1,2-Bis[N-(N′-alkylimidazolium)]ethane salts as new guests for crown ethers and cryptands

Article abstract of DOI:10.1016/j.tet.2010.07.010

1,2-Bis[N-(N′-alkylimidazolium)ethane salts form complexes presumed to be pseudorotaxanes with crown ether and cryptand hosts. The association constants of 1,2-bis[N-(N′-butylimidazolium)]ethane bis(hexafluorophosphate) with dibenzo-24-crown-8 and a dibenzo-24-crown-8-based pyridyl cryptand were estimated as 24 (±1) and 348 (±30) M ^-1, respectively, in acetonitrile at 25 °C. The pseudorotaxane-like structure of the 1:2 complex of the N′-methyl analog with the cryptand was observed by X-ray crystallography. Replacement of the ethylene spacer with propylene and butylene spacers resulted in K_a values an order of magnitude smaller.

Full text of DOI:10.1016/j.tet.2010.07.010

Tetrahedron 66 (2010) 7077e7082
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
0
1
,2-Bis[N-(N -alkylimidazolium)]ethane salts as new guests for crown ethers
and cryptands
Minjae Lee, Zhenbin Niu, Daniel V. Schoonover, Carla Slebodnick, Harry W. Gibson *
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
a r t i c l e i n f o
a b s t r a c t
0
Article history:
1,2-Bis[N-(N -alkylimidazolium)ethane salts form complexes presumed to be pseudorotaxanes with crown
Received 17 May 2010
Received in revised form 4 July 2010
Accepted 5 July 2010
0
ether and cryptand hosts. The association constants of 1,2-bis[N-(N -butylimidazolium)]ethane bis(hexa-
fluorophosphate) with dibenzo-24-crown-8 and a dibenzo-24-crown-8-based pyridyl cryptand were es-
ꢁ1
ꢂ
timated as 24 (ꢀ1) and 348 (ꢀ30) M , respectively, in acetonitrile at 25 C. The pseudorotaxane-like
0
structure of the 1:2 complex of the N -methyl analog with the cryptand was observed by X-ray crystal-
a
lography. Replacement of the ethylene spacer with propylene and butylene spacers resulted in K values an
order of magnitude smaller.
Keywords:
Imidazolium salt
Imidazolium ionic liquid
Imidazolium dication
Host-guest complexation
X-ray crystal structure
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
There is a need for structural diversity in supramolecular build-
ing blocks; discovery of new pseudorotaxane systems drives new
supramolecular structures and applications. On another front, for
application of imidazolium salts and other ionic liquids in electro-
Imidazolium salts have been and are going to be significant not
only in organometallic chemistry as precursors of N-heterocyclic
carbenes, but also in organic chemistry and material science areas
1
7
active actuators, ion conductivity is a key parameter. In earlier work
as ionic liquids due to their unique chemical, physical, and electrical
properties.2 For 1,3-disubstituted imidazolium ionic liquid salts,
all of the protons on the imidazolium ring are quite acidic; for
we demonstrated that pseudorotaxane complexes of dibenzyl-
ammonium salts with DB24C8 and its derivatives are not ion
paired. Similarly, we reported that the pseudorotaxane complex of
8,9
10
0
0
example, the pK
a
of the C2 proton is 16e24, depending on the
N,N -dimethyl-4,4 -bipyridinium (‘paraquat’ or ‘dimethyl viologen’)
3
ꢁ
9,11
nature of the substituents on the two imidazolium nitrogens. The
acidic protons are attractive in supramolecular chemistry, since
more acidic protons provide stronger hydrogen bonds in the for-
2PF
6
with DB24C8 likewise is not ion paired;
this was sub-
12
sequently corroborated by other workers. It therefore appeared to
us that pseudorotaxanes derived from DB24C8 and imidazolium
ionic liquids would possess a larger fraction of ‘free’ ions than the
ionic liquids themselves and thus possess enhanced performance in
electroactive bending actuators. Driven by these two separate but
related goals, here we report new ionic guests, alkylene 1,2-bis[N
4
mation of supramolecular complexes, such as pseudorotaxanes.
1
-Alkyl-3-methylimidazolium bromides bind with cucurbit[6]uril;
the binding constant depends on the number of carbons in the alkyl
5
0
chain. Schmitzer et al. reported that N,N -disubstituted (benzyl or
phenyl) methylene diimidazolium salts also act as guest molecules
for various macrocycles with high association constants
0
(N -alkylimidazolium)] salts with different alkyl substituents, and
their complexation with crown-type host molecules. We also in-
ꢁ
1
(K
a
¼4200e7500 M ) in water:
b
-cyclodextrin, cucurbit[7]uril, and
2 3 4
vestigated the effect of C , C , and C spacers on their complexation
6
tetrapropoxycalix[4]arene. However, they reported lower
K
a
with DB24C8.
ꢁ
1
0
values (56 and 120 M ) for methylene bis[N-(N -benzylimidazo-
lium)] and methylene bis[N-(N -phenylimidazolium)] bis(hexa-
0
2. Results and discussion
3
fluorophosphate)s with dibenzo-24-crown-8 (DB24C8) in CD CN.
The preparations of the imidazolium dicationic salts 1e5 were
done in two steps (Scheme 1): the coupling reaction of the 1-alkyli-
midazole (2 M equiv) with a dibromoalkane (1 M equiv) and anion
exchange in water. Non-commercial 1-hexylimidazole and 1-dode-
*
Corresponding author. Tel.: þ1 540 231 5902; fax: þ1 540 231 8517; e-mail
13
address: hwgibson@vt.edu (H.W. Gibson).
cylimidazole were prepared before quaternization. Most of the
0
040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.07.010

7078
M. Lee et al. / Tetrahedron 66 (2010) 7077e7082
1
. MeCN, reflux
. MX, H O
R
N
N
Br(CH
2
)
n
Br
R
N
N
(CH
2
)
n
N
N
R
-
2
2
X-
2
-
1
3
: R = CH , n = 2, X = PF6
: R = n-butyl, n = 2, X = PF6
: R = n-butyl, n = 3, X = PF6
: R = n-butyl, n = 4, X = PF6
2
: R = n-dodecyl, n = 2, X = Tf N
2
-
-
3
4
-
5
Scheme 1.
