A halogen-bonding catenane for anion recognition and sensing

Article abstract of DOI:10.1002/anie.201108404

The anion-templated synthesis of a halogen-bonding catenane, which recognizes chloride and bromide ions solely by halogen bonding, is described. The catenane's ability to optically sense halide anions using fluorescence spectroscopy is demonstrated.

Full text of DOI:10.1002/anie.201108404

.
Angewandte
Communications
DOI: 10.1002/anie.201108404
Halogen Bonding
A Halogen-Bonding Catenane for Anion Recognition and Sensing**
Antonio Caballero, Fabiola Zapata, Nicholas G. White, Paulo J. Costa, Vꢀtor Fꢁlix, and
Paul D. Beer*
The field of anion supramolecular chemistry has expanded
enormously during the past few decades, inspired in large part
by the realization of the many fundamental roles negatively
charged species play in a range of chemical, biological,
medical and environmental processes.[1] Through the incor-
poration of complementary electrostatic, hydrogen bonding,
Lewis acid–base[2] and anion–p non-covalent interactions[3]
into acyclic and macrocyclic molecular framework design,
numerous efficient synthetic anion receptors and sensors have
been developed. However, the challenge to mimic the degree
of selectivity exhibited by biological anion-binding proteins
still remains. Halogen bonding (XB) is the attractive non-
covalent interaction between an electron-deficient, positively
polarized halogen atom, commonly bromine or iodine, and a
Lewis base.[4] Anions have been exploited extensively as XB
acceptors in the solid state crystal engineering[5] of conduct-
ing, magnetic and liquid-crystalline materials.[6] Given XBꢀs
complementary analogy to hydrogen bonding in terms of
stringent directionality and bond strength,[7] it is surprising
that solution phase applications of XB[8] in molecular
recognition processes such as protein–ligand[9] complexes,
anion receptor chemistry,[10] and catalysis[11] are only now
beginning to emerge.
selectively recognizes iodide through the combination of
halogen and hydrogen bonding interactions from respectively
the axle and macrocyclic components of the interlocked host
system.[13] Herein we describe the anion-templated synthesis
of the first XB catenane which selectively recognizes chloride
and bromide anions solely by halogen bonding, through
cooperative action of two bromine halogen bond donor
atoms. Furthermore we demonstrate the XB catenaneꢀs
ability to optically sense anions using fluorescence spectros-
copy.
The target acyclic precursor bromine-functionalized imi-
dazolium compound 6+ was designed to incorporate comple-
mentary supramolecular interactions to facilitate the assem-
bly of an orthogonal 2:1 host-to-guest stoichiometric complex
around a halide anion template using XB (Figure 1).
By using anion templation, we have constructed three-
dimensional interpenetrated and interlocked molecular host
systems that exhibit a high degree of selectivity towards the
templating anion through primarily electrostatics and con-
vergent hydrogen bonding.[12] In an effort to influence
significantly the steric, electronic and lipophilic character-
istics, and hence anion recognition properties, of the inter-
locked binding pocket, we recently reported the first XB-
bonding rotaxane containing an iodotriazolium axle which
Figure 1. Schematic representation of halogen bonding anion tem-
plated assembly of an orthogonal 2:1 stoichometric complex.
The acyclic precursor molecule contains a bromine
halogen bond-donating imidazolium group covalently linked
through naphthalene and hydroquinone motifs to vinyl
functional groups, such that highly directional, linear coop-
erative XB halide interactions would favor formation of the
orthogonal assembly. In addition, p–p stacking interactions
between the intercalated electron deficient bromoimidazo-
lium motif and electron-rich hydroquinones are designed to
further stabilize the XB-associated assembly prior to ring
closing metathesis (RCM) double cyclization catenane syn-
thesis. Importantly, the naphthalene spacer unit of precursor
6+ not only serves as a rigid linker, but also has the potential
to act as a fluorescent reporter group for catenane host anion
sensing.
