Synthesis and complexation studies between trifluoromethylammonium threads and dibenzo[24]Crown-8

Article abstract of DOI:10.1002/ejoc.201001277

Two new ammonium threads were synthesized and characterized. They are composed of either one or two bis-alkylammonium sites that can act as recognition elements for dibenzo[24]crown-8 (DB24C8) and are both appended with two electron-withdrawing trifluorom

Full text of DOI:10.1002/ejoc.201001277

FULL PAPER
DOI: 10.1002/ejoc.201001277
Synthesis and Complexation Studies between Trifluoromethylammonium
Threads and Dibenzo[24]Crown-8
Carsten Johnsen,[a] Paul C. Stein,[a] Kent A. Nielsen,[a] Andrew D. Bond,[a] and
Jan O. Jeppesen*[a]
Keywords: Crown compounds / Ammonium salts / Rotaxanes / Self-assembly / Supramolecular chemistry
Two new ammonium threads were synthesized and charac-
terized. They are composed of either one or two bis-alkyl-
ammonium sites that can act as recognition elements for di-
benzo[24]crown-8 (DB24C8) and are both appended with
two electron-withdrawing trifluoromethyl (CF3) groups. The
pseudorotaxanes formed between the trifluoromethyl ammo-
nium threads and DB24C8 were characterized by single-
crystal X-ray diffraction analyses and studied by NMR spec-
troscopy. The NMR spectroscopic data reveal that attach-
ment of two electron-withdrawing trifluoromethyl groups to
the single-site ammonium thread enhances the binding
strength between the DB24C8 ring and the ammonium
thread. Investigations carried out between the double-site
ammonium thread and DB24C8 reveal that the relative
amount between the [2]pseudorotaxane and the [3]pseudoro-
taxane can be controlled by changing the ratio between the
double-site ammonium thread and DB24C8. Quantitative
studies carried out by NMR titrations indicate at weak
(K2/K1 = 0.24) anticooperative effect in the formation of the
[3]pseudorotaxane.
the ammonium thread increases the binding strength of the
complex formed between the crown ether and the ammo-
nium thread.
In this paper, we present the synthesis of the novel am-
monium threads 3-H·PF6 and 4-H2·2PF6 (Figure 1) ap-
pended with two CF3 groups. Two different synthetic ap-
proaches were applied for the preparation of 4-H2·2PF6.
Thereafter, we describe and discuss some complexation
studies carried out between the bis-alkyl ammonium
threads and dibenzo[24]crown-8 (DB24C8).
Introduction
Crown ethers have, since their discovery in 1967 by Ped-
ersen,[1] been an important constituent of supramolecular
chemistry, and they have been widely used as the ring com-
ponent in catenanes[2] and rotaxanes[2a,2d,3] or as the re-
cognition element in cation sensors.[4] In particular, the
complexation between crown ethers and ammonium
threads has been intensively studied during the last decades.
Pedersen demonstrated that 18-crown-6 (18C6) was capable
of forming complexes with different alkyl ammonium salts,
and it was found that primary alkyl ammonium salts can
penetrate sufficiently deep into the crown ether cavity al-
lowing complexation to take place. On the other hand, sec-
ondary and tertiary alkyl ammonium salts are sterically
hindered, which prevents them in forming complexes with
18C6. However, Kolchinski et al. and Ashton et al. later
showed[5] that secondary alkyl ammonium salts can pen-
etrate into the cavity of crown ethers when the crown ether
ring consists of 24 atoms or more, leading to the formation
of pseudorotaxanes. Further investigations carried out by
Ashton et al. revealed[6] that the presence of electron-with-
drawing and electron-donating groups (EWGs and EDGs)
on the ammonium thread impact the binding strength be-
tween the crown ether and the alkyl ammonium threads. In
particular, investigations conducted by Loeb et al.[7] indi-
cated that attachment of a trifluoromethyl (CF3) group to
Figure 1. Molecular formulas of ammonium threads 1-H·PF6, 2-
H2·2PF6, 3-H·PF6, and 4-H2·2PF6.
Results and Discussion
Synthesis
[a] Department of Physics and Chemistry, University of Southern
Denmark,
The syntheses of ammonium threads 1-H·PF6 and 2-
H2·2PF6 shown in Figure 1 and their complexation with
DB24C8 have already been reported.[5] For comparison
Campusvej 55, 5230 Odense M, Denmark
Fax: +45-6615-8780
E-mail: joj@ifk.sdu.dk
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C. Johnsen, P. C. Stein, K. A. Nielsen, A. D. Bond, J. O. Jeppesen
FULL PAPER
reasons, 1-H·PF6 and 2-H2·2PF6 were synthesized accord- Ph3P in excellent yield (96%). Treatment of 14 with 4-tri-
ing to literature procedures,[8] whereas 3-H·PF6 was synthe- fluoromethylbenzylamine (6, 2 equiv.) afforded compound
sized as shown in Scheme 1. Here, 4-trifluoromethylbenzal- 15 in 54% yield. Subsequently, the Boc protecting group in
dehyde (5) and 4-trifluoromethylbenzylamine (6) were con- 15 was removed by using trifluoroacetic acid (TFA), provid-
densed to imine 7 (99%), followed by reduction with ing bisamine 16 in 64% yield. After protonation and anion
NaBH4 to afford amine 8 in 56% yield. Finally, after pro- exchange with NH4PF6, desired bis-ammonium thread 4-
tonation and anion change with NH4PF6, 3-H·PF6 was ob- H2·2PF6 was obtained in quantitative yield.
tained in 92% yield.
Although most of the yields in route 1 were rather high,
the overall yield (20%) of this route is not very impressive.
Consequently, an alternative route was devised for the prep-
aration of compound 16, as illustrated in Scheme 3. Route 2
uses a similar strategy to that used for the synthesis of 2-
H2·2PF6.[8] Terephthal dicarboxaldehyde (17) was treated
with 4-trifluoromethylbenzylamine (6, 2 equiv.) to provide
diimine 18 in quantitative yield, which subsequently was re-
duced with NaBH4 to give amine 16 in 68% yield. Finally,
protonation and anion exchange with NH4PF6 gave 4-
H2·2PF6 in quantitative yield. The overall yield of this alter-
native route is 68%, which is an obvious improvement to
the overall yield obtained from route 1.
Scheme 1. Synthesis of compound 3-H·PF6.
Ammonium thread 4-H2·2PF6 was synthesized by two
different synthetic routes. Route 1 is shown in Scheme 2 and
was inspired by earlier work carried out by Ashton and
Loeb.[7,9] First, 4-trifluoromethylbenzylamine (6) and
methyl-4-formylbenzoate (9) were condensed to afford im-
ine 10 in quantitative yield, which subsequently was re-
duced (74%) to amine 11 by using NaBH4. To prevent any
unwanted alkylation of the amino functionality in the fol-
lowing syntheses, the amine functionality of 11 was pro-
tected by using di-tert-butyl dicarbonate (Boc2O) to afford
Boc-protected amine 12 in 97% yield. Reduction of methyl
ester 12 with LiAlH4 in THF gave primary alcohol 13 in
Scheme 3. Synthesis of compound 16 (route 2).