0
6
intermediate alkylene bis[N-(N -alkylimidazolium)] bromide salts
using a single point NMR integration, which could be subject to
significant error.
ꢁ
8e11
were very hygroscopic; however, the imidazolium salts of PF
6
and
The two imidazolium rings of methylene bis-
ꢁ
2
Tf N
salts were non-hygroscopic due to their hydrophobic anions.
The complexation of 2 and DB24C8 was qualitatively observed
by H NMR in CD CN. After adding excess DB24C8 to a solution of 2
3
in CD CN at 25 C, the imidazolium protons moved upfield. This
system undergoes fast-exchange; only time-averaged signals ap-
pear in the 1H NMR spectrum (Fig. 1). The fast-exchange in this
complexation (Fig. 2) is reasonable due to the rod-like geometry of
(imidazolium) salts cannot be parallel due to the tetrahedral ge-
ometryof the methylene spacer. In the case of the two-carbon spacer
the trans-conformation of the ethylene unit is preferred (at room
temperature, Fig. 3). The previously reported interactions of dica-
1
ꢂ
3
1
5
tionic guests, bis(pyridinium)ethane and bis(benzimidazolium)
16
ethane salts, with DB24C8, are also good examples to support this
hypothesis. We predicted that the two imidazolium rings of 2 would
2
, which in solution probably has a trans-conformation pre-
be parallel and individually interact with the
matic rings of DB24C8, leading to -stacking interactions and higher
values. Contrary to our expectations based on these conforma-
tional, i.e., entropic arguments, the observed higher K values for the
p-electron rich aro-
dominantly, consistent with the solid-state structure of 1 (Fig. 3).
The fast exchange behavior exhibited by these systems is in con-
trast to the slow exchange reported for the methylene bridged bis
p
K
a
a
imidazoium) salts.6
methylene versus the ethylene spacer can be attributed to the
6
(
Figure 1. Partial 400 MHz 1H NMR spectra of (a) 2 (2 mM), (b) 2þDB24C8 (1:5 M ratio, 2 mM of 2), and (c) DB24C8 (10 mM) in CD
CN at 25 C.
ꢂ
3
Isothermal microcalorimetric titration (ITC) was used to quantify
the complexations of the host and guest compounds in solution. The
higher acidity of the methylene protons as a result of the two
neighboring positively charged nitrogen atoms (Fig. 2).
14
results aresummarizedinTable 1. All experiments wereperformed
The K
spacer) with DB24C8 are an order of magnitude smaller than
those of the analogous compound with a C spacer (2). From
a 3
values for complexations of 3 (C spacer) and 4
ꢂ
under ambient conditions at 25 C. The association constants and
(C
4
0
0
enthalpies of N,N -dimethyl ethylene linked 1 and N,N -dibutyl
ethylene linked 2 with DB24C8 in MeCNare similar toeach otherand
2
a molecular structural viewpoint, three and four carbons between
the imidazolium moieties make it less likely that both can interact
with the aromatic rings of DB24C8. The spatial requirements for
formation of hydrogen bonds are also less likely to be met when the
two imidazolium rings are far from each other. Furthermore, only
the protons of the methylene units attached to the positively
charged nitrogen atoms are acidic enough to form hydrogen bonds
with the crown ether oxygens; additional carbons in the spacer
only increase the number of unfavorable binding conformations.
Indeed the results support this analysis. The enthalpic changes for
complexes of DB24C8 with 2, 3, and 4 are identical within experi-
ꢁ
1
0
slightly lower than the K
lium)]methane 2PF
a
(56 M ) of bis[N-(N -benzylimidazo-
with DB24C8 determined by Schmitzer et al.
ꢁ
6
Table 1
ITC results: complexation of imidazolium salts by DB24C8 and Cryptand 6
(Mꢁ1)
Imidazolium Macrocycle Solvent
Salt
K
a
DH (kcal/mol)
ꢁ
DS
(cal/mol deg)
a
1
2
3
4
5
2
DB24C8
DB24C8
DB24C8
DB24C8
DB24C8
6
MeCN
MeCN
MeCN
MeCN
27ꢀ3
24ꢀ1
ꢁ2.7ꢀ0.3
ꢁ2.5ꢀ0.1
2.3ꢀ1.1
2.0ꢀ0.3
5.2ꢀ1.4
6.8ꢀ2.0
16ꢀ1
a
a
a
4.3ꢀ0.8 ꢁ2.4ꢀ0.3
2.2ꢀ0.7 ꢁ2.5ꢀ0.4
a
mental error; the order of magnitude decreases in K from the two
carbon spacer to the four carbon spacer are entirely due to the
a
CHCl
MeCN
3
30ꢀ1
ꢁ6.7ꢀ0.1
ꢁ8.0ꢀ0.2
b
348ꢀ30
15ꢀ1
increased entropic penalty for complexation, ꢁ2.0 to ꢁ5.2 to
a
All of the complexes are of 1:1 stoichiometry; other assumed stoichiometries
ꢁ
6.8 eu, respectively.
