Modulating Chalcogen Bonding and Halogen Bonding Sigma-Hole Donor Atom Potency and Selectivity for Halide Anion Recognition

Article abstract of DOI:10.1002/anie.202108591

A series of acyclic anion receptors containing chalcogen bond (ChB) and halogen bond (XB) donors integrated into a neutral 3,5-bis-triazole pyridine scaffold are described, in which systematic variation of the electronic-withdrawing nature of the aryl sub

Full text of DOI:10.1002/anie.202108591

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Accepted Article
Title: Modulating Chalcogen Bonding and Halogen Bonding Sigma-
Hole Donor Atom Potency and Selectivity for Halide Anion
Recognition
Authors: Andrew Docker, Charles Guthrie, Heike Kuhn, and Paul D.
Beer
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Modulating Chalcogen Bonding and Halogen Bonding Sigma-
Hole Donor Atom Potency and Selectivity for Halide Anion
Recognition
[
a]
Andrew Docker, Charles H. Guthrie, Heike Kuhn and Paul Beer*
[
a] A. Docker, C.H. Guthrie, H. Kuhn, Prof. P.D. Beer
Department of Chemistry, University of Oxford
Chemistry Research Laboratory
Mansfield Road, Oxford OX1 3TA (United Kingdom).
E-mail: paul.beer@chem.ox.ac.uk
homologous ChB, XB and HB acyclic receptor systems. The first
generation receptors investigated, 1·ChB, 1·XB and 1·HB,
comprise of a 3,5-bistriazole pyridine motif covalently appended
Abstract: A series of acyclic anion receptors containing chalcogen
bond (ChB) and halogen bond (XB) donors integrated into a neutral
3,5-bis-triazole pyridine scaffold are described, in which systematic
with either phenyl or highly electron deficient perfluorophenyl
variation of the electronic-withdrawing nature of the aryl substituents
reveal a dramatic modulation in sigma-hole donor atom potency for
anion recognition. Incorporation of strongly electron-withdrawing
perfluorophenyl units appended to the triazole heterocycle telluro- or
iodo- donor atoms, or directly linked to the tellurium donor atom
dramatically enhances the anion binding potency of the sigma-hole
receptors, most notably for the ChB and XB receptors displaying over
thirty-fold and eight-fold increase in chloride anion affinity respectively,
relative to unfluorinated analogues. Linear free energy relationships
for a series of ChB based receptors reveal the halide anion
recognition behaviour of the tellurium donor is highly sensitive to local
electronic environments. This is especially the case for those directly
1
(
PFP) substituents (Figure 1a). Extensive H NMR halide anion
titration studies reveal the integration of PFP groups dramatically
enhance ChB, and to a lesser extent XB, anion binding potency,
whereas HB analogues exhibited no measureable effect.
Systematically varying the electron-withdrawing/-donating
characteristics of aryl substituents appended to the triazole
heterocycle 2·ChB (Figure 1b) or directly bonded to the tellurium
donor atom 3·ChB (Figure 1c) facilitated the construction of
Hammett plots, establishing linear free energy relationships
(LFER), which crucially highlight ChB halide anion affinity and
selectivity is highly sensitive to electronic through-bond
modulation
of
chalcogen
bond
donor
potency.
appended to the Te centre (3·ChB), where
a remarkable
enhancement of strength of binding and selectivity for the lighter
halides is observed as the electron-withdrawing ability of the Te-
bonded aryl group increases, highlighting the exciting opportunity to
fine-tune anion affinity and selectivity in ChB-based receptor systems.
