Guanidinium-based artificial receptors for binding orthophosphate in aqueous solution

Article abstract of DOI:10.1002/ejoc.201301867

Receptors based on guanidinium- and pyrrole-containing binding sites were designed and synthesized for selective recognition of orthophosphate in aqueous solution. Various analytical techniques were used to understand solution behavior and anion binding p

Full text of DOI:10.1002/ejoc.201301867

Job/Unit: O31867
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Date: 27-02-14 12:40:07
Pages: 8
FULL PAPER
DOI: 10.1002/ejoc.201301867
Guanidinium-Based Artificial Receptors for Binding Orthophosphate in
Aqueous Solution
Evgeny A. Kataev,*[a] Christoph Müller,[a] Grigory V. Kolesnikov,[b] and
Victor N. Khrustalev[b]
Keywords: Supramolecular chemistry / Molecular recognition / Receptors / Anions / Phosphate
Receptors based on guanidinium- and pyrrole-containing
binding sites were designed and synthesized for selective re-
cognition of orthophosphate in aqueous solution. Various an-
alytical techniques were used to understand solution behav-
ior and anion binding properties of the receptors in methanol
and methanol/water mixtures. Binding studies revealed that
the structure of pyrrole-containing binding sites has great in-
fluence on selectivity of receptors for orthophosphate. The
receptor with bulky substituents in the meso-position of the
dipyrrolylmethane core demonstrated the highest selectivity
for orthophosphate over other inorganic anions.
methane- and bipyrrole-based acyclic and macrocyclic re-
ceptors bind orthophosphate with high selectivity, albeit in
organic solvents.[11] In the present work, we have combined
dipyrrole binding units and two guanidinium fragments to
provide heterotopic interactions in the receptor-anion com-
plex and achieve selectivity and efficient coordination of or-
thophosphate in aqueous solution. Various analytical tech-
niques were used to understand the binding behavior of the
receptors in different methanol/aqueous buffer mixtures,
and to understand the influence of structure of the pyrrole-
containing binding site on selectivity of the receptors.
Introduction
Recognition of phosphates has attracted considerable at-
tention in recent years primarily because phosphates are an
essential part of nature and living organisms.[1] Most of the
mined orthophosphate is used in fertilizer products, thus
there is a demand to detect, bind and recycle orthophos-
phate in water.[2] This goal can be achieved through the de-
sign of artificial receptors.[3] Although a number of recep-
tors already developed effectively bind orthophosphate,[3a]
there is still a great challenge to bind orthophosphate selec-
tively and directly in aqueous solution, so that real-world
applications can be possible.[4] This challenge is part of an
intriguing research field, namely supramolecular chemistry
in water.[5] Known approaches to bind orthophosphate in
water include receptors that interact with the anion through
hydrogen bonds,[6] electrostatic interaction provided by am-
monium[2,7] or guanidinium groups,[8] and metal cations.[9]
Heterotopic receptors, which bind the anion through sev-
eral different supramolecular interactions, are also proven
as selective binders.[10] To the best of our knowledge, none
of these receptors possesses high selectivity for phosphate
in aqueous solution over general inorganic anions.
Results and Discussion
Synthesis and Structure of Receptors
Target receptors 1, 2a and 2b were constructed from 3,3Ј-
dimethyl-4,4Ј-diphenyl-2,2Ј-bipyrrole,[11b] bis(3-methyl-4-
phenylpyrrol-2-yl)(dimethyl)methane and bis(pyrrol-2-yl)-
(diprop-1-yl)methane,[12] to which (2-aminobenzyl)guanid-
inium arms were attached (Figure 1). The synthesis of re-
ceptors was accomplished by acylation of tert-butyl carb-
amate (BOC)-protected (2-aminobenzyl)guanidine with the
corresponding in situ generated dicarboxylic acid chlorides
of the pyrrole fragments, followed by subsequent deprotec-
tion of the BOC-groups in the presence of trifluoroacetic
acid (TFA; Figure 1). Therefore, TFA salts of receptors 1,
2a and 2b were used in binding studies. Three different pyr-
role-containing binding sites were chosen with the aim to
vary rigidity of the structure and to elucidate the effect of
substituents in the pyrrole ring and in the meso-position of
the dipyrrolylmethane core on the binding properties of the
receptors.
Non-metallic artificial receptors are usually more specific
in their recognition of target species relative to metal-based
receptors, although they often have lower affinity.[3a] Re-
cently, we have successfully shown that neutral dipyrrolyl-
[a] Institute of Chemistry, Faculty of Natural Sciences, Technische
Universität Chemnitz,
09107 Chemnitz, Germany
E-mail: evgeny.kataev@chemie.tu-chemnitz.de
http://www.tu-chemnitz.de/chemie/supchem/
[b] A. N. Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences,
Vavilov st. 28, Moscow 119991, Russian Federation
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301867.
