Cucurbit[7]uril host-guest and pseudorotaxane complexes with α,ω-bis(pyridinium)alkane dications

Article abstract of DOI:10.1039/b910322h

The host molecule cucurbit[7]uril forms very stable host-guest complexes (1:1 and 2:1) and [2]pseudorotaxanes with α,ω-bis(pyridinium)alkane dications in aqueous solution. With the dications containing pyridinium, 2- and 3-picolinium, and 4-dimethylaminop

Full text of DOI:10.1039/b910322h

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Cucurbit[7]uril host-guest and pseudorotaxane complexes with
a,x-bis(pyridinium)alkane dications†
Ian W. Wyman and Donal H. Macartney*
Received 26th May 2009, Accepted 23rd June 2009
First published as an Advance Article on the web 27th July 2009
DOI: 10.1039/b910322h
The host molecule cucurbit[7]uril forms very stable host-guest complexes (1:1 and 2:1) and
[2]pseudorotaxanes with a,w-bis(pyridinium)alkane dications in aqueous solution. With the dications
containing pyridinium, 2- and 3-picolinium, and 4-dimethylaminopyridinium end groups and a hexyl
chain, [2]pseudorotaxanes are formed with one equivalent of CB[7] positioned over the central chain. A
second equivalent of CB[7] encapsulating one of the end groups results in the translocation of the first
CB[7] to the other end group. For the dications with these end groups, the 2:1 host-guest complexes
exhibit stability constants which are considerably smaller than the 1:1 values as the result of steric and
electronic repulsions between the two CB[7] hosts.
diffuse cations, such as tetraalkylammonium and tetraalkylphos-
phonium guests, may reside partially or entirely within the
cavity.10
Introduction
The cucurbit[n]uril (CB[n], where n = 5–8 and 10) family of
macrocyclic host molecules, comprised of n glycoluril units
bridged by 2n methylene groups, have been demonstrated to form
exceedingly stable host-guest complexes with cationic organic and
organometallic guests in aqueous solution, as a result of a hy-
drophobic cavity accessed through two restrictive polar carbonyl-
lined portals.1 The CB[7] host (Scheme 1),2 in particular, has the
appropriate cavity size to include guests such as aromatic rings,3,4
metallocenes,5 and platinum(II) antitumor agents.6 Cucurbiturils
have been applied to the recognition of amino acids, peptides,7
and drugs,8 including vitamin B12.8b The encapsulation of guests
can significantly affect chemical properties, such as the pKa
of nitrogen- and carbon-centered acids and their absorption
and fluorescence spectra.9 While the majority of cationic guests
are included with the more hydrophobic portion of the guest
within the cavity of CB[7] and the cationic charge(s) outside
the cavity adjacent to the carbonyl oxygens on the portal(s),
we have demonstrated that more hydrophobic and charge-
Cucurbiturils have also been employed in the preparation of
a significant number of mechanically-interlocked supramolec-
ular complexes,11 including [n]rotaxanes, [n]semirotaxanes,
[n]pseudorotaxanes, and [n]catenanes with CB[5],12 CB[6],11a,13
CB[7],14,15 and CB[8]16 acting as the cyclic component. With the
polar carbonyl groups lining the rims of the cucurbiturils, the end
groups on the threads can prevent dissociation of the rotaxanes
into their cyclic bead and linear thread components through
steric and/or electrostatic means. With the polypseudorotaxanes
and polyrotaxanes, molecular switches and shuttles have been
demonstrated11 using proton transfer,11b,c,13b,c,d electron transfer,11c
light,14i,n and heat13d as external stimuli. In many instances poly-
mers are used as threads in the cucurbituril polypseudorotaxane
and polyrotaxanes. The encapsulation of polyaniline by CB[6] or
CB[7], for example, leads to the stabilization of the conductive rad-
ical cationic form.12i,13b,c We have shown that the tetracationic guest
[CH3bpy(CH2)6bpyCH3]4+ (bpy2+ = 4,4¢-bipyridinium) forms a
[2]pseudorotaxane with the addition of one equivalent of CB[7],
with the host residing over the central hexyl chain.14f The addition
of a second equivalent of the host and its encapsulation of one of
the viologen end groups results in the movement of the first CB[7]
to the other end group. Liu et al.14g and Schmitzer and coworkers14d
have each demonstrated that addition of cyclodextrins to 1:1
complexes between CB[7] and a cationic guest with multiple
binding sites can result in the movement of the CB[7] host between
binding positions.