Introduction
In the context of anion recognition, during the last few decades,
anion receptor design has been dominated by the employment of
hydrogen bonding (HB).[1,2] Indeed, the variety and wealth of HB
receptors that utilise a diverse range of HB donor types, have
contributed enormously to the current understanding of HB-anion
interactions. In more recent years, sigma-hole based interactions
including halogen bonding (XB) and chalcogen bonding (ChB) are
emerging as promising additions to the supramolecular host-
[
3–11]
anion guest tool-box,
as well as for self-assembly and
catalysis.[
12–21]
However, elucidating the influence of key structural
Figure 1. a) Perfluorophenyl and phenyl appended receptor series 1·ChB, 1·XB
and 1·HB b) Aryl triazole functionalised of telluromethyl 2·ChB receptor series.
c) Te-bound aryl bis-triazole 3·ChB receptor series.
and electronic factors for modulating and importantly raising the
potency, and molecular engineering selectivity in sigma-hole
based anion recognition remains largely underdeveloped.[12,20,22]
We have recently demonstrated the potency of XB and ChB Results and Discussion
interactions for ion-pair recognition is highly sensitive to the local
electronic environment of the sigma hole donor atom.[23,24]
Synthesis
Specifically, co-bound sodium cation crown ether binding in XB
and ChB heteroditopic receptor systems dramatically switches on
the recognition of sigma hole donor halide anion interactions.
Motivated by these observations, we sought to further investigate
the mechanism of sigma hole through-bond polarisation. To this
end we report herein, the synthesis, anion binding studies and
linear free energy relationship analysis of a library of structurally
The target ChB, XB and HB receptors were prepared via CuAAC
methodology (Scheme 1). The requisite iodo- and tellurium-
functionalised alkyne precursors, 2 and 4 were synthesized
according to Scheme 1, from 3,5-diethynyl pyridine 1. Treatment
of the proto-alkyne with N-iodomorpholine hydroiodide in the
1
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RESEARCH ARTICLE
Scheme 1. a) Synthesis of the requisite functionalised alkyne derivatives. CuAAC mediated synthesis of the b) 1·ChB, 1·XB and 1·HB series c) 2·ChB series d)
·ChB series.
3
presence of catalytic copper(I) iodide gave the bis-iodoalkyne 2 in
1% yield. Reaction of 1 with freshly pulverised AgNO in
NH OH(aq)/MeOH mixtures facilitated precipitation and isolation of
bis-Ag(I)-acetylide 3.
achieved by treatment of the appropriate dialkyl- or diaryl-
ditelluride with a 1 M Br CH Cl solution to afford the organo
8
3
Preliminary 1H NMR spectroscopic titration experiments in
acetone-d were performed to investigate the anion binding
properties of the novel 1·ChB, 1·XB and 1·HB receptor series.
4
[25,26]
6
Synthesis of the bis-telluro alkynes was
2
2
2
tellurium bromide which was reacted immediately with a THF
suspension of 3, affording the series of bis-telluro functionalized
alkynes 4 and 5R after rapid chromatographic purification. In
general the synthesis of all target ChB, XB and HB based
receptors was achieved by a CuAAC reaction between the
appropriately functionalised alkyne and aryl-azide, in the
4 6
presence of catalytic [Cu(MeCN) ]PF and the rate accelerating
ligand TBTA in dichloromethane. Subsequent aqueous work-up
procedures of the reaction mixtures and purification of the crude
material by column chromatography gave the receptors in yields
in the range 41-81%. The 20 novel anion receptors were
characterised by H NMR, 13C NMR, and high resolution ESI-MS.
Crystals of 1·XBPFP, 1·ChBPFP, 1·HBPFP, 3·ChBpCl, 3·ChB2F and
1
3
·ChB3F suitable for X-ray structural analysis were grown from
slow evaporation of chloroform solutions (Figure 2). The
structures show the σ-hole donors participating in intermolecular
N···I XB or N···Te ChB interactions respectively whilst the
1
·HBPFP forms linear chains in the solid state directed by triazole
C-H···N interactions and 1·HBPFP
.
Figure 2. X-ray crystallographic structures of a) 1·ChBPFP, b) 1·XBPFP and c)
·ChBPFP d) 3·ChBpCl e) 3·ChB2F f) 3·ChB3F showing the formation of
1
ChB/XB/HB interactions in the solid state, represented by dashed lines. Grey =
carbon, blue = nitrogen, white = hydrogen, green = fluorine, dark green =
chlorine, orange = tellurium, purple = iodine.
