Fluorescent Chemosensors for Selective and Sensitive Detection of Phosmet/Chlorpyrifos with Octahedral Ni2+ Complexes

Article abstract of DOI:10.1021/acs.inorgchem.6b00332

The hexadentate ligands H₂L1-L3 with mixed S, N, O donor sites and possessing substituents having either "no" or electron-releasing/withdrawing nature at terminal ends are synthesized. The ligands H₂L1-L3 were tested for binding with

Full text of DOI:10.1021/acs.inorgchem.6b00332

Article
pubs.acs.org/IC
Fluorescent Chemosensors for Selective and Sensitive Detection of
2+
Phosmet/Chlorpyrifos with Octahedral Ni Complexes
†
†
†
‡
§
Pushap Raj, Amanpreet Singh, Kamalpreet Kaur, Thammarat Aree, Ajnesh Singh,
,†
and Narinder Singh*
†
Department of Chemistry, Indian Institute Technology, Ropar, Punjab 140001, India
‡
Department of Chemistry, Faculty of Science, Chulalongkorn University, Phyathai Rd., Pathumwan, Bangkok 10330, Thailand
Department of Applied Sciences and Humanities, Jawaharlal Nehru Government Engineering College, Sundernagar, Mandi (H.P.)
§
175018, India
*
S Supporting Information
ABSTRACT: The hexadentate ligands H L1−L3 with mixed S, N, O
2
donor sites and possessing substituents having either “no” or electron-
releasing/withdrawing nature at terminal ends are synthesized. The
ligands H L1−L3 were tested for binding with library of metal ions,
2
2
+
wherein maximum efficiency was observed with Ni , and it motivated us
to prepare the Ni complexes. The ligand H L1 underwent
2
+
2
deprotonation and formed binuclear complex when interacted with
2+
Ni as evident from its crystal structure. The H L2 and H L3 having
2
2
electron-withdrawing/electron releasing groups, respectively, were also
deprotonated; however, they afforded mononuclear complexes with Ni2+
ion. This signifies the importance of steric parameters instead of
electronic factors in these particular cases. Impressed by differential
2+
behavior of complexes of H L1 and H L2/H L3 with Ni and their photophysical and electrochemical properties, all the metal
2
2
2
complexes were studied for their chemosensing ability. Nowadays with increased use of organophosphate, there is alarming
increase of these agents in the environment, and thus we require efficient technique to estimate the level of these agents with high
sensitivity and selectivity in aqueous medium. The Ni2+ complexes with hydrophobic nature were suspended into aqueous
2
+
medium for testing them as sensor for organophosphate. The (L1) .(Ni ) could sense phosmet with detection limit of 44 nM,
2
2
whereas L2.Ni2 and L3.Ni exhibited the detection limits of 62 and 71 nM, respectively, for chlorpyrifos.
+
2+
17,18
INTRODUCTION
labile ligand.
The strategy is explored reasonably; however,
■
many reports are limited to anion sensing only. Contrarily,
these activities are applied hardly to warfare chemicals,
poisonous materials such as nerve agents, and organo-
phosphates. The sensing of organophosphates is among the
most focused areas with respect to environment, as they are the
neurotoxins and are poisonous substances to be known as
The metal complexes of imine-linked receptors have been
extensively reported because of their pronounced application in
research arena of catalysis, biology, mimicking, sensing, and
1
−4
materials science.
real impulse in last two decades, when materials were explored
comprehensively for modulation of surface area, porous nature,
These metal complexes have gained the
1
9−21
5
−7
pesticides for agriculture.
The inherent toxicity of
and labile metal−ligand interactions.
In this context,
organophosphates lies with their affinity to bind irreversibly
with acetylcholinesterase, and this leads to prevent their
biological activities; consequently, the accumulation of
acetylcholine in the nervous system causes organ failure and
multinuclear metal complexes are reported, which mimics the
metalloenzyme for their catalytic function such as urease,
phosphotase, hemerythrin, catechol oxidase, arginase, ribonu-
8
−10
clease reductase.
plexes are intensively being used for sensing of anions,
biomolecules, and organophosphate with better optical and
Similarly, nowadays these metal com-
22,23
eventually death.
In spite of severe toxicity of organo-
phosphate, they are the far-most choice of farmers as a
24
1
1−13
pesticide, herbicide, and rodenticide. However, the surplus
amounts of organophosphate released to environment has
alarming consequences, as they have become a part of food
chain and ultimately cause great risk to human health,
ecosystem, and homeland security. Further to protect our
electrochemical properties.
The sensing in aqueous
medium is a challenging task due to competition between the
solvent and guest for receptor binding sites, and the issue is
very severe, if the binding is realized with hydrogen
1
4−16
bonding.