Attempts to grow crystals were successful only for acid-
free 2a (Figure 2) crystallized from chloroform and for 2b
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E. A. Kataev, C. Müller, G. V. Kolesnikov, V. N. Khrustalev
FULL PAPER
Figure 1. Synthesis and structures of the investigated receptors and 5(6)-carboxyfluorescein used for the indicator displacement assay.
(Figure 3) crystallized from methanol in the presence of a dimer and the trifluoroacetate anion. It is suggested that 2a
6-fold excess of tetrabutylammonium dihydrogenphosphate. does not form a dimer because of the bulky phenyl substitu-
To our surprise, only three of the trifluoroacetate anions ents in the 4-position of the pyrrole ring and the neutral
were exchanged with dihydrogen phosphate, whereas 2b was charge. The interesting structure shown in Figure 3 provides
found in a self-associated form held by six H-bonds be- evidence that strong self-association is a feature of our re-
tween the receptor molecules and two H-bonds between the ceptors at concentrations above ca. 10–4 m in methanol.
Finding Optimal Conditions for Measurements
1
The properties of the receptors were investigated by H
NMR, UV/Vis, and fluorescence spectroscopic titrations in
pure methanol, a 1:1, and 1:9 methanol/buffer (10 and
100 mm acetate buffer pH 6.0) mixtures. The latter method
was possible only for receptor 1 because the conjugated bi-
pyrrole subunit emits light at 420 nm under excitation at
343 nm. Dilution of receptors in different solvent mixtures
and following the absorption spectra of these solutions
showed that aggregation is present for all receptors in meth-
anol. In contrast, in water/methanol mixtures no aggrega-
tion was observed for 2a and 2b up to 10–3 m. Receptor 1
aggregated in all solvent mixtures, but less in aqueous solu-
tions, probably because of the lower solubility relative to
dipyrrolylmethane-based hosts. Dilution of the bipyrrole re-
ceptor was additionally followed by fluorescence spec-
troscopy and showed linear behavior between 7ϫ10–5–
10–7 m (Figure S20, see the Supporting Information). Such
behavior of receptor 1 is identical to that of recently investi-
gated bipyrrole-based receptors in chloroform and dimethyl
sulfoxide solutions.[13]
Figure 2. Pov-ray rendered molecular structure of acid free receptor
2a according to single-crystal X-ray analysis. Ellipsoids are shown
in 50% probability level. Most of the hydrogen atoms and solvent
molecules are omitted for clarity.
We also conducted time-dependent absorption and fluo-
rescence measurements of the receptors in water/methanol
mixtures, which are important experiments to check for and
obtain reliable results. For example, the absorption of recep-
tors in a 10% methanol/buffer solution slowly decreases
over 60 min indicating slow aggregation/deaggregation pro-
cesses. If the portion of methanol is increased to 50%, the
fluorescence and absorption intensities stay constant and
reproducible. Acetate buffer at 100 mm concentration dem-
onstrated better pH stability in the presence of phosphate
anions relative to 10 mm acetate buffer. Thus, the best con-
ditions for the determination of affinity constants are a 1:1
methanol/100 mm acetate buffer (pH 6.0) mixture.
Figure 3. Pov-ray rendered molecular structure of the product of
complexation between 2b and (TBA)H2PO4 according to the sin-
gle-crystal X-ray analysis. Ellipsoids are shown in 30% probability
level. One of the receptor molecules is highlighted in yellow for
clarity. Most of the hydrogen atoms, solvents and one dihydrogen
phosphate anion are omitted for clarity.
2
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Guanidinium-Based Artificial Receptors for Binding Orthophosphate
Anion Binding in a 1:1 Methanol/Buffer Solution
UV/Vis titrations of receptors with different anions were
carried out under the same conditions (1:1 methanol/buffer
mixture). Spectral changes showed clean isosbestic points.
However, much smaller changes of absorption were ob-
served relative to the titrations conducted by us with other
pyrrole-based receptors in organic solvents.[11] The obtained
data was sufficient to calculate binding constants with good
precision. Small changes in absorbance were attributed to
small changes in electronic properties of the receptor upon
binding in an aqueous solution. Because both water and
anions interact with the receptor by means of hydrogen
bonds and the receptor is strongly solvated, the total
change in the electronic surroundings of the pyrrole chro-
mophores upon anion binding is minimal. Several groups,
that used UV/Vis titrations in aqueous solution, reported
similarly small changes.[14]
Figure 4. Binding isotherms (experimental values and fitting
curves) obtained from UV/Vis titrations of receptors 1, 2a and 2b
with phosphate (Pi) and pyrophosphate (PPi) anions in a 1:1 mix-
ture of 100 mm pH 6.0 acetate buffer/methanol at 23 °C. Anion
concentration is 0.0006 m. Absorption spectra were normalized to
start at the same value.