Anion Binding Studies of 1·ChB, 1·XB and 1·HB receptor
series.
2
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RESEARCH ARTICLE
Figure 3. a) Chloride anion binding equilibria for 1·ChBPFP. b) Stacked H NMR titration spectra for 1·ChB with increasing equivalents of TBACl (acetone-d
1
PFP
, 500
6
MHz, 298K). c) Anion Binding titration isotherm for 1·ChBPFP with chloride, bromide and iodide.
Table 1. Halide anion association constants and calculated binding enhancement factors for phenyl and pefluorophenyl appended ChB, XB and HB acyclic anion
receptor systems.
Anion association constants K
a
/ M-1 [a]
1·HBPFP[c]
Binding enhancement factors α = (K
a a
PFP/ K Ph)
Anion[b]
1·ChBPFP[c]
18,500[e]
4,620
1·ChBPh[a]
590
1·XBPFP[d]
2,920
1·XBPh[d]
416
1·HBPh[c]
394
ChB
XB
HB
Cl-
Br-
I-
386
116
37
32 ± 4
7.0 ± 0.2
4.4 ± 0.3
3.6 ± 0.3
0.86 ± 0.06
0.76 ±0.03
0.86 ±0.05
304
4,600
1,060
1,450
191
15 ± 0.5
5.2 ± 0.4
1,230
234
5,230
49
[
a] Association constants determined by Bindfit analysis of internal pyridine proton signal, errors < 5% unless otherwise specified. [b] Anions added as their
tetrabutylammonium salts. [c] Conducted in acetone-d [d] Conducted in 2.5% D O-acetone-d (v/v) [e] Error < 13%.
6
2
6
Aliquots of halides as their tetrabutylammonium (TBA) salts were
titrated against solutions of the six receptors, with significant
perturbations in various proton resonances commonly observed.
Typically, with the XB and ChB receptors the external pyridyl
signal underwent an upfield shift on addition of the TBA halide
salts, whereas the internal pyridyl proton universally shifted
downfield. Concomitant downfield perturbations in the chemical
shift of the triazole C-H and TeMe protons for the HB and ChB
receptors respectively, were also observed upon addition of anion
titrant, indicating that the binding event occurs in the bis-triazole
cleft (Figure 3a and 3b). Isotherm binding titration data, obtained
by monitoring the chemical shift of the respective receptor’s
internal pyridyl proton b local to the anion binding cavity as a
function of anion concentration (Figure 3c), was analysed using
Bindfit[27] to determine 1:1 stoichiometric host:guest halide
association constants displayed in Table 1. Halide anion binding
affinities correlate with charge density and basicity of the halide
As anticipated, the XB receptors display the greatest halide
affinities, followed by the ChB hosts: XB > ChB > HB. Notably, the
incorporation of perfluorophenyl substituents in receptors 1·XBPFP
and 1·ChBPFP produces a dramatic enhancement in halide
association constant magnitudes relative to their unperfluorinated
receptor analogues. Indeed, the remarkable halide anion affinities
of perfluorophenyl-substituted 1·XBPFP (>105 M ) precluded
accurate quantification of its halide association constants,
necessitating the more competitive solvent mixture of 2.5%
-1
D
2
O/acetone-d
6
(v/v) for their evaluation which resulted in a
Hofmeister bias for anion afffinity; I-
>
Br-
>
Cl .
- [28]
The
incorporation of spacer perfluorophenyl groups to the receptor
produces a dramatic enhancement in its halide affinities relative
Ph [27]
to 1·XB
.
Inspection of Table 1 also reveals a significant
enhancement in halide affinity for 1·ChBPFP relative to 1·ChBPh
,
illustrating the influence of the perfluorophenyl electron-
withdrawing groups on the halide affinities of both XB and ChB
sigma-hole type receptors. This is in stark contrast to that
observed with the HB receptors, which surprisingly appear
-
-
-
ion: Cl > Br > I .
