Benzimidazole-based anion receptors: Tautomeric switching and selectivity

Article abstract of DOI:10.1039/c1ob06800h

Tautomeric switching is observed in a series of benzimidazole-based anion receptors upon addition of basic anions. An N-methylbenzimidazole based receptor selectively interacts with dihydrogen phosphate over a variety of other putative anionic guests via a combination of donated and accepted hydrogen bonds.

Full text of DOI:10.1039/c1ob06800h

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PAPER
Benzimidazole-based anion receptors: tautomeric switching and selectivity†
Philip A. Gale,*a Jennifer R. Hiscock,a Noémie Lalaoui,a,b Mark E. Light,a Neil J. Wellsa and Marco Wenzela
Received 26th October 2011, Accepted 17th November 2011
DOI: 10.1039/c1ob06800h
Tautomeric switching is observed in a series of benzimidazole-based anion receptors upon addition of
basic anions. An N-methylbenzimidazole based receptor selectively interacts with dihydrogen phosphate
over a variety of other putative anionic guests via a combination of donated and accepted hydrogen
bonds.
water.35 We have previously explored benzimidazole moieties in
Introduction
receptors designed on complex ureas.36 We wished to study the
Anion complexation processes1–3 can often be more complex
effect of anions on a series of simple benzimidazole urea-based
than they first appear. For example, we have found examples of
deprotonation of receptors by basic anions4,5 (as have other
groups),6 deprotonation of bound anions by free anions in solu-
tion,7,8 and studied the effect of anions on tautomerism processes
in acridinone-based anion receptors.9
Imidazole and benzimidazole groups have been used in a
variety of anion receptors. Often these receptors contain an imi-
dazolium or benzimidazolium group in which the imidazole
nitrogen atoms are both covalently bound to sp3 hybridized
carbon atoms. In these systems the positively charged imidazo-
lium CH group functions as a hydrogen bond donor with guests
bound by a combination of hydrogen bonding and electrostatic
interactions.10–21
anion receptors. These systems were based on our diindolylurea
scaffold37 that has proven highly effective as a receptor for
phosphate anions. In the benzimidazole derivatives the urea
group will donate a hydrogen bond to the benzimidazole
nitrogen. We wished to ascertain the effect of anions on
these systems and in particular whether the receptors would
tautomerise to offer a convergent array of urea and imidazole
NH hydrogen bond donors to particular anions – potentially
the most basic anionic guests, increasing the selectivity for these
guests over other anionic species (Scheme 1).
Results and discussion
Less common are systems in which neutral imidazole or
benzimidazole units are employed as NH hydrogen bond donors.
In these systems, as noted by Causey and Allen in their 2002
report on the anion complexation properties of fluorescent
biimidazole diamides,22 tautomerism processes may affect the
nature of the hydrogen bonding array presented to an anionic
guest. Several receptors containing 2-aminobenzimidazole have
been reported which act as effective receptors for anions.23–28
Imidazoles, benzimidazoles and related compounds have also
been employed in colorimetric,29–31 fluorescent32 and electro-
chemically active33,34 anion sensors. Imidazoles have also been
employed by Merschky and Schmuck as an intramolecular
buffer in low-weight organocatalysts for phosphate hydrolysis in
Compounds 1 and 2 were synthesised by the reaction of
4-aminobenzimidazole with either pentafluorophenylisocyanate
or hexylisocyanate in pyridine, with the pure product obtained
via recrystallisation from methanol or ethyl acetate affording
yields of 76% and 46% for compounds 1 and 2 respectively.
Compound 3 was synthesised by the formation in dichloro-
methane of the carbonyldiimidazole-activated derivative of
4-aminobenzimidazole which was then reacted with a second
equivalent of the amine in pyridine. The product precipitated
from the reaction solution in 45% yield.
Compound 4 was synthesised by coupling 1-methyl-4-amino-
benzimidazole with triphosgene in a mixture of dichloromethane
and a saturated aqueous solution of NaHCO3. Two crops of the
product were isolated, the first by precipitation from the reaction
mixture. The remaining solution was reduced in vacuo and the
residue recrystallised from methanol affording the product as a
white crystalline material in 42% overall yield.
aChemistry, University of Southampton, Southampton, UK, SO17 1BJ.
