Enhancing the anion affinity of urea-based receptors with a Ru(terpy) 22+ chromophore

Article abstract of DOI:10.1021/ic400196a

Covalent linking of a Ru(terpy)₂²⁺ substituent improves recognition and sensing properties of the urea subunit toward anions. Urea's anion affinity is enhanced by the electrostatic attraction exerted by the Ru^II cation and by the electron-withdrawing effect exerted by the entire polypyridine-metal complex. Such an enhancement of the anion affinity, which results from the combination of a through-space and a through-bond effect, is greater than that exerted by the classical neutral electron-withdrawing substituent nitrophenyl. Small yet significant modifications of π-π* and MLCT bands of the Ru(terpy)₂²⁺ chromophore, detected through UV-vis spectrophotometric titrations, allowed the determination of the constants for the formation of receptor-anion H-bond complexes in diluted MeCN solution. On ¹H NMR titration experiments, carried out under more concentrated conditions, the interaction of a second Cl^- ion was observed, taking place through an outer-sphere mechanism. The Ru(terpy) ₂²⁺ substituent favors the deprotonation of a urea N-H fragment on addition of a second equivalent of fluoride, with formation of HF₂^-.

Full text of DOI:10.1021/ic400196a

Article
pubs.acs.org/IC
Enhancing the Anion Affinity of Urea-Based Receptors with a
2+
Ru(terpy)2 Chromophore
Giorgio Baggi,† Massimo Boiocchi,‡ Carlo Ciarrocchi,† and Luigi Fabbrizzi*,†
‡
†Dipartimento di Chimica and Centro Grandi Strumenti, Universita
̀
di Pavia, 27100 Pavia, Italy
S
* Supporting Information
2+
ABSTRACT: Covalent linking of a Ru(terpy)2 substituent improves
recognition and sensing properties of the urea subunit toward anions. Urea’s
anion affinity is enhanced by the electrostatic attraction exerted by the RuII
cation and by the electron-withdrawing effect exerted by the entire
polypyridine−metal complex. Such an enhancement of the anion affinity,
which results from the combination of a through-space and a through-bond
effect, is greater than that exerted by the classical neutral electron-withdrawing
substituent nitrophenyl. Small yet significant modifications of π−π* and
2+
MLCT bands of the Ru(terpy)2 chromophore, detected through UV−vis
spectrophotometric titrations, allowed the determination of the constants for
the formation of receptor−anion H-bond complexes in diluted MeCN solution. On 1H NMR titration experiments, carried out
under more concentrated conditions, the interaction of a second Cl− ion was observed, taking place through an outer-sphere
mechanism. The Ru(terpy)22+ substituent favors the deprotonation of a urea N−H fragment on addition of a second equivalent
−
of fluoride, with formation of HF2 .
INTRODUCTION
■
The affinity toward inorganic anions of organic receptors
behaving as H-bond donors is typically assessed through
titration experiments in aprotic media (CHCl3, CH3CN, or
DMSO, in order of increasing polarity).1 Observed quantities
are in general the chemical shifts, δ, of selected protons (1H
NMR titrations) and the energy and the intensity of absorption
bands in the UV−vis region (spectrophotometric titrations).
20 000 M−1 cm−1 at λmax = 480 nm). In particular, we
1
2+
considered the derivative [L1H]2+ in which a RuII(terpy)2
The H NMR titration in deuterated solvents is convenient in
different aspects, because it provides also useful pieces of
information on the atoms directly involved in the anion binding
and on the structural rearrangements, if any, accompanying the
complexation process. An intrinsic disadvantage associated with
this method is related to the relatively high concentrations
required for the 1H NMR experiment (≥10−3 M), which
prevents a safe determination of binding constants K > 104. In
spectrophotometric titrations, the receptor must be equipped
with a covalently linked chromophore the spectral features of
which should be modified as a consequence of anion
interaction. As an example, the classical chromophore nitro-
phenyl (ε ≈ 20 000 M−1 cm−1 at λmax = 380 nm), which can be
covalently linked to a receptor’s backbone through simple
synthetic procedures, ensures the observation of significant
spectral modifications for concentrations as low as 10−6 M. This
allows the determination of binding constants as high as 107
(being very careful in avoiding the interference of impurities,
including water).
subunit has been linked to a phenylurea moiety.
Urea is a classical neutral receptor capable of recognizing
anions by hydrogen bonding whose interaction with anions has
1
been extensively investigated through H NMR spectroscopy,
spectrophotometry, spectrofluorimetry, and voltammetry.2
Urea can interact with a single acceptor atom (e.g., that of a
halide ion), thus forming a six-membered chelate ring, or with
two adjacent oxygen atoms of an oxoanion to give an eight-
membered chelate ring. Since the appearance of the seminal
papers by Wilcox3 and Hamilton,4 a variety of receptors
containing one or more urea moieties have been synthesized
over the last 2 decades. In particular, attention has been
devoted to N,N′-substituted ureas, with the aim of controlling
through the choice of the substituents the polarization of the
N−H fragments and, ultimately, the receptor’s binding
tendencies toward anions.
In this perspective, we consider here also the urea derivative
[L2H]2+, containing a nitrophenyl group as an additional
substituent. The nitrophenyl group is both a sensitive
In this work, we consider as a reporter of receptor−anion
2+
interaction an inorganic chromophore, RuII(terpy)2 (terpy =
2,2′:6′,2″-terpyridine), which shows a rather intense metal-to-
ligand charge transfer (MLCT) band in the visible region (ε ≈
Received: January 25, 2013
© XXXX American Chemical Society
A
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chromophore and a powerful electron-withdrawing substituent
capable of highly polarizing the adjacent urea N−H fragment, a
feature that, in the presence of the F− ion, may lead to N−H
deprotonation and formation of the HF2 ion.5 This study is
−
aimed (i) to evaluate the capability of the RuII(terpy)22+ subunit
to act as an optical reporter (absorbance) and (ii) to assess its
ability to polarize the N−H fragment and control the H-bond
tendencies toward anions. Combined 1H NMR and spec-
trophotometric titration experiments helped to sketch a clear
picture of the interaction of [L1H]2+ and [L2H]2+ with anions
and to throw further light on the intriguing topic of the solution
behavior of N−H-containing receptors.6
Figure 1. The crystal and molecular structure of the [L1H]-
(PF6)2·1.75H2O complex salt (ellipsoids are drawn at the 30%
probability level; one PF6− counterion and disordered water molecules
have been omitted for clarity; atom names are reported only for atom
sites bonded to the metal center and for atoms involved in H-bonds;
only H atoms of the urea N−H fragments are shown). Selected
features of the distorted octahedral RuII coordination are Ru(1)−N(1)
2.04(1), Ru(1)−N(2) 2.00(1), Ru(1)−N(3) 2.04(1), Ru(1)−N(6)
2.09(2), Ru(1)−N(7) 2.01(1), Ru(1)−N(8) 2.07(2) Å; N(1)−
Ru(1)−N(2) 78.2(6)°, N(1)−Ru(1)−N(6) 90.0(6)°, N(1)−Ru(1)−
N(7) 102.7(6)°, N(1)−Ru(1)−N(8) 93.1(5)°, N(2)−Ru(1)−N(3)
79.8(6)°, N(2)−Ru(1)−N(6) 102.1(7)°, N(2)−Ru(1)−N(8)
100.2(7)°, N(3)−Ru(1)−N(6) 94.5(6)°, N(3)−Ru(1)−N(7)
99.4(6)°, N(3)−Ru(1)−N(8) 91.0(6)°, N(6)−Ru(1)−N(7)
78.0(8)°, N(7)−Ru(1)−N(8) 79.6(7)°.
