Cooperative loading and release behavior of a metal-organic receptor

Article abstract of DOI:10.1021/ja5120437

In order to design artificial chemical systems that are capable of achieving complex functions, it is useful to design synthetic receptors that mimic their biological counterparts. Biological functions are underpinned by properties that include specific binding with high affinity and selectivity, cooperativity, and release triggered by external stimuli. Here we show that a metal-organic receptor constructed through subcomponent self-assembly can selectively and cooperatively load and release oxocarbon anions. The flexible coordination spheres of its cadmium(II) centers allow the receptor to dynamically adjust its structure upon exchanging four triflate or triflimide counterions for two oxocarbon anions, resulting in strong cooperativity and very tight binding, with an apparent association constant for C₅O₅^2- of 5 × 10¹⁰ M^-1. Substituting the cadmium(II) ions for copper(I) by switching solvent prompted a structural reorganization and release of the oxocarbon anions. Its cooperative behavior allows the receptor to carry a greater payload than would be possible in a noncooperative analogue.

Full text of DOI:10.1021/ja5120437

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Communication
Cooperative Loading and Release Behavior of a Metal-Organic Receptor
Quan Gan, Tanya K Ronson, David A. Vosburg, John D. Thoburn, and Jonathan R. Nitschke
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Quan Gan, Tanya K. Ronson, David A. Vosburg, John D. Thoburn, and Jonathan R. Nitsch-
ke*
,†
†
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
‡
Department of Chemistry, Harvey Mudd College, 301 Platt Boulevard, Claremont, CA 91711, United States
Department of Chemistry, Randolph-Macon College, Ashland, VA 23005, United States
§
Supporting Information Placeholder
6
tive, selective uptake, but also controlled, complete release
under well-defined conditions.
ABSTRACT: In order to design artificial chemical systems
that are capable of achieving complex functions, it is useful
to design synthetic receptors that mimic their biological
counterparts. Biological functions are underpinned by prop-
erties that include specific binding with high affinity and
selectivity, cooperativity, and release triggered by external
stimuli. Here we show that a metal-organic receptor con-
structed through subcomponent self-assembly can selectively
and cooperatively load and release oxocarbon anions. The
flexible coordination spheres of its cadmium(II) centers al-
low the receptor to dynamically adjust its structure upon
exchanging two oxocarbon anions for four triflate or triflim-
ide counterions, resulting in strong cooperativity and very
tight binding, with an apparent association constant for
Subcomponent A, based on a pseudo-fourfold-symmetric
(cyclopentadienyl)(cyclobutadienyl) cobalt (CpCoCb) moiety,
was designed and synthesized based on literature precedent
7
for related molecules. Four anilines provide four ‘arms’ to
react with 2-formylpyridines, generating a tetrakis(bidentate)
ligand. Meta-substituted anilines were chosen to bring the
ligand arms close enough to form a sandwich structure
through free rotation between the anilines and the para-
phenylene rings (Figure 1). Symmetrical sandwich structure 1
II
formed during the reaction of
A
with Cd and 2-
formylpyridine B in a 2 : 4 : 8 ratio in acetonitrile. NMR and
ESI-MS spectra of 1 were consistent with the formation of a
highly symmetric M L assembly in quantitative yield (Fig-
2−
10
-1
C O of 5 × 10 M . Substituting the cadmium(II) ions for
5
5
4 2
copper(I) by switching solvent prompted a structural reor-
ganization and release of the oxocarbon anions. Its coopera-
tive behavior allows the receptor to carry a greater payload
than would be possible in a non-cooperative analogue.
ures S1-S4).
The cooperative binding and release of substances in re-
sponse to external stimuli is a key feature of metabolic and
1
signaling functions in biological systems. A canonical exam-
ple is the dioxygen-transport protein hemoglobin, where
dioxygen binding at one of the four sites increases the oxy-
gen affinity of the other sites, resulting in efficient delivery of
2
oxygen from respiratory organs to tissues where it is needed.
