A Kamikaze Approach for Capturing Hg2+ Ions through the

Article pubs.acs.org/IC

A Kamikaze Approach for Capturing Hg2+ Ions through the Formation of a One-Dimensional Metal−Organometallic Polymer Sk. Atiur Rahaman, Biswajit Roy, Soumik Mandal, and Subhajit Bandyopadhyay* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata Mohanpur, Nadia, West Bengal 741246, India S Supporting Information *

ABSTRACT: Efficient uptake of Hg2+ ions in mercuryresistant bacteria is attributed to the presence of cysteine thiolates in the Mer proteins. In this work, a pyridineappended pyridine-fused imidazolyl-2-thione scaffold was used as a mimic for the cysteinyl residues for efficient binding of the Hg2+ ions. In the presence of Hg2+ ions, an aryl C−H bond of the ligand is activated. The sulfur and nitrogen donors on the other end of the ligand coordinate with a second Hg2+ ion. This motif in the presence of acetate ions forms a onedimensional polymeric crystalline network characterized by singal-crystal X-ray diffraction studies. The formation of this polymeric structure leads to efficient removal (∼99%) of Hg2+ ions from aqueous solutions through an underexplored “kamikaze” approach involving a small-molecule ligand as a sacrificial agent for trapping the ion.

1. INTRODUCTION Mercury is a toxic metal. The use of mercury salts and organomercury compounds is still widespread, although their toxicity and perils to health and the environment are wellknown. Anthropological activities including the use of fossil fuels and industrial and agricultural uses add mercury to the environment.1 Natural sources such as volcanic and underwater thermal vents act as natural sources for mercury as well. The U.S. Environmental Protection Agency allowed upper limit for the concentration of mercury ions in drinking water is 2 ppb.2 A number of medications such as mercurochrome, ear and eye drops, and even vaccines for infants and children contain preservatives, such as thiomersal, that contain mercury.3 Dental amalgam can contain more than 40% mercury.4 Interestingly, in mercury-resistant bacteria, nature has evolved its own detoxification mechanism via binding, degrading of organomercury compounds, and eliminating mercury. Summers and co-workers have immensely contributed to the microbiological aspects of these species.5 In addition, the work of Walsh,6,7 Omichinski,8−10 Parkin,11−14 O’Halloran,15a,b and co-workers have contributed to an understanding of the chemical and biochemical mechanisms of the organomercury-degrading proteins as well as the chemistry behind the toxicity of mercury compounds in general. The cysteine residues of the mercurybinding proteins are crucial, as expected from the classical hard−soft-acid−base (HSAB) principle.16 For example, the active site of MerB of the organomercury protonolytic bacteria Escherichia coli R831b, an enzyme that breaks down the Hg−C bonds of organomercury species, contains three cysteines.17 Out of these, only two of the cysteines (Cys96 and Cys159) are actually involved in the cleavage of HgC, and these two residues © XXXX American Chemical Society

are essential. Mutation of either one completely abolishes the activity. The third cysteine (Cys160) is believed to be involved in shuttling either methylmercury into or Hg2+ out of the active site and is not as highly conserved as the other two. This Cys160 is actually located outside the active site and is not essential for activity, and it is not even geometrically possible for it to be coordinated to methylmercury during the HgC cleavage reaction. Mutating it to Ala only reduces the value of the catalytic efficiency, kcat/Km, by a little more than 2-fold. The third key residue in MerB is the catalytic acid, Asp99, which is required for enzymatic activity. With computational studies using the newly determined crystal structures, Parks et al. showed that two Cys and one Asp residues are necessary and sufficient for the actual HgC cleavage step.18 MerB coordinates methylmercury with only two cysteines, and it uses Asp99 as the catalytic acid that protonates the leaving group and cleaves the Hg−C bond. Interestingly, prior to knowledge of the crystal structure of MerB, the active site of the enzyme was mimicked by the Parkin group with a model system containing three sulfur donors of a tris(2-mercapto-1-tert-butylimidazolyl)hydroborato ligand that efficiently cleaved a stable Hg−C bond.11 Therefore, although Parkin’s mimic works best with a three-sulfur-containing ligand bonded to methylmercury, that is not how HgC cleavage in MerB works. Indeed, Melnick and Parkin mentioned this possibility in the reference notes of their paper.11 Our group has been involved in developing receptors for mercuric ions.19−22 Although the focus of this study is not cleavage of a Hg−C bond, we made use of the 2Received: September 11, 2015

