Chemosensing of chloride based on a luminescent platinum(II) NCN pincer complex in aqueous media

Article abstract of DOI:10.1021/om4007054

The new luminescent platinum(II) pincer complex [Pt(NCN)(S)]TfO (I; NCN = 1,3-bis(2-N-phenylbenzimidazolyl)benzene, S = solvent, and TfO^- = triflate anion) was synthesized and studied as a chemosensor for chloride in aqueous media. In 50 vol % aqueous DMF or CH₃CN chloride quenches the fluorescence with association constants of 1.2 × 10³ and 81 M^-1, respectively. On the other hand, in the micellar medium of cetyltrimethylammonium hydrogensulfate at pH 7.0 additions of inorganic anions to I enhance the fluorescence with a pronounced selectivity toward chloride, which shows also much tighter binding to the receptor with association constant 7.9 × 10⁴ M^-1 in comparison to that in mixed organic solvents. On basis of ¹H NMR titration experiments and the crystal structure of the neutral chloro complex of I the binding mode of chloride is proposed involving the coordination of chloride to the Pt(II) atom with simultaneous formation of intramolecular short C-H···Cl-Pt contacts. The combination of the cyclometalated platinum complex I with a cationic surfactant allows for the detection of chloride in the micromolar concentration range in samples of bottled mineral water.

Full text of DOI:10.1021/om4007054

Article
pubs.acs.org/Organometallics
Chemosensing of Chloride Based on a Luminescent Platinum(II) NCN
Pincer Complex in Aqueous Media
Alejandro Dorazco-Gonzalez*
́
́
Centro Conjunto de Investigacion
́
en Quımica Sustentable UAEM-UNAM, Instituto de Quımica, Universidad Nacional Auton
́
oma
de Mexico, Carretera Toluca-Atlacomulco Km 14.5, C. P. 50200, Toluca, Estado de Mex
́
́
ico, Mexico
́
S
* Supporting Information
ABSTRACT: The new luminescent platinum(II) pincer
complex [Pt(NCN)(S)]TfO (I; NCN = 1,3-bis(2-N-
phenylbenzimidazolyl)benzene, S = solvent, and TfO−
=
triflate anion) was synthesized and studied as a chemosensor
for chloride in aqueous media. In 50 vol % aqueous DMF or
CH3CN chloride quenches the fluorescence with association
constants of 1.2 × 103 and 81 M−1, respectively. On the other
hand, in the micellar medium of cetyltrimethylammonium
hydrogensulfate at pH 7.0 additions of inorganic anions to I
enhance the fluorescence with a pronounced selectivity toward chloride, which shows also much tighter binding to the receptor
1
with association constant 7.9 × 104 M−1 in comparison to that in mixed organic solvents. On basis of H NMR titration
experiments and the crystal structure of the neutral chloro complex of I the binding mode of chloride is proposed involving the
coordination of chloride to the Pt(II) atom with simultaneous formation of intramolecular short C−H···Cl−Pt contacts. The
combination of the cyclometalated platinum complex I with a cationic surfactant allows for the detection of chloride in the
micromolar concentration range in samples of bottled mineral water.
between 100 and 200 M−1.8 Recently a series of RhIII aqua
INTRODUCTION
■
complexes of general formula [Cp*Rh(N−N-chelate)(OH2)]2+
showed higher values of up to 660 M−1.9 This work is based on
the idea that the capture of chloride in water is possible with a
transition-metal-based receptor with a high affinity for this
halide. In this context, PtII complexes containing one available
coordination site occupied by a solvent molecule or a
noncoordinating anion are potential receptors and sensors for
chloride. The results obtained for a green luminescent platinum
pincer complex, including synthesis, X-ray crystal structures,
and spectroscopic studies, are summarized below.
