A tridentate halogen-bonding receptor for tight binding of halide anions

Article abstract of DOI:10.1002/anie.200906488

Chemical Equation Presented It's In the I's: Three suitably oriented electron-deficient iodoaryl groups form the basis for the first anion receptor (see structure; white H, gray C, red O, blue F, purple I, green Cl) that employs the halogen-bonding interaction alone to achieve high-affinity molecular recognition in dilute solution. The anion selectivity of this tridentate host differs from those of similar receptors based on hydrogen bonding.

Full text of DOI:10.1002/anie.200906488

Communications
DOI: 10.1002/anie.200906488
Anion Recognition
A Tridentate Halogen-Bonding Receptor for Tight Binding of Halide
Anions**
Mohammed G. Sarwar, Bojan Dragisic, Sandeep Sagoo, and Mark S. Taylor*
In memory of Keith Fagnou
The selective recognition of anions by synthetic receptors is a
problem that continues to fascinate chemists.[1] Hydrogen
bonding has been the most frequently employed noncovalent
interaction for the design of such receptors: molecular
scaffolds that place H-bond donor groups in geometries
suitable for an anion of interest demonstrate remarkable
levels of selectivity and affinity.[2] Nonetheless, anion recep-
tors that rely upon other noncovalent forces, including Lewis
acid–base[3] and anion–p[4] interactions, have been investi-
gated, with considerable success. Such studies have provided
insight into the interactions employed, and have offered new
opportunities to achieve selectivity in anion recognition.
Here, we describe simple receptors capable of tight binding of
halide anions through multidentate halogen bonding inter-
actions (Figure 1). These represent the first systems in which
the cooperative action of multiple halogen-bond donors is
employed to achieve high-affinity binding in dilute solution.
The selectivities of these receptors differ substantially from
those of related receptors based on hydrogen bonding.
Figure 1. Structures of the multivalent halogen-bond donors 1a–1d,
Although halogen bonds between electron-deficient orga-
nohalides and electron donors were observed decades ago, it
is only recently that the generality and utility of this non-
covalent interaction have gained widespread appreciation.[5]
Halogen bonding has now been established as a powerful
strategy for self-assembly in condensed phases, and its
implications in biological systems are emerging.[6] Anions,
particularly halides, participate readily as halogen-bond
acceptors in the solid state,[7] including examples of crystalline
networks in which a single anion accepts multiple halogen
bonds.[8] This last observation suggests that a multidentate
halogen-bond donor capable of donating several halogen
bonds in a convergent fashion might be capable of anion
binding in dilute solution.[9]
and the ion-pair receptor 2a of Resnati and co-workers (Ref. [11a]).
Designing such a receptor presented a challenge. Appli-
cations of halogen bonding in self-assembly have relied
extensively on para-substituted iodotetrafluorobenzene
derivatives prepared by nucleophilic aromatic substitution
(for example, 2a).[10] This strategy results in divergent arrays
of halogen-bond donors useful for constructing noncovalent
polymers as well as two- and three-dimensional networks. In
contrast, it is poorly suited for orienting multiple donors in a
convergent fashion for binding to a single acceptor as is
generally required of a high-affinity host.[1,2] The most
successful example of anion binding by a halogen-bond
donor achieved to date makes use of ion-pair recognition:
receptor 2a shows a 20-fold higher affinity for sodium iodide
than does control receptor 2b, indicating a modest but
measurable increase in affinity resulting from halogen bond-
ing of iodide to the iodoarene groups (Figure 1).[11]
We chose to explore ortho-substituted iodoperfluoroar-
enes as the basis for receptors capable of multidentate
halogen bonding. Esters of 2,3,4,5-tetrafluoro-6-iodobenzoic
acid (which is easily prepared on multigram scale in one
step)[12] emerged as attractive targets (1a–1d, Figure 1): we
anticipated that the electron-withdrawing carboxy group
would promote halogen-bond donor ability, while enabling
a straightforward receptor synthesis through coupling reac-
tions of readily available diols and triols.[13] Receptors 1b and
1c were prepared to test the feasibility of bidentate halogen
[*] M. G. Sarwar, B. Dragisic, S. Sagoo, Prof. M. S. Taylor
Department of Chemistry, Lash Miller Laboratories
University of Toronto
80 St. George St., Toronto, ON M5S 3H6 (Canada)
Fax: (+1)416-978-8775
E-mail: mtaylor@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/MST/taylorgroup
[**] This work was funded by NSERC (Discovery Grants Program), the
Canadian Foundation for Innovation, the Province of Ontario,
Merck Research Laboratories (Canadian Academic Development
Program), and the University of Toronto. We are grateful to Prof.
