A schiff base and its copper(II) complex as a highly selective chemodosimeter for mercury(II) involving preferential hydrolysis of aldimine over an ester group

Article abstract of DOI:10.1021/ic403149b

The syntheses of a new Schiff base, diethyl-5-(2-hydroxybenzylidene) aminoisophthalate (HL), and a copper complex, [Cu(L₂)] (1), imparting L^-, have been described. Both the ligand HL and complex 1 have been thoroughly characterized b

Full text of DOI:10.1021/ic403149b

Article
pubs.acs.org/IC
A Schiff Base and Its Copper(II) Complex as a Highly Selective
Chemodosimeter for Mercury(II) Involving Preferential Hydrolysis of
Aldimine over an Ester Group
Ashish Kumar,† Mrigendra Dubey,† Rampal Pandey,‡ Rakesh Kumar Gupta,† Amit Kumar,†
Alok Ch. Kalita,§ and Daya Shankar Pandey*,†
†Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, U.P., India
‡Department of Chemistry, Dr. Hari Singh Gour University, Sagar 470 003, M.P., India
§Department of Chemistry, Indian Institute of Technology, Bombay 400 076, Maharashtra, India
S
* Supporting Information
ABSTRACT: The syntheses of a new Schiff base, diethyl-5-
(2-hydroxybenzylidene)aminoisophthalate (HL), and a copper
complex, [Cu(L2)] (1), imparting L−, have been described.
Both the ligand HL and complex 1 have been thoroughly
characterized by elemental analyses, electrospray ionization
mass spectrometry, FT-IR, NMR (1H and 13C), electronic
absorption, and emission spectral studies and their structures
determined by X-ray single-crystal analyses. Distinctive
chemodosimetric behavior of HL and 1 toward Hg2+ has
been established by UV/vis, emission, and mass spectral
studies. Comparative studies further revealed that the
chemodosimetric response solely originates from selective
hydrolysis of the aldimine moiety over the ester group and 1 exhibited greater selectivity toward Hg2+ relative to HL while the
sensitivity order is reversed. Further, these followed different hydrolytic pathways but ended up with the same product analyzed
for diethyl-5-aminoisophthalate (DEA). Hg2+-induced displacement of Cu2+ and subsequent hydrolysis of the −HCN− moiety
in 1 affirmed the identity of the actual species undergoing hydrolysis as HL. The occurrence of Cu2+ displacement and Hg2+
detection via hydrolytic transformation has been supported by various physicochemical studies.
monitoring Hg2+-induced changes in the UV/vis or fluo-
INTRODUCTION
■
rescence of a chromophore.10
Studies pertaining to the detection and determination of heavy-
and transition-metal ions have been attractive because of their
extremely toxic impact on the environment and biological
Considering these issues, various novel molecules (fluo-
rophores,11−17 proteins,18 and metal nanoparticles19) have been
synthesized and used toward the development of techniques for
systems.1 Among these, mercury presents one of the most
the detection of Hg2+. These methods also bear a scope of
hazardous and toxic pollutants that upon inhaling or ingestion
overcoming some unavoidable limitations such as poor
poses serious health problems.2,3 Regardless of the toxicity,
solubility, cross-sensitivity of other metal ions, and cost
mercury and its salts find wide applications in diverse industrial
effectiveness of the probe synthesis. Continual efforts in this
processes and products like paints, electronic equipment, and
batteries.4,5 Although strict regulations have decreased its use in
direction have led to the manifestation of a chemodosimetric
approach for the detection of Hg2+.20 Efficacy of this approach
many industrial processes, a high concentration of mercury is
still present in many environmental segments.6 Amid different
largely depends upon the elegant design of probes capable of
exhibiting highly selective and sensitive spectroscopic responses
forms of mercury distributed in the environment (Hg0, Hg2+,
to Hg2+-induced chemical transformations. In this context, a
and CH3Hg+), methyl mercury easily enters the food chain of
few systems have been devised that can selectively detect Hg2+
in an aqueous/mixed-aqueous media via hydrolytic trans-
formation of the Schiff base.21 Moreover, to best of our
knowledge, none of the available systems involve Hg2+-assisted
preferential cleavage of the aldimine bond over ester groups. In
search of such a “turn-on” fluorescent chemodosimeter for
aquatic organisms and ultimately in human beings.7 Bio-
accumulation of methyl mercury occurs to a greater extent
relative to its elemental and inorganic forms, and its buildup in
the liver, kidney, and spleen even at lower concentrations leads
to DNA damage, mitosis impairment, and nervous system
defects.8 Therefore, the development of suitable techniques for
its detection and determination has been a challenging task.9
Further, owing to its d10 configuration, Hg2+ lacks an optical
spectroscopic signature and its optical detection is achieved by
Received: December 26, 2013
Published: April 28, 2014
© 2014 American Chemical Society
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Chart 1
Hg2+ in mixed-aqueous media, a new Schiff base, diethyl-5-(2-
hydroxybenzylidene)aminoisophthalate (HL), and a copper(II)
complex, [Cu(L2)] (1), containing L− have been synthesized.
Through this contribution, we present aldimine-based
systems HL and 1, which accomplish the detection of Hg2+
through hydrolysis at a specific aldimine site without affecting
the ester groups under a suitable pH range (∼7.2−3.5) in
mixed-aqueous media. The mechanistic pathways (Chart 1)
have been profoundly established by various methods, which
strongly support Hg2+-induced Cu2+ displacement cum explicit
hydrolysis at the −HCN− bond and accountability of the
resulting species (diethyl-5-aminoisophthalate, DEA) for a
fluorescence “turn-on” response toward Hg2+.
obtained was filtered, washed several times with water, extracted with
dichloromethane, and dried using anhydrous Na2SO4. After decant-
ation, the filtrate was concentrated to dryness under reduced pressure
on a rotary evaporator to afford the desired product. Yield: 4.7 g, 72%.