Recognition and sensing of biological anions such as chloride
remains an active and important area in supramolecular
chemistry.1 Chloride anion recognition has been dominated
by the use of synthetic receptors containing arrays of hydrogen
bond donors as association sites. However, such receptors are
restricted to nonaqueous media,2,3 which limits their practical
applications in water. On the other hand, the optical detection
of chloride in water can be achieved with fluorescent dyes,
nanoparticle-based fluorescent probes, and luminescent iridium
complexes, which undergo a collision-induced quenching
process in the presence of chloride.4−6 Typically, these dyes
or sensors show Stern−Volmer quenching constants between 1
and 300 M−1. Consequently, they are suitable to sense chloride
in the millimolar concentration range, but not significantly
below. Furthermore, these probes are not particularly selective,
and interference from other halides, pseudohalides, heavy-
atom-containing molecules, and oxygen can be a problem.4,6 In
principle, it should be possible to overcome these limitations by
using an optical chemosensor with a specific, high-affinity
binding site for chloride. However, the creation of a potent and
selective receptor for chloride in pure water is an ongoing
challenge. The difficulty in making receptors for chloride
recognition in water is due to the high solvation energy of
chloride (ΔG° = −340 kJ mol−1).7 Water is thus an efficient
competitor for any chloride-binding molecule. Reports in the
context of medicinal inorganic chemistry have shown that the
organometallic aqua complexes [(arene)Ru(en)(OH2)]2+ are
able to bind chloride in water with association constants
RESULTS AND DISCUSSION
■
Platinum pincer complexes of the general formula Pt(NCN)Cl
(NCN = N,C,N-chelating ligand) are often highly luminescent,
and these have been used in catalysis, as biomarkers for
proteins, as probes for DNA, and as phosphorescent
compounds for new materials.10,12b,c,13,14 However, anion
recognition still remains largely unexplored. The chloride
ligand in these complexes can be abstracted efficiently with
silver salts (e.g., AgOTf).12a The resulting cationic complexes
[Pt(NCN)(S)]+ (S = solvent) display a strong affinity for
chloride.11 The aqua complex [Pt{C6H3(CH2NMe2)2}-
(OH2)]+, for example, was shown to bind chloride in water
with an association constant of Ka ≈ 280 M−1.11 An interesting
platinum aqua complex with a bis(imidazolinyl)phenyl ligand,
Received: July 29, 2013
© XXXX American Chemical Society
A
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on the other hand, was reported to abstract chloride from
CH2Cl2 in the crystallization process.12a These findings
suggested that [Pt(NCN)(S)]+X− (X = noncoordinating
anion) complexes could be used as receptor units for chloride
sensors. It therefore appears possible to obtain a luminescent
platinum(II) complex with interesting properties, to sense
chloride without the need for an additional signaling unit. For
these investigations, the ionic complex 4 was successfully
obtained following the procedure described in Scheme 1, which
Scheme 1
is similar to that reported by Song.12a Complex 4 is structurally
close to cyclometalated complexes reported by Yam.12b The
synthesis of 4 was accomplished by metalation of 1,3-bis(2-N-
phenylbenzimidazolyl)benzene (1) with K2PtCl4 in acetic acid
to give the neutral chloro complex 2. Halide exchange with KI
gave the iodo complex 3, which was converted to 4 by reaction
with AgOTf in DMF/CH3OH (1/9 v/v).12d−f Complex 4 was
isolated in good yield as a light brown crystalline powder, pure
according to 1H NMR in CD2Cl2 (Figure S9, Supporting
Information). Direct abstraction of the chloride ligand from
complex 2 using AgOTf did not produce the pure complex 4.
Complexes 2 and 3 are insoluble in CH3OH; in contrast, the
complex 4 is moderately soluble at room temperature. The
mass spectrum of 4 by positive electrospray ionization showed
essentially two peaks that can be assigned to the cationic
complex [4]+ (656.16 m/z) and the methanolic complex
[4(CH3OH)]+ (697.17 m/z). Elemental analysis (C, N, H) was
rather consistent with the methanolic adduct. The 19F NMR
spectrum of 4 showed one signal at −80.0 ppm (Figure S10,
Supporting Information), which can be assigned to a
noncoordinated triflate anion by comparison with the spectrum
of NaOTf.
Single crystals of 2 and 3 were obtained by slow evaporation
from CH2Cl2 solutions. Crystallographic analysis showed that
both complexes display a distorted-square-planar geometry
(Figure 1). The platinum−halide bond distances Pt−Cl =
2.4213(19) and 2.4150(19) Å (two independent molecules in
the unit cell) and Pt−I = 2.6911(4) Å are significantly longer
than what was found for Pt−terpyridine complexes such as
[Pt(terpy)Cl]OTf (2.307(1) Å),14 [Pt(terpy)Cl]ClO4
(2.301(6) and 2.302(7) Å),15 or [Pt(terpy)I][AuI2]
(2.5930(5) Å)16 (Table 1). This difference can be ascribed to
the strong trans influence of the phenyl ligand opposite the
halide.17 Both complexes feature intramolecular short CH···X−
Pt contacts between hydrogen atoms of the benzimidazole
groups and the halide ligands (1-CH···Cl−Pt = 2.744(4) and 4-
CH···Cl−Pt = 2.752(4) Å; 1-CH···I−Pt = 2.863(4) and 4-
CH···I−Pt = 2.891(5) Å). In the case of 2, the lengths of the
Figure 1. Ball and stick models of the molecular structures of complex
2 (top) and 3 (bottom) in the crystal. Solvent molecules are omitted
for clarity. For complex 2, two independent molecules are found in the
unit cell, only one of which is shown.