The imidazolium salt 5 with n-dodecyl arms was synthesized to
resulted in poorer fits.
b
For 1:1 assumed stoichiometry. 2:1H:G stoichiometry yielded: K
a
¼166ꢀ8;
D
H¼18.1 kcal/mol;
D
S¼ꢁ50 cal/mol deg.
a
observe the complexation in a less polar solvent. However, the K of

M. Lee et al. / Tetrahedron 66 (2010) 7077e7082
7079
O
O
O
O
O
O
O
O
O
O
PF -
PF -
6
6
O
Ka
N
9
N
C H
4
ON
N
O
C H
4 9
O
O
O
N
N
N
N
C H
4
9
C H
4 9
2
PF -
6
O
O
O
O
O
2
O
O
O
O
O
Ka
N
O
O
6
Figure 2. Schematic diagram of complexation of 2 with DB24C8 and DB24C8-based pyridyl cryptand 6.11
þ
6
] , calcd for
5
and DB24C8 in CHCl
3
at room temperature was almost the same
ion from the 1:1 complex ([2þDB24C8ePF
PF 869.4062). A solution of 5 and DB24C8 gave m/z
1228.5961, corresponding to the 1:1 complex after loss of one
as the K of the analogous methyl analog 1 in acetontrile. Appar-
a
C
40
H
60
N
4
O
8
6
ently the longer terminal chains are detrimental and offset the
lower polarity of the solvent. This is reflected by the fact that the
enthalpy of complexation of 5 with DB24C8 is actually 2.5 times
more exothermic than that of 1, as a result of the lower polarity
solvent, but 5 suffers a seven-fold larger entropic penalty, appar-
ently due to the long alkyl side chains.
ꢁ
þ
Tf
2
N
anion ([5þDB24C8eTf
2 92 6 5 12 2
N] , calcd for C58H F N O S
Complexation of N-butyl compound 2 with the DB24C8-based
pyridyl cryptand 611 was also observed by ITC. Its K
is >10 times
a
higher and the exothermic enthalpy change is 3.2-fold larger than
the binding of 2 with DB24C8. In spite of the sizeable entropic
Figure 3. A capped-sticks view of the X-ray structure of 1. PF
clarity. Ring plane/ring plane inclination of the two imidazolium rings: 0 .
6
anions are omitted for
penalty (ꢁ15 eu), the larger K
a
and enthalpy change are due to the
ꢂ
presence of more binding sites on the cryptand. We tried to in-
1
vestigate the stoichiometry of the complex of 2 and 6 by H NMR in
CD
3
CN; however, to our surprise the chemical shift changes were
1228.6082). Solutions of cryptand 6 and the imidazolium salts
within experimental error across a range of ratios with a total
concentration of 2.2 mM.
The formation of these complexes was also confirmed by high
resolution ESI TOF mass spectrometry (see Supplementary data).
also gave mass numbers corresponding to the 1:1 complexes: m/z
17
þ
976.3349 ([1þ6ꢁPF
6
] , calcd for C43
H
53
N
5
O
12PF
6
976.3333), and
C H N O12PF
49 65 5 6
þ
m/z 1060.4311 ([2þ6ꢁPF
6
] , calcd for
1060.4281). Every host-guest solution revealed only 1:1 host/
guest complex peaks, even with an excess of the host.
A solution of 1 and DB24C8 yielded m/z 785.3108, which corre-
ꢁ
sponds to the 1:1 complex after loss of one PF
6
ion
A crystal structure of 1$(6)
rotaxane-like structure
guest
2
(Fig. 4) shows that the pseudo-
in the solid state was formed from one
1 and two hosts 6. This observation of different
þ
18e20
(
[1þDB24C8ꢁPF
6
] , calcd for C34
H
48
N
4
O
8
PF
6
785.3114). A solu-
tion of 2 and DB24C8 yielded m/z 869.4050, after loss of one PF
ꢁ
6
Figure 4. Two capped-sticks views of the X-ray structure of 1$(6)
2
: side view (left) and end view (right). There are two complexes in the unit cell and they are similar but not
ꢁ
ꢀ
identical. These views are one of the complexes. PF
6
anions, MeCN molecules and some hydrogens are omitted for clarity. One PF
6
anion is disordered. H/O (or N) distances (A),
CeH/O (or N) angles (degrees) of: (a) 2.60, 165.4; (b) 2.89, 126; (c) 2.64, 138; d 2.71, 155; (e) 2.68, 122; (f) 2.79, 166; (g) 2.30, 135; (h) 2.23, 145.

7080
M. Lee et al. / Tetrahedron 66 (2010) 7077e7082
stoichiometries in solution and the solid state is consistent with
several other crown ether and cryptand complexes: both cryptands
3.2. General anion exchange procedure
2
0
11
7
a and 7b with paraquat, DB24C8 with paraquat, bis(p-xylyl)-
Into a solution of bromide salt (1 equiv) in deionized water, KPF
6
(or LiTf N, 2e4 equiv) was added with stirring, which continued for
2
1e2 h at room temperature. The precipitate was filtered and
21
0
2
6-crown-8 with paraquat,
DB24C8 with N,N -dialkyl para-
and DB24C8 with ‘diquat’ (N,N -ethylene-2,2 -bipyr-
2
2
0
0
quats,
ꢁ
).23
idinium 2PF
6
washed with deionized water twice. Drying in a vacuum oven gave
ꢁ ꢁ
6 2
(or Tf N ) salt.
the pure imidazolium PF
3.3. Synthetic procedures or alkylene bis-
(N-alkylimidazolium) salts
0
3
.3.1. 1,2-Bis[N-(N -methylimidazolium)]ethane bis(hexafluorophos-
phate) (1). A solution of 1-methylimidazole (2.49 g, 30 mmol) and
,2-dibromoethane (2.82 g, 15 mmol) in MeCN (20 mL) was refluxed
1
for 3 days. The precipitate was filtered after cooling, and washed with
tetrahydrofuran (THF) three times. Drying in a vacuum oven gave the
The structure of guest 1 in the present complex (Fig. 4) is
somewhat different from the crystal structure of 1 itself (Fig. 3); the
imidazolium planes are twisted at a 44 angle, to accommodate the
ꢂ
colorlesscrystallinebromidesalt(4.54 g,86%), mp231.1e233.8 C(lit.
