Novel uranyl(VI) complexes incorporating ethynyl groups as potential halide chemosensors: an experimental and computational approach

Article abstract of DOI:10.1080/10610278.2017.1361036

The synthesis of two novel Uranyl-salophen complexes, 1 and 2, decorated with ethynyl substituents, and the study in chloroform of their binding properties toward three different tetrabutylammonium halide salts, i.e. fluoride, chloride, bromide, are here

Full text of DOI:10.1080/10610278.2017.1361036

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Novel uranyl(VI) complexes incorporating ethynyl
groups as potential halide chemosensors: an
experimental and computational approach
Silvia Bartocci, Ferran Sabaté, Francesco Yafteh Mihan, Ramon Bosque,
Laura Rodríguez & Antonella Dalla Cort
To cite this article: Silvia Bartocci, Ferran Sabaté, Francesco Yafteh Mihan, Ramon Bosque,
Laura Rodríguez & Antonella Dalla Cort (2017): Novel uranyl(VI) complexes incorporating
ethynyl groups as potential halide chemosensors: an experimental and computational approach,
Supramolecular Chemistry, DOI: 10.1080/10610278.2017.1361036
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Date: 05 August 2017, At: 06:01

Supramolecular chemiStry, 2017
https://doi.org/10.1080/10610278.2017.1361036
Novel uranyl(VI) complexes incorporating ethynyl groups as potential halide
chemosensors: an experimental and computational approach
Silvia Bartoccia, Ferran Sabatéb, Francesco Yafteh Mihana, Ramon Bosqueb, Laura Rodríguezb,c and Antonella Dalla
Corta
b
aDipartimento di chimica and imc-cNr Sezione meccanismi di reazione, università la Sapienza, roma, italy; Secció de Química inorgànica,
Departament de Química inorganica i Química orgànica, universitat de Barcelona, Barcelona, Spain; cinstitute of Nanoscience and
Nanotechnology (iN2uB), universitat de Barcelona, Barcelona, Spain
ABSTRACT
ARTICLE HISTORY
received 15 may 2017
accepted 25 July 2017
The synthesis of two novel Uranyl-salophen complexes, 1 and 2, decorated with ethynyl substituents,
and the study in chloroform of their binding properties toward three different tetrabutylammonium
halide salts, i.e. fluoride, chloride, bromide, are here reported. Such derivatives proved to be efficient
halide receptors. The presence of two ethynyl groups in the para position, with respect to the
phenoxide oxygens, seemed to be accountable for the moderate emission shown by complex 1.
Surprisingly, instead, complex 2 does not show such property. The possibility of 1 to form dimers
in non-coordinating solvents provides an explanation for such difference, since emission can be
induced by the aggregation. This finding provides an unprecedented example of aggregation
induced emission (AIE) for metal salophen derivatives. Moreover DFT calculations provide theoretical
insight to the formation of host-guest complexes. Their stabilities were calculated in vacuum and in
chloroform and the results are perfectly in agreement with the experimental data.
KEYWORDS
halide recognition; metal
salophen; uranyl
Introduction
number of artificial anion receptors that can act as chem-
osensors, efficiently changing their photophysical proper-
ties in the presence of anions, and showing high sensitivity
and low detection limit. Different types of non-covalent
interactions are exploited to achieve recognition, ranging
from hydrogen bond, anion-π interactions to hydropho-
bic effects etc. (2–4). Within these, an important binding
motif is metal coordination, that has its roots in classical
coordination chemistry (5).
Nowadays a main focus of supramolecular chemistry is the
design and study of luminescent and colorimetric sensors
for anions (1). It is well known that anions play an essential
role in many chemical and biological processes. Inorganic
and biotic anions such as acetate, phosphate, and halide
are involved in the activity of enzymes, transport of hor-
mones, protein synthesis, and DNA regulation. Moreover,
environmentally important anions such as nitrate and
phosphate constitute a large part of current pollutants
that cause eutrophication of rivers. For these reasons, over
recent years, we have seen the development of a huge
Among the different metals that can be coordinated by
these ligands, there is the hexavalent uranyl dication,UO22+
which displays a pentagonal bipyramidal coordination
CONTACT antonella Dalla cort
antonella.Dallacort@uniroma1.it
Supplemental data for this article is available online at https://doi.org/10.1080/10610278.2017.1361036.
© 2017 informa uK limited, trading as taylor & Francis Group

