An anion-modulated three-way supramolecular switch that selectively binds dihydrogen phosphate, H2PO4-

Article abstract of DOI:10.1002/anie.201302929

An anionic three-way switch: A bipyridyl bis(urea)-based anion receptor that is highly selective for dihydrogen phosphate demonstrates spectroscopically distinct anion-bound conformations toward halides and select oxoanions. ¹H NMR studies show the differing anion-induced conformations are reversible allowing this system to function as a three-way molecular switch. Copyright

Full text of DOI:10.1002/anie.201302929

.
Angewandte
Communications
DOI: 10.1002/anie.201302929
Molecular Devices
An Anion-Modulated Three-Way Supramolecular Switch that
À
Selectively Binds Dihydrogen Phosphate, H2PO4 **
Jesse V. Gavette, Nancy S. Mills, Lev N. Zakharov, Charles A. Johnson II, Darren W. Johnson,*
and Michael M. Haley*
Molecular switches have received considerable attention
owing to interest in studying the miniaturization of macro-
scopic machines and the desire to emulate biological sys-
tems.[1] A firm understanding of cationic molecular interac-
tions has resulted in numerous examples of supramolecular
switches in which cations serve as the major stimulus.[2] As the
subtle interactions of anions in supramolecular systems have
become better understood, anion-stimulated conformational
changes and supramolecular switches have also gained in
importance.[3]
Several supramolecular systems have utilized conforma-
tional changes to target anions,[4] inspired by the concept of
the induced-fit model proposed for enzymatic selectivity.[5]
This has been implemented by engineering molecules with
controlled rotational freedom in conjunction with strategic
placement of specific anion-coordinating functional groups.
The phosphate anion is known to significantly affect
biological and ecological systems, and a great deal of interest
focuses on further understanding its impact. Phosphorylated
molecules are key components in biological processes such as
energy storage and signaling pathways. Phosphate has sig-
nificant environmental impact with its role in the eutrophi-
cation process.[6] As a result, phosphate-binding receptors
have become a highly desirable target in supramolecular
chemistry, with several recent reports of systems designed to
selectively coordinate phosphate.[7]
phosphate coordination. This receptor features an order of
magnitude increase in association constant for dihydrogen
phosphate over halides and a series of other competing
oxoanions. The presence of two different induced-fit binding
conformations is in part responsible for this selectivity.
Receptor 1 was designed to coordinate anions by exploiting
the potential of the bis(urea) compounds to converge multi-
ple binding units on a single guest. Replacement of the
pyridine core in our previously reported class of arylethynyl
anion receptors[8] with a bipyridyl unit is expected to provide
a flexible structural unit to aid in forming a discrete binding
pocket, offering two hydrogen bond acceptors to specifically
target protic oxoanions. This design results in a receptor with
divergent and guest-specific anion binding conformations,
allowing this receptor to function as a three-way molecular
switch influenced strictly by anions with an “off” (unbound)
state and two spectroscopically distinct “on” states. Addi-
tionally, we show that it is possible to reversibly switch
between the bound conformational states by altering the
relative equivalents of chloride and hydrogen sulfate anions.
Receptor 1 was prepared according to Scheme 1. The
known compounds 6,6’-dibromo-2,2’-bipyridine[9] and 2-
alkynyl-4-tert-butyl aniline[10] were reacted in a double Sono-
gashira cross-coupling reaction yielding 2. Direct coupling of
two equivalents of 4-nitrophenylisocyanate to diamine 2
Herein we report a bisurea-based anion receptor bearing
a bipyridine core unit that is capable of selective dihydrogen
[*] J. V. Gavette, Dr. C. A. Johnson II, Prof. Dr. D. W. Johnson,
Prof. Dr. M. M. Haley
Department of Chemistry and Biochemistry &
Materials Science Institute
University of Oregon, Eugene, OR 97403-1253 (USA)
E-mail: dwj@uoregon.edu
haley@uoregon.edu
Prof. Dr. N. S. Mills
Department of Chemistry, Trinity University
San Antonio, TX 78212 (USA)
Dr. L. N. Zakharov
CAMCOR—Center for Advanced Materials Characterization in
Oregon, University of Oregon
Eugene, OR 97403-1443 (USA)
[**] This work was supported by NIH grant R01-GM087398, which also
funded early stage intellectual property that was licensed by
SupraSensor Technologies, a company co-founded by the principal
investigators. We appreciate extremely helpful conversations with
Prof. Pablo Ballester at ICIQ, Spain in revising some of the complex
solution speciation presented herein. University of Oregon NMR
facilities are supported by a grant from the National Science
Foundation (CHE-0923589).