Rebecca Jockusch for assistance and instrumentation used for the
ESI-MS binding studies.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906488.
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bonding of anions. Based on the well-studied conformational
behavior of hexasubstituted benzene derivatives and the
application of these species in receptor design,[14] we antici-
pated that 1d would act as a preorganized tridentate
receptor.[15] Calculations (HF, 6-31 + G*-LANL2DZdp[16]
)
suggested that multipoint binding of halides by such hosts
would be geometrically feasible (Figure 2, left).[17] The
Figure 2. Left: Calculated structure of the 1d–Clꢀ complex (HF/6-
31+G**-LANL2DZdp:[17] H white; C gray; O red; F blue; I purple;
Cl green). Right: Electrostatic potential surface of 1d (chloride-bound
conformation, B3LYP/6-31G*-LANL2DZdp: blue indicates sites of
partial positive charge and red sites of partial negative charge).
Figure 3. Top : 19F NMR titration of 1d (acetone, 295 K; change in
chemical shift jDdj (in ppm) vs [nBu4N+Clꢀ] (in mm)). Curve
represents the fit of a 1:1 binding isotherm. Bottom: Job plot
([1d+nBu4N+Clꢀ]=11 mm, jcDdj vs mole fraction c).
Table 1: Binding constants of 1a–d with nBu4N+Xꢀ (acetone, 295 K).
calculated gas-phase geometry of the 1d–Clꢀ complex is
characterized by I···Cl halogen-bond distances of 3.20 ꢀ and
CI···Clꢀ angles of 1788, results that are consistent with data
from the crystallographic literature.[18] The computed electro-
static potential surface of 1d in its chloride-bound conforma-
tion (B3LYP/6-31G*-LANLdp: Figure 2, right) illustrates the
extended region of partial positive charge created by the three
electron-deficient iodo groups. The relationship between such
sites of partial positive charge (termed the “s hole”) and the
halogen-bond interaction has been elucidated by Politzer and
co-workers.[19]
Entry
Halogen-bond
donor
Xꢀ
Ka [mꢀ1 [a]
]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1d
1d
1d
1d
1d
Clꢀ
Clꢀ
70
1.1ꢀ103
1.8ꢀ103
1.9ꢀ104
3.8ꢀ103
7.6ꢀ102
10
Clꢀ
Clꢀ
Brꢀ
Iꢀ
TsOꢀ
HSO4
ꢀ
<10
<10
ꢀ
NO3
The addition of monovalent anions as their tetrabutylam-
monium salts to 1a–d (acetone, 295 K) resulted in downfield
shifts of the 19F NMR spectra that are diagnostic of halogen
bonding (Figure 3, top).[20,21] Binding constants calculated by
curve-fitting 1:1 binding isotherms to these data are assem-
bled in Table 1. The Job plots of the 1c–Clꢀ (see the
Supporting Information) and 1d–Clꢀ interactions (Figure 3,
bottom) display maxima at a mole fraction c of 0.5, consistent
with a 1:1 binding stoichiometry.[22] Electrospray ionization
mass spectrometry (ESI-MS) provided additional support for
the formation of a 1d–Clꢀ complex of 1:1 stoichiometry (m/z
1151 amu; see the Supporting Information). Adducts 1d-Brꢀ
(m/z 1195) and 1d–Iꢀ (m/z 1243) were also observed by ESI-
MS.
[a] Binding constants Ka were determined by curve-fitting 1:1 binding
isotherms to 19F NMR titration data. Each reported binding constant
reflects an average of two or three independent determinations, with an
estimated error of ꢁ10%. See the Supporting Information.
probe the possibility that receptors 1a–1d interact with
halides through anion–p interactions rather than halogen
bonding, a modified version of receptor 1c was prepared in
which the two iodo groups were replaced by fluorine
substituents (see the Supporting Information). This modified
receptor showed no detectable affinity for chloride in acetone
(Ka < 10mꢀ1), suggesting that halogen bonding is the domi-
nant contributor to the observed binding.[24] This observation
is consistent with previous observations of anion recognition
by electron-deficient, uncharged p systems, which have shown
that such interactions are generally weaker than those studied
here.[25]
The significant effect of “chelate cooperativity”[23] in
halogen bonding is evident from the binding data shown in
Table 1: the chloride affinities of the bidentate receptors 1b
and 1c are more than an order of magnitude higher than that
of benzyl ester 1a, which serves as the control experiment in
this study. Likewise, the additional halogen-bonding inter-
action possible in tridentate receptor 1d results in a further
increase in association constant of an order of magnitude in
comparison to the bidentate systems (Ka = 1.9 ꢁ 104 mꢀ1). To
While the 1d–Clꢀ association constant is modest in
comparison to those of the best synthetic chloride recep-
tors,[26,27] it represents, by several orders of magnitude, the
highest-affinity receptor developed to date that operates in
dilute solution using halogen bonding alone. The affinity of 1c
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1997; b) Anion Receptor Chemistry (Eds.: J. L. Sessler, P. A.