CH···Cl−Pt interaction are longer than a typical hydrogen
bonding interaction such as Cl3C−H···Cl− (2.39(3) Å).19 This
difference of 0.35 Å is possibly a result of the low flexibility of
the NCN pincer ligand and the chloride ligand in the complex.
1
Despite this, in the H NMR spectra of 2 and 3, these CH
atoms are characterized by a pronounced downfield shift (δ
9.05 ppm for 2 and 9.58 ppm for 3 in CD2Cl2; see the
Supporting Information). Similar short CH···Cl−Pt contacts
and 1H NMR spectroscopic features were observed for a
Pt(NCN)Cl complex with a pinene-derived pincer ligand.18
Attempts to get suitable crystals of 4 in CH3OH were not
successful; however, the crystallization of 4 in CH3CN/CH2Cl2
(1/1 v/v) at room temperature generated suitable yellow single
crystals. X-ray diffraction showed a cocrystal of general formula
[Pt(NCN)CH3CN]OTf·[Pt(NCN)Cl] (5; NCN = ligand 1)
(Figure S11, Supporting Information), where two different
platinum complexes are inside the same unit cell: one cationic
adduct with one molecule of CH3CN in a trans position with
respect to the carbon atom and a second complex which is the
neutral chloro complex 2. This finding is interesting because it
shows that complex 4 can abstract chloride from CH2Cl2, which
illustrates the high affinity of the platinum atom for this anion.
Anion Binding Studies. The first evidence for the high
affinity of complex 4 for chloride was obtained by NMR
measurements. Solutions of 4 in a mixture of CD2Cl2 and
CD3OD (1/1 v/v) were stable over a prolonged period of time.
In pure CD2Cl2, however, the formation of the neutral chloro
complex 2 was observed after several hours (Figure S12,
1
Supporting Information). A new and modest H NMR signal
gradually appeared (δ 9.05 ppm). This new signal is consistent
B
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Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 2 and 3
2
3
Pt1−C1
Pt1−N3
Pt1−N1
Pt1−Cl1
C1−Pt1−N3
C1−Pt1−N1
N3−Pt1−Cl1
N1−Pt1−Cl1
1.933(8)
2.039(6)
2.042(6)
2.4213(19)
79.8(3)
Pt2−C33
Pt2−N5
Pt2−N7
Pt2−Cl2
C33−Pt2−N5
C33−Pt2−N7
N5−Pt2−Cl2
N7−Pt2−Cl2
1.927(7)
2.037(6)
2.042(6)
2.4150(19)
79.6(3)
Pt1−C1
Pt1−N1
Pt1−N3
P1−I1
C1−Pt1−N1
C1−Pt1−N3
N1−Pt1−I1
N3−Pt1−I1
1.947(3)
2.047(3)
2.050(3)
2.6911(4)
79.72(11)
79.67(11)
100.04(7)
100.57(7)
79.9(3)
80.0(3)
100.36(18)
99.84(19)
101.10(18)
99.31(18)
with that of chloro complex 2. On the other hand, the addition
of NaCl (1.0 equiv) to a solution of 4 (2.0 mM) induces a
notable downfield shift (Δδ = 1.20 ppm at saturation) of the 1-
CH proton (see Figure 1 for the numbering of the structure)
and small upfield changes for protons 5-CH and 6-CH, as
shown in Figure 2. The 1H NMR spectrum of 4-triflate + NaCl
shift for both emission bands (Δλ = 7−9 nm; Figure 3, top).
These spectral changes are in line with the (partial) conversion
Figure 2. 1H NMR spectral changes observed of 4 (2.0 mM) in
CD2Cl2/CD3OD (1/1 v/v) during the addition of up 1.5 equiv of
NaCl.
(1.0 equiv), corresponds to the chloro complex 2. In this
context, the NMR changes support the formation of the Pt−Cl
bond and the involvement of the intramolecular C−H···Cl−Pt
interaction, which was observed in the crystal structure of 2
(Figure 1).
Complex 4-triflate displays poor solubility in pure water.
However, it can be dissolved in aqueous solutions containing
50 vol % DMF or CH3CN, and those mixtures were used for
further studies. When excited at 360 nm, solutions of complex 4
in DMF/water are luminescent with emission maxima at 500
and 537 nm. Similar results are obtained in acetonitrile/water.
