A uniquely fabricated Cu(II)-metallacycle as a reusable highly sensitive dual-channel and practically functional metalloreceptor for Fe3+ and Ca2+ ions: An inorganic site of cation detection

Article abstract of DOI:10.1039/c7nj03294c

The present study deals with the formation of cyanuric chloride·1,2-di(pyridin-2-yl)disulfane 1 co-crystal and trimethylcyanuric acid 2 through unusual reactions of cyanuric chloride (CC). Reaction of co-crystal 1 with Co(ii) and Cu(ii) led to the formation of di(pyridin-2-yl)sulfane 1′ through a desulfurization reaction and complexes [Co(1′)Cl₂] (3) and [Cu(1′)₂-(μ₂-SO₄)₂(H₂O)₂] (4). The structures of 1-4 were verified using X-ray single crystal analysis. The Cu(ii)-metallacycle 4 acts as a dual-channel metalloreceptor for Fe³⁺ and Ca²⁺ through a fluorescence 'turn-on' response for Ca²⁺, a 'turn-off' response for Fe³⁺, and changes in cyclic voltammograms in pure aqueous-buffer medium (1.0 M Tris-HCl, pH 7.4). The limits of detection (LOD) for Fe³⁺ and Ca²⁺ were estimated to be 30 ppb and 0.2 ppm, respectively. For the first time, the EPR technique was applied to detect a paramagnetic ion (Fe³⁺) using a paramagnetic probe (4), which exhibited considerable change upon interaction with Fe³⁺. Further, structural elucidation of 4, use of Co(ii)-complex 3 as a model compound, and FTIR and mass spectral studies suggested that the detection of cations by Cu(ii)-metallacycle 4 occurs through its unique inorganic detection site (six oxygen atoms from sulphate and water groups). Due to its higher selectivity of Fe³⁺ over Ca²⁺ and other metal ions as well as its LOD for Fe³⁺, which is lower than the US Environmental Protection Agency permitted maximum level in drinking water (5.4 μmol L^-1), 4 was successfully used for practical detection of Fe³⁺ in environmental water samples. The exceptionality of this report comprises the detection of vital cations using a binuclear Cu(ii) metallacycle probe, utilizing the EPR technique, binding through an inorganic site and the practical application of 4 toward Fe³⁺ sensing.

Full text of DOI:10.1039/c7nj03294c

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Uniquely fabricated Cu(II)ꢀmetallacycle as reusable highly sensitive dualꢀ
channel and practically functional metalloreceptor for Fe3+ and Ca2+ ions: An
inorganic site of cation detection
Neha Thakur, Mrituanjay D. Pandey* and Rampal Pandey*
Department of Chemistry, Dr. Harisingh Gour Central University, Sagar (MP), India
Abstract:
Present work deals with the formation of cyanuric chloride·1,2ꢀdi(pyridinꢀ2ꢀyl)disulfane 1 coꢀ
crystal and trimethylcyanuric acid 2, through unusual reactions of cyanuric chloride (CC).
Reaction of coꢀcrystal 1 with Co(II) and Cu(II) leads to the formation of di(pyridinꢀ2ꢀyl)sulfane
1' through desulfurization reaction and complexes [Co(1')Cl2] (3) and [Cu(1')2ꢀ(ꢀ2ꢀSO4)2(H2O)2]
(4). Structures of 1ꢀ4 have been verified using Xꢀray single crystal analysis. The Cu(II)ꢀ
metallcycle 4 acts as dualꢀchannel metalloreceptor for Fe3+ and Ca2+ through fluorescence ‘turnꢀ
on’ response for Ca2+, ‘turnꢀoff’ for Fe3+, changes in cyclic voltammograms in pure aqueousꢀ
buffer medium (1.0 M TrisꢀHCl, pH 7.4). The limit of detection (LOD) for Fe3+ and Ca2+
estimated to be 30 ppb and 0.2 ppm, respectively. For the first time, EPR technique has been
applied to get insights in detecting a paramagnetic ion (Fe3+) using a paramagnetic probe 4 which
exhibited considerable change upon interaction of 4 with Fe3+. Further, structural elucidation of
4, using a Co(II)ꢀcomplex 3 as a model compound, FTIR and mass spectral studies, suggested
detection of cations by a Cu(II)ꢀmetallcycle 4 occurs through the unique inorganic detection site
(six oxygen atoms from sulphate and water groups). The higher selectivity of Fe3+ over Ca2+ and
other metal ions as well as due to the LOD lower than U.S. Environmental Protection Agency
permitted maximum Fe3+ level in drinking water (5.4 ꢁmol L−1), 4 was successfully used for
practical detection of Fe3+ in environmental water samples. Exceptionality of this report
comprises setection of these vital cations using a binuclear Cu(II) metallacycle probe, utilizing
EPR technique, binding through inorganic site and practical application of 4 toward Fe3+ sensing.
Email: mrituanjaypandey@gmail.com (MDP); rppandeysu@gmail.com (RP)

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Introduction
Enormous interest has been drawn toward development of fluorogenic and electrochemical
receptors for biologically, clinically, and environmentally important analytes.1ꢀ2 In this context,
metalloreceptors (receptors comprising bound metal ions) have recently paid considerable
attention owing to their functionality in selective guest binding, regulation, and molecular
sensing.3 Metalloreceptors are generally recognize only anions or neutral species however these
can also detect cationic entities, depending on the preꢀorganized recognition site and/or
coordination environment of the metal center.4 With these criteria, detection of cations using
metalloreceptor are generally highly selective4 though fluorescent metalloreceptors for cation
detection are seldom reported.5 In particular, Cu(II)ꢀcomplex based fluorescent metalloreceptors
for biologically and environmentally important cations functional in aqueous media, are
extremely rare.6
Amongst metals of biological significance, iron and calcium have received substantial
interest. Iron is widely distributed in nature and most abundant vital metal ion in human beings.7
It induces cellular toxicity that leads to Huntington’s, Alzheimer’s, and Parkinson’s diseases.8 In
9
particular, Fe3+ ion is important for several biochemical processes at the cellular level , and both
its overload and deficiency can cause various diseases like liver cirrhosis, arthritis, liver cancer,
heart failure and diabetes.10 Consequently, intense research efforts have been made in developing
the sensitive and selective chemosensors for Fe3+.11 The U.S. Environmental Protection Agency
permitted maximum Fe3+ level in drinking water is 5.4 ꢁmol L−1 therefore; determination of Fe3+
especially with lower detection limit is useful for the evaluation of water quality.12 Likewise,
calcium ion (Ca2+), the fifth most abundant element in the human body, also plays important role
in biological systems.13 It triggers and regulates the most physiological and pathological
phenomena in the human body like extraꢀ and intracellular signaling14a, muscle contraction14b
and nerve impulse transmission14c including cell damage and death14d, tendon matrix
degeneration14e, and psychiatric disorders14f. Consequently, a great deal of attention has been
paid toward development of highly sensitive and selective chemosensors for Ca2+.14a, 15
In addition, it becomes more promising if the formation of chemosensor occurs through
unique chemical reactions. One of the most pertinent instances is the formation of novel metal
complex comprising special in situ generated ligands.16ꢀ17 In situ metal/ligand reactions result in