Interestingly, the changes in absorption spectra were in-
duced only by iodide, oxalate, phosphate and pyrophos-
phate, among other studied inorganic anions [Cl–, Br–, I–,
To obtain additional evidence for the strong affinity of
–
2–
–
,
–, SO42–, ClO4 , (COO)22–]. This fact is in
1
receptor 2b for orthophosphate we conducted H and 31P
NO3 , CO3
agreement with the precipitation trend of receptor-anion
complexes from a 1:9 (v/v) methanol/buffer solution. Ac-
cording to the Job’s Plot analysis the coordination of ortho-
phosphate occurs in a stepwise 1:1 and 1:2 stoichiometry,
whereas pyrophosphate binds in a 1:1 stoichiometry. This
fact is in agreement with the charge state of orthophos-
NMR titration experiments in a 1:1 CD3OD/D2O (100 mm
acetate buffer, pH6.0) mixture at a receptor concentration
of 10–3 m (Figures 5 and 6). Addition of sodium phosphate
to the receptor led to precipitation of the host-guest com-
plexes. Thus, the titrations were conducted with the help
of tetrabutylammonium dihydrogen phosphate to keep the
formed host-guest complexes soluble. In the proton spectra
most signal shifts were less than 0.01 ppm. However, the
shifts of the methylene group, which connects the guanid-
inium and the phenylene fragments, allowed us to estimate
the approximate value of the first binding event, which is
about Ͼ104 m–1. A more accurate calculation of the K11
binding constant was obtained by fitting the measured val-
ues from 31P NMR titrations, in which the shifts of the
orthophosphate signal were much stronger (Ͼ0.1 ppm).
The analysis of two independent measurements with the
help of the HypNMR program resulted in a binding con-
stant logK11 = 4.53Ϯ0.1. The second binding event could
only be roughly assessed with the value less than 2 logarith-
mic units. The obtained data is in good agreement with the
results calculated from UV/Vis and fluorescence titration
–
phate, which exists in a diprotonated (H2PO4 ) form at pH
6.0. The obtained titration curves were fitted by using the
HypSpec program (Table S1, see the Supporting Infor-
mation).[15] As can be seen from Table 1, the three receptors
bind phosphate anions with affinity ≈ 105 m–1. Other anions
induced negligible spectroscopic changes, except iodide
(logK11 = 2.12Ϯ0.03) and oxalate (logK11 = 3.89Ϯ0.14,
logK12 = 3.16Ϯ0.15). Interestingly, receptor 2b showed a
particular affinity for binding orthophosphate at almost
three orders of magnitude higher than pyrophosphate. This
can be clearly seen from the analysis of binding isotherms
of the receptors in Figure 4.
Table 1. Binding constants (logK) were determined by UV/Vis ti-
trations in a 1:1 mixture of 100 mm pH 6.0 acetate buffer/methanol experiments.
at 23 °C.
Based on the analysis of the X-ray structure of receptor
2b, we propose that the reason for the high selectivity of 2b
for orthophosphate is the orientation of pyrrole-binding
sites induced by bulky substituents in the meso-position of
dipyrrolylmethane. Receptors 1 and 2a, in contrast to 2b,
are not preorganized for binding because of the presence of
bulky phenyl groups, which orient the pyrrole-binding sites
in opposite directions. We used a similar approach for in-
creasing binding selectivity in the design of a sulfate-selec-
tive neutral receptor, in which we introduced a p-tolyl sub-
stituent in the meso-position of the dipyrrolylmethane
core.[16] However, in the latter case the meso-p-tolyl di-
pyrrolylmethane-based receptor was not stable in air be-
cause of slow oxidation to dipyrrolylomethene. Thus, meso-
Host/Guest
1
2a
2b
Orthophosphate– UV/Vis 5.84Ϯ0.04[a,d] 4.99Ϯ0.03[a,d] 6.01Ϯ0.03[a,d]
Fluo.
5.32Ϯ0.03[b] 4.12Ϯ0.01[b] 5.00Ϯ0.02[b]
5.58Ϯ0.03[c]
Pyrophosphate
UV/Vis 4.45Ϯ0.03[a] 4.00Ϯ0.01[a] 3.11Ϯ0.02[a]
Fluo.
n.d.[b]
3.91Ϯ0.02[b] 3.63Ϯ0.02[b]
4.16Ϯ0.02[c]
[a] Values were obtained from UV/Vis titration. [b] Values were ob-
tained from fluorescence indicator displacement measurements
with 5(6)-carboxyfluorescein; n.d.: no changes were detected upon
titration with anions. [c] Values were obtained from direct fluores-
cent titrations of receptor 1 (em. 420 nm, ex. 343 nm). [d] For all
receptors the second binding constant (K12) was too low to assess
it by using the HypSpec program.