This cooperative behavior enables hemoglobin to deliver 1.7
times more oxygen than would be possible if the binding
3
sites were non-cooperative. The cooperative loading phe-
nomenon in natural biomacromolecules has inspired the
study of smaller synthetic analogues, both to better under-
stand the mechanisms involved in natural systems and also
to develop functional systems that may benefit from such
binding interactions. Several groups have fruitfully investi-
Figure 1. Synthesis and schematic representation of metal-
4
gated artificial guest catch-and-release molecular systems,
organic receptor 1 and a (2) •1 complex. The reaction of A (2
2
2+
including both open architectures and closed capsules, as
well as demonstrating artificial cooperative binding behav-
equiv.) and B (8 equiv.) with Cd (4 equiv.) led to the for-
mation of 1. Addition of croconate anion (2) to 1 gave rise to
5
ior. The present work builds upon these foundations by
a (2) •1 complex.
2
demonstrating a receptor that is capable of not only coopera-
1
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The crystal structure of 1•(NTf ) was consistent with the
sion-ordered H NMR spectroscopy (DOSY) was also con-
2
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approximate D symmetry observed in solution by NMR, in
sistent with the two species having similar sizes (Figure S7).
4
which two ligands are brought together by formation of co-
ordination bonds, with the Cp rings on the exterior faces of
the complex (Figure 2a,b). Each ligand coordinates to four
The enthalpy and entropy of transformation from the D to
4
C isomer in acetonitrile were determined using a van ’t Hoff
i
−
1
−1 −1
plot to be 6.7 kJ mol and 4.5 J mol K , respectively (Figure
II
Cd ions, with an average distance of 16.5 Å between adjacent
S6). The positive values of both ꢀH and ꢀS illustrate that the
II
metal centers. Each Cd is bound by a pyridyl-imine ‘arm’
D symmetric conformation of receptor 1 is enthalpically
4
from each ligand, leaving two free coordination sites that are
occupied by acetonitrile or water to complete the octahedral
preferred while the C conformation is favored entropically.
i
We attribute the dynamic behavior of compound 1 to the
II
II
coordination sphere of Cd .
flexible coordination sphere of the large Cd ions, which
8
allows them to tolerate distorted coordination geometries.
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We hypothesized that 1 would be a suitable receptor for
anions, whereby the weakly coordinated solvent or anions
II
occupying the two vacant coordination sites at each Cd cen-
ter would be displaced by more strongly coordinating anions.
Furthermore, we envisaged that the dynamic reconfiguration
of 1 might allow it to bind anions cooperatively via rear-
rangement of the coordination sphere of the metal centers
upon addition of a suitable anion. NMR titration experiments
of receptor 1 with the tetrabutylammonium salts of croconate
2−
2−
2−
(
C O , 2), squarate (C O , 3) and rhodizonate (C O , 4)
5 5 4 4 6 6
were carried out. In all cases, complexation was inferred to
1
occur, as new signals in the H NMR spectra emerged, and
free receptor signals decreased, consistent with slow ex-
change between complexed and uncomplexed states on the
1
H NMR time scale (Figures S8-S10). ESI mass spectra dis-
played ions corresponding to 1 : 2 complexes (Figures S16 and
S17). During the titration process, we observed two new sets
of peaks. One set of signals arose first but then decreased
with further anion addition, while the other species gradually
increased and remained following saturation; we therefore
assigned these to the 1 : 1 and 1 : 2 complexes, respectively.
DOSY NMR further showed comparable diffusion rates for
1
the two complexes, suggesting that each H NMR resonance
belongs to the framework of an intact M L receptor rather
4
2
than to smaller fragments (Figure S13).