Anal. Calcd for [C12H15NO4]: C, 60.75; H, 6.37; N, 5.90. Found: C,
60.63; H, 6.31; N, 5.86. IR (KBr pellets, cm−1): 3457 and 3363 (s, N−
Hstr, ArNH2), 2997−2905 (m, νstr C−Haliphatic), 1698 and 1689 (s, νstr
COester). 1H NMR (CDCl3, 300 MHz, δH, ppm): 8.05 (s, 1H, ArH),
7.51 (s, 2H, ArH), 4.37 (m, 2H, J = 6.9 Hz, CH2), 3.97 (br, 2H, NH2),
1.39 (t, 3H, J = 6.9 Hz, CH3). 13C NMR (CDCl3, 75.45 MHz, δc,
ppm): 166.1, 146.6, 131.7, 120.5, 120.4, 119.6, 61.2, 14.3. UV/vis [c,
50 μM; 9:1 (v/v) MeOH/H2O; λmax, nm (ε, M−1 cm−1)]: 330 (7.70 ×
103), 252 (1.67 × 104).
Synthesis of Diethyl-5-(2-hydroxybenzylidene)-
aminoisophthalate (HL). DEA (1.185 g, 5.0 mmol) dissolved in
MeOH (50 mL) was added dropwise to a methanolic solution (10
mL) of salicylaldehyde (0.53 mL, 5.0 mmol) and the resulting solution
heated under reflux for 12 h. The reaction mixture was filtered while
hot and its volume reduced to ∼20 mL. A few drops of acetonitrile was
added to the above solution and allowed to cool at rt. This afforded
diffraction-quality yellow block-shaped crystals within 5 h. Yield: 1.2 g,
69%. Anal. Calcd for [C19H19NO5]: C, 66.85; H, 5.61; N, 4.10. Found:
C, 66.79; H, 5.59; N, 4.02. IR (KBr pellets, cm−1): 2993−2876 (m, νstr
EXPERIMENTAL SECTION
■
Materials. Solvents were dried and distilled prior to their use
following standard procedures.22 Reagent-grade chemicals were used
throughout, and HPLC-grade solvents were employed for spectro-
scopic studies. The metal nitrates Hg(NO3)2·H2O (98.0% purity) and
Cu(NO3)2·2.5H2O were purchased from Sigma-Aldrich Chemical Co.
Pvt. Ltd. and used as received. Diethyl-5-aminoisophthalate (DEA)
was prepared following a literature procedure with slight modifica-
tions.23
1
C−Haliphatic), 1722−1710 (vs, νstr COester), 1619 (s, νstr CN). H
NMR (CDCl3, 300 MHz, δH, ppm): 12.82 (s, 1H, H9), 8.71 (s, 1H,
H4), 8.58 (s, 1H, H1), 8.12 (s, 2H, H2 and H3), 7.42 (t, 2H, H5 and
H8), 6.99 (m, 2H, H6 and H7), 4.44 (q, 4H, J = 6.9 Hz, H10), 1.44 (t,
6H, J = 6.9 Hz, H11). 13C NMR (CDCl3, 75.45 MHz, δC, ppm):
165.3, 164.4, 161.1, 148.9, 133.8, 132.7, 132.2, 128.5, 126.2, 119.2,
118.8, 117.3, 61.6, 14.2. ESI-MS (rel intens). Calcd, found: m/z
342.1341, 342.1518 [(M + H)+, 100%]. UV/vis [c, 50 μM; 9:1 (v/v)
MeOH/H2O; pH ∼7.2; λmax, nm (ε, M−1 cm−1)]: 338 (1.24 × 104),
274 (1.77 × 104).
Physical Measurements. Elemental analyses on the samples were
performed in the Microanalytical Laboratory of the Department of
Chemistry, Faculty of Science, Banaras Hindu University, Varanasi,
India, on an Exeter Analytical Inc. CE-440 analyzer. IR and electronic
absorption spectra were acquired on PerkinElmer 577 FT-IR and
1
Shimadzu UV-1601 spectrophotometers, respectively. H (300 MHz)
and 13C (75.45 MHz) NMR spectra were obtained at room
temperature (rt) on a JEOL AL300 FT-NMR spectrometer using
tetramethylsilane [TMS, Si(CH3)4] as an internal reference. Emission
spectra at rt were recorded on a PerkinElmer fluorescence
spectrometer (LS-55) in methanol (MeOH)/H2O (9:1, v/v).
Electrospray ionization mass spectrometric (ESI-MS) data were
acquired on Bruker Micro TOF QII and THERMO Finningan LCQ
Advantage Max ion-trap mass spectrometers.
Synthesis of [Cu(L2)] (1). To a deprotonated solution of HL
[obtained by treating HL (1.023 g, 3.0 mmol) with KOH (0.168 g, 3.0
mmol) in MeOH (50 mL) and stirring for 20 min to afford a clear
yellow solution] was added with stirring a methanolic solution of
Cu(NO3)2·2.5H2O (0.350 g, 1.5 mmol, 5 mL). Immediately, the color
of the reaction mixture turned olive green, and a precipitate started to
appear within 10 min. This was stirred for an additional 3 h and the
resulting precipitate filtered and washed with cooled MeOH and
diethyl ether. The crude solid thus obtained upon crystallization from
tetrahydrofuran (THF)/methyl cyanide (MeCN) (9:1, v/v) afforded
block-shaped green crystals within 2 days. These were separated and
Preparation of DEA. A solution of 5-aminoisophthalic acid (5.0 g,
27.62 mmol) in dry ethanol (50 mL) was treated with concentrated
sulfuric acid (4 mL) and the resulting solution heated under reflux for
16 h. The reaction mixture was poured into ice−water containing
sodium hydrogen carbonate (10.0 g), and the white precipitate thus
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Scheme 1. Synthesis of HL and 1
dried in vacuo. Yield: 0.621 g, 55%. Anal. Calcd for [C38H36CuN2O10]:
C, 61.32; H, 4.88; N, 3.76. Found C, 61.24; H, 4.96; N, 3.69. IR (KBr
pellets, cm−1): 2954 (m, νstr C−Haliphatic), 1723 (vs, νstr COester),
1613 (s, νstr CN). ESI-MS (rel intens). Calcd, found: m/z 745.2538,
745.2052 [(M + H)+]. UV/vis [c, 50 μM; 9:1 (v/v) MeOH/H2O; pH
∼7.4; λmax, nm (ε, M−1 cm−1)]: 392 (1.09 × 104), 281 (3.16 × 104).
