the formation of the [2]pseudorotaxanes 3a and 3b revealed activation volumes (ΔV^-) of -2.5 and -4.6 cm³ mol^-1, respectively, at 303 K and zero pressure. We also investigated the effect of pressure on the decomplexation of the [2]pseudorotaxanes 3a and 3b in a polar solvent (DMSO-d₆/CDCl₃), obtaining activation volumes of -0.9 and -0.4 cm³ mol^-1, respectively, at 303 K and zero pressure. Moreover, we calculated the activation parameters for the decomplexation processes on the basis of transition state theory at each pressure.

Full text of DOI:10.1021/jo1003955

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Effect of Pressure on [2]Pseudorotaxane Formation and Decomplexation
and Their Corresponding Activation Volumes
Yuji Tokunaga,*,† Nanae Wakamatsu,† Norihiro Ohiwa,† Osamu Kimizuka,†
Akihiro Ohbayashi,† Koichiro Akasaka,† Susumu Saeki,† Kenji Hisada,‡ Tatsuhiro Goda,†
and Youji Shimomura†
†Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, Fukui,
910-8507, Japan, and ‡Department of Fiber Amenity Engineering, Faculty of Engineering,
University of Fukui, Fukui, 910-8507, Japan
tokunaga@matse.u-fukui.ac.jp
Received March 5, 2010
In this study, we investigated the effect of pressure on the formation and decomplexation of
[2]pseudorotaxanes. High pressure accelerated the formation of [2]pseudorotaxanes in an aprotic
nonpolar solvent (CDCl3/CD3CN) via the slipping approach when using two crown ether/secondary
ammonium salt systems: dibenzo[24]crown-8/bis(cyclohexylmethyl)ammonium salt (1a/2a) and
tetrabenzo[24]crown-8/dibenzylammonium salt (1b/2b). The influence of pressure on the rate
constants for the formation of the [2]pseudorotaxanes 3a and 3b revealed activation volumes
(ΔVq) of -2.5 and -4.6 cm3 mol-1, respectively, at 303 K and zero pressure. We also investigated
the effect of pressure on the decomplexation of the [2]pseudorotaxanes 3a and 3b in a polar solvent
(DMSO-d6/CDCl3), obtaining activation volumes of -0.9 and -0.4 cm3 mol-1, respectively, at
303 K and zero pressure. Moreover, we calculated the activation parameters for the decomplexation
processes on the basis of transition state theory at each pressure.
Introduction
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Published on Web 07/01/2010
DOI: 10.1021/jo1003955
r
2010 American Chemical Society

Tokunaga et al.
JOCArticle
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chemical reduction and oxidation,8b,9 temperature,10
pH,9a,11 solvent polarity,12 and chemical additives.11k,13 In
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Pressure is a fundamental physical stimulus that influences
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Tokunaga et al.
SCHEME 1. Thermal Formation of a [2]Pseudorotaxane
via Slipping
JOCArticle
state from the total volume of the substrate(s). The relation-
ship between ΔV (or ΔVq) and the pressure dependence of
the equilibrium (or rate) constant originates from the funda-
mental thermodynamic equations. The equilibrium of a
reversible chemical reaction is affected by the applied pres-
sure, as described using the well-known eq 1
d ln K=dP ¼ - ΔV=RT
ð1Þ
where K represents the equilibrium constant, T and P are the
system’s absolute temperature and pressure, and R is the
universal gas constant.