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the development of new exceptional reactions/products18 wherein, the oxidative disulfide bond
formation and reductive disulfide bond cleavage are the best instances.19 Such systems can
produce unique metalꢀcomplexes through S–S bond formation cum concurrent S–S and S–C(sp2)
bond cleavage and rearrangement.20ꢀ24
We report the development of a novel fluorescent Cu(II)ꢀcomplex based reusable dualꢀ
channel metallorecptor 4 for Ca2+ and Fe3+ ions with high sensitivity, selectivity and distinct
signaling in pure aqueous phase. Metallorecptor 4 offers a unique inorganic receptor site (four
oxygen atoms of sulphate groups and two of coordinated water molecules). Further, the detection
of paramagnetic Fe3+ ion using a paramagnetic Cu(II)ꢀmetallacycle 4 has been supported by EPR
technique unprecedentedly involved for cation detection. Notably, formation of 4 unexpectedly
occurred through a couple of reactions; oxidative SꢀS bond formation in a coꢀcrystallized species
and metal mediated desulfurization (Scheme 1 and Scheme 2; see experimental section).25
Although, such sulfur reactions are known21, 26ꢀ27 but reports where coꢀcrystallized disulfide
moiety fabricates fluorescent Cu(II)ꢀmetallacycle that detects cations in aqueousꢀbuffer solution
(pH 7.4), is not yet known to the best of our knowledge. Eventually, 4 has been successfully used
as fluorescent sensor for practical determination of Fe3+ directly in real water samples.
Scheme 1. Preparation of 1 (coꢀcrystal) and 2.

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Experimental Section
Reagents. The solvents were dried and distilled prior to their use.28 Metal nitrates viz., NaNO3,
KNO3, Mg(NO3)2ꢂ6H2O, Ca(NO3)2ꢂ4H2O, Fe(NO3)2ꢂ9H2O, Co(NO3)2ꢂ6H2O, Ni(NO3)2ꢂ6H2O,
Zn(NO3)2ꢂ6H2O, AgNO3, Cd(NO3)2ꢂ4H2O, Hg(NO3)2 and other common reagents and solvents
were procured from HiMedia Laboratories Pvt. Ltd., India. 2ꢀmercaptopyridine/ pyridineꢀ2ꢀthiol
and cyanuric chloride were procured from Sigma Aldrich Chemical Co., USA. The reagents
were procured from commercial sources and used as received. Solvents were dried and distilled
following standard literature procedures.28
Physical measurements. IR and electronic absorption spectra were acquired on a Shimadzu FT
13
IRꢀ8400 S and Systronics double beam UV/vis spectrophotometers, respectively. 1H and C
NMR spectra in CDCl3 were obtained on a JEOL AL 300 FTꢀNMR machine at an operating
frequency of 300 MHz (1H) and 75.45 MHz (13C). Chemical shifts (δ) are given in parts per
million (ppm) relative to tetramethylsilane (TMS, δ 0.00 ppm) and splitting patterns are
designated as s (singlet), d (doublet) and t (triplet). Elemental analyses (C, H and N) were
performed on an Exter CEꢀ440 CHN analyser. Fluorescence spectra were acquired on a RFꢀ5301
spectrophotometer. Electrospray ionizationꢀhigh resolution mass spectra (ESIꢀHRMS) were
obtained from MICROMASS QUATTRO II triple quadruple mass spectrometer. The ESI
capillary was set at 3.5 kV and the cone voltage was 40 V. Cyclic voltammetric measurements
were carried out in a 797 VA Computrace Metrohem SwissꢀModel electrochemical analyzer.
The three electrode cell comprised of a reference Ag/AgCl, auxiliary platinum and working
glassy carbon electrodes. The 0.01 M KCl was used as an electrolyte for entire electrochemical
study under aqueous media. Xꢀband electron paramagnetic resonance (EPR) measurements were
performed at room temperature for methanolic solution of samples in standard quartz EPR tubes
on a Bruker BioSpin GmbH A300 electron spin resonance spectrometer (power = 0.5 mW,
frequency = 9.5 GHz).
Synthesis
Preparation of cyanuric chloride·1,2ꢀdi(pyridinꢀ2ꢀyl)disulfane Coꢀcrystal (1). To a
stirring solution of pyridineꢀ2ꢀthiol (0.333 g, 3.0 mmol) in methanol (10 mL) was added a
methanol (5 mL) solution of cyanuric chloride (0.183 g, 1.0 mmol). The reaction mixture was