In the present study, a series of dicationic organic guests
(Scheme 2) containing unsubstituted and substituted (2- and
3-methyl, 4-dimethylamino, and 4-tert-butyl) pyridinium end
groups and aliphatic linkers (ethyl, hexyl and p-xylyl) have been
encapsulated by one or two CB[7] host molecules in aqueous
solution, to form 1:1 and 2:1 host-guest complexes in the forms
of [2]- and [3]-pseudorotaxanes, respectively. The stoichiometries
and nature of the inclusions, and the stability constants and their
dependencies on the nature of the end groups and linkers, have
Scheme 1 Cucurbit[7]uril.
Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston,
ON K7L 3N6, Canada. E-mail: donal@chem.queensu.ca; Fax: +1 613 533
6669; Tel: +1 613 533 2617
† Electronic supplementary information (ESI) available: 1H NMR spectra
and HR-MS spectral data of the host-guest complexes, titration data for
2+
{BPH·nCB[7]} complexes. See DOI: 10.1039/b910322h
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Scheme 2 Structures of the guests and complexation induced chemical shifts (Ddlim in ppm) for the 1:1 and 2:1 (italics) host-guest complexes with
cucurbit[7]uril.
1
been investigated using H NMR and UV spectroscopy and ESI
mass spectrometry.
protons upon additions of more than one equivalent of CB[7]
and the ESI mass spectrum displays no peaks associated with a
2:1 host-guest complex. All of the guests, with the exception of
BPE2+, exhibit binding by a second CB[7] host at host-guest ratios
greater than 1:1, which can be demonstrated using 1H NMR and
UV spectroscopy and ESI mass spectrometry.
Results and discussion
The formations of host-guest and pseudorotaxane complexes of
CB[7] with the dicationic guests in this study were confirmed
by the use of 1H NMR and UV spectroscopy and ESI mass
spectrometry.17 1H NMR spectroscopy is particularly useful for
determining the position(s) of the guest which is encapsulated by
the CB[7] host(s), as the complexation-induced chemical shifts
(Ddlim = dbound - dfree) in the guest proton resonances are indicative
of their location within the cavity (Ddlim < 0 ppm) or outside
the cavity near the carbonyl-lined portal (Ddlim > 0 ppm). The
titrations of the guests in this study with CB[7] reveal a range of
guest exchange rates, with respect to the 1H NMR timescale, from
slow (separate resonances for the free and bound guests) to fast
(observed resonances represent an average of free and bound guest
resonances), with intermediate (considerable broadening of the
averaged resonances) exchange observed with the BBPX2+ guest.
The 1,2-bis(1-pyridinium)ethane dication (BPE2+) forms only
a 1:1 complex with upfield shifts in the pyridinium and ethane
protons, suggesting that the CB[7] is moving over both the
pyridinium and ethane regions rapidly on the 1H NMR timescale.
There is no additional change in the chemical shifts of the guest
With the 1,2-bis(4-dimethyaminopyridinium)ethane dication
guest (BAPE2+) there are two possible resonance structures as
indicated in Fig. 1. The first has the positive charges localized
on the pyridinium nitrogens, with the other bearing the posi-
tive charges on the amine nitrogens, rendering the pyridinium
groups neutral. Substantial double-bond character in the exo-
cyclic carbon-nitrogen bond has been established in 1-methyl-4-
alkylaminopyridinium cations at room temperature in aqueous
solution by means of 13C NMR measurements.18 In the latter
resonance structure, the encapsulation of the guest by CB[7] would
place the positive charges adjacent to the polar carbonyl groups
on the portals of the host and stabilize the resonance structure.
