New guests for the cucurbit[8]uril host. Formation of G2H ternary complexes

Article abstract of DOI:10.1002/poc.2928

On the basis of the highly stable G₂H (2:1) ternary complex formed by two methyl viologen cation radicals inside the cavity of cucurbit[8]uril, we prepared three monocationic 4-phenylpyridinium derivatives: 1-(hydroxyethyl)-4-phenyl-pyridinium

Full text of DOI:10.1002/poc.2928

Research Article
Received: 15 December 2011,
Revised: 27 January 2012,
Accepted: 31 January 2012,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.2928
New guests for the cucurbit[8]uril host.
Formation of G2H ternary complexes
Sanem Senlera, Lu Cuia, Ana Michelle Broomesa, Erika L. Smitha,
James N. Wilsona and Angel E. Kaifera*
On the basis of the highly stable G2H (2 : 1) ternary complex formed by two methyl viologen cation radicals inside the
cavity of cucurbit[8]uril, we prepared three monocationic 4-phenylpyridinium derivatives: 1-(hydroxyethyl)-4-phenyl-
pyridinium (1+) bromide, 1-(octaethyleneglycol)-4-phenyl-pyridinium (2+) chloride, and 4-[4-(methoxymethoxy)phenyl]
pyridinium (3+) iodide, as possible guests for 2 : 1 complexation inside cucurbit[8]uril. We also investigated a fourth
monocationic guest (4+), in which a central vinylidene group is inserted to elongate the 4-phenyl-pyridinium residue.
1
Using H NMR and UV–Vis spectroscopic data and mass spectrometric data, we obtained unequivocal evidence for
the formation of G2H (2: 1) ternary complexes in all cases. The stoichiometry of the complexes was further verified by
continuous variation (Job) plots, and in some cases, high resolution ESI-MS spectrometric data. Diffusion coefficient
measurements, using 1H NMR pulse gradient spin echo techniques, yielded values consistent with the formation and
expected structures of the ternary complexes. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: cucurbituril hosts; host–guest complexes; inclusion complexes; ternary complexes
stability of the ternary complexes that result from two viologen
radical cations forming a p–p dimer inside the cavity of CB8.
INTRODUCTION
The main idea is shown in Scheme 1. Although methyl viologen
forms a stable 1 : 1 complex with CB8, its one-electron reduced
form selectively yields a 2 : 1 complex with the same host.[13] As
mentioned before, this complex is a clear example of a G2H com-
plex, in which two identical guests are held inside the host cav-
ity. We reasoned that this ternary (2 : 1 or G2H) complex derives
stabilization from the ion–dipole interactions between the two
positive charges on the radical cation dimer and the rims of car-
bonyl oxygens lining the host cavity portals. Contacts between
the aromatic surfaces of the viologen radical cations and
between the dimer and the inner surface of the cavity, coupled
with hydrophobic forces, also contribute significantly to the
stability of the ternary complex. As shown in Scheme 1, we pro-
pose to replace the viologen radical cations by two 4-phenyl-
pyridinium residues. Each 4-phenyl-pyridinium group provides
the same overall charge (+1) as that on the methyl viologen rad-
ical cation and a similarly sized aromatic surface. Our hypothesis
is that 4-phenyl-pyridinium derivatives will form stable 2 : 1 (G2H)
complexes inside the cavity of CB8 with an overall stability simi-
lar to that exhibited by viologen radical cation dimer complexes.