Scheme 1. Synthesis of receptor 1. Assignments determined by 1H–1H
COSY, 1H–13C HSQC, and 1H–13C HMBC NMR spectroscopy.
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produced bis(urea) product 1 as a light yellow powder.
Repeated precipitation of 1 from hot THF gave analytically
pure product in up to 83% yield. The final product could be
further purified by flash column chromatography using
CH2Cl2 containing 3% v/v of a 9:1 v/v MeOH/NH4OH
mixture.
Taken as a whole, the observed anion binding trend is
H2PO4À > OAcÀ > HSO4À ꢀ ClÀ > BrÀ ꢀ NO3À > IÀ. The most
noteworthy result is the high affinity of receptor 1 for
À
dihydrogen phosphate: H2PO4 is bound over an order of
magnitude more strongly than the more basic OAcÀ, and over
two orders of magnitude greater than similarly shaped
HSO4À. This selectivity may result from a short, strong
hydrogen bond between guest and host, or perhaps even
a proton transfer to the host and concomitant hydrogen
The ability of receptor 1 to bind anions in solution was
1
initially probed by H NMR titrations in 10% [D6]DMSO/
CDCl3 or by UV/Vis titrations in 10% DMSO/CHCl3
(Table 1). In all cases, association constants (Ka) were
bonding between the resulting protonated bipyridine and
2À
HPO4
.
Further analysis of the binding data revealed distinctly
different proton shifting patterns of 1 upon addition of
various anionic guests. The addition of halides as the
tetrabutylammonium salts expectedly caused steady down-
field shifting of the urea resonances Hg and Hh, but surpris-
ingly the Hc aryl resonance residing on the bipyridine unit also
exhibited a drastic shift downfield by 0.64 ppm. The response
of Hc to increasing halide concentration suggests the involve-
ment of Hc in hydrogen bonding toward halide guests
(Figure 1a). Though unexpected, the importance of aryl
Table 1: Association constants (Ka) obtained by UV/Vis or 1H NMR
titrations of 1 with various anions in 10% DMSO/CHCl3 or the
perdeutero-equivalent at 298 K and data fit to an apparent 1:1 binding
model.[a]
Halides [LmolÀ1
]
Oxoanions [LmolÀ1
]
1
1
ClÀ
BrÀ
IÀ
140Æ10
60Æ5
H2POÀ4
HSO4
OAcÀÀ
NO3
78000Æ2000[b]
170Æ20
À
40Æ15
3200Æ500
60Æ5
À
C H hydrogen bonding by neutral receptors to halides in
[a] Anions were added as the tetrabutylammonium salts. Association
constants represent an average of at least three titrations. Data were
fitted to a 1:1 binding model using HypNMR 2006 or Hyperquad 2006
and based on goodness of fit, results of Job plots, and extensive non-
linear least squares modeling (see the Supporting Information). It is
clear from the increased goodness of fit that 1:2 association does occur
for HSO4À and the halides, specifically ClÀ, at least to some extent during
titrations (Supporting Information, Figure S27, S28 for 1:2 fits). Only 1:1
apparent Ka values are given, as 1:2 fits are less reliable owing to the
need to fit too many unknown variables. Nevertheless, association
constant fits (b11, b12) are provided and discussed for 1:2 association in
the Supporting Information. [b] Association constant obtained from
UV/Vis titrations.
solution has been thoroughly demonstrated by Flood and
Lee,[12] and a similar interaction was observed by Jeong and
Kwon.[7f] This phenomenon was most prominent for ClÀ
followed by BrÀ. The binding of IÀ was too weak to determine
if shifting trends definitively followed that of the other
halides. The synchronous downfield movement of the urea
and aryl protons upon addition of the spherical halides are
consistent with a binding conformation similar to Z (Fig-
ure 1c), where a cleft formed by the urea unit rotates circa
1808 away from the bipyridine nitrogen atoms about the
alkyne bond. This binding conformation would be expected to
offer two equivalent binding sites. A 1:2 host–guest complex is
suggested from representative fits and speciation diagrams of
ClÀ at high equivalents of guest (Supporting Information,
1
determined from H NMR titrations by simultaneous fitting
of the downfield shifting of the urea protons; in the case of
titrations with ClÀ and BrÀ, the Hc aryl resonances were
included in the fit. UV/Vis binding studies were carried out by
measuring the bathochromic shifting of the host absorbance.
Binding curves were fit using the Hyperquad 2006 suite of
non-linear curve fitting software.[11] All titrations of the
bipyridine-based receptor were fit to a 1:1 host–guest model,
which was confirmed by a Job plot analysis.