The contribution of this resonance structure may be seen in the
1H NMR spectrum upon addition of one equivalent of the host
(Scheme 2 and Fig. 2).
The changes in the chemical shifts of the guest protons
upon inclusion by one CB[7] host would be a combination
of complexation-induced chemical shift changes and chemical
shifts arising from a change in the relative contributions of the
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Fig. 1 The quinoidal (top left) and pyridinium (bottom left) resonance structures of the BAPE2+ dication and an energy-minimized structures of the
2+
2+
{BAPE·CB[7]} (top right) and {BAPE·2CB[7]} (bottom right) complexes.
Fig. 2 1H NMR spectra of BAPE2+ in the (a) absence and presence of
(b) 0.73, (c) 1.14, (d) 1.86, and (e) 2.34 equivalents of CB[7] in D2O.
Fig. 3 UV titration of BAPE2+ (5.1 ¥ 10-5 mol dm-3) with CB[7]. Inset:
plot of absorbance at 314 nm as a function of [CB[7]]/[BAPE2+].
two resonances structures of the guest. The shift in the relative
contributions of the resonance structures from the pyridinium
structure to the quinoidal structure would be expected to result in
upfield shift shifts in all of the proton resonances. The slight upfield
shift in the methyl resonances is thus a result of the upfield shift
for the quinoidal resonance structure combined with an expected
downfield shift in the peak when it is placed outside of the portals
of the CB[7] host in the 1:1 complex. The 1H NMR spectra indicate
that the formation/dissociation of the 1:1 host-guest complex with
BAPE2+ is slow on the NMR time scale (observance of peaks for
both the free and bound guest), while the 2:1 complex exhibits
exchange which is fast on the NMR timescale (peaks represent
average of 1:1 and 2:1 resonances). This would be consistent with
the location of the CB[7] hosts on the pyridinium rings in the 2:1
complex.
as shown in Fig. 3. With up to one equivalent of the host,
the lmax value increases from 286 nm to 298 nm for the 1:1
complex, with an isosbestic point at 292 nm. With more than one
equivalent, the peak moves to 294 nm, with an isosbestic point
at 296 nm. The second, weaker binding results in the formation
of the 2:1 host-guest complex. The initial bathochromic shift
is likely a result of the stabilization of the quinoidal resonance
structure by the first CB[7], which is somewhat diminished when
a second CB[7] is attached, resulting in a slight hypsochromic
shift as the resonance equilibrium is shifted back towards the
pyridinium form. Loeb and coworkers have observed the opposite
behaviour in the interaction of the diprotonated 1,2-bis(4-(N,N¢-
dimethylamino)bipyridinium)ethane tetracation with dibenzo-24-
crown-6, in which the crown host only forms a complex with the
protonated pyridinium resonance structure and not the deproto-
nated pseudo-quinoidal structure.19 In this host-guest complex,
The UV spectrum of the BAPE2+ guest exhibits noticeable
changes upon the addition of increasing amounts of CB[7],
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it is the central +NCH2CH2N+ moiety which forms ion-dipole
interactions with the crown ether, while in the present system, the
pseudo-quinoidal resonance structure provides the right match
between the positive charges on the guest and the polar carbonyl
groups on the portals in the 1:1 CB[7] host-guest complex.
With the hydrophobic 4-tert-butyl groups on the pyridinium
rings, CB[7] forms a very stable 1:1 complex with BBPE2+, with the
host residing over one of the tert-butylpyridinium end groups, but
causing small complexation-induced chemical shifts in the non-
complexed end group. The slow exchange of the ligand on the 1H
NMR timescale is evident in the spectra in Fig. 4, with separate
aromatic resonances for the free guest, the non-complexed and
complexed rings of the 1:1 complex, and the fully complexed 2:1
host-guest complex. The addition of the second CB[7] has little
effect on the chemical shifts of the protons on the pyridinium ring
which was first complexed.