The quantum yields of 4 in these solvent mixtures are Φ =
0.054 (CH3CN/H2O) and Φ = 0.15 (DMF/H2O). The
lifetimes calculated upon excitation at λ 360 nm were 2.0 and
2.2 μs, respectively. These values are close to those of other
NCN and NCCN platinum(II) complexes.13b,c In the context
of chemosensing, luminescent sensors which display long-lived
emission in the microsecond range are of interest in the analysis
of bioactive ions or molecules in solution, since their emission
may be discriminated readily from scattered light and shorter-
lived background fluorescence normally present in biological
and clinical samples.20
Figure 3. Changes of the emission spectra (λex 360 nm) of buffered
aqueous solutions (40 mM, MOPS at pH 7.0) of 4 (20 μM)
containing 50 vol % DMF (top) or CH3CN (bottom) upon addition
of increasing amounts of NaCl. The insets show the emission intensity
at λ 512 nm for different chloride concentrations. The lines were
obtained by fitting the data to a 1/1 binding model.
of 4 in the chloro complex 2 (Scheme 2). Due to the strong
trans effect of the phenyl ligand,17 the dynamic equilibrium
between 4 and 2 is established quickly and measurements can
be performed within minutes. A similar behavior was observed
for solutions of complex 4-triflate in CH3CN/H2O (Figure 3,
bottom).
The addition of NaCl (0−3.5 mM) to a buffered aqueous
solution of complex 4-triflate (20 μM) in DMF/H2O (40 mM
MOPS, pH 7.0) induced a quenching at the maxima and a red
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this issue, we decided to explore the behavior of complex 4 in
aqueous solutions containing a micelle-forming surfactant. The
utilization of micelles for chemosensors is an emerging topic in
analytical chemistry.21 The group of Dalla Cort has shown that
a salophen−UO2 complex is able to bind fluoride in aqueous
solutions containing a cationic surfactant,22a and Severin has
demonstrated that the binding affinity of a Cp*Rh-based
receptor for chloride is significantly increased upon addition of
Scheme 2
9b
−
cetyltrimethylammonium hydrogensulfate (CTA+HSO4 ).
̈
Recently, Gabbaı has shown an interesting fluorescent system
with high selectivity for fluoride using cetyltrimethylammonium
bromide (CTA+Br−) and an antimony-based receptor, which is
able to sense F− in the parts per million range using drinking
water samples.22b
The variations in emission intensity with increasing NaCl
concentration could be well fitted to a 1/1 chemical
equilibrium. The resulting association constants are Ka(DMF/
H2O) = (1.2 0.20) × 103 M−1 and Ka(CH3CN/H2O) = 81
M−1. The binding constants were also determined by
spectrophotometric titration experiments under the same
conditions (40 mM MOPS, pH 7.0), but with more
concentrated solutions of 4 (60 μM). Figure 4 illustrates the
The surfactant CTA+HSO4 was used for the present study.
−
Complex 4-triflate can be dissolved in buffered aqueous
solutions containing CTA+HSO4 ([CTA] = 5.0 mM, 40
−
mM MOPS, pH 7.0) without the need of an organic cosolvent,
which is interesting for our purposes. The aqueous solutions of
4-triflate with CTA+HSO4 display a modest increase of
−
lifetime 2.9 μs (at λ 360 nm) with emission maxima at 500 and
537 nm. The intensity of the emission increased, which is
expected due to the encapsulation of the active species inside
the hydrophobic core of the micelle. Interestingly, upon
addition of NaCl, a distinct behavior was observed: instead of
emission quenching, the intensity of the emission bands
increased along with a red shift of the maximum (Figure 5)
Figure 4. Spectrophotometric titration of buffered aqueous solutions
(40 mM MOPS at pH 7.0) of 4 (60 μM) containing 50 vol % CH3CN.
Arrows show the direction of the spectral changes on addition of
increasing amounts of NaCl. The insets show the profile of absorbance
at 404 nm for different chloride concentrations. The lines were
obtained by fitting the data to a 1/1 binding model.
family of spectra obtained when a solution of 4 in CH3CN/
H2O is titrated with NaCl. The presence of two distinct
isosbestic points at 335 and 378 nm indicates that only two
species are present in the equilibrium (Scheme 2). The inset in
Figure 4 shows the increase of absorbance at 404 nm on
progressive addition of Cl− (0−30 mM). The titration profile
could be fitted to a 1/1 binding model using a nonlinear least-
squares treatment. The calculated association constants are
Ka(CH3CN/H2O) = 260 16 M−1 and Ka(DMF/H2O) = (2.0
0.2) × 103 M−1. The Ka values determined by two
spectroscopic techniques are similar, which supports a 1/1
chemical equilibrium. In the case of DMF/H2O, the addition of
NaCl (0−8 mM) increases modestly the absorbance and the
course of the titration does not show isosbestic points.
