Weakly associated TFPB anions are superior to PF6 anions when preparing (pseudo)rotaxanes from crown ethers and secondary dialkylammonium ions

Article abstract of DOI:10.1002/chem.201103464

Making the right choice: Tetrakis(3,5-trifluoromethylphenyl)borate (TFPB) counter anions can facilitate the threading of dibenzylammonium (DBA ⁺) ions through macrocycles in cases where the corresponding PF ₆^- salts fail to exhibit complexation.

Full text of DOI:10.1002/chem.201103464

DOI: 10.1002/chem.201103464
Weakly Associated TFPB Anions Are Superior to PF6 Anions When
Preparing (Pseudo)Rotaxanes from Crown Ethers and Secondary
Dialkylammonium Ions**
Nai-Chia Chen,[a] Chun-Ju Chuang,[a] Liang-Yun Wang,[a] Chien-Chen Lai,[b] and
Sheng-Hsien Chiu*[a]
The potential applications of rotaxane-type molecular
switches and actuators—in sensing,[1] organogelation,[2] drug
delivery,[3] fluid transportation,[4] and molecular memory[5]—
have driven the development of new threading systems and
methods for synthesizing their interlocked structures effi-
ciently.[6] In the past two decades, crown ethers and dibenzy-
lammonium (DBA+) ions have been among the most fre-
quently used recognition systems for rotaxane synthesis,
mainly because of their ready accessibility and structural
modification.[7] The binding affinity of a DBA+ ion to a
crown ether is, however, strongly related to the nature of
the counter anion, because ion pairing significantly affects
the complexation of DBA+ to the macrocyclic host.[8] As a
result, DBA+ ions are commonly prepared in the form of
in NMR spectra. One approach toward taking advantage of
such weakly associated recognition systems featuring DBA+
ions to facilitate the synthesis of corresponding rotaxanes
would be to enhance the solubility of DBA+ salts in less-
polar solvents while maintaining weak ion pairing; that is,
allowing stronger hydrogen bonds to form between the host
and guest units with less disruption from solvent molecules
À
and anions. Ideally, the additional benefits of PF6 ions—the
resulting rotaxanes are relatively easily crystallizable solids
and the lack of hydrogen atoms make them silent in
1H NMR spectra—will not be lost; indeed, it might be prac-
tically useful if weakly associated, nonpolar-soluble DBA+
salts could be exchanged back to [DBA]ACHTNURTGEN[UNG PF6] salts after
completing the syntheses of interlocked molecules. To dem-
onstrate this concept, we selected the tetrakis(3,5-trifluoro-
methylphenyl)borate (TFPB) anion, which coordinates cat-
À
hexafluorophosphate (PF6 ) salts, weakly ion-paired systems
that are highly useful for rotaxane synthesis. Because the
primary interactions between DBA+ ions and macrocyclic
hosts are generally hydrogen bonds, less-polar solvents (e.g.,
CHCl3, CH2Cl2) favor strong binding; unfortunately, many
À
ionic metal complexes more weakly than does the PF6
ion.[10] Herein, we report the enhanced binding affinities of
pseudorotaxanes and greater reaction yields of rotaxanes
obtained when switching the counter anions of DBA+ salts
of a reported very weakly interacting recognition system
from PF6 to TFPB, and the successful exchange of the coun-
ter anions of the corresponding rotaxanes from TFPB back
to PF6. We also report a new recognition system in which
the formation of a pseudorotaxane from a macrocycle and a
DBA+ ion is not possible in CDCl3 in the PF6 salt form, but
the synthesis of the corresponding rotaxane was successful
when using TFPB as the counter anion. The dissociation of
derivatives of [DBA]
ACHTUNGTRENNUNG
Therefore, the binding of [DBA]
AHCTUNGTRENNUNG
macrocyclic hosts and the synthesis of rotaxanes from the
resulting pseudorotaxanes are typically performed in more-
polar solvents (e.g., CH3CN, CH3NO2, or their mixtures with
CHCl3 and CH2Cl2) to improve the solubility while compro-
mising binding affinity.[9] In more-polar solvents, however, it
is possible that many potentially useful, but relatively
weakly associated, host–guest recognition systems will be
overlooked because of the negligible changes in their signals
À
this [2]rotaxane occurred after adding PF6 ions to the
CDCl3 solution, providing direct evidence that the interac-
tion of the DBA+ ion with the PF6 anion is stronger than
that with the TFPB anion, suggesting the preferable use of
TFPB counter anions for DBA+ salts when synthesizing ro-
taxanes in less-polar solvents.