Further solution characterization of the complex was hin-
1
Figure 2. Top and side views of the crystal structures of
dered by the H NMR silence of the oxocarbon anions. The
1
•(NTf ) (a & b), 1•(OTf) (c & d), and (2) •1 complex (e & f).
solid-state structure (Figure 2e,f), however, revealed the orig-
inal M L architecture with two bound molecules of 2. Each
4 2
2
8
8
2
Oxocarbon anions are shown as CPK representations (C gray,
O red). Hydrogen atoms, uncoordinated counterions, solvent
croconate anion is coordinated in a bridging bis-bidentate
14
II
molecules and disorder have been omitted for clarity.
manner to two Cd centers, with an average Cd-O bond
II
length of 2.3 Å. Each Cd center in (2) •1 is thus coordinated
2
The crystal structure of 1•(OTf)8 (Figure 2c,d), in con-
by two pyridyl-imine ligands and one croconate anion in a
trigonal prismatic geometry, in contrast to the distorted oc-
tahedral coordination observed in the absence of the guest.
Binding of the croconate anions thus induces a structural
reorganization of the M L architecture. The bridging anions
trast, possesses C symmetry, in which two opposing corners
i
are similar to those of 1•(NTf ) , with the two pyridyl donors
2 8
II
occupying trans coordination sites. The other two Cd cen-
ters adopt a less symmetric coordination geometry, having
pyridyl donors cis to each other. The two central cyclobuta-
dienyl rings, which are stacked atop each other in 1•(NTf2)8
4
2
II
bring pairs of Cd centers closer together, with the Cd···Cd
distance shrinking from 16.5 Å to 7.4 Å, and the ‘arms’ of the
receptor become eclipsed in order to reposition the free co-
ordination sites to bind to the anions.
(
centroid-centroid distance 4.5 Å), adopt a slipped, offset
arrangement in 1•(OTf) (centroid-centroid distance 5.3 Å).
8
NMR measurements in solution provided evidence for the
presence of two configurations of 1 for both the triflimide
and triflate salts, with symmetries consistent with the two
forms observed in the solid state. The triflimide salt was used
in all of our solution studies due to the greater solubility of
The squarate anion 3 was chosen for UV-Vis titration stud-
ies as the absorbances of both croconate and rhodizonate
9
overlap with that of receptor 1. Nonlinear regression analysis
of the titration data (Figure S15) based on a 1 : 2 binding
4
model gave macroscopic binding constants of 6.3 ± 0.3 × 10
the resultant complexes in CD CN. At 278 K, NMR data were
5
−1
3
and 7.90 ± 0.08 × 10 M , respectively, for K and K in the 1 :
1
2
consistent with a D -symmetric structure, whereas a second
4
2
complexation of 1 with squarate anion 3 (binding constants
set of signals appeared when the temperature was increased
to 333 K (Figure S5). We infer the new species to be the Ci-
symmetric conformation of 1, since it possesses twice as
averaged from three runs). The ratio of the corresponding
microscopic binding constants, K /K = 50, clearly indicates
2μ
1μ
the positive cooperative nature of this complexation and
corresponds to a cooperative Gibbs free energy of –9.7 kJ
many resonances as the major D -symmetric species. Diffu-
4
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-
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II
mol . The measured apparent association constant (K ) for 3
and the subsequent addition of Cd (4 equiv.). Each conver-
a
1
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sion step was monitored by H NMR (Figure 4).
binding to 1 was calculated to be 5.2 ± 1.1 × 10 M (Figure
S15), representing an affinity value that is a composite of the
two binding events.
This cycle implements a strategy of cooperative catch and
release of an anion in two different environments through
altering the properties of the receptor upon addition of
chemical stimuli. The considerable manipulation of com-
pounds and solutions required by this strategy would be
most worthwhile in the context of high-value extractions, as
The observed high cooperativity may be interpreted to re-
1
0
sult from an induced-fit process during recognition: when
one oxocarbon anion docks into the receptor, displacing and
exchanging for the less-well-bound counteranions, a confor-
mational change is induced to fit the oxocarbon anion. The
remaining binding site is thereby preorganized to have in-
creased affinity for a second anion. The crystal structures
provide insight as to how this process might occur. A key
point is that the staggered ‘arms’ of the receptor become
eclipsed following the binding of the first croconate. This
eclipsing brings the other two arms of the complex together
to bind the second croconate with high affinity.