However the modest spectral change at 400 nm (ΔA = 0.03)
was used to get a fit (Figure S14, Supporting Information). The
association constants obtained by spectroscopy make evident
that complex 4-triflate is able to bind chloride in buffered
aqueous solutions with high affinity. To obtain a sensitive
chemosensor, however, a higher value was required. To address
Figure 5. Changes of the emission spectra (λex 360 nm) of buffered
aqueous solutions (40 mM MOPS buffer at pH 7.0) of 4 (20 μM)
−
containing the surfactant CTA+HSO4 (5.0 mM) upon addition of
increasing amounts of NaCl (0−0.35 mM). The inset shows the
emission intensity at λ 512 nm for different chloride concentrations.
The solid line was obtained by fitting the data profile to a 1/1 binding
model.
with quantum yields from Φ = 0.17 for 4+OTf− to Φ = 0.25 for
̈
chloro complex 2. Previous work of Gabbaı has demonstrated a
fluorescence turn-on response upon anion binding using a
micellar system with an anthryltriphenylstibonium-based
receptor. This effect may result from the decreased solvation
of the neutral complex 2 in comparison to that of the cationic
precursor 4.
In addition, this effect has been attributed to the
disappearance of any intramolecular charge transfer processes
in the receptor−anion complex.22b The intensity variations at
512 nm were well fitted to a 1/1 binding model to give an
D
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apparent binding constant of Ka(CTA) = (7.9
0.8) × 104
M−1. This value is nearly 2 orders of magnitude higher than
that observed in DMF/H2O. On the other hand, UV−vis
titration experiments in this micellar system exhibit a very small
spectral change under additions of NaCl (Figure S16,
Supporting Information).
Next, the anion selectivity of the micellar system with 4 was
analyzed. Sodium salts of simple inorganic anions (halides,
−
−
−
−
−
2−
AcO−, NO3 , NO2 , H2PO4 , HCO3 , H3P2O7 , and SO4
;
[X−]final = 100 μM) were added to a buffered aqueous solution
of complex 4 (20 μM) and CTA+HSO4 (5.0 mM), and the
−
emission intensity increase at 512 nm was recorded. All
oxoanions and fluoride gave a very low response (Figure 6).
Figure 7. Profiles of emission titration experiments at 512 nm (λex 360
nm) of buffered aqueous solutions (40 mM, MOPS at pH 7.0, 5 mM
−
CTA+HSO4 ) of 4 (20 μM) by NaCl (0−0.3 mM) and NaBr (0−0.5
mM). The lines were obtained by fitting the data to the equilibrium
model 1/1.
The chloride concentration was then calculated using the fitting
equation derived from the emission titration experiment
(Figure 5). A comparison between reported and determined
chloride concentrations is given in Figure 8. The match
Figure 6. Relative emission intensity at 512 nm (λex 360 nm) of
buffered aqueous solutions (40 mM MOPS at pH = 7.0) of 4 (20 μM)
−
containing CTA+HSO4 (5.0 mM) upon addition of different anions
([X−] = 100 μM).
The addition of NaBr resulted in a modest increase in emission
intensity, but it was still significantly lower than that observed
for NaCl. Addition of NaI resulted in the formation of a
precipitate immediately. The interference of the heavier halide
anions is not unexpected, especially for bromide. Still, it is
remarkable that the sensing ensemble containing CTA+HSO4
−
with 4 is selective for chloride over bromide because bromide
has a lower solvation energy,7 and because bromide has been
shown to form more stable complexes with [Pt(NCN)(OH2)]+
complexes.11 The association constant with bromide was
calculated under the same conditions as in Figure 5. The
Figure 8. Reported (black bars) and determined (white bars) chloride
concentrations of commercial mineral water samples. The measure-
ments were carried out as described in the main text. The estimated
error is 10 μM.
resulting value of Ka.Br(CTA) = (6.3
0.4) × 103 M−1 is 1
between the two values is very good. It should be noted that the
mineral waters contain several competing inorganic anions,
some in the millimolar concentration range (e.g., bicarbonate
and sulfate).
order of magnitude lower than for chloride. The profiles of
emission data at 512 nm (λex 360 nm) for both anions are
shown in Figure 7. The high affinity of 4 for chloride over
bromide could be the result of having two directed arene CH
groups to the anion-binding site. It expects that these CH
groups form stronger hydrogen bonds with a more electro-
negative halide such as Cl−. In the case of oxoanions, it is
noteworthy that complex 4 has a higher affinity for chloride
than for more basic anions such as acetate and pyrophosphate.
In order to probe the utility of this platinum-based sensor, we
have determined the chloride concentration in commercial
mineral water as described below. A portion of degassed
mineral water (700 μL) was added to 1.4 mL of a buffered
aqueous solution of complex 4 and CTA (final concentrations:
CONCLUSION
■
A cationic platinum pincer complex with the N,C,N-chelating
ligand 4 can be used as an intrinsic luminescent chemosensor
for the detection of chloride in aqueous solutions containing 50
vol % of an organic cosolvent (DMF or CH3CN). Under these
conditions, the addition of NaCl quenches the emission of 4.