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stirred at around 5 °C for four hours which turned the pale yellow solution to light green. The
volume of solution was reduced to half under vacuum and left for slow evaporation. The light
green needle shaped single crystals appeared within 24 h which were separated, washed quickly
with mixture of MeOH and petroleum ether (1:20, v/v). Complete evaporation of solution
indicated presence of some unreacted pyridineꢀ2ꢀthiol (verified by TLC and IR) which was
separated using diethyl ether (3×5 mL). Yield (0.394 g, 97%): mp 143ꢀ145 ºC. Analytical data:
Anal. Calc. for C13H8Cl3N5S2 (404.72): C, 38.58; H, 1.99; N, 17.30. Found: C, 38.61; H, 2.00; N,
17.34. FTꢀIR (KBr; cm−1): 3058 (w), 2987 (w), 2962 (m), 2919 (m), 1566 (vs), 1538 (s), 1495
1
(s), 1431 (s), 1332 (s), 1269 (s), 870 (m), 851 (s), 795 (vs), 705 (s) 465 (w). H NMR (CDCl3,
300 MHz, δH, ppm): 8.52 (d, J = 4.2 Hz, 1H); 7.84 (t, 1H); 7.39 (d, J = 4.8 Hz, 1H); 7.22 (t, 1H).
13C NMR (CDCl3, 75 MHz, δC, ppm): 172.65, 160.15, 149.22, 130.40, 124.29, 121.42. ESIꢀMS
(m/z) for C13H8Cl3N5S2: [M+] calcd 402.9287; found 402.9287.
Preparation of trimethylcyanuric acid (2). Compound 2 was prepared following procedure
adopted for 1 except pyridineꢀ2ꢀol was used instead of pyridineꢀ2ꢀthiol (0.285 g, 3.0 mmol). The
unreacted pyridineꢀ2ꢀol (monitored by TLC) was separated using excess amount of diethyl ether
(3×10 mL) which dissolves trimethylcyanuric acid (2) leaving the pyridineꢀ2ꢀol in the reaction
pot. The diethyl ether solution was slow evaporated to give white color compound which was reꢀ
crystallized using MeOH. The white colored block shape single crystals were appeared in the
slow evaporation of MeOH solution. Yield (0.139 g, 81 %): mp 132ꢀ135 ºC. Analytical data:
Anal. Calc. for C6H9N3O3 (171.15): C, 42.10; H, 5.30; N, 24.55. Found: C, 42.10; H, 5.28; N,
24.51. FTꢀIR (KBr; cmꢀ1): 2962 (m), 2937 (s), 2854 (m), 1591 (s), 1567 (vs), 1458 (s), 1364 (vs),
1
1204 (s), 1111 (vs), 940 (m), 817 (w). H NMR (CDCl3, 300 MHz, δH, ppm): 3.68 (s, 9H). 13C
NMR (CDCl3, 75 MHz, δC, ppm): 172.8, 55.6. ESIꢀMS (m/z) for C6H9N3O3: [M+] calcd
171.0648; found 171.0648; [2M+] calcd 342.1296; found 342.1289.
Preparation of [Co{di(pyridinꢀ2ꢀyl)sulfane}Cl2] (3). To a stirring solution of 1 (0.111 g,
1.0 mmol) in methanol (5 mL) was added anhydrous CoCl2 (0.129 g, 1.0 mmol) in methanol (5
mL). The reaction mixture was further stirred for two hours at rt. The solution was concentrated
to half under reduced pressure and left for slow evaporation to afford pink color block shaped
single crystals within 24 h. The crystals were separated by filtration and air dried. Yield (0.267 g,
84 %): mp 216ꢀ218 ºC. Analytical data: Anal. Calc. for C10H8Cl2CoN2S (318.08): C, 37.76; H,

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2.53; N, 8.81. Found: C, 37.71; H, 2.50; N, 8.77. FTꢀIR (KBr; cmꢀ1): 3057 (w), 2959 (w), 2916
(w), 1581 (s), 1546 (m), 1452 (s), 1400 (m), 1266 (s), 1015 (m), 869 (m), 790 (vs), 464 (w).
Preparation of [Cu{di(pyridinꢀ2ꢀyl)sulfane}2ꢀ(ꢀ2ꢀSO4)2] (4). A methanol (5 mL) solution
of CuSO4—5H2O (0.249 g, 1.0 mmol) was added to a stirring solution of 1 (0.111 g, 1.0 mmol) in
methanol (5 mL) and the resulting solution was further stirred for two hours at rt. The volume of
solution was reduced to half under vacuum and left for slow evaporation which gave a dark
green precipitate. It was filtered, washed with methanol (5 mL) followed by diethyl ether (2 × 5
mL) and air dried. The product was reꢀcrystallized in MeOH/ MeCN (1:1) to afford block shaped
single crystals within 48 h. Yield (0.284 g, 78 %): mp 202ꢀ204 ºC. Analytical data: Anal. Calc.
for C20H20Cu2O10N4S4 (731.74): C, 32.83; H, 2.75; N, 7.66. Found: C, 32.79; H, 2.75; N, 7.62.
FTꢀIR (KBr; cmꢀ1): 3267 (w), 3060 (w), 2961 (w), 2919 (w), 1578 (s), 1536 (m), 1489 (s), 1398
(m), 1261 (s), 1022 (s), 1017 (m), 968 (m), 865 (m), 787 (vs).
UVꢀvis and fluorescence experiments. Stock solution of 4 for electronic absorption and
emission studies was prepared in buffer/H2O solution a (10 ꢀM; 1.0 M TrisꢀHCl, pH = 7.4). The
stock solution for various metal ions (Li+, Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+,
Cu2+, Zn2+, Cd2+, Hg2+ and Ag+) were prepared by dissolving their nitrate salts in triple distilled
water (100 mM). In the titration experiments, 2.0 mL solution of 4 (c, 10 ꢀM; 1.0 M TrisꢀHCl,
pH = 7.4) were taken in a quartz cuvette (3 mL, path length, 1 cm) and the solution of metal ions
(100 mM) were added gradually with the help of a microꢀpipette.
Electrochemical experiments. Redox properties of 4 at rt were followed by cyclic voltammetry
in buffer/H2O containing 0.1 M KCl as a supporting electrolyte in the potential range + 2.0 ꢀ 2.0
V. Stock solution for abovementioned metal ions (nitrate salts) were prepared in triple distilled
water (100 mM). The cyclic voltammetric titrations were performed using 10.0 mL solution of 4
(10 ꢀ
M; 1.0 M TrisꢀHCl, pH = 7.4) and 100 mM solutions of Ca2+ and Fe3+ as their nitrate salts.
The redox potentials were referenced relative to K3[Fe(CN)6] couple as a reference (0.1 V).
Preparation of Water Samples for Fe3+ Detection. Real water samples taken from Sagar lake
(LW), bore well water (BW) from university, tap/drinking water (TW/DW) from civil lines,
Sagar, and waste water (WW) from motor garage civil lines, Sagar, were collected and processed
prior to their Fe3+ content analysis by using metalloreceptor 4. All the raw water samples were