1
1
in the extraction of sulfate from nuclear waste.
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The cooperative binding observed for squarate also oc-
curred for croconate and rhodizonate. For each of these oxo-
carbon anions, the intermediate 1 : 1 complexes were never
significantly populated during NMR titration experiments.
Among these anions, croconate 2 bound most tightly to 1, as
inferred from NMR experiments where two anions were al-
lowed to compete to bind (Figure S11); croconate was thus
chosen for subsequent studies. The binding of receptor 1
with other anions was tested, but the cooperative binding
behavior seen for 2, 3, and 4 was not observed in the cases of
‒
‒
‒
‒
PF , BF , Br or I (Figure S12).
6
4
Having established cooperative loading of anions into 1, we
sought to release the anions completely without disturbing
I
the overall structure. For this purpose the Cu L analogue 5
4
2
II
4
of Cd L receptor 1 was synthesized and studied (Figure S18).
Unlike receptor 1, each Cu metal center in complex 5 is fully
saturated by two pyridyl-imine ligands, since Cu ions favor a
2
I
I
Figure 3. Schematic representation of loading and release of
croconate with receptors 1 and 5 by switching the metal cen-
tetrahedral coordination environment; the metal centers of 5
thus exhibited no tendency to coordinate anions. Upon the
addition of excess croconate (4 equiv.) to 5, no new peaks or
ters. (i) Trapping 2 as (2) •1. (ii) DMSO addition prompted
2
metal exchange and release of anion 2. (iii) Addition of di-
2+
ethyl ether gave solid complex 5, with Cd •2 in the filtrate.
shifts in the signals were observed in either DMSO-d or
2+
6
1
(iv) Redissolution of 5 in CD CN followed by addition of Cd
3
CD CN by H NMR (Figure S21). Further addition of crocon-
3
regenerated the initial state.
ate resulted in broadened signals and precipitation of 5 due
to anion exchange.
I
The addition of Cu to a solution of (2) •1 (Figure S22) re-
2
I
II
sulted in its clean conversion to 5 and free 2, without forming
When both Cu and Cd were present, we found that 1 and
interconverted upon variation of the solvent: complex 1 was
I
II
Cu -Cd hybrid structures. This observation suggests that
both metal substitution and anion release are strongly coop-
erative. If the metal substitution process was not cooperative,
partially exchanged receptor signals would be significant and
would greatly complicate the NMR spectra. Interestingly, in
the absence of oxocarbon anions, complicated spectra were
observed during metal substitution (Figure S23), which we
attribute to mixed-metal complexes.
5
preferred in acetonitrile, whereas 5 dominated in DMSO. As
I
a result, 5 could be obtained by adding Cu to 1 in DMSO,
II
and 5 could be converted into 1 via addition of Cd in ace-
II
tonitrile. We infer the stronger solvation of free Cd by
I
DMSO, and Cu by acetonitrile, to drive the solvent-
dependent metal exchange that allows 1 and 5 to intercon-
vert. The combination of different solvent stability and dif-
ferential oxocarbon anion recognition ability allows us to
envisage a cycle of loading and release of anions by solvent-
mediated interconversion between 1 and 5. The sequence of
steps is shown in Figure 3: (i) To an acetonitrile solution of
The observed high-affinity cooperative catch and release
behavior provides 1 with advantages in comparison with oth-
5
-12
er artificial systems in which no cooperativity occurs. At a
−
6
concentration of 5.0 × 10 M, 1 would be half-saturated with
−
6
I
3 at an anion concentration of 8.0 × 10 M. In the absence of
cooperativity, however, the hypothetical maximum amount
of anion that could be loaded under the same conditions is
receptor 1 containing free Cu (4 equiv.) was added anion 2
(
2.4 equiv.), leading to formation of the (2) •1 complex, i.e.