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E. A. Kataev, C. Müller, G. V. Kolesnikov, V. N. Khrustalev
FULL PAPER
ments in pure methanol, binding constants for receptor 1
were high even for hydrogen sulfate, e.g. affinities of 1 for
(TBA)H2PO4 and (TBA)HSO4 are 105 and 106 m–1, respec-
tively (Figure S23, see the Supporting Information).
1H NMR titrations carried out for 2b in CD3OD resulted
in lower affinity (104 m–1) for (TBA)H2PO4 relative to spec-
troscopic titrations because dimerization is present at higher
concentrations. NMR titrations also provided evidence of
the strong interaction of phosphate with all NH-donor sites
in the receptor in the presence of an excess of the anion.
Upon titration of 2b with (TBA)H2PO4 the strongest NMR
signal shifts were observed for the methylene group, that
bridges the phenylene ring and the guanidinium group, as
well as for hydrogen atoms on the phenylene ring. The sig-
nals of the phenylene ring observed as a multiplet split into
four signals indicating a destruction of the dimer and bind-
ing of phosphate. The pyrrole CH-signals were strongly
broadened after addition of phosphate indicating that coor-
dination of phosphate to pyrrole subunits occurs (Figure
S22, see the Supporting Information). The formation of
complexes was additionally supported by using MS mea-
surements. Major signals correspond to 1:1 and 1:2 com-
Figure 5. Experimental values and the fitting curve for 1H NMR
titration of 2b with tetrabutylammonium dihydrogenphosphate in
a 1:1 methanol/buffer mixture. The predicted changes in concentra-
tion [mol/L] of 1:1 and 1:2 receptor – orthophosphate (Pi) com-
plexes are also shown.
–
plexes of receptors with H2PO4 anion. For receptor 1 m/z
=
791.32 was observed for the 1:1 complex [1 –
–
2(CF3COOH) + 2H+ + H2PO4 ] and m/z = 911.27 for the
–
1:2 complex [1 – 2(CF3COOH) + 2H+ + 2(H2PO4 ) + Na+];
for receptor 2b m/z = 709.32 was observed for the 1:1 com-
–
plex [2b-2(CF3COOH) + 2H+ + H2PO4 ] and m/z = 829.29
–
for the 1:2 complex [2b-2(CF3COOH) + 2H+ + 2(H2PO4 )
+ Na+], respectively (Figure S24 and S25, see the Support-
ing Information).
Figure 6. Experimental values and the fitting curve for 31P NMR
titration of 2b with tetrabutylammonium dihydrogenphosphate in
a 1:1 methanol/buffer mixture.
Fluorescence Titrations in a 1:1 Methanol/Buffer Mixture
Direct fluorescence titrations and the fluorescent indi-
cator displacement assay were conducted to assess binding
affinities of receptors. Receptor 2a and 2b bind 5(6)-car-
boxyfluorescein (CF) in a 1:1 stoichiometry with binding
constants 104 m–1, whereas bipyrrole-based receptor 1
shows a slightly higher value of 105 m–1. The complexes be-
tween receptors and CF were generated in a 1:1 methanol/
buffer solution and titrated with different anions. The fluo-
rescence of the dye was quenched by receptors but restored
by addition of anions. From a series of anions, only phos-
phate and pyrophosphate were able to displace CF from the
complexes 1·CF, 2a·CF and 2b·CF. To obtain evidence for
the key role of the dipyrrolylmethane core in recognition,
we synthesized control compound 3. Two guanidinium
fragments were connected in this receptor through a flexible
adipic acid. According to fluorescence measurements, nei-
ther quenching upon mixing of 3 with CF, nor enhance-
ment of fluorescence upon addition of phosphate to com-
plex 3·CF was detected (Figure 7). This fact indicates that
3 does not bind CF under the conditions studied. Because
dipropyl-substituted dipyrrolylmethanes solve not only the
chemical stability problem, but also increase selectivity in
the binding of anions.
As can be seen from Figure 4, addition of orthophos-
phate to receptors 1 and 2b induces a decrease of absorp-
tion of the major band. However, changes in UV/Vis spec-
tra of receptor 2a are quite different from those of 1 and
2b. Addition of the first equivalent of orthophosphate leads
to a decrease, whereas further addition of the anion resulted
in an increase of absorption. Interestingly, addition of pyro-
phosphate induces only an increase in absorption. This ob-
servation allows one to suggest that conformational reorga-
nization of the receptor structure differs in case of ortho-
phosphate or pyrophosphate coordination.