E-mail: philip.gale@soton.ac.uk; Fax: +44 23 8059 3332;
Tel: +44 23 8059 6805
bUniversité Paris Descartes, CNRS UMR 8601, Laboratoire de Chimie
et de Biochimie pharmacologiques et toxicologiques, 45, rue des
Saint-Pères, 75270 Paris Cedex 06, France
Compound 5 was synthesised via the formation of an activated
carbonyldiimidazole derivative of 7-aminoindole. This com-
pound was reacted with 1-methyl-4-aminobenzimidazole in a
chloroform, triethylamine and dimethylformamide solution. The
product was isolated by precipitation with water and hexane in a
yield of 27%.
†Electronic supplementary information (ESI) available: Experimental
details and characterisation data. CCDC reference numbers
851118–851121. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c1ob06800h
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The interactions of compounds 1–5 with various anionic
1
guests were studied by H NMR titration techniques in DMSO-
d6/0.5% water solution. Compound 1 was studied with a variety
of anions added as tetrabutylammonium salts. Upon addition of
chloride, continuous downfield shifts of one of the urea NH
resonances and the benzimidazole NH were observed whilst the
signal from the second urea NH group does not shift (Fig. 1).
This is evidence that leads us to suggest that the receptor adopts
a conformation in which the urea NH2 forms an intramolecular
hydrogen bond to the benzimidazole nitrogen atom leaving the
other NH groups to interact weakly via a single hydrogen bond
with added chloride (Fig. 2).
Fig. 1 1H NMR titration of compound 1 with tetrabutylammonium
chloride.
Scheme 1 A dibenzoimidazoleurea compound binds anion A1 via two
hydrogen bonds but anion A2 via four. The tautomerism triggered by A2
increases the number of hydrogen bond donors offered to the anion and directs
the lone pairs on the benzimidazole nitrogen atoms away from the anion-
binding site so increasing the affinity of the receptor for A2 over that of A1.
Fig. 2 The conformation of compound 1 in the presence of chloride.
HMBC NMR solution studies of compound 1 in DMSO/0.5%
water provide further evidence that this conformation is adopted
in solution. In the absence of anionic guests the benzimidazole
NH proton (12.5 ppm) and the urea NH1 proton (9.3 ppm)
appear as broad resonances which do not couple to any carbon
nuclei. This is evidence that these NH groups are freely exchan-
ging. However the urea NH2 proton (9.2 ppm) appears as a
sharp resonance that couples to various carbon nuclei. This indi-
cates that this NH is not freely exchanging. The most likely
reason for this is the formation of an intramolecular hydrogen
bond from the urea NH to the adjacent benzimidazole nitrogen
(analogous to that shown in Scheme 1). Upon addition of one
equivalent of tetrabutylammonium sulfate, significant changes
are observed in the HMBC spectrum (see ESI†). Both urea NH
resonances have shifted downfield, broadened and resonate at the
same chemical shift (11.7 ppm). The urea NH2 no longer
couples to any carbon signals and the benzimidazole NH reson-
ance has also shifted downfield (14.2 ppm). Taken together, this
is evidence that leads us to suggest all three of the NH groups
are involved in hydrogen bonding to sulfate with the broad
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resonances indicating that there is no longer any intramolecular
hydrogen bonding.
By increasing the temperature at which the NMR experiments
with compound 1 were performed, larger shifts of the NH reson-
ances were observed upon addition of chloride. This is presum-
1
ably due to a change of binding mode. The H NMR spectra of
compound 1 in a DMSO-d6/H2O 0.5% solution at various
temperatures are shown in Fig. 3. As the temperature is increased
a
broadening is observed with all three NH signals
(9.0–13.0 ppm), which may be indicative of the intramolecular
hydrogen bonds breaking and the NH groups consequently
1
freely exchanging. To test this theory the H NMR titration with
tetrabutylammonium chloride was repeated at 333 K. In this case
the urea NH1 proton resonance broadens out and cannot be fol-
lowed however both the benzimidazole and urea NH2 proton
resonances can be followed during the titration. This time unlike
the room temperature experiments shown in Fig. 1 the urea NH2
proton shifts downfield at the same rate as the benzimidazole
NH proton. This is evidence that both NH groups are involved in
forming the receptor : anion hydrogen bonded complex and that
intramolecular hydrogen bonding and the tautomeric event have
a limited effect on the anion binding process at this temperature
(see ESI†).
Fig. 4 The single crystal X-ray structure of compound 1. Non-acidic
hydrogen atoms have been omitted for clarity. Data were collected on a
Rigaku AFC12 and Saturn724+ mounted at the window of RA-FRE+
HFM with Varimax optics; standard structure solution and refinement
procedures were followed.37
conformation adopted in the solid state matches the confor-
mation of the compound in solution in the absence of anions.