ꢂ
ꢂ
25
mp 230e234 C). Compound 1 was obtained by following the
general anion exchange procedure; from bromide salt (4.50 g), KPF
5.52 g, 30 mmol), and deionized water (40 mL) colorless crystalline
pep stacking with the aromatic rings of 6. Centroid to centroid
6
ꢀ
distances of the phenylene and imidazolium rings are 3.6e3.9 A
(
and dihedral angles of the phenylene and imidazolium planes are
ꢂ
solid 1 (5.26 g, 73%based on 1,2-dibromoethane), mp 213.8e214.9 C,
ꢂ
1
2e23 . The catechol rings of different host molecules in the 1:2
1
ꢂ
was isolated. H NMR (400 MHz, CD
H), 7.33 (t, J¼2, 2H), 7.42 (t, J¼2, 2H), 8.40 (s, 2H). C NMR (100 MHz,
3
CN 23 C): d 3.85 (s, 6H), 4.62 (s,
complex are significantly tilted, which causes the twisted imida-
zolium rings. All of the hydrogens of 1 are hydrogen-bonded with
oxygen atoms (or the nitrogen atom) of 6. One of the imidazolium
hydrogens interacts with the nitrogen atom and the other two bind
with oxygens in the ethyleneoxy arms. Interestingly ester carbonyl
oxygens seem to strongly interact with the ethylene protons of 1;
13
4
CD
ꢂ
þ
6
] ,
3
CN, 23 C):
d
37.2, 49.7,123.5,125.6,137.6. HRMS (ESI): [MꢁPF
16 4 6
found 337.1022. C10H N PF requires 337.1017.
0
3
.3.2. 1,2-Bis[N-(N -butylimidazolium)]ethane bis(hexafluorophos-
phate) (2). The same procedure was used as for 1. From 1-butylimi-
dazole(6.21 g, 50 mmol)and1,2-dibromoethane(4.69 g, 25 mmol)in
ꢀ
the H/O distances are 2.23 and 2.30 A (g and h in Fig. 4).
0
In summary, the complexation of alkylene 1,2-bis[N-(N -alkyl-
MeCN (25 mL) colorless crystalline bromide salt (9.64 g, 88%), mp
1
imidazolium)] salts with DB24C8 was confirmed by H NMR
ꢂ
1
66.5e168.2 C, was produced. The general anion exchange pro-
spectroscopy, mass spectrometry and ITC titrations. The association
constants for pseudorotaxane (or pseudorotaxane-like complex)
cedure was used; from the bromide salt (1.30 g, 3.0 mmol) and KPF
6
(
1.65 g, 9 mmol) in deionized water (10 mL) colorless crystalline 2
0
formation from the 1,2-bis[N-(N -alkylimidazolium)]ethanes with
ꢂ
1
(1.54 g, 91%), mp 181.6e182.5 C, was isolated. H NMR (400 MHz,
ꢁ
1
DB24C8 in solution are K
a
¼24 to 30 M in MeCN or CHCl
3
and
ꢂ
acetone-d
4
acetone-d
6
23 C):
d
0.94 (t, J¼7, 6H),1.33 (m, 4H),1.78e1.85 (m, 4H),
13
ꢁ
1
K
a
¼348 M in MeCN for 2 with cryptand 6. However, alkylene bis-
.14 (m, 4H), 5.04 (s, 4H), 7.39 (m, 4H), 8.42(s, 2H). C NMR (100 MHz,
0
[
N-(N -alkylimidazolium)] salts with a C
3
or C
4
spacer show weaker
ꢂ
6
, 23 C):
d
13.5, 19.8, 32.4, 49.9, 50.6, 123.6, 124.2, 137.5.
binding with DB24C8. The host/guest threaded structure was
confirmed by the X-ray crystal structure of the 1:2 complex of 1 and
þ
HRMS (ESI): [MꢁPF
] , found 421.1966. C16H N PF
6 28 4 6
requires
4
21.1950.
6
; every proton of the guest molecule participates in hydrogen
bonding with oxygen or nitrogen atoms of the cryptand 6. These
readily accessible imidazolium salts are expected to be valuable in
the construction of larger supramolecular systems, such as poly-
pseudorotaxanes,24 and in construction of pseudorotaxane or
rotaxane actuators based on ionic liquids and polyelectrolytes. Our
results in these areas will be published in due course.
0
3
.3.3. 1,3-Bis[N-(N -butylimidazolium)]propane bis(hexafluorophos-
phate) (3). The same procedure was used as for 1, except the bro-
mide salt could not be filtered because it was a viscous oil. From
-butylimidazole (3.73 g, 30 mmol) and 1,3-dibromopropane
3.03 g, 15 mmol) in MeCN (20 mL), a light brown viscous liquid
bromide salt, 5.81 g, 86%) was isolated. The general anion ex-
1
(
(
change procedure was used; from the oily bromide salt (5.00 g,
11.1 mmol) and KPF (6.20 g, 33 mmol) in deionized water (40 mL)
colorless crystalline solid 3 (6.25 g, 97% from the bromide salt),
3
3
3
. Experimental section
6
ꢂ
1
ꢂ
.1. General comments
mp 96.4e97.7 C, resulted. H NMR (400 MHz, acetone-d , 23 C):
6
d
0.94 (t, J¼7, 6H), 1.38 (m, 4H), 1.94 (m, 4H), 2.74 (m, 2H), 4.36
13
.1.1. Materials. Dibenzo-24-crown-8 was purchased from Aldrich
(t, J¼7, 4H), 4.55 (t, J¼7, 2H), 7.81 (m, 4H), 9.12 (s, 2H). C NMR
ꢂ
Chem. Co., Inc. and used as received. Acetone for the quaternization
reactions was dried with anhydrous CaSO and then distilled.
