Solution and Solid-State Studies on the Halide Binding Affinity of Perfluorophenyl-Armed Uranyl–Salophen Receptors Enhanced by Anion–π Interactions

Article abstract of DOI:10.1002/chem.201604313

The enhancement of the binding between halide anions and a Lewis acidic uranyl–salophen receptor has been achieved by the introduction of pendant electron-deficient arene units into the receptor skeleton. The association and the occurrence of the elusive anion–π interaction with halide anions (as tetrabutylammonium salts) have been demonstrated in solution and in the solid state, providing unambiguous evidence on the interplay of the concerted interactions responsible for the anion binding.

Full text of DOI:10.1002/chem.201604313

A Journal of
Accepted Article
Title: Solution and Solid State Studies on Anion-π Interaction
Enhanced Halide Binding with a Perfluorophenyl armed Uranyl
Salophen Receptor
Authors: Antonella Dalla Cort, Kari Rissanen, Luca Leoni, Rakesh
Putreddy, Ondřej Jurček, Ilaria Giannicchi, Francesco Yafteh
Mihan, and Andrea Mele
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To be cited as: Chem. Eur. J. 10.1002/chem.201604313
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Solution and Solid State Studies on Anion- Interaction Enhanced Halide
Binding with a Perfluorophenyl armed Uranyl Salophen Receptor
Luca Leoni,[a] Rakesh Puttreddy,[b] Ondřej Jurček,[b] Andrea Mele,[a] Ilaria Giannicchi,[a] Francesco
Yafteh Mihan,[a] Kari Rissanen*[b] and Antonella Dalla Cort*[a]
Abstract: The enhancement of the binding between halide anions
and a Lewis acidic uranyl salophen receptor has been achieved by
the introduction of pendant electron-deficient arene units into the
receptor skeleton. The association and the occurrence of the elusive
anion-π interaction with halide anions (as tetrabutylammonium salts)
has been demonstrated in solution and in the solid state, providing
an unambiguous evidence about the interplay of the concerted
interactions responsible for the anion binding.
Anion recognition is an active area of research in
supramolecular chemistry since anions are important and
Figure 1. The chemical structure of the ortho-disubstituted benzyloxy uranyl
ubiquitous in biological processes. Thus, the development of
novel artificial anion receptors represents an important research
salophen complex 1 for cooperative ion pair binding.
field, and the study of the non-covalent interactions on which
host-anion recognition relies is crucial for the understanding of
electron-deficient arenes, are attracting considerable attention
the biological processes and designing systems for
environmental monitoring.[1]
Such interactions, which take place between anions and
within the supramolecular community, and appear as an
appealing non-covalent tool with interesting applications in the
design of highly selective anion receptors[6], channels[7] and
more recently in catalysis.[8]
Among these receptors that are recently used to this purpose,
are the strongly Lewis acidic uranyl salophen [(N,N-phenylene-
bis(salicylimine)] complexes. They are easily available through
the condensation of two equivalents of salicylaldehyde and one
equivalent of phenylenediamine in the presence of uranyl
acetate.[2] The large number of synthetic routes to ortho-
substituted phenols and to the corresponding ortho-
salicylaldehydes gives access to a variety of structures with
subtle sterical and electronical variations on the receptor
structure, thus making them perfect candidates for systematic
studies. Clearly the main driving force for anion complexation in
these systems is the hard Lewis acidic uranyl center, where the
fifth equatorial coordination site of the uranium atom acts as the
binding site. Nevertheless, these complexes do have an
additional feature as they can act as multitopic receptors.[3] For
example, the presence of appended benzyloxy groups in the
ortho-positions of the salicylimine shown in Figure 1 results in
the cooperative ion pair binding. The Lewis acidic uranyl center
binds the anions, while the appropriately spatially situated
Although this interaction has been underscored in many crystal
structures,[9] the unambiguous proof of its occurrence in solution
is still a challenging task.[10] Furthermore the majority of artificial,
neutral anion receptors reported in the literature so far, combine
hydrogen bonding and anion- interactions, while the uranyl
salophen complexes herein described, to the best of our
knowledge, are the first example of receptors that manifest
Lewis acid-base and anion-interactions simultaneously to
reach the goal.