Table 1 The 1:1 and 2:1 host-guest stability constants for the CB[7]
complexes in D2O (pH = 4.75 (0.050 mol dm-3 NaOAC/HOAc) at 25 ◦
C
Guest
K
1:1, dm3 mol-1
K
2:1, dm3 mol-1
K1:1/K2:1
a
a
BPE2+
(3.1 0.6) ¥ 106
(7.4 1.3) ¥ 107
(1.0 0.2) ¥ 1011
(4.8 1.1) ¥ 108
(2.4 0.4) ¥ 107
(1.6 0.3) ¥ 108
(1.5 0.5) ¥ 108
(5.2 1.2) ¥ 1010
(1.2 0.3) ¥ 1010
b,c
d,e
BAPE2+
BBPE2+
BPH2+
(1.6 0.7) ¥ 105
(8.1 2.3) ¥ 109
(8 2) ¥ 101
460
12
d,e
d
6 ¥ 106
750
7000
220
25
a
b
B2PH2+
B3PH2+
BAPH2+
BBPH2+
BBPX2+
(3.2 1.5) ¥ 104
(2.3 1.2) ¥ 104
(6.8 2.6) ¥ 105
(2.1 0.4) ¥ 109
(7.9 1.9) ¥ 108
a
b
a,f
a,b,c
d,e
d
d
f
15
+
a Using 3-(trimethylsilyl)propionic acid as competitor.2b b Using N(CH3)4
as competitor.10 c Using N(CH2CH3)4 as competitor.10 d Using p-
+
xylenediamine as competitor.2b e Using FeCp(CpCH2N(CH3)3)+ as
+
competitor.2b f Using BzN(CH3)3 as competitor.10
chain, resulting in the formations of [2]pseudorotaxanes. Upon
the addition of the second equivalent of CB[7], the original CB[7]
is translocated from the hexyl chain to being positioned over the
picolinium end group while the second CB[7] encapsulates the
other end group in a stable 2:1 host-guest complex. A similar
process is observed with BAPH2+, while for BBPH2+ BBPX2+
there are no formations of a [2]pseudorotaxane species, but rather
1:1 and 2:1 host-guest complexes, with sequential binding of the
4-tert-butylpyridinium end groups occurring. This is indicated by
a doubling of the Ddlim values for the methyl and aromatic proton
resonances on going from the 1:1 species (average of complexed
and non-complexed end groups) to the 2:1 host-guest species
(see the H3 proton in Fig. 4).
The 1:1 and 2:1 (with the exceptions of BPE2+ and BPH2+)
CB[7] host-guest stability constants with the guests in this study
1
were too large to be determined by conventional H NMR or
UV-visible spectroscopic titrations. Instead, 1H NMR competition
experiments, using a variety of competitor guests, were carried out
at 25 ◦C and pH 4.75 (0.05 mol dm-3 NaOAc/HOAc buffer) with
a deficiency of CB[7]. In the case of BPH2+, a Benesi-Hildebrand
double reciprocal plot of 1/Dd against 1/[CB[7]] afforded the
stability constant from the intercept/slope ratio.17 The magnitudes
of the 1:1 and 2:1 host-guest stability constants (Table 1) for the
guests in this study depend on the natures of the substituents on the
pyridinium end groups and the nature and length of the aliphatic
linkers.
The largest 1:1 stability constant of 1.1 ¥ 1011 dm3 mol-1 is
observed for BBPE2+, likely the result of the hydrophobic nature of
the tert-butyl substituent and the possibility of the second positive
charge interacting in an ion-dipole fashion with the polar portal
of CB[7]. The flexibility in the ethane bridge would allow for the
pyridinium ring to approach the portal, accounting for the small
downfield shifts in the non-complexed end of the guest molecule.
The other guests bearing the tert-butyl substituents, BBPH2+ and
BBPX2+, have only slightly smaller 1:1 stability constants.