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centrifuged (5,000 rpm) and filtered through a 0.45ꢀꢁm membrane to remove any suspended
particles and then used directly for Fe3+ detection and determination.
Results and discussion
Synthesis and characterization. Synthesis of disulfide 1 is result of unprecedented reactions
between cyanuric chloride (CC) and pyridineꢀ2ꢀthiol (2ꢀPySH), respectively without using a
reagent/base.27ꢀ29 Considering the use of 1,3,5ꢀtriazine derivatives as supramolecular synthon, as
herbicide, DꢀπꢀA structures with eahanced electronic communication and numerious biological
application, the reactions were carried out to prepare star shape ligand with 1,3,5ꢀtriazine core.30
Notably, the desired reaction was not occurred rather unique coꢀcrystal, CC·2ꢀPySꢀSpy 1 was
formed (Scheme 1; see experimental section).25a Likewise, reaction between and CC and 2ꢀ
PyOH ensued trimethylcyanuric acid 2.25b
Scheme 2. Formation of complexes 3 and 4 from uniquely coꢀcrystallized ligand 1.
Both 1 and 2 have been analyzed by FTꢀIR, NMR (1H & 13C) and mass (ESIꢀMS)
spectroscopic techniques25 and their structures have been authenticated by Xꢀray single crystal
analysis. Further, metal mediated reactions were investigated using CoCl2 and CuSO4—5H2O with

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1, which led to the formation of mononuclear Co(II)ꢀcomplex [Co{2ꢀPySPy}Cl2] (3) and dimeric
Cu(II)ꢀcomplex [Cu(2ꢀPySPy)(SO4)(H2O)]2 (4), via one atom desulfurization (2ꢀPySPy = 1')
(Scheme 2).27b Since a coꢀcrystallized system used as a ligand, the reproducibility of complexes
was examined by reacting 1 with Co(II) and Cu(II) which led to the formation of exclusive
complexes 3 and 4 analyzed by FTꢀIR and Xꢀray single crystal analysis. Complexes 3 and 4 are
soluble in H2O, MeOH, EtOH, CH3CN, DMF and DMSO while insoluble in CHCl2, CHCl3,
THF, ether etc. The mechanisms for disulfide bond formation and desulfurization have been
given in ESI† (pp S5).
Crystal structures
Compound 1 crystallized in monoclinic crystal system with C2/c space group and detailed
crystallographic parameters are given in Table S1. Single crystal analysis revealed coꢀ
crystallization of 2ꢀPySSPy with CC (Fig. 1a). The pyridyl rings in 2ꢀPySSPy oriented transoid
to one another and involved in intermolecular H———bonding with CC which might be driving
force for coꢀcrystallization of CC with 2ꢀPySSPy. The SꢀS bond length in 2ꢀPySSPy dimer is
2.03 Å while other bond lengths are comparable to the known analogous systems.21ꢀ26 In
addition, crystal cell parameters of 2 are known previously (Table S1; Fig. S4a).31 Detailed
crystallographic discussions of 1 and 2 are available in ESI† (pp S7).
Co(II)ꢀcomplex 3 crystallizes in monoclinic crystal system with P21/c space group (Table
S1). The Co(II) centre assumes distorted Td geometry which is completed by N,N donor sites of
1' and two Cl¯ ligands. The structure exhibits good symmetry which is reflected by the ClꢀCoꢀCl
plane that divides ligand 1' into two equal parts.21ꢀ26ꢀ27b Complex 4 crystallized in triclinic crystal
system with P
−
1 space group (Table S1). Crystallographic analysis revealed dimeric structure of
2−
4 comprising two units of each; Cu(II), 1', SO4 , coordinated H2O and lattice H2O (Fig. 1b)
wherein Cu(II) centers assume almost ideal square pyramidal (SPy) geometry. The
Cu(II)———Cu(II) distance in 4 is 4.93 Å32 bridged by sulphate ligands which maintain almost
regular Td geometry despite in ꢀ
ꢀsulphato coordination mode (Fig. 1b).21ꢀ29,31 The cavity created
by sulphate ligands, Cu(II) and coordinated H2O having size dimensions ~3.17 × 4.93 × 5.35 Å
which can be suited to accommodate guest molecule (vide supra). Detailed crystallographic
discussions for 3 and 4 are available in Supporting Information (pp S7). Since 1 and 4 are the
compounds find application (vide supra), important most discussions on these two have been

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focused to the main text body whereas complete details of these as well as 2 and 3 are available
in ESI†.
Fig. 1. (a) Crystal structure of (a) 1 and (b) 2 with 30% ellipsoid probability (hydrogen atoms
have been omitted for clarity).
UV/vis and fluorescence studies. The UV/vis spectra of 1ꢀ4 displayed prominent bands in the
range of 215ꢀ350 nm in buffer/H2O solution (Fig. S7a). Detailed discussion on absorption
spectral studies on 1ꢀ4 is given in ESI† (pp S11). Compound 1 exhibited three bands at 280 nm
(ε, 53,963 M−1cm−1), 232 nm (ε, 67,542 M−1cm−1) and 210 nm (ε, 67,860 M−1cm−1) assignable to
the nꢀπ∗ and πꢀπ∗ transitions in 1. Complex 4 displayed LE band at 282 nm (ε, 74, 808 M−1cm−1)
and strong HE band at 232 nm (ε, 1,11,497 M−1cm−1). Upon excitation (λex) at 280 nm, 1
exhibited moderate emission at 328 nm with quantum yield (Φfl) determined to be 0.37 and
Stokes shift (SS) of 5226 cm−1 (Fig. S7b). Complex 4 exhibited rather distinct dual emissions
having maxima at 427 nm (SS, 11064 cm−1) and 355 nm (SS, 6314 cm−1) and the Φfl determined
to be 0.60. It is worth mentioning that fluorescent Cu(II)ꢀcomplexes are quite scarce due to
inherent quenching nature of Cu(II) ions (d9 configuration) therefore development of Cu(II)ꢀ
complexes particularly with good quantum yield are of special importance.33 Further, the cause
of dual emission in 4 may be associated with the formation of intermolecular aggregates as
suggested by single crystal Xꢀray structure and concentration (10−5 M to 10−7 M) based
fluorescence studies (Fig. S8). Emission spectral data have been thoroughly discussed in ESI†
(pp S11).