Characterization
All new compounds described in this paper were charac-
good yield (84%). Thereafter, the hydroxy group in 13 was terized by NMR spectroscopy, mass spectrometry, and ele-
converted into bromide 14 through the use of CBr4 and mental analysis. The 1H NMR spectrum (200 MHz, 298 K)
Scheme 2. Synthesis of compound 16 (route 1) and its corresponding bis-ammonium thread 4-H2·2PF6.
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Complexation Studies between Ammonium Threads and DB24C8
recorded (Figure 2) in CD3CN of bis-ammonium thread 4- as evidenced by the appearance of several new signals in the
1H NMR spectrum (300 MHz, 298 K), a situation which is
characteristic of superstructures containing an ammonium
thread located inside DB24C8. The exchange between the
complexed and uncomplexed species in the 1:1 complex
formed between thread 3-H·PF6 and DB24C8 occurs slowly
H2·2PF6 revealed two singlets at δ = 4.29 and 4.34 ppm,
which can be assigned to the resonances associated with the
benzylic CH2N+ protons H5 and H4, respectively. In the
aromatic region, the inner p-xylyl protons (i.e., H6) appear
as a singlet at δ = 7.55 ppm, whereas the outer p-xylyl pro-
tons (i.e., H2 and H3) are observed as an AB system at δ =
7.67 and 7.79 ppm. Matrix-assisted laser-desorption/ioniza-
tion time-of-flight (MALDI-TOF) mass spectrometry of 4-
H2·2PF6 gave peaks at m/z = 599.09, 453.12, and 227.09
corresponding to the [M – PF6]+, [M – HPF6]+, and [M –
2PF6]2+ ion, respectively.
1
on the H NMR timescale, allowing the association con-
stant Ka to be determined (vide infra) by using the single-
point method.[10,11]
A comparison of the 1H NMR spectra (Figure 3) of
DB24C8 and ammonium thread 3-H·PF6 with that re-
corded of a 1:1 mixture of DB24C8 and 3-H·PF6 reveals,
as expected, that the formation of [2]pseudorotaxane 3-
H·PF6ʚDB24C8 induces a significant shift in the reso-
nances associated with the protons in both the thread and
the crown ether component.[5] In the aromatic region, all
signals of [2]pseudorotaxane 3-H·PF6ʚDB24C8 are shifted
upfield relative to its free constituents. The signals associ-
ated with the aromatic protons H2 and H3 of 3-H·PF6 shift
from 7.67/7.79 ppm (AB system) to 7.43 ppm (singlet) upon
complexation, whereas the signal associated with the aro-
matic protons Hc of DB24C8 shift from 6.89 ppm (singlet)
to 6.73/6.85 ppm (two multiplets). These observations can
most likely be accounted for by the presence of π–π stack-
ing interactions between the aromatic rings in thread 3-
H·PF6 and the DB24C8 ring. In the aliphatic region of the
1H NMR spectrum, the complexation event results in a
downfield shift of the benzylic H4 protons from 4.34 to
4.78 ppm, whereas the signals associated with the glycolic
protons (α, β, and γ) of DB24C8 all shift upfield from 4.13,
3.88, and 3.78 ppm to 4.06, 3.80, and 3.55 ppm, respec-
tively, a situation which can be explained by the formation
of hydrogen-bonding interactions between the ammonium
protons in 3-H·PF6 and the oxygen atoms in the glycol
chains of DB24C8. Formation of hydrogen bonds deshield
the benzylic CH2N+ protons (i.e., H4) of 3-H·PF6, while
shielding the glycolic OCH2 protons (i.e., α, β, and γ) of
DB24C8 at the same time. All relevant chemical shifts
are summarized in Table 1. [2]Pseudorotaxane 3-H·
PF6ʚDB24C8 was observed to be in slow exchange with
Figure 2. Partial 1H NMR spectrum (200 MHz, CD3CN, 298 K)
of 4-H2·2PF6. The applied numbering of protons on the bis-ammo-
nium thread will be used throughout the article.
Complexation Studies
The inclusion of ammonium threads inside the cavity of
DB24C8 is well documented[5–7] and leads to the formation
of pseudorotaxanes under thermodynamic control upon
mixing of their cyclic and acyclic components in solution.
In the present study, the complexation between DB24C8
and ammonium threads 1-H·PF6, 2-H·PF6, 3-H·PF6, and 4-
1
H2·2PF6 was investigated by using H NMR spectroscopy.
Single-Site Ammonium Threads
1
its free constituents on the H NMR timescale (300 MHz,
Addition of ammonium thread 3-H·PF6 (1 equiv.) to a 298 K), as both complexed and uncomplexed resonances
CDCl3/CD3CN (3:1) solution of DB24C8 leads (Scheme 4) are present in the 1H NMR spectrum (Figure 3d). As noted
to the formation of [2]pseudorotaxane 3-H·PF6ʚDB24C8, above, several of the protons in both the thread and the
Scheme 4. Formation of [2]pseudorotaxane 3-H·PF6ʚDB24C8.
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C. Johnsen, P. C. Stein, K. A. Nielsen, A. D. Bond, J. O. Jeppesen
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ring component show significant shifts (Table 1) in their
resonances upon complexation, which makes it possible to
determine the association constants (Ka) by employing the
1H NMR single-point method.[12] The signal associated
with the benzylic CH2N+ protons (i.e., H4) of 3-H·PF6 was
found to be the most useful probe and a Ka value[13] of
3700 m–1 for the complexation of 3-H·PF6 by DB24C8 in
CDCl3/CD3CN (1:1) at 298 K was obtained. By employing
a similar approach and for comparison reasons, we also de-
termined the Ka value[13] for the complexation of 1-H·PF6
by DB24C8 in CDCl3/CD3CN (1:1) to be 2000 m–1 at
298 K. These results indicate that attachment of two CF3
groups to the single-site ammonium thread enhances the
binding strength between the DB24C8 ring and the ammo-
nium thread.
Double-Site Ammonium Threads
Double-site ammonium thread 4-H2·2PF6 contains two
ammonium recognition sites for the DB24C8 ring. Conse-
quently, addition of DB24C8 to 4-H2·2PF6 can lead
to the formation (Scheme 5) of both [2]pseudorotaxane
4-H2·2PF6ʚDB24C8
and
[3]pseudorotaxane
4-H2·
2PF6ʚ(DB24C8)2. In common with the single-site ammo-
nium threads, the exchange between the complexed and un-
complexed species formed between 4-H2·2PF6 and DB24C8
1
occurs slowly on the H NMR timescale [CDCl3/CD3CN
(1:1), 300 MHz][14] at 298 K. Thus, both the resonances for
the complexed and uncomplexed species can be observed in
the 1H NMR spectrum (Figure 4) recorded on a mixture of
4-H2·2PF6 and DB24C8 (2 equiv.) in CDCl3/CD3CN (1:1).
