Electrochemically induced intermolecular anion transfer

Article abstract of DOI:10.1002/chem.200900878

Two macrocyclic anion receptors with the proper signaling units were designed to control intermolecular anion transfer using an electrochemical stimulus. The solubility of both receptors drastically improved when one equivalent of benzoate was added to the suspension in the NMR tube. Macrocycles 1 and 2 were prepared via multistep synthetic pathways. An important enhancement of the MLCT emission intensity was initially seen when benzoate was added to a solution of macrocycles 1. Spectroelectrochemical experiments were conducted for a mixture which contained receptors 1,2 and benzoate in equimolar concentration in acetone. The reversibility of the anion transfer could be shown and hence, electrochemical reduction of ferrocenium to ferrocene at -0.2 V was simultaneously accompanied by an enhancement of the emission intensity.

Full text of DOI:10.1002/chem.200900878

DOI: 10.1002/chem.200900878
Electrochemically Induced ACHTNUTRGNEIUNG ntermolecular Anion Transfer
David Curiel,*[a] Paul D. Beer,[b] Alberto Tꢀrraga,[a] and Pedro Molina[a]
Control over molecular motion still represents a challenge
for supramolecular chemists. Although great advances have
been reported during the last two decades,[1] the application
of molecular machines into real devices is still to come.
Supramolecular interlocked structures are among the most
studied systems to test molecular motion since they facilitate
the detection of relative displacement between their compo-
nents due to their topological restrictions. Very imaginative
examples of chemical,[2] photochemical[3] and electrochemi-
cally-driven[4] devices have been described in the literature.
Although the field of anion-coordination chemistry has
expanded greatly in recent years,[5] the application of nega-
tively charged species to the area of molecular machines has
not been exploited to any great extent. Of the rare examples
reported to date the majority focus on rotaxane systems,
where competition between axle and macrocycle for anion
recognition chemically switches movement between the in-
terlocked components.[6]
Redox-switched systems have mainly been restricted to
shuttling in interlocked structures by influencing donor–ac-
ceptor electronic interactions[7] and coordination geometry
change.[8] For example, Fabbrizzi and co-workers have re-
ported the redox-driven intramolecular nitrate anion trans-
location between a copper metal centre and a bis-imidazoli-
um anion-recognition site.[9] In this regard, with a few excep-
tions,[10] ferrocene has not been commonly utilized in molec-
ular machinery, despite its well known electrochemical be-
havior in supramolecular receptors.[11] This context
motivated us to design a supramolecular system which could
involve the displacement of an anionic component, electro-
chemically regulated by the oxidation state of a ferrocene
unit.
Herein we report a novel approach to the control of inter-
molecular motion between two receptors where the location
of an anion can be regulated by the redox-controlled rela-
tive strength of non-covalent interactions between two rec-
ognition sites. Initially, the anion is bound solely by hydro-
gen bonds. Upon electrochemical oxidation, the anion relo-
cates to be complexed by hydrogen bonds and by favourable
electrostatic attractive interactions.
In the design of our system we intended to incorporate a
luminescent signalling unit as well as an electrochemically
active unit, into two macrocyclic receptors, in order to
define a bistable arrangement. In the search for such com-
ponents, photo-active ReI–2,2’-bipyridine-4,4’-diamide, and
redox-active ferrocene-1,1’-dithiourea motifs, were chosen
(see below). These fragments are known to be efficient
anion recognition agents[12] and will enable spectroelectro-
chemical techniques to be used in the monitoring of inter-
molecular anion transfer and switching of the device. Mac-
rocycles 1 and 2 were prepared via multistep synthetic path-
ways and characterized as described in the Supporting Infor-
mation.
[a] Dr. D. Curiel, Prof. A. Tꢀrraga, Prof. P. Molina
Departamento de Quꢁmica Orgꢀnica
Facultad de Quꢁmica, Universidad de Murcia
Campus de Espinardo, 30100 Murcia (Spain)
Fax : (+34)968364149
E-mail: davidcc@um.es
[b] Prof. P. D. Beer
Chemistry Research Laboratory
University of Oxford, Mansfield Road
Oxford, OX1 3TA (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900878.
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Preliminary anion-titration experiments were independ-
ently carried out with 1 and 2, by both absorption and emis-
sion spectroscopy in acetone solution.[13] Receptors 1 and 2
revealed their ability to bind different anions, as could be
quantified by the determination of their stability constants
(Table 1).[14] All titrations fitted well a 1:1 stoichiometry
model, with most of the anions forming stronger complexes
with 1.
Table 1. Anion-binding constants (logKass)
[a] of 1 and 2.