On the other hand, a particularly high chloride affinity (Ka =
(7.9
0.8) × 104 M−1) was found for buffered solutions
containing the cationic surfactant CTA+HSO4 . The green
−
[4], 20 μM; [CTA+HSO4 ], 5.0 mM; [MOPS], 20 mM; pH
luminescence of neutral aqueous solutions of 4 containing
−
−
7.0), and the emission intensity of the solution was recorded.
CTA+HSO4 was turned on by addition of salts of inorganic
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110.6. ESI-MS: [2]+ 656.42. Anal. Calcd for C32H21ClN4Pt (691.11):
C, 55.53; H, 3.06; N, 8.10. Found: C, 55.51; H, 3.05; N, 8.04.
Complex 3. The iodo complex 3 was obtained by reaction of
complex 2 (300 mg, 0.43 mmol) with 20 equiv of KI in DMF/CH3OH
(1/9 v/v, 30 mL) at 70 °C for 48 h. After the mixture was cooled, a
yellow precipitate was isolated by filtration. Recrystallization from
dichloromethane and diethyl ether gave the product in the form of
yellow crystals suitable for X-ray diffraction (310 mg) in 92% yield. 1H
NMR (400 MHz, CD2Cl2): δ 9.58 (d, J = 8.6 Hz, 2H), 7.75−7.69 (m,
6H), 7.60−7.54 (m, 4H), 7.47 (t, J = 7.5 Hz, 2H), 7.35 (t, J = 7.9 Hz,
2H), 7.13 (d, J = 8.4 Hz, 2H), 6.71 (t, J = 7.6 Hz, 1H), 6.40 (d, J = 7.7
Hz, 2H). 13C NMR (100 MHz, CD2Cl2): δ 162.8, 162.6, 142.2, 136.6,
134.5, 131.6, 131.2, 130.8, 128.3, 125.3, 124.8, 124.5, 122.9, 121.6,
111.0. ESI-MS: [3]+ 656.42. Anal. Calcd for C32H21IN4Pt (783.05): C,
49.05; H, 2.70; N, 7.15. Found: C, 49.01; H, 2.79; N, 7.09.
Complex 4. A mixture of complex 3 (200 mg, 0.25 mmol) and
silver triflate (70 mg, 0.27 mmol) in DMF/CH3OH (1/9 v/v, 20 mL)
was stirred at room temperature overnight. Subsequently, the reaction
mixture was filtered through Celite 540 and the solvent was removed
under reduced pressure to give a brown powder. Complex 4 was
obtained by recrystallization from CH3OH in 81% yield (169 mg). 1H
NMR (400 MHz, CD2Cl2/CD3OD, 1/1 v/v): δ 7.94 (m, 3H), 7.82−
7.72 (m, 6H), 7.64−7.54 (m, 7H), 7.46 (t, J = 7.5 Hz, 2H), 7.28 (d, J =
7.9 Hz, 2H), 6.76 (t, J = 8.4 Hz, 1H), 6.42 (d, J = 7.7 Hz, 2H). 19F
NMR: −80.0 ppm. 13C NMR (100 MHz, CD2Cl/CD3OD, 1/1 v/v): δ
161.4, 140.2, 136.5, 134.5, 133.1, 132.1, 131.6, 128.5, 126.7, 125.8,
125.7, 125.1, 121.4 (q, JCF = 310 Hz; only the two major signals of the
quartet were observed), 116.3, 112.7 (one signal was not detected).
ESI-MS: [4]+ 656.42; [4(CH3OH)]+ 697.17. Anal. Calcd. for
C34H25F3N4O4PtS (837.12): Cal. C, 48.75; H, 3.01; N, 6.69. Found:
C, 48.73; H, 3.05, N, 6.61.
anions, especially by NaCl. The optical change allows for
detection of chloride in the micromolar concentration range
with good selectivity over other common anions such as
bromide, phosphate, and acetate. NMR experiments show that
chloride anion is coordinated to platinum atom; additionally
two short CH···Cl−Pt contacts are formed, which could result
in the high affinity. This is supported by the crystal structure of
chloro complex 2. As a representative application, the chloride
concentration in mineral water samples was determined.
Overall, these results further highlight the utility of transition-
metal-based receptors for anion recognition and sensing in
water.