Here, we report the study of the binding of tetrabutylammonium
halide salts TBAX (X= Cl-, Br-, I-) with three uranyl-salophen
receptors decorated with appended aromatic arms, differently
substituted, 2a-c. The data obtained in chloroform solution and
the analysis of the solid-state structures of [TBA][2aCl] and
[TBA][2aBr] provide
a
comprehensive view about the
contribution of the anion- interactions to the overall binding
event in both media.
electron-rich
π
system binds cations (typically tetra-
For this investigation, we prepared the uranyl salophen
complexes as depicted in Scheme 1. The fact that only one
appended aromatic substituent is present is due to our previous
observations on similar derivatives for which we found,
specifically in the solid state, that only one side arm is involved
in the interaction with the bound substrate.[11] We synthesised
the properly functionalized salicylaldehyde derivatives by
palladium-catalyzed cross-coupling reaction (Suzuki coupling) of
either pentafluorophenyl boronic acid and 2-bromoanisole or
alkylammonium) via multiple cation- and C-H∙∙∙ interactions.
This has been demonstrated by extensive studies in solid state
and in solution.[4]
As the distance and electronic features of the pendant aromatic
arms of the salophen skeleton is easy to tune, this gives rise to a
possibility to enhance the interactions with the anion bound to
the metal through weaker, secondary interactions, namely
anion- interactions.[5]
3,5-dimethoxyphenylboronic
iodobenzene, followed by the deprotection of the phenolic
groups and ortho formylation.
acid
and
1-benzyloxy-2-
[a]
Luca Leoni, Andrea Mele, Dr Ilaria Giannicchi, Dr Francesco Yafteh
Mihan and Prof Antonella Dalla Cort
Dipartimento di Chimica and IMC-CNR, Università di Roma La
Sapienza Box 34, Roma 62, Piazzale Aldo Moro 5, 00185 Roma, Italy
[b]
Dr Rakesh Puttreddy,Dr Ondřej Jurček, and Prof Kari Rissanen
University of Jyväskylä, Department of Chemistry, Nanoscience Center,
P.O. Box 35, FIN-40014, University of Jyväskylä, Finland.
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They behave quite similarly although 2c is slightly more efficient
for chloride binding than 2b confirming the strong electron-
donating ability of the methoxy groups. Thus the further
stabilization via anion -interactions takes place only with the
receptor 2c that displays a -acceptor character that enhances
halide binding in solution.[15] The same behavior is observed with
bromide, while the association with iodine is so weak that no
remark can be done. The directionality of the complexation, the
halide is hosted in the fifth equatorial site of the uranyl, provides
a suitable geometrical orientation to establish intramolecular
contacts with the -cloud of the pendant arm. The potential
electronic effect of the substituents on the Lewis acidity of the
uranyl center can be easily ruled out since a direct interaction
between the sidearm and the uranyl center is prevented by the
marked curvature imposed to the coordinated salophen ligand
by the large atomic radius of uranium. This is clearly highlighted
by
a
number of X-ray structures[4,11,16] and also easily
reproduced by molecular mechanics calculations.[17] Slight
variations in the resonances of the TBA salt during titrations
seem to indicate that the uranyl complexes do not behave as
ditopic receptors, Fig S28.
Scheme 1. Synthesis of the ortho- mono-substituted aromatic uranyl salophen
complexes.