To demonstrate the ability of TFPB anions to enhance
the binding affinity between hosts and DBA+ guests, we
chose reported weakly binding systems because the synthe-
ses of their corresponding rotaxanes are much less efficient
and have great room for improvement. The association con-
stant (determined by using 19F NMR spectroscopy) for the
interaction of bis(4-fluorobenzyl)ammonium hexafluoro-
[a] N.-C. Chen, C.-J. Chuang, L.-Y. Wang, Prof. S.-H. Chiu
Department of Chemistry and Center for Emerging Material
and Advanced Devices, National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan, 10617 (R.O.C.)
Fax : (+886)2-33661677
E-mail: shchiu@ntu.edu.tw
[b] Prof. C.-C. Lai
Institute of Molecular Biology
National Chung Hsing University and
Department of Medical Genetics
China Medical University Hospital, Taichung, Taiwan (R.O.C.)
[**] TFPB=Tetrakis(3,5-trifluoromethylphenyl)borate anion.
phosphate ([DFA]ACTHNUTRGNE[UNG PF6]) with the 4-methyldianilino[24]-
crown-8 (4-Me-DA24C8) in CDCl3/CD3NO2 (2:1) at 298 K
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103464.
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Chem. Eur. J. 2012, 18, 1896 – 1900

COMMUNICATION
after proving that the TFPB
anion can improve the yield of
a
[2]rotaxane prepared from
two weakly associated recogni-
tion elements—that it is possi-
ble for the TFPB counter
anions of these rotaxanes to be
À
exchanged back to PF6 ions, if
necessary. We suspected that a
basic anion exchange resin
would be useful for the removal
of highly organic-soluble and
weakly
coordinated
TFPB
anions, simply by associating a
solution of the [2]rotaxane with
the positively charged resin and
then filtering off the [2]rotax-
ane through either ClÀ ion ex-
change or neutralization. After
using the anion exchange resin
to remove the TFPB anions, we
treated the solution with
2 equivalents of HCl (1m) and
then subjected the [2-H]Cl salt
to saturated aqueous NH4PF6;
Figure 1. Partial 19F NMR spectra for the complexation of 4-Me-DA24C8 to [DFA]
The descriptors c and uc refer to complexed and uncomplexed states of the components.
ACHUTGTNERNGUN[PF6] and [DFA]ACHTUNGTNER[NUGN TFPB].
is only 15mÀ1 (DG8=À1.61 kcalmolÀ1).[11] As indicated in
subsequent chromatographic purification (SiO2) afforded
the desired rotaxane [2-H][PF6] in good yield (92%).
Figure 1, replacing the guest salt [DFA]
N
ACHTUNGTRENNUNG
A
Next, we investigated the complexation of a macrocyclic
host with the TFPB and PF6 salts of a threadlike cation in
more-polar solvents. We mixed the macrocycle 3[14] with the
sociation constant for the interaction of [DFA]ACTHNUTRGENUG[N TFPB] with
4-Me-DA24C8 to be increased to 36mÀ1 (DG8=À2.13 kcal
salts [4-H]ACTHUNGTRENU[NG PF6] and [4-H]ACTHUNGTRENUN[NG TFPB], individually, in combina-
molÀ1). Because the binding of [DBA]
N
tions of CDCl3 and CD3CN in different ratios, but at a con-
stant concentration (5 mm, Scheme 2). Because of the slow
rates of exchange for the complexation and decomplexation
processes in this host–guest system under these conditions,
we used a single-point method[15] to obtain most of the asso-
ciation constants. Table 1 reveals that the association con-
stant for the complexation of the threadlike salt [4-H]-
DA24C8 is even weaker than that of [DFA]
ous study we developed a particularly mild stoppering
method to synthesize the [2]rotaxane [2-H][PF6] in 40%
yield from 4-Me-DA24C8 and the threadlike salt [1-H]-
[PF6]in CH2Cl2.[12] The reaction of the macrocycle 4-Me-
DA24C8, P(OEt)3, and the new threadlike salt [1-H][TFPB]
under the same conditions provided the corresponding
[2]rotaxane [2-H][TFPB] in a significantly higher isolated
ACHTUNGTRENNUNG
N
AHCTUNGTRENNUNG
N
ACHTUNGTRENNUNG
yield of 69% (Scheme 1). To take advantage of the unique
characteristics of the PF6 ion, we wished to demonstrate—
that TFPB is a better anion than PF6 for the complexation
À
Scheme 1.