The most classical of luminescent metal complexes,
2+
RuII(bpy)3 (bpy = 2,2′-bipyridine), has been covalently
linked to a variety of receptors containing amide N−H
fragments as H-bond donors and has been extensively
investigated as an optical reporter for anion sensing.7−11
Interaction with anions induced in most cases a moderate
bathochromic shift and a substantial increase of the intensity of
the emission band, which was ascribed to an increased rigidity
of the molecular system and to the consequent decreased
possibility of a vibrational deactivation. However, chirality
2+
makes complexes of the Ru(bpy)3 family unsuitable for the
construction of metallosupramolecular systems. On the other
hand, RuII(terpy)22+ is achiral and allows easy functionalization
at position 6 of the terpy molecule, which opens the way to the
synthesis of symmetrical derivatives and favors a thorough
angles from the ideal value of 90° are most pronounced for the
angles involving the N atom of a terminal ring (N in 2-position,
N2) and the N atom of a central ring (N in 6-position, N6); the
observed N2−Ru−N6 angles fall in the range 78.0(8)−
102.7(6)°. The mean Ru−N2 distances (2.038(14) Å for the
nonsubstituted terpyridine moiety and 2.081(17) Å for the
other one) are longer than the two Ru−N6 distances
(1.996(13) Å for the nonsubstituted terpyridine moiety and
2.007(14) Å for the other one). All these features can be
ascribed to the rigidity of the terdentate ligand and are
commonly observed in similar ruthenium(II) complexes in the
literature. For instance, inspection of structures of bis-
(2,2′:6′,2″-terpyridine)−ruthenium(II) complexes occurring
in the CSD database system (2012 release)18 shows that the
Ru−N2 distances are in the range 2.05−2.10 Å, whereas the
Ru−N6 distances fall in the range 1.97−2.02 Å.
The two terpy moieties are essentially planar, the dihedral
angles between the central and terminal pyridine rings of each
terpyridine moiety lying in the range 2.6(12)−5.1(11)°.
Moreover, the two terpy subunits are nearly orthogonal: in
fact, the dihedral angle between the best planes of two
coordinating terpyridine moieties is 88.7(2)°. The RuII metal
center lies almost in these best planes, being displaced by
0.04(1) Å with respect to the best plane of the nonsubstituted
terpy moiety and by 0.08(1) Å with respect to the other one.
The entire terpy substituted molecule, coordinated to RuII, is
flat and roughly planar. In fact, the dihedral angle between the
best plane of the terpy moiety and the urea subunit is
11.2(13)°; the dihedral angle between the urea subunit and the
terminal phenyl ring is 5.7(17)°; the dihedral angle between the
terpyridine moiety and the phenyl ring is 16.8(11)°. On the
other hand, the dihedral angle a−b−N4−c (terpy-urea) is
2(3)°, while the dihedral angle c−N5−d−e (urea-phenyl) is
5(3)°. On these structural bases, an extended π-delocalization
over the entire molecular framework can be anticipated.
It is finally observed that one of the two PF6− counterions is
definitely involved in a H-bond interaction with the urea
subunit. In particular, each of the two contiguous fluorine
investigation of substituent effects.12 However, RuII(terpy)2
2+
cannot compete with RuII(bpy)3 as a luminophore for
2+
sensing, because it is not luminescent in solution at room
temperature, displaying a definite emission spectrum only at
low temperature (e.g., in a matrix at 77 K).13 Nevertheless,
insertion in position 6 of the terpy ligand of an electron-
withdrawing/electron-donor substituent may induce an in-
3
crease of the lifetime of the MLCT excited state and an
increase of the emission intensity.14 Thus, appropriate
substitution at position 6 has produced some luminescent
homo- and heteroleptic RuII(terpy)2 derivatives suitable for
2+
anion sensing. In particular, the heteroleptic complex
RuII(terpy)(terpy-L)2+, in which a CuII(cyclam)2+ moiety has
be covalently linked at position 6 of a terpy group,15 is not
luminescent in aqueous MeCN at room temperature, due to
the occurrence of an electron transfer process from the CuII
center to the photoexcited luminophore, which quenches
emission. Added anions (halides, hydroxide) stabilize the
CuII(cyclam)2+ complex through apical coordination, which
alters the CuII/CuI redox potential and prevents the electron
transfer process. Thus, anion recognition is signaled by a sharp
revival of luminescence.16 More recently, one terpy moiety of
RuII(terpy)2 has been functionalized with a side chain
2+
containing an imidazole ring, whose N−H fragment acts as
the binding site for anions.17 The complex is luminescent and is
quenched on interaction with F−.
RESULTS AND DISCUSSION
■
The X-ray Structure of the [L1H](PF6)2·1.75H2O
Complex Salt. Crystals of [L1H](PF6)2·1.75H2O were
obtained by diffusion of diethyl ether on an MeCN solution
of the complex salt, whose structure is shown in Figure 1.
The two 2,2′:6′,2″-terpyridine moieties behave as terdentate
ligands and the RuII metal center exhibits a distorted octahedral
geometry, with the two ligands arranged according to a
meridional coordination. Deviations for the N−Ru−N bond
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Figure 2. (a) Family of spectra taken over the course of the titration of a 2.07 × 10−5 M solution of [[L1H](PF6)2 with a standard solution of
[Bu4N]Cl; (b) titration profiles at selected wavelengths; (c, d) details of the spectral modifications observed over the course of the titration in the
π−π* region; (e) MLCT region.
1
Figure 3. H NMR titration profiles, based on the chemical shifts δ, for a CD3CN solution of 3.5 × 10−3 M [L1H](PF6)2, titrated with a standard
solution of [Bu4N]Cl.