1
The H NMR spectrum reveals that new signals appear as
Figure 3. Partial 1H NMR spectra recorded at 298 K of
(a) DB24C8 (300 MHz, CDCl3/CD3CN, 3:1), (b) a 1:1 mixture of
DB24C8 and 3-H·PF6 (300 MHz, CDCl3/CD3CN, 3:1), and (c) 3-
H·PF6. (200 MHz, CD3CN). (d) Complete assignment of all signals
in the 1H NMR spectrum recorded of a 1:1 mixture of DB24C8
and 3-H·PF6 [300 MHz, CDCl3/CD3CN (3:1), 298 K]. The descrip-
tions c and u refer to complexed and uncomplexed components,
respectively.
a result of complexation. The resonances associated with
aromatic protons H2, H3, and H6 in thread component 4-
H2·2PF6 shift from 7.80, 7.67, and 7.55 ppm to 7.49, 7.41,
and 7.05 ppm, respectively, upon complexation with
DB24C8. In the aliphatic region, two multiplets at δ = 4.54
and 4.75 ppm are observed, which can be assigned to the
complexed benzylic CH2N+ protons (i.e., H4 and H5). Al-
Table 1. Selected 1H NMR spectroscopic data (δ values) for [2]pseudorotaxanes 1-H·PF6ʚDB24C8 and 3-H·PF6ʚDB24C8 and [3]pseudo-
rotaxanes 2-H2·2PF6ʚ(DB24C8)2 and 4-H2·2PF6ʚ(DB24C8)2, together with their corresponding free constituents at 298 K. s = singlet,
d = doublet, t = triplet, m = multiplet, AB = AB system.
Compound/complex
Crown ether
α-OCH2
Ammonium thread
C6H4
o-O2C6H4
β-OCH2
γ-OCH2
CH2N+
DB24C8[a]
6.89 (s)
–
6.78–6.83 (m), 6.87–6.93 (m)
4.13 (m)
3.88 (m)
3.78 (s)
–
3.44 (s)
–
–
–
[b]
1-H·PF6
–
4.10 (m)
–
4.06 (m)
–
3.98 (m)
–
3.98 (m)
–
4.23 (s)
4.62 (m)
4.34 (m)
4.78 (m)
7.46(s)
7.17–7.36 (m)
1-H·PF6ʚDB24C8[a]
3.76 (m)
[b]
3-H·PF6
–
–
7.67 and 7.79 (AB)
7.43 (s)
7.53 (s), 7.46 (s)
6.92 (s), 7.20–7.40 (m)
7.55 (s), 7.67 and 7.80 (AB)
7.07 (s), 7.41 and 7.49 (AB)
3-H·PF6ʚDB24C8[c]
2-H2·2PF6
6.70–6.76 (m), 6.82–6.89 (m)
3.80 (m)
3.55 (s)
[b]
–
–
–
3.40–3.65 (m)
–
4.23 (m)
[a]
[c]
2-H2·2PF6ʚ(DB24C8)2
6.68–6.76 (m), 6.78–6.86 (m)
–
6.66–6.76 (m), 6.78–6.87 (m)
3.68–3.72 (m)
–
3.63–3.82 (m)
4.63 (m), 4.45 (m)
4.29 (s), 4.34 (s)
4.50–4.60 (m),
4.70–4.80 (m)
[b]
4-H2·2PF6
4-H2·2PF6ʚ(DB24C8)2
3.56 (s)
[a] 1H NMR spectra were recorded in CDCl3/CD3CN (3:1) at 300 MHz. [b] 1H NMR spectra were recorded in CD3CN at 200 MHz.
[c] 1H NMR spectra were recorded in CDCl3/CD3CN (1:1) at 300 MHz.
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Complexation Studies between Ammonium Threads and DB24C8
Scheme 5. Formation of [2]pseudorotaxane 4-H2·2PF6ʚDB24C8 and [3]pseudorotaxane 4-H2·2PF6ʚ(DB24C8)2, together with cartoon
representations of bis-ammonium thread 4-H2·2PF6, [2]pseudorotaxane 4-H2·2PF6ʚDB24C8, and [3]pseudorotaxane 4-
H2·2PF6ʚ(DB24C8)2.
1
though the H NMR spectrum (Figure 4) clearly indicates the increased complexity of the multiplet resonating at δ =
that complexation between 4-H2·2PF6 and DB24C8 takes 4.75 ppm. These observations can most likely be ascribed
place, it does not give a clear picture on whether the com- to the formation of a new species in solution, namely, [3]
plexation event leads to the formation of [2]pseudorotaxane pseudorotaxane 4-H2·2PF6ʚ(DB24C8)2. This was further
1
4-H2·2PF6ʚDB24C8 or to [3]pseudorotaxane 4-H2· substantiated by the fact that the H NMR spectrum (Fig-
2PF6ʚ(DB24C8)2 – or a mixture of both. Consequently, ure 5c and Figure 6c) recorded on a 1:5 mixture of 4-
further 1H NMR spectroscopic investigations (Figures 5 H2·2PF6 and DB24C8 leads to simplification in the signals
and 6) of the complexation between 4-H2·2PF6 and observed in the region from 4.5 to 4.8 ppm, an observation
DB24C8 were carried out by changing the ratio between 4- which is consistent with the conversion of [2]pseudorotax-
H2·2PF6 and DB24C8 from 5:1 to 1:1 and then finally to ane 4-H2·2PF6ʚDB24C8 into [3]pseudorotaxane 4-H2·
1
1:5. A comparison of the H NMR spectra (Figures 5 and 2PF6ʚ(DB24C8)2. Consequently, the multiplets at δ =
6) reveal that the molar amount of DB24C8 has a signifi- 4.67–4.75 and 4.78–4.86 ppm can be used as a signature
cant impact on the appearances of the spectra, both in the
1
aromatic and in the aliphatic region. In the H NMR spec-
trum recorded (Figure 5a and Figure 6a) of a 5:1 mixture
of 4-H2·2PF6 and DB24C8, ammonium thread 4-H2·2PF6
is as expected the dominant species, but it should be noted
that virtually no uncomplexed DB24C8 is present. These
results indicate that the complexation of 4-H2·2PF6 by
DB24C8 is strong at this temperature and that the equili-
brated solution predominantly consist of a mixture of un-
complexed thread 4-H2·2PF6 and [2]pseudorotaxane 4-
H2·2PF6ʚDB24C8.[15] By changing the molar ratio from
5:1 to 1:1, 4-H2·2PF6 is no longer observed to be the pre-
1
Figure 4. Partial H NMR spectrum recorded of a 1:2 mixture of
dominant species in the 1H NMR spectrum (Figure 5b).
4-H2·2PF6 and DB24C8 [300 MHz, CDCl3/CD3CN (1:1), 298 K].
The descriptions c and u refer to complexed and uncomplexed com-
The most important thing to notice, however, is the appear-
ance (Figure 6b) of a signal resonating at δ = 4.54 ppm and ponents, respectively.
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for the presence of [2]pseudorotaxane 4-H2·2PF6ʚDB24C8,
whereas the multiplets at δ = 4.50–4.60 and 4.72–4.80 ppm
can be used as a signature for the presence of [3]pseudorot-
axane 4-H2·2PF6ʚ(DB24C8)2.