2
anion loading. (ii) Addition of DMSO stimulated the re-
II
I
−6
placement of the Cd ions of the complex with Cu ions, re-
sulting in transformation of 1 into 5, and thus release of the
two equivalents of 2. (iii) Addition of diethyl ether to the
above solution followed by filtration of the precipitated 5
3.1 × 10 M. Thus, the cooperative binding behavior of the
receptor enables it to deliver 1.6 times as much anion as it
would if the sites were independent, a value comparable
to the cooperative advantage enjoyed by hemoglobin in
II
4
allowed its separation from the released 2 and Cd left in
transporting oxygen. Furthermore, 1 efficiently catches and
solution. (iv) Finally, complex 5 transformed back into recep-
tor 1 following dissolution of the precipitate in acetonitrile
releases anions at low concentrations within a narrow re-
sponse region, where small changes in conditions lead to a
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complete transformation between free and bound
states. These features are of considerable benefit in the con-
ACKNOWLEDGMENT
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This work was supported by the Marie Curie Academic-
Industrial Initial Training Network on Dynamic Molecular
Nanostructures (DYNAMOL) of the European Union's Sev-
enth Framework Programme (FP7) and the UK Engineering
and Physical Sciences Research Council (EPSRC). We thank
Diamond Light Source (UK) for synchrotron beam time on
I19 (MT8464), the EPSRC National Crystallography Service
for X-ray data collection, and the EPSRC Mass Spectrometry
Service at Swansea for high-resolution electrospray ioniza-
tion experiments.
text of chemical sensors or drug delivery systems. Alt-
hough we did not carry out studies on a sufficiently large
scale to allow yield determination, NMR spectra did not
show substantial impurity buildup (estimated < 5%) even
after four turns around the cycle of Figure 3. This observation
is consistent with an efficient strategy.
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Freeman: New York, 2002.
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4) (a) Han, M.; Michel, R.; He, B.; Chen, Y. S.; Stalke, D.; John, M.;
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Freudenreich, J.; Barry, N. P.; Juillerat-Jeanneret, L.; Süss-Fink, G.;
Therrien, B. J. Am. Chem. Soc. 2011, 134, 754.
1
Figure 4. Partial H NMR spectra (400 MHz, 298 K) of the
catch and release of anions following the cycle of Figure 3. a)
+
Receptor 1•(NTf ) (0.25 mM) with Cu (4 equiv.) in CD CN
2
8
3
(
500 ꢁL); b) addition of 2 (2.4 equiv.) to solution a; c) addi-
(
5) (a) Pistner, A. J.; Lutterman, D. A.; Ghidiu, M. J.; Ma, Y.-Z.;
tion of DMSO-d (50 ꢁL) to solution b; d) precipitate ob-
6
Rosenthal, J. J. Am. Chem. Soc. 2013, 135, 6601. (b) Ikeda, M.; Tanida,
T.; Takeuchi, M.; Shinkai, S. Org. Lett. 2000, 2, 1803. (c) Ikeda, T.;
Hirata, O.; Takeuchi, M.; Shinkai, S. J. Am. Chem. Soc. 2006, 128,
16008. (d) Deutman, A. B.; Monnereau, C.; Moalin, M.; Coumans, R.
G.; Veling, N.; Coenen, M.; Smits, J. M.; de Gelder, R.; Elemans, J. A.;
Ercolani, G. Proc. Natl. Acad. Sci. USA 2009, 106, 10471. (e)
Nabeshima, T.; Yoshihira, Y.; Saiki, T.; Akine, S.; Horn, E. J. Am.