To address this problem of sensing in aqueous
medium, the transition metal complexes have gained good
reputation through utilizing electrostatic interaction, vacant d-
orbital to form covalent bond or sometimes replacement of
Received: February 17, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00332
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Inorganic Chemistry
Article
natural resources, flora, and fauna, the scientific community has
shown seriousness through developing the assay for qualitative
and quantitative analysis of organophosphates in soil, water,
ties were evaluated using absorption and emission spectrosco-
py. The UV−vis absorption spectrum of ligand H L1 has
2
shown a broad absorption band (λ ) at 350 nm (ε = 35 000 L
abs
o
2
5
−1
−1
and foodstuff. The chromatography and mass and NMR
mol cm ) due to ∏−∏* transition. However, the ligands
H L2 and H L3 have shown broad absorption band at 420 (ε
= 55 000 L mol cm ) and 400 nm (ε = 40 000 L mol
2
6,27
spectroscopies
have been used with good success; however,
2
2
o
28
−1
−1
−1
there is always room for improvement. These techniques are
time-consuming, require expensive sample preparation, and
need experts to handle the instrument. The other set of assay
including chemosensors involving enzymatic assays, colorimet-
ric method, metal−organic frameworks, electrochemical
sensors, fluorescence organic molecules, and interferometry is
o
−1
cm ), respectively, again due to ∏−∏* transition. The
interaction of ligands H L1−L3 with a library of metal ions
2
+
+
2+
2+
2+
2+
3+
2+
3+
2+
(such as Na , K , Mg , Ca , Ba , Sr , Cr , Mn , Fe , Co ,
2
+
2+
2+
2+
2+
+
2+
3+
Ni , Cu , Zn , Cd , Hg , Ag , Pb , and Al ) exhibited the
most significant changes upon binding with Ni , as interpreted
2+
29−33
mostly operative in organic solvents.
To overcome this
from the modulation in the absorption spectrum of H L1. The
2
limitation of operation in organic solvents, we developed novel
strategies for detection of organophosphate in aqueous
medium. Under present manuscript, we developed novel
metal complexes and suspended them in aqueous medium.
The skeleton of organic receptors is designed in such a way that
it is easy to engineer the sensor system, offer modulation in
fluorescence emission, and fabricate with multiple binding sites
that can afford flexible coordination sphere. Our strategy for
metal center glares the open shell, which should reversibly
absorption band for H L1 (10 μM) in DMSO (originally at 350
2
2
+
nm) was shifted to 410 nm on addition of 50 μM of Ni ion,
2+
clearly showing the binding of Ni in the coordination sphere
2
+
of ligand H L1 (Figure S13). To investigate the effect of [Ni ]
2
on the modulation of absorption profile of ligand H L1, a
2
titration was performed through the addition of 0−50 μM of
2
+
Ni to the fixed concentration of H L1 (10 μM). The
2
2+
successive addition of Ni to the solution of ligand H L1 led to
2
the increase in absorbance at 410 nm and decrease in
absorption at 350 nm with isosbestic points at 325 and 380
nm (Figure 1A). In the competitive binding experiment, the 50
μM of each selected metal ion was added to Ni2+ solution of
“switch off” the fluorescence intensity, and the interaction of
metal complex with analyte must regain the original
fluorescence signature of receptor. We compared our result
with literature-reported sensor for organophosphate and found
that present sensor has multiple advantages as shown in Table
S5.
H L1, and absorbance was measured. The absorbance
2
measurements showed no interference in the binding of nickel
with any of tested ions (Figure S14). Similarly, the L2 (10 μM)
was tested with the same set of metal ions, and the significant
2+
RESULTS AND DISCUSSION
change was observed only with Ni (Figure S15). The titration
■
2
+
of H L2 with Ni (0−50 μM) leads to enhance the absorbance
Syntheses and Characterization of Ligands and Metal
2
at 370 nm and decrease the absorbance at 405 nm with
isosbestic point at 390 nm (Figure1B). Further, the competitive
binding studies were performed to confirm the selective
Complexes. The ligand H L1 was synthesized through
2
condensation reaction between 2,2′-disulfanediyldianiline and
salicylaldehyde in methanol, whereas 2,2′-disulfanediyldianiline
was synthesized through the aerial oxidation of 2-amino-
2
+
binding of Ni , and the experiment revealed that H L2
2
selectively binds with the nickel ion (Figure S16). The H L3
thiophenol. The ligands H L2−L3 were synthesized using the
2
2
was then treated with similar library of metal ions; the only
change in absorbance was noticed upon the addition of nickel
similar condensation procedure as was used for the synthesis of
H L1. All the ligands H L1−L3 were fully characterized with
2
2
2
+
ion. Finally, the titration of H L3 with Ni (0−50 μM) has
spectroscopic methods, and the purity of samples was
established with elemental analysis. The formation of imine
2
pronounced hyperchromic shift at 305 nm and hypochromic
shift at 400 nm with isosbestic points at 375 and 425 nm
linkages in H L1−L3 were established from signals at 8.6, 9.1,
2
1
13
(
Figure1C). The interference experiment was performed to
and 8.5 ppm in H NMR, and C NMR depicted these signals
at 163.28, 165.40, and 163.28 ppm; similarly, the imine linkages
were characterized with a band between 1600 and 1630 cm− in
IR spectra. To prepare the required metal complexes, the
reactions of ligand H L1/H L2/H L3 were performed with
confirm the selectivity of nickel with the H L3. It was observed
2
1
from the studies that the nickel ion selectively binds with the
H L3 (Figure S17). Thus, these significant changes in
2
2
+
absorption profiles of ligands H
in solution state highlights the pivotal role of mixed S/N/O
donor sites in the hexadentate framework of H
L1−L3 on reacting with Ni
2
2
2
2
nickel nitrate, and upon completion of reaction, the metal
complexes were separated from reaction mixture. All the
complexes were characterized with spectroscopic methods;
photophysical properties were measured with absorption and
L1−L3 and also
2
inspired us to synthesize and investigate the metal complexes.