Anion Binding in Methanol
The presence of water in solvent mixtures plays a crucial receptor 1 emits light upon excitation, it allowed us to ob-
role in determining the selectivity for phosphates. Accord- tain binding constants from direct titration. The binding
ing to UV/Vis and fluorescence spectroscopic measure- constants in general are of the same order of magnitude as
4
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Guanidinium-Based Artificial Receptors for Binding Orthophosphate
spectra were recorded with a Q-TOF 6540 UHD mass spectrometer
from Agilent. Melting points were obtained with a melting point
measuring device PHMK 74/0032 manufactured by “VEB Analytik
obtained from UV/Vis titrations and confirm that receptor
2b is selective for orthophosphate over pyrophosphate
(Table 1).
Dresden”. Fluorescence measurements were recorded with
a
FluoroMax® Spectrofluorometer from Horiba Scientific. UV/Vis-
measurements were recorded with an UV/Vis/NIR-Spectrometer
Lambda 900 manufactured by Perkin–Elmer. Particle size (DLS)
measurements were recorded with a Zetasizer Nanoseries Nano-
S90 DLS-device from von Malvern.
UV/Vis and Fluorescence Binding Studies: Anion titrations were
carried out by a stepwise addition of 10–3–10–4 m stock solution of
guest to a 10–5–10–6 m stock solution of receptor. The experimental
data were fitted to binding models by using the HYPERQUAD or
HypSpec program.[15] All the titration experiments were carried out
in 10 or 100 mm acetate (pH 6.0) buffer containing 50% v/v MeOH
for solubilizing of receptors. The complexes between receptors and
CF were generated prior to titration experiments.
Bis(pyrrol-2-yl)(diprop-1-yl)methane: According to a modified syn-
thesis,[17] the target dipyrrolylmethane was synthesized as follows.
To boiling water (100 mL), 4-heptanone (20 mL, 16.34 g,
0.143 mol) and concd. HCl (2 mL) were added. To this mixture
heated to reflux pyrrole (2.5 mL, 2.42 g, 0.036 mol) was added
dropwise. The reaction mixture was heated to reflux for three
hours. Then it was cooled to room temperature and extracted four
times with ethyl acetate (100 mL). The combined organic phases
were dried with potassium carbonate. After removal of the solid
residue most of the ethyl acetate was removed on the rotary evapo-
rator until a brown, oily slurry was obtained. The oily residue was
further dried to remove traces of ethyl acetate and 4-heptanone in
vacuo until an orange-brown solid was obtained. The material was
then distilled by using a short-path distillation apparatus and col-
lected in a trap cooled with liquid nitrogen. The crude product
was recrystallized from ethanol. The product was obtained as pale
orange crystals (542.3 mg, 13%). (A fast conversion to the more
stable bis-trichloro methylketone derivative is recommended.) 1H
NMR (CDCl3): δ = 0.86 (t, 6 H, J = 7.3 Hz), 1.03–1.16 (m, 4 H),
1.84–1.92 (m, 4 H), 6.08–6.11 (m, 2 H), 6.11–6.14 (m, 2 H), 6.59–
Figure 7. Analysis of quenching efficiency of receptors 1, 2b and 3
of 5(6)-carboxyfluorescein.
Conclusions
In summary, we have synthesized new receptors based on
guanidinium and pyrrole binding sites and evaluated their
binding properties towards inorganic anions in aqueous
solution. The binding properties were determined by using
NMR and fluorescence spectroscopy, mass spectrometry,
and an indicator displacement assay with CF. The receptors
showed a general preference to bind orthophosphate and
pyrophosphate. The structure of the dipyrrolylmethane core
has a significant influence on both the binding constants
and selectivity. Dipropyl-functionalized dipyrrolylmethane- 6.63 (m, 2 H), 7.66 (br. s, 2 H) ppm. 13C NMR (CDCl3): δ = 14.7,
17.3, 40.1, 42.9, 105.6, 107.5, 116.9, 137.4 ppm.
based receptor 2b demonstrates the highest selectivity for
orthophosphate. This property has been attributed to ap-
propriate orientation of the pyrrole-binding sites induced
by its steric interaction with propyl groups. Thus, new gua-
nidinium-based receptors possess high selectivity and bind
orthophosphate with at least three orders of magnitude
higher affinity than other inorganic anions. This is much
better selectivity than that reported in the literature for re-
ceptors containing only guanidinium binding sites.[3a] Cur-
rently we are investigating macrocyclic receptors, which are
more soluble in water and containing both guanid-
inium- and pyrrole-binding sites.