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and 1d for chloride compare favorably to binding constants of
neutral, hydrogen-bond donors that employ two or three
contacts to halide anions, respectively.[28]
The anion selectivity displayed by 1d differs significantly
from those of hydrogen-bonding-based receptors: while 1d
interacts with bromide and iodide more weakly than with
chloride, it shows negligible affinity for p-toluenesulfonate,
nitrate, and bisulfate (Table 1).[29] In contrast, hydrogen-
bonding receptors with similar geometries show measurable
affinity for such oxoanions.[30] We speculate that receptor 1d is
sufficiently flexible to accommodate a variety of guests, and
that the differences in 1d–-anion association constants do not
result from discrimination based on anion size. Instead, we
propose that the selectivity of 1d reflects the intrinsic
preference of the halogen-bonding interaction for this series
of anions. Quantitative thermodynamic data for halogen
bonding of anions in solution are not available,[31] and even
qualitative comparisons are lacking. Crystallographic data
and the behavior of receptor 2a have been used to support the
trend in acceptor ability Iꢀ > Brꢀ > Clꢀ,[5b,11a] while other data
from the solid state suggest the reverse (Clꢀ > Brꢀ > Iꢀ).[32]
Calculations support the latter proposal.[33]
We measured the binding constants of iodopentafluoro-
benzene to the anions shown in Table 1 (see the Supporting
Information), and found that the affinities decrease in the
order Clꢀ > Brꢀ > Iꢀ @ TsOꢀ, NO3ꢀ, HSO4ꢀ, in accordance
with the data of Brammer and co-workers.[32] The preference
of the halogen-bond donor for the anion having the highest
charge density is consistent with the formulation of halogen
bonding as an interaction involving a dominant electrostatic
component.[19,34] However, the weak binding to oxoanions
known to act as hydrogen-bond acceptors points towards a
fundamental difference between halogen bonding and hydro-
gen bonding. A non-negligible contribution of dispersion and/
or charge transfer to the halogen-bond interaction is a
plausible explanation. Additional experiments and calcula-
tions are underway to clarify this point.
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Clearly, improvements are needed if receptors of this type
are to be employed in “real-world” applications where high
selectivity and the ability to function in competitive media are
required. Nonetheless, the ability to develop anion sensors
based on a class of interaction that differs from hydrogen
bonding in its geometric and energetic features represents an
important fundamental step. Our ongoing efforts are aimed at
developing halogen-bonding-based receptors with lower
conformational mobility and altered anion selectivity, and at
exploring the potential for cooperativity or complementarity
between halogen- and hydrogen-bonding-based molecular
recognition in solution.
[12] R. D. Richardson, J. M. Zayed, S. Altermann, D. Smith, T. Wirth,
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Received: November 18, 2009
Published online: January 29, 2010
[13] See the Supporting Information for details.
[14] a) For a review: G. Hennrich, E. V. Anslyn, Chem. Eur. J. 2002, 8,
2218 – 2224; b) K. V. Kilway, J. S. Siegel, Tetrahedron 2001, 57,
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[15] Although trisubstituted mesitylene derivatives show a lower
degree of preorganization than trisubstituted triethylbenzenes,
the former have been used productively in receptor design. For
examples, see: a) K. Sato, S. Arai, T. Yamagishi, Tetrahedron
Lett. 1999, 40, 5219 – 5222; b) K. H. Ahn, S.-G. Kim, J. Jung, K.-
Keywords: anions · halogens · molecular recognition ·
noncovalent interactions · supramolecular chemistry
.
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[26] The “cholapods” of Davis and co-workers show Ka values as high
as 1011 with chloride in chloroform: J. P. Clare, A. J. Ayling, J.-B.
Joos, J.-A. L. Sisson, G. Magro, M. N. Pꢅrez-Payꢆn, T. N.
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[27] Receptors capable of binding chloride in water or mixed
aqueous/organic solutions have also been developed. For
charged receptors: a) E. Graf, J.-M. Lehn, J. Am. Chem. Soc.
1976, 98, 6403 – 6405; b) M. Staffilani, K. S. B. Hancock, J. W.