Figure S27).
1H NMR studies of 1 with oxoanions (H2PO4À, HSO4
,
À
À
OAcÀ, and NO3 ) produced distinctly different shifting
patterns from those with the halides (Figure 1b). The urea
resonances of 1 experienced the most intense shifts when
treated with oxoanions, as was seen with the halides. The most
significant differences in the shifting pattern of halides and
oxoanions pertained to the Ha and Hc protons. In the case of
the oxoanions, the Hc resonance does not exhibit the same
magnitude of downfield shift that was observed with the
halides, suggesting that the aryl proton is no longer signifi-
cantly involved in the complexation of guests. The Ha
The halides in general exhibited minimal affinity toward
1 trending ClÀ > BrÀ > IÀ. Acetate (OAcÀ) anion gave an
association constant of 3200 LmolÀ1, whereas HSO4 and
À
NO3 had binding constants of 170 and 60 LmolÀ1, respec-
À
tively. Initial attempts to determine association constants
resonance of 1 also shifts much further upfield with
À
from 1H NMR titrations of 1 with H2PO4 resulted in
H2PO4À, HSO4À, and OAcÀ versus the halides. The binding
of NO3À is too weak to induce a significant perturbation of the
Ha resonance. The lack of Hc shifting is consistent with
binding conformations similar to either S or U (Figure 1c).
The upfield shifting of the 2,2’-bipyridine cleft resonance has
been shown to be indicative of rotation about the bipyridine
bridging bond during the transition of an “anti” to a “syn”
conformation with respect to the bipyridine nitrogen atoms,
which is more suggestive of a U-like conformation.[14] Addi-
tionally, DFT calculations[13] indicate that the most stabilized
broadening of the urea resonances to approx. 1 equiv of
guest, and an eventual sharpening of the urea peaks beyond
1 equiv was observed (Supporting Information, Figure S8).
The broadening of the urea resonances is likely a result of
either exchange on the NMR timescale or a coordination-
induced proton exchange event, similarly observed by Gale
et al.[7a] As a result, reliable binding constants of 1 and
À
H2PO4 (Ka = 78000 LmolÀ1) were ultimately obtained by
UV/Vis titrations (Supporting Information, Figure S21).
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anion complex is in a U-like conformation (see the Support-
ing Information). An S-like conformation is expected in
formation of 1:2 complexes in large excesses of guest, though
À
reliable evidence for this currently exists only with the HSO4
system. Unfortunately, NOESY and ROESY NMR experi-
ments did not yield any additional information on the nature
of the anion-bound conformation.[15] The differences in the
trends of the NMR data between halides and oxoanions are
definitive evidence for the emergence of a binding conforma-
tion for oxoanions that is distinct spectroscopically from that
of the halides. This yields a unique system with the possibility
of acting as a three-way molecular switch influenced solely by
anionic stimuli with an “off” (unbound) state and two
different “on” (bound) states that are identifiable as either
halide- or oxoanion-bound.
Single crystals suitable for X-ray analysis were obtained
from slow evaporation of solutions of 1 containing excess
tetrabutylammonium bromide in 10% [D6]DMSO/CDCl3
(Figure 2).[16] The solid-state studies revealed that the recep-
Figure 2. ORTEP representations of the X-ray crystal structure of the
1·2BrÀ hydrogen-bonded complex. Ellipsoids set at 50% probability;
TBA+ counterions have been omitted for clarity.
tor adopts the anticipated “anti” conformation with respect to
the bipyridine unit. The receptor is able to coordinate
bromide in the solid state forming a 1·2BrÀ host–guest
complex in the expected Z conformation (Figure 1c). Recep-
tor 1 forms two weak hydrogen bonds through the urea
À
moieties, and another weak C H hydrogen bond with the
À
bipyridine unit to each bromide with distances Ng Br
À
À
3.432(1) ꢀ, Nh Br 3.232(1) ꢀ, and Cc Br, 3.85(1) ꢀ and
angles Ng Hg···Br 156.8(4)8, Nh Hh···Br 167.1(7)8, and
Cc Hc···Br 141.8(4)8. Though it is often dangerous to correlate
solid-state and solution-phase data, the correlation of the X-
ray hydrogen bonding network and the previously described
solution studies is in excellent agreement. Both sources of
data provide a convincing picture for halide binding in this
receptor.