1-(2-Aminobenzyl)-2,3-di[(tert-butoxy)carbonyl]guanidine: To
a
solution of 2-aminobenzylamine (467 mg, 3.8264 mmol) was added
1,3-bis(-tert-butoxycarbonyl)-2-methyl-2-thiopseudourea (1.00 g,
3.4438 mmol, 0.9 equiv.). The solution was stirred for 18 h at room
temperature (caution! thiomethanol is set free). Next, the solvent
and thiomethanol were removed in vacuo at room temp. The ob-
tained solid was dried under high vacuum. The crude product was
purified by flash column chromatography on silica gel by using
CH2Cl2. The product is obtained as a second fraction after non-
reacted 1,3-bis(-tert-butoxycarbonyl)-2-methyl-2-thiopseudourea.
After the removal of the solvents, the product was obtained as a
pale yellow solid (930.1 mg, 74%), m.p. 122 °C. 1H NMR ([D6]-
DMSO): δ = 1.40 (s, 9 H), 1.46 (s, 9 H), 4.34 (d, J = 5.8 Hz, 2 H),
3
1
5.27 (s, 2 H), 6.50 (td, J = 7.4, J =. 1.2 Hz, 1 H), 6.63 (dd, J =
3
1
3
Experimental Section
8.0, J = 1.0 Hz, 1 H), 6.97 (td, J = 7.6, J = 1.4 Hz; 1 H), 7.05
(dd, 1J = 7.5, J = 1.4 Hz, 1 H), 8.57 (t, J = 5.8 Hz, 1 H), 11.47 (s,
3
Materials and Methods: All solvents were reagent grade and pur-
chased commercially. All commercial reagents were used in the
quality as purchased without further purification. NMR spectra
were recorded on with Varian UNITY INOVA 400 MHz spectrom-
eter. NMR spectra were referenced to the chemical shift of residual
signal of the NMR solvent. IR spectra were recorded with a FT-
IR spectrometer NICOLET IR 200 FT-IR from Thermo Electron.
All substances were measured in potassium bromide matrix. Mass
1 H) ppm. 13C NMR ([D6]DMSO): δ = 27.6, 28.0, 40.8, 78.3, 83.0,
115.0, 115.9, 120.9, 128.2, 129.5, 146.6, 152.0, 155.2, 162.9 ppm.
HRMS (ESI): calcd. for [C18H29N4O4]+ [M + H]+ 365.2183; found
365.2186. C 59.32, H 7.74, N 15.37; found C 59.29, H 7.718, N
15.33.
Compound 5b: To a suspension of 5,5Ј-(heptane-4,4-diyl)bis(1H-
pyrrole-2-carboxylic acid) (62.3 mg, 0.1975 mmol) in absolute
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E. A. Kataev, C. Müller, G. V. Kolesnikov, V. N. Khrustalev
FULL PAPER
CH2Cl2 under a nitrogen atmosphere a solution of oxalyl chloride
(0.49 mL, 0.7452 g, 5.871 mmol) in dry CH2Cl2 (3 mL) was added.
To this mixture three drops of absolute dimethylformamide were
added and the mixture was stirred at room temperature for 3.5 h.
The remaining oxalyl chloride and solvents were removed in vacuo
followed by drying at high vacuum at 35 °C for several hours. For
the following aminolysis the crude product from the first synthetic
step was used without further purification. The solid was dissolved
in absolute CH2Cl2 (5 mL) and added dropwise to a solution of 1-
(2-aminobenzyl)-2,3-di[(tert-butoxy)carbonyl]guanidine (156.9 mg,
0.4305 mmol), 4-(dimethylamino)pyridine (DMAP, 6 mg) and pyr-
idine (0.24 mL) in absolute CH2Cl2 (6.2 mL). The reaction mixture
was stirred under a nitrogen atmosphere at room temperature for
at least two days. Next, all volatiles were removed in vacuo. The
brown crude product was purified by flash column chromatography
with silica gel by using a mixture of 90% CH2Cl2 and 10% meth-
anol as eluent. The product was obtained as a cream-colored solid
+ 2H]2+. C47H48F6N10O6 (962.95): calcd. C 58.62, H 5.02, N 14.55;
found C 58.48, H 5.10, N 14.33. UV/Vis (λ = 312 nm): ε = 35200
Lmol–1 cm–1 {MeOH(50%)-[H2O(50%), pH 6.0, 100 mm acetate
buffer]}.
Compound 2b: Yield 59.1 mg, 89%, m.p. 146–149 °C (dec.). 1H
NMR (CD3OD): δ = 0.91 (br. s, 6 H), 1.14 (br. s, 4 H), 2.10 (br. s,
4 H), 4.39 (d, 4 H), 6.17 (br. s, 2 H), 6.98 (m, 2 H), 7.23–7.32 (m,
2 H), 7.33–7.44 (m, 6 H), 7.83 (br. s, 1 H), 10.36 (br. s, 1 H) ppm.
13C NMR (CD3OD): δ = 14.7, 18.5, 42.4, 44.9, 109.8, 113.5, 116.7,
119.7, 125.1, 127.7, 128.7, 128.9, 129.4, 135.1, 136.0, 143.6, 158.8,
162.6, 163.0 ppm. HRMS (ESI): calcd. for [C33H43N10O2]+ [M +
H]+ 611.3565; found 611.3565. calcd. for [C33H44N10O2]2+ [M +
2H]2+ 306.1822; found 306.1821. UV/Vis (λ = 287 nm): ε = 33800
Lmol–1 cm–1 {in MeOH(50%)-[H2O(50%), pH 6.0, 100 mm acetate
buffer]}.