Steed, K. T. Holman, J. L. Atwood, R. K. Juneja, R. S. Burkhal-
ter, J. Am. Chem. Soc. 1997, 119, 6324 – 6335; for neutral
receptors: c) S. Kubik, R. Kirchner, D. Nolting, J. Seidel, J. Am.
Chem. Soc. 2002, 124, 12752 – 12760; d) D.-W. Yoon, D. E. Gross,
V. M. Lynch, C.-H. Lee, P. C. Bennett, J. L. Sessler, Chem.
Commun. 2009, 1109 – 1111.
[28] Comparisons of Ka values determined in different solvents may
be problematic. However, we note that the association constant
for 1c–Clꢀ is within an order of magnitude of bidentate H-
bonding receptors: a) K. Kavallieratos, S. R. de Gala, D. J.
Austin, R. H. Crabtree, J. Am. Chem. Soc. 1997, 119, 2325 –
2326; b) M. A. Hossain, S. O. Kang, D. Powell, K. Bowman-
James, Inorg. Chem. 2003, 42, 1397 – 1399. The 1d–Clꢀ associ-
ation is three orders of magnitude weaker than that of a
structurally similar tridentate H-bond donor: c) A. H. McKie, S.
Friedland, F. Hof, Org. Lett. 2008, 10, 4653 – 4655.
[16] LANL2DZdp refers to the LANL2DZ effective core potential
augmented with diffuse functions of p symmetry and polar-
ization functions of d symmetry, optimized for calculations
involving hypervalent iodine. For the LANL2DZdp basis set:
a) C. E. Check, T. O. Faust, J. M. Bailey, B. J. Wright, T. M.
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For the LANL2DZ effective core potential: b) P. J. Hay, W. R.
Wadt, J. Chem. Phys. 1985, 82, 270 – 283.
[17] Geometry optimization of the 1d–Clꢀ complex (HF,
LANL2DZdp basis set for iodine and 6-31 + G** for all other
elements: see ref. [16]) was carried out with Gaussian03
(RevisionC.02): M. J. Frisch et al., see the Supporting Informa-
tion.
[18] S. Triguero, R. Llusar, V. Polo, M. Fourmiguꢅ, Cryst. Growth Des.
2008, 8, 2241 – 2247.
[19] T. Clark, M. Hennemann, J. S. Murray, P. Politzer, J. Mol. Model.
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[20] P. Metrangolo, W. Panzeri, F. Recupero, G. Resnati, J. Fluorine
Chem. 2002, 114, 27 – 33.
[21] The changes in 19F NMR chemical shift of the fluorine sub-
stituent ortho to the iodo group upon addition of anions were
used to calculate the Ka values for all interactions studied. The
magnitudes of these chemical shift changes ranged from 1.2 to
2 ppm for 1a–1d. The fluorine substituents that are not adjacent
to the iodo group showed changes in chemical shift of a similar
magnitude during these titrations. We have observed similar
behavior in 19F NMR titrations of iodopentafluorobenzene with
a range of neutral halogen-bond acceptors (M. G. Sarwar, B.
Dragisic, L. Salsberg, C. Gouliaras, M. S. Taylor, J. Am. Chem.
Soc., in press).
[29] Decomposition was observed in the presence of fluoride and
acetate.
[30] For example, the tridentate host of Hof and co-workers
(ref. [28c]) that employs tetrazole H-bond donors on a triethyl-
benzene scaffold binds to anions in the order Clꢀ > Brꢀ > TsOꢀ
> NO3ꢀ, HSO4 > Iꢀ. The Gibbs free energies of transfer of
ꢀ
anions from water to acetonitrile, another metric related to
hydrogen-bond-accepting ability, follow the order Clꢀ > Brꢀ
>
NO3 > Iꢀ: see Y. Marcus, Pure Appl. Chem. 1983, 55, 977 –
ꢀ
1021.
ꢀ
[31] For the thermodynamics of a C6F5I···F NiLn interaction: S. Libri,
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[24] The 19F NMR signals of this compound changed by less than d =
0.2 ppm upon titration with tetrabutylammonium chloride in
acetone.
[25] For example, a receptor that employs a pentafluorophenyl group
in cooperation with a sulfonamide hydrogen-bond donor binds
chloride with a Ka of 30mꢀ1 (CDCl3, ref. [4e]), while a receptor
employing three 3,5-dinitrobenzoyl groups binds chloride with a
Ka of 50mꢀ1 (C6D6, ref. [4f]).
[34] The preference of ion-pair receptor 2a for iodide over chloride
and bromide is puzzling, in light of these data. Perhaps this
selectivity reflects the higher energetic penalty for ion separation
of NaCl relative to NaI.
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