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Cation detection behavior of 4. Interaction of 4 with a variety of cations has been investigated
by UV/vis and fluorescence spectral studies. Absorption spectrum of 4 (10 ꢀM, 1.0 M TrisꢀHCl/
H2O, pH = 7.4) in the presence of 10.0 equiv of various metal ions (Li+, Na+, Ca2+, Mg2+, Mn2+,
Fe3+, Co2+, Ni2+, Zn2+, Cd2+ and Hg2+) is illustrated in Fig. 2a. Evidently, only Fe3+ and Ca2+
caused considerable change in the spectral feature. Addition of Ca2+ could not induce significant
change in the LE band of 4 however it caused hyperchromic shift (ꢁA, 6.36 × 104 M−1cm−1) to
the HE band (Fig. 2a) therefore, titration experiments with Ca2+ was abandoned. Notably, Fe3+
addition resulted in the considerable hyperchromic shift in the both LE band (ꢁA, 1.02 × 104
M−1cm−1) and HE band (ꢁA, 8.89 × 104 M−1cm−1) with a noticeable shift in the HE band.
However, to have deep insight on interaction with 4 with Fe3+, UV/vis titration experiments were
performed. Gradual addition of Fe3+ (0.1ꢀ1.0 equiv, 0.3ꢀ3.0
ꢀL) led to the incessant
hyperchromic shift both in the LE and HE bands (Fig. S9). Moreover, it induced considerable red
shift (1781 cm−1) in the HE band upon addition of just 0.1 equiv of Fe3+ while caused
insignificant shift in the LE band.
The fluorescence performance of 4 in the presence of aforesaid cations has also been
examined (Fig. 2b). Addition of 10.0 equiv of various metal ions could not cause significant
change in the fluorescence feature of 4 except to that of Fe3+ and Ca2+. Addition of Fe3+ lead to
the substantial fluorescence quenching in the both LE band (~73%) and HE band (~51%)
whereas Ca2+ induced only small fluorescence enhancement both in HE and LE (~20%) bands. It
is noteworthy to mention here that, although addition of Fe2+ could not cause instant change in
the fluorescence of 4 but leaving the solution mixture of 4 + Fe2+ (5.0 equiv) overnight (~18 hr)
followed by rescanning the solution resulted in the fluorescence quenching of 4 (~32%). The
quenching of fluorescence in 4 + Fe2+ solution after long time may be attributed to the well
known oxidation of Fe2+ into Fe3+ in water therefore this redundant data was ignored. To
understand the binding affinity of 4 with Fe3+, fluorescence titration experiments were carried out
(Fig. 3a). Gradual addition of Fe3+ (10 mM, 0.15ꢀ3.0
at both LE band (~ 73 %) and HE band (~ 52 %) without any significant wavelength shift in the
band maxima. Metalloreceptor 4 recognizes Fe3+ at 0.15
L (0.05 equiv) addition within an
ultrafast timescale (5.0 sec). Total amount of Fe3+ required for optimum quenching of
fluorescence of 4 was 3.0 L (1.0 equiv). Although, both the emission band intensity decreases
incessantly but the rate of quenching at LE band is greater than that of HE band. It suggests that
ꢀL) led to the large fluorescence quenching
ꢀ
ꢀ

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quenching of fluorescence may likely be associated to the change in the geometry of 4 upon
complexation with Fe3+ along with the paramagnetic quenching nature of bound Fe3+ or chelation
enhanced quenching (CHEQ) effect.34b
Fig. 2. UV/vis (a) and fluorescence (b) spectra of 4 (10 ꢀM) in presence of various metal ions in
buffer/H2O solution (1.0 M TrisꢀHCl pH = 7.4).
The quenching constant (Ksv) has been found to be 1.17 × 105 M−1 derived from the Sternꢀ
Volmer plot (Fig. S10). Conversely, addition of Ca2+ (0.3ꢀ3.0
ꢀL) to the soution of 4 resulted in
the fluorescence ‘turnꢀon’ response (~20%) in the LE band whereas HE band exhibited a blue
shift (816 cm−1) (Fig. 3b) along with appearance of two clear isoꢀemissive points appeared at 292
nm and 346 nm indicated more than two species in the medium, within 5.0 sec. of timescale. The
Job’s plot analysis revealed 1:1 binding stoichiometry for both Fe3+ and Ca2+ with 4 (Fig. S11).
The association constants (Ka) for 4 — Fe3+ and 4 — Ca2+ were calculated using BenesiꢀHildebrand
method and found to be 2.68 × 107 mol−1 and 1.53 × 106 mol−1, respectively (Fig. S12).35 To have
insight whether changes in 4 in presence of these cations are associated with pH change, we
looked into pH effect on 4. It is observed that in UV/vis spectra of 4 is insensitive toward basic
pH but shows decrease in absorbance at acidic pH (in particular <pH 4). Notably, 4 does not
display considerable change in pH range (pH ~3.0ꢀ11.0) which reveals that UV/vis and
fluorescence changes of 4 is due to interaction with Fe3+.
Selectivity and sensitivity assessment for cations and anions. The interference of metal ions
was examined by dualꢀway addition i) addition of various cations followed by Ca2+ and