UV/vis and Fluorescence Studies. A stock solution of HL and 1
for electronic absorption (50 μM) and fluorescence (10 μM) studies
was prepared in 9:1 (v/v) MeOH/H2O. Solutions of the respective
metal ions (Na+, K+, Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Ag+, Zn2+,
Cd2+, Hg2+, and Pb2+) were prepared by dissolving their nitrate salts in
triply distilled water (100 mM). In a typical titration, a solution of HL
and 1 was taken in a quartz cuvette (3.0 mL; path length, 1 cm), and
diluted stock solutions of the metal ions were added gradually with the
help of a micropipet. The spectral changes for HL and 1 were noted as
a function of the equivalents of the metal ions added after 2 min, and
all of the experiments were repeated thrice.
Crystal Structure Determinations. Suitable crystals for single-
crystal X-ray diffraction analyses for HL were obtained directly from
the reaction mixture and for 1 using THF/MeCN (9:1, v/v). The
crystals were mounted on glass fibers. All of the geometric and
intensity data were collected at 150 K using a Rigaku Saturn 724+
CCD diffractometer with a fine-focus 1.75 kW sealed tube Mo Kα (λ =
0.71075 Å) X-ray source, with increasing ω (width of 0.3 per frame) at
a scan speed of either 3 or 5 s/frame. After the initial solution,
refinement was performed with a WinGX environment using direct
methods (SHELXS 97) and by full-matrix least squares on F2 (SHELX
97).24 All of the non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were geometrically fixed and refined as per the riding
model.
afford the mononuclear complex 1 in good yield (55%). A
simple scheme showing the synthesis of HL and 1 is depicted in
Scheme 1.
Characterization of HL and 1 has been achieved by
satisfactory elemental analyses and spectral (IR, NMR, UV/
vis, fluorescence, and ESI-MS) studies. Crystal structures of
both HL and 1 have been authenticated by X-ray single-crystal
analyses.
The 1H NMR spectrum of HL is expected to exhibit singlets
due to the phenolic (−OH) and aldimine (−HCN−)
protons in downfield side along with other protons. Expectedly,
it displayed two distinct singlets at δ 12.82 (−OH) and 8.71
(−CHN−). The isophthalate ring protons resonated as
singlets at δ 8.58 (H1) and 8.12 (H2 and H3), while phenolic
ring protons (H5, H6, H7, and H8) resonated as a multiplet at
∼δ 7.45−6.95. In addition, methylene and methyl protons
appeared as a quartet and a triplet at δ 4.44 (4H) and 1.44
(6H), respectively. The position and integrated intensity of
various signals in the 1H NMR spectrum strongly supported the
formulation of ligand HL. 13C NMR spectral data (summarized
in the Experimental Section) also corroborated well with the
formation of HL (SI, Figure S2). The composition of both HL
and 1 has also been supported by ESI-MS studies (SI, Figure
S4). Molecular ion peaks at m/z 342.1518 [(M + H)+, 100%]
and 745.2052 [(M + H)+, 25%], respectively, in the ESI-MS
spectra of HL and 1 strongly supported their formulations
(vide supra).
Crystal Structures of HL and 1. The ligand HL
crystallizes in an orthorhombic system and space group Pca21
and 1 in a monoclinic system with space group P21/c. Selected
crystallographic data and geometrical parameters for these are
summarized in Tables 1 and S1 (SI), respectively, and their
pertinent views along with the atom numbering scheme
(partial) are depicted in Figures 1 and 2. The crystal structure
of HL revealed that it is a nonplanar molecule, wherein
isophthalate and phenolic rings are mutually twisted with a
dihedral angle of 43.32° (SI, Figure S5). This twisting may
enhance the photoinduced electron-transfer (PET) phenom-
enon responsible for fluorescence quenching.25
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The ligand HL was
synthesized by condensation of DEA with salicylaldehyde (1:1)
in MeOH under refluxing conditions in ∼69% yield
[Supporting Information (SI), Figures S1 and S2]. After
cooling to rt, the reaction mixture, upon treatment with a few
drops of acetonitrile and allowing it to remain undisturbed,
gave yellow block-shaped crystals within 5 h. The deprotonated
ligand L− [obtained by treating HL with potassium hydroxide
(KOH) in MeOH and stirring for 20 min; see the Experimental
Section] reacted with hydrated Cu(NO3)2·2.5H2O (2:1) to
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Table 1. Crystal Data and Structure Refinements for Ligand
HL and Complex 1
HL
1
identification code
empirical formula
fw
SHELXL
SHELXL
C19H19NO5
341.35
C38H36CuN2O10
744.23
temperature (K)
wavelength (Å)
cryst syst
150(2)
150(2)
0.71075
0.71075
orthorhombic
Pca21
monoclinic
P21/c
space group
unit cell dimens
a (Å)
7.616(2)
14.870(10)
7.459(6)
14.326(9)
90
b (Å)
13.133(5)
c (Å)
16.922(6)
Figure 2. ORTEP view of 1 (50% probability and hydrogen atoms
α (deg)
90
omitted for clarity).