The freshly prepared and purified metal complexes were
dissolved in DMSO, and the electronic absorption spectrum of
emission spectroscopy, and structure of two complexes [(L1) .
2
2+
2+
2+
(
Ni ) and L2.Ni ] was established with crystallography. The
(L1)
2
.(Ni )
2
complex showed a broad absorption band (λabs),
2
−1
−1
−1
IR spectra of all the complexes have shown the bands at 1500−
1
with maxima at 24 390 cm (ε
Table 1), which is assigned to the spin-allowed transition A2g
F) → T1g (P), and this supports the distorted octahedral
geometry. The electronic absorption spectra of L2.Ni and
o
= 14 000 L mol cm ; see
640 cm− due to CHN stretching vibration.
1
3
3
(
2
+
2
+
−1
L3.Ni showed broad band (λ ) with maxima at 27 027 cm
abs
−1
−1
−1
(
ε = 11 000 L mol cm ) and 25 000 cm (ε = 8000 L
o
o
−
1
−1
mol cm ), respectively, and are assigned spin-allowed
3
3
transition A2g (F) → T (P) in both the complexes and
1g
34
again highlight the distorted octahedral geometry. The
Photophysical Properties of Ligands and Metal
luminescence properties of all the three lignads H2L1−L3
2
+
Complexes. All the three ligands H L1−L3 were soluble in
were measured in DMSO, and subsequently the effect of Ni
2
dimethyl sulfoxide (DMSO), and their photophysical proper-
coordination was evaluated on the fluorescence signature of
B
DOI: 10.1021/acs.inorgchem.6b00332
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Inorganic Chemistry
Article
2
+
Figure 1. Changes in UV−vis absorption spectra upon addition of Ni (0−50 μM) to the DMSO solution (10 μM) of (A) ligand H L1, (B) ligand
2
H L2, and (C) ligand H L3. (D) A comparison of luminescence spectra recorded with 10 μM solution of ligands H2L1−L3 and corresponding
2
2
2
+
2+
2+
metal complexes (L1) .(Ni ) , L2.Ni and L3.Ni .
2
2
Table 1. Photophysical Parameters of Ligand and Metal Complexes
ε (L mol− cm−1
1
)
absorption transition
λem (nm)
quantum yield (Φs)
sample
ligand/metal complexes
λabs
0
1.
2.
3.
4.
5.
6.
H L1
350 nm
405 nm
35 000
55 000
40 000
14 000
11 000
8000
∏ → ∏*
∏ → ∏*
∏ → ∏*
A2g (F) → T (P)
3A2g (F) → T (P)
407
398
398
470
469
470
0.84
0.82
0.82
0.54
0.53
0.55
2
H L2
2
H L3
2
400 nm
2
+
24 390 cm−1
27 027 cm−1
25 000 cm−1
3
3
[(L1) .(Ni ) ]
L2.Ni2+
L3.Ni2+
2
2
1g
3
1g
3A2g (F) → T (P)
3
1g
each ligand (Figure 1D). The excitation of nickel complex
will be realized through the change in fluorescence intensity as
usually happen in “Cation Displacement Assay”/“Anion
Displacement Assay”.
2+
(
L1) .(Ni ) at 350 nm exhibited photoluminescence at 470
2 2
2
+
2+
35−41
nm, whereas the L2.Ni and L3.Ni complexes exhibited the
emission at 469 and 470 nm, respectively. All the three ligands
Crystal Structure of Metal Complexes. X-ray structure
2
+
(
H L1−3), when excited with same excitation wavelength as
determination revealed that complex (L1) .(Ni ) crystallizes
2
2
2
that used to excite the metal complexes at same physical
in triclinic crystal system with space group P1
̅
and that the unit
2
+
conditions, the ligand H L1 emitted at 407 nm; ligands H L2
cell consists of two (L1) .(Ni ) neutral complexes and 6.4
2
2
2 2
and H L3 exhibited emission at 398 nm. A comparison of
cocrystallized water molecules. The ORTEP view of complex
2
2
+
emission profile of a metal complex with respective ligand
revealed that the intensity of metal complexes gets quenched as
evident from quantum yields. This shows the importance of
open shell of metal ion, which causes the quenching of
fluorescence intensity. This metal ion mediated modulation of
fluorescence intensity may make the sensor system ideal for the
recognition of organophosphates. In other words, when these
analytes will bind with the metal ions, then these binding events
(L1) .(Ni ) along with atom numbering is shown in Figure
2 2
2A (for clarity only one complex is shown). In dinuclear
complex, the Ni2+ is surrounded by one sulfur, two nitrogen,
and three oxygen atoms, of which one oxygen atom is bridged
between two Ni2+ ions. These ligands are arranged in
octahedral coordination fashion (Figure 2B) with average
Ni−O, Ni−N, and Ni−S bond distances of 2.058(4), 2.066(5),
and 2.489(2) Å. The maximum angular deviation from
C
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Inorganic Chemistry
Article
2
+
Figure 2. (A) ORTEP diagram along with atom numbering scheme of complex (L1) .(Ni ) with 40% probability thermal ellipsoids (for clarity
2
2
2
+
only one diatomic complex is shown). (B) Geometry of dinuclear nickel complex (L1) .(Ni ) . (C) ORTEP diagram along with atom numbering
2
2
2
+
scheme of complex L2.Ni with 40% probability thermal ellipsoids (for clarity only complex is shown, and water molecules are removed). (D)
2
+
Geometry of mononuclear nickel complex L2.Ni .
expected octahedral geometry is 78.27(14) and 158.83(17) in
case of angle N(7)−Ni(4)−S(7) and N(6)−Ni(3)−O(8),
respectively. The different complex moieties are held in three-
dimensional crystal lattice with the help of number of hydrogen
bonding interactions of type O−H···O, C−H···O, and C−H···
N. The hydrogen bonding parameters are shown in Table S2.
Additional crystallographic parameters are given in Table 2.