Upon addition of acetate to the NMR solution of the receptor,
all three of the NH resonances broaden. After the addition of one
equivalent of acetate a single NH peak begins to reappear and
sharpens with further equivalents of anion. Although this makes
it impossible to follow the NH resonances it is possible to follow
the aromatic CH resonances of the benzimidazolium group as
can been seen from the stack plot in Fig. 5. This shows a typical
titration profile for the formation of a 1 : 1 complex and is also
supported by the Job plot analysis. This is evidence that a differ-
ent mode of binding occurs upon addition of acetate than was
observed with the addition of the chloride anion. This could be
due to the more basic acetate anion binding favourably to the
other tautomer of the receptor in which all three hydrogen bonds
are directed to form a single anion-binding site (Fig. 6).
Fig. 3 1H NMR spectra of compound 1 in DMSO-d6/H2O 0.5% at
variable temperatures.
A single crystal X-ray structure of receptor 1 was obtained
from a crystal grown from a solution of the compound in
DMSO.‡ It shows that in the solid state in the absence of anions
the receptor adopts a conformation with the benzimidazole NH
group oriented away from the urea group (Fig. 4). This com-
pound forms a dimer, held together by the formation of inter-
molecular hydrogen bonds N1⋯O11 2.825(2) Å; N12⋯O11
3.037(2) Å; N13⋯N24 2.881(3) Å; N21⋯O21 2.956(2) Å;
N22⋯O21 2.752(2) Å; N23⋯O11 2.821(2) Å. Thus the
‡Crystal data for compound 1: M = 342.24, Monoclinic, a = 16.627(4),
b = 8.9654(18), c = 17.878(4) Å, α = 90.00°, β = 93.051(4)°, γ =
90.00°, U = 2661.3(10) Å3, T = 100(2)K, space group P21/n, Z = 8,
μ(Mo K\α) = 0.160 mm−1, 18 568 reflections measured, 6004 unique
reflections (Rint = 0.0351). The final R1 values were 0.0571 (I > 2σ(I)).
The final wR(F2) values were 0.1081 (I > 2σ(I)). The final R1 values
were 0.0680 (all data). The final wR(F2) values were 0.1130 (all data).
The goodness of fit on F2 was 1.143.
Fig. 5 1H NMR stack plot of compound 1 vs. tetrabutylammonium
acetate in DMSO-d6/H2O 0.5%.
Addition of benzoate under the same conditions resulted in a
slow exchange process on the NMR timescale with resonances
for the free ligand broadening and disappearing and new broad
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tautomerism processes occurring within the receptor. Although
these sigmoidal curves can be seen more clearly with these par-
ticular anions the sigmoidal binding curves are also observed
with acetate and benzoate and to some extent bicarbonate with
the remaining compounds 2–5. Sigmoidal curves are not
observed with anions that have a lower affinity for the receptors
such as hydrogen sulfate, nitrate and chloride. Presumably these
less basic anions do not induce the tautomeric switch. Addition-
ally, slow exchange processes occur most commonly with dihy-
drogen phosphate, sulfate and benzoate anions with compounds
1–4. Some of these examples such as compound 2 and sulfate
show several different processes reflected in a combination of
slow and fast exchange in the 1H NMR titration experiments.
Compound 2 contains a hexyl chain instead of a pentafluoro-
phenyl group. The interaction of this receptor with chloride was
very similar to that of compound 1. In general the interaction
with the other anions was similar to that of compound 1 however
in a number of cases more significant broadening of the NH
resonances and signal overlap of the urea NH and aromatic CH
resonances made it impossible to follow these resonances
throughout parts of the titration experiments.
Fig. 6 Proposed mode of binding of acetate in DMSO-d6/H2O 0.5%
solution.
resonances for the benzoate complex appearing over the course
of the titration. In this case all three of the NH groups had
shifted downfield and hence we believe that the same binding
mode observed with acetate occurs in this case.
Crystals of the benzoate complex of compound 1 were grown
by slow evaporation of a methanol solution of the receptor in the
presence of excess tetrabutylammonium benzoate.§ The structure
(Fig. 7) reveals that in the solid-state benzoate is bound to the
tautomer in which the benzimidazole NH is convergently
directed with the urea NH groups N1⋯O3 2.799(5); N2⋯O3
2.795(6); N3⋯O2 2.768(5); N3⋯O3 3.397(6) Å. Two of these
complexes are held together by hydrogen bonding interactions
with water molecules to create a dimer containing receptor :
anion : water 2 : 2 : 2 O4⋯N4 2.903(6); O4⋯O2 2.878(5) Å.