−
−
−
atoms of PF6 interacts with one N−H fragment (see Figure
[L1H]2+ and [L2H]2+ with HSO4 and H2PO4 could not be
investigated due to formation of a precipitate.
1). Features of these H-bond interactions are N(4)···F(1)
3.08(2) Å, H(4N)···F(1) 2.19(14) Å, N(4)−H(4N)···F(1)
152(11)°; N(5)···F(2) 3.03(3) Å, H(5N)···F(2) 2.14(13) Å,
and N(5)−H(5N)···F(2) 155(11)°. The plane of the urea
subunit and that containing the F−P−F atoms involved in the
interaction form an angle of 63.4(10)°.
[L1H]2+ in MeCN shows an absorption spectrum substan-
tially similar to that of the model complex [RuII(terpy)2]2+ in
the same medium. In particular only a moderate bathochromic
shift of the MLCT band is observed (see Figure S1, Supporting
Information). Figure 2 shows the family of spectra recorded
over the course of the titration of a solution of 2.07 × 10−5
M
The Interaction of [L1H]2+ with Anions in Solution. The
[L1H]2+ receptor was isolated as an hexafluorophosphate salt
[L1H](PF6)2 with a standard solution of [Bu4N]Cl. Small yet
significant spectral changes have been observed both in the
π−π* region (see Figure 2c,d) and in the MLCT region
(Figure 2e). Titration profiles at selected wavelengths show
saturation on addition of 1 equiv of chloride (see Figure 2b).
The formation of a receptor−anion complex of 1:1
stoichiometry was confirmed by a Job plot at 500 nm (see
Figure S2a, Supporting Information). On fitting of titration
−
because PF6 is a rather poor H-bond acceptor and, in
equimolar concentration, does not compete in solution with
most anions (e.g., halides) for the receptor. Moreover, in
titration experiments in MeCN, anions were added as [Bu4N]+
salts, in order to minimize the cation−anion electrostatic
interaction and the formation of ion pairs. The interaction of
C
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Figure 4. 1H NMR spectra taken over the course of the titration with [Bu4N]Cl of a CD3CN solution of 3.49 × 10−3 M [L1H](PF6)2. Equivalent of
Cl− added (from the bottom): 0.00, 0.21, 0.41, 0.62, 0.83, 1.03, 1.24, 1.66, 2.69, 3.72, 5.17, and 7.24.
data through a nonlinear least-squares procedure,19 over the
250−600 nm interval, log K = 5.66 0.01 was calculated for
the equilibrium:
by proton 5′ is distinctly larger than that experienced by proton
7′, symmetrically positioned on the other side of the urea
subunit. This strengthens the hypothesis of the higher acidity of
the N−H fragment close to the RuII center.
1
1
[LH]2+ + Cl− ⇆ [LH···Cl]+
(1)
The behavior of protons 1 and 5, belonging to the plain terpy
molecule, is peculiar: they undergo a moderate upfield shift on
0−1 equiv addition of chloride but experience a pronounced
downfield shift on further addition of Cl− (see profiles in Figure
2c).
More detailed pieces of information on the interaction of
1
[L1H]2+ with Cl− were obtained from a H NMR titration
experiment. In particular, a CD3CN solution of 3.49 × 10−3
M
[L1H](PF6)2 (i.e., 175-fold more concentrated than that
investigated in the UV−vis studies) was titrated with a standard
solution of [Bu4N]Cl. N−H protons of the urea subunits were
observed at ∼8.75 ppm (N1) and at 8.13 ppm (N2) (see Figure
S8 in the Supporting Information). On chloride addition, both
signals experienced a drastic downfield shift, ascribed to an
electrostatic effect. The greater shift felt by the N1 proton with
respect to N2 indicates establishment of a more intense H-bond
interaction with Cl−, ascribed to the polarizing effect exerted by
It is suggested that such an effect is exerted by a second Cl−
ion electrostatically interacting with the Ru(terpy)22+ moiety in
a definite outer-sphere complex, which forms according to
equilibrium 2:
1
1
[LH···Cl]+ + Cl− ⇆ {[LH···Cl]···Cl}
(2)
It has to be noticed that formation of an outer-spher complex
with Cl− takes place also in the case of the model complex
1
[RuII(terpy)2]2+, as indicated by a H NMR titration experi-
2+
the Ru(terpy)2 substituent on the nearby N−H fragment.
ment on a CD3CN solution of 4.04 × 10−3 M [RuII(terpy)2]-
(PF6)2. In particular, Figure 5 shows the titration profiles based
on the chemical shift of protons 1 and 5 of each coordinated
terpy molecule, which undergo a definite downfield shift,
indicative of a through-space electrostatic interaction with Cl−,
within the {[RuII(terpy)2]···Cl}+ outer sphere complex. On
nonlinear least-squares treatment of titration data,14 log K =
1.96 0.01 was calculated for equilibrium 3:
Figure 3a shows the titration profiles based on the chemical
shifts δ of N1 and N2 protons: the two profiles reach a plateau
on addition of 1 equiv of anion, which confirms the occurrence
of equilibrium 1. The absence of a smooth curvature in both
profiles prevented a safe evaluation of the association constant.
Figure 4 shows the family of spectra (C−H portion) taken
over the course of the titration. Protons 4′, 5′ (belonging to the
terpy subunit linked to the urea moiety), and 7′ (belonging to
the phenyl substituent of urea) undergo a significant downfield
shift over the 0−1 equiv addition of chloride, then reach a
definite plateau (see profiles in Figure 3b). Such a behavior is
consistent with Cl− binding to urea and with the establishment
of an electrostatic interaction between the anion and the nearby
protons. It is worth noting that the downfield shift experienced
[Ru(terpy)2]2+ + Cl− ⇆ {[Ru(terpy)2]···Cl}+
(3)
On the other hand, on curve fitting the titration profiles
reported in Figure 3, which pertain to the outer-sphere
interaction of a second Cl− ion with the [L1H···Cl]+ complex,
log K2 = 1.7 0.1 was obtained for equilibrium 2. A lower value
D
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satisfactorily on the concentration profiles of the H-bond
complex (proton 5′) and of the outer-sphere complex (proton
5, see Figure 6b). Thus, it appears that the two experiments are
complementary and help to define accurately the [L1H]2+/Cl−
equilibrium: the UV−vis titration of the more diluted solution
allows the determination of the association constant for the H-
1
bond complex [L1H···Cl]+, while the H NMR titration of the
more concentrated solution makes possible the discovery and
characterization of the outer-sphere complex {[L1H···Cl]···Cl}.