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eventually adding Fe3+ ion (Fig. S13a) and ii) addition of various cations followed by Fe3+ and
finally adding Ca2+ ion (Fig. S13b). Notably, none of the other tested metal ions interfere to the
selectivity of 4 toward Fe3+. In contrast, selectivity of Ca2+ was affected by Fe3+ only which
induced significant fluorescence quenching both at LE band (57%) and HE band (39%). It is
−
worth mentioning that interference of a set of anions (F−, Cl−, Br−, I−, NO3 ,
2−
3−
−
2−
ΗPO4 , PO4 , HCO3 , CO32− and SO3 ) were also examined by addition of these anions to 4
followed by addition of Ca2+ and Fe3+ ions wherein none of the anions could interfere to the Fe3+
and Ca2+ ions selectivity (Fig. S13c). Overall UV/vis and fluorescence results suggest that 4
detects Fe3+ and Ca2+ ions with distinct output signaling along with high selectivity and
sensitivity. Although, probe 4 exhibits much better selectivity for Fe3+ over Ca2+ in pure aqueousꢀ
buffer medium.
To measure the sensitivity of 4 toward Fe3+ and Ca2+, the varying concentrations of Fe3+
(10−12 M − 10−6 M) and Ca2+ (10−12 M − 10−4 M) were added to solutions containing of 4 (1.0
ꢀM) (Fig. S14). A plot between fluorescence intensity vs concentration displayed significant
change at 10−8 M concentration for Fe3+ and 10−7 M for Ca2+. To get better accuracy, 10−8 M (1.0
× 10−8 –10.0 × 10−8 M) solution of Fe3+ and 10−7 M (1.0 × 10−7 –10.0 × 10−7 M) solution of Ca2+
were added to the solution of 4 (1.0 ꢀ
M). A plot between fluorescence intensity (F0/F) and Fe3+
concentration displayed linear relationship (R2 = 0.99) and indicated limit of detection (LOD) of
30 ppb (Fig. S15a) whereas (F/F0) vs [Ca2+] plot exhibited LOD of 0.2 ppm (R2 = 0.98) (Fig.
S15b). The LOD for Fe3+ ion is much lower than its maximum concentration (5.4 ꢁM)
acceptable in drinking water by the US Environmental Protection Agency, which clearly
indicated that metalloreceptor 4 can be potentially useful for practical detection of Fe3+ ion.
Furthermore, the reusability of 4 as a probe was investigated by addition of a strong metal
ion chelator EDTA (5.0 equiv) separately to the solutions containing 4+Fe3+ and 4+Ca2+ (10 ꢀM)
which lead to the revival of fluorescence characteristics of 4 (Fig. S16). The extraction of pure 4
was not possible due to its high water solubility therefore stock solutions were evaporated at rt
and pure 4 was extracted using MeCN (20 mL) and dried at rt. The probe 4 thus obtained was
further dissolved in pure H2O for further use as a probe. Notably, addition of Fe3+ and Ca2+ to the
solution of 4 led to the exactly similar changes as using fresh 4. The fluorescence response of

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recycled 4 toward Fe3+ and Ca2+ in pure aqueousꢀbuffer medium strongly claims it to be reused as
chemosensor for the detection of these cations.
Fig. 3. Fluorescence titration spectra of 4 in presence of (a) Fe3+ and (b) Ca2+ ions (1.0 M Trisꢀ
HCl/ H2O, pH = 7.4).
Binding site studies by structural examination and using model compound. It is essential to
explore the chemistry involved in the detection of Fe3+ and Ca2+ under aqueous conditions. The
selective response of 4 toward Fe3+/Ca2+ and insignificant response toward sulphurꢀphilic metals
Cu2+, Cd2+, Hg2+, Pb2+ etc. indicated the interaction site other than ‘S’ꢀatom(s). To have deep
insight, we acutely examined the single crystal structure of 4 which suggested six oxygen atoms
of dimeric 4 as a quite possible interaction site for Fe3+ and Ca2+. As a matter of fact, four
oxygens from two bridged sulphate groups (O1 and O3 of dimer) and two oxygen atoms from
coordinated water molecules (O5 of dimer) create an environment similar to the six coordination
site offered by pseudoꢀcrownꢀ6 ether system for cations to assume distorted octahedral (Oh)
geometry (Fig. S17). Structure based complete discussion is given in ESI† (pp S18). To get deep
insight about binding of cations through sixꢀoxygen donor set of 4, we used Co(II)ꢀcomplex 3 as
a model metalloreceptor since it comprises same ligand but does not entail sixꢀoxygen donor set
like 4 (Scheme 2 and Fig. S4b). As expected, none of the metal ion (10.0 equiv) could induce
considerable change in the fluorescence spectral feature of 3 (Fig. S18) which strongly supported
the proposed interaction site of 4 toward Fe3+ and Ca2+ ions.34
The plausible mechanism of distinct fluorescence response of 4 toward Fe3+ (turnꢀoff) may be
enhanced ICT upon binding of Fe3+ with 4 through its oxygen donor set (like pseudo crown
ethers).36a The paramagnetic high spin Fe3+ (d5) promotes charge/electron transfer process and

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leads to the fluorescence quenching. Large fluorescence quenching resulting from Fe3+ binding is
also ascribed to the adjustment of oxygen atoms in 4 as per coordination environment of Fe3+
which tends to ‘turn off’ the excited state through nonradiative process.36b Conversely, binding of
diamagnetic Ca2+ through six oxygen atoms of 4 does not enhance charge/electron transfer
process rather it increases the rigidity in 4 and controls vibrational decay thereby leading to the
‘turn on’ fluorescence.36c However, enhancement in fluorescence is moderate which may be
attributed to the competitive effect of increased regidity and positional adjustment of oxygen
atoms according to the geometrical requirement of Ca2+.
Electrochemical studies. The redox behavior of 1ꢀ4 was investigated by cyclic voltammetry in
the potential range −2.0 to +2.0 in aqueousꢀbuffer medium (Fig. S18). Detailed electrochemical
discussion on 1ꢀ4 is given in ESI† (pp S20). Cyclic voltammogram of 1 exhibits an irreversible
single reductive wave −1.36 V attributable to the reduction of pyridyl ring nitrogens whereas a
single oxidative wave at 0.37 V may be associated with the oxidation of sulfur atom. Notably, 4
displays irreversible double reduction waves at −0.12 V and −1.36 V wherein previous is
ascribed to the Cu2+/ Cu+ redox couple and the latter may be associated to the ligand centered
reduction. Further, a single oxidative wave at 0.45 V may be ascribed to the ligand centered
oxidation.
Electrochemical detection of cations. The metal ion detection studies on 4 (10 ꢀM, 1.0 M Trisꢀ
HCl/ H2O, pH = 7.4) have been investigated by cyclic voltammetry in presence of various metal
ions (Na+, K+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Ag+ and Hg2+) (Fig. 4
and Fig. S20). Addition of various metal ions (10.0 equiv; 100 mM) to the solution of 4 could
not cause considerable potential shift except to the Fe3+ and Ca2+ ions which lead to the
significant potential shift as well as current intensity change (Fig. 4). Addition of Fe3+ in 4
caused negative potential shift to the reductive wave from −1.36 V (ꢁV, 0.19; ꢁI, 1.30 × 10−5)
along with emergence of a new band at −0.90 V. Moreover, the oxidative wave of 4 shifted
toward negative potential to appear at 0.35 V (ꢁV, 0.10; ꢁI, 9.8 × 10−5) in presence of Fe3+. In
addition, interaction of 4 with Ca2+ induced positive potential shift of 0.18 V (ꢁI, 8.40 × 10−6) in
the reductive wave and 0.50 V (ꢁI, 5.52 × 10−6) in the oxidative wave. However, current
intensity changes in 4 were observed in presence of some other metal ions as well though
insignificant potential shift is observed. However, Hg2+ caused appearance of a new oxidative