Compound 3 again showed a similar interaction with chloride
although in this case as both urea NH groups are involved in
intramolecular hydrogen bonding, the anion was observed to
only interact with the benzimidazole NH groups (i.e. continuous
downfield shifts of the benzimidazole NH groups were observed
upon addition of chloride whilst the urea NH group resonance
was not perturbed).
Upon addition of acetate the benzimidazole NH resonances
broaden immediately but do not shift. Between one and two
equivalents of acetate the urea NH resonances also broaden into
the baseline and then re-appear shifted downfield as 2 equiva-
lents of acetate are added. This may be indicative of initial com-
plexation to the benzimidazole NH groups which at a certain
tipping point switches over to a 1 : 1 complex with the anion
bound to the urea NH groups and the benzimidazole NH groups
(Scheme 2). Interestingly, benzoate behaves in a similar
fashion to chloride in this case with interactions only with the
benzimidazole NH groups.
Fig. 7 The single crystal X-ray structure of the benzoate complex of
compound 1. Non-acidic hydrogen atoms and the tetrabutylammonium
counter cation have been omitted for clarity. Data were collected on a
Rigaku AFC12 and Saturn724+ mounted at the window of RA-FRE+
HFM with Varimax optics; standard structure solution and refinement
procedures were followed.37
Proton NMR titration studies show that tetrahedral or pseudo-
tetrahedral anions such as sulfate or dihydrogen phosphate bind
to the three NH groups, presumably in the convergent hydrogen
bond donor tautomer. The proton NMR titration curves are
sigmoidal in these cases. This is presumably related to the
Scheme 2 An equilibrium between acetate complexed to the benzimi-
dazole NH groups and acetate bound in a convergent array of four
hydrogen bond donors by the alternate tautomer.
§Crystal data for the benzoate complex of compound 1: M = 723.82,
Monoclinic, a = 21.464(15), b = 8.350(6), c = 20.873(14) Å, α =
90.00°, β = 98.375(12)°, γ = 90.00°, U = 3701(4) Å3, T = 100(2)K,
space group P21/c, Z = 4, μ(Mo K\α) = 0.102 mm−1, 24 627 reflections
measured, 6506 unique reflections (Rint = 0.0982). The final R1 values
were 0.1173 (I > 2σ(I)). The final wR(F2) values were 0.1791 (I >
2σ(I)). The final R1 values were 0.1445 (all data). The final wR(F2)
values were 0.1917 (all data). The goodness of fit on F2 was 1.269.
Addition of sulfate or dihydrogen phosphate results in a slow
exchange complexation process on the NMR timescale indicative
of the formation of a 1 : 1 complex. The 1H NMR spectra for the
sulfate titration are shown in Fig. 8.
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Crystals of a methanol solvate of compound 4 were grown by
slow evaporation of a methanol solution containing compound
4.¶ The structure (Fig. 11) reveals that in the solid-state the
methanol molecules are bound to the receptor through both the
hydrogen bond donating and accepting groups within the recep-
tor’s cleft, this produces a dimer supported by the formation of
CH hydrogen bonds N3⋯O2B 2.850(3); N3⋯O2A 2.986(3);
N4⋯O2B 2.856(3); N4⋯O2A 2.970(3); O2A⋯N5 2.825(3);
O2B⋯N5 2.767(3); C16⋯N2 3.402(2) Å.
Fig. 8 1H NMR titration with compound 3 vs. tetrabutylammonium
sulfate shows slow exchange on the NMR timescale.
By adding two equivalents of HPF6 to the NMR titration solu-
tion and then repeating the NMR titrations a contrasting set of
results were obtained. The now diprotonated receptor binds
chloride more strongly via the urea NH groups (and presumably
the benzimidazole NH groups) and anions such as nitrate which
do not interact with the other receptors now form weak com-
plexes with the protonated system. More complex 1H NMR titra-
tion profiles are obtained with more basic anions possibly due to
proton transfer from the protonated receptor to the added anion.
Compound 4 is analogous to compound 3 except that the ben-
zimidazole groups are methylated. This removes the possibility
of these groups donating hydrogen bonds. Proton NMR titrations
with a variety of anionic guests shows remarkably high selectiv-
ity for dihydrogen phosphate (Fig. 9). Presumably the anion
donates two hydrogen bonds to the benzimidazole groups as
shown in Fig. 10.