Compound 1: Green powder (68.9 mg, 94%), m.p. 152–155 °C
(dec.). 1H NMR (CD3OD): δ = 1.98 (s, 6 H); 3.96 (br. s, 4 H);
7.22–7.35 (m, 6 H); 7.41–7.50 (m, 4 H); 7.50–7.60 (m, 8 H); 7.72
(t, J = 5.7 Hz, 1 H); 11.40 (br. s, 1 H) ppm. 13C NMR (CD3OD):
δ = 10.6, 42.1, 120.9, 123.2, 126.2, 126.5, 127.8, 128.3, 129.3, 129.7,
129.8, 130.4, 131.8, 131.9, 136.0, 136.3, 158.8, 162.2 ppm. HRMS
(ESI): calcd. for [C40H41N10O2]+ [M + H]+ 693.3408; found
693.3407. calcd. for [C40H41N10O2]2+ [M + 2H]2+ 347.1744; found
347.1747. C44H42F6N10O6 (920.87): calcd. C 57.39, H 4.60, N
15.21; found C 57.14, H 4.75, N 15.00. UV/Vis (λ = 287 nm): ε =
29500 Lmol–1 cm–1 {in MeOH(50%)-[H2O(50%), pH 6.0, 100 mm
acetate buffer]}.
1
(109.9 mg, 56%), m.p. 258 °C (dec.). H NMR ([D6]DMSO): δ =
0.89 (t, 6 H), 1.10 (m, 4 H), 1.26 (s, 18 H), 1.46 (s, 18 H), 2.09–
2.20 (m, 4 H), 4.44 (d, J = 5.8 Hz, 4 H), 5.96 (m, 2 H), 6.87 (m, 2
1
3
1
3
H), 7.19 (td, J = . 7.5, J = 1.2 Hz, 2 H), 7.29 (td, J = 7.7, J =
1
3
1
1.5 Hz, 2 H), 7.39 (dd, J = 7.6, J = 1.3 Hz, 2 H), 7.48 (dd, J =
7.9, 3J = 0.9 Hz, 2 H), 8.85 (t, J = 5.8 Hz, 2 H), 9.89 (s, 2 H), 11.16
(br. s, 2 H), 11.46 (s, 2 H) ppm. 13C NMR ([D6]DMSO): δ = 14.4,
16.8, 27.6, 27.7, 28.2, 42.2, 77.0, 78.6, 83.0, 106.1, 111.4, 125.2,
125.3, 126.1, 127.4, 128.8, 132.3, 135.8, 142.1, 151.8, 155.5, 159.4,
162.8 ppm. HRMS (ESI): calcd. for [C53H76N10O10]2+ [M + 2H]2+
506.2871; found 506.2871. calcd. for [C53H75N10O10]+ [M + H]+
1011.5662; found 1011.5670. C 61.85, H 7.44, N 13.61; found C
62.02, H 7.425, N 13.69.
Compound 3: The synthesis was conducted in two steps. In the first
step, adipic acid dichloride (0.018 mL, 0.1235 mmol) was added to
a solution of pyridine (0.022 mL) and BOC-protected amine
(100 mg, 0.274 mmol) in dry acetone (10 mL). The mixture was
stirred for 3 h and the solvent was evaporated. To the oily residue
CH2Cl2 (≈ 25 mL) was added and washed with NaHCO3 (1ϫ
25 mL) followed by washing with water (3ϫ 25 mL) and drying
with MgSO4. The product was further purified by using column
chromatography (CH2Cl2/MeOH 4%, v/v). To the resulting bisam-
ide , CH2Cl2 (10 mL) and TFA (5 mL) were added and the mixture
was stirred for 3 h. The solvent was evaporated. Overall yield 90%,
m.p. 197 °C. 1H NMR (CD3OD): δ = 7.84 (m, 1 H), 7.35–7.25 (m,
8 H), 4.35 (s, 4 H), 3.31 (m, 8 H) 1.84 (m, 4 H) ppm. 13C NMR
(CD3OD): δ = 175.4, 158.7, 136.1, 133.8, 129.7, 128.6, 128.5, 128.1,
42.5, 36.7, 26.4 ppm. HRMS (ESI): calcd. for C22H31N8O2 [M +
H]+ 439.2570; found 439.2571.