Furthermore, a reaction proceeding via a transition state
possessing a highly negative activation volume is signifi-
cantly accelerated under high-pressure conditions. Many
examples of this phenomenon have been reported for reac-
tions in which the molecularity decreases along the reaction
coordinate, such as cycloadditions and condensations.14
The relationship between the rate constant k and pressure
is given by
d ln k=dP ¼ - ΔVq=RT
ð2Þ
Therefore, the application of high pressure to a reaction
mixture containing self-assembled molecules can be used as
an external stimulus to control their molecular dynamics
kinetically and thermodynamically. Furthermore, the ki-
netic and thermodynamic effects of pressure on supramole-
cular complexation can be quantified in terms of reaction
and activation volumes. Herein, we describe the effects of
pressure on thermal [2]pseudorotaxane formation, from
secondary ammonium ions and crown ethers, using the slip-
ping method; we provide detailed kinetic data regarding the
enhancement of [2]pseudorotaxane formation and decom-
plexation under various applied pressures.15,16
afforded the concentrations of the three species (i.e., host,
guest, and complex). Plotting 1/[2a] with respect to time
provided a straight line at low conversion (Figure S1,
Supporting Information).20,21 The transformation closely
resembled a second-order reaction; therefore, the reverse
process could be ignored until the [2]pseudorotaxane forma-
tion reached 10% conversion.
Effect of Pressure on [2]Pseudorotaxane Formation. A
solution of the crown ether 1a (15 mM) and the ammonium
salt 2a (15 mM) in CDCl3/CD3CN (1:1) was subjected to a
pressure of 200 MPa at 303 K and the extent of [2]pseudo-
rotaxane formation was monitored at low conversion
(<10%) with 1H NMR spectroscopy. The second-order
plots for the complexation of 1a and 2a reveal that high
pressure accelerated the [2]pseudorotaxane formation rela-
tive to that under ambient conditions. In the same manner,
we subjected equimolar mixtures of the two components to
pressures ranging from 250 to 500 MPa. In each case, the plot
of 1/[2a] with respect to time at low conversion could be fit to
a straight line, the slope of which afforded the second-order
rate constant k,20 which was affected by the applied pressure
(Figure S2, Supporting Information).21 We also performed
these [2]pseudorotaxane formation processes in the same
manner at 313, 323, and 333 K; we found that the complexa-
tion was accelerated upon increasing the applied pressure
and temperature (Figures S3-5, Supporting Information).21
Next, we used tetrabenzo[24]crown-8 (1b) and dibenzyl-
ammonium hexafluorophosphate (2b) as substrates to in-
vestigate the effects of pressure on the reaction rate. We
employed a 10 mM mixture of both reactants in CDCl3/
CD3CN (2:1), in consideration of the reaction rate and
the solubilities of the crown ether 1b and the resulting
Results and Discussion
Formation of [2]Pseudorotaxanes at Ambient Pressure. The
formation of 1:1 complexes from [24]crown-8 derivatives and
secondary ammonium ions in solution is well-established.17
Indeed, the formation of [2]pseudorotaxanes from the crown
ethers 1a and 1b and the ammonium ions 2a and 2b,
respectively, proceeds slowly at ambient pressure and tem-
perature (Scheme 1).18,19 For our present studies, we chose
CDCl3/CD3CN (1:1) as the reaction solvent. 1H NMR spec-
tra of 50 mM solutions of the host and guest components 1a
and 2a, respectively, were recorded at 303 K. We confirmed
the formation of the [2]pseudorotaxane 3a by comparing the
1H NMR spectrum of the mixture with that described
previously;18 integrations of the intensities of various signals
(15) Part of this study has been published as a preliminary communica-
tion: Tokunaga, Y.; Wakamatsu, N.; Ohbayashi, A.; Akasaka, K.; Saeki, S.;
Hisada, K.; Goda, T.; Shimomura, Y. Tetrahedron Lett. 2006, 47, 2679–
2682.
(16) The molar volumes of a catenane and its non-interlocked rings have
been discussed in terms of measurement of their densities; see: Morel-
Desrosiers, N.; Morel, J.-P.; Dietrich-Buchecker, C. O.; Weiss, J.; Sauvage,
J.-P. New J. Chem. 1988, 12, 205–208.
(17) Ashton, P. R.; Campbell, P. J.; Glink, P. T.; Philp, D.; Spencer, N.;
Stoddart, J. F.; Chrystal, E. J. T.; Menzer, S.; Williams, D. J.; Tasker, P. A.
Angew. Chem., Int. Ed. 1995, 34, 1865–1869.