1
Figure 6. Partial H NMR spectra (400 MHz, 298 K) recorded in
CDCl3/CD3CN (1:1) of 4-H2·2PF6 and DB24C8 at different molar
ratios: (a) 5:1, (b) 1:1, and (c) 1:5. The multiplets centered at δ =
4.71 (upside-down filled grey triangle) and 4.81 (filled black square)
ppm correspond to the benzylic CH2N+ resonances when DB24C8
encircles one of the ammonium recognition sites in thread 4-
H2·2PF6, whereas the multiplets centered at δ = 4.54 (filled grey
diamond) and 4.75 (filled grey circle) ppm correspond to the benz-
ylic CH2N+ resonances when DB24C8 encircles both ammonium
recognition sites in thread 4-H2·2PF6. The labels (upside-down
filled grey triangle, filled black square, filled grey circle, and filled
grey diamond) used to assign the different signals refer to the benz-
ylic CH2N+ protons as shown in the cartoons in Scheme 5.
1
Figure 5. Partial H NMR spectra (400 MHz, 298 K) recorded in
CDCl3/CD3CN (1:1) of 4-H2·2PF6 and DB24C8 at different molar
ratios: (a) 5:1, (b) 1:1, and (c) 1:5.
In the case of the complexation between 4-H2·2PF6 and
DB24C8, the simple single-point method (vide supra) is no
longer a valid method for determination of the binding con-
stants (i.e., K1 and K2). Instead K1 and K2 were obtained
by taking the weighted average of a series of K1 and K2
values obtained through single-point determinations of two
independent titration curves. For the complexation of 4-
H2·2PF6 with DB24C8 recorded in CDCl3/CD3CN (1:1) at
298 K, this yielded K1 = 2500 m–1 and K2 = 600 m–1.[16]
These findings indicate a weak anticooperative effect in the
formation of 4-H2·2PF6ʚ(DB24C8)2. A similar approach
was employed for the complexation of 2-H2·2PF6 with
DB24C8, affording K1 = 2900 m–1 and K2 = 1100 m–1.[16]
When comparing the two bis-ammonium threads, it is evi-
groups is absent in the case of 4-H2·2PF6. This observation
can most likely be accounted for by the fact that in the
single-site ammonium thread 3-H·PF6 both CF3 groups are
appended in close proximity to the ammonium recognition
site, whereas in the double-site ammonium thread 4-
H2·2PF6 only one of the CF3 groups is located close to the
ammonium recognition site.
X-ray Diffraction Analysis
Diffraction-grade single crystals of [3]pseudorotaxane 4-
dent that the K1 values are comparable in size. The anti- H2·2PF6ʚ(DB24C8)2 were obtained by slow vapor dif-
cooperative effect on K2 is also present in this case, but for fusion of iPr2O into a solution of 4-H2·2PF6 and DB24C8
4-H2·2PF6ʚ(DB24C8)2 the size of K2 is reduced to approxi- (2 equiv.) in CH3CN. The resulting X-ray crystal structure
mately half that of 2-H2·2PF6ʚ(DB24C8)2. A comparison (Figure 7) confirms the 2:1 ratio between DB24C8 and 4-
of the K1 and K2 values for the complexation between 4- H2·2PF6 and reveals that the two DB24C8 adopt an S-
H2·2PF6 and DB24C8 with those for the complexation be- shaped conformation around the ammonium centers. The
tween 2-H2·2PF6 with DB24C8 reveals that the positive ef- complex is centrosymmetric and occupies a crystallographic
fect on the binding strength exhibited by the appended CF3 inversion center in the solid state. Four N+–H···O hydrogen
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Complexation Studies between Ammonium Threads and DB24C8
layer chromatography (TLC) was carried out by using aluminum
sheets precoated with silica gel 60F (Merck 5554). The plates were
inspected under UV light (254 nm) and developed with I2 vapor.
Column chromatography was carried out by using silica gel 60F
(Merck 9385, 0.040–0.063 mm). Melting points were determined
with a Büchi melting point apparatus. 1H NMR spectra were re-
corded with a Varian (500 MHz) instrument, a Bruker Avance III
(400 MHz) instrument, a Varian Gemini instrument (300 MHz), or
a Varian Mercury instrument (200 MHz) by using the residual sol-
vent as internal standard. 13C NMR spectra were recorded at room
temperature with a Varian Gemini instrument (75 MHz) or a
Bruker Avance III (100 MHz) by using the residual solvent as in-
ternal standard. Samples were prepared by using CDCl3 and
CD3CN purchased from Cambridge Isotope Labs. Electron ioniza-
tion mass spectrometry (EI MS) was performed with a Finnegan
MAT TSQ 700 instrument, matrix-assisted laser-desorption/ioniza-
tion time-of-flight mass spectrometry (MALDI-TOF-MS) was per-
formed with an Ionspec 4.7 Tesla Ultima Fourier Transform instru-
ment by utilizing a 2,5-dihydroxybenzoic acid matrix, whereas elec-
trospray ionization (ESI) analysis was performed with a Bruker
microTOF-Q II ESI-Qq-TOF mass spectrometer. Microanalyses
were performed by the Atlantic Microlab, Inc., Atlanta, Georgia.
bonds are formed between the two ammonium centers and
DB24C8. In addition, face-to-face π–π stacking interactions
between the central p-xylyl ring on bis-ammonium thread
4-H2·2PF6 and one of the catechol rings on each DB24C8
can be observed. The distance between the centroid of each
catechol ring and that of the central p-xylyl ring is
3.731 Å.[17]
Figure 7. X-ray crystal structure of [3]pseudorotaxane 4-
H2·2PF6ʚ(DB24C8)2. The dashed lines denote N+–H···O hydrogen
bonds.
N-(4-Trifluoromethylbenzylidene)-4-trifluoromethylbenzylamine (7):
A solution of 4-trifluoromethylbenzaldehyde (5; 2.03 g, 11.7 mmol)
and 4-trifluoromethylbenzylamine (6; 2.03 g, 11.6 mmol) was
heated under reflux in PhMe (100 mL) overnight, and the water
liberated from the reaction was removed from the reaction mixture
by means of a Dean–Stark apparatus. The organic phase was re-
moved in vacuo, yielding imine 7 as a clear yellow oil, which crys-
tallized upon cooling to a white solid (3.78 g, 99%). M.p. 34–35 °C.