Acetonitrile was dried with anhydrous K CO and then distilled. All
other chemicals and solvents were used as received.
Instruments. H and C NMR spectra were obtained on Varian
Inova 400 MHz and Unity 400 MHz spectrometers. High resolution
electrospray ionization time-of-flight mass spectrometry (HR ESI
TOF MS) was carried out on an Agilent 6220 Accurate Mass TOF
LC/MS Spectrometer in positive ion mode. Melting points were
(100 MHz, acetone-d
6
, 23 C):
d
13.6, 20.0, 31.5, 32.6, 47.5, 50.4,
þ
4
123.6, 123.9, 136.9. HRMS (ESI): [MꢁPF
6
] , found 435.2143.
2
3
C
17
30
H N
4
PF
6
requires 435.2112.
1
13
0
3.3.4. 1,4-Bis[N-(N -butylimidazolium)]butane bis(hexafluorophos-
phate) (4). The same procedure was used as for 3; from 1-butyl-
imidazole (3.73 g, 30 mmol) and 1,4-dibromobutane (3.24 g,
15 mmol) in MeCN (20 mL), a light brown viscous liquid (bromide
salt, 5.71 g, 82%) resulted. The anion exchange procedure was used;
ꢂ
observed on a Büchi B-540 apparatus at a 2 C/min heating rate.
6
from the oily bromide salt (5.70 g, 2.3 mmol) and KPF (5.56 g,
ITC titrations were run on a Microcal MCS ITC. Raw isotherm data
were collected using the Microcal Observer software. Integration
30 mmol) in deionized water (40 mL) 4 was isolated as an off-white
viscous liquid (7.05 g, 96% from the bromide salt), which solidified in
ꢂ
1
ꢂ
3
CN, 23 C):
and fitting of the isothermal data (K
a
and
D
H) were accomplished
a vacuum oven, mp 50.4e52.4 C. H NMR (400 MHz, CD
0.94 (t, J¼7, 6H), 1.34 (m, 4H), 1.78e1.85 (m, 8H), 2.74 (m, 4H), 4.14
using Origin software with a one set of sites algorithm.
d

M. Lee et al. / Tetrahedron 66 (2010) 7077e7082
7081
m, 8H), 7.38 (m, 4H), 8.42 (s, 2H).13C NMR (100 MHz, CD
CN, 23 C):
ꢂ
chosen crystal was centered on the goniometer of an Oxford Dif-
(
3
d
13.7, 20.0, 27.2, 32.5, 49.8, 50.4, 123.4, 123.6, 136.3. HRMS (ESI):
fraction Gemini A Ultra diffractometer operating with Mo K
a ra-
þ
[MꢁPF
6 32 4 6
] , found 449.2304. C18H N PF requires 449.2269.
diation. The data collection routine, unit cell refinement, and data
2
7
processing were carried out with the program CrysAlisPro. The
Laue symmetry and systematic absences were consistent with the
monoclinic space groups C2/c and Cc. The centric space group C2/c
was chosen based on the E-statistics. The structure was solved by
3
1
.3.5. 1-Dodecylimidazole. To
a
solution of imidazole (6.81 g,
00 mmol) in NaOH (50%) solution (8.80 g, 110 mmol), 1-bromo-
dodecane (24.92 g, 100 mmol) and THF (30 mL) were added. The
mixture was refluxed for 3 days. After the mixture had cooled, THF
was removed by a rotoevaporator. The residue was extracted with
dichloromethane/water three times. The combined organic layer
was washed with water and dried over Na SO . The drying agent
2 4
was filtered and the filtrate was concentrated. Column chroma-
tography through a short silica-gel column with THF gave a clear
yellow oil 20.51 g (87%). H NMR (CDCl
1
1
2
8
direct methods and refined using SHELXTL NT. The asymmetric
unit of the structure comprises 0.5 crystallographically in-
dependent molecules. The final refinement model involved aniso-
tropic displacement parameters for non-hydrogen atoms and
a riding model for all hydrogen atoms.
3
Crystal data: prism, colorless, crystal size¼0.21ꢃ0.21ꢃ0.09 mm ,
1
ꢂ
ꢀ
3
, 22 C):
d
0.88 (t, J¼7, 3H),
C
10
16 4 6
H N $2PF ,
M¼482.21, wavelength¼0.71073 A, monoclinic,
ꢀ
ꢀ
ꢀ
3
ꢀ
.29 (m, 18H), 1.76 (m, J¼7, 2H), 3.91 (t, J¼7, 4H), 6.90 (s, 1H), 7.05 (s,
space group C2/c, a¼22.186 (3) A, b¼6.4852 (3) A, c¼14.4065 (13) A,
H), 7.45 (s,1H). 13C NMR (100 MHz, CDCl
, 22 C):
ꢂ
d
14.0, 22.6, 26.4,
a
¼90 ,
ꢂ
b
¼122.621(15) ,
ꢂ
g
¼90 , V¼1745.8(3) A , Z¼4, D
ꢂ
3
c
¼1.835 Mg/
1
3
ꢁ1
2
9.0, 29.2, 29.3, 29.4, 29.5, 31.0, 31.8, 46.9, 118.6, 129.2, 136.9. The H
m , T¼100 (2) K,
m
¼0.376 mm , 11,193 reflections collected, 2562 [R
2
6
NMR spectrum was exactly identical to the literature report.