[*] Dr. A. Caballero, Dr. F. Zapata, N. G. White, Prof. P. D. Beer
Chemistry Research Laboratory, Department of Chemistry
University of Oxford, Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: paul.beer@chem.ox.ac.uk
Dr. P. J. Costa, Prof. V. Fꢀlix
Departamento de Quꢁmica, CICECO and Secżo Autꢂnoma de
CiÞncias da Safflde, Universidade de Aveiro, 3810-193 Aveiro
(Portugal)
[**] A.C. thanks the European Union for a Marie Curie Postdoctoral
Fellowship. F.Z. thanks the Ministry of Education of Spain for a
postdoctoral contract (Programa Nacional de Movilidad y Recursos
Humanos del Plan Nacional I+D+I 2008-2011). N.G.W. thanks
the Clarendon Fund and Trinity College for a studentship. P.J.C.
thanks FCT for the postdoctoral grant SFRH/BPD/27082/2006. We
express our gratitude to Diamond Light Source for the award of
beamtime on I19 (MT1858).
Precursor compound 6+·BrÀ was prepared in a stepwise
procedure shown in Scheme 1. The reaction of 4-(2-(allylox-
y)ethoxy)phenol (1)[14] and 2,7-bis(bromomethyl)naphtha-
lene (2) in the presence of base produced the bromomethyl-
naphthalene derivative 3 in 31% yield. Alkylation of
bromoimidazole 4[15] with 3 under basic conditions gave 5 in
near quantitative yield. The coupling of 5 and 3 afforded the
desired bis-vinyl appended precursor 6+·BrÀ in a yield of 60%
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108404.
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Scheme 2. Synthesis of XB catenane 72+·BrÀ PF6À. Reagents and
conditions: a) Grubbs’ 2nd generation catalyst (10 wt%), dichloro-
methane, RT, 16 h (24%).
Scheme 1. Synthesis of acycle precursor 6+·BrÀ. Reagents and condi-
tions: a) K2CO3, acetonitrile, RT, 16 h (31%); b) NaOH (1m in water),
acetonitrile, RT, 16 h (98%); c) 3, acetonitrile, 608C, 48 h (60%).
(Scheme 1), and anion exchange with NH4PF6 (aq) gave the
corresponding hexafluorophosphate salt 6+·PF6
.
À
Catenane 72+ was synthesized by RCM reaction of
À
equimolar amounts of the BrÀ and PF6 salts of 6+ in
dichloromethane, using Grubbsꢀ 2nd generation catalyst (10
wt%). The catenane 72+BrÀ·PF6 was isolated by silica gel
À
preparatory thin layer chromatography in 24% yield
(Scheme 2). Bromide anion template removal was achieved
by the addition of a saturated aqueous solution of NH4PF6 to
Scheme 3. Synthesis of the macrocycle 8+·PF6À. Reagents and condi-
tions: 1) Grubbs’ 2nd generation catalyst (10 wt%), dichloromethane,
RT, 16 h (25%).
À
À
72+·BrÀ·PF6 in acetonitrile to afford the catenane 72+·2PF6
in almost quantitative yield.
It is noteworthy that the bromide anion template was
found to be crucial to the synthesis of the XB catenane.
Repeating the RCM reaction by using 6+·PF6À in the presence
of 10 wt% Grubbsꢀ 2nd generation catalyst in dichloro-
methane gave only the macrocycle 8·PF6À which was isolated
in 25% yield by silica gel preparatory thin layer chromatog-
raphy (Scheme 3).
[M2+ + PF6À = 1825.5012] was also detected (see Supporting
Information).
Crystals of 8+ suitable for single-crystal X-ray structural
analysis were grown by the slow evaporation of an aqueous
solution of the chloride/bromide salt of the macrocycle. The
asymmetric unit contains two monocationic macrocycles as
well as one bromide, one chloride counterion and several
water solvates (see Supporting Information). The bromoimi-
dazolium rings are twisted out of the plane of the macrocycle
(Figure 2).