Analogous UV−vis titration experiments were carried out
−
with the tetrabutylammonium salts of Br−, I−, NO2 , and
−
NO3 . In all cases, best fitting of titration data was obtained
assuming the formation of a H-bond complex of 1:1
stoichiometry, [L1H···X]+. The log K values for the
corresponding equilibria are shown in Table 1. The trend of
Figure 5. 1H NMR titration of a 3.89 × 10−3 M solution of
[Ru(terpy)2](PF6)2 in CD3CN with [Bu4N]Cl; profiles based on the
chemical shift (δ, ppm) of protons 1 and 5 (open symbols, lower
horizontal axis)). Full symbols refer to the chemical shift of protons 1
and 5 of [L1H]2+ observed in the titration in Figure 3, after the
addition of 1 equiv of Cl− (upper horizontal axis). δ values taken from
spectra in Figure S7, Supporting Information, and Figure 4.
Table 1. The log K Values for the Formation of H-Bond
Receptor−Anion Complexes Involving [L1H]2+ and [L2H]2+
in MeCN at 25 °C
a
log K, [L1H]2+ + X− log K, [L2H]2+ + X− log K, L4H]2+ + X−
⇆ [L1H···X]+
⇆ [L2H···X]+
⇆ [L4H···X]+
Cl−
Br−
I−
5.66 0.01
4.84 0.01
3.98 0.01
5.51 0.02
4.38 0.01
6.31 0.02
5.13 0.01
4.44 0.01
5.93 0.01
4.53 0.01
4.55
3.22
2.00
4.33
3.65
of K2 (eq 2) with respect to K (eq 3) is expected on a mere
statistical basis: [L1H···Cl]+ makes available to the electrostatic
interaction with Cl− roughly one-half of the RuII coordination
sphere, due to the presence of the urea···Cl− substituent on one
of the terpy moieties. Moreover, an electrostatic factor can play
some role. In fact, the outer-sphere Cl− ion interacts with a
monopositive species, [L1H···Cl]+, compared with the dication
[Ru(terpy)2]2+.
−
−
NO2
NO3
a
Values from ref 5a.
anion affinities is that typically observed for the interaction with
H-bond donating receptors, in the absence of relevant steric
constraints. For monatomic anions, the affinity decreases along
the series Cl− > Br− > I−, which parallels the decrease of the
Figure 6 shows the distribution of the species present at the
equilibrium over the course (i) of the spectrophotometric
titration (concentration of [L1H]2+ = 2.0 × 10−5 M, Figure 6a)
charge density of the anion.20 In the case of oxoanions, affinity
1
and (ii) of the H NMR titration (concentration of [L1H]2+
=
−
decreases with the decreasing basicity of the anion: NO2
>
3.5 × 10−3 M, Figure 6b).
21
−
NO3 .
The Interaction of [L2H]2+ with Anions in Solution. The
spectrophotometric response of receptor [L1H]2+ can be
improved by covalently linking to the urea subunit a second
powerful chromophore, a 4-nitrophenyl substituent, thus giving
[L2H]2+.
In the more diluted solution used in the spectrophotometric
titration, the outer-sphere complex {[L1H···Cl]···Cl} does not
form in a significant amount and the titration profile based on
the absorbance of the MLCT band (at 500 nm) fits quite well
the concentration profile of the H-bond complex [L1H···Cl]+
(see Figure 6a). In the more concentrated solution, both
[L1H···Cl]+ and {[L1H···Cl]···Cl} form over the course of the
1H NMR titration, and titration profiles based on selected
protons of the two coordinated terpy moieties overlap
Urea derivatives containing 4-nitrophenyl substituents
typically show an intense absorption band at ∼350 nm,
which results from a charge transfer transition from the urea
nitrogen atom to the −NO2 group, across the phenyl ring. The
Figure 6. (lines) Distribution of the species present at the equilibrium over the course of the titration with chloride of a solution of [L1H]2+ (a) 2.0 ×
10−5 M (UV−vis titration) and (b) 3.5 × 10−2 M (1H NMR titration,). Symbols: (a) absorbance at 500 nm; (b) chemical shift for protons 5 and 5′.
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1
On the other hand, H NMR titration with [Bu4N]Cl of a
CD3CN solution of 3.47 × 10−3 M [L2](PF6)2 disclosed
features similar to those observed with [L1H]2+.
In particular, the N2 proton undergoes downfield shift, which
sharply ends on addition of 1 equiv of chloride, as shown in
Figure 9a. The N1 proton shows a broad and poorly defined
signal, which does not allow the drawing of a clear titration
profile. The interaction of a first Cl− ion with the urea subunit
can be also monitored through the moderate downfield shift of
protons 4′ and 5′. Moreover, also in the present case, the
formation of an outer-sphere complex {[L2H···Cl]···Cl} is
observed, as demostrated by the behavior of protons 1 and 5,
which experience a very moderate upfield shift over the 0−1
equiv addition of chloride, then undergo a definite downfield
shift on excess addition of the anion (see Figure 9c). Nonlinear
least-squares treatment of titration data gave log K2 = 1.6 0.1
for the equilibrium
spectrum of 1-(4-nitrophenyl)-3-phenylurea, L3H, is shown as
an example in Figure 7 (blue solid line).
2
2
[L H···Cl]+ + Cl− ⇆ {[L H···Cl]···Cl}
(4)
Spectrophotometric titration experiments on diluted MeCN
solutions of [L2H]2+ indicated the formation of 1:1 complexes
−
−
with Br−, I−, NO2 , and NO3 , whose association constants are
reported in Table 1. These values are slightly larger (0.3−0.5
log units) than those observed for [L1H]2+, which may be
ascribed to the increased acidity of the urea subunit due to the
additional electron-withdrawing effect exerted by the nitro-
phenyl substituent. Table 1 reports also log K values referring
to the anion complex formation of the neutral urea-based
receptor L4H, bearing two nitrophenyl substituents.5a These
values are 1−2 log units lower than those determined for L2H,
which suggests that the combined electrostatic effect (through-
space) and the electron-withdrawing effect (through-bond)
Figure 7. Spectra in MeCN solution of L3H, [Ru(terpy)2](PF6)2, and
[L2H](PF6)2. Black dotted line results from the sum of spectra of L3H
and [Ru(terpy)2](PF6)2.
The spectrum of [L2H]2+ (pink dotted line in Figure 7),
2+
which contains both a Ru(terpy)2 and a 4-nitrophenyl
substituent, is evidently very close to that resulting from the
sum of the spectra of L3H and of [Ru(bpy)2]2+ (black dotted
line). In particular, the charge transfer transition associated with
the nitrophenyl substituent appears as a shoulder at 350 nm,
partially masked by a more intense π−π* transition from terpy
moieties.