To test this idea, we prepared three monocationic compounds
containing the 4-phenyl-pyridinium residue. Their structures are
shown in Fig. 1. Guests 1+ and 2+ are simple 4-phenyl-pyridinium
derivatives with N-substituents of variable length, intended to
Cucurbit[8]uril (CB8) is a macrocyclic host formed by the conden-
sation in acidic medium of glycoluril and formaldehyde.[1,2]
A
member of the cucurbit[n]uril family of hosts,[3–11] it is character-
ized by its ability to bind simultaneously two aromatic guests in
its cavity. Kim et al. first reported the formation of charge transfer
complexes – between a p-donor (dihydroxynaphthalene) and a
p-acceptor (methyl viologen) – inside the cavity of CB8.[12] These
two guests are obviously different, albeit complementary in their
p-donor/acceptor characters. Thus, the corresponding ternary
complexes can be classified as GG′H complexes, where G and
G′ represent the two distinct aromatic guests and H symbolizes
the host. Jeon et al. also showed that CB8 sharply stabilizes
and includes the dimer formed by methyl viologen radical
cations,[13] which illustrates a different class of ternary complex
(G2H) with two identical guests. His group has made extensive
use of these unique binding properties to design and demon-
strate the operation of a number of interesting CB8-based,
switchable molecular systems.[6,7] We have used CB8 to mediate
the redox-controlled dimerization of dendrimers containing
viologen residues[14] and extended this concept to the redox
control and size selection of dimeric assemblies formed between
two types of dendrimers, containing either (p-acceptor) viologen
groups or (p-donor) dialkoxybenzene groups.[15] Recently, Appel
et al. have shown that CB8 can be used to mediate the formation
of supramolecular polymers.[16] Vincil and Urbach also investi-
gated the effects of the number and placement of positive
charges on viologen-cucurbit[n]uril interactions.[17] This entire
body of work relies on the pronounced stabilization of aromatic
donor–acceptor charge transfer complexes or viologen radical
cation dimers inside the cavity of CB8.
* Correspondence to: Angel E. Kaifer, Chemistry, University of Miami Coral
Gables, FL, USA 33124-0431
E-mail: akaifer@miami.edu
a
S. Senler, L. Cui, A. M. Broomes, E. L. Smith, J. N. Wilson, A. E. Kaifer
In this work, we propose an expansion of this chemistry and
investigate new guests for CB8 inspired by the considerable
Center for Supramolecular Science and Department of Chemistry, University of
Miami, Coral Gables, FL, 33124-0431, USA
J. Phys. Org. Chem. 2012
Copyright © 2012 John Wiley & Sons, Ltd.

S. SENLER ET AL.
complexes and mass spectrometric data were also useful in this
regard. Finally, pulse gradient spin echo (PGSE) NMR techniques
were used to determine the diffusion coefficients of the various
guests, the CB8 host, and the corresponding supramolecular
host–guest complexes.
Figure 2 shows the 1H NMR spectrum of guest 3+ in D2O solu-
tion as the concentration of CB8 was increased from 0 to 0.6
equiv. The most noticeable host-induced shifts are experienced
by the aromatic protons on the guest, which move upfield, as
expected from their inclusion in the cavity of CB8. The N–CH3
protons also shift upfield by ~0.3 ppm, the O–CH2–O protons
barely shift and the OCH3 protons experience a small downfield
shift. Interestingly, all these host-induced shifts level off and
reach saturation values as soon as the concentration of CB8
reaches 0.5 equiv., that is, host additions beyond this level have
no further effect on the 1H NMR resonances of the guest. Taken
together, this set of experimental data indicates the formation of
a G2H (2 : 1) complex, in which the aromatic residues of the
guests are included within the host cavity.
Scheme 1. Schematic comparison of the G2H ternary complexes formed
by CB8 with (A) a viologen radical cation dimer and (B) two identical
4-phenyl-pyridinum guests
Figure 3 shows similar NMR data obtained with guest 2+. The
analysis of the data leads to a similar conclusion, that is, the for-
mation of a G2H complex, with two 2+ guests bound to CB8 by
inclusion of their aromatic units inside the host cavity. However,
an important difference between the two complexes is the
behavior of the highest field aromatic proton (at ~9.0 ppm),
which in the case of guest 2+ undergoes a minimal upfield shift
upon addition of CB8. This NMR resonance corresponds to the
protons on the carbon adjacent to the pyridinium nitrogen on
Figure 1. Structures of the cationic guests and the host used in this work
increase their solubility in aqueous solution. Guest 3+ was
designed to increase the p-donor character of the 4-phenyl ring
by attaching –OCH2OCH3 as the terminal group. If we assume
that, in the CB8 inclusion complex, the positively charged pyridi-
nium rings interact with the host portals, deriving stabilization
from ion-dipole forces, each of the 4-phenyl rings should be in
close contact with the pyridinium ring on the other guest
(see Scheme 1B). Therefore, increasing the p-donor character of
the 4-phenyl ring should favor the development of charge trans-
fer interactions and enhance the overall stability of the complex.