β (deg)
90
90.468(11)
90
γ (deg)
90
volume (Å3)
1692.6(10)
1588.9(19)
2
bonding with an interaction distance of 2.605 Å. This lies
within the range for O−H···N hydrogen bond distances.26 In
the crystal lattice, molecular units adopt a zigzag arrangement
originating from an alternate array of successive isophthalate
ring planes (SI, Figure S6). Various bond distances and angles
fall within the typical range reported for analogous systems.27
The copper(II) center in 1 assumes a square-planar geometry
completed by phenolate oxygen and aldimine nitrogen from
deprotonated ligand L− in a trans N2O2 fashion (Figure 2). The
dihedral angle between the isophthalate and phenolate ring
planes in a particular ligand unit is 46.48°, which is comparable
to that in HL (SI, Figure S5). The Cu−O (1.889 Å) and Cu−N
(2.021 Å) bond distances and angles lie within the acceptable
range and are comparable to those in other related systems.28
The aldimine bond lengths C13−N1 in both HL (1.282 Å) and
1 (1.305 Å) also corroborated well with analogous systems.27
Further, HL lacks any solvent molecules and intermolecular
weak interactions like hydrogen bonding and π···π stacking,
while 1 shows strong stacking between the phenolate−
isophthalate and phenolate−phenolate rings with stacking
distances of 3.631 and 3.577 Å, respectively.
Z
4
density (calcd)(mg m−3
)
1.340
1.556
abs coeff (mm−1
)
0.097
0.756
F(000)
cryst size (mm3)
720
774
0.10 × 0.10 × 0.04
2.86−24.99
0.08 × 0.04 × 0.02
2.74−25.00
θ range for data collection
(deg)
reflns collected
indep reflns
11887
11476
2957 [R(int) =
0.0319]
2787 [R(int) =
0.0572]
completeness to θ = 24.99° 99.90
(%)
99.70
abs corrn
semiempirical from
equivalents
semiempirical from
equivalents
max and min transmission
0.9961 and 0.9903
0.9850 and 0.9420
refinement method
full-matrix least
full-matrix least
squares on F2
squares on F2
data/restraints/param
GOF on F2
2957/1/226
0.976
2787/23/232
1.132
final R indices [I > 2σ(I)]
R1 = 0.0353, wR2 = R1 = 0.0881, wR2 =
0.0789 0.2255
R indices (all data)
R1 = 0.0380, wR2 = R1 = 0.1005, wR2 =
0.0805
0.2355
Absorption and Emission Studies. The electronic
absorption spectrum of HL [c, 50 μM; 9:1 (v/v) MeOH/
H2O; pH ∼7.2] exhibited two prominent bands at 338 nm (ε,
1.24 × 104 M−1 cm−1) and 274 nm (ε, 1.77 × 104 M−1 cm−1).
The low-energy (LE) band at 338 nm has been tentatively
assigned to n−π* transitionsand the high-energy (HE) band at
274 nm to the π−π* transitions.29 To examine the interaction
of HL with various cations, a solution of this compound was
treated with 10.0 equiv of nitrate salts of Na+, K+, Mg2+, Ca2+,
Mn2+, Co2+, Ni2+, Zn2+, Cd2+, Ag+, Pb2+, Cu2+, and Hg2+. This
exhibited insignificant changes in the presence of the aforesaid
metal ions except for Cu2+ and Hg2+ (Figure 3a). The addition
of Hg2+ induced significant hypso- and hypochromic shifts (Δλ,
08 nm; Δε, 4.22 × 103 M−1 cm−1) for the LE band. On the
other hand, the HE band (λ, 274 nm) disappeared, and
simultaneously a new band appeared at 252 nm (ε, 1.79 × 104
M−1 cm−1). Likewise, the addition of the Cu2+ led to a decrease
in the intensity of the LE band (Δε, 6.02 × 103 M−1 cm−1); at
the same time, the optical density of the HE band (Δε, 6.00 ×
102 M−1 cm−1) was enhanced with concomitant emergence of a
new band at 392 nm (ε, 4.98 × 103 M−1 cm−1). A significant
shift in the position of the LE band and vanishing of the HE
band in the presence of Hg2+ suggested noteworthy changes in
the structure of HL. Conversely, in the presence of Cu2+, the
largest diff peak and hole (e 0.126 and −0.153
0.929 and −1.323
Å−3
)
Figure 1. ORTEP view of HL showing intramolecular hydrogen
bonding (50% ellipsoid probability and hydrogen atoms omitted for
clarity).
The phenolic −OH (donor) and azomethine nitrogen
(acceptor) atoms are involved in intramolecular hydrogen
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Figure 3. (a) Absorption spectra of HL (c, 50 μM) and in the presence of various metal ions. (b) Absorption spectra of 1 (c, 50 μM) with various
metal ions (10.0 equiv) in MeOH/H2O (9:1, v/v).
Figure 4. (a) Comparison of the absorption spectrum of HL + Hg2+ (green) with that of DEA (violet). (b) Resemblance of HL and 1 with each
other after the addition of Hg2+ and their resemblance with DEA (violet).
intensity of the LE band appreciably decreased probably
because of its interaction with HL through azomethine nitrogen
and phenolic oxygen to form complex 1.
followed by hydrolysis at the aldimine linkage of the resulting
HL.
With an objective of establishing the role of Cu2+ and Hg2+,
we intended to synthesize the Cu2+ complex, imparting HL.
The key idea behind this objective was to look into comparative
affinities of the “aldimine” and “ester” functionalities toward the
hydrolytic detection of Hg2+, particularly, in a protected
aldimine site coordinated to Cu2+. In this regard, the desired
copper complex 1 was synthesized and thoroughly charac-
terized (vide supra). The electronic absorption spectrum of 1
comprises two intense bands at 392 and 281 nm due to n−π*
and π−π* transitions, respectively (Figure 3b).