Scheme 2.
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S.-H. Chiu et al.
Table 1. Association constants Ka for the complexation of the macrocycle
3 and the threadlike salts [4-H]+ in various solvents.[a]
Solvent
[b]
Salt
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CD3CN
+
+
+
+
CD3CN CD3CN CD3CN CD3CN
(9:1) (3:1) (1:1) (1:3)
A
E
N
ACHTUNGTRENNUNG
A
E
6900
1560
1300
3000
370
470
160
190
110
100
N
ACHTUNGTRENNUNG
[TFPB] 6.3ꢂ106 27000
[a] Experiments were performed by using an equimolar mixture of the
macrocycle and corresponding threadlike salt (5 mm). [b] Determined by
using isothermal titration calorimetry (ITC) at
a concentration of
0.5 mm.[13]
of this ammonium cation to the macrocycle 3 in this solvent.
The difference in association constants for the macrocycle 3
complexing the threadlike salts [4-H]ACTHNUTRGENNUG[PF6] and [4-H]ACHTUGNTREN[NUGN TFPB]
became increasingly smaller upon increasing the content of
the more-polar solvent (CD3CN). Thus, in the complexation
of the macrocycle 3 and the threadlike salt [4-H]ACTHNUGRTNEUNG[TFPB],
the presence of the more-polar solvent molecules counter-
acted the enhancing effect of the TFPB anionꢁs weak associ-
ation with the DBA+ ion, possibly because the degrees of
ion pairing of the two salts became similar upon increasing
the content of CD3CN, which is good at solvating ions. Thus,
if more-polar solvents are unavoidable, it might not be real-
istic to expect significant increases in binding affinity for
crown ether/DBA+ pairs merely by changing the counter
anion of the DBA derivatives from PF6 to TFPB.
Scheme 3.
cies in solution, we heated a mixture of the macrocycle 5
(400 mm) and the threadlike salt [6-H][TFPB] (200 mm) in
CHCl3 at 323 K for 48 h and isolated the corresponding
AHCTUNGTRENNUNG
[2]rotaxane [7-H]ACTHNUTRGNE[NUG TFPB] in 30% yield (Scheme 3). Thus,
Interestingly, we found that TFPB counter anions can fa-
cilitate the threading of DBA+ cations through macrocycles
in cases where the corresponding PF6 salts fail to exhibit
complexation. We replaced the two benzylic oxygen atoms
in macrocycle 3—which forms pseudorotaxane complexes
the threadlike cation [6-H]+ could form a [2]pseudorotax-
ane with the macrocycle 5 in CHCl3 as long as its counter
anion was TFPB, not PF6.[17] This finding suggests that using
À
only PF6 salts to investigate the binding of DBA+ deriva-
tives with macrocycles can result in the omission of possi-
ble—and potentially useful—recognition systems.
+
with the DBA+ ion mainly through interactions of the NH2
center with the diethylene glycol unit of the macrocycle[14]
—
The addition of more tightly binding anions, such as FÀ or
ClÀ, to DBA+ salts can induce the transfer of the originally
DBA+-encircling interlocked macrocycle to another station,
thereby generating anion-controllable molecular switches.[18]
We found that the addition of tetra-n-butylammonium chlo-
ride (NBu4Cl) to a CDCl3 solution of the [2]rotaxane [7-H]-
with sulfur atoms, with the expectation that the association
of the DBA+ ion with the resulting macrocycle 5 would be
significantly weakened through loss of the primary modes of
+
+
À
À À
stabilization (i.e., [ N H···O] and [ N C H···O] hydrogen
bonds). We initially tested the complexation of the macrocy-
cle 5 with DBA+ ions using the threadlike salt [6-H]
[PF6],
ACHTUNGTREN[NGNU TFPB], led to dethreading of the macrocycle 5 from the
not only because it is soluble in CDCl3 but also because we
had used it previously for rotaxane synthesis with the mac-
rocycle 3 through a mild “swelling” process.[16] Notably, how-
dumbbell-shaped component, as evidenced in 1H NMR
spectra by gradual increases in the intensities of the signals
of the free macrocycle 5 and the free dumbbell-shaped
thread [8-H]+, together with decreases in the intensities of
1
ever, the H NMR spectrum of an equimolar mixture of the
1
threadlike salt [6-H]
N
those of the [2]rotaxane [7-H]+. We used H NMR spectros-
298 K featured only negligible shifts of the signals relative
to those of their individual components, suggesting that