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wave at 1.16 V which may be attributed to the change in the oxidation behavior of sulfur atom of
4 upon interaction with Hg2+. The interference of Hg2+ with 4—Fe3+/4—Ca2+ was also investigated
by separate addition of Fe3+ and Ca2+ to the solution containing 4+Hg2+ which almost reproduced
the electrochemical characteristics of 4—Fe3+ and 4—Ca2+ complexes (Fig. S21). It suggests high
electrochemical selectivity of 4 toward Fe3+ and Ca2+ ions though it is better for Fe3+. However,
the shape of irreversible reductive waves is sharp which may be attributed to the irreversible
behavior of metalloreceptor 4, redox silent Ca2+ ion and one electron reduction of complexed
Fe3+ ion in aqueous solution.37
Fig. 4. Cyclic voltammogram of 4 in presence of 10.0 equiv of Ca2+ (blue), Fe3+ (red) and Hg2+
(dark yellow), in reduction (left) and oxidation (right) window (1.0 M TrisꢀHCl/ H2O, pH = 7.4).
Electrochemical titration experiments were performed by addition of Fe3+ (0.3
solution of 4 which led to the considerable negative potential shift (ꢁV, 0.05) and current
intensity change to the reductive wave (Fig. 5a). Further, gradual additions of Fe3+ (0.6ꢀ2.1
L)
caused incessant potential shift and current intensity increase. Remarkably, upon increasing Fe3+
amount (2.4ꢀ3.0 L), the reductive single wave at −1.37 V was converted to the double waves at
ꢀL) to the
ꢀ
ꢀ
−1.51 V (ꢁV, 0.15 V) and −0.90 V (ꢁV, 0.46 V). The Fe3+ induced negatively shifted wave is
ascribed to the ligand based reduction whereas new reductive wave (−0.90 V) may be associated
with the Fe3+/Fe2+ redox couple.38 It suggests that reduction of 4 requires higher potential in Fe3+
bound state. Moreover, additions of Fe3+ (0.3ꢀ3.0
(0.16 V) to the oxidative wave of 4 (Fig. 5b). Conversely, addition of Ca2+ (0.3ꢀ3.0
significant current intensity change at −1.36 V without any considerable potential shift along
ꢀ
L) led to the incessant nehative potential shift
L) caused
ꢀ

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with appearance of a strong oxidative wave at 0.95 V (Fig. 6). Overall, electrochemical studies
also claimed 4 to be used as an appropriate electrochemical metalloreceptor for Fe3+ and Ca2+
ions.
Fig. 5. Electrochemical titration plot of 4 (1.0 ꢀM) in presence of Fe3+ (0.3ꢀ3.0 ꢀL) both in
reduction (a) and oxidation (b) window (1.0 M TrisꢀHCl/ H2O, pH = 7.4).
Fig. 6. Electrochemical titration plot of 4 (1.0 ꢀM) in presence of Ca2+ both in reduction (left)
and oxidation (right) window (1.0 M TrisꢀHCl/ H2O, pH = 7.4).
Detection of Fe3+ by electron paramagnetic resonance (EPR) technique. Detection of
paramagnetic Fe3+ by a paramagnetic Cu(II) complex based probe 4, prompted us to explore the
detection of Fe3+ using electron paramagnetic resonance (EPR) spectroscopy. EPR spectra of 4
was acquired in methanol solution at 295K which exhibits two signals corresponding to gII =
2.24 and g⊥ = 2.07, comparable to Cu(II)ꢀcomplexes in significantly distorted tetrahedral, square

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planar and square pyramidal geometry (Fig. 7).39 The isotropic g value using equation giso = (gII
+ 2g⊥)/3 has been calculated to be 2.12. Further, g values following the order gII > g⊥ > ge
(2.0023), suggests that unpaired electron lies is in the dx2ꢀy2 orbital of Cu(II).40 Under identical
conditions, Fe3+ (~1.0 equiv) was added to the solution of 4 which led to the significant change
in the band intensities as well as shift in the gII and g⊥ values to appear at 2.26 (gII) and 2.04 (g⊥).
In addition, a broad signal appeared about g = ~ 4.0, which is ascribed to a typical, distorted
rhombic HS Fe3+ (Fig. 7 and Fig. S17).41 The considerable change in the g values of 4 along with
emergence of a broad signal due to distorted rhombic Fe3+ strongly suggested interaction of 4
with Fe3+ through proposed six oxygen donor set. The EPR technique has been applied for the
detection of paramagnetic cation using paramagnetic probe, for the first time.
Fig. 7. Xꢀband EPR spectra of 4 and 4+ Fe3+ in MeOH at 295K.
ESIꢀMS and FTIR analyses. The interaction of Ca2+ and Fe3+ with 4 was also scrutinized by
performing a 1:1 reaction between 4 and Ca2+/Fe3+ separately at rt in H2O and the ensuing
products were dried at rt. The compounds were subjected to the ESIꢀMS analysis which strongly
supported the formation of 4+Ca2+ and 4+Fe3+ complexes. Mass spectrum of diꢀcationic complex
4+Ca2+ displayed molecular ion peak at m/z 384.4013 assignable to the [4+Ca]2+ species and its
relatively less abundant monocationic species at m/z 768.8561 corresponding to [4ꢀH+Ca]+ (Fig.
S22a). Likewise, ESIꢀMS spectrum of tricationic complex 4+Fe3+ exhibited molecular ion peak
at m/z 262.0746 assignable to the [4+Fe]3+ entity. Its dicationic [4ꢀH+Fe]2+ and monocationic [4ꢀ
2H+Fe]+ counterparts displayed peaks at m/z 393.0117 and 784.8001 (Fig. S22b). In addition,