1H NMR (300 MHz, CDCl3): δ = 4.90 (s, 2 H, NCH2), 7.47 (d,
Conclusions
In summary, we have successfully prepared ammonium
threads 3-H·PF6 and 4-H2·2PF6 appended with two elec-
tron-withdrawing CF3 groups. In both cases, the threads are
able to bind DB24C8, leading to the formation of [2]-
3
3JH,H = 8.0 Hz, 2 H, 2 Ar-H), 7.61 (d, JH,H = 8.0 Hz, 2 H, 2 Ar-
H), 7.69 (d, 3JH,H = 8 Hz, 2 H, Ar-H), 7.90 (d, 3JH,H = 8.0 Hz, 2 H,
Ar-H), 8.46 (s, 1 H, N=CH) ppm. 13C NMR (100 MHz, CDCl3): δ
1
3
= 64.6, 124.2 (q, JF,C = 270 Hz, CF3), 125.6 (q, JF,C = 4 Hz),
125.8 (q, 3JF,C = 4 Hz), 128.3, 128.6, 129.6 (q, 2JF,C = 32 Hz), 132.7
1
pseudorotaxanes and [3]pseudorotaxanes. H NMR spec-
troscopic studies show that attachment of two electron-
withdrawing CF3 groups to single-site ammonium thread 3-
H·PF6 enhances the binding strength between the DB24C8
ring and the ammonium thread by a factor of two. In the
case of double-site ammonium thread 4-H2·2PF6, the rela-
tive amount of the [2]pseudorotaxane and the [3]pseudorot-
axane can be controlled by changing the ratio between 4-
H2·2PF6 and DB24C8. Quantitative analysis reveal that
thread 4-H2·2PF6 exhibits negative cooperativity towards
DB24C8 – an observation that most likely can be accounted
2
(q, JF,C = 32 Hz), 139.1, 143.1, 161.2 ppm. MS (ESI): m/z (%) =
332 (4) [M]+, 159 (100). C16H11F6N (331.3): calcd. C 58.01, H 3.35,
N 4.23; found C 57.77, H 3.17, N 4.25.
N,N-Bis(4-trifluoromethylbenzyl)amine (8): Imine
7
(2.04 g,
6.16 mmol) was dissolved in anhydrous MeOH (70 mL), whereafter
NaBH4 (1.14 g, 30.1 mmol) was added. The reaction mixture was
stirred for 2 h at room temperature, followed by the addition of
another portion of NaBH4 (1.14 g, 30.1 mmol). After stirring the
reaction mixture overnight under N2, the excess amount of NaBH4
was destroyed by slow addition of an aqueous solution of HCl (2 m,
for by steric hindrance upon binding of a second DB24C8 200 mL). After washing with CHCl3 (50 mL), an aqueous solution
of NaOH (2 m) was added until pH≈14. The aqueous phase was
extracted with CHCl3 (5ϫ100 mL), whereafter the combined or-
ganic phases were dried (MgSO4) and concentrated in vacuo, af-
ring to the [2]pseudorotaxane 4-H2·2PF6ʚDB24C8. The
findings reported herein are undoubtedly not unimportant
when it comes to the future design and construction of [2]-
rotaxanes and [3]rotaxanes incorporating bis-alkyl ammo-
nium recognition elements and DB24C8.
1
fording 8 as a clear yellow oil (1.13 g, 56%). H NMR (300 MHz,
CDCl3): δ = 2.72 (br. s, 1 H, NH), 3.87 (s, 4 H, 2 NCH2), 7.48 (d,
3
3JH,H = 8.0 Hz, 4 H, 4 Ar-H), 7.59 (d, JH,H = 8.0 Hz, 4 H, 4 Ar-
1
H) ppm. 13C NMR (100 MHz, CDCl3): δ = 52.8, 124.4 (q, JF,C
=
=
3
2
270 Hz, 2 CF3), 125.5 (q, JF,C = 4 Hz), 128.4, 129.6 (q, JF,C
32 Hz), 144.3 ppm. MS (ESI): m/z (%) = 332 (4) [M + H]+, 159
(100). C16H13F6N (333.3): calcd. C 57.66, H 3.93, N 4.20; found C
57.88, H 3.80, N 4.23.
Experimental Section
General Methods: Chemicals were purchased from Aldrich and
used as received unless indicated otherwise. The compounds di-
benzylammonium hexafluorophosphate (1-H·PF6)[5d] and α,αЈ-bis-
N,N-Bis(4-trifluoromethylbenzyl)ammonium Hexafluorophosphate
(3-H·PF6): A concentrated aqueous solution of HCl was added to
a solution of amine 8 (1.02 g, 3.06 mmol) in MeOH (50 mL) until
pH≈2, and the mixture was stirred for 1 h. The solvent was evapo-
benzylammonium-p-xylene
bis(hexafluorophosphate)
(2-H2·
2PF6)[8] were prepared according to literature procedures. Thin-
Eur. J. Org. Chem. 2011, 759–769
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
765

C. Johnsen, P. C. Stein, K. A. Nielsen, A. D. Bond, J. O. Jeppesen
(br. s, 2 H, NH2), 4.48 (br. s, 2 H, NCH2), 7.20–7.35 (m, 4 H, 4
FULL PAPER
rated, and the residue was dissolved in Me2CO (50 mL). Upon
dropwise addition of an aqueous solution (20 mL) of NH4PF6
(1.63 g, 10 mmol), the product precipitated. The organic solvent
was removed in vacuo, and the product was isolated by filtration,
washed with H2O (50 mL), and air dried, yielding 3-H·PF6 as a fine
3
3
Ar-H), 7.58 (d, JH,H = 8.4 Hz, 2 H, 2 ArH), 8.00 (d, JH,H
=
8.4 Hz, 2 H, 2 ArH) ppm. 13C NMR (100 MHz, CDCl3): δ = 28.5,
1
3
49.6, 52.3, 80.9, 124.2 (q, JF,C = 271 Hz, CF3), 125.7 (q, JF,C
=
3 Hz), 130.1, 142.1, 143.1, 156.0, 167.0 (five signals missing/over-
lapping) ppm. MS (EI): m/z (%) = 424 (2) [M + H]+, 392 (3) [M –
CH3O]+, 367 (61) [M – (CH3)3C]+, 57 (100) [(CH3)3 C]+.
1
white powder (1.34 g, 92%). M.p. Ͼ230 °C. H NMR (300 MHz,
CD3CN): δ = 4.34 (s, 4 H, 2 NCH2), 7.41 (br. s, 2 H, NH2+), 7.67
3
3
(d, JH,H = 8.3 Hz, 4 H, 4 Ar-H), 7.78 (d, JH,H = 8.3 Hz, 4 H, 4 C22H24F3NO4 (423.4): calcd. C 62.40, H 5.71, N 3.31; found C
Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ = 52.0, 125.1 (q, 1JF,C
62.33, H 5.72, N 3.31.
3
2
= 270 Hz, CF3), 126.9 (q, JF,C = 3.5 Hz), 132.1 (s), 132.1 (q, JF,C
= 32 Hz), 135.5 ppm. MS (ESI): m/z (%) = 336 (100) [M + H]+,
159 (88) [CF3Bn]+. C16H14F12NP (479.24): calcd. C 40.10, H 2.94,
N 2.91; found C 40.01, H 2.85, N 2.95.