Table 2. Crystal Refinement Data
2+
L2.Ni2+
compound
empirical formula
Mw
(L1) .(Ni )
2 2
C104H N Ni O S
C H N O S Ni
26 20
84
8
4
14
8
4
8 2
2161.11
639.29
temperature, [K]
crystal system
space group
a, [Å]
293(2) K
triclinic
293(2) K
monoclinic
2
+
P1
̅
P2 /c
1
The complex L2.Ni crystallizes in monoclinic crystal
14.0785(8)
14.2423(8)
27.2243(13)
76.500(2)
88.452(2)
74.831(2)
5119.6(5)
2
13.7815(17)
13.8543(18)
27.356(4)
90
system with space group P2 /c. It consists of two neutral
1
b, [Å]
mononuclear Ni2 complexes and two water molecules of
+
c, [Å]
2
+
crystallization. The central metal ion, Ni , adopted the
octahedral geometry (Figure 2D), which is satisfied by the
two nitrogen, one sulfur, and two oxygen atoms originating
from the ligand and one from water molecule. The ORTEP
diagram along with atom numbering scheme of complex is
shown in Figure 2C (for clarity only one complex is shown).
The two water molecules of crystallization are involved in
hydrogen bonding with coordinated water molecules, and
oxygen atoms of the nitro groups of the ligand to provide
robustness to the crystal lattice (hydrogen bonding parameters
are shown in Table S4).
α, [deg]
β, [deg]
γ, [deg]
97.566(4)
90
V, [Å3]
5177.7(11)
8
Z
D , [Mg m−3]
1.402
1.640
c
μ, [mm−1
]
0.953
0.971
reflections collected
data/restraints/parameters
unique reflections, [Rint]
GOF = Sall
40354
90381
14 333/1/1292
8896/0/748
14 333 [ 0.0929]
0.965
8896 [0.1256]
1.025
Electrochemical Properties. The electrochemical behav-
ior of mononuclear and dinuclear complexes was studied with
cyclic voltammetry in DMSO containing 0.1 M tetrabutyl
ammonium perchlorate. The redox behavior of dinuclear
final R indices
R , wR [I > 2σI]
0.0579, 0.1088
0.1436, 0.1258
0.553 /-0.345
0.0772, 0.1411
0.1461, 0.1572
1.695/-0.558
1
2
R , wR (all data)
1
2
3
Δρmax/Δρmin, [Å ]
2
+
complex of nickel (L1) .(Ni ) showed quasi-reversible
2
2
4
2
II II
reduction wave at Epc = −0.40 V corresponding to Ni Ni /
Ni Ni . However, the second reduction peak corresponding to
II
I
D
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Figure 3. (A) Cyclic voltammogram (scan rate = 100 mV/s) of (L1) .(Ni ) , L2.Ni and L3.Ni2+ in DMSO with supporting electrolyte
[
2
+
2+
2
2
n
2+
2+
2+
Bu N][ClO ]. (B) TGA graph of (L1) .(Ni ) , L2.Ni and L3.Ni in the range from 20 to 700 °C at heating rate of 10 °C per minute.
4 4 2 2
II
I
I
I
Ni Ni /Ni Ni was not observed possibly due to fast reduction
process. The coulometric experiment with potentiostatic
exhaustive electrolysis was performed at −0.30 V (lower than
recent years due to the potential applications in the field of
chemistry, material science, biological imaging, and diagnos-
48
tics. The numbers of approaches are reported in literature;
however, the most successful approach involves the control
over sensitivity, selectivity, solubility, stability, and application
at the target. Among all the reported methods, the fluorescence
is one of straightforward, least time-consuming, and highly
1
00 mV as compared to cathodic peak), and the reduction peak
consumed one electron (n = 0.91) for redox reaction. This
peak was attributed to quasi-reversible signal of Ni Ni /Ni Ni
redox couple. The effect of scan rate on the redox behavior of
43
II II
II
I
4
4
2+
49
(
s
L1) .(Ni ) was measured in the scan range of 50−300 mV
reproducible methods. In present manuscript, we prepared
2
2
−
1
(Figure S27). The ΔE value (200 mV) is greater than the
the weakly fluorescent nickel complexes and further suspended
2
+
5
9/n mV and increase with the increase in scan rate. The ratio
them in aqueous medium. The nickel complexes (L1) .(Ni )
2 2
of cathodic to anodic peak current is greater than one (i /i >
were dissolved into DMSO, and the solution was injected to
doubly deionized water with sonication at 25 ± 1 °C. After
injection, the sonication was continued for 30 min at constant
rate to establish the stable sensor system of the nickel complex
pc pa
1
(
−
). The redox behavior of both mononuclear complexes
L2.Ni2 and L3.Ni ) showed irreversible reduction at E
+
2+
=
pc
II
I
0.70 and −0.60 V corresponding to Ni /Ni as shown in
2
+
2+
Figure 3A. The reduction potential (E = −0.70 V) of L2.Ni
(L1) .(Ni ) . The same procedure was used for the develop-
pc
2
2
2
+
2+
is higher than the reduction potential of (L1) .(Ni ) and
ment of sensors system of nickel complexes L2.Ni and
L3.Ni . The UV−vis absorption spectra of (L1) .(Ni ) ,
L2.Ni , and L3.Ni in DMSO showed broad absorption band
2
2
2+
2+
2+
L3.Ni , which is most likely due to the less distorted
octahedral geometry
2
2
4
5−47
2+
2+ 2+
in the L2.Ni as compared to
2+
2+
(L1) .(Ni ) and L3.Ni . In general the electronic density on
at 410, 370, and 400 nm, respectively, which was assigned to
2
2
3
3
metal ion and geometry distortion are the governing factors for
the reduction process. The electron-withdrawing group
attached to metal complexes reduces at lower negative potential
than the electron-releasing group bonded metal complexes.