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DOI: 10.1021/acs.inorgchem.5b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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3(2H)-thione compound 1 was synthesized, and its interaction with the Hg2+ ions was studied. Interestingly, besides the expected coordination to the mercuric ion of compound 1, an aryl C−H bond of compound 1 was also activated, resulting in a C−Hg bond formation on one end and an S−Hg−N coordination motif on the other end of the ligand. The acetate counteranion of the Hg2+ salt gave rise to bridging interactions between the [Hg-1-Hg] units, thereby forming an extended self-assembled one-dimensional (1D) coordination polymer. The structure of the organic ligand as well as the onedimensional coordination polymer has been supported by single-crystal X-ray diffraction (SCXRD) structures. There are a few reports of mercury sequestering by coordination polymers.24−27 Recently, Stock and Xu have reported thiol-laced metal− organic polymers based on 2,5-dimercapto-1,4-benzenedicarboxylic acid and ZrIV or AlIII metal centers. The zirconiumbased polymer absorbs Hg2+ ions and consequently reduces its concentration in water below 0.01 ppm.28 The organic nanotrap by Ma and co-workers, based on a thiol-containing organic polymer, is remarkable in terms of its efficiency for the removal of mercury ions from aqueous contamination.29 However, in this work, trapping and removal of mercuric ions have been achieved through an underexplored route where the ions are incorporated in the polymeric backbone by a dual mode via coordination and also by formation of an organometallic bond. The efficiency of removal was ∼99%.

mercaptoimidazolyl scaffold in an effort to provide a strong binding motif for the mercuric ions. Additionally, to monitor the Hg2+ binding, we have chosen to study the fluorescence response of a mercaptoimidazole-containing receptor. However, because the parent mercaptoimidazole compound does not possess any fluorophore, a highly fluorescent pyridine-fused system was designed.23 Another key reason for the design of pyridine-fused imidazolylthione was to impart high electron density to the CS sulfur so that its binding affinity to the Hg2+ ion is not compromised compared to the parent mercaptoimidazole system. The high electron density on the sulfur can be rationalized by the simple argument that the C− S− system having 10π electrons is stable owing to its aromatic nature (see Figure 1), similar to the parent 6π imidazolyl

2. RESULTS AND DISCUSSION Recently, thiocarbamate-linked dipicolylamine-based fluorescence chemosensors for the detection of Ag+ ions have been reported by us.20 During the synthesis involving dipicolylamine and carbon disulfide, compound 1 was obtained as a byproduct in only 13% yield. The yield of compound 1 was further improved by modifying the procedure, as described in the Experimental Section. Wang and co-workers, in the meantime, have reported the compound with partial characterization for chemodosimetric detection of hypochlorous acid through an oxidative desulfurization mechanism.30 Colorless needle-shaped crystals of compound 1 were grown from (1:1, v/v) CH2Cl2/hexane through slow evaporation.

Figure 1. Formation of the polymeric structure with mercuric ions in the presence of compound 1 and acetate ions under aqueous conditions at pH 7 and 25 °C.

system. This offers a close resemblance of these systems with the cysteine thiolates that are present in the Mer enzymes of the mercury-resistant bacterial species. In our ligand, the appended methylpyridinyl donor unit provided an additional binding motif for mercury. Thus, the imidazo[1,5-a]pyridine-

Figure 2. (A) Changes in the UV−vis spectra of compound 1 (10 μM) upon the addition of various metal ions (50 μM) in aqueous media [99:1 (v/ v) H2O/DMSO; see the main text]. (B) Fluorescence intensity of compound 1 (10 μM) upon the addition of various metal ions (50 μM; λex = 320 nm; slit widths = 5 and 5 nm). B