N-(tert-Butoxycarbonyl)-N-(4-trifluoromethylbenzyl)-4-hydroxy-
methylbenzylamine (13): A solution of 12 (1.35 g, 3.19 mmol) dis-
solved in anhydrous THF (30 mL) was added dropwise to a solu-
tion of LiAlH4 (0.36 g, 9.49 mmol) in anhydrous THF (20 mL) over
a period of 15 min. After stirring overnight at room temperature,
the mixture was treated with H2O (1 mL), an aqueous solution of
NaOH (5 m, 1 mL), and finally more H2O (2 mL). The resulting
suspension was stirred for another 5 min and subsequently filtered
through a thin layer of Celite 545 (1 cm). The filtrate was concen-
trated in vacuo, whereafter the residue was dissolved in CH2Cl2
(50 mL) and washed with H2O (50 mL). Thereafter, the aqueous
phase was extracted with CH2Cl2 (2ϫ50 mL). Finally, the organic
phases were combined, dried (MgSO4), and concentrated in vacuo,
affording 13 as a colorless oil (1.06 g, 84%), which was used with-
out any further purification. 1H NMR (300 MHz, CDCl3): δ = 1.48
(s, 9 H, 3 Boc-CH3), 1.80 (br. s, 1 H, OH), 4.37 (br. s, 2 H, NCH2),
4.43 (br. s, 2 H, NCH2), 4.69 (s, 2 H, OCH2), 7.20 (br. s, 2 H, 2
N-(4-Carboxymethoxybenzylidene)-p-trifluoromethylbenzylamine
(10): A solution of 4-trifluoromethylbenzylamine (6; 1.77 g,
10 mmol) and methyl-4-formylbenzoate (9; 1.64 g, 10 mmol) was
heated under reflux in PhMe (160 mL) overnight, and the H2O
liberated from the reaction was removed from the reaction mixture
by means of a Dean–Stark apparatus. The organic phase was co-
oled to room temperature, whereafter the solvent was removed in
vacuo, yielding imine 10 as a white solid (3.21 g, quant.). M.p. 78–
1
80 °C. H NMR (300 MHz, CDCl3): δ = 3.94 (s, 3 H, CH3), 4.89
3
(s, 2 H, NCH2), 7.47 (d, JH,H = 8.0 Hz, 2 H, 2 Ar-H), 7.76 (d,
3JH,H = 8.0 Hz, 2 H, 2 Ar-H), 7.85 (d, JH,H = 8.4 Hz, 2 H, 2 Ar-
3
3
H), 8.10 (d, JH , H = 8.4 Hz, 2 H, 2 Ar-H), 8.46 (s, 1 H,
N=CH) ppm. 13C NMR (100 MHz, CDCl3): δ = 52.4, 64.6, 125.6
(q, 3JF,C = 4 Hz), 124.4 (q, 1JF,C = 271 Hz, CF3), 128.3, 128.3, 129.6
(q, 2JF,C = 32 Hz), 130.1, 132.3, 139.9, 143.2, 161.8, 166.7 ppm. MS
(EI): m/z (%) = 321 (100) [M]+, 306 (15) [M – CH3]+, 262 (8) [M –
CF3]+. C17H14F3NO2 (321.3): calcd. C 63.55, H 4.39, N 4.36; found
C 63.70, H 4.34, N 4.37.
3
Ar-H), 7.31 (br. s, 2 H, 2 Ar-H), 7.33 (d, JH,H = 8.1 Hz, 2 H, 2
3
Ar-H), 7.56 (d, JH,H = 8.1 Hz, 2 H, 2 Ar-H) ppm. MS (EI): m/z
(%) = 396 (4) [M + H]+, 339 (66) [M + H – (CH3)3C]+, 57 (100)
[(CH3)3C]+. C21H24F3NO3 (395.4): calcd. C 63.79, H 6.12, N 3.54;
found C 63.89, H 6.05, N 3.43.
N-(tert-Butoxycarbonyl)-N-(4-trifluoromethylbenzyl)-4-bromo-
methylbenzylamine (14): To a solution of 13 (0.90 g, 2.28 mmol)
and CBr4 (1.59 g, 4.79 mmol) in anhydrous THF (75 mL) was
added Ph3P (1.51 g, 5.76 mmol) in small portions over a period of
1 h. After stirring overnight at room temperature, the reaction mix-
ture was concentrated in vacuo. The resulting residue was purified
by column chromatography (SiO2; n-hexane/EtOAc, 5:1; Rf = 0.6),
affording 14 as a yellow oil (1.00 g, 96%). 1H NMR (300 MHz,
CDCl3): δ = 1.49 (s, 9 H, 3 Boc-CH3), 4.42 (br. s, 4 H, NCH2),
4.49 (s, 2 H, CH2Br), 7.17 (br. s, 2 H, 2 Ar-H), 7.29 (br. s, 2 H, 2
N-(4-Trifluoromethylbenzyl)-4-carboxymethoxybenzylamine (11):
Imine 10 (2.90 g, 9.03 mmol) was dissolved in a mixture of anhy-
drous THF/MeOH (1:1, 100 mL), whereafter NaBH4 (0.35 g,
9.25 mmol) was added. The reaction mixture was stirred for 4 h at
room temperature, followed by the addition of another portion of
NaBH4 (0.43 g, 11.4 mmol). After stirring the reaction mixture
overnight at room temperature under N2, the excess amount of
NaBH4 was destroyed by slow addition of an aqueous solution of
HCl (4 m, 40 mL). The solvent was removed in vacuo, providing a
white powder, which was partitioned between an aqueous solution
of NaOH (1 m, 100 mL) and CH2Cl2 (100 mL). The aqueous phase
was extracted with CH2Cl2 (2ϫ100 mL), whereafter the combined
organic phases were dried (MgSO4) and concentrated in vacuo, af-
fording 11 as a pale-yellow oil (2.16 g, 74%). 1H NMR (300 MHz,
CDCl3): δ = 1.68 (s, 1 H, NH), 3.86 (s, 4 H, 2 NCH2), 3.91 (s, 3
3
3
Ar-H), 7.35 (d, JH,H = 8.1 Hz, 2 H, 2 Ar-H), 7.57 (d, JH,H
=
8.1 Hz, 2 H, 2 Ar-H) ppm. MS (EI): m/z (%) = 458 (15)
[M + H]+, 402 (29) [M + H – (CH3)3C]+, 57 (100) [(CH3)3C]+.
α-[N-(tert-Butoxycarbonyl)-4-trifluoromethylbenzylamino]-αЈ-(4-tri-
fluoromethylbenzylamino)-p-xylene (15): Compound 14 (0.85 g,
1.85 mmol) and 4-trifluoromethylbenzylamine (6; 0.65 g,
3.71 mmol) were dissolved in MeCN (50 mL), and the mixture was
heated under reflux for 3 h. After cooling to room temperature, the
reaction mixture was filtered and the filtrate was concentrated in
vacuo. The resulting residue was subjected to column chromatog-
raphy (SiO2; CHCl3/n-hexane, 4:1; Rf = 0.1), yielding 15 as a yellow
3
3
H, OCH3), 7.41 (d, JH,H = 8.4 Hz, 2 H, 2 Ar-H), 7.46 (d, JH,H
=
8.1 Hz, 2 H, 2 Ar-H), 7.58 (d, 3JH,H = 8.1 Hz, 2 H, 2 Ar-H), 8.00 (d,
3JH,H = 8.4 Hz, 2 H, 2 Ar-H) ppm. 13C NMR (100 MHz, CDCl3): δ
1
3
= 52.2, 52.7, 52.9, 124.4 (q, JF,C = 272 Hz, CF3), 125.5 (q, JF,C
=
2
4 Hz), 128.1, 128.5, 129.2, 129.6 (q, JF,C = 32 Hz), 129.9, 144.2,
145.4, 167.1 ppm. MS (EI): m/z (%) = 324 (100) [M + 1]+, 264 (22)
[M – COOMe]+. C17H16F3NO2 (323.3): calcd. C 63.15, H 4.99, N
4.33; found C 62.88, H 5.09, N 4.17.