We also decided to test a structurally related phenyl-vinylidene-
pyridinium compound (4+), which was available to us.
RESULTS AND DISCUSSION
The binding studies between the cationic guests shown in Fig. 1
and the CB8 host were carried out using several experimental
1
techniques. H NMR spectroscopy was useful at elucidating the
main binding sites and gave us insight into the stoichiometry
of the complexes. Electronic absorption spectroscopy was
extremely useful, because its higher sensitivity allows the inves-
tigation of binding interactions at lower guest/host concentra-
tions than is possible with nuclear magnetic resonance (NMR)
spectroscopy. Continuous variation (Job) plots based on UV–Vis
data were important to establish the stoichiometry of the
Figure 2. Partial 1H NMR spectra (500 MHz, D2O) of guest 3+ (1.0 mM) in
the absence (a) and in the presence of (b) 0.1 mM, (c) 0.2 mM, (d) 0.3 mM,
(e) 0.4 mM, (f) 0.5 mM, and (g) 0.6 mM CB8
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J. Phys. Org. Chem. 2012

NEW TERNARY COMPLEXES FORMED BY THE CUCURBIT[8]URIL HOST
very well defined maximum at a molar fraction of guest equal
to 0.67 (Fig. 5), providing unequivocal evidence for the expected
2 : 1 stoichiometry in the supramolecular complex. Similar UV–Vis
data were obtained with all the other guests, and Job plots
confirmed the 2:1 stoichiometry in all cases (see Figs S2–S5).
It is important to note here that these UV–Vis experiments
were performed with guest concentrations in the range 1–5 Â
10–5 M. The fact that the electronic absorption spectroscopic
data show essentially quantitative formation of the G2H (2 : 1)
complexes at these low concentrations indicates that the ther-
modynamic stability of the complexes is substantial. On the basis
of the spectroscopic data, we estimate a minimum value of
~1 Â 108 M–2 for the equilibrium association constant (K)
corresponding to the overall complexation process: 2 G + H ⇄ G2H.
The stability of the complexes led us to attempt their detec-
tion in the gas phase using mass spectrometric methods. Cucur-
bituril complexes are usually stable enough to allow their
detection by mass spectrometry and our group has reported a
number of examples in recent work.[18] Rauwald et al. have
developed a method to estimate the stability of cucurbituril
inclusion complexes from mass spectrometric data.[19] Using
high-resolution electrospray mass spectrometry (ESI-MS) we
detected the presence of the (3+)2•CB8 complex in the gas phase,
at an m/z ratio of 894.8210. The experimental and simulated
spectra are shown in the supporting information (Fig. S6). We
also detected an intense ESI-MS peak for (4+)2•CB8 (Fig. S7).
Attempts to obtain ESI-MS evidence for the (2+)2•CB8 complex
failed, but we could detect a clear peak for the 2+•CB8 1 : 1
complex under matrix-assisted laser desorption ionization mass
spectrometry (MALDI MS) conditions (data not shown). Although
this is a qualitative result and we cannot discard the possible
observation of (2+)2•CB8 complex using different mass spectro-
metric conditions, the relatively easy detection of the (3+)2•CB8
and (4+)2•CB8 complexes suggest that these two complexes are
more stable than (2+)2•CB8. This is in qualitative agreement with
the NMR spectroscopic results that suggest a more shallow inclu-
sion of the aromatic residues of guest 2+ inside the CB8 cavity.
As discussed before, we also measured diffusion coefficients
(Do’s) using NMR PGSE techniques. The recorded values are given
in Table 1 and are generally consistent with the formation of 2 : 1
complexes. For instance, the Do value recorded for the (3+)2•CB8
complex is ~10% lower than the value recorded for uncom-
plexed CB8 and much lower than the value obtained for the free
guest (3+). These findings are consistent with the postulated
structure of the 2 : 1 complex, in which the guest aromatic
Figure 3. Partial 1H NMR spectra of guest 2+ (1.0 mM) in the absence (a) and
in the presence of (b) 0.1 mM, (c) 0.25 mM, (d) 0.5 mM, and (e) 0.75 mM CB8
the guest. The very small upfield shift induced by the presence of
CB8 suggests a more shallow inclusion of the aromatic residue of this
guest inside the host cavity in the (2+)2•CB8 complex as compared
with the (3+)2•CB8 complex. This difference must be the result of
the long ethyleneglycol N-substituent chain present in the former
guest. The NMR spectroscopic data for the guest 4+ resemble those
obtained with guest 3+, suggesting deeper inclusion of the aromatic
residue (see Fig. S1). Unfortunately, the simpler guest 1+ was found
to be poorly soluble in aqueous solution, which made impossible the
collection of suitable NMR spectroscopic data.