(18) For the pairing of 1a and 2a, see: Ashton, P. R.; Baxter, I.; Fyfe,
M. C. T.; Raymo, F. M.; Spencer, N.; Stoddart, J. F.; White, A. J. P.;
Williams, D. J. J. Am. Chem. Soc. 1998, 120, 2297–2307.
(20) The second-order rate equation would be next:
- d½1aꢀ=dt ¼ - d½2aꢀ=dt ¼ k½1aꢀ½2aꢀ
In the case of [1a]0 = [2a]0:
1=½2aꢀ ¼ kt þ 1=½2aꢀ0
(19) For the pairing of 2a and 2b, see: Tokunaga, Y.; Goda, T.;
Wakamatsu, N.; Nakata, R.; Shimomura, Y. Heterocycles 2006, 68, 5–10.
(21) See the Supporting Information.
4952 J. Org. Chem. Vol. 75, No. 15, 2010

Tokunaga et al.
JOCArticle
[2]pseudorotaxane 3b. Again, we observed enhancements
in the rate of [2]pseudorotaxane formation upon increasing
the applied pressure from 0.1 to 350 MPa and the tempera-
ture from 303 to 333 K; the results were similar to those for
the complexation of 1a and 2a (Figures S6-9, Supporting
Information).21
In general, reactions in which the molecularity decreases
upon proceeding from reactants to products have negative
activation volumes. Indeed, the activation volume for rotax-
ane formation via the slipping method, in which the mole-
cularity decreases, is negative.
Gibson reported that ion pairing of the ammonium salt 2a
affects the formation of [2]pseudorotaxanes derived from
ammonium ion/crown ether pairs; thus, determination of the
experimental association constant for [2]pseudorotaxane
formation must take ion pair decomplexation (ipd) into
consideration as a pre-equilibrium step [eq 3].22,23
FIGURE 1. Plots of ln k, measured for the formation of the
[2]pseudorotaxanes 3a and 3b, plotted as a function of pressure; solid
lines are best fits for the data based on eq 4. 1a/2a: (O) 303, (Δ) 313, (0)
323, and (]) 333 K. 1b/2b: (b) 303, (2) 313, (9) 323, and ([) 333 K.
TABLE 1. Temperature Dependence of the Activation Volumes for the
Complexation of 1 and 2 at Zero Pressure
temp [K]
1a-2a [cm3 mol-1
]
1b-2b [cm3 mol-1
]
303
313
323
333
-2.5
-3.1
-3.4
-3.2
-4.6
-5.5
-5.9
-3.6
temperature.
ln k ¼ aP2 þ bP þ c
ð4Þ
Figure 1 presents the plots of ln k with respect to pressure;
Table 1 lists the activation volumes of the complexation
processes at each temperature.26 We estimated the activation
volumes for formation of the [2]pseudorotaxanes 3a and 3b
to be -2.5 and -4.6 cm3 mol-1, respectively, at zero pressure
and 303 K. Although not as strongly negative as those of
typical reactions in which the molecularity decreases during
the process, large negative activation volumes usually exist
for transition states in which a covalent bond is formed; for
example, simple radical decompositions generally have acti-
Ion pairing has two considerable effects during pseudo-
rotaxane formation. The first is a change in the concentra-
tions of ammonium ions and salt (2aþ and 2a) during the
complexation process. According to Gibson, the value of
Kipd for 2a in CDCl3/CD3CN (3:2) at 295 K is 2.6 ꢁ 10-2 M.
This value suggests that the ionic form of 2aþ comprises ca.
80% of the ammonium species present in solution at 295 K
and ambient pressure; additionally, this percentage would
barely change at conversions of less than 10%. The second
effect of ion pairing is its pressure dependence. Eckert
reported the effects of pressure on the ionic associations of
several ammonium salts in aprotic polar solvents, such as
acetone and acetonitrile,24 finding that the ion pairing
association constants decrease upon increasing pressure in
the range from 0.1 to ca. 200 MPa. Even though high pre-
ssure would accelerate the decomplexation of the ion pairs of
the ammonium salts 2, the maximum enhancement in the
rate of pseudorotaxane formation would be ca. 1.25-fold,
because ca. 80% of the ammonium species already exist in
ionic form at ambient pressure.25
vation volumes of ca. þ10 cm3 mol-1 14a,e
.