Compound 4: The synthesis of 4 was conducted analogously to the
synthesis of 5b. The crude product was purified by flash column
chromatography with silica gel by using a mixture of 85% CH2Cl2
and 15% ethyl acetate as eluent. The product was obtained as a
lemon-yellow solid in 60% yield, m.p. 275 °C (dec.). 1H NMR
([D6]DMSO): δ = 1.22 (s, 18 H), 1.47 (s, 18 H), 1.94 (s, 6 H), 3.74
(d, J = 5.4 Hz, 4 H), 7.09 (m, 2 H), 7.23 (m, 2 H), 7.26–7.33 (m, 4
H), 7.33–7.44 (m, 8 H), 7.84 (J = 8 Hz; 2 H, d), 8.66 (t, J = 5.7 Hz,
2 H), 9.21 (s, 2 H), 11.34 (s, 2 H), 11.51 (s, 2 H) ppm. 13C NMR
([D6]DMSO): δ = 10.8, 27.6, 27.7, 78.2, 83.0, 117.4, 123.6, 123.7,
123.9, 124.5, 126.6, 127.2, 127.3, 128.1, 129.2, 130.2, 130.5, 135.0,
136.1, 151.7, 154.7, 159.6, 162.5 ppm. HRMS (ESI): calcd. for
[C60H73N10O10]+ [M + H]+ 1093.5506; found 1093.5514. calcd. for
[C60H74N10O10]2+ [M + 2H]2+ 547.2793; found 547.2793. C 64.85,
H 6.71, N 12.60; found C 65.16, H 6.67, N 12.43.
CCDC-945174 (for 2a) and -945175 (for 2b) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of Compounds 1, 2a and 2b: BOC-protected starting mate-
rial (e.g. 4) (70 mg, 0.0692 mmol) was dissolved in CH2Cl2 (5 mL)
under a nitrogen atmosphere. To this solution trifluoroacetic acid
(2.5 mL) was added dropwise. The reaction mixture was stirred
overnight at room temperature under a nitrogen atmosphere. The
solvents were removed under high vacuum to obtain the target gua-
nidinium salt as a light-grey powder.
Supporting Information (see footnote on the first page of this arti-
1
cle): H NMR, 13C NMR and IR spectra for all new compounds,
detailed X-ray data figures, spectra from fluorescent and UV/Vis
titrations, figures from dilution experiments and mass spectra of
host–guest complexes.
Compound 2a: The synthesis of 2a was conducted without isolation
of BOC-protected intermediate 5a. Compound 5a was prepared
analogously to the synthesis of 5b and then deprotected by using
trifluoroacetic acid as described above. The crude product was pre-
cipitated from a CH2Cl2/diethyl ether solution, yield 87%. 1H
NMR (CDCl3): δ = 1.45 (s, 6 H), 2.08 (s, 6 H), 3.86 (d, 4 H), 7.10–
7.60 (m, 18 H), 7.89 (d, 2 H), 8.32 (s, 2 H), 10.70 (br. s, 2 H) ppm.
13C NMR ([D6]DMSO): δ = 9.9, 28.1, 31.1, 40.6, 115.5, 119.4,
125.0, 125.8, 127.2, 127.4, 128.2, 128.7, 129.8, 130.9, 130.9, 135.6,
135.8, 137.8, 157.4, 158.8, 159.0, 159.9 ppm. MS (ESI): 368.5 [2a
Acknowledgments
This work was financially supported by Fonds der Chemischen In-
dustrie, Deutsche Bundesstiftung Umwelt, Technische Universität
Chemnitz and COST action “Supramolecular Chemistry in water”
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Guanidinium-Based Artificial Receptors for Binding Orthophosphate
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Received: December 17, 2013
Published Online: ᭿
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7

Job/Unit: O31867
/KAP1
Date: 27-02-14 12:40:07
Pages: 8
E. A. Kataev, C. Müller, G. V. Kolesnikov, V. N. Khrustalev
FULL PAPER
Orthophosphate Recognition
Selective recognition of orthophosphate in
aqueous solution was achieved by receptors
based on guanidinium and pyrrole binding
sites. The structure of pyrrole-containing
binding sites has great influence on the se-
lectivity of receptors for orthophosphate.
The receptor with bulky substituents in the
meso-position of the dipyrrolylmethane
core demonstrated the highest selectivity
for orthophosphate
E. A. Kataev,* C. Müller, G. V. Kolesnikov,
V. N. Khrustalev ................................ 1–8
Guanidinium-Based Artificial Receptors
for Binding Orthophosphate in Aqueous
Solution
Keywords: Supramolecular chemistry
/
/
Molecular recognition
Anions / Phosphate
/
Receptors
8
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