On the basis of the disparity of spectral features, it may be
concluded that HL interacted with Hg2+ and Cu2+ in a distinct
manner. Changes in the UV/vis spectrum of HL selectively in
the presence of Hg2+ and Cu2+ motivated us to investigate the
ultimate product under harsh competition between Hg2+ and
Cu2+. In this context, Hg2+ (10.0 equiv) and Cu2+ (10.0 equiv)
were added to a solution of HL + Cu2+ (10.0 equiv) and HL +
Hg2+ (10.0 equiv), respectively. In the first instance, spectral
features were found to be similar to that of HL + Hg2+, while
the second one remained unperturbed. This observation
strongly suggested that the addition of Hg2+ causes similar
effects on HL and HL + Cu2+. At this stage, we assume that
Cu2+ is being displaced from the HL + Cu2+ complex by Hg2+
liberating the ligand under a reaction medium, which, in turn,
undergoes hydrolytic cleavage.13 Moreover, HL possesses two
dissimilar hydrolyzable sites in the form of an aldimine and two
ester groups. Therefore, to validate the site of hydrolysis, the
UV/vis spectrum of DEA itself was acquired. Notably, it came
out to be similar to that of HL + Hg2+ or HL + Cu2+ + Hg2+,
indicating hydrolytic cleavage of the aldimine bond (Figure 4a).
Irrefutably, the addition of Hg2+ directly to a solution of HL
induces hydrolysis at the aldimine bond, generating DEA,
whereas it displaces Cu2+ from the HL + Cu2+ complex,
The addition of tested metal ions (10.0 equiv) to a solution
of 1 displayed insignificant alterations in the spectral features of
this complex except for Hg2+, which led to the loss of both LE
and HE bands and the emergence of new bands at 330 and 252
nm, resembling that of DEA (Figure 3b). This observation
strongly suggested that DEA is the ultimate product that arises
from the interaction of both HL and 1 with Hg2+ (Figure 4b).
This further strengthened the possibility of Hg2+-induced
decomplexation of 1 followed by liberation of HL in the
reaction medium, which, in turn, undergoes site-selective
hydrolysis at the aldimine bond. Also, it strongly supported
the Hg2+-selective chemodosimetric behavior of HL and 1
involving site-specific hydrolysis.20,21,30
The ligand HL [c, 10 μM; 9:1 (v/v) MeOH/H2O; pH ∼7.2]
and 1 [c, 10 μM; 9:1 (v/v) MeOH/H2O; pH ∼7.4] upon
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Figure 5. Fluorescence spectra of (a) HL (c, 10 μM) and (b) 1 (c, 10 μM) in the presence of various metal ions (10.0 equiv; c, 10 mM) in MeOH/
H2O (9:1, v/v).
Figure 6. Absorption titration of (a) HL (c, 50 μM) and (b) 1 (c, 50 μM) in [9:1 (v/v) MeOH/H2O] in the presence of Hg2+ (10 mM) at rt.
excitation at 338 nm weakly fluoresce (λem = 423 nm; Φ, 0.014,
HL; λem = 423 nm; Φ, 0.011, 1) with a Stokes shift of 85 nm.
To compare and contrast the results, solutions of both HL and
1 were treated with tested metal ions under analogous
conditions and excited at the same wavelength (λex = 338
nm).13,31 Additionally, 1 was not excited at the LE band (392
nm) because it overlaps with its own emission bands (λem = 423
nm).
H2O; pH ∼7.2] exhibited a hypochromic shift (Δε, 1.54 × 103
M−1 cm−1) for the band at 338 nm with a negligible blue shift
and substantial shrinkage of the band at 274 nm (Δε, 4.04 ×
103 M−1 cm−1). Further additions of Hg2+ (0.3−2.0 equiv) led
to complete loss of the band at 274 nm. At the same time, the
optical density of the band at 338 nm significantly diminished
with a concomitant hypsochromic shift (330 nm; Δλ, 08 nm;
Δε, 4.22 × 103 M−1 cm−1) and a new band emerged at 252 nm
(ε, 1.79 × 104 M−1 cm−1) (Figure 6a).
Both HL and 1 illustrated insignificant changes in their
fluorescence spectra in the presence of tested metal ions (10.0
equiv) except for Hg2+. Further, HL + Hg2+ leads to a ∼30-fold
enhancement (Φ, 0.43) in the fluorescence quantum yield
(QY); however, a very small quenching was observed with
Cu2+. Likewise, the addition of Hg2+ to a solution of 1 causes
∼55-fold (Φ, 0.62) emission enhancement (Figure 5a,b).
Although both HL and 1 displayed good selectivity toward
Hg2+, comparative spectral (UV/vis and fluorescence)
responses revealed that the latter exhibits greater selectivity
and the former higher sensitivity toward Hg2+. This may be
ascribed to the unavailability of the aldimine moiety for
interaction with other metal ions because of its involvement in
complexation with the Cu2+ center.
The isosbestic point at 262 nm signifies the existence of more
than two species in the system. Saturation was attained at 2.0
equiv of Hg2+ quantification that may be accredited to
generation of the hydrolyzed amine counterpart (DEA) to its
maximum, through specific hydrolysis of the aldimine linkage of
HL. The resulting species after isolation and thorough
1
characterization by H NMR and mass spectral studies was
confirmed as DEA (SI, Figures S15a and S16). To ascertain
applicability of HL as a selective chemodosimeter for Hg2+,
interference studies were carried out that demonstrated
insignificant changes in the spectral features due to HL +
Hg2+ and showed almost the same optical density at 338 nm
(Figure 7a,b).