their association was extremely weak under these condi-
tions. When mixing the macrocycle 5 with the threadlike salt
copy to monitor, over time, the slow dissociation of the com-
ponents of the [2]rotaxane [7-H]ACTHUNGRTNE[NUG TFPB] in CDCl3 in the
presence of NBu4Cl (Figure 2). After integration of the sig-
nals at d=7.69 and 6.66 ppm, for the aromatic protons of
TFPB and the phenolic protons of the free macrocycle 5, re-
spectively, we determined the rate constant (kd), the half-life
(t1/2), and the free energy of activation (DG°) for this disso-
ciation process to be 2.5ꢂ10À4 sÀ1, 0.8 h and, 22.3 kcalmolÀ1,
respectively. Thus, the cycloheptadiene unit is not a true
stopper for the macrocycle 5, and anions that disrupt the hy-
drogen bonding between the dumbbell-shaped cation and
[6-H]ACHTUNGTRENNUNG[TFPB] (10 mm each) under the same conditions, a
new set of signals belonging to a 1:1 complex of the two
1
components appeared in the H NMR spectrum recorded at
298 K. Using a single-point method, we determined the as-
sociation constant for the interaction between the macrocy-
cle 5 and the threadlike salt [6-H]
prove that a [2]pseudorotaxane formed from these two spe-
[TFPB] to be 50mÀ1. To
ACHTUNGTRENNUNG
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Chem. Eur. J. 2012, 18, 1896 – 1900

Anion Choices in (Pseudo)Rotaxane Synthesis
COMMUNICATION
from a mixture of the macrocy-
cle 5 and the threadlike salt [6-
H]ACTHNUTRGNEUNG
[PF6].[21] These results reveal
À
that although PF6 is generally
considered to be an anion that
associates only weakly with
threadlike
dialkylammonium
cations in CDCl3, these ion
pairing interactions remain suf-
ficiently strong to prevent
threading of the dialkylammo-
nium ions through the cavities
of macrocyclic hosts that pro-
vide little stabilization energy.
Thus, it can be worthwhile to
test the binding affinities of
DBA+ derivatives with poten-
tial hosts in less-polar solvents
in the form of their TFPB salts,
even if negative results have
À
Figure 2. 1H NMR spectra (400 MHz, CDCl3, 298 K) revealing the dissociation of the macrocycle 5 from an
been obtained from their PF6
equimolar mixture of Bu4NCl and [7-H]ACTHUNTGRNEUNG[TFPB] (4 mm) after a) 4, b) 7, c) 13, d) 19, e) 28, and f) 43 min.
salts.
We have demonstrated that
À
the macrocycle can induce the dissociation of the compo-
nents of the [2]rotaxane [7-H][TFPB]. We monitored the
replacing the counter anion of DBA+ ions from PF6 to
TFPB can significantly enhance their affinity toward macro-
cyclic hosts and, thereby, increase the yields of their corre-
sponding rotaxanes. Indeed, when using TFPB as the coun-
ter anion for DBA+ species, we could synthesize rotaxanes
from macrocycles that displayed negligible binding to the
AHCTUNGTRENNUNG
same dissociation events induced by other introduced anions
and at other temperatures, allowing us to calculate the en-
thalpic (DH°) and entropic (DS°) contributions to the disso-
ciation process (Table 2).[19] The rates of dissociation of the
[2]rotaxane [7-H]+ in CDCl3 followed the order ClÀ > BrÀ
> IÀ, which correlates well with the associated affinities of
these anions to the DBA+ ion in this solvent.[8a,20] This find-
ing suggested to us the opportunity to directly compare the
corresponding [DBA]ACTHNUTRGNEUNG[PF6] salts. Dissociation experiments
provided direct evidence that the interactions of DBA+ ions
À
with PF6 ions in CDCl3 are stronger than those with TFPB
anions. We have also demonstrated that the TFPB anions of
the resulting rotaxanes can be exchanged to the correspond-
À
affinities of PF6 and TFPB anions toward the [2]rotaxane
À
[7-H]+ in CDCl3. As expected, the addition of Bu4NPF6 to a
ing PF6 salts, if necessary. When performing binding studies
CDCl3 solution of the [2]rotaxane [7-H]
[TFPB] led to disas-
and rotaxane syntheses with DBA+ ions and potentially
weakly interacting hosts in less-polar solvents, we suggest
that the corresponding TFPB salts be examined to prevent
possible dismissal of potentially useful recognition systems.