2
S. BARTOCCI ET AL.
geometry in which the apical positions are occupied by
the two oxygen atoms, while four of the five equatorial
positions are engaged with the N2O2 donor atoms. Thus,
an equatorial position remains available for substrates
that can be complexed through Lewis acid-base inter-
actions (6). It was found that such complexes behave
as highly efficient receptors for anions. The recognition
event can be easily detected by UV-vis or NMR spectros-
copy following variations induced by the presence of the
anion. Moreover uranyl cation and salophen ligands are
fluorescent on their own, while the corresponding com-
plexes hardly emits (7). Therefore only few examples of
luminescent uranyl salophen complex based receptors
have been reported till now (8).
In the literature several approaches have been pursued
to solve the problem of having very good receptors for
specific targets, though weakly emissive, to use them in
sensing. Different strategies have been adopted includ-
ing for example the attachment to the ligand molecular
framework of anthracene (9) and/or pyrene units (10) with
high fluorescence quantum yields (ΦF), or the extension of
π-conjugation and the introduction of electron withdraw-
ing groups (11) as substituents (12).
5-bromo-2-hydroxybenzaldehyde is commercially avail-
able, while 5-bromo-3-tert-butyl-2-hydroxybenzaldehyde,
3b, was prepared according to Scheme 2. Compounds 4a
and 4b were obtained through Sonogashira coupling reac-
tions (13) between the proper hydroxybenzaldehyde, 3a
or 3b, ethynyltrimethylsilane in the presence of bis(triphe-
nylphosphine)palladium(II) dichloride and copper iodide
in triethylamine. After deprotection, derivatives 5a and 5b
(14) were reacted with 1,2-phenylendiamine in the pres-
ence of UO2(OAc)2·2 H2O in methanol. After 24 h at room
temperature, a bright orange powder, 1, or a red powder, 2,
were obtained and isolated without further purification (SI).
Results and discussion
Absorption spectra of the uranyl complexes display differ-
ent bands between 300 and 450 nm which can be assigned
to π→π* transitions of the phenyl ring and of the azome-
thine chromophore. The farthest energy band is assigned
to n→π* transition involving the promotion of the lone pair
electrons of nitrogen atom to the anti-bonding π* orbital6a.
The appearance of an emission band in the case of 1
with a maximum at 479 nm characterised by moderate
quantum yield (ΦF = 0.021 0.001) in chloroform, Figure 2,
let us think that this was due to the introduction of ethynyl
groups.
Such considerations have prompted us toward the
synthesis of two new uranyl-salophen complexes, 1–2,
bearing ethynyl substituents as chromophoric groups.
Here we report the synthesis of such derivatives and how
the introduction of the ethynyl substituents on the ligand
skeleton affects their association behavior toward a series
of halides, fluoride, chloride and bromide, and their pho-
toluminescent properties (Figure 1).
Results and discussion
Synthesis
The new compounds were obtained according to
Scheme 1.
Scheme 1. Synthetic route to uranyl-salophen complex 1–2.
Scheme 2. Synthetic route to 5-bromo-3-tert-butyl-2-
hydroxybenzaldehyde, 3b.
Figure 1. Structure of compounds 1 and 2.

SUPRAMOLECULAR CHEMISTRY
3
of 5·10−5–5·10−6 M, the calculated association constants
assume values between 3·103 and 2·104 M−1.
In 2007 Ikeda et al. reported that in non-coordinating
solvents like chloroform, Uranyl-salophen complexes with-
out substituents in the ortho position to the phenolic oxy-
gens are present as dimeric complexes [(UO2(salophen)]2
even at low concentration (10−5–10−6 M) (16). In the dimer,
the two salophen units are held together through the
coordination of the phenoxide oxygen of one salophen
ligand to the fifth equatorial coordination site of the other
Uranyl center. Likely, this occurs also in the case of complex
1. This assumption is confirmed by the 1H-NMR spectrum
in CDCl3 in which the aromatic signals appear quite broad.
Indeed the addition of a small amount of a competitive
guest, for example one or two drops of a coordinating sol-
vent such as pyridine, leads to a well resolved spectrum
characterised by sharp signals. To avoid dimerization, a
likely reason for the irreproducibility of measurements in
the case of weaker binders, we introduced two tert-butyl
groups in the 3,3′ positions, 2. UV-vis titrations with the
three TBA halides, F−, Cl−, and Br−, were performed using
this time complex 2. Very good affinities for fluoride and
chloride, with association constants higher than 106 and
105 M−1, respectively, were measured, Table 1, and accu-
rate, although lower values, as expected, were obtained
in the case of bromide, i.e. 7·103 M−1 (Figure 4). These
data are perfectly reproducible and independent of host
concentration. Moreover, unexpectedly no emission was
observed in this case.
Figure 2. (colour online) uV-vis (red) and emission (blue) spectra
of compound 1, c = 8.62 × 10−5 m, in chcl3.
Before starting an exploratory study of the affinity of
1 towards a series of halides based on emission varia-
tions, we performed standard UV-vis titrations measuring
absorption variations in CHCl3 using tetrabutylammonium
(TBA) salts of fluoride, chloride and bromide.
The addition of increasing amount of standard solu-
tions of the TBA salt in chloroform to the solution of 1,
caused variations of the absorbance spectrum, see Figure 3
due to the well known strong affinity of uranyl salophen
complexes towards halides (15).
Typical titration experiments are reported in Figure 3.
Reproducible absorbance changes in the 280–550 nm
region caused by the enhancement of anion concentra-
tion at 25 °C were observed. The presence of sharp isos-
bestic points and the close adherence of titration data to
the binding isotherm of a 1:1 complexation phenomenon
lead to estimate association constants higher than 106 M−1
for fluoride and 105 M−1 for chloride. Instead in the case
of TBABr, the titrations data were not reproducible and
the values obtained appeared to be dependent on com-
plex concentration, i.e. in the host concentration range
Thus, the absence of substituents close to the metal
center in complex 1 favors the formation of dimers even
in diluted solution. Since fluoride and chloride bind very
strongly to uranyl, their additions immediately disrupt
dimeric aggregates shifting the equilibrium completely
toward the monomeric species. This leads to reliable and
reproducible data for titrations as the model we apply
Figure 3. (colour online) uV-vis titration curves in chcl3 at 25 °c of host 1 withtBaF ([1] = 1.02·10−5 m) left, and tBacl ([1] = 2.98·10−5 m)
right.
insets: variation of the absorption at 420 nm against concentration; points are experimental, curves are calculated.