The [2]pseudorotaxanes formed by the 1:1 complexation of the
central chains by CB[7] in the cases of the BHP2+, B2PH2+, B3PH2+,
and BAPH2+ have stability constants in the range of 2 ¥ 107 to
5 ¥ 108 dm3 mol-1 (Table 1). We have determined a somewhat higher
value of (3.9 0.9) ¥ 109 dm3 mol-1 for the 1:1 CB[7] complex of the
1,6-diammoniumhexane dication, which exhibits no formation of
a 2:1 complex.20 Kaifer and coworkers have reported a lower limit
Fig. 4 Downfield portions of the 1H NMR spectra of BBPE2+ (left,
(a) 0.00, (b) 0.30, (c) 0.73, (d) 1.57, and (e) 2.19 equiv of CB[7]), BBPH2+
(middle, (a) 0.00, (b) 0.34, (c) 0.90, (d) 1.57, and (e) 2.48 equiv of CB[7]),
and BBPX2+ (right, (a) 0.00, (b) 0.56, (c) 1.13, (d) 1.36, and (e) 2.53 equiv
of CB[7]) in the presence of CB[7] in D2O.
When the aliphatic linker is lengthened from ethyl to hexyl, the
addition of one equivalent of CB[7] may result in the formation
of a 1:1 host-guest complex in the form of a [2]pseudorotaxane.
A crystal structure of the 1:1 host-guest complex between CB[6]
and the 1,6-bis(1-pyridinium)hexane dication (BPH2+) reveals that
the host is positioned over the hexyl linker, with the cationic
pyridinium end groups located near the polar portals.13h The
BPH2+ dication exhibits strong 1:1 complex formation with CB[7],
followed by a much weaker 2:1 complex formation as excess
CB[7] is added.17 This is manifested in significant upfield shifts
in the hexyl chain protons, with small downfield shifts in the
meta and para protons of the pyridinium rings. With the weak
complexation of the guest by a second CB[7], the trend in the
chemical shift changes, with upfield shifts in the pyridinium
protons and downfield shifts in the hexyl protons. While it is not
possible to extract accurate limiting chemical shifts for the 2:1
complex, the directions of the chemical shift changes support the
inclusion of one or both pyridinium end groups.
1
With both of the B2PH2+ and B3PH2+ guests, the H NMR
spectra also indicate that the first CB[7] resides over the hexyl
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for the binding of the a,a-bis(1-pyridinium)-p-xylene dication to
CB[7] of 1.0 ¥ 106 dm3 mol-1 in D2O containing 0.20 mol dm-3
NaCl.15 Without the 4-tert-butyl substituents found in BBPX2+,
the CB[7] complexes the internal p-xylene linker, rather than the
pyridinium head groups. Even with 4-pyridylpyridinium groups
attached to the p-xylene core, Kaifer observed binding to the center
of the guest forming a [2]pseudorotaxane, allowing for capping of
the thread with bulky stoppers to form a [2]rotaxane.
with the shorter ethyl bridge. This appears to hold true in the cases
of BPE2+ vs BPH2+ and BAPE2+ vs. BAPH2+, however the value
of K2:1 for BBPE2+ vs. BBPH2+ (and BBPX2+) follows an opposite
trend with slightly more stable complexes with the shorter ethyl
linker than with the hexyl (or p-xylyl) linker (Table 1).
Experimental
1
The 2:1 stability constants were also determined by H NMR
Materials
competition experiments in solutions with CB[7]:guest concen-
tration ratios of approximately 1.5:1, assuming strong binding of
the 1:1 host-guest complex. If a statistical binding phenomenon
is operating, with no inhibitory effect of the first host molecule
on the strength of the binding of the second host molecule, then
the ratio of the 1:1 stability constant to the 2:1 stability constant
(K1:1/K2:1) would be 4.21 This would assume that the first and
second host molecules occupy similar binding sites. With all of
the guests in this study, the decrease in the strength of the second
binding exceeds a factor of 4 indicating the first host molecule has
a detrimental effect on the binding of the second host (Table 1).