Figure 8a shows the spectra taken over the course of the
titration of a MeCN solution of 2.00 × 10−5 M [L2H](PF6)2
with a standard solution of [Bu4N]Cl. On chloride addition, a
significant bathochromic shift of the shoulder at ∼350 nm is
observed (see Figure 8b). The corresponding titration profile,
shown in Figure 8c, indicates the formation of a 1:1 receptor−
anion complex, to which log K = 6.32 0.02 corresponds.
2+
exerted by the Ru(terpy)2 substituent is more effective than
the simple through-bond effect by the neutral nitrophenyl
group.
Interaction with Fluoride: Complex Formation and
N−H Deprotonation. The fluoride ion plays a unique role in
the interaction with urea-based receptors equipped with
electron-withdrawing substituents: in a first step, as the anion
of the most electronegative element, it establishes strong
hydrogen-bonding interactions with the N−H fragments,
forming a very stable 1:1 complex. Then, on addition of a
second F−, an N−H proton is abstracted from the receptor,
with simultaneous formation of the most stable of H-bond
Figure 8. (a) Spectra taken over the course of the titration of a MeCN solution of 2.00 × 10−5 M [L2H](PF6)2 with a standard solution of [Bu4N]Cl;
(b) close-up of the charge transfer band involving the nitrophenyl moiety; (c) titration profile based on the nitrophenyl band.
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Figure 9. 1H NMR titration profiles, based on the chemical shifts δ, for a CD3CN solution 3.47 × 10−3 M in [L2H](PF6)2 titrated with a standard
solution of [Bu4N]Cl. δ values taken from spectra in Figure S9, Supporting Information.
Figure 10. (a) Family of spectra (solid lines) recorded over the course of the titration of a 2.00 × 10−4 M solution of [L2H]2+ in MeCN with a
standard solution of DBU; spectra in MeCN of L4H (red dotted line) and of its deprotonated form [L4]− (blue dotted line); (b) concentration
profiles (lines) of the species at the equilibrium over the course of the titration; absorbances (symbols) at 400 nm and at 500 nm measured during
the titration with DBU.
a
Scheme 1. The Two Forms of [L2]+
a
Charge delocalization over the two urea substituents, through a π mechanism, is tentatively illustrated.
−
complexes, HF2 . Such a behavior was first documented for the
interaction with the urea derivative L4H,5a for which the two
stepwise equilibria were observed:
Specifically, the bands at 375 and 475 nm originate from
charge-transfer transitions from the deprotonated urea frag-
ment to the nitrophenyl subsituent.
The occurrence of similar equilibria is expected for the
interaction of fluoride with receptors [L1H]2+ and [L2H]2+,
which are more acidic than L4H. However, in order to
characterize the nature and properties of the deprotonated
forms [L1]+ and [L2]+, preliminary investigations were carried
out on the reactions of [L1H]2+ and [L2H]2+ with 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU). DBU is a very strong
base, with pKa = 24.13, in MeCN at 25 °C.22 In particular, a
2.00 × 10−4 M solution of [L2H]2+ in MeCN was titrated with a
standard solution of DBU. The family of spectra recorded over
the course of the titration is shown in Figure 10a. Addition of
the strong base induces significant spectral changes, with the
4
4
L H + F− ⇆ [L H···F]−
(5)
(6)
4
4 −
[L H···F]− + F− ⇆ [L ] + HF −
2
In particular, the occurrence of the two processes could be
characterized through spectrophotometric titrations in MeCN.
Figure 10a shows the spectrum of a MeCN solution of L4H
(red dotted line). Formation of the [L4H···F]− H-bond
complex induces a bathochromic shift (of ca. 20 nm) of the
band at 375 nm. Then, the occurrence of the second step is
signaled by the development of a new band at 475 nm,
pertinent to the [L4]− species (blue dotted line in Figure 10a).
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Figure 11. 1H NMR titration profiles, based on the chemical shifts, δ, for a CD3CN solution of 3.21 × 10−3 M [L2H](PF6)2 titrated with a standard
solution of DBU. Values taken from spectra in Figure S11, Supporting Information.
Figure 12. (a) Family of spectra (solid lines) taken over the course of the titration with fluoride of an MeCN solution of 2.00 × 10−4
M
[L2H](PF6)2; spectra of an MeCN solution of L4H (red dotted line) and of [L4]− (pink dotted line). (b) Concentration of the species present at the
equilibrium over the course of the titration (lines, left vertical axis), calculated assuming log K1 = 9.0 and log K2 = 3.5; titration profile (symbols, right
vertical axis) at 350 nm.
development of new intense bands at 400 and 500 nm. It is
suggested that the new bands pertain to the deprotonated
species [L4]+, which forms in the neutralization equilibrium 7:
course of the titration, as calculated from the log K value.
Notice that absorbances at 400 and 500 nm (symbols in Figure
10b) fit well the concentration profile of [L2]+.
1
However, H NMR titration allowed us to define more
2
2 +
[L H]2+ + DBU ⇆ [L ] + DBUH+
clearly how deprotonation of [L2H]2+ takes place, directly
indicating the coexistence of the two species a and b. In
particular, Figure 11 shows the significant chemical shifts, δ, of
some selected protons.
(7)
Notice that [L2]+ may exist as an equilibrium mixture of two
forms, a and b, in which deprotonation has occurred at either
N−H fragment, as illustrated in Scheme 1.
On titration, a defined upfield shift is observed for proton 8′,
which indicates that in the form deprotonated at N2, formula a
in Scheme 1, electrons are delocalized on the aromatic ring of
the nitrophenyl substituent via a through-bond mechanism. On
the other hand, the upfield shift experienced by proton 3′
suggests that a similar mechanism operates in the [L2]+ form
deprotonated at N1 (formula b in Scheme 1). Very significantly,
a chemical shift of the same extent is observed also for proton
4, belonging to the coordinated plain terpy molecule, which
implies that transfer of some electron charge takes place
through the RuII center. Moreover, proton 4′ undergoes a
significant downfield shift (Figure 11b), pointing toward the
predominance of a through-space effect. This may be due to the
electrostatic interaction of proton 4′ with the carbonyl oxygen
atom, which may assume a partial negative charge on N−H
deprotonation (through a delocalization mechanism not
illustrated in Scheme 1).