À
À
PhCO2
AcOÀ
HSO4
BrÀ
1
2
6.1
4.1
5.3
3.9
5.2
3.8
4.7
5
[a] Kass [MÀ1] determined by UV/Vis spectroscopy with an error <10%,
at 258C. [1]=5ꢃ10À5 m, [2]=10À4 m in acetone. Association constants, for
1, have been further confirmed by emission spectroscopy at 258C, lexc
420 nm and [1]=5ꢃ10À5 m.
=
Figure 1. 1H NMR spectra of A) 1; B) 1/benzoate 1:1; C) 1/2/benzoate
1:1:1; D) 2; E) 2/benzoate 1:1 in deuterated acetone.
Since the initial condition of our dynamic system demand-
ed an unbalanced distribution of the complexed anion be-
tween both macrocycles, we focused our study on the ben-
zoate anion, which showed the larger binding constant ratio
between 1 and 2.
Although the low solubility of compounds 1 and 2 in deu-
terated acetone thwarted any possibility of quantifying bind-
1
ing constants by H NMR analysis, a qualitative study could
be indeed carried out (Figure 1).[15] It is worth highlighting
how the solubility of both receptors is remarkably improved
when one equivalent of benzoate is added to the suspension
in the NMR tube. This can be interpreted as an initial evi-
dence of the host–guest interaction. Furthermore, a down-
field shift is detected for the signals corresponding to the
protons more directly involved in the complexation of the
anion, in both macrocycles 1 and 2, as commonly results
from the hydrogen bonding interaction between the recep-
tor and the complexed anion (Figure 1).
Figure 2. Emission spectra (lexc =420 nm) of (A) 1 (5ꢃ10À5 m) in acetone
(c), 1/benzoate 1:1 (c) and B) 1/benzoate 1:1 (c), 1/2/benzoate
1:1:1 (c).
benzoate is added to a solution of 1 (5ꢃ10À5 m in acetone)
88% of the anion is bound by the macrocyclic receptor.
However, when 1, 2 and TBA-benzoate are dissolved to-
gether, at the same concentration, the ratio of anion com-
plexed by receptor 1 drops to 85%.[17]
The most important condition to be fulfilled by our
system in order to achieve an efficient intermolecular anion
transfer is that the affinities towards anionic guests can be
reversed. In our case, this will be done by a controlled elec-
trochemical potential. A square wave voltammetry titration
(Figure 3) of compound 2 with TBA-benzoate showed a
double wave behaviour with a cathodic shift in the oxidation
potential (DEp =0.218 V). This double wave observation is
the usual response to strong anion binding with slow kinetic
decomplexation on the voltammetric timescale. According
to the square Scheme applied to the equilibria of redox
Interestingly, an upfield shift of the protons, which are
complexing the anion in 1, could be detected after the addi-
tion of one molar equivalent of 2 to a 1:1 mixture of recep-
tor 1 and benzoate. Simultaneously, a downfield shift of the
1
thiourea protons is observed in the H NMR spectra of the
ternary mixture 1/benzoate/2. The structural information,
which can be drawn from this experiment, indicates that
competitive anion complexation equilibria are established
between both macrocycles.
Control experiments carried out by emission spectroscopy
also confirmed this result (Figure 2). An important enhance-
ment of the MLCT emission intensity was initially observed
when benzoate was added to a solution of 1. Subsequently,
the luminescence intensity of the anion complex was partial-
ly quenched after the addition of 2. In agreement with what
has been discussed for the 1H NMR experiments, the de-
crease in the luminescence can be related to the removal of
part of the complexed anion from 1.[16]
switchable host–guest systems,
a binding enhancement
factor of 4860 could be estimated for the complexation of
benzoate by 2 in its oxidized state.[18] Thus, the resulting
binding constant for the oxidized receptor would be approx-
imately logKass =7.8, which is significantly larger than the
binding constant determined for 1, logKass =6.1 (Table 1).
This promising result may mean that, when 1 and 2 are dis-
The anion distribution can be determined from the values
of the binding constants. Initially, when one equivalent of
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D. Curiel et al.
Figure 4. Cartoon representation of the electrochemically driven anion
transfer between macrocycles 1 and 2, accompanied by a variation in the
emission intensity of receptor 1. The size of the encircled anion is related
to the percentage of complexed anion by macrocycles 1 and 2, which is
depicted as a column plot.
Figure 3. Square wave voltammetry of 2 (Ep =0.41 V) upon addition of
benzoate (0–1 equivalents). Scan rate 100 mVsÀ1, [Host]=5ꢃ10À4 m in
acetone. Supporting electrolyte: TBAPF6 (0.05m). Potentials are referred
to decamethylferrocene (0 V).
solved together, an anion initially bound to receptor 1 upon
oxidation could be transferred to receptor 2.