However, in our work it was observed that the geometry of
complexes is the dominant factor to modulate the reduction
potential then the electronic factors. This was also earlier
observed with photoluminescence studies, which have shown
that the electron-releasing and electron-withdrawing group has
spin-allowed transition A2g (F) → T (P). However, the
1g
2
+
2+
2+
absorption band of (L1) .(Ni ) , L2.Ni , and L3.Ni in
2
2
aqueous medium was shifted drastically due to the change in
band gap between highest occupied molecular orbitals and
lowest unoccupied orbitals (Figure S26). The chemosensory
2
+
2+
2+
activities of (L1) .(Ni ) , L2.Ni , and L3.Ni toward various
2
2
organophosphates were evaluated using emission spectroscopy.
2
+
The emission spectra of (L1) .(Ni ) recorded with 10 μM
2
2
concentration in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic
acid (HEPES) buffer at pH 7.4 showed a peak at 470 nm. On
addition of tested species (such as ATP, AMP, NADP, NADH,
azamethiphos, parathion-methyl, parathion, fenitrothion, chlor-
2
+
not affected the emission profile of Ni complexes.
Thermal Studies. The thermal stability of the complexes
2+
2+
2+
(
L1) .(Ni ) , L2.Ni , and L3.Ni were analyzed through
2 2
2
+
TGA in the range from 20 to 700 °C at heating rate of 10 °C/
pyrifos, and azinphos-methyl) to the solution of (L1) .(Ni ) ,
2 2
2
+
min. TGA results showed that the complexes (L1) .(Ni )
no significant change in fluorescence emission profile was
2
2
and L2.Ni2 were stable and showed adequate crystallinity at
room temperature. It was analyzed that the organic ligand in
both the complexes decomposed at 450 °C with the loss of
+
shown except for phosmet. The emission profile of (L1) .
2
2
+
(Ni ) showed fourfold enhancement in the fluorescence
2
intensity upon binding with phosmet; thus concluded the
2
+
6
5% mass of the complex, and 35% residue was found to be
selective binding of phosmet with (L1) .(Ni ) (Figure 4A).
2 2
2
+
NiO. In the complex L3.Ni , it was observed that the organic
ligand decomposed up to 350 °C with loss of 54% mass of
complex, and remaining NiO residue was observed (Figure 3B).
Chemosensor Development Using Metal Complexes.
The development of chemosensors becomes passionate in
Upon progressive addition of phosmet (0−50 μM) into (L1) .
2
2
+
(Ni ) solution, the gradual enhancement takes place in the
2
emission profile of metal complex (Figure 4B). The species
distribution of metal complex−phosmet was evaluated using
50
hyperspec software, and the nonlinear regression plot is
E
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2
+
2+
2+
Figure 4. Changes in fluorescence spectra of (A) (L1) .(Ni ) ; (C) L2.Ni ; (E) L3.Ni upon addition of various organophosphate (50 μM) in
2
2
2+
2+
HEPES buffered aqueous medium (pH = 7.4). Fluorescence titration of (B) (L1) .(Ni ) with phosmet (0−50 μM); (D) L2.Ni with chlorpyrifos
2
2
2
+
(
0−50 μM); (F) L3.Ni with chlorpyrifos (0−50 μM) in HEPES buffered aqueous medium (pH = 7.4). The nonlinear plot between fluorescence
intensity vs concentration of organophosphate and their species distribution are depicted as inset of respective titration figure.
shown as inset of Figure 4B. It was found that 1:1 species of
due to selective binding of chlorpyrifos with corresponding
2
+
phosmet and (L1) .(Ni ) dominate in the solution at higher
metal complexes. It was further confirmed with titration
experiment: on adding an aliquot of chlorpyrifos (0−50 μM)
to metal complexes the regular enhancement in the
fluorescence intensity takes place (Figure 4D,F). A nonlinear
plot was drawn between fluorescence intensity versus
2
2
2
+
2+
concentration. Similarly, the L2.Ni and L3.Ni showed
enhancement in the emission profile upon interaction of
chlorpyrifos rather than any other tested organophosphate
(Figure 4C,E). The enhancement in emission peak occurred
F
DOI: 10.1021/acs.inorgchem.6b00332
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Inorganic Chemistry
Article
Figure 5. Possible mechanism of interaction of (A) phosmet with dinuclear nickel complex and (B) chlorpyrifos with mononuclear nickel complex.