À
À
À
We were interested in the ability of this system to act
reversibly with regard to the two “on” states. As a result, the
interconversion of the binding conformations was determined
by a series of competition experiments. Chloride and hydro-
gen sulfate were chosen as the stimuli since they yielded
Figure 1. Partial 1H NMR spectra of receptor 1 upon titration with
a) TBACl and b) TBAHSO4. Numbers next to NMR spectra represent
[anion]/[1]. c) Chemdraw representations and calculated (wB97X-D/
6-31G(d,p))[13] wireframe representations of possible anion binding in
the putative S, Z, (truncated for clarity) and U conformations.
1
differing conformations by H NMR spectroscopy and pro-
duced similar association constants. A sample of free receptor
was incrementally saturated with ClÀ. As mentioned previ-
ously, this caused significant downfield shifting of the urea
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resonances (Hg and Hh) and the bipyridine aryl proton (Hc).
The chloride saturated system was then treated with up to
in increased downfield shifting of the urea and Hc resonances
as well as an overall downfield shift of the Ha resonance.
These results show the proton resonances diagnostic of the
varying conformations can be influenced reversibly by ClÀ
and HSO4À. Furthermore, this system is able to revert back to
the “off” state from the chloride complex by the addition of
1 equivalent of NaBPh4 (Supporting Information, Fig-
ure S31), suggesting these conformational states can indeed
be reversibly achieved.
about 3 equiv of HSO4 relative to ClÀ, which caused
À
a retreating of urea and bipyridine Hc resonances back
upfield as well as continued upfield shifting of the Ha
resonance suggesting complexation of HSO4 over ClÀ
À
(Figure 3). Figure 3b shows the speciation diagram of free
host 1, 1·ClÀ, and 1·HSO4 during the course of this
À
competition experiment. The reverse experiment was also
conducted where the free receptor was initially treated with
In conclusion, through binding studies we have demon-
strated a supramolecular receptor that preferentially coordi-
nates the biologically and environmentally relevant dihydro-
gen phosphate anion an order of magnitude greater than the
more basic acetate, and several orders of magnitude greater
than other anions tested. Binding studies also revealed the
emergence of spectroscopically different halide- and oxoan-
ion-dependent binding conformations. The halide conforma-
tion in solution is further confirmed by solid-state data. The
distinctly different conformations demonstrate the ability of
this receptor to function as a strictly anion-influenced
supramolecular three-way switch. Furthermore, it has been
shown that the different bound conformations can be
reversibly modulated by changing the identity of the anionic
stimuli between chloride and hydrogen sulfate. The various
conformational outcomes exhibited by this receptor influ-
enced only by the subtle differences in anionic guests
represents a potentially interesting system for use in supra-
molecular machinery, and shows that it is possible to allow
conformationally flexible hosts to “select” their preferred
anionic guests.
À
HSO4 to saturation of the receptor, with the oxoanion
causing significant upfield shifting of the Ha resonance and
moderate downfield shifting of the urea protons (Supporting
Information, Figure S30). This system was then slowly treated
with an up to threefold excess of ClÀ to HSO4À, which resulted
Received: April 9, 2013
Revised: June 29, 2013
Published online: August 9, 2013
Keywords: anions · host–guest systems ·
.
molecular recognition · receptors · supramolecular chemistry
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À
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[15] A combination of the high symmetry of the receptor, borderline
molecular weight for NOESY and ROESY experiments, signifi-
cant overlap of aryl peaks of the strongest complex (1·H2PO4À),
and weak binding in all of the other systems prevented any
conclusive conformational assignment of anion complexes in
solution by these methods.
[16] X-ray data for 1·(TBABr)2: C80H114Br2N10O6, Mr = 1471.63,
3
¯
crystal size 0.34 ꢂ 0.10 ꢂ 0.01 mm , Triclinic, space group P1,
a = 8.496(8), b = 12.527(12), c = 19.598(18) ꢀ, a = 100.299(16),
b = 93.450(15), g = 92.059(14)8, V= 2046(3) ꢀ3, Z = 1, Z’ = 0.5,
1calcd = 1.194 gcmÀ3, m = 1.045 mmÀ1, F(000) = 782, MoKa radi-
ation, l = 0.71073 ꢀ, T= 173(2) K, 2Vmax = 45.008, 11442 reflec-
tions measured [Rint = 0.1774], 5324 reflections observed, 443
refined parameters, R1 = 0.0926, wR2 = 0.1626, and GOF =
1.003 for reflections with I > 2s(I), R1 = 0.2375, wR2 = 0.2146,
and GOF = 1.003 for all data, max/min residual electron density
+ 0.490/À0.714 eꢀÀ3. CCDC 930545 contains the supplemen-
tary 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.
10274 www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 10270 –10274