In particular, the new bands at 400 and 500 nm seem to
pertain to form a, resulting from charge-transfer transitions
from the deprotonated N−H fragment to the adjacent
nitrophenyl substituent. On the other hand, the negative
charge originated by the deprotonation of the N−H fragment
2+
close to the Ru(terpy)2 subunit, in form b, may delocalize to
the nitrogen atoms of the terpy substituent and, from there,
through the metal center, to the coordinated plain terpy
molecule. While this mechanism should alter MO levels of the
2+
RuII(terpy)2 subunit, thus modifying π−π* and MLCT
transitions, corresponding spectral changes cannot be un-
ambiguously assigned and characterized. In any case, titration
profiles based on absorbances at 400 and 500 nm, reported in
Figure 10b, clearly indicated the 1:1 stoichiometry of
equilibrium 7. Through nonlinear least-squares treatment of
spectrophotometric data,19 log K = 5.87 0.01 was determined
for the neutralization equilibrium 7. Lines in Figure 10b
represent the concentrations of [L2H]2+ and [L2]+ over the
The spectroscopic features of the deprotonated form [L2]+
being clear, titrations of [L2H]2+ with fluoride were carried out.
H
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Figure 13. 1H NMR titration of CD3CN solutions of (a) 3.21 × 10−3 M [L2H](PF6)2 and (b) 3.36 × 10−3 M [L1H](PF6)2 with a standard solution
of [Bu4N]F·3H2O. Titration profiles based on the chemical shift, δ, of proton 5′. Values in panel a taken from spectra in Figure S13, Supporting
Information; values in panel b taken from spectra in Figure S12, Supporting Information. Panel a also displays the concentration of the species
present at equilibrium over the course of the titration (solid lines), calculated for log K (eq 8) = 9.0 and log K (eq 9) = 3.5.
2
2
[L H]2+ + F− ⇆ [L H···F]+
Figure 12a (solid lines) shows the family of spectra taken over
the course of the spectrophotometric titration with fluoride of
an MeCN solution of 2.00 × 10−4 M [L2H](PF6)2. The dotted
line represents the spectrum of a 2.00 × 10−4 M [L2H](PF6)2
solution plus 5 equiv of DBU, thus containing 100% of the
deprotonated species [L2]+, which has been reported for
comparative purposes.
(8)
(9)
2
2 +
[L H···F]+ + F− ⇆ [L ] + HF −
2
More detailed pieces of information on the nature of these
1
species could be obtained from the H NMR titration with
fluoride of a solution of 3.21 × 10−3 M [L2H](PF6)2 in CD3CN
(Figure S13 in Supporting Information). The most significant
change of chemical shift is that pertaining to proton 5′ and is
shown in Figure 13a.
On titration with F−, a general increase of the absorption is
observed in the 350−550 nm interval, which can be ascribed to
the development of the low-energy bands pertinent to the
deprotonated nitrophenyl−urea moiety. However, π−π* and
MLCT based absorptions overlap with the absorption due to
the nitrophenyl chromophore, making a quantitative inter-
pretation of titration data difficult and preventing an accurate
description of the system. The most significant spectral changes
are observed at 330−350 nm, where a bathochromic shift of the
shoulder assigned to the urea-to-nitrophenyl transition takes
place on addition of the first equivalent of F− to form a defined
band. Then, on addition of more F−, the intensity of this band
distinctly decreases. The change of the absorption intensity at
350 nm (symbols in Figure 12b) is especially indicative: a steep
increase of the absorbance is observed on addition of the first
equivalent of F−, which is followed by a more gentle decrease
on excess anion addition. It is suggested that the absorbance at
350 nm pertains to the H-bond complex, [L2H···F]+, which
reaches its highest concentration (presumably 100%) with the
addition of 1 equiv of fluoride. Then, deprotonation of one N−
H fragment takes place, with which a decrease of the band at
350 nm and of the concentration of the [L2H···F]+ complex, are
associated. The sharp discontinuity of the ascending and
descending portions of the profile implies a very high value for
the association constant of the H-bond complex. On the other
hand, a particularly high value is expected, if one considers (i)
that the corresponding log K value for L4H is 7.6 and (ii) that
[L2H]2+ forms with halide ions complexes 1−2 orders of
magnitude more stable than L4H. Figure 12b reports also the
variation of the concentration (%) of the species at the
equilibrium during the titration, calculated tentatively assuming
log K1 = 9.0 (formation of the H-bond complex, eq 8) and log
K2 = 3.5 (deprotonation equilibrium, eq 9). This couple of
values ensures the best superimposition of absorbance at 350
nm on the concentration profile of [L2H···F]+, thus confirming
the occurrence of the stepwise equilibria 8 and 9:
On addition of the first equivalent of F−, the signal undergoes
a well-defined downfield shift, a behavior consistent with the
formation of the H-bond complex, as described by eq 9. In
particular, the through-space effect due to the electrostatic
interaction with the H-bonded fluoride ion (and on the
carbonyl oxygen atom, on which partally negative electrical
charge has been transferred) predominates over the through-
bond effect. On addition of excess of F−, the signal undergoes a
neat upfield shift, a behavior in agreement with the occurrence
of eq 9. In fact, the negative charge originated from the
deprotonation of the N−H fragment substantially delocalizes,
through a conjugative mechanism, onto the central heterocyclic
ring of the terpy subunit linked to urea, inducing a pronounced
upfield effect. Noticeably, the profile of the chemical shift fits
quite reasonably the calculated concentration profile of the
[L2H···F]+ complex (see symbols in Figure 13a), confirming
the reliability of the model.
Titration with DBU showed that [L1H]2+ is a monoprotic
acid (log K of the neutralization equilibrium = 5.15 0.01),
slightly weaker than [L2H]2+, which contains the nitrophenyl
substituent. The limiting spectrum obtained in the presence of
5 equiv of DBU, thus pertaining to [L1]+, is shown in Figure 14,
as a dotted line.
The spectrophotometric titration of [L1H](PF6])2 in MeCN
with fluoride induced spectral changes less significant than
those observed with the [L2H]2+ derivative yet more
pronounced than those observed in the titration of [L1H]2+
with chloride and other inorganic anions. The family of spectra
obtained on titration of a solution of 2.07 × 10−4
M
[L1H](PF6])2 with F− is shown in Figure 14 (solid lines).
Occurrence of deprotonation is suggested by the increase of
absorbance at ca. 350 and 530 nm. However, spectral changes
are not definite enough to allow a quantitative asssessment of
the equilibria that take place. On the other hand, the
I
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substitution at position 6 by electron-withdrawing/electron-
donor substituents.14 In any case, the absence of a luminescent
emission at room temperature prevents or makes rather difficult
the accomplishment of titration experiments with anions.