(int)¼0.0300] independent reflections, 2562/0/128 data/restraints/
parameters, F(000)¼968, ¼0.0358, wR ¼0.0780 (all data),
R
s
1
2
0
2
3
.3.6. 1,2-Bis[N-(N -dodecylimidazolium)]ethane di[bis(trifluorome-
R
1
¼0.0288, wR
2
¼0.0759 [I>2 (I)], and goodness-of-fit on F ¼1.051.
thanesulfonyl)imide] (5). A solution of 1-dodecylimidazole (3.78 g,
6 mmol) and 1,2-dibromoethane (1.50 g, 8 mmol) in MeCN (15 mL)
This file corresponds to CCDC 768151.
1
was refluxed for 3 days. After the mixture had cooled, the volatile
materials were removed under vacuum. The residue was dispersed
in THF (30 mL) and the insoluble bromide salt was filtered. The filter
cake was washed with THF three times. Drying in a vacuum oven
2
3.5.2. Crystal growing and crystallography of 1$(6) . A colorless
plate of 1$(6) was obtained in two steps. First, a 1:2 mixture of 1
2
and 6 was dissolved in MeCN and then the solvent was removed by
evaporation in an open vial at room temperature. Then, the residue
was re-dissolved in acetone and the crystal was grown by vapor
diffusion of pentane into the acetone solution at room temperature.
The colorless plate was centered on the goniometer of an Oxford
gave colorless a crystalline solid (bromide salt, 4.54 g, 86%), mp
ꢂ
2
49.3e252.9 C (dec). The general anion exchange procedure was
ꢂ
used, except the mixture was stirred at 50 C for 24 h; from the
bromide salt (2.70 g, 6.4 mmol), LiTf
2
N (4.32 g, 15 mmol), and
Diffraction SuperNova diffractometer operating with Cu Ka radia-
deionized water (60 mL), crystalline 5 (3.89 g, 97% from the bromide
tion. The data collection routine, unit cell refinement, and data
processing were carried out with the program CrysAlisPro. The
Laue symmetry and systematic absences were consistent with the
ꢂ
1
27
salt), mp 45.5e47.7 C, was isolated. H NMR (400 MHz, CDCl
3
ꢂ
2
4
2
2
3 C):
d
0.87 (t, J¼7, 6H), 1.25e1.31 (m (br), 36H), 1.86 (t, J¼7, 4H),
.15 (t, J¼7, 4H), 4.74 (s, 4H), 7.33 (t, J¼2, 2H), 7.60 (t, J¼2, 2H), 8.84 (s,
1
monoclinic space group P2 /n. The structure was solved by direct
13
ꢂ
28
H). C NMR (100 MHz, CDCl
3
, 23 C):
d
14.2, 22.8, 26.3, 29.0, 29.4,
methods and refined using SHELXTL NT. In the final refinement
9.5, 29.6, 29.7, 32.0, 48.5, 50.7, 123.0, 123.3, 136.0. HRMS (ESI):
model, the asymmetric unit included four host molecules, two
þ
[
MꢁTf
2
N] , found 780.3997. C34
H
60
F
6
N
5
O
4
S
2
requires 780.3985.
guest salts, seven CH
sition disorder model was used for one PF
of a host molecule, and one CH
3
CN and one partially occupied H
2
O. A 2 po-
ꢁ
6
, two C-atoms in the arm
CN is disor-
3
.4. Example of ITC titration methods
3
CN molecule. The CH
3
dered across an inversion center and relative occupancies were
constrained to 0.5. The relative occupancies of the remaining dis-
ordered atoms refined to 0.59 and 0.41. For steric reasons, the water
Stock solutions of titrant 1 (175 mM) and 6 (6.11 mM) in MeCN
were prepared using volumetric flasks. All titrations were run at
ꢂ
2
5 C. The high gain (high sensitivity) system was used, and the
molecule could only be present at the same time as the disordered
ꢁ
reference offset was set at 50% of the maximum. During each ti-
tration, 10 injections of 2.0 L, 20 injections of 2.5 L, 25 injections
of 3.0 L, and 26 injections of 4.0 L of 1 were made consecutively
PF
6
with occupancy 0.59; consequently, the water occupancy was
ꢁ
m
m
tied to the PF
rameters suggest additional PF
6
occupancy of 0.59. Anisotropic displacement pa-
ꢁ
m
m
6
and CH
3
CN disorder, but attempts
into the cell containing 6. Injections were made every 180 s with
a primary filter period of 2 s, and a secondary filter period of 4 s.
The switch time for the filter periods was at 120 s. After the titration
isotherm was recorded, the raw data were reduced using com-
mercial software and the supplied algorithms from the manufac-
turer. A control titration was also completed so that the heat of
dilution from the titrant could be subtracted from the original ti-
tration isotherm. The parameters of the control titration were the
same as those used in the original titration with the exception that
the cell solution was replaced with pure MeCN. The control iso-
therm was integrated using the same method as the original ti-
tration and the energy values obtained were subtracted from the
integrated point plot of the original titration. The integration data
from the titrations were fit using the ‘One Set of Sites’ model; other
stoichiometries yielded unsatisfactory fits of the data.
to model this disorder did not improve the model and were
abandoned. All non-hydrogen atoms were refined anisotropically
except for two CH CN molecules and the partially occupied water
3
molecule. A riding model was used for all hydrogen atoms. The H-
atoms of the water molecule were not included in the refinement.