Both catenane 72+ and macrocycle 8+ were characterized
1
by H and 13C NMR spectroscopy and high resolution mass
spectrometry. The high resolution mass spectra reveal that the
isotopic distribution of the macrocycle 8+ [M+] and the
catenane 72+ [M2+] are in good agreement with their
simulated spectra and the peak corresponding to 72+·PF6
À
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upfield, which suggests that the respective co-conformations
of the catenane salts are similar.[16]
The anion recognition properties of the XB catenane
72+·2PF6 and macrocycle 8+·PF6 were investigated by
À
À
À
fluorescence spectroscopy. Catenane 72+·2PF6 exhibits one
broad and structureless emission band at l = 309 nm in
CH3CN (c = 1 ꢁ 10À5 m), exciting at l = 280 nm, which is
attributed to the monomer emission of the naphthalene
motif. The macrocycle 8+·PF6 (c = 1 ꢁ 10À5 m in CH3CN,
À
lexc = 280 nm) displays two emission bands at l = 309 nm,
assigned to the monomer emission, and l = 425 nm character-
istic of the excimer emission of the naphthalene.[17]
Figure 2. Solid state structure of macrocycle 8+. Counterions and
hydrogen atoms are omitted for clarity; ellipsoids are shown at 50%
probability. Colour scheme: gray=carbon, blue=nitrogen, red=oxy-
gen, brown=bromine.
The addition of increasing amounts of FÀ, ClÀ, BrÀ, IÀ,
AcOÀ, H2PO4À, NO3 as tetrabutylammonium (TBA) salts
À
and HCO3À as tetraethylammonium (TEA) salt to macrocycle
Comparison of the 1H NMR spectra in CD3CN of the
8+·PF6 (c = 1 ꢁ 10À5 m in CH3CN) caused no significant
À
À
À
catenane salts 72+·BrÀ·PF6 and 72+·2PF6 with macrocycle
perturbation in the emission spectra of the macrocycle (see
Supporting Information). By contrast the addition of ClÀ and
BrÀ to catenane 72+·2PF6À (c = 1 ꢁ 10À5 m in CH3CN) showed
a progressive decrease of the intensity of the emission band
located at l = 309 nm concomitant with the appearance and
increase of a new broad band at l = 445 nm (Figure 4a). The
À
8+·PF6 provides evidence of the interlocked nature of the
catenane (Figure 3).
Importantly the hydroquinone protons of the macrocycle
(Figure 3a) have split significantly in the catenane (Fig-
ure 3b) with hydroquinone protons g moving upfield which is
indicative of p–p stacking interactions between the bromoi-
midazolium groups of one ring and the hydroquinone ring of
the other.
In addition the naphthalene protons j, k, m, and n of the
catenane are shifted upfield in comparison with the same
protons of the macrocycle and there is a large downfield shift
in the naphthalene proton i suggesting the proximity of the
bound bromide. Upon exchange to the hexafluorophosphate
salt of the catenane (Figure 3c), there is an upfield shift of the
naphthalene proton i, due to the removal of the bromide
anion template. However, the hydroquinone protons g and h
remains split and the naphthalene protons j, k, m, and n stay
Figure 4. a) Changes in the emission spectra of the catenane 72+·2PF6
À
(1”10À5 m in CH3CN) upon the addition of increasing amount of ClÀ;
b) Changes of emission at 445 nm upon addition of ClÀ.
titration profiles revealed a 1:1 catenane/anion stoichiometry
(Figure 4b) and association constants were determined using
Specfit[18] (Table 1). Both ClÀ and BrÀ are bound strongly,
with the smaller halide anion being preferentially complexed
by a significant factor of 25-times. Remarkably the addition of
excess amounts of FÀ, IÀ, AcOÀ, H2PO4À, NO3À, and HCO3
À
À
to a solution of the catenane 72+·2PF6 caused no significant
Figure 3. Comparison of the 1H NMR spectra in CD3CN at 208C of the
perturbations of the interlocked hostꢀs emission spectrum
(see Supporting Information). This suggests the unique
preorganized interlocked cavity of catenane 72+ is of exclusive
a) macrocycle 8+·PF6À, b) catenane 72+·PF6À·BrÀ, and c) catenane
72+·2PF6
.