2+
The Ru(terpy)2 complex undergoes electrochemically
reversible metal-centered one-electron oxidation at a rather
anodic potential. Thus, the electrode potential associated with
2+
the RuIII(terpy)23+/RuII(terpy)2 couple of the [L1H]2+
receptor, measured in voltammetric titration experiments,
could monitor the interaction with anions, provided that they
are resistant to the oxidation. Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) studies at a platinum
working electrode on an MeCN solution of 1.00 × 10−4
M
[L1H]2+ and 0.05 M background electrolyte ([Bu4N]PF6)
showed the occurrence of a very moderate cathodic shift of E1/2
on addition of [Bu4N]NO3 (∼20 mV in the presence of 20
equiv of anion, see Figure S15, Supporting Information).
Indeed, one would expect that a +2 to +3 increase of the
electrical charge of the proximate metal center induces a
substantial enhancement of the association constant, to which a
significant decrease of E1/2 should correspond. However, in the
present experiments, the anion of the background electrolyte,
Figure 14. Family of spectra (solid lines) taken over the course of the
titration with fluoride of an MeCN solution of 2.07 × 10−4
M
[L1H](PF6)2; spectrum of an MeCN solution of [L1H]2+ in the
presence of an excess of DBU (dotted line), thus containing 100% of
the deprotonated form [L1]+.
occurrence of a two-step interaction of [L1H]2+ with fluoride,
−
PF6 , present in a 500-fold excess, can compete for the
described by equilibria of type 8 and 9, is clearly indicated by
1
interaction at the urea subunit, thus decreasing the affinity for
the H NMR titration, in particular by the profile of the
−
NO3 of both reduced and oxidized forms of the receptor. On
chemical shift experienced by proton 5′, reported in Figure 13b.
The profile is analogous to that observed for the [L2H]2+,
showing an ascendent (up to 1 equiv of F−), then a descendent
arm, with some significant differences: (i) the extent of the
upfield shift observed on addition of excess fluoride is distinctly
greater for [L1H]2+ than for [L2H]2+; this may be because in the
absence of the electron-withdrawing nitrophenyl substituent,
deprotonation takes place mainly at the N−H fragment close to
titration with the other investigated oxidation-resistant anion,
fluoride, anion additions induced a broadening of the DPV peak
and a decrease of the current intensity. At the same time, the
formation of a dark powder on the working electrode was
observed, which indicated the formation of an insoluble
oxidation product and prevented any reliable investigation.
2+
CONCLUSION
The Ru(terpy)2 substituent undergoes small yet significant
the Ru(terpy)2 subunit, which induces a more pronounced
■
2+
delocalization of negative charge on the heterocyclic rings of
the covalently linked terpy; (ii) the chemical shift δ(5′) in
[L1H]2+, after the addition of 1 equiv of F−, decreases less
steeply than for [L2H]2+, a behavior ascribed to a less
pronounced formation of the deprotonated species in the
case of the less acidic receptor [L1H]2+ compared with [L2H]2+.
Luminescence and Redox Behavior of [L1H]2+. The
modifications of its π−π* and MLCT bands on anion
interaction with a covalently linked urea moiety, a circumstance
that allows the safe determination of especially high anion−
receptor association constants in MeCN. However, Ru-
2+
(terpy)2 cannot compete as an optical reporter with classical
organic chromophores like the nitrophenyl substituent, whose
charge transfer transitions are directly and profoundly modified,
in energy and intensity, by the anion−urea interaction. On the
other hand, Ru(terpy)22+ enhances the affinity toward anions of
the linked urea subunit to a larger extent than the powerful
2+
Ru(terpy)2 complex is not luminescent at room temperature
because the sterically constrained terpy molecule does not exert
an especially strong ligand field (as bpy does).23 As a
3
consequence, the metal-centered excited state, MC, is not
2+
destabilized enough and a thermally activated transition from
the 3MLCT excited state to 3MC can take place, with
subsequent nonradiative decay to the ground state. On
electron-withdrawing group nitrophenyl. The Ru(terpy)2
advantage seems to result from (i) an electrostatic (through-
space) effect and (ii) a covalent (through-bond) effect. As far as
point i is concerned, it is suggested that the urea bound anion
(through a H-bond interaction) experiences an additional
Coulombic attraction by the nearby positively charged metal
center. Noticeably, the Ru(terpy)22+ substituent itself is capable
of interacting with a second chloride ion via an outer-sphere
mechanism, through a purely electrostatic interaction. Con-
cerning point ii, 1H NMR experiments have demonstrated that
the electronic charge brought by the H-bonded anion (e.g.,
Cl−) is transferred not only to the nearby terpy fragment but
also to the ancillary terpy ligand, according to a transmission
mechanism that must involve the RuII center. The positive
3
3
temperature decrease, the LMCT → MC thermal transition
2+
is made less efficient and luminescence of Ru(terpy)2 is
revived in a matrix at 77 K. On the other hand, insertion at
position 6 of the terpy ligand of an electron-withdrawing/
electron-donor substituent may induce an increase of the
3
lifetime of the MLCT excited state and an increase of the
emission intensity.14 Apparently, the urea substituent in
[L1H]2+ and [L2H]2+ does not behave as a strong electron-
withdrawing group, because the two RuII complexes do not
show luminescence in an MeCN solution at room temperature.
However, both [L1H]2+ and [L2H]2+ display luminescence in a
frozen butyronitrile solution at 77 K. The emission spectrum of
2+
charge and the electron-withdrawing properties of Ru(terpy)2
[L1H](PF6)2 (λexc = 490 nm, see Figure S14, Supporting
favor the deprotonation of one N−H fragment in the presence
of excess fluoride, a process that has been independently
characterized by both spectrophotometric and 1H NMR
titration experiments.
3
Information) shows that the MLCT emission band (λmax
=
614 nm) is bathochromically shifted with respect to Ru-
2+
(terpy)2 (λmax = 598 nm), a feature typically imparted by
J
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Inorganic Chemistry
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(7) Szemes, F.; Hesek, D.; Chen, Z.; Dent, S. W.; Drew, M. G. B.;
Goulden, A. J.; Graydon, A. R.; Grieve, A.; Mortimer, R. J.; Wear, T.;
Weightman, J. S.; Beer, P. D. Inorg. Chem. 1996, 35, 5868−5879.
(8) Beer, P. D. Acc. Chem. Res. 1998, 31, 71−80.
There exists a vast body of literature on anion recognition
and sensing based on the direct interaction of the analyte with a
coordinatively unsaturated metal center.24,25 It has been shown
here that metals, even when coordinatively saturated, can play a
valuable role in anion recognition, by enhancing the H-bond
donor tendencies of a covalently linked N−H containing
receptor. Such an enhancing effect can be modulated by varying
the nature of the metal and of its ligand. Studies along this line
are underway in our laboratory.