Despite the
absence of covalent bond formation in the complexation
events leading to pseudorotaxane creation, these processes
presumably exhibit negative activation volumes because the
passage of the stopper groups through the cavities of the
crown ethers is the rate limiting step.
Interestingly, [2]pseudorotaxane formation from 1b and
2b, which contain a larger number of rigid benzene rings,
exhibited a more negative activation volume than that from
1a/2a. We suspect that two factors might be responsible for
this phenomenon: (1) the rigid aromatic units of both sub-
strates 1b and 2b undergo stacking to form a more compact
transition state and (2) differences in the degrees of solvation
of the substrates 1a/2a and 1b/2b and/or their transition
states.
Activation Volumes for [2]Pseudorotaxane Formation. We
determined rate constants for [2]pseudorotaxane formation
from 1 and 2 via the slipping method in solution at various
temperatures and pressures. The activation volumes at zero
pressure were determined from a least-squares fit of the rate
constants to eq 4,14 using data obtained with eq 2 at pressures
of up to 450-500 MPa for 1 and 300-350 MPa for 2 at each
Effect of Pressure on the Decomplexation of the [2]Pseu-
dorotaxanes. The decomplexation of [2]pseudorotaxanes
formed from ammonium salts and crown ethers resembles
a first-order reaction,27 with polar solvents accelerating the
(22) Jones, J. W.; Gibson, H. W. J. Am. Chem. Soc. 2003, 125, 7001–7004.
(23) The importance of ion pairing on the association of ammonium ions
with crown ethers has also been reported previously; see: Montalti, M.
Chem. Commun. 1998, 1461–1462.
(24) (a) Glugla, P. G.; Byon, J. H.; Eckert, C. A. J. Chem. Eng. Data 1981, 26,
80-84. See also: (b) Everaert, J.; Persoons, A. J. Phys. Chem. 1982, 86,
546-552 and references cited therein.
(25) We did not estimate the association constants (Kipd) for the ion
pairing of the ammonium salts 2 in our solvents at various temperatures and
pressures. All of the reported values are those obtained when ion pairing is
ignored.
(26) The activation volumes of the complexation processes were already
reported.15 The difference of the values between previous studies and this
report is due to the addition of another experiment and disparity in pressures.
(27) (a) Chiu, S.-H.; Rowan, S. J.; Cantrill, S. J.; Glink, P. T.; Garrell,
R. L.; Stoddart, J. F. Org. Lett. 2000, 2, 3631–3634. (b) Tachibana, Y.;
Kihara, N.; Furusho, Y.; Takata, T. Org. Lett. 2004, 6, 4507–4509. (c)
Tokunaga, Y.; Akasaka, K.; Hashimoto, N.; Yamanaka, S.; Hisada, K.;
Shimomura, Y.; Kakuchi, S. J. Org. Chem. 2009, 74, 2374–2379.
J. Org. Chem. Vol. 75, No. 15, 2010 4953

Tokunaga et al.