In the cases where 1:1 and 2:1 binding is limited to the
end groups of the guest, such as with BBPE2+, BBPH2+, and
BBPX2+, the ratios are in the range of 12–25, indicating a
minor deviation from statistical binding. Kaifer and coworkers
have observed that the K1:1/K2:1 ratios for CB[7] binding to
HO2C(CH2)nbpy(CH2)nCO2H2+ guests (n = 6, 7, and 10) are
in the range of 25–45, also evidence of a degree of negative
cooperativity.11c,14l Sun and coworkers have recently shown that
with tetracationic threads composed of methyl viologen and N,N-
dimethyl-3,3¢-dimethyl-4,4¢-bipyridinium end groups and either
propyl or hexyl linkers, the CB[8] host will form 1:1 and 2:1
host-guest complexes with the host residing exclusively over the
bipyridinium end groups.22 A weaker (and more dynamic) 2:1
complex is observed for the shorter propyl linker compared with
the hexyl linker, based on 1H NMR spectra.
The cucurbit[7]uril was prepared by the method of Day and
co-workers.2b The bis(pyridinium)alkane dibromide salts were
prepared by heating a mixture of an excess of the appro-
priate substituted pyridine (pyridine, 2-picoline, 3-picoline, 4-
dimethylaminopyridine, or 4-tert-butylpyridine (typically 15–
30 mmol, Aldrich)) with 1,2-dibromoethane, 1,6-dibromohexane,
or a,a-dibromo-p-xylene (typically 4–5 mmol Aldrich) in 50 mL of
acetonitrile (15 mL of DMF for reaction with 2-picoline) at 70 ◦C
for 24 hr. The precipitate was washed with methanol, followed by
ether and then recrystallized from methanol/ether mixtures.
1,2-Bis(1-pyridin◦ium)ethane dibromide ([BPE]Br2). Yield:
1
26%, mp 240–245 C. H NMR (D2O, 400 MHz) d 8.82 (d, 4H,
H2, J = 5.6 Hz), 8.66 (t, 2H, H4, J = 8.0 Hz), 8.12 (t, 4H, H3, J =
6.0 Hz), 5.32 (s, 4H, Ha) ppm. 13C NMR (D2O) d 147.59 (C4),
144.76 (C2), 129.23 (C3), 60.16 (Ca) ppm. HR-ESIMS calcd for
C12H14N2 [M - 2Br]2+ m/z = 93.0573; found 93.0571.
1,2-Bis(4-dimethylamino-1-pyridinium)ethane
◦ dibromide
([BAP◦E]Br2). Yield: 38%, mp > 300 ◦C (Lit. 301 C (dec),23
1
>300 C (dec)24). H NMR (D2O, 400 MHz) d 7.74 (d, 2H, H2,
J = 7.2 Hz), 6.79 (d, 2H, H3, J = 7.2 Hz), 4.56 (s, 4H, Ha), 3.15
(s, 2H, CH3) ppm. 13C NMR (D2O, 100 MHz) d 156.46 (C2),
141.05 (C3), 56.72 (Ca), 39.60 (CH3) ppm. HR-ESIMS calcd for
C16H24N2 [M - 2Br]2+ m/z = 136.0994; found 136.0995.
1,2-Bis(4-tert-butyl-1-pyridinium)ethane dibromide ([BPPE]Br2).
In situations where there is movement of the first CB[7] host
molecule from the center of the guest to an end group, upon
binding of the second CB[7] over the other end group, the
difference between K1:1 and K2:1 will depend on the natures of
the center and end groups and the distance between the two end
groups. We have previously reported that for the tetracationic guest
[CH3bpy(CH2)6bpyCH3]4+, the K1:1/K2:1 ratio for CB[7] binding is
approximately 9. This relatively small decrease can be attributed
to the fact that both the central unit and the viologen end groups
provide two positive centers for the hosts. In the present study, the
movement of first CB[7] from the hexyl chains in BPH2+, B2PH2+,
B3PH2+, and BAPH2+ to the end groups results in reductions in
◦
◦
1
Yield: 29%, mp > 300 C (Lit.25 > 300 C). H NMR (D2O,
400 MHz) d 8.51 (d, 4H, H2, J = 6.5 Hz), 8.00 (d, 4H, H3, J =
6.5 Hz), 5.12 (s, 4H, Ha), 1.38 (s, 18H, CH3) ppm. 13C NMR (D2O,
125 MHz) d 174.37 (C4), 144.05 (C2), 126.66 (C3), 59.70 (Ca),
36.76 (Ctert), 29.39 (CH3) ppm. HR-ESIMS calcd for C20H30N2
[M - 2Br]2+ m/z = 149.1199; found 149.1199.