Crystal data: plate, colorless, crystal size¼0.4460ꢃ0.1156ꢃ
3
0.0205 mm , 2{[(C33
H
37NO12
)
2
$C10
H
16
N
4
][PF
6
]
2
}$7C
2
H
3
N$0.59H
2
O,
ꢀ
M¼3820.97, wavelength¼1.54178 A, monoclinic, space group P2
1
/n,
ꢀ
ꢀ
ꢀ
ꢂ
a¼30.5666 (14) A, b¼14.1326 (5) A, c¼42.0175 (18) A,
a¼90 ,
ꢂ
ꢂ
ꢀ
3
3
b
¼99.972 (4) .,
g
¼90 , V¼17876.7 (13) A , Z¼4, D ¼1.420 Mg/m ,
c
ꢁ
1
T¼100 (2) K, ¼1.356 mm , 89614 reflections collected, 34633 [R
m
(int)¼0.0563] independent reflections, 34633/66/2413 data/re-
straints/parameters, F(000)¼7984, R ¼0. 1248, wR ¼0. 2297 (all
data), R (I)], and goodness-of-fit on
1
2
1
¼0820, wR
2
¼0.2095 [I>2
s
2
F ¼1.030. This file corresponds to CCDC 768150.
3
.5. X-ray crystallography
Acknowledgements
3.5.1. Crystal growing and crystallography of 1. Colorless prisms
This material is based upon work supported in part by the U.S.
crystallized from acetone/pentane at room temperature. The
Army Research Office under grant number W911NF-07-1-0452,

7082
M. Lee et al. / Tetrahedron 66 (2010) 7077e7082
Ionic Liquids in Electro-Active Devices (ILEAD) MURI. We thank the
National Science Foundation for funds to purchase the Varian Unity
and Inova NMR spectrometers (DMR-8809714 and CHE-0131124),
the Agilent 6220 Accurate Mass TOF LC/MS Spectrometer (CHE-
T. E. Macromolecules 2010, 43, 790e796; Liu, S.; Liu, W.; Liu, Y.; Lin, J.-H.; Zhou, X.;
Janik, M. J.; Colby, R. H.; Zhang, Q. Polym. Int. 2010, 59, 321e328.
. Jones, J. W.; Gibson, H. W. J. Am. Chem. Soc. 2003, 125, 7001e7004.
8
9
. Huang, F.; Jones, J. W.; Gibson, H. W. J. Org. Chem. 2007, 72, 6573e6576.
10. Huang, F.; Jones, J. W.; Gibson, H. W. J. Am. Chem. Soc. 2003, 125,
4458e14464.
1. Gibson, H. W.; Wang, H.; Slebodnick, C.; Merola, J.; Kassel, S.; Rheingold, A. L. J.
Org. Chem. 2007, 72, 3381e3393.
1
0
722638) and the Oxford Diffraction Gemini diffractometer (CHE-
1
0131128).
12. Gasa, T. B.; Spruell, J. M.; Dichtel, W. R.; Sorensen, T. J.; Philp, D.; Stoddart, J. F.;
Kuzmic, P. Chem.dEur. J. 2009, 15, 106e116.
3. Tosoni, M.; Laschat, S.; Baro, A. Helv. Chim. Acta 2004, 87, 2742e2749.
Supplementary data
1
1
4. For simplicity these analyses assume that either (1) both the salts and the
complexes are not ion paired or less likely, (2) both the salts and the complexes
are completely ion paired. For complexes of DB24C8 and 6 with other guests, it
has been shown that although the guest salts are mostly ion paired, the
complexes are not ion paired; complete analysis then requires extensive NMR,
UVevis or fluorescence measurements to estimate the ion pair dissociation
constant of the guest salt and the binding constant for the free cation. See:
Refs. 8e11.
NMR spectra for all synthesized compounds, ITC titration data,
mass spectrometric results and X-ray crystal structures. Supple-
mentary data associated with this article can be found in online
version at doi:10.1016/j.tet.2010.07.010. These data include MOL
files and InChIKeys of the most important compounds described in
this article.
1
5. Loeb, S. J.; Wisner, J. A. Angew. Chem., Int. Ed. 1998, 37, 2838e2840; Loeb, S. J.;
Tiburcio, J.; Vella, S. J.; Wisner, J. A. Org. Biomol. Chem. 2006, 4, 667e680; Loeb,
S. J. Chem. Soc. Rev. 2007, 36, 226e235.
References and notes
16. Li, L.; Clarkson, G. J. Org. Lett. 2007, 9, 497e500.
1
7. This result highlights a potential weakness of NMR in such studies, but points
out the power of calorimetry.
1
. For some recent examples and reviews of N-heterocyclic carbene ligands from
imidazolium salts see: Tapu, D.; Owens, C.; VanDerveer, D.; Gwaltney, K.
Organometallics 2009, 28, 270e276; Raynal, M.; Pattacini, R.; Cazin, C. S. J.;
Valle ꢁc e, C.; Olivier-Bourbigou, H.; Braunstein, P. Organometallics 2009, 28,
18. Ashton, P. R.; Philip, D.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.;
Stoddart, J. F.; Williams, D. J. J. Chem. Soc., Chem. Commun. 1991, 1680e1683.
19. Bryant, W. S.; Guzei, I. A.; Rheingold, A. L.; Gibson, H. W. Org. Lett. 1999, 1,
4028e4047; Walton, J. C.; Brahmi, M. M.; Fensterbank, L.; Lacôte, E.; Malacria, M.;
47e50.
Chu, Q.; Ueng, S.-H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc. 2010, 132,
2
0. Huang, F.; Gibson, H. W.; Bryant, W. S.; Nagvekar, D. S.; Fronczek, F. R. J. Am.
Chem. Soc. 2003, 125, 9367e9371.
2
2
350e2358; Knappke, C. E. I.; Neud o€ rfl, J. M.; Wangelin, A. J. Org. Biomol. Chem.
010, 8, 1695e1705; Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903e1912;
21. Cheng, P.-N.; Lin, C.-F.; Liu, Y.-H.; Lai, C.-C.; Peng, S.-M.; Chiu, S.-H. Org. Lett.
Kuhl, O. Functionalised N-Heterocyclic Carbene Complexes; J. Wiley and Sons: New
York, NY, 2010.