1
oil (0.55 g, 54%). H NMR (300 MHz, CDCl3): δ = 1.48 (s, 9 H, 3
Boc-CH3) 1.62 (s, 1 H, NH) 3.80 (s, 2 H, NCH2), 3.87 (s, 2 H,
NCH2), 4.38 (br. s, 4 H, 2 NCH2), 7.18 (br. s, 2 H, 2 Ar-H), 7.29
N-(tert-Butoxycarbonyl)-N-(4-trifluoromethylbenzyl)-4-carboxy-
methoxybenzylamine (12): Compound 11 (1.64 g, 5.07 mmol),
Boc2O (1.26 g, 5.77 mmol), and DMAP (6 mg, 0.05 mmol) were
dissolved in CH2Cl2 (50 mL), and the mixture was stirred overnight
under N2 at room temperature. Thereafter, the solvent was evapo-
rated in vacuo to yield 12 as a colorless oil (2.08 g, 97%), which
was used without any further purification. 1H NMR (300 MHz,
3
3
(d, JH,H = 8.1 Hz, 4 H, 4 Ar-H), 7.47 (d, JH,H = 8.1 Hz, 2 H, 2
Ar-H), 7.51–7.62 (m, 4 H, 4 Ar-H) ppm. MS (MALDI): m/z (%) =
553 (8) [M + H]+, 519 (37), 497 (14) [M + H – (CH3)3C]+, 322
(100), 304 (34). C29H30F6N2O2 (552.6): calcd. C 63.04, H 5.47, N
5.07; found C 63.22, H 5.60, N 4.91.
α,αЈ-Bis(4-trifluoromethylbenzylamino)-p-xylene (16) (Route 1): A
CDCl3): δ = 1.48 (s, 9 H, 3 Boc-CH3), 3.92 (s, 3 H, OCH3), 4.40 solution of 15 (0.27 g, 0.65 mmol) in CHCl3 (20 mL) and TFA
766
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 759–769

Complexation Studies between Ammonium Threads and DB24C8
3
(1.04 g, 9.1 mmol) was stirred at 70 °C for 3 d. The organic phase
was washed with an aqueous solution of NaOH (1 m, 2ϫ50 mL),
dried (MgSO4), and concentrated in vacuo, affording 16 as a white
3JH,H = 8 Hz, 4 H, 4 Ar-H), 7.80 (d, JH,H = 8 Hz, 4 H, 4 Ar-
H) ppm. 13C NMR (100 MHz, CD3CN): δ = 52.0, 52.1, 125.1 (q,
1JF,C = 271 Hz, CF3), 127.0 (q, JF,C = 4 Hz), 132.0, 132.1, 132.9
3
solid (0.14 g, 64%). The product was used without any further puri-
(q, 2JF,C = 32 Hz), 132.9, 135.6 ppm. MS (MALDI): m/z (%) = 599
fication. M.p. 41–43 °C. H NMR (300 MHz, CDCl3): δ = 1.64 (s, (50) [M – PF6]+, 453 (45) [M + H – 2PF6]+, 295 (100), 159 [CF3Bn]
1
2 H, 2 NH), 3.80 (s, 4 H, 2 NCH2), 3.86 (s, 4 H, 2 NCH2), 7.31 (s,
+. C24H24F18N2P2 (744.4): calcd. C 38.72, H 3.25, F 45.94, N 3.76;
found C 38.60, H 3.21, F 45.82, N 3.82.
3
4 H, 4 Ar-H), 7.47 (d, JH,H = 8.0 Hz, 4 H, 4 Ar-H), 7.58 (d, 3JH,H
= 8.0 Hz, 4 H, 4 Ar-H) ppm. MS (EI): m/z (%) = 452 (10) [M]+,
General Procedure for Preparation of Pseudorotaxanes for 1H NMR
Spectroscopic Studies: The crown ether DB24C8 and the investi-
gated ammonium thread (10 mm) in question were mixed in the
required molar ratio and dissolved in CDCl3/CD3CN (3:1, 800 μL)
or CDCl3/CD3CN (1:1). Subsequently, a 1H NMR spectrum was
acquired at 298 K based on 64 scans.
Determination of Binding Constants (Ka Values) by using the 1H
NMR Single-Point Method: An equimolar amount of the ammo-
nium thread in question and DB24C8 were dissolved in CDCl3/
CD3CN (1:1, 800 μL) to achieve a 10 mm solution (solution A).
This solution was diluted to 1 mm by transferring an aliquot of
solution A (80 μL) to a vial containing the solvent mixture
159 (100) [CF3Bn]+.
α,αЈ-Bis(4-trifluoromethylbenzylimino)-p-xylene (18): A solution of
terephthaldehyde (17; 1.34 g, 9.99 mmol) and 4-trifluoromethyl-
benzylamine (6; 3.61 g, 20.6 mmol) in PhMe (100 mL) was heated
under reflux overnight; H2O was liberated from the reaction was
removed from the reaction mixture by means of a Dean–Stark ap-
paratus. The organic phase was cooled to room temperature,
whereafter the solvent was removed in vacuo, affording 18 as a
white powder (4.49 g, quant.), which was used without any further
purification. M.p. 120–122 °C. 1H NMR (300 MHz, CDCl3): δ =
3
4.88 (s, 4 H, 2 NCH2), 7.47 (d, JH,H = 8.1 Hz, 4 H, 4 Ar-H), 7.61
3
1
(d, JH,H = 8.1 Hz, 4 H, 4 Ar-H), 7.85 (s, 4 H, 4 Ar-H), 8.44 (s, 2
(720 μL). Thereafter, a H NMR spectrum (400 MHz, 298 K) was
H, 2 N=CH) ppm. 13C NMR (100 MHz, CDCl3): δ = 64.7, 124.4
(q, 1JF,C = 271 Hz, CF3), 125.6 (q, 3JF,C = 4 Hz), 128.3, 128.8, 129.5
recorded. The integrals of the signals in the 1H NMR spectrum
associated with the benzylic CH2N+ protons in both uncomplexed
and complexed species were determined. These integrals, together
with the initial molar concentrations of the ammonium thread
(≈1 mm) in question and DB24C8 (≈1 mm), were used to determine
the Ka value according to Equation (1).
2
(q, JF,C = 32 Hz), 138.3, 143.4, 162.1 ppm. MS (EI): m/z (%) =
448 (27) [M]+, 429 (5), 289 (56), 262 (15), 186 (4), 159 (100)
[CF3Bn]+, 109 (14). C24H18F6N2 (448.4): calcd. C 64.29, H 4.26, N
6.25; found C 64.37, H 3.97, N 6.10.