1
concentration of chlorpyrifos; species formed in the solution at
different concentration were checked with hyperspec software,
and 1:1 species dominate in the solution at higher
geometry. In H NMR spectrum both complexes show the
change in chemical shift value of respective proton with
addition of chlorpyrifos. This attribute to change the symmetry
of metal complexes when bind with the chlorpyrifos. The
possible binding site of chlorpyrifos with metal complex
concentration. The limit of detection (LOD) of (L1) .
2
2+
(
Ni ) toward phosmet was determined by 3σ method,
2
2
+
using LOD = 3 × SD/m, and it was found to be 44 nM, where
L2.Ni is the replacement of water molecule by sulphur
atom of thiophosphate (PS) group and simultaneously
electrostatic interaction with the pyridine nitrogen to the nickel
center (Figure S31). In the P NMR of chlorpyrifos the δ value
at 61.08 ppm was shifted to δ value 60.90 ppm when
chlorpyrifos interacted with metal complexes (Figure S34). The
SD is the standard deviation of blanks signal and m is the slope
2
+
2+
of the curve. Similarly, the LOD for L2.Ni and L3.Ni
31
toward chlorpyrifos was calculated to be 62 and 71 nM,
respectively. The binding events of metal complexes for
organophosphates were also evaluated with UV−vis absorption
spectroscopy, and the results are shown in Figures S28 and S29.
Mechanism of Sensing of Organophosphate. To study
the binding mechanism of interaction of phosmet/chlorpyrifos
with metal complexes, the NMR titrations were conducted in
2
+
binding behavior of chlorpyrifos with L3.Ni was same as that
2
+
of chlorpyrifos with L2.Ni (Figure S32). This was also
confirmed from 31P NMR spectrum, which revealed that the
phosphorus signal of chlorpyrifos shifted from (δ) 61.08 to
2
+
DMSO-d . NMR spectra were recorded by equimolar addition
60.74 ppm, when interacting with L3.Ni (Figure S35). From
51,54
6
1
31
1
of respective analytes to the host solution. The H NMR
the H NMR and P NMR study the possible mechanism
2
+
2+
of interaction of L2.Ni /L3.Ni with chlorpyrifos are given
titration revealed (a) significant shift in proton signal in (L1) .
2
2+
below.
(
Ni ) upon addition of phosmet and (b) considerable shift in
2
the aromatic proton of phosmet on interacting with (L1) .
2
2+
1
(
Ni ) . The H NMR titration concluded that the metal center
EXPERIMENTAL SECTION
General Information. The analytical-grade chemicals were
2
■
binds with the carbonyl group of phosmet, and the other metal
center underwent interaction with the sulfur moiety of the
thiophosphate group prevailing in phosmet (Figure S30). This
purchased from Sigma-Aldrich, and bulk chemicals were supplied by
1
13
SD Fine India. H and C NMR spectra were measured on JEOL
1
3
1
instrument operated at 400 MHz (for H NMR) and 100 MHz (for
C NMR). Fourier transform infrared (FT-IR) spectra of dried
was further confirmed through P NMR spectrum that the
phosphorus signal of phosmet originally at δ = 94.90 ppm was
13
samples were measured on a Bruker Tensor 27 spectrophotometer
using solid cell technique. Elemental analyses were measured through
a Fisons instrument (Model EA 1108 CHN). Electrospray ionization
mass spectra (ESI-MS) were analyzed on ES-MS Q-TOF mass
spectrometer. TGA was performed on a TGA/DSC 1 STAR SYSTEM
from Mettler Toledo with temperature increments of 10 °C/min
split into δ = 94.53−94.71 ppm on interaction with (L1) .
2
2
+
1
31
(
Ni ) (Figure S33). On the basis of H NMR and P NMR
2
5
1−53
titrations, the possible mechanism
of binding of phosmet
with nickel complex are shown as follows (see Figure 5).
The L2.Ni2 and L3.Ni have shown binding affinity with
+
2+
chlorpyrifos because both have same orientation of ligand and
under N stream. The photophysical properties were evaluated with
2
G
DOI: 10.1021/acs.inorgchem.6b00332
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
1
UV−vis absorption spectrophotometer (Shimadzu UV-2400) and
fluorescence spectrophotometer (Perkin L55). The analysis range was
fixed as 200−900 nm, and measurements were performed at room
temperature using a 1 cm path length quartz cuvette. The
electrochemical properties were recorded using a potentiostat
galvanostat BASI EPSILON using a platinum disc as working
electrode, Ag/AgCl as reference electrode (3 M KCl), and platinum
wire as counter electrode.
50.29; H 3.01; N, 9.09%; IR (cm ) 1615 (ν
620.9 (M+1) .
); ESI-MS (m/z):
CHN
+
Synthesis and Characterization of Ni2+ Metal Complex of L3. The
560 mg (2 mmol) of NiNO ·6H O and 1196 mg (2 mmol) of L3 were
dissolved in THF, and the reaction mixture was stirred for 8 h at 150
°C. A light greenish-brown colored precipitate was formed, which was
washed with THF, filtered, and dried to obtain a greenish-brown
powder. Yield: 35%. Anal. Calcd for C H N O S Ni: C, 60.63; H,
5.69; N, 8.32; found C, 60.61; H, 5.67; N, 8.30%; IR (cm ) 1634
(ν ); ESI-MS (m/z): 672 (M+).