2006, 8, 435e438.
2
2
2. Nikitin, K.; Müller-Bunz, H. New J. Chem. 2009, 33, 2472e2478.
3. Li, S.; Huang, F.; Slebodnick, C.; Ashraf-Khorassani, M.; Gibson, H. W. Chin. J.
Chem. 2009, 27, 1777e1781.
2. Welton, T. Chem. Rev. 1999, 99, 2071e2083; Davis, J. H., Jr.; Fox, P. A. Chem.
Commun. 2003, 1209e1212; Xu, W.; Cooper, E. I.; Angell, C. A. J. Phys. Chem. B
2003, 107, 6170e6178; Miao, W.; Chan, T. H. Acc. Chem. Res. 2006, 39, 897e908;
2
4. For recent examples see: Gibson, H. W.; Ge, Z.; Huang, F.; Jones, J. W.; Lefebvre,
H.; Vergne, M. J.; Hercules, D. M. Macromolecules 2005, 38, 2626e2637; Sato, T.;
Takata, T. Macromolecules 2008, 41, 2739e2742; Kang, S.;Berkshire,B.M.;Xue, Z.;
Gupta, M.; Layode, C.; May, P. A.; Mayer, M. F. J. Am. Chem. Soc. 2008, 130,
Lee, S.-G. Chem. Commun. 2006, 1049e1063; El Seoud, O. A.; Koschella, A.;
Fidale, L. C.; Dorn, S.; Heinze, T. Biomacromolecules 2007, 8, 2629e2647; Headly,
A. D.; Ni, B. Aldrichimica Acta 2007, 40, 107e117; Forsyth, S. A.; Fr o€ hlich, U.;
Goodrich, P.; Gunaratne, H. Q. N.; Hardacre, C.; McKeown, A.; Seddon, K. R. New
J. Chem. 2010, 34, 723e731; Bara, J. E.; Camper, D. E.; Gin, D. L.; Noble, R. D. Acc.
Chem. Res. 2010, 43, 152e159.
15246e15247; Nam, S.; Leisen, J.; Breedveld, V.; Beckham, H. W. Macromolecules
2009, 42, 3121e3128; Gibson, H. W.; Farcas, A.; Jones, J. W.; Ge, Z.; Huang, F.;
Vergne, M.; Hercules, D. M. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3518e3543;
Lee, M.; Schoonover, D.; Gies, A.; Hercules, D. M.; Gibson, H. W. Macromolecules
3. Ennis, E.; Handy, S. T. Curr. Org. Synth. 2007, 4, 381e389.
4
. For recent reviews of pseudorotaxanes and rotaxanes see: Collin, J.-P.; Heitz, V.;
Sauvage, J.-P. Top. Curr. Chem. 2005, 262, 26e62; Huang, F.; Gibson, H. W. Prog.
Polym. Sci. 2005, 30, 982e1018; Wenz, G.; Han, B.-H.; Mueller, A. Chem. Rev.
2009, 42, 6483e6494; Bria, M.; Bigot, J.; Cooke, G.; Lyskawa, J.; Rabani, G.; Rotello,
V. M.; Woisel, P. Tetrahedron 2009, 65, 400e407; Gibson, H. W.; Ge, Z.; Jones, J.
W.; Harich, K.; Pederson, A.; Dorn, H. C. J. Polym. Sci., Part A: Polym. Chem.
2006, 106, 782e817; Griffiths, K.; Stoddart, J. F. Pure Appl. Chem. 2008, 80,
485e506; Harada, A.; Hashidzume, A.; Yamaguchi, H.; Takashima, Y. Chem. Rev.
2009, 109, 5974e6023; Harada, A.; Takashima, Y.; Yamaguchi, H. Chem. Soc. Rev.
2009, 38, 875e882; Crowley, J. D.; Goldup, S. M.; Lee, A.-L.; Leigh, D. A.;
2009, 47, 6472e6495; Gibson, H. W.; Yamaguchi, N.; Niu, Z.; Jones, J. W.;
Rheingold, A. L.; Zakharov, L. N. J. Polym. Sci., Part A: Polym. Chem. 2010, 48,
975e985; Taura, D.; Li, S.; Hashidzume, A.; Harada, A. Macromolecules 2010,
4
3, 1706e1713; Ogoshi, T.; Nishida, Y.; Yamagishi, T.; Nakamoto, Y. Macro-
McBurney, R. T. Chem. Soc. Rev. 2009, 38, 1530e1541.
molecules 2010, 43, 3145e3147.
5
6
. Liu, L.; Zhao, N.; Scherman, O. A. Chem. Commun. 2008, 1070e1072.
. Noujeim, N.; Leclercq, L.; Schmitzer, A. R. J. Org. Chem. 2008, 73, 3784e3790.
25. Lee, H. M.; Lu, C. Y.; Chen, C. Y.; Chen, W. L.; Lin, H. C.; Chiu, P. L.; Cheng, P. Y.
Tetrahedron 2004, 60, 5807e5825.
7
. Ahn, S.-K.; Kasi, R. M.; Kim, S.-C.; Sharma, N.; Zhou, Y. Soft Matter 2008, 4,
151e1157; Duncan, A. J.; Leo, D. J.; Long, T. E. Macromolecules 2008, 41,
765e7775; Green, M. D.; Long, T. E. Polym. Rev. 2009, 49, 291e314; Brown, R. H.;
Duncan, A. J.; Choi, J.-H.; Park, J. K.; Wu, T.; Leo, D. J.; Winey, K. I.; Moore,R. B.; Long,
26. Varma, R. S.; Namboodiri, V. V. Chem. Commun. 2001, 643e644.
27. CrysAlisPro v171.33.51; Oxford Diffraction: Wroclaw, Poland, 2009.
28. Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112e122.
1
7