Chemodosimetric Detection of Hg2+. To scrutinize the
quantitative chemodosimetric feasibility and sensitivity of HL
and 1 toward Hg2+, electronic absorption and fluorescence
titration studies have been carried out. The addition of Hg2+
(0.2 equiv) to a solution of HL [c, 50 μM; 9:1 (v/v) MeOH/
Similarly, the gradual addition of Hg2+ (1.0 equiv) to a
solution of 1 [c, 50 μM; 9:1 (v/v) MeOH/H2O; pH 7.42] led
to a hypochromic shift for the bands at 392 nm (Δε, 5.78 × 103
M−1 cm−1) and 281 nm (Δε, 1.15 × 104 M−1 cm−1). This was
accompanied by the surfacing of a new band at 338 nm (ε, 9.32
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Figure 7. (a) Interference of various metal ions with Hg2+ in HL. (b) Bar diagram showing negligible interference of the other analytes with HL +
Hg2+ at λ = 338 nm. (c) Interference of various metal ions with Hg2+ in 1. (d) Absorption spectrum of 1 after the addition of a Hg2+ solution (1.0
equiv) and its resemblance with HL.
Figure 8. (a) Fluorescence titration of HL [9:1 (v/v) MeOH/H2O; c, 10 μM]. The inset shows the intensity saturation curve for HL with
equivalents of Hg2+. (b) Fluorescence titration of 1 [9:1 (v/v) MeOH/H2O c, 10 μM] in the presence of Hg2+ (10 mM) at rt. The inset shows the
fluorescence response toward decomplexation of 1 to “in situ”-generated HL.
× 103 M−1 cm−1), which has been attributed to the presence of
HL, arising because of decomplexation of 1. The presence of
two isosbestic points at ∼362 and ∼320 nm also suggested the
existence of a new chemical entity (HL) in the solution besides
1 and Cu2+ (Figure 6b). At this stage, the band due to the
n−π* transition almost disappeared, leaving a residual hump,
while the one at 281 nm (π−π*) blue-shifted to appear at 274
nm (Δλ, 07 nm). Furthermore, in the presence of 1.0 equiv of
Hg2+, the spectrum showed good resemblance with HL;
however, a residual hump indicated either incomplete
decomplexation of Cu2+ from 1 or a competitive approach of
Cu2+ with Hg2+ toward complexation (Figure 7d). Further
additions of Hg2+ (1.0−3.0 equiv) exhibited changes similar to
that arising from the interaction of Hg2+ with HL (Figure 6a,b).
It clearly suggested that, after decomplexation, HL is generated
in situ and further undergoes Hg2+-induced hydrolysis at the
aldimine core. Ultimately, at saturation, the spectra resembled
well the one obtained from the treatment of Hg2+ with HL or
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Figure 9. Fluorescence response of HL [c, 10 μM; 9:1 (v/v) MeOH/H2O] toward various metal ions (100 mM). Blue cones represent the emission
intensities of free HL, brown bars intensity change in the presence of tested metal ions (10.0 equiv), and green cylindrical bars the subsequent
addition of Hg2+ (1.0 equiv) to the same solution. The intensities were recorded at λem = 423 nm at rt.
DEA itself (Figures 6b and 4b). It is noteworthy to mention
that the resulting species was isolated and characterized as DEA
using H NMR and mass spectrometry (SI, Figures S15b and
S16). Also, interference studies demonstrated insignificant
changes in the spectral features of 1 + Hg2+ in the presence of
competing metal ions, which establishes 1 as a selective
chemodosimeter for Hg2+ (Figure 7c).
cumulative effect of the background presence of 1.0 equiv of
Hg2+ and externally added Hg2+, which simultaneously interacts
with HL. To ascertain this viewpoint, a time-dependent
fluorescence experiment was performed on HL and 1 using
an excess of Hg2+ (10.0 equiv). It was observed that a ∼80%
increase in the fluorescence intensity occurred in just ∼2.0 min
and virtual saturation after ∼6.0 min (SI, Figure S9).
1
To gain deep insight into the proficiency of HL and 1 toward
a “turn-on” response for Hg2+, fluorescence titration experi-
ments have been performed under analogous conditions
(Figure 8). The addition of Hg2+ (0.2 equiv) to a solution of
HL induced a ∼3-fold increase in the fluorescence intensity.
The limit of quantification of HL for Hg2+ was ∼1.0:0.2 to
∼1.0:2.0, and maximum fluorescence enhancement (∼20-fold)
was observed at ∼1.0:2.0 (HL/Hg2+), wherein Φ was
considerably enhanced to 0.43 (Figure 8a). Observed emission
enhancement has been attributed to the formation of DEA via
selective hydrolysis of the aldimine linkage despite the presence
of ester groups in HL (SI, Figure S7). Job’s plot analysis
revealed 1:1 stoichiometry between HL and Hg2+ (SI, Figure
S8).