Spectroelectrochemical experiments were carried out for
a mixture which contained receptors 1, 2 and benzoate in
equimolar concentration (5ꢃ10À5 m) in acetone. Initially,
blank experiments were recorded in order to confirm that
no side effects interfered with the experimental require-
ments of the spectroelectrochemical analysis, such as
common ion effect from the supporting electrolyte or emis-
sion quenching caused by the presence of 2+ (see the Sup-
porting Information for experimental details).
Figure 5. Evolution of the emission intensity (lexc =420 nm, 5ꢃ10À5 m, T=
258C) of the mixture 1/benzoate/2/TBAPF6 1:1:1:200. A) Emission
quenching upon oxidation of 2; B) emission enhancement upon reduction
of 2+.
Interestingly, a noticeable decrease of the luminescence of
the 1/2/benzoate/TBAPF6 (1:1:1:200) mixture could be de-
tected when controlled oxidation of the macrocyclic ferroce-
nophane 2 was carried out by electrolysis at 0.38 V. This
emission decrease can be accounted for by the relocation of
the benzoate anion from receptor 1 to the oxidized ferroce-
nium containing receptor 2+. The ferroceniumꢄs positive
charge makes a favourable and significant electrostatic con-
tribution to the overall anion-recognition process. Moreover,
the positively charged ferrocenium group will also increase
the relative acidity of the appended thiourea hydrogen-bond
donor groups.[19] Therefore, the combination of stronger hy-
drogen bonding and electrostatic attraction enhances the
anion-binding affinity which facilitates the benzoate anion
transferring from receptor 1 to oxidized receptor 2+. Al-
though the emission does not decrease exactly to the level
of the free macrocycle 1, which may indicate that not all the
anion is transferred, calculation of the anion distribution
shows that the extent of anion transfer is quite significant,
with only 12% of the benzoate being now complexed by the
ReI–bipyridinediamide macrocyle (Figure 4).[20]
to control intermolecular anion transfer by means of an
electrochemical stimulus. We have also described a new ap-
proach to the control of molecular motion based on hydro-
gen bonding versus hydrogen bonding plus electrostatic in-
teraction. Currently we are investigating the possibility of
integrating these preliminary results, about a forward and
back mass transfer, into more elaborated devices.
Experimental Section
See Supporting Information for experimental details.
Acknowledgements
D.C. acknowledges Ministry of Science and Innovation for funding his
work through a contract of the Ramꢅn y Cajal Programme. D.C., A.T.
and P.M. also acknowledge the financial support from Fundaciꢅn Sꢆneca
(Agencia de Ciencia y Tecnologꢁa de la Regiꢅn de Murcia) projects
02970/PI/05 and 04509/GERM/06 (Programa de Ayudas a Grupos de Ex-
celencia de la Regiꢅn de Murcia, Plan Regional de Ciencia y Tecnologꢁa
2007/2010).
The reversibility of the anion transfer could be demon-
strated (Figure 5). Hence, electrochemical reduction of fer-
rocenium to ferrocene at À0.2 V was simultaneously accom-
panied by an enhancement of the emission intensity. Once
the receptor 2 recovers charge neutrality, the anion goes
back to macrocycle 1 restoring the original anion load distri-
bution.
In conclusion, we have designed and synthesized two mac-
rocyclic anion receptors with the appropriate signaling units
Keywords: luminescence · macrocycles · molecular devices ·
molecular recognition · voltammetry
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Intermolecular Anion Transfer
COMMUNICATION
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[14] Binding constants were calculated by non-linear regression of the ti-
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[15] Solubility problems came up when working at the concentration
range of the NMR technique (10À3 m). Conversely, the solubility of 1
and 2 was good enough when working at more dilute concentration
(10À4–10À5 m).
[16] Blank experiments revealed that the emission of 1 was not affected
by the presence of 2 under our experimental conditions ([1]=[2]=
5ꢃ10À5 m in acetone; lexc =400 nm). Thus, we consider that the
major cause for the above mentioned emission quenching process
would be the removal of the anion from 1. See the Supporting Infor-
mation.
[17] For a ternary mixture of 1, 2 and TBAbenzoate (AÀ) in equimolar
concentration (5ꢃ10À5 m) in acetone, having assumed a total dissoci-
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be studied are: 1+AÀ Q 1AÀ (K1A) and 2+AÀ Q 2AÀ (K2A). Using
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D. Curiel et al.
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[20] We assume that compound 2 has been completely oxidised to 2+.
Therefore, the later and 1 are the two receptors taking part in the
anion binding equilibria.
Received: April 3, 2009
Published online: July 2, 2009
7538
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Chem. Eur. J. 2009, 15, 7534 – 7538