JOCArticle
process by weakening the hydrogen bonds between the
ammonium ion and the crown ether. For example, Stoddart
reported that the [2]pseudorotaxane 3a dissociated comple-
tely in less than 18 h at ambient temperature and pressure
when dissolved in DMSO-d6.27a We used 1H NMR spectros-
copy to monitor the decomplexation of 3a (5 mM) in CDCl3/
DMSO-d6 (1:1) at 303 K and atmospheric pressure. The plot
of the concentration of the [2]pseudorotaxane 3a provided a
first-order curve; we obtained the decomplexation rate con-
stant k0 from the slope of the straight line of the plot of ln
([3a]t/[3a]0) against time (t), where [3a]0 and [3a]t are the
initial concentration of 3a and that at time t, respectively;
the half-life (τ1/2) of 3a was 14.6 h. Using the same approach,
we determined the value of τ1/2 (303 K, 0.1 MPa) for 3b
(2 mM) in CDCl3/DMSO-d6 (2:1) to be 12.3 h.28 Next, we
investigated the effect of pressure on the decomplexation of
the [2]pseudorotaxanes 3a and 3b. We monitored these
decomplexation processes using a 5 mM solution of 3a in
CDCl3/DMSO-d6 (1:1) and a 2 mM solution of 3b in CDCl3/
DMSO-d6 (2:1) at temperatures from 303 to 333 K and
pressures ranging up to 500 and 300 MPa, respectively. All
of the processes exhibited typical first-order behavior, with
higher pressure accelerating the decomplexation of the
[2]pseudorotaxanes 3 (Figure 2).21
FIGURE 2. (a) First-order plots of ln ([3a]/[3a]0) with respect to
time for the decomplexation of 3a (5 mM) in CDCl3/DMSO-d6 (1:1)
at 303 K. (b) First-order plots of ln ([3b]/[3b]0) with respect to time
for the decomplexation of 3b (2 mM) in CDCl3/DMSO-d6 (2:1) at
303 K. Pressure: (O) 0.1, (b) 100, (Δ) 200, (2) 300, (0) 400, and (9)
500 MPa.
Activation Volumes for the Decomplexation of the [2]-
Pseudorotaxanes. The plots of ln k0 with respect to pressure
for the decomplexation of the [2]pseudorotaxanes 3 are
nonlinear (Figure 3). We determined the activation volumes
at 0 MPa from a least-squares fit of the data to eqs 2 and 4,
using data obtained at pressures of up to 300-500 MPa for
3a and 300 MPa for 3b at each temperature. Interestingly, the
calculated activation volumes (ΔVq) for these two decom-
plexation processes are also negative at zero pressure and
303 K: -0.9 cm3 mol-1 for 3a and -0.4 cm3 mol-1 for 3b
(Table 2). Negative or positive activation volumes are mainly
observed for organic reactions involving covalent bond
formation or cleavage; notably, however, no covalent bonds
are formed or cleaved during the decomplexation of these
[2]pseudorotaxanes. Because the substrates 3 have compact
structures, because of strong hydrogen bonding between
the ammonium ions and the crown ethers, we suspect that
the negative activation volumes for these decomplexation
processes arise from (1) the complementary sizes of the
cavities of the crown ethers and the stopper units, similar
to those during [2]pseudorotaxane formation, and (2) the
compression of the molecules, caused by electrostatic inter-
actions with the polar solvent;29 note that ion pairing
decreases upon increasing the pressure in polar solvents, as
described above.24 In the [2]pseudorotaxanes, the ammo-
nium ion moieties are encapsulated by the crown ether units;
in contrast, these moieties interact electrostatically with
polar solvents in the transition states, resulting in volume
changes.30
FIGURE 3. Pressure dependence of ln k0 for the decomplexation
of the [2]pseudorotaxanes (a) 3a (5 mM) in CDCl3/DMSO-d6
(1:1) and (b) 3b (2 mM) in CDCl3/DMSO-d6 (2:1) at (O) 303, (Δ)
313, (0) 323, and (]) 333 K; solid lines are best fits for the data based
on eq 4.
Effect of Pressure on the Kinetic Parameters for the De-
complexation of [2]Pseudorotaxanes. Because we determined
the rate constants for [2]pseudorotaxane decomplexation at
various temperatures and pressures, we could calculate the
kinetic parameters associated with these systems at each
pressure based on transition state theory. Eyring plots of
the data for the [2]pseudorotaxane decomplexation pro-
cesses yielded straight lines (Figure 4a,b); Table 3 lists the
corresponding kinetic parameters. The activation free en-
ergies (ΔGq) were similar at each pressure; the activation
enthalpy (ΔHq) and entropy (ΔSq) increased upon increasing
the pressure.
(28) The concentrations of the two substrates are different because of
the low solubility of the [2]pseudorotaxane 3b in a mixture of CDCl3 and
DMSO-d6.
(29) Whalley, E. J. Chem. Phys. 1963, 38, 1400–1405.