A competitive metal-ion selectivity study was performed by
adding Hg2+ to a solution of HL and tested metal ions (10.0
equiv). A fluorescence response was noted at λem = 423 nm (λex
= 338 nm) and is presented as a bar diagram in Figure 9. This
study revealed that the addition of individual metal ions causes
an insignificant change in the fluorescence intensity of HL,
which substantially increased upon the addition of Hg2+ (1.0
equiv). Conversely, the addition of tested metal ions (10.0
equiv) to solutions of HL + Hg2+ and 1 + Hg2+ exhibited
insignificant changes (SI, Figure S10). These experiments not
only suggested a minor background effect of the other metal
ions but also indicated appreciably good selectivity of HL and 1
for Hg2+. They also eventually proved that only a small amount
of Hg2+ (1.0 equiv) is sufficient over a large quantity of other
metal ions (10.0 equiv). Further, it has been concluded that the
emission intensity is proportional to the concentration of the
hydrolyzed product, in turn, generated (DEA) and may be
attributed to the lowering of PET. Thus, the overall results
from the fluorescence titration studies are consistent with the
conclusions drawn from UV/vis studies.32
The association constants for HL and 1 in the presence of
Hg2+ have been deduced following the Benesi−Hildebrand
method. It came out to be KaHL = 7.1 × 104 mol−1 and Ka1 = 1.8
× 106 mol−1 (SI, Figure S11). The higher association constant
for 1 may be associated with the greater affinity of 1 toward
Hg2+ relative to HL. To examine the sensitivity of HL toward
Hg2+, the limit of detection (LOD) has been evaluated by
adding a fixed volume (50 μL) of Hg2+ (10−12−10−9 M) to a
solution of HL (c, 10 μM). The LOD was estimated to be 8 ×
10−10 M for Hg2+, below the acceptable level in drinking water
(SI, Figure S12).5a
Likewise, the addition of Hg2+ (0.0−1.0 equiv) to a solution
of 1 exhibited gradual fluorescence enhancement (∼3-fold) and
a small increase in the QY (Φ, 0.016; ΔΦ = 0.005; Figure 8b,
inset). The slower response of 1 toward Hg2+ may be ascribed
to decomplexation of Cu2+ to generate HL. A further addition
of Hg2+ (1.2 equiv) displayed a sharp fluorescence enhance-
ment (∼7-fold; Φ, 0.077), probably because of initiation of
hydrolysis of “in situ”-generated HL. An increase in the
concentration of Hg2+ (3.0 equiv) led to enormous
fluorescence enhancement (Figure 8b). Notably, at the
saturation stage (maximum hydrolysis), the intensity assumed
∼80-fold enhancement and QY increased by a factor of ∼55
(Φ, 0.62; SI, Figure S7b). Although it may be presumed that
decomplexation and hydrolysis phenomena occur simulta-
neously in the solution, a sharp enhancement was observed
only after the addition of more than 1.0 equiv of Hg2+. This
indicated that the mentioned amount (1.0 equiv) of Hg2+ is
required for competitive Cu2+ displacement from 1. Further,
the very rapid response toward Hg2+ may be associated with the
One can presume anonymously that the selectivity of HL and
sensitivity of 1 toward Hg2+ may be affected by harsh
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1
Figure 10. H NMR titration spectra of HL with Hg2+. The −OH proton may not be visible because of fast exchange with a deuterated solvent.
competition between Hg2+ and Cu2+. The spectral studies
substantiated that the addition of only Hg2+ (hydrolysis) and
Cu2+ (complex formation) to a solution of HL leads to
significant spectral changes; these do not vie for each other
because they are involved in two different phenomena. Ideally,
the selectivity of HL for Hg2+ (hydrolysis) should not be
significantly affected by the addition of Cu2+ (complex
formation). On the other hand, the lower sensitivity of 1 for
Hg2+ relative to HL has been ascribed to the use of the first
mole ratio of Hg2+ for the decomplexation process, which, in
turn, facilitates hydrolysis. Hg2+ does not tend to directly
replace the coordinated CuII for complex formation, but rather
it interacts only through the aldimine nitrogen (−NCH−) to
engage its lone pair of electrons for hydrolysis. It enhances
polarity of the aldimine moiety, making the −NCH− carbon
electropositive, and favors nucleophilic attack by H2O. This
reasonably explains negligible prospects for [Hg(L2)] for-
mation, which has also been supported by the absence of the
molecular ion peak associated with [Hg(L2)], or any species
parallel to it in mass spectra of 1 + Hg2+.
To gain deep insight into the selectivity/sensitivity of HL/1,
a model compound (LM) involving benzaldehyde instead of
salicylaldehyde has been designed and synthesized (see the SI
for details, Scheme S1 and Figure S3). Strategically, to eliminate
the possibility of complex formation, the nitrogen/oxygen-
donor chelating site was removed; however, to promote
hydrolysis the aldimine bond was kept intact. As expected,
the UV/vis and fluorescence spectra of LM were significantly
influenced only by Hg2+ and not Cu2+/tested metal ions. The
resultant spectral feature resembled to that of DEA, which
notably indicates the selectivity of LM for Hg2+ similar to 1 and
not HL (an isolated absorption band like HL + Cu2+ did not
appear). Conversely, the aliquot addition of Hg2+ led to
significant changes in the UV/vis and fluorescence spectra of
LM, with the very first addition of Hg2+ (0.3 equiv) akin to HL
and saturated at 3.0 equiv (SI, Figure S13). This strongly
suggested the significant role of Hg2+ in decomplexation of 1
and the comparable hydrolytic response of the aldimine core in
LM to that of HL. Notably, LM required higher equivalents of
Hg2+ (3.0 equiv) than HL (2.0 equiv of Hg2+), which may be
attributed to the absence of the o-OH substituent. In HL, the
−OH group offers greater electron density to ortho −N
CH− nitrogen and facilitates interaction with Hg2+, in turn,
enhancing the sensitivity toward hydrolysis.
Effect of the pH. The pH sensitivity of aldimine linkages
toward hydrolysis is well documented.33 However, for a
particular aldimine system, a short pH range may be effective
and its stability may also depend on other factors including the
effect of the substituents.34 The compounds under inves-
tigation, HL and 1, contain ester groups that stabilize that
isophthalate ring with respect to the aldimine core. Conversely,
ester groups also provide a potential site for hydrolysis under
the influence of Hg2+. During the UV/vis and fluorescence
studies in the presence of Hg2+ (10.0 equiv), the pH of the
systems varied from ∼7.2 to ∼5.2 for HL and from ∼7.4 to
∼4.9 for 1. To investigate the effect of the pH in the absence of
Hg2+, pH titrations have been performed using 0.1 M HCl. The
aliquot addition of HCl led to a gradual decrease in the pH of
HL and 1 to ∼2.5 and displayed no significant change in their
absorption/emission spectra up to pH ∼3.5 (SI, Figure S14).