(9) Beer, P. D.; Dent, S. W.; Wear, T. J. Chem. Soc., Dalton Trans.
1996, 2341−2346.
(10) Beer, P. D.; Cadman, J. New J. Chem. 1999, 23, 347−350.
(11) Beer, P. D.; Timoshenko, V.; Maestri, M.; Passaniti, P.; Balzani,
V. Chem. Commun. 1999, 1755−1756.
(12) Constable, E. C. Chem. Soc. Rev. 2007, 36, 246−253.
(13) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.;
Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L.
Chem. Rev. 1994, 94, 993−1019.
EXPERIMENTAL SECTION
■
General methods and procedures, synthesis, and characterization of
receptors [L1H]2+ and [L2H]2+ and crystal structure determination
studies on [L1H](PF6)2 are described in detail in the Supporting
Information, in which families of spectra from titration experiments
(both UV−vis and 1H NMR) are also shown. CCDC 916540 contains
the supplementary crystallographic data for this paper. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.
html (or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB21EZ, UK; fax (+44) 1223-336-033 or e-mail
deposit@ccdc.cam.ac.uk).
(14) (a) Constable, E. C.; Cargill Thompson, A. M. W.; Armaroli, N.;
Balzani, V.; Maestri, M. Polyhedron 1992, 11, 2707−2709. (b) Maestri,
M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill Thompson, A.
M. W. Inorg. Chem. 1995, 34, 2759−2767. (c) Wang, J.; Fang, Y. Q.;
Hanan, G. S.; Loiseau, F.; Campagna, S. Inorg. Chem. 2005, 44, 5−7.
́
(15) Padilla-Tosta, M. E.; Lloris, J. M.; Martínez-Manez, R.; Pardo,
̃
T.; Soto, J.; Benito, A.; Marcos, M. D. Inorg. Chem. Commun. 2000, 3,
45−48.
́
(16) Padilla-Tosta, M. E.; Lloris, J. M.; Martínez-Manez, R.; Pardo,
̃
T.; Sancenon
1221−1226.
́
, F.; Soto, J.; Marcos, M. D. Eur. J. Inorg. Chem. 2001,
ASSOCIATED CONTENT
■
(17) (a) Bhaumik, C.; Saha, D.; Das, S.; Baitalik, S. Inorg. Chem.
2011, 50, 12586−12600. (b) Bhaumik, C.; Maity, D.; Das, S.; Baitalik,
S. Polyhedron 2013, 52, 890−899.
S
* Supporting Information
X-ray crystallographic files in CIF format for the [L1H]-
(PF6)2·1.75H2O salt and details of the synthesis and character-
ization of [L1H](PF6)2 and [L2H](PF6)2 and of spectrophoto-
(18) Allen, F. H. Acta Crystallogr. 2002, B58, 380−388.
(19) Using the Hyperquad package: Gans, P.; Sabatini, A.; Vacca, A.
Talanta 1996, 43, 1739−1753. http://www.hyperquad.co.uk/index.
htm; accessed 24 January, 2013.
1
metric and H NMR titration experiments. This material is
available free of charge via the Internet at http://pubs.acs.org.
(20) Amendola, V.; Bergamaschi, G.; Boiocchi, M.; Fabbrizzi, L.;
Milani, M. Chem.Eur. J. 2010, 16, 4368−4380.
(21) Baggi, G.; Boiocchi, M.; Fabbrizzi, L.; Mosca, L. Chem.Eur. J.
2011, 17, 9423−9439.
(22) Kaljurand, I.; Rodima, T.; Leito, I.; Koppel, I. A.; Schwesinger,
R. J. Org. Chem. 2000, 65, 6202−6208.
(23) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: New York, 1991.
AUTHOR INFORMATION
Corresponding Author
*E-mail: luigi.fabbrizzi@unipv.it.
■
Notes
The authors declare no competing financial interest.
(24) O’Neil, E. J.; Smith, B. D. Coord. Chem. Rev. 2006, 250, 3068−
ACKNOWLEDGMENTS
The financial support of the Italian Ministry of University and
Research (PRIN−Infochem) is gratefully acknowledged.
■
3080.
(25) Fabbrizzi, L.; Poggi, A. Chem. Soc. Rev. 2013, 42, 1681−1699.
REFERENCES
■
(1) (a) Anion Coordination Chemistry; Bowman-James, K., Bianchi, A.,
García-Espana, E., Eds; Wiley-VCH: Weinheim, Germany, 2011.
̃
(b) Santos-Figueroa, L. E.; Moragues, M. E.; Climent, E.; Agostini, A.;
Martínez-Manez, R.; Sancenon, F. Chem. Soc. Rev. 2013, 42, 3489−
́
́
̃
3613. (c) Wenzel, M.; Hiscock, J. R.; Gale, P. A. Chem. Soc. Rev. 2012,
41, 480−520.
(2) (a) Li, A.-F.; Wang, J.-H.; Wang, F.; Jiang, Y.-B. Chem. Soc. Rev.
2010, 39, 3729−3745. (b) Amendola, V.; Fabbrizzi, L.; Mosca, L.
Chem. Soc. Rev. 2010, 39, 3889−3915. (c) Hossain, M. A.; Begum, R.
A.; Day, V. W.; Bowman-James, K. Supramol. Chem.: Mol. Nanomater.
2012, 3, 1153−1178.
(3) Smith, P. J.; Reddington, M. V.; Wilcox, C. S. Tetrahedron Lett.
1992, 33, 6085−6088.
(4) Fan, E.; van Arman, S. A.; Kincaid, S.; Hamilton, A. D. J. Am.
Chem. Soc. 1993, 115, 369−370.
́
(5) (a) Boiocchi, M.; Del Boca, L.; Esteban-Gomez, D.; Fabbrizzi, L.;
Licchelli, M.; Monzani, E. J. Am. Chem. Soc. 2004, 126, 16507−16514.
(b) Boiocchi, M.; Del Boca, L.; Esteban-Gomez, D.; Fabbrizzi, L.;
Licchelli, M.; Monzani, E. Chem.Eur. J. 2005, 11, 3097−3104.
(c) Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M. J. Org. Chem. 2005,
70, 5717−5720. (d) Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M.;
Monzani, E. Org. Biomol. Chem. 2005, 3, 1495−1500.
(6) Amendola, V.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M.
Acc. Chem. Res. 2006, 39, 343−353.
́
́
́
́
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dx.doi.org/10.1021/ic400196a | Inorg. Chem. XXXX, XXX, XXX−XXX