1,6-Bis(1-pyridin◦ium)hexane dibromide ([BPH]Br2). Yield:
1
93%, mp 140–143 C. H NMR (D2O, 400 MHz) d 8.80 (d, 4H,
H2, J = 5.6 Hz), 8.62 (t, 2H, H4, J = 8.0 Hz), 8.03 (t, 4H, H3, J =
7.2 Hz), 4.57 (t, 4H, Ha, J = 7.4 Hz) 1.99 (m, 4H, Hb), 1.37 (m,
4H, Hg) ppm. 13C NMR (D2O) d 145.62 (C4), 144.20 (C2), 128.26
(C3), 61.73 (Ca), 30.29 (Cb), 24.81 (Cg) ppm. HR-ESIMS calcd
for C16H22N2 [M - 2Br]2+ m/z = 121.0886; found 121.0881.
K
2:1, relative to K1:1, in the range of 2 ¥ 102 to 6 ¥ 106 fold, with
the diminution factor being inversely related to the strength of
the binding to the end groups. While the guest in the 1:1 host-
guest species provides a positive charge located near each portal
of the CB[7] in the [2]pseudorotaxanes, there is only one positive
charge forming an ion-dipole interaction with each CB[7] in the
2:1 complexes. The stability of the latter species is thus dependent
on the hydrophobicity of the pyridinium end group and follows
the order of pyridinium < 2-picolinium ª 3-picolinium < 4-
dimethylaminopyridinium << 4-tert-butylpyridinium. In terms of
the electronic repulsions between two neighbouring CB[7] hosts in
a 2:1 complex, the largest effects might be expected in the guests
1,6-Bis(2-methyl-1-pyridinium)hexane dibromide ([B2PH]Br2).
Yield: 15%, mp 230–235 ◦C. 1H NMR (D2O) d 8.66 (d, 2H, H6, J =
6.4 Hz), 8.33 (t, 2H, H4, J = 8.0 Hz), 7.87 (d, 2H, H3, J = 8.0 Hz),
7.81 (t, 2H, H5, J = 6.4 Hz), 4.49 (t, 4H, Ha, J = 7.9 Hz), 2.80
(s, 6H, CH3), 1.93 (m, 4H, Hb), 1.46 (m, 4H, Hg) ppm. 13C NMR
(D2O, 125 MHz) d 155.63 (C2), 145.42 (C4), 145.07 (C6), 130.61
(C3), 125.98 (C5), 58.19 (Ca), 29.54 (Cb), 25.50 (Cg), 19.89 (CH3)
ppm HR-ESIMS calcd for C18H26N2 [M - 2Br]2+ m/z = 135.1042;
found 135.1043.
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1,6-Bis(3-methyl-1-pyridinium)hexane dibromide ([B3PH]Br2).
1.00 cm pathlength quartz cells. The electrospray mass spectra
were obtained on a QstarXL MS/MS system, with an ESI
source, with the samples prepared in distilled water. The energy-
minimization calculations (Fig. 1) were carried out with the MM2
program in the Chem3D Pro (Version 11.0.1, CambridgeSoft)
software.