X-ray Data Collection and Refinement. After repeated attempts
the crystals were obtained, and the data were collected for some of the
3
2
34
38
4
3 2
−1
Synthesis and Characterization of H L1. The ligand H L1 was
2
2
synthesized via condensation reaction between 2,2′-disulfanediyldiani-
line (496 mg, 2 mmol) with salicylaldehyde (488 mg, 4 mmol) in
methanol. A yellow colored solid separated out after stirring for 6 h.
The precipitates were washed and recrystallized with methanol to give
CHN
2+
single crystals. The X-ray diffraction data for (L1) .(Ni ) and
2
2
−1
2+
pure product in 91% yield. mp ≥ 270 °C; IR (cm ) 3373 (O−Hsym),
L2.Ni were collected on a Bruker X8 APEX II KAPPA CCD
diffractometer and Bruker D8 Venture PHOTON 100 CMOS CCD
diffractometer, respectively, at 293 K using graphite/mirrors
monochromatized Mo Kα radiation (λ = 0.710 73 Å). The crystals
were positioned at 50 mm from the CCD, and the diffraction spots
were measured using a counting time of 15s. Data reduction and
multiscan absorption were performed using the APEX II program suite
(Bruker, 2007). The structures were solved by direct methods with the
SIR97 program [S1] and refined using full-matrix least-squares with
SHELXL-97[S2]. Anisotropic thermal parameters were used for all
1
1
611 (CHN ); H NMR (400 MHz in CDCl ) δ (ppm) = 8.62 (s,
sym
3
CHN, 1H), 7.65 (d, Ar−H, 1H), 7.4 (m, Ar−H, 2H), 7.23 (d, Ar−
H, 1H), 7.18 (t, Ar−H, 1H), 7.14 (d, Ar−H, 1H), 7.05 (d, Ar−H,
13
1
1
1
H), 6.95 (t, Ar−H, 1H); C NMR (100 MHz in CDCl ) δ (ppm):
3
62.9, 161.2, 146.4, 133.9, 132.8, 131.6, 127.8, 127.7, 127.5, 127.3,
19.3, 117.7, 117.5. Anal. Calcd for C H N O S : C, 68.40; H, 4.42;
26
20
2
2 2
N, 6.14; found: C, 68.46; H, 4.39; N, 6.18%; ESI-MS (m/z): 457 (M
+
+
1) .
Synthesis and Characterization of H L2. Ligand H L2 was
2
2
2+
synthesized via condensation reaction between 2,2′-disulfanediyldiani-
line (496 mg, 2 mmol) with 5-nitro-salicylaldehyde (668 mg, 4 mmol)
in methanol. A light yellow colored solid was separated out after 6 h.
The precipitate was washed and recrystallized with methanol to form
non-H atoms. In case of complex (L1) .(Ni ) three water molecules
O4W, O5W, and O6W were found disordered. O4W was disordered
over two positions, while O5W and O6W were disordered over three
2 2
2+
positions. In the case of L2.Ni , two water molecules of crystallization
O1W and O2W are disordered over two positions each. The hydrogen
atoms of C−H groups were with isotropic parameters equivalent to 1.2
times those of the atom to which they were attached. All other
calculations were performed using the programs WinGX [S3] and
PARST [S4]. The molecular diagrams were drawn with DIAMOND
[S5]. Final R-values together with selected refinement details are given
in Table 1. Selected bond lengths and bond angles for complexes
−1
the pure product having 89% yield. mp ≥ 277 °C; IR (cm ) 3377
1
(
O−H ), 2973 (Ar−H ), 1615 (CHN ); H NMR (400 MHz,
sym
sym
sym
DMSO-d ) δ (ppm) = 9.11 (s, CHN, 1H), 8.69 (d, Ar−H, 1H), 8.3
6
(
dd, Ar−H, 1H), 7.6 (d, Ar−H, 1H), 7.45 (d, Ar−H, 1H), 7.3 (m, Ar−
13
H, 2H), 7.1(d, Ar−H, 1H); C NMR (100 MHz, DMSO-d ) δ (ppm)
6
=
165.9, 161.9, 146.3, 140.2, 135.9, 131.2, 129.0, 128.3, 127.9, 124.9,
1
18.5, 116.5, 115.3; Anal. Calcd for C H N O S : C, 57.13; H, 3.32;
26
18
4
6 2
2
+
2+
N, 10.25; found: C, 57.20; H, 3.25; N, 10.28%; ESI-MS (m/z): 547
(L1) .(Ni ) and L2.Ni are given in Tables S2 and S4.
Evaluation of Photophysical Properties. The recognition
2 2
+
(
M+1) .
Synthesis and Characterization of H L3. Ligand H L3 was
properties of ligand H L1−L3 were conducted at 25 ± 1 °C, and
2
2
2
synthesized via condensation reaction between 2,2′-disulfanediyldiani-
line (496 mg, 2 mmol) with 4,4′-diethylamino-2-hydroxybenzaldehyde
772 mg, 4 mmol) in methanol. A deep yellow colored solid was
before recording the experiment, the solution of ligand (10 μM) was
shaken well to ensure the uniformity. The metal binding behavior of
(
ligand H L1−L3 was checked with a library of different metal ions in
2
separated out after 6 h. The precipitate was treated and recrystallized
DMSO and was investigated through change in absorbance spectrum.