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both 4+Ca2+ and 4+Fe3+ complexes display a peak at m/z 729.8220 assignable to the 4 which
ruled out the possibility of transmetallation reaction between Cu2+ and Ca2+/Fe3+ in 4. Moreover,
the FTIR spectra of 4 and its complexes (4+Fe3+ and 4+Ca2+) are also significantly different
however major peak profile is matching with each other (Fig. S23). These studies suggested
formation of Ca2+/Fe3+ complexes with 4, responsible for optical and electrochemical changes.
Practical detection of Fe3+ in environmental water samples. To test practical application of
metalloreceptor 4 for Fe3+ in environmental analysis, we used aforementioned real water samples
LW, BW TW/DW and WW. Initially real water samples in buffer (1.0 M TrisꢀHCl, pH 7.4) were
directly used for the detection of Fe3+ (without manual addition). For analysis, 2.5 mL of these
real water samples were taken in quartz cuvette and the solution of 4 (250 ꢀL, 100 ꢀM) was
added into these solutions. The addition of 4 was done separately to the real water samples and
mixed properly for one minute. Remarkably, all the real water samples exhibited quenching of
fluorescence in the order WW > LW > TW/DW ≈ BW (Fig. S24). The largest quenching (61 %)
obtained using motor garage waste water (WW) suggested optimum Fe3+ content in it. Further,
manual addition of Fe3+ (50 ꢀL, 10 ꢀM) to the lake water (LW) led to maximum quenching of
fluorescence of 4 (Fig. S24). Eventually, to get recovery information and R.S.D. values, a known
Fe3+ concentration (20 ꢀM, 50 ꢀL) was added to all the real samples and the total Fe3+
concentrations were then determined under same condition (Table 1).
Conclusion
In summary, this work comprises development of a Cu(II)ꢀmetallacycle based
metalloreceptor 4 through two unique coꢀcrystallization cum metal mediated desulphurization
reactions wherein new ligand di(pyridinꢀ2ꢀyl)sulfane (1') is inꢀsitu generated. Notably, Cu(II)ꢀ
complex 4 is fluorescent (which is scarce) and it also acts as dualꢀchannel metalloreceptor for
Fe3+and Ca2+ ions through fluorescence ‘turnꢀon’ signaling for Ca2+ and ‘turnꢀoff’ response for
Fe3+, in pure water buffer solution at physiological pH. Selectivity of 4 has been found excellent
for Fe3+ over Ca2+ and other cations/anions. Further, 4 also detects Fe3+ and Ca2+ via
electrochemical changes. For the first time, EPR experiment has been used to the detection of a
paramagnetic ion (Fe3+) using a paramagnetic probe (4). Based on chemical structure of 4, Job’s
plot, ESIꢀMS, EPR and using Co(II) complex as model compound, we proposed that 4 interacts

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with Fe3+ and Ca2+ through six oxygen atoms oriented like pseudoꢀcrown ethers. The Fe3+ ion has
been practically identified to the real water samples especially in waste water. Overall, this
unique report deals with the development of dimeric Cu(II)ꢀcomplex based reusable, dualꢀ
channel, and practically functional metalloreceptor for cations in pure aqueous medium through
distinct signaling wherein the responsible detection site is of inorganic nature.
Acknowledgments. RP acknowledges Department of Science and Technology for financial
assistance through DSTꢀInspire Faculty scheme (IFA12ꢀCHꢀ66) and Department of Chemistry,
Dr. Harisingh Gour Central University Sagar, (M.P) India. NT acknowledges DST for financial
assistance for JRF through DSTꢀInspire Faculty scheme (IFA12ꢀCHꢀ66). MDP is thankful to the
university grants commission for the UGCꢀstartup grant [F. No. 30ꢀ56/2014(BSR)] and DST
India for SERB research grant (EMR/2016/001779).
Supporting Information Available: Supplementary data associated with this article can be
found, in the online version, at doi: XXXX. CCDC numbers 1505819ꢀ1505822 (1ꢀ4) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road,
Cambridge CB21EZ, UK; Fax: +44ꢀ1223ꢀ336033; Eꢀmail: deposit@ccdc.cam.ac.uk).

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Table 1. Determination and recovery of Fe3+ in real water samples (N = 3).
Sample
Fe3+ added
(ꢁmol Lꢀ1)
Fe3+ found
(ꢁmol Lꢀ1)
Recovery ±
RSD
(%)
Bore well
0.00
20.0
1.20
21.00
105 ± 0.2
Tap/drinking water
Lake water
0.00
20.0
0.00
20
1.20
21.10
1.85
105 ± 0.3
102 ± 0.8
108 ± 0.4
21.40
3.40
Waste water
0.00
20.0
23.15

Page 27 of 27
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DOI: 10.1039/C7NJ03294C
Graphical Abstract
Uniquely fabricated Cu(II)-metallacycle as reusable highly sensitive dual-
channel and practically functional metalloreceptor for Fe3+ and Ca2+ ions: An
inorganic site of cation detection
Neha Thakur, Mrituanjay D. Pandey* and Rampal Pandey*
Department of Chemistry, Dr. Harisingh Gour Central University, Sagar (MP), India
Dimeric Cu(II)-complex developed from disulfane ligand, serves as dual-channel
metalloreceptor for Fe3+/Ca2+ and detection of Fe3+ in real water samples.