3
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RESEARCH ARTICLE
unaffected by the incorporation of electron-withdrawing
perfluorophenyl groups in their receptor design. To quantitatively
evaluate the effects of perfluorophenyl group incorporation in
these receptor systems, halide anion binding enhancement
aryl substituents on halide association constant values is
quantified, as represented by the ρ value, which serves as a
measure of the sensitivity of the ChB-anion interaction to the
changing electronic properties of an aryl substituent. Summarised
in Table 4, the ρ > 0 value for all halides indicates that anion
binding potency increases as the sum of the electron withdrawing
substituent constants (Σσ) increases. The sensitivity of this
through-bond inductive polarisation appears to most pronounced
for the lighter halides, Cl- and Br- with ρ = 0.91 and 0.81
factors (α = K
a a
PFP/ K Ph) were calculated, facilitating comparison
of the relative augmentation of association constants. Importantly,
a survey of the binding enhancement factors (α) in Table 1
demonstrates that for all halides the ChB receptor exhibits the
largest α value relative to XB and HB analogues, as graphically
illustrated
in
Figure
4.
a
respectively. Analogous plots constructed with K values for
iodide, revealed a much less pronounced correlation with Σσ,
possibly due to steric effects implicated in binding the larger halide.
Table 2. Halide anion association constants and calculated ρ values from
Hammett plot analysis of the 2·ChB receptor series.
Anion association constants K
M-1 [a] for 2·ChBR series
a
/
Anion[b]
mF
2F
3F
ρ[c]
r[d]
Cl-
Br-
I-
987[e]
592
2070
966
547
2950[e]
145
0.91 ± 0.11
0.81 ± 0.11
0.53 ± 0.21
0.99
0.98
0.87
217
497
[a] Association constants determined by Bindfit analysis of internal pyridine
proton signal, errors ± ≤ 7% conducted in acetone-d [b] Anions added as their
6
tetrabutylammonium salts. [c] ρ refers to the value from the Hammett equation,
errors indicated in parenthesis. [d] Pearson’s correlation coefficient.
Figure 4. Plot showing the halide anion enhancement factors for the 1·ChB,
·XB and1·HB receptor series.
1
In particular, α values calculated for chloride reveal over a thirty-
fold enhancement for ChB, whereas XB demonstrates an eight-
fold enhancement. The pronounced increase of anion association
constant magnitude by the perfluorophenyl substituent may be
rationalised by the increased polarisability of the heavier ChB and
XB donor atoms in the C-X bond (X = TeMe, I), relative to the
proto-triazole HB donor. Interestingly, the HB analogue’s halide
affinities appear surprisingly unperturbed upon incorporation of
the highly electron-withdrawing aryl substituents (α
≈ 1),
suggesting an apparent insensitivity of the HB donors to electronic
substitution. Whilst somewhat surprising and contradictory to the
well established doctrine of correlating HB donor acidity with
anion binding potency, the observed trend most likely is a
consequence of solvation effects and competitive HB formation
between the acidic triazole C-H donors and acetone solvent
molecules, which is anticipated to be a more pronounced effect in
the perfluorinated receptor.
Figure 5. Hammett plot for the 2·ChBR receptor series.
R
R
Anion Binding Studies of 2·ChB receptor series
Anion Binding Studies of 3·ChB receptor series
In order to assess the effect of replacing the tellurium methyl
group with a more sterically demanding aromatic substituent,
preliminary 1H NMR spectroscopic titration experiments in
The halide anion affinities of the complementary 2·ChB receptor
series were determined via 1H NMR spectroscopic titration
experiments. As expected, increasing the extent of fluorine
substitution on the triazole linked aryl substituent results in a
concomitant increase in halide association constant (Table 2).