4
S. BARTOCCI ET AL.
is that of a 1:1 complexation phenomenon (17). Instead
for bromide, that has a lower affinity for the metal, at
least three orders of magnitude less, the equilibrium
between monomeric and dimeric species, Equation (1),
is more important and concentration starts playing a
crucial role. Indeed for complex 1 the binding constant
with bromide decreases upon increasing complex con-
centration while for complex 2, in which dimer forma-
tion is prevented, the host-halide complex is formed
immediately, and concentration is not influencing the
measurements.
Molecular modelling
In order to provide more theoretical insight into the geom-
etry of the host-guest complexes as well as into the energy
of their formation, molecular modelling studies were per-
formed at the DFT level, using the B3LYP functional (19, 20)
(see below). The minimum energy geometries of receptors
1 and 2 are displayed in Figure 5.
As previously reported, the U–O distance, is significantly
longer than the one corresponding to the axial oxygens
(U=O) due to the overlap between the 6d and 5f orbitals
of the uranium atom and the three p orbitals (or two p and
one hybrid sp orbitals) of each axial oxygen providing the
linear structure (21).
(1)
The O=U=O moiety is almost linear, Table 2. The two
derivatives, 1–2, are expected to be folded with the aro-
matic substituted rings with a torsion angle of 43–45° with
respect to the main plane of the U–N2O2 atoms (see Figure
S1). This conformation would favour the coordination with
guest molecules as no steric hindrance is expected around
the fifth equatorial binding site of the metal.
Calculations on the halide complexes were also per-
formed and the resulting structures and main distances
and angles are shown in Figures 6, S2 and Table 3.
Coordination with halides affects the environment
around the metal atom and induces a slight deviation
from O=U=O linearity (the angle becomes less than 180°).
It should be noted that the U···X (X = F−, Cl−, Br−) calculated
distances reproduce quite nicely those obtained from X-ray
crystal diffraction in analogous host-guest uranyl-halide
complexes (22, 23).
The unexpected finding that complex 2 does not show
emissive properties suggests that the fluorescence that
we observe for 1 should be ascribed to aggregation. Such
phenomenon is known as aggregation-induced emission
(AIE) and originates from the restriction of intramolecular
rotation (18).
The energies for the formation of the adducts with the
different halides (fluoride, chloride and bromide) were cal-
culated by DFT in the gas phase and in chloroform. The
results are summarised in Table 4.
Inspection of Table 3 confirms the experimental data.
The affinity for fluoride anion is the highest for both
receptors, following the general trend F− > Cl− > Br− and
indeed, the predicted energy for the complex formation
with the bromide anion is clearly lower in all cases. The
energies predicted for the host: guest interactions with
receptor 2 are larger than those calculated for 1. This is in
Figure 4. (colour online) uV-vis titration curve in chcl3 at 25 °c of
complex 2 (inset, points are experimental, curve is calculated) and
corresponding spectral variation with tBaBr ([2] = 9.36·10−6 m).
Figure 5. (colour online) optimised structures of 1 (left) and 2 (right).
Notes: carbon (grey); oxygen (red); uranium (cyan); nitrogen (blue). hydrogens are omitted for clarity.