Fig. 11 The single crystal X-ray structure of the methanol complex of
compound 4. Non-acidic hydrogen atoms and parts of the disordered
methanol molecule have been omitted for clarity. Data were collected on
a Bruker Nonius KappaCCD with a Mo rotating anode generator, stan-
dard structure solution and refinement procedures were followed.
A second crystal structure containing a pentameric water
cluster was obtained by slow evaporation of a DMSO solution
containing compound 4.k This water cluster was found to be
supported by intermolecular hydrogen bond formation between
the water molecules themselves and two receptor molecules
N5⋯O2W 2.822(2); N6⋯O2W 3.254(2); O1W⋯O3W 2.792(2);
O3W⋯N2 2.896(2); O1W⋯N3 3.086(3); O2W⋯O1W 2.825
(3); O2W⋯N2 3.084(2) Å. Fig. 12 shows this pentameric water
cluster while Fig. 13 shows a bottom view of the structure.
Compound 5 contains one benzimidazole and one indole
group and was found to bind anions with 1 : 1 stoichiometry.
The titration curves that were obtained with this receptor
and various anionic guests are more typical of simple 1 : 1
Fig. 9 1H NMR titrations with compound 4 and a variety of anionic
guests added as the tetrabutyl/tetraethyl ammonium salts following the
urea NH.
¶Crystal data for the methanol solvate of compound 4: M = 352.40,
Monoclinic, a = 7.2914(2), b = 13.7723(3), c = 17.2777(4) Å, α =
90.00°, β = 99.684(2)°, γ = 90.00°, U = 1710.29(7) Å3, T = 120(2)K,
space group P21/c, Z = 4, μ(MoKα) = 0.094 mm−1, 18 453 reflections
measured, 3025 unique reflections (Rint = 0.0426). The final R1 values
were 0.0444 (I > 2σ(I)). The final wR(F2) values were 0.1151 (I >
2σ(I)). The final R1 values were 0.0523 (all data). The final wR(F2)
values were 0.1222 (all data). The goodness of fit on F2 was 1.024.
kCrystal data for the water solvate of compound 4: M = 730.8, monocli-
nic, a = 29.489(5), b = 7.2109(11), c = 18.264(3) Å, α = 90.00°, β =
119.114(8)°, γ = 90.00°, U = 3392.9(9) Å3, T = 120(2)K, space group
C 2/c, Z = 4, μ(MoKα) = 0.104 mm−1, 9118 reflections measured, 3888
unique reflections (Rint = 0.0365). The final R1 values were 0.0567 (I >
2σ(I)). The final wR(F2) values were 0.1514 (I > 2σ(I)). The final R1
values were 0.0693 (all data). The final wR(F2) values were 0.1606 (all
data). The goodness of fit on F2 was 1.080.
Fig. 10 The proposed binding mode of compound 4 with dihydrogen
phosphate.
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phosphate, sulfate, acetate and benzoate anions to the receptor
solutions, indicative of the tautomeric switch. However this is
not observed with the addition of chloride or nitrate anions
which have a weaker affinity to the receptor and therefore do not
initiate the switch. Interestingly there is no evidence of this event
in the titration curve of 5 with bicarbonate. As with compound 4
and dihydrogen phosphate this is most likely due to both the
receptor and the anion being able to form a combination of
hydrogen bond donating and accepting interactions. This may
also be the case for compound 2 with the addition of this anion.
It is because of these more simplistic results that we have found
it acceptable to calculate binding constants from these two
sets of titration data to give binding constants of 130 M−1 and
422 M−1 for compounds 2 and 5 respectively.39
Conclusions
Tautomeric switching of this type of benzimidazole-based anion
receptor offers a method of reducing the affinity for certain
anionic guests which do not trigger the switch whilst still
strongly interacting with more basic anions which do bind to the
alternate tautomer. Methylation of these systems provides recep-
tors that cannot switch and consequently show selectivity
towards anions which can both accept and donate hydrogen
bonds. We are continuing to explore the chemistry of these inter-
esting receptor systems.
Fig. 12 The single crystal X-ray structure of compound 4. Non-acidic
hydrogen atoms have been omitted for clarity. Data were collected on a
Rigaku AFC12 and Saturn724+ mounted at the window of RA-FRE+
HFM with Varimax optics; standard structure solution and refinement
procedures were followed.37
Acknowledgements
We thank the EPSRC for postdoctoral fellowships (JRH and
MW) and for access to the crystallographic facilities at the
University of Southampton.
Notes and references
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