Furthermore, the additive nature of Hammett substituent
constants (σ) facilitated the construction of LFER plots (Figure 5)
in which the effect of the increasing electron withdrawing ability of
6
acetone-d were performed on the 3·ChBseries, which, in general,
revealed significant perturbations observed in the receptors’ aryl
and pyridyl proton resonances, with the largest chemical shift
perturbations seen with the more electron-deficient aryl
substituent systems, a representative example is shown in Figure
S108.
4
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RESEARCH ARTICLE
ࣿ
Table 3. Halide anion association constants for the 3·ChBR receptor systems.
Anion association constants K
/ M-1 for 3·ChBR series [a]
a
Anion[b]
pOMe
pMe
Ph[c]
pF
pCl
mF
mCF3
pCF3
2F[c]
3F[c]
>105
2CF3
ρ[g]
r[h]
Cl-
[d]
[d]
[d]
[d]
[d]
450
1140
518
1,360
822
1,610
863
4,790
14,000[f]
2,740
592
>105
>105
>105
>105
2.20 ± 0.29
1.59 ± 0.13
1.28 ± 0.23
0.97
0.97
0.90
Br-
I-
230
2,540
674
7,540
7,270
1,620
5,180
1,040
11,800[f]
1,750
19,900[f]
3,980
[e]
NB
[e]
NB
119
398
1450
[
a] Association constants determined by Bindfit analysis of Te-aryl chemical shift perturbations unless otherwise specified, errors ≤ 10% unless otherwise
specified, conducted in acetone-d
6
. [b] Anions added as their tetrabutylammonium salts. [c] Determined by monitoring of internal pyridine proton signal. [d] Too
1
large to be accurately quantified by H NMR titration. [e] No binding. [f] Error ≤ 26%. [g] ρ refers to the value from the Hammett equation [h] Pearson’s correlation
coefficient.
Analysis of the anion binding isotherms using the Bindfit
programme determined 1:1 stoichiometric association constants
summarised in Table 3. Interestingly, comparison of the anion
binding properties of 3·ChBPh relative to methyl functionalised
analogue, 1·ChBPh, reveal the incorporation of the phenyl group
directly linked to the Te donor significantly attenuates the halide
of the Te-bonded aryl group increases. This pronounced
polarisation-induced halide anion discrimination presents
unique opportunity for a non-covalent interaction-based approach
to engineering selectivity in ChB anion receptors.
a
affinity, presumably as
a consequence of the sterically
encumbering aryl group. The anion affinities of the ChB receptors
-
-
-
decrease in the series Cl > Br > I , which correlates with the
decreasing basicity of the halide guests. More importantly, on
traversing Table 3 from left to right, a large enhancement in the
association constant magnitudes are observed as the electron-
withdrawing ability of the aryl substituents increase. Furthermore,
the para-methyl and para-methoxy receptors 3·ChBpMe and
3
·ChBpMeO displayed no measurable chemical shift perturbation
upon the addition of 10 equivalents of iodide, whereas the
5
-1
remarkable halide affinity (K
a
> 10 M ) of highly electron-deficient
receptors, such as 3·ChB3F and 3·ChB , precluded accurate
3F
quantification of their chloride association constants. LFER plots
a
of halide K values versus Hammett aryl substituent constants (σ)
(
Figure 6) reveal in all cases that receptor-halide binding affinity
correlates well with the aryl substituent σ-parameters.
Several trends can be observed from the LFER analysis. As
expected from the relative association constant values, the
positive gradients indicate that incorporation of more potent
electron-withdrawing aryl substituents in the receptor design
universally increases halide anion binding affinities. Importantly,
the ρ values summarised in Table 3, demonstrate the significant
influence of electron-withdrawing and donating groups on ChB
anion binding strength. Moreover, relative to the ρ values
Figure 6. Hammett plot for the 3·ChBR receptor series.