(30) Merbach groups reported volume profiles for complexation of
R-cyclodextrin with diphenyl azo dyes using high-pressure kinetic experi-
ments. The complexation consists of two processes, and generally the
activation volumes of both processes were more negative than those of the
pseudorotaxane formation; see: Abou-Hamdan, A.; Bugnon, P.; Saudan, C.;
Lye, P. G.; Merbach, A. E. J. Am. Chem. Soc. 2000, 122, 592–602.
4954 J. Org. Chem. Vol. 75, No. 15, 2010

Tokunaga et al.
JOCArticle
TABLE 2. Temperature Dependence of the Activation Volumes for
[2]Pseudorotaxane Decomplexation at Zero Pressure
TABLE 3. Kinetic Data for the Decomplexation of the
[2]Pseudorotaxanes (a) 3a and (b) 3b at Various Pressuresa
temp [K]
3a [cm3 mol-1
]
3b [cm3 mol-1
]
pressure MPa ΔHqkJ mol-1 ΔSq J mol-1 K-1 ΔGq (303 K) kJ mol-1
303
313
323
333
-0.9
-1.9
-2.4
-2.8
-0.4
-0.5
-0.9
-0.9
(a) reaction conditions: 3a (5 mM) in CDCl3/DMSO-d6 (1:1)
0.1
100
46 ( 0.5
64 ( 2.2
75 ( 0.9
83 ( 2.5
-187 ( 1.7
-124 ( 7.1
-86 ( 2.9
-58 ( 7.8
102.4 ( 0.6
101.1 ( 0.2
100.7 ( 0.4
100.3 ( 0.4
200
300
(b) reaction conditions: 3b (2 mM) in CDCl3/DMSO-d6 (2:1)
0.1
100
200
300
80 ( 2.7
93 ( 2.5
101 ( 1.9
117 ( 2.5
-73 ( 8.6
-30 ( 7.8
1 ( 6.0
102.2 ( 0.1
101.6 ( 0.1
101.8 ( 0.1
101.7 ( 0.1
52 ( 8.0
aThese parameters were determined from the Eyring plots in Figure 4.
FIGURE 5. Enthalpy/entropy compensation plots for the decom-
plexation of the [2]pseudorotaxanes 3a (O) and 3b (0).
FIGURE 4. Eyring plots for the decomplexation of the [2]-
pseudorotaxanes (a) 3a (5 mM) in CDCl3/DMSO-d6 (1:1) and (b)
3b (2 mM) in CDCl3/DMSO-d6 (2:1) at (þ) 0.1, (O) 100, (0) 200, and
(]) 300 MPa.
Conclusion
We have studied the formation and dissociation of
[2]pseudorotaxanes comprising crown ethers and secondary
ammonium ions in aprotic nonpolar solvents at high pres-
sure. Complexation was enhanced under high pressure; the
relationships between the rate constant and the pressure
revealed negative activation volumes. Decomplexation of
the [2]pseudorotaxanes in polar solvents was also enhanced
under high pressure. Kinetic studies of variable-temperature
and -pressure data provided us with the kinetic parameters
for the decomplexation processes, revealing that they also
occurred with negative activation volumes.
Figure 6 displays the volume profiles for the formation
and decomplexation of the [2]pseudorotaxanes. The forma-
tion process is characterized by a substantial decrease in
volume upon proceeding from the free species to the transi-
tion states, followed by an increase in volume upon reaching
the complexes. Because the solvent systems that we emp-
loyed for the formation and decomplexation processes were
different, the degree of solvation of the transition states
cannot be compared directly. Nevertheless, one possibility
for the negative volumes for both the complexation and
decomplexation processes can be attributed to the comple-
mentary sizes of the terminal units of the dumbbell-like axles
and the cavities of the crown ethers; another is that solvation
might contribute to the negative activation volumes.