The above observations clearly indicated that hydrolysis of HL
and 1 may be subjected to two different sites, aldimine
(−CHN−) and ester (−COOC2H5), yet Hg2+ plays a crucial
role in triggering preferable hydrolysis of the −CHN−
linkage over an ester group in a wide pH range. The solitary
effect of an acidic pH imposed insignificant changes in the
absorption/emission spectra of HL and 1.35 However, some
irrelevant changes were observed in their fluorescence spectra
below pH ∼3.5. These changes are entirely different from that
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Figure 11. Plausible mechanism for Hg2+ detection via chemodosimetric transformations: complex formation (turn off); subsequent Hg2+ addition in
two stages; stage 1, Hg2+-induced CuII displacement, leading to decomplexation, i.e., in situ generation of HL (weak fluorescence); stage 2, Hg2+-
assisted hydrolysis, leading to the formation of DEA (turn on).
observed with Hg2+, which, in turn, supported the key role
played by Hg2+ in hydrolytic transformations.36
completely disappeared (Figure 10). This clearly indicated
Hg2+-induced hydrolysis of the aldimine bond. Moreover, it is
worth mentioning that the addition of Hg2+ (1.0 equiv) leads to
an almost negligible change for the signals due to the −CH2−
(H10) and −CH3 (H11) moieties of the ester resonating at δ
4.45 (q, 4H) and δ 1.46 (t, 6H). With an increase in the
concentration of Hg2+, the signals due to the −CH2− protons
apparently merged with the solvent and retained its identity. In
addition, aromatic protons corresponded to the characteristic
resonances due to DEA and salicylaldehyde. The peak
assignable to the −NH2 proton was not observable, probably
1H NMR Studies. To have an idea about the mechanism of
1
fluorescence enhancement in the presence of Hg2+, H NMR
titrations have been performed using HL (CD3OD) and an
aqueous solution of Hg2+ (0.01 mM) as an analyte. Upon the
addition of Hg2+ (1.0 equiv) to a solution of HL, resonances
due to the aldimine proton −CHN− (H4, δ 8.82)
significantly diminished and a new signal assignable to free
aldehyde appeared at δ 9.97. Further additions of Hg2+ (∼2.0
equiv) led to prominence of the resonances due to free
aldehyde, while signals due to the azomethine proton
1
because of its merger with the solvent peak. A H NMR
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titration study clearly suggested that the addition of Hg2+ could
impose significant changes, particularly for protons due to the
aldimine linkage, and the hydrolyzable ester group remained
ASSOCIATED CONTENT
* Supporting Information
Characterization data, UV/vis and fluorescence spectra related
to this work, and crystal data file in CIF format (CCDC 955431
and 955432 for HL and complex 1, respectively). This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
1
intact. On the other hand, a well-resolved H NMR spectrum
for 1 could not be obtained because of a paramagnetic Cu2+(d9)
system.37
Mass Spectral Studies. ESI-MS of HL displayed a
molecular ion peak at m/z 342.1518 [(M + H)+; 100%]
along with other peaks at m/z 382.1441 [(M + Na + H2O)+;
40%], 364.1343 [(M + Na)+; 28%], and 705.2711 [(2M +
Na)+; 4%] (SI, Figure S4a). Similarly, 1 in its ESI-MS displayed
a molecular ion peak at m/z 745.2052 [(M + H)+; 15%] and
prominent peaks at m/z 342.1473 [(L + 2H)+; 100%] and
705.2671 [{2(L + H) + Na}+; 45%] (SI, Figure S4b). The
presence of various peaks and their position strongly supported
the formulation of HL and 1. To have an idea about the
composition of the species resulting from the interaction of HL
+ Hg2+ and 1 + Hg2+, ESI-MS of the isolated products were
acquired. Notably, products from both the former and latter
exhibited a 100% intense peak at m/z 238.1012 and 238.1107,
respectively (SI, Figure S15a,b). This signifies that HL and 1
upon treatment with Hg2+ ended up with the same species, i.e.,
DEA (calcd m/z 238.1035), and strongly suggested that Hg2+-
induced hydrolysis of the aldimine linkage occurred in HL as
well as in 1. Therefore, one may conclude that these exhibit
100% feasibility of being hydrolyzed by Hg2+ in mixed-aqueous
media.
On the basis of the above observations, a plausible
mechanism for Hg2+-assisted hydrolysis of HL and 1 has
been proposed (Figure 11). In the presence of Hg2+, HL
directly undergoes hydrolysis at the aldimine linkage, while 1 in
the very first step undergoes decomplexation (stage 1) and the
resulting species passes through hydrolytic cleavage (stage 2) at
the aldimine linkage. Stage 2 is mechanistically similar to direct
hydrolysis of HL, and that is why HL and 1 exhibit analogous
spectral features under the influence of Hg2+ and end up with
the same product, DEA, which is liable for a highly fluorescent
“switch-on” response.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dspbhu@bhu.ac.in. Tel.: + 91 542 6702480. Fax: + 91
542 2368174.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Council of Scientific and Industrial
Research, New Delhi, India, for financial assistance through
Scheme HRDG 01 (2361/10/EMR-II) and the Department of
Science and Technology, New Delhi, India, through Scheme
IFA12-CH-66. A.K. thanks the University Grants Commission,
New Delhi, India, for a Senior Research Fellowship [R/Dev./
Sch.(UGC-JRF-SRF)/S-01:29.2.29]. We are also thankful to
the Head, Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi (U.P.), India, for extending
laboratory facilities. The authors also thank Prof. R. Murugavel
for extending his X-ray Single Crystal Diffraction Facility,
established through a DAE-SRC Outstanding Investigator
Award.
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CONCLUSIONS
■
In summary, through this work, the synthesis and spectral and
structural characterization of a new Schiff base, HL, and a
copper(II) complex, 1, derived from it have been described.
These exhibit appreciable chemodosimetric behavior toward
Hg2+ via hydrolytic cleavage of the aldimine linkage. The actual
moiety involved in chemodosimetric detection has been
identified as HL, and the concept of hydrolysis is well
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chemodosimeters.
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