Conclusion
In summary, a library of acyclic anion receptors containing
chalcogen bond (ChB) and halogen bond (XB) donors integrated
into a neutral 3,5-bis-triazole pyridine scaffold appended with
various phenyl and electron-withdrawing fluoroaryl substitutents
have been prepared for halide anion recognition investigation. In
the first receptor series 1·ChB, 1·XB and 1·HB, the replacement
of the triazole-phenyl substituents with perfluorinated analogues
elicits dramatic halide anion affinity enhancements for the sigma-
hole based, ChB and XB receptors, demonstrating an impressive
obtained by variation of the triazole-appended aryl substituent
-
(
ρ(Cl ) = 0.91) in the 2·ChB series, modulating the electronic
donating/withdrawing nature of the tellurium directly appended
aryl substituent (3·ChB) has a considerably larger influence on
-
anion binding potency (ρ(Cl ) = 2.20). Intuitively, this is consistent
with conventional wisdom regarding the distance dependence of
inductive effects. However, it is noteworthy that comparison of the
most electron withdrawing and the most electron donating
substituents, has the effect of increasing the chloride association
constant value by 4 orders of magnitude. Most importantly, the
3
2-fold and 8-fold increase in chloride anion affinity respectively.
Importantly, systematic variation of the electron-withdrawing
potency of aryl-triazole and Te-bound-aryl groups facilitated the
construction of Hammett plots, establishing linear free energy
relationships between Σσ and determined anion K
Determined ρ values of the 2·ChB and 3·ChB receptor series
-
-
-
trend ρ(Cl ) > ρ(Br ) > ρ(I ) translates to an enhancement of
a
values.
selectivity for the lighter halides as the electron-withdrawing ability
5
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
-
(
ρ(Cl ) = 0.91 and 2.20 respectively), demonstrate that the
[20] S. Benz, A. I. Poblador‐Bahamonde, N. Low‐Ders, S.
Matile, Angew. Chem. Int. Ed. 2018, 57, 5408–5412.
tellurium ChB donor is highly sensitive to modulation of local
electronic environments, which is most pronounced for those
directly appended to the Te centre. In the case of 3·ChB the
[
[
[
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-
-
-
observed trend ρ(Cl ) > ρ(Br ) > ρ(I ) translates to a remarkable
enhancement of selectivity for the lighter halides as the electron-
withdrawing ability of the Te-bonded aryl group increases. Most
importantly, these results serve to highlight that through
considered selection of electron-withdrawing aryl substituents
covalently linked to the ChB donor atom, exquisite control over
both anion binding potency and selectivity is achievable, providing
an exciting opportunity for tuning non-covalent interactions in
anion receptor design.
[
[
[
24] T. Bunchuay, A. Docker, A. J. Martinez‐Martinez, P. D.
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[
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http://app.supramolecular.org/bindfit/.
28] Although enhancement values for 1·XB were determined
Acknowledgements
in 2.5% D
only the magnitudes of the determined K
the α values see supporting information for further details.
2
O-acetone-d
6
solvent mixtures, this affected
A.D. thanks the EPSRC for a studentship (Grant reference
number EP/N509711/1).
a
values and not
Keywords: Chalcogen bonding • Halogen bonding • Anion
receptors • Linear free energy relationship • Hammett plot
analysis
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Angewandte Chemie International Edition
10.1002/anie.202108591
RESEARCH ARTICLE
Entry for the Table of Contents
Chalcogen bonding (ChB) and halogen bonding (XB) anion receptors functionalised with phenyl and electron-withdrawing aryl
substituents exhibit a dramatic modulation in sigma-hole donor atom potency for anion recognition. The systematic variation of
electron-withdrawing/-donating properties of triazole appended- or directly Te-bound- aryl-substituents facilitated linear free
energy relationship (LFER) Hammett plot analysis revealing a remarkable four orders of magnitude range in ChB donor atom
potency, as well as electronic modulation of halide anion selectivity behaviour in sigma-hole anion receptor design.
7
This article is protected by copyright. All rights reserved.