In order to quantify the contribution in solution of the anion-
interaction for these systems, we calculated G values, being
The obtained aldehyde derivatives were then reacted with the
G = GX @Host • GX @2c),[18] with respect to the control
receptor 2c, in which the appended phenyl ring does not have
any substituent. The free energies relative to anion binding are
obviously the sum of two different types of intermolecular
interactions: Lewis acid-base interaction with the metal center
and anion- interaction with the pendant aromatic wall. We
estimated for receptor 2a an attractive free energy of -1.0
kcal/mol for chloride binding and -0.7 kcal/mole for bromide;
while for receptor 2b a repulsive free energy of 0.5 kcal/mol for
chloride and 1.1 kcal/mol for bromide was calculated. The
observed trend is in agreement with the existence of a weak
non-covalent stabilizing interaction for the perfluorinated
derivative, whose magnitude is in accordance with the values
reported by Ballester and coworkers[19] for a series of “two-wall”
-
-
monoimine
prepared
from
o-phenylenediamine
and
salicylaldehyde in the presence of uranyl acetate.[12] For
compound 2c, the ortho formylation of 1,1’-biphenyl-2-ol leads to
the corresponding salicylaldehyde, that was reacted with the
monoimine as previously described (See ESI for details).
Binding constants for the 1:1 complexes of TBAX, X= F-, Cl-, Br-,
with the uranyl-salophen complexes were obtained in CDCl3 at
25°C, from UV-vis titrations. UV-vis spectra and titration curves
are reported in ESI. Binding constants Ka (M-1) are listed in Table
1. To evaluate the binding constant with iodine we had to
undertake 1H NMR titrations.
Table 1. Association constant values, Ka (M-1), for the complexation of TBAX
with receptors 2a-c obtained by UV-vis titrations in CHCl3 at 298 K.
calix[4]pyrrole
electronically different aromatic rings.
receptors
bearing
two
six-membered
X
F-
2a
>106
140000 ± 10000
2090 ± 15
<20[a]
2b
>106
11000 ± 1200
100 ± 5
<20[a]
2c
>106
24000 ± 200
650 ± 65
<20[a]
Cl-
Br-
I-
[a] Obtained by 1H NMR titrations in CDCl3.
Inspection of the results shows that the binding order of the
complexes with the TBA salts, namely, F->>Cl- > Br- > I-, is in
good agreement with the hard Lewis acidic character of the
uranyl center. The strong affinity of the TBA fluoride salt is
dominated by the interaction of the hard fluoride anion with the
metal center. Such binding is so high for all the receptors (Ka
>106) that it prevents the observation of any difference, if
present, caused by the nature of the substituents on the
aromatic appended arm. Instead lower binding affinities in the
case of chloride allow interesting considerations. The highest
stabilization of the host@chloride complex was found with
receptor 2a, decorated with a perfluorophenyl unit. Indeed
fluorine is the most electronegative element, and it is known that
when present as a substituent in hexafluorobenzene it induces a
large positive quadrupole moment (Qzz (C6F6) = +9.50 B).[13] For
this reason electron-deficient fluorinated arenes are known to
act as -acceptors.[14] The measured binding affinity towards
chloride shown by the other two receptors either displaying two
methoxy groups, 2b, or no substituents, 2c, are definitely lower.
Figure 2. X-Ray crystal structures of [TBA][2aCl] (a) and (b) [TBA][2aBr] with
the thermal displacement parameters at 50 % propability level (top) and in
CPK (bottom) presentations.
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Crystals of the complexes [TBA][2aCl] and [TBA][2aBr] were
obtained by slow evaporation of CHCl3 and CHCl3:CCl4, 50:50
solutions, respectively. The crystal structures are isomorphous
and crystallize in the monoclinic space group P21/c, Fig. 2.
The biphenyl unit formed by the salicylaldehyde and the pendant
perfluoro arene ring displays a very similar twist angle, 63.3(8)°
and 64.8(8)°, in the chloride complex and the bromide one,
respectively. For comparison, in the 2,3,4,5,6-pentafluoro-2’,4’-
dimethoxy-1.1’-biphenyl compound[20] the twist angle is 53.6°.