α,αЈ-Bis(4-trifluoromethylbenzylamino)-p-xylene (16) (Route 2): To
a solution of 18 (4.41 g, 9.83 mmol) in MeOH (150 mL) was added
NaBH4 (1.87 g, 49.4 mmol) cautiously in small portions. After stir-
ring for 2 h at room temperature another portion of NaBH4
(1.84 g, 49 mmol) was added, and the reaction mixture was stirred
overnight at room temperature, whereafter the solvent was removed
in vacuo. The residue was cautiously treated with an aqueous solu-
tion of HCl (2 m, 200 mL), and the resulting solution was washed
with CHCl3 (150 mL). Subsequently, the aqueous phase was treated
with an aqueous solution of NaOH (2 m, until pH ≈ 14) and then
extracted with CHCl3 (5ϫ100 mL). The combined organic phases
were dried (MgSO4), and the solvent was removed in vacuo, yield-
ing 16 as a white solid (3.04 g, 68%). M.p. 41–43 °C. 1H NMR
(300 MHz, CDCl3): δ = 1.64 (s, 2 H, 2 NH), 3.79 (s, 4 H, 2 NCH2),
3.86 (s, 4 H, 2 NCH2), 7.30 (s, 4 H, 4 Ar-H), 7.46 (br. s, 4 H, 4 Ar-
H), 7.56 (br. s, 4 H, 4 Ar-H) ppm. 13C NMR (100 MHz, CDCl3):
(1)
where Ic and Iu are the integrals of the benzylic CH2N+ protons
(i.e., H4) in the complexed (c) and uncomplexed (u) species, respec-
tively and cDB24C8 and cThread are the initial molar concentrations
of DB24C8 and the ammonium thread in question, respectively.
Single-Crystal X-ray Diffraction: X-ray analysis of 4-H2·
2PF6ʚ(DB24C8)2 was performed with
a
Bruker-Nonius
X8-APEXII instrument at 180(2) K by using graphite-monochro-
2+
mated Mo-Kα radiation (λ
=
0.7107 Å): C24H24F6N2 ·
–
1
3
2PF6 (C24H32O8)2·2C2H3N, Mr = 1723.49, F(000) = 3592, colorless
plate crystals, 0.25ϫ0.25ϫ0.05 mm, monoclinic, space group C2/
c, Z = 4, a = 15.7109(7) Å, b = 16.1101(7) Å, c = 32.3046(14) Å, β
δ = 52.7, 53.0, 124.4 (q, JF,C = 270 Hz, CF3), 125.4 (q, JF,C
=
3 Hz), 128.4, 128.5, 129.4 (q, 2JF,C = 32 Hz), 139.0, 144.6 ppm. MS
(EI): m/z (%) = 452 (8) [M]+, 413 (2), 361 (2), 293 (7), 279 (20),
264 (18), 188 (18), 174 (9), 159 (100) [CF3Bn]+, 139 (4), 118 (12),
104 (22), 91 (17). C24H22F6N2 (452.4): calcd. C 63.71, H 4.90, N
6.19; found C 63.50, H 4.80, N 6.10.
= 93.084(2)°, V = 8164.6(6) Å3, Dcalcd. = 1.402 gcm–3, θmax
=
26.39°. Min/max transmission 0.84/0.99, μ = 0.161 mm–1. From a
total of 32559 reflections, 8315 were independent (merging R =
0.039) and were used to refine 512 parameters. 8 restraints were
placed on the geometry of the acetonitrile solvent molecules. R1 =
0.065 [4557 data with IϾ2σ(I)], wR2 = 0.216 (all data), GOF =
1.07. Max/min residual electron density –0.65/0.93 eÅ–3. CCDC-
791631 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
α,αЈ-Bis(4-trifluoromethylbenzylammonium)-p-xylene Bis(hexa-
fluorophosphate) (4-H2·2PF6): A concentrated aqueous solution of
HCl was added to a solution of 16 (0.30 g, 0.66 mmol) in MeOH
(30 mL) until pH Ͻ 2 and then stirred for 1 h. The solvent was
evaporated, affording a white powder, which was suspended in
Me2CO (30 mL), whereupon an aqueous solution of NH4PF6
(6.11 g, 37.5 mmol) was added dropwise until all the suspended ma-
terial was dissolved. The solvent was removed in vacuo, and the
product was isolated by filtration, washed with H2O (20 mL), and
air dried, providing 4-H2·2PF6 as a fine-white powder in (0.49 g,
quant.). The product can be recrystallized from Me2CO/H2O to Acknowledgments
afford 4-H2·2PF6 as a white powder. M.p. Ͼ230 °C. 1H NMR
(400 MHz, CD3CN): δ = 4.30 (s, 4 H, 2 NCH2), 4.35 (s, 4 H, 2
This work was funded in part by a Ph.D. Scholarship from the
NCH2), 7.18 (br. s, 4 H, 2 NH2+), 7.55 (s, 4 H, 4 Ar-H), 7.67 (d, University of Southern Denmark to C. J. O. We also gratefully
Eur. J. Org. Chem. 2011, 759–769
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C. Johnsen, P. C. Stein, K. A. Nielsen, A. D. Bond, J. O. Jeppesen
FULL PAPER
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acknowledge the Strategic Research Council in Denmark through
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IT (project no. 2106-07-0031 to J. O. J.), the Danish Natural Science
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(no. 2117-05-0115 to J. O. J.) for financial support.
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1
The H NMR single-point method has often been used to de-
termine the binding constants between ammonium threads and
crown ethers, see for example: Ref.[3,6,7] and P. R. Ashton,
E. J. T. Chrystal, P. T. Glink, S. Menzer, C. Schiavo, N. Spencer,
J. F. Stoddart, P. A. Tasker, A. J. P. White, D. J. Williams,
Chem. Eur. J. 1996, 2, 709–728.
A more detailed procedure for the use of the single-point 1H
NMR method, although on a different system, is described by
M. B. Nielsen, J. O. Jeppesen, J. Lau, C. Lomholt, D.
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[12]
[13]
Estimated error on Ka: 3-H·PF6, Ϯ10%; 1-H·PF6, Ϯ20%.
768
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Eur. J. Org. Chem. 2011, 759–769

Complexation Studies between Ammonium Threads and DB24C8
[14] The solvent ratio between CDCl3 and CD3CN was changed
from 3:1 to 1:1 on account of the low solubility of thread 4-
H2·2PF6 in a 3:1 mixture of CDCl3 and CD3CN.
[16]
Estimated error on K1 and K2: 4-H2·2PF6, Ϯ15 and Ϯ10%,
respectively; 2-H2·2PF6, Ϯ10% for both.
[17] The X-ray crystal structure of 4-H2·2PF6ʚ(DB24C8)2 is sim-
ilar to that of earlier reported systems; see ref.[3e,3m]
Received: September 12, 2010
[15] Careful examination of the 4.5–4.8 ppm region in the 1H NMR
spectrum shown in Figure 6a reveals that a small amount of
[3]pseudorotaxane 4-H2·2PF6ʚ(DB24C8)2 is present in the 5:1
mixture of 4-H2·2PF6 and DB24C8.
Published Online: December 17, 2010
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