◦
1
Yield: 55%, mp 157–162 C. H NMR (D2O, 400 MHz) d 8.62
(s, 2H, H2), 8.57 (d, 2H, H6, J = 6.0 Hz), 8.30 (d, 2H, H4, J =
8.0 Hz), 7.87 (t, 2H, H5, J = 7.0 Hz), 4.49 (t, 4H, Ha, J = 7.9 Hz),
2.48 (s, 6H, CH3), 1.94 (m, 4H, Hb), 1.32 (m, 4H, Hg) ppm. 13C
NMR (D2O, 100 MHz) d 145.98 (C4), 143.66 (C2), 141.23 (C6),
139.91 (C3), 127.39 (C5), 61.45 (Ca), 30.24 (Cb), 24.78 (Cg), 17.60
(CH3) ppm. HR-ESIMS calcd for C18H26N2 [M - 2Br]2+ m/z =
135.1042; found 135.1043.
Conclusions
A series of 1:1 (including [2]pseudorotaxanes) and 2:1 host-
guest complexes may be formed between cucurbit[7]uril as and
dicationic a,w-bis(pyridinium)alkane chains in aqueous solution.
The relative 1:1 and 2:1 host-guest stability constants are related
to the natures of the cationic head groups and the alkyl linkers.
The CB[7] cyclic component in the [2]pseudorotaxane may be
moved along the dicationic thread to one of the end groups by the
complexation by a second CB[7] host on other end group.
1,6-Bis(4-dimethylamino-1-pyridinium)hexane
dibromide
([BAPH]Br2). Yield: 80%, mp 275–276 ◦C. 1H NMR (D2O,
400 MHz) d 7.92 (d, 2H, H2, J = 7.6 Hz), 6.80 (d, 2H, H5, J =
7.6 Hz), 4.05 (t, 4H, Ha, J = 7.0 Hz), 3.14 (s, 12H, CH3), 1.78
(m, 4H, Hb), 1.25 (m, 4H, Hg) ppm. 13C NMR (D2O, 100 MHz)
d 156.29 (C4), 141.25 (C2), 107.47 (C3), 57.46 (Ca), 39.33 (CH3),
29.68 (Cb), 24.83 (Cg) ppm. HR-ESIMS calcd for C20H32N2 [M -
2Br]2+ m/z = 164.1308; found 164.1309.
1,6-Bis(4-tert-butyl-1-pyridinium)hexane dibromide ([BBPH]Br2).
Yield: 74%, mp 274–277 ◦C. (Lit.26 299–300 ◦C). 1H NMR (D2O,
500 MHz) d 8.63 (d, 4H, H2, J = 7.0 Hz), 8.01 (d, 4H, H3, J =
7.0 Hz), 4.49 (t, 4H, Ha, J = 7.2 Hz) 1.95 (m, 4H, Hb), 1.36 (s,
18H, CH3), 1.34 (m, 4H, Hg) ppm. 13C NMR (D2O, 125 MHz)
d 171.96 (C4), 143.67 (C2), 125.74 (C3), 61.00 (Ca), 36.32 (Ct),
30.59 (Cb), 29.49 (CH3), 25.18 (Cg) ppm. HR-ESIMS calcd for
C24H38N2 [M - 2Br]2+ m/z = 177.1512; found 177.1514.
Acknowledgements
The financial support of this research by the Natural Sciences
and Engineering Research Council of Canada (Discovery Grant
to DHM), the Ontario Ministry of Training and the Walter
C. Sumner Foundation (scholarships to IWW) is gratefully
acknowledged.
a,a¢-Bis(4-tert-butyl-1-pyridinium)-p-xylene
dibromide
([BBPX]Br2). Yield: 89%. mp > 300 ◦C. 1H NMR (D2O)
d 8.66 (d, 4H, H2, J = 5.6 Hz), 8.01 (d, 4H, H3, J = Hz), 7.44
(s, 4H, H2¢), 5.72 (s, 4H, Ha), 1.33 (s, 18H, CH3) ppm. 13C
NMR (D2O, 125 MHz) d 172.72 (C4), 143.93 (C2), 134.98 (C1¢),
130.11 (C2¢), 126.03 (C3), 63.30 (CH2), 36.44 (Ct), 29.45 (CH3).
HR-ESIMS calcd for C26H34N2 [M - 2Br]2+ m/z = 187.1355;
found 187.1356.
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◦
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