We believe that this approach will aid in the discovery of
new hosts for DBA+ ions and, thereby, new interlocked
structures and molecular switches derived from them.
sembly of the interlocked structure at a rate much slower
than that induced by the halogen anions (Table 2), consis-
tent with our inability to synthesize the [2]rotaxane [7-H]+
Table 2. Kinetic data for the dissociation of [7-H]ACHTNUTRGNEUNG[TFPB] into its compo-
nents in the presence of various anions.[a,b]
[c,d]
Anion
kd
A
DG°[d,e]
DDH°[f]
DDS°[f]
[sÀ1
]
[kcalmolÀ1
]
[kcalmolÀ1
]
[calmolÀ1 KÀ1
]
ClÀ
BrÀ
IÀ
2.5ꢂ10À4
1.1ꢂ10À4
2.2ꢂ10À5
4.1ꢂ10À6
22.3
22.8
23.8
24.8
17.1
17.5
18.8
18.5
À17.3
À17.9
À16.9
À21.2
Acknowledgements
We thank National Science Council (Taiwan) for financial support (NSC-
98-2113M-002-004-MY3 and NSC-100-2119M-002-026).
À
PF6
[a] Experiments were performed in CDCl3 with an initial concentration
À
of [7-H]
N
Keywords: anion effect · dissociation · host–guest systems ·
and 6 equiv, respectively, in the form of their tetra-n-butylammonium
salts; see ref. [20]. [c] Values of kd were obtained from the slope of the
straight line in the plot of ln([A0]/[At]) against t, using the relationship
ln([A0]/[At])=kt. [d] Calculated at 298 K. [e] The values of DG° were cal-
rotaxanes
·
supramolecular chemistry
·
tetrakis(3,5-
trifluoromethylphenyl)borate
culated using the relationship DG° =ÀRT ln
ACHTUNGTREN(NGNU kh/kBT), where R, h, and kB
correspond to the gas, Planck, and Boltzmann constants, respectively.
[f] The values of DH° and DS° were obtained from the intercept and
slope, respectively, of the straight lines in the plots of DG° against T,
using the relationship DG° =DH°ÀTDS°.
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[19] The dissociations of the [2]rotaxane [7-H]
ACHTUNGERTN[NUNG TFPB] after introducing
F
À and AcOÀ anions as TBA salts to its CDCl3 solution under simi-
lar conditions were too rapid to obtain meaningful data using this
approach.
[20] Because of the strong ion-pairing tendency of halogen anions to
DBA+, we applied first-order rate law analysis to their equimolar
mixtures with the [2]rotaxane [7-H]ACTHNUGTRNEUNG[TFPB] based on the assumption
that the TFPB anion would be replaced rapidly and completely by
these halogen anions upon mixing. This assumption might be less
À
appropriate in the case of the weakly coordinated PF6 ion. We
found that the dissociation rate of [7-H]
significantly when the amount of [TBA][PF6] introduced to the solu-
tion was greater than six equivalents. Therefore, our kinetic studies
were based on the addition of six equivalents of [TBA][PF6] to the
CDCl3 solution of [7-H][TFPB]; these conditions might provide data
ACHTUNGTRENUN[NG TFPB] in CDCl3 varied in-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
inconsistent with those obtained in the halogen cases because the
ion strengths and counter ion effects were different.
[21] A CDCl3 solution of the [2]rotaxane [7-H]ACTHUNRTGNEUNG[TFPB] (4 mm) and
NBu4TFPB (24 mm) in a sealed NMR tube was heated at 323 K for
16 h. No signals corresponding to the dissociation of the [2]rotaxane
1
or decomposition of the anions were evident in the H and 19F NMR
spectra, suggesting that the dethreading of the components of the
[2]rotaxane after adding various anions at different temperatures
was not initiated by the increased thermal energy or the ionic
strength under the experimental conditions. To avoid possible dis-
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À
ruptive effects caused by ClÀ ions or by the decomposition of PF6
into PF5 and FÀ in CDCl3, the deuterated solvent used in the dissoci-
ation experiments was treated with K2CO3 and Na2S2O3 and distilled
prior to performing these studies. For an observation of the decom-
À
position of PF6 ions under specific conditions, see: S. J. Cantrill,
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Received: November 4, 2011
Published online: January 16, 2012
1900
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