À
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Table 1: Association constants Ka for catenane 72+·2PF6 with different
anions in CH3CN at 208C.
À
illustrated with this particular snapshot. Also, one bromoimi-
dazolium unit is stacked over another hydroquinone group.
The stability of the halogen bonded assembly in the
catenane system was accessed by monitoring both the Br···ClÀ
distances and the C-Br···ClÀ angles, since the halogen bonds
are characterized by short and almost linear contacts. The
histograms built with 3 ꢁ 30 ns data were projected on a 2D
surface and are shown in the Supporting Information (Fig-
ure S24) for both macrocycles. The areas with larger densities
Receptor
Anion[a]
Ka [mÀ1 [b]
]
72+·2PF6
FÀ
NB[c]
3.71”106
1.48”105
NB
NB
NB
À
ClÀ
BrÀ
IÀ
AcOÀ
H2PO4
NO3
HCO3
À
À
NB
NB
À
À
À
(red) are located at Br···Cl = 3.4 ꢂ and C Br···Cl = 172.58,
which in addition to the relatively narrow area of the density
maxima, is consistent with the continuous presence of two
simultaneous Br···ClÀ halogen bonds.
À
À
[a] Anions have been used as tetrabutylammonium salts, except HCO3
which was added as a tetraethylammonium salt. [b] Error <10%. [c] NB:
,
No evidence of binding.
In conclusion, the first halogen-bonding catenane has
been prepared by bromide anion templation. Upon removal
of the bromide anion template, fluorescence spectroscopic
investigations reveal the catenane host system to bind
strongly and sense selectively chloride and bromide, where
impressively, no significant optical response is detected with
complementary size for recognition of ClÀ and BrÀ anions
through intra-ring cooperative XB bromoimidazolium–halide
interactions.
Molecular dynamics (MD) simulations were performed in
an explicit acetonitrile solvent model using a suitable
structure of catenane 72+·PF6À·ClÀ found by quenched
dynamics (see Supporting Information). The MD simulations
were carried out with Amber11[19] using the pmemd.cuda
AMBER executable, able to accelerate explicit solvent
Particle Mesh Ewald (PME) calculations through the use of
GPUs.[20] Halogen bonding is not described by the standard
force field parameters, but recently two similar additions to
the General Amber Force Field (GAFF)[21] were proposed to
describe this interaction.[22,23] We followed a similar approach
and parameterized a dummy atom (DU) optimally located at
a Br–DU distance of 2.3 ꢂ. Three MD runs of 30 ns each were
performed in a NPT ensemble leading to a total of 90 ns
sampling time. The remaining simulation and parameteriza-
tion details are given in the Supporting Information. A
representative snapshot of the three replicas was extracted by
clustering the MD trajectories and is represented in Figure 5.
Two simultaneous halogen bonds are established between
the bromine atoms of both macrocycles and the chloride
anion. Furthermore, the p–p stacking interaction between
one hydroquinone and a naphthalene group was also
observed throughout the course of MD simulation as
FÀ, IÀ, AcOÀ, H2PO4À, and HCO3 anions. These observa-
À
tions indicate the unique, preorganized topological nature of
the catenane host cavity facilitates the efficacy of cooperative
XB recognition of chloride and bromide guest species. The
incorporation of XB donor groups into interlocked host
structural frameworks is continuing in our laboratories.
Received: November 29, 2011
Published online: January 16, 2012
Keywords: anions · catenane · halogen-bonding ·
.
molecular dynamics · supramolecular chemistry
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Angew. Chem. Int. Ed. 2012, 51, 1876 –1880