The electronic absorption spectrum of 3+ shows two main
bands at 225 and 321 nm. In the presence of increasing concen-
trations of CB8 the intensity of both bands is depressed and the
absorption maxima undergo bathochromic shifts (Fig. 4). Similar
depressions of the guest’s molar absorptivity coefficients have
been previously observed with other absorbing guests bound
to cucurbituril hosts. In the case of guest 3+, these host-induced
effects are more pronounced on the lower energy band at
321 nm. A plot of absorbance (at 321 nm) as a function of the
added concentration of CB8 is shown in the inset of Fig. 4. The
decreasing absorbance values level off quickly after the addition
of 0.5 equiv. of CB8, again suggesting the formation of a G2H
(2 : 1) complex. To further confirm the stoichiometry of the com-
plex, we recorded the corresponding Job plot, which exhibits a
0.7
0.6
0.7
0.5
0.0025
0.0020
0.4
0.6
0.3
0.2
0.5
0.1
geust
0.0
0
5
10 15 20 25 30 35 40
0.4
0.3
0.2
0.1
0.0
0.0015
0.0010
0.0005
0.0000
c (CB8)
*X
0
A
A/
200
250
300
350
400
450
500
0.00
0.20
0.40
0.60
0.80
1.00
wavelength
Xguest
Figure 5. Job-plot of guest 3+ with CB8
Figure 4. UV-titration guest 3+ (30 mM) upon addition of increasing con-
centrations of CB8 in aqueous solution
J. Phys. Org. Chem. 2012
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S. SENLER ET AL.
Table 1. Diffusion coefficients (Do’s) recorded using 1H NMR
(two times) and brine. The organic layer was dried over MgSO4 and filtered.
After solvent evaporation the desired compound was obtained as an oil.
1H NMR (CD2Cl2, 500MHz): d = 2.44 (s, 3H), 3.32 (s, 4H), 3.43-3.68 (m, PEG),
4.11 (s, 2H), 7.37 (d, J= 7.46 Hz, 2H), 7.77 (d, J= 7.54 Hz, 2H) ppm. HRMS (ESI):
calcd for C24H42O11S [M]+[Na] 561.2340; found 561.2358
PGSE techniques in D2O solution at 25 ꢀC
Compound
Do (cm2 s–1)
Complex
Do (cm2 s–1)
CB8
2+
2.63 Â 10–6
3.17 Â 10–6
5.56 Â 10–6
5.36 Â 10–6
—
—
(2+)2•CB8
(3+)2•CB8
(4+)2•CB8
1.64 Â 10–6
2.38 Â 10–6
2.70 Â 10–6
Synthesis of 1-(octaethyleneglycol)-4-phenylpyridinium (2+)
3+
The reaction was carried out according to the literature procedure[23]
reported for the synthesis of N-alkylpyridinium podands. 4-Phenylpyridinium
(0.58 mmol, 90 mg) and octaethylene glycol monomethyl ether tosylate
(0.72 mmol, 388 mg) were boiled in 5.0 mL of dioxane for 5 h with reflux
and the reaction progress was monitored by thin layer chromatography
(TLC). Upon completion, the solvent was evaporated. Excess octaethylene
glycol monomethyl ether tosylate was removed by reversed phase chroma-
tography. The counter ion (OTs–) was exchanged to Cl– by using an ion-
exchange resin. The desired 4-phenylpyridinium salt was obtained as a
viscous liquid.
4+
residues are included inside the host cavity. Therefore, the com-
plex is expected to exhibit a molecular volume slightly larger
than that of the free host, with the volume increment reflecting
the small substituents on the guests that are not included inside
the cavity. The Do value recorded for the (4+)2•CB8 complex is
identical within experimental error to that of CB8. This is proba-
bly a result of the rigid character of guest 4+, which decreases
the potential dragging effects of any small groups protruding
outside of the CB8 cavity in the 2 : 1 complex. In contrast to these
complexes, the (2+)2•CB8 complex shows a Do value significantly
lower than that of free CB8, a result of the two long ‘PEGylated’
chains protruding from the cavity in the 2 : 1 complex.