The wider twist angle we found can be ascribed to the presence
in the equatorial coordination plane of the bound anion, close to
the pentafluoro phenyl ring, with which it establishes stabilizing
anion -interactions. The Cring∙∙∙X (X = Cl- and Br-) distances
range between 3.82 – 3.94 Å in [TBA][2aCl] and 3.77 – 3.94 Å in
[TBA][2aBr]. However, remarkably, the π(centroid)∙∙∙X, [X = Cl-
and Br-] has the shortest distances of 3.63 and 3.60 Å,
respectively, indicating clear η6 anion-π interactions.[14] The
normal U-Cl bond distances in similar uranyl salophen
complexes vary between 2.713 – 2.760 Å, the same distance in
[TBA][2aCl] 2.843(2) Å is markedly longer (0.083 - 0.130 Å) and
statistically significantly different. On the contrary the U-Br
distance 2.930(1) Å is only slightly longer that the only reported
U-Br distance (HEYXEP, 2.902 Å) in salophen complexes found
on CCDC.[4a,11a,11d] The more electron-rich chlorine atom tends to
have stronger (yet weak) attractive interactions to the perfluoro
group than the corresponding bromide thus supporting the
anion-π interactions in solution state in a form of higher
association constants. On the other hand there are no observed
short contacts between the aromatic ring and TBA cations, like
cation-π interactions. The packing of [TBA][2aCl] and
[TBA][2aBr] starts through numerous intermolecular π-π, C-
H∙∙∙π, C-H∙∙∙O and C-H∙∙∙F interactions (See Fig S1-2). The X-
ray structures provide additional proof that the perfluoro group,
though having five highly electron-withdrawing fluorine
substituents, does not influence the Lewis acidity of the uranyl
center. The C-O and O-U bond distances are equal in
[TBA][2aCl] and [TBA][2aBr] (See ESI for bond parameters
comparison), and are close to the unsubstituted uranyl
salophen@tetraethylammonium chloride (TEACl) complex,[16]
Fig.3:
C20-C19-C21 bond angle. In [TBA][2aCl] and [TBA][2aBr] the
angle is 117.1° and 115.1°, markedly smaller than in the tert-
butylpyridine complex of 3,3’-bis(o-methoxy)UO2-salophen,
where the same bond angle is 124.2°.[21]
Hence, despite of unsuccessful attempts to isolate single
crystals of 2b and 2c, the experimental evidence obtained from
solution state, together with the X-ray analysis of 2a complexes
with TBACl and TBABr, provide an unbiased view for the
existence of interactions between anions and charge-neutral
arenes. Strongly electron-deficient systems attract the anions,
while in the presence of more electron-rich units the interaction
becomes repulsive. The values of the free energies estimated in
solution for the interaction of chloride and bromide with the -
aromatic systems are in the range of those already reported,
and underscore the role played by them in the recognition event.
Moreover, this is the first comprehensive set of data, in solution
and in the solid state, for anion recognition relying on the
combination of Lewis acid-base and anion- interactions.
Acknowledgements
The authors acknowledge the European Cooperation in Science
and Technology (COST Actions “Supramolecular Chemistry in
Water”,
CM1005
and
Crystallize
CM1402).
A.D.C.
acknowledges the financial support of Università La Sapienza,
project “Ricerca scientifica di Ateneo 2014” and MIUR “PRIN
2010CX2TLM”. K.R. gratefully acknowledges the financial
support from the Academy of Finland (prof. no.’s 263256,
265328 and 292746) and University of Jyväskylä.
Keywords: anion- interactions• Lewis acid-base interactions •
uranyl-salophen • halide binding • host-guest complexes
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Entry for the Table of Contents (Please choose one layout)
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COMMUNICATION
The combination of Lewis
acid-base and anion-
interactions enhances the
binding affinity of uranyl-
Luca Leoni, Rakesh
Puttreddy, Ondřej Jurček,
Andrea Mele, Ilaria
Giannicchi, Francesco Yafteh
Mihan, Kari Rissanen* and
Antonella Dalla Cort*
salophen
toward
complexes
halides.
Evidences in solution and
in solid state are
presented.
Solution and Solid State
Studies on Anion-
Interaction Enhanced
Halide Binding with a
Perfluorophenyl armed
Uranyl Salophen Receptor
Layout 2:
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Author(s), Corresponding Author(s)*
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Title
Text for Table of Contents
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