EXPERIMENTAL SECTION
■
General Considerations. All reagents for synthesis and analysis
were of analytical grade and were used without further purification: N-
Phenyl-o-phenylenediamine (Sigma-Aldrich), isophthaloyl dichloride
(Sigma-Aldrich), K2PtCl4 (99%, Precious Metals Online), silver triflate
(Sigma-Aldrich), 3-(N-morpholino)propanesulfonic acid (MOPS,
Sigma-Aldrich), cetyltrimethylammonium hydrogensulfate
−
(CTA+HSO4 , Aldrich), sodium chloride extra pure (Acros Organics
99.99%), sodium bromide (Fluka 99.9%), sodium fluoride (Fluka,
99.9%), sodium acetate (Sigma-Aldrich, 99.5%), sodium nitrate
(Aldrich, 99.9%), sodium nitrite (99.9% ACS reagent), sodium
phosphate monobasic dihydrate (Aldrich 99.0%), sodium sulfate
(Aldrich 99.9% ACS reagent), sodium bicarbonate (Sigma-Aldrich
99.5%), sodium pyrophosphate decahydrate (Sigma-Aldrich 99.0%).
MOPS buffer solution (40 mM, pH 7.0) was prepared with double-
distilled water. Stock solutions of anions were prepared in MOPS
buffer (40 mM, pH 7.0). A stock solution of complex 4 (2.0 mM)
containing CTA (5.0 mM) was made with double-distilled water.
Luminescence spectra were recorded on a Varian Cary Eclipse
Luminescence Emission Measurements. Titration experiments
were performed by adding aliquots of stock solutions of anions as
sodium salts to buffered aqueous solutions (40 mM, MOPS at pH 7.0)
−
of complex 4 (20 mM) containing CTA+HSO4 (5.0 mM). After
1
spectrophotometer equipped with a thermostated cell holder. H and
13C NMR spectra were recorded on a Bruker Advance DPX 400
spectrometer at 400 and 100 MHz, respectively. Electrospray
ionization mass spectra were obtained with a Waters CapLC-coupled
Micromass Q-ToF Ultima ESI instrument. Combustion analysis was
performed with a Thermo Scientific Flash 2000 Organic Elemental
Analyzer.
addition of anions, the solution was equilibrated for 5 min at room
temperature before recording the emission spectrum (excited at 360
nm) using a quartz cuvette. Experiments in DMF/H2O or CH3CN/
−
H2O were carried out accordingly, but without CTA+HSO4 . The
resulting data were fitted to an equation describing a 1/1
complexation24 using the nonlinear least-squares treatment with
Microcal Origin version 8.0 program. The UV−visible spectra were
recorded on a Hewlett-Packard 8453 diode array spectrometer at room
temperature. Luminescence quantum yields were determined using an
aqueous solution of quinine sulfate containing H2SO4 (0.5 M) as
standard (Φ = 0.546; excited at 360 nm).25 The refractive indexes used
for the mixtures CH3CN/H2O (1/1 v/v, xMeCN = 0.345) and DMF/
H2O (1/1 v/v, xDMF = 0.180) were taken from the literature.26
Phosphorescence lifetime measurements were performed on the same
Cary Eclipse fluorescence spectrophotometer and off-gate detection.
The selectivity experiments were performed with stock solutions of the
respective NaX and Na2X salts (100 mM) in MOPS buffer (40 mM,
pH 7.0). The analyses of commercial mineral water samples were
performed as follows: an aliquot (700 μL) of the degassed water
sample was added to a buffered aqueous solution containing complex 4
and CTA (final concentrations: [4], 20 μM; [XCTA], 5.0 mM;
[MOPS], 20 mM; pH 7.0). The emission intensities at 512 nm
(excited at 360 nm) were recorded, and the chloride concentration
was calculated using the fitting equation derived from the emission
titration experiment (Figure 5).
Crystallographic Investigations. The relevant details of the
crystals, data collection, and structure refinement can be found in
Table S1 (Supporting Information). Data collections for complex 2
and 3 were performed at low temperature using Mo Kα radiation on a
Bruker APEX II CCD instrument. Semiempirical27 absorption
correction was applied to all data sets. Structure solutions, refinements,
and geometrical calculations have been carried out by SHELXTL.28 All
structures were refined using full-matrix least-squares methods on F2
with all non-H atoms anisotropically defined. The hydrogen atoms
were placed in calculated positions using the “riding model” with Uiso
1,3-Bis(2-N-phenylbenzimidazolyl)benzene (1). Ligand 1 was
prepared by a reported procedure:23 isophthaloyl dichloride (1.02 g,
5.0 mmol) was added to a solution of N-phenyl-o-phenylenediamine
(1.85 g, 10 mmol) in N-methyl-2-pyrrolidinone (25 mL). The reaction
mixture was stirred under an atmosphere of dinitrogen at room
temperature for 2 h, and then the temperature was raised to 50 °C for
another 2 h. After it was cooled, the mixture was poured into cool
water (100 mL). The white precipitate was filtered off, washed with
water, and dried under vacuum to give the diamide in 90% yield.