1H NMR (D2O, 500 MHz): d = 3.42 (s, 3H), 3.57–3.85 (m, PEG), 4.12
(s, 2H), 7.43 (s, 1H) , 7.74 (d, J = 3.92 Hz, 2H), 8.05 (d, J = 5.5 Hz, 2H), 8.41
(d, J = 3.62 Hz, 2H), 8.91 (d, J = 3.85 Hz, 2H) ppm. 13 C NMR (in D2O,
500 MHz):
d
=68.62(ÀOCH3), 69.53(ÀOCH2CH2O–), 69.62(ÀNCH2–),
69.79, 71.67, 124.71, 127.95, 129.76, 132.31, 133.68, 144.58, 156.80. HRMS
(ESI): calcd for C28H44O8N [M]+ 522.3061; found 522.3084
Synthesis of 4-[4-(methoxymethoxy)phenyl]pyridinium iodide (3+)
CONCLUSIONS
The precursor pyridine derivative was synthesized using a literature
procedure.[24] To a solution of 4-[4-(methoxymethoxy)phenyl]pyridine
(0.30 mmol, 65 mg) in acetone (5.0 mL), iodomethane (1.2 mmol, 0.17 g)
was added. The reaction mixture was stirred under reflux for 4 h. After-
wards, the mixture was cooled down overnight and the desired phenyl-
pyridinium salt was filtered out and isolated as a yellowish solid.
1H NMR (D2O, 500 MHz): d = 3.34 (s, 3H), 4.14 (s, 3H), 5.19 (s, 2H), 7.11
(d, J = 8.0 Hz, 2H), 7.78 (d, J = 8.2 Hz, 2H), 8.05 (d, J = 5.5 Hz, 2H), 8.50 (d,
J = 5.5 Hz, 2H) ppm. 13 C NMR (MeOD, 500 MHz): d = 46.34(ÀNCH3), 55.22
(ÀOCH3), 93.91(ÀOCH2O–), 116.98, 123.31, 126.54, 129.50, 144.92,
155.55, 160.99. HRMS (ESI): calcd for C14H16O2N [M]+ 230.1176; found
230.1196
We have shown that two new 4-phenyl-pyridinium derivatives
(2+ and 3+) and a related vinylidene compound (4+) are suitable
guests for the formation of G2H (2 : 1) ternary complexes with the
CB8 host. Our experimental results show that these guests form
ternary complexes readily at submillimolar concentration levels
of the individual components, reflecting the considerable stabil-
ity of these supramolecular complexes. Somewhat unexpectedly,
1
the H NMR data provide experimental evidence for shallower
penetration of guest 2+, as compared with guests 3+ and 4+,
inside the host cavity. This finding may be attributable to the
presence of an oligo(ethyleneglycol) chain attached to the posi-
tively charged nitrogen atom. The likely electrostatic repulsions
between the chain oxygen atoms and the carbonyl oxygen on
the host portal weaken the N+-portal ion–dipole interactions,
leading to less penetration of guest 2+ and increased distance
between the nitrogen atom on the guest and the portal
entrance on the host. This experimental finding may be of gen-
eral importance in the design of other guests for inclusion by
cucurbit[n]uril hosts.
All electronic absorption spectra were recorded using a 1-cm quartz
cuvette. Mass spectrometric data were obtained either with a high-
resolution electrospray ionization time-of-flight (ESI-TOF) mass spectrometer
or a matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)
1
mass spectrometer. Diffusion coefficients were measured by H NMR PGSE
techniques as previously reported by our group.[25,26]
Acknowledgements
The authors are grateful to the US National Science Foundation
(to AEK, CHE-0848637) for the generous support of this work
and for an instrumentation grant (CHE-0946858) that supported
the purchase of the high resolution ESI mass spectrometer. We
acknowledge Song Yi for assistance with the preparation of the
image for the graphical table of contents.
EXPERIMENTAL
Chemicals and experimental techniques
Guests 1+ and 4+ were prepared according to published reports.[20,21]
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