Chem. Soc. 2003, 125, 28. (f) Willans, C. E.; Anderson, K. M.; Potts, L.
C.; Steed, J. W. Org. Biomol. Chem. 2009, 7, 2756. (g) Huang, W.-H.;
Liu, S.; Zavalij, P. Y.; Isaacs, L. J. Am. Chem. Soc. 2006, 128, 14744. (h)
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Nielsen, K. A.; Johnsen, C.; Jeppesen, J. O. Chem. Eur. J. 2010, 16, 848.
tained from addition of diethyl ether to c following dissolu-
2+
tion in CD3CN (500 ꢁL); e) addition of Cd (4 equiv.) to
solution d.
In summary, we have shown how oxocarbon anions can be
cooperatively loaded and released by regulating the proper-
ties of an artificial receptor. The binding of the first oxocar-
bon anion to receptor 1 increases the binding affinity of the
second one, while both are released completely in the reverse
process triggered by chemical stimuli (exchanging the sol-
vent resulting in switching the metal centers). This system
also illustrates induced-fit recognition behavior: the flexibil-
ity of the arms of the complex allows dynamic adjustment of
its binding sites via rearrangement of the metal coordination
geometry to cooperatively accommodate anions with high
affinity.
(
i) Sato, H.; Tashiro, K.; Shinmori, H.; Osuka, A.; Murata, Y.;
Komatsu, K.; Aida, T. J. Am. Chem. Soc. 2005, 127, 13086. (j)
Takezawa, H.; Murase, T.; Resnati, G.; Metrangolo, P.; Fujita, M. J.
Am. Chem. Soc. 2014, 136, 1786. (k) Jiang, W.; Nowosinski, K.; Löw, N.
L.; Dzyuba, E. V.; Klautzsch, F.; Schäfer, A.; Huuskonen, J.; Rissanen,
K.; Schalley, C. A. J. Am. Chem. Soc. 2012, 134, 1860.
(
6) (a) Kremer, C.; Lützen, A. Chem. Eur. J. 2013, 19, 6162. (b)
Chakrabarty, R.; Mukherjee, P. S.; Stang, P. J. Chem. Rev. 2011, 111,
810.
7) Harrison, R. M.; Brotin, T.; Noll, B. C.; Michl, J.
ASSOCIATED CONTENT
Supporting Information
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Organometallics 1997, 16, 3401.
(8) Meng, W.; Ronson, T. K.; Clegg, J. K.; Nitschke, J. R. Angew.
Chem. Int. Ed. 2013, 52, 1017.
Full experimental procedures, NMR and ESI-MS spectra, X-
ray crystallographic data including CIF files (also deposited
with the Cambridge Crystallographic Data Centre as entries
CCDC 1012306-1012309). This material is available free of
charge via the Internet at http://pubs.acs.org.
(9) Thordarson, P. Chem. Soc. Rev. 2011, 40, 1305.
(
(
10) Koshland, D. E. Jr. Angew. Chem. Int. Ed. 1995, 33, 2375.
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AUTHOR INFORMATION
(
12) Vajpayee, V.; Song, Y. H.; Lee, M. H.; Kim, H.; Wang, M.;
Stang, P. J.; Chi, K. W. Chem. Eur. J. 2011, 17, 7837.
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Corresponding Author
jrn34@cam.ac.uk
(
Notes
The authors declare no competing financial interests.
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Partial 1H NMR spectra (400 MHz, 298 K) of the catch and release of anions following the cycle of Figure 3.
a) Receptor 1•(NTf2)8 (0.25 mM) with Cu+ (4 equiv.) in CD3CN (500 uL); b) addition of 2 (2.4 equiv.) to
solution a; c) addition of DMSO-d6 (50 uL) to solution b; d) precipitate obtained from addition of diethyl
ether to c following dissolution in CD3CN (500 uL); e) addition of Cd2+ (4 equiv.) to solution d.
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