Ligand 1 was obtained by cyclodehydration at 290 °C under an
atmosphere of dinitrogen for 2 h, followed by sublimation at 300−310
°C at reduced pressure. Yield: 1.25 g, 60%. 1H NMR (400 MHz,
CD2Cl2): δ 7.90 (s, 1H), 7.81 (d, J = 7.4 Hz, 2H), 7.55−7.44 (m, 8H),
7.36−7.20 (m, 11H). 13C NMR (100 MHz, CD2Cl2): δ 152.1, 143.7,
137.9, 137.3, 131.1, 130.7, 130.4, 129.2, 128.6, 127.9, 123.9, 123.4,
120.4, 111.0 (one signal was not detected). ESI-MS: [M + H]+ 463.18.
Anal. Calcd for C32H22N4 (462.18): C, 83.09; H, 4.79; N, 12.11.
Found: C, 83.01 H, 4.83; N, 12.06.
Complex 2. A mixture of N,C,N-chelating ligand 1 (462 mg, 1.0
mmol) and K2PtCl4 (421 mg, 1.0 mmol) in acetic acid (40 mL) was
heated under reflux for 24 h. After cooling and concentrating under
vacuum, the residue was purified by column chromatography on silica
gel with dichloromethane/chloroform (1/1 v/v) as eluent to give
1
complex 2 as a red crystalline powder (590 mg) in 85% yield. H
NMR (400 MHz, CD2Cl2): δ 9.05 (d, J = 9.6 Hz, 2H), 7.72−7.68 (m,
6H), 7.61−7.57 (m, 4H), 7.47 (t, J = 7.7 Hz, 2H), 7.35 (t, J = 7.7 Hz,
2H), 7.16 (d, J = 8.1 Hz, 2H), 6.68 (t, J = 7.6 Hz, 1H), 6.43 (d, J = 7.7
Hz, 2H). 13C NMR (100 MHz, CD2Cl2): δ 162.0, 161.2, 141.2, 135.8,
134.0, 131.7, 130.5, 130.4, 127.8, 124.9, 124.4, 124.0, 122.1, 118.6,
F
dx.doi.org/10.1021/om4007054 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
= aUeq (where a is 1.5 for −CH3 and −OH moieties and 1.2 for
others). Crystallographic data for the two crystal structures have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication nos. CCDC 969988−969989. X-ray
crystallographic data in CIF format are available in the Supporting
Information.
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Alcaz
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ASSOCIATED CONTENT
* Supporting Information
■
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S
Figures, tables, and CIF files givingcrystallographic data for 2
and 3, 1H and 13C NMR spectra of 1−4, 19F NMR spectrum of
4, UV−vis absorption spectra of complex 4 in aqueous media,
emission spectra of 4 in the presence of inorganic anions in
neutral micellar aqueous system, time-dependent 1H NMR
spectra of 4 in CD2Cl2, spectrophotometric titration of 4 by
NaCl in DMF/water, and UV−vis absorption spectra of
aqueous solutions of 4 in the presence and absence of chloride
containing the surfactant CTA+HSO4 . This material is
−
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: adg@unam.mx.
Zamparo, I.; Lodovichi, C. Org. Lett. 2012, 14, 2984−2987. (b) Jagt, R.
B. C.; Kheibari, M. S.; Nitz, M. Dyes Pigm. 2009, 81, 161−165.
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C. D.; Apperson, K.; Karolin, J.; Birch, D. J. S. Anal. Biochem. 2001,
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85, 49−57. (f) Goodall, W.; Williams, J. A. G. Dalton Trans. 2000,
2893−2895.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(6) For chloride recognition/sensing in aqueous solution see:
(a) Hancock, L. M.; Marchi, E.; Ceroni, P.; Beer, P. D. Chem. Eur.
J. 2012, 18, 11277−11283. (b) Ambrosi, G.; Formica, M.; Fusi, V.;
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Chem. Eur. J. 2011, 17, 1670−1682. (c) Hancock, L. M.; Gilday, L. C.;
A.D.-G. thanks the CONACyT for the repatriation fellowship
́
203539 and Professor Jesus Valdes
́
-Martınez and Professor Kay
́
́
Severin for all support. A.D.-G. thanks Ms. C. Marıa de las
́
Nieves Zavala Segovia and Dr. Diego Martınez Otero for their
technical assistance.
́
Carvalho, S.; Costa, P. J.; Felix, V.; Serpell, C. J.; Kilah, N. L.; Beer, P.
D. Chem. Eur. J. 2010, 16, 13082−13094. (d) Zhang, Y.-M.; Lin, Q.;
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