The titration was recorded with the standard solution of ligand (10
μM) with constant addition of nickel nitrate (0−50 μM) solution, and
respective changes in absorbance were observed.
in methanol to get pure product having 85% yield. mp ≥ 296 °C; IR
−
1
1
(
cm ) 3373 (O−H ), 1615 (CHN ); H NMR (400 MHz in
sym
sym
CDCl ) δ (ppm) = 8.48 (s, CHN, 1H), 7.7 (d, Ar−H, 1H), 7.23 (t,
3
Ar−H, 2H), 7.15 (d, Ar−H, 2H), 6.25 (m, Ar−H, 2H), 3.4 (q, CH ,
Electrochemical Measurement of Metal Complexes. All
electrochemical properties were recorded on a BASI EPSILION. All
studies were performed on a single compartment under nitrogen
2
13
4
1
1
H), 1.3 (t, CH , 6H); C NMR (100 MHz in CDCl ) δ (ppm) =
3
3
63.2, 160.4, 152.1, 146.4, 134.1, 131.2, 127.0, 126.3, 126.1, 117.1,
09.2, 103.9, 97.8, 44.7, 12.3; Anal. Calcd for C H N O S : C, 68.19;
−1
atmosphere at 100 mV s scan rate (25 ± 2 °C), with a platinum disc
as working electrode, Ag/AgCl as reference electrode (3 M KCl), and
platinum wire as counter electrode. The solution of metal complexes
was prepared with concentration of 50 μM in DMSO along with
tertiary ammonium perchlorate as supporting electrolytes.
34
38
4
2 2
H, 6.40; N, 9.36; found: C, 68.24; H, 6.29; N, 9.31%; ESI-MS (m/z):
+
5
99.1 (M+1) .
Synthesis and Characterization of Ni2+ Metal Complex of L1. The
12 mg (2 mmol) of ligand L1 dissolved in tetrahydrofuran (THF)
9
and aqueous solution of nickel nitrate 560 mg (2 mmol) was mixed
and stirred at 100 °C for 30 min. The green colored solution was
obtained, which was filtered and kept for slow evaporation at room
temperature to get crystalline material. After 10 d, the formation of
green colored crystals occurred, which are suitable for X-ray
crystallography, and yield was 59%. Anal. Calcd (%) for
C H N 0 S Ni : C, 60.84; H, 3.53; N, 5.46; found: C, 60.76; H,
Chemosensors Activities of Metal Complexes. The metal
complexes were suspended in aqueous solution using re-precipitation
5
5
methods. In this typical method, the metal complex was dissolved in
DMSO (1.0 mL), and solution was injected into 99.0 mL of doubly
distilled deionized water with a constant injection rate and under
vigorous sonication. All organophosphate binding studies were
performed at 25 ± 1 °C, and before recording the experiment enough
time was given to ensure the uniformity of the solution. The
organophosphate binding behavior was evaluated from the change in
the UV−vis absorption and fluorescence spectra of metal complex (10
μM) on addition of tested organophosphate (50 μM) in aqueous
medium. To further confirm the experiment, the reproducibility of the
titration was performed. For titration, volumetric flasks were taken for
each standard solution of metal complex along with various amounts
of organophosphate (0−50 μM) in aqueous buffered solution.
Fluorescence quantum yield (Φs) was determined by using an
optically matching solution as standard (9,10-diphenyl anthracene in
5
2
36
4
4
4
2
−1
3
.50; N, 5.49%; IR (cm ): 1613 (ν
), 1303 (νS−S); ESI-MS (m/
CHN
+
z): 1025.2 (M+1) .
2
+
Synthesis and Characterization of Ni Metal Complex of L2. The
60 mg (2 mmol) of Ni(NO ) ·6H O and 1092 mg (2 mmol) of L2
5
3
2
2
were dissolved in THF, and the mixture was stirred at 110 °C for 2 h.
A light green colored solution was formed, which was kept for slow
evaporation at room temperature to achieve crystallization. After a few
days green colored crystals are formed, which were washed with THF
and were suitable for X-ray crystallographic studies. Yield: 49%; Anal.
Calcd(%) for C H N O S Ni: C, 50.26; H, 2.92; N, 9.02; found: C,
26
18
4
7 2
H
DOI: 10.1021/acs.inorgchem.6b00332
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ethanol) at excitation wavelength of 350−370 nm, and quantum yield
ACKNOWLEDGMENTS
56
■
is calculated by the following equation as
We are thankful to DST [Project No. EMR/2014/000613] for
financial support. P.R. is thankful to UGC (New Delhi) for a
junior research fellowship.
I ODr η′
Φs = Φr ×
Ir OD η′ r
Whereas Φs and Φr are the quantum yield of sample and reference,
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ASSOCIATED CONTENT
■
(
2
(
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00332.
(
9
(
Spectroscopic data including NMR, IR, and mass spectra;
graphs for photophysical properties. (PDF)
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AUTHOR INFORMATION
■
*
(
31) Lum, K. T.; Huebner, H. J.; Li, Y.; Phillips, T. D.; Raushel, F. M.
Corresponding Author
E-mail: nsingh@iitrpr.ac.in. Phone: +91-1881242176.
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I
DOI: 10.1021/acs.inorgchem.6b00332
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Inorganic Chemistry
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7
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