SUPRAMOLECULAR CHEMISTRY
5
Figure 6. (colour online) molecular modelling structure of 2-F (left), 2-cl (middle) and 2-Br (right).
Notes: hydrogens are omitted for clarity. carbon (grey); oxygen (red); uranium (cyan); nitrogen (blue); fluoride (yellow); chloride (green) and bromide (brown).
Table 1. Binding constants, K (m−1) of 1 and 2 toward tBaX salts
(X = F−, cl−, Br−) in chcl3 at 25 °c.
of their binding properties toward three different
tetrabutylammonium halide salts, i.e. fluoride, chloride,
bromide.
Complex
F−
Cl−
Br−
The presence of two ethynyl groups in the para posi-
tion with respect to the phenoxide oxygens, seemed to be
accountable for the appearance of moderate emission in 1.
UV-vis titration experiments highlighted the good affinity
for TBAF (K > 106 M−1) and TBACl (K > 105 M−1) salts, while
for TBABr measurements resulted to be not reproducible,
depending on host concentration.
The possibility for 1 to form dimeric species in non-co-
ordinating solvents provides an explanation for the irre-
producibility as well as for the observed emission that can
be induced by aggregation (AIE).
These considerations are supported by the fact that
complex 2, in which the presence of two tert-butyl groups
in the ortho position with respect to the phenoxide oxy-
gens prevents dimer formation, shows association con-
stants for fluoride and chloride, comparable with those
obtained for 1. The binding affinity of 2 toward TBABr,
K = 7·103 M−1, was, in this case, reproducibly measured
and proved to be independent of receptor concentration.
Moreover complex 2 did not have any emission spectrum
confirming that the moderate fluorescence shown by 1 is
indeed due to dimerization.
1
2
>106
>106
–
>105
>105
7 × 103
Table 2. main distances calculated for optimised geometries of
compounds 1 and 2.
Distance (Å)
U-O
Angle (°)
Complex
U=O
U-N
O=U=O U-O-C U-NH-C
1
2
1.787
1.787
2.271
2.274
2.579
2.563
179
179
135.36
137.69
125.06
124.86
Table 3. main distances calculated for optimised geometries of
complexes 1 and 2 with fluoride, chloride and bromide.
Distance (Å)
Angle (°)
Complex
U-X U=O U-O U-N O=U=O U-O-C U-NH-C
1·F
2.115 1.797 2.380 2.757 172.26
2.708 1.788 2.345 2.748 172.51
2.923 1.787 2.336 2.740 172.91
2.141 1.798 2.372 2.713 173.46
2.726 1.787 2.368 2.696 173.44
2.936 1.786 2.369 2.689 173.63
136.16 125.08
136.44 125.11
136.45 125.11
137.16 124.31
141.24 126.96
141.52 126.34
1·cl
1·Br
2·F
2·cl
2·Br
Table 4. Δe values (kcal/mol) calculated for the formation of 1 and
2:halide adducts in gas phase and in chcl3.
DFT calculations provided theoretical insight into the
formation of host-guest complexes. Their stabilities were
calculated in vacuum and in chloroform and the results
are in agreement with the experimental data since the
energies for the host:guest interactions calculated in
the case of receptor 2 are higher than those calculated
for 1 where the different affinity strength towards hal-
ides, F− > Cl− >> Br−, influences the efficiency of dimer
dissociation.
Gas
CHCl3
1-F
−129.76
−59.01
−44.87
−135.35
−61.71
−46.12
−71.42
−15.93
−7.04
−83.73
−24.94
−14.83
1-cl
1-Br
2-F
2-cl
2-Br
agreement with the more stable monomeric species of
complex 2, where the presence of the tert-butyl group pre-
cludes dimer formation. Additional non common halide···H
hydrogen bonding interactions, that could be established
between the different halides and the tert-butyl groups,
may favor the stability of the resulting adducts (24).
Acknowledgements
L. R. and R. B. are grateful to the Ministerio de Ciencia e Inno-
vación of Spain (AEI/FEDER, UE Projects CTQ2016-76120-P and
CTQ2015-65040-P). S. B. gratefully acknowledges the CIRCC,
Interuniversity Consortium of Chemical Catalysis and Reactivi-
ty for the fellowship. A.D.C. acknowledges the financial support
of Università La Sapienza, project ‘Ricerca scientifica di Ateneo
2015’.
Conclusions
Here, we reported the synthesis of two novel Uranyl-
salophen complexes, 1 and 2, and the study in chloroform

6
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Disclosure statement
No potential conflict of interest was reported by the authors.
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