Although the activation free energies of 3a and 3b were
almost identical, the activation enthalpy and entropy of 3a
were both smaller than those of 3b. The entropic destabiliza-
tion of the transition state for the decomplexation of 3a
became an enthalpic one upon increasing the pressure. In the
decomplexation of 3b, enthalpy contributed to the destabi-
lization of the transition state, even at ambient pressure. We
suspect that because the [2]pseudorotaxane 3b possesses six
rigid benzene rings, which fix the conformation in the ground
state, its loss of conformational freedom in the transition
state is relatively small.31
Figure 5 presents TΔSq plotted against ΔHq. We obtained
straight lines having slopes (3a: 1.05; 3b: 1.02) representing
the extent to which the enthalpic gain was canceled by the
accompanying entropic loss; the intercepts (3a: -105 kJ
mol-1; 3b: -104 kJ mol-1) represent the complex stabilities
obtained at a value of ΔHq of 0. The good relationships
between ΔHq and TΔSq suggest that the decomplexation
mechanisms were independent of the pressure and occurred
with similar kinetic processes.
(31) We also estimated the kinetic data for the formation of the
[2]pseudorotaxanes, but the correlation between the activation enthalpy
and the activation entropy was poor, presumably because we neglected to
consider the changes in the association constants for ion pairing between the
ammonium and hexafluorophosphate ions at elevated temperatures and
pressures. Nevertheless, the tendency of the parameters upon increasing
pressure were similar to those for [2]pseudorotaxane decomplexation: (1) the
activation free energies (ΔGq) for [2]pseudorotaxane formation were similar
at all pressures; (2) the activation enthalpy (ΔHq and entropy (ΔSq) increased
upon increasing pressure; (3) the activation enthalpy and entropy of 3a were
both smaller than those of 3b. See the Supporting Information for additional
details.
Experimental Section
Kinetic Measurement of the Formation of the [2]Pseudo-
rotaxanes (3). Solutions of 30 mM 1a in CDCl3 and 30 mM 2a
in CD3CN were prepared separately. Mixtures of the two
solutions (2 mL each) were pressed at various pressures and
J. Org. Chem. Vol. 75, No. 15, 2010 4955

Tokunaga et al.
JOCArticle
Kinetic Measurement of the Decomplexation of the [2]-
Pseudorotaxanes (3). Solutions of 3a (5 mM) and 3b (2 mM)
in CDCl3/DMSO-d6 (1:1 for 3a; 2:1 for 3b) were prepared
separately and then samples (2 mL) were pressed at various
pressures and temperatures. For each pressure and temperature,
more than four samples were taken and analyzed with 1H NMR
spectroscopy.
Acknowledgment. We thank Professor T. Uchida (Univer-
sity of Fukui) for helpful comments and advice and the
Innovation Plaza Ishikawa of the Japan Science and Techno-
logy Agency for financial support.
Supporting Information Available: kinetic data for the
complexation of 1a and 2a at various pressures, kinetic data
for the complexation of 1b and 2b at various pressures, kinetic
data for the dissociation of the [2]pseudorotaxane 3a at various
temperatures, kinetic data for the dissociation of the [2]pseu-
dorotaxane 3b at various temperatures, k values for the dis-
sociation of pseudorotaxanes 3a and 3b, Eyring plots for the
formation of the [2]pseudorotaxanes 3a and 3b, kinetic para-
meters for the complexation of the [2]pseudorotaxanes 3a and 3b
under various pressures, and NMR spectra of compounds 1b,
2a, and 3a and 3b. This material is available free of charge via the
Internet at http://pubs.acs.org.
FIGURE 6. Volume profile for the formation (a) and decomplex-
ation (b) of the [2]pseudorotaxanes at 298 K and zero pressure.
temperatures. For each pressure and temperature, more than
four samples were taken and analyzed (1H NMR spectroscopy)
after dilution of the sample twice with CDCl3/CD3CN (1:1). The
concentrations of 2a were calculated through integrations of
1
benzylic protons of 2a and 3a in the H NMR spectra. In a
similar manner, 10 mM solutions of 1b and 2b in CDCl3/
CD3CN (2:1) were prepared and the rate constants measured
by monitoring H NMR spectra at appropriate times (dupli-
cated at least four times).
1
4956 J. Org. Chem. Vol. 75, No. 15, 2010