DOI: 10.1021/acs.inorgchem.5b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (A) Fluorescence spectra (λex = 320 nm; slit widths = 5 and 5 nm) of compound 1 (10 μM) upon the addition of 0−10 equiv of Hg2+ ions. (B) Job’s plots indicating 1:2 complexation of compound 1 with Hg2+ ions in aqueous media [99:1 (v/v) H2O/DMSO] at a total concentration of 0.1 mM of 1 and Hg2+.

equiv of Hg2+ ions. The method of continuous variation, popularly known as the Job’s plot experiment, carried out at a total concentration of 10−4 M of the metal ion and the compound, indicated a 1:2 binding of compound 1 with Hg2+ ions (Figure 3B). This was somewhat baffling because it was difficult to envisage the coordination of two metal ions to one molecule of compound 1. The NMR titration of compound 1 against a varying amount of Hg2+ displayed a distinct change in the proton resonances. To our surprise, the signal for the Hi proton at a value of 7.22 ppm in the compound completely disappeared after the addition of Hg2+ ions (see Figure 4). In

Selected bond lengths and the crucial bond angles are listed in Table S2 in the Supporting Information. In the structure, it was observed that the C−S distance in 1 was 1.675 Å compared to an average CS bond distance of 1.599 Å reported for the non-hydrogen-bonded CS bonds.31 This is an indication of the reduction of the CS double-bond character and a partial negative charge on the sulfur atom, which is an assisting factor for efficient coordination. Compound 1 is greenish in color and fluorescent in aqueous media [99:1 (v/v) H2O/dimethyl sulfoxide (DMSO)] with a quantum yield (Φ) of 0.16. To understand the effect of a compound with various metal ions, UV−vis and fluorescence experiments were initially carried out using salts (50 μM; see the Supporting Information) of Cr3+, Mn2+, Fe2+, Fe3+, Cd2+, Mg2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Hg2+, Al3+, Ba2+ Na+, K+, and Ag+ with compound 1 (10 μM) in 99:1 (v/v) H2O/DMSO in the presence of N-(2-hydroxyethyl)piperazine-N′-2-ethanesulfonic acid (1 mM, buffered at pH 7) at 25 °C. Significant changes in the UV−vis spectra were observed upon the addition of Hg2+ and Ag+ ions. A new broad absorption band at 324 nm was obtained (Figure 2A) in the aqueous medium. The emission spectra of compound 1 (10 μM; λex = 320 nm; excitation and emission slit widths = 5 and 5 nm) upon the addition of various metal ions (50 μM) were recorded (Figure 2B). A strong quenching of the fluorescence with Hg2+ was observed, displaying a 63-fold reduction of the intensity with no shift of the fluorescence band, which brought the signal down to almost the baseline (Figure 2B). This strong quenching corresponded to a drastic 94-fold reduction of the fluorescence quantum yield to Φ = 0.0017, indicating a strong interaction of the metal ion with compound 1. The disappearance of the green color and fluorescence (under 366 nm UV light) of a solution of compound 1 upon the addition of the Hg2+ solution indicated complexation (see the Supporting Information, Figure S4). Although in the presence of Ag+ ions a decrease in the emission intensity was also observed, the reduction was only 2-fold, accompanied by a 29 nm blue shift. To investigate the nature of the interaction between compound 1 and Hg2+ ions, fluorimetric titration of the compound with the metal ion was carried out (Figure 3). The titration plot revealed a strong quenching with virtual disappearance of the fluorescence upon the addition of 2

Figure 4. Partial 1H NMR spectra in CDCl3 at 25 °C of (A) compound 1 (1 × 10−3 M) with Hg2+ (2 × 10−3 M) and (B) compound 1 (1 × 10−3 M) alone.

addition, there were significant changes for the signals of the Hd, Hc, Hh, Hg, He, and Hf protons at δ values of 7.66, 7.44, 7.22, 7.11, 6.73, and 6.53, respectively. The chemical shift of the methylene protons Hj also underwent a downfield shift from δ 5.64 to 5.78 (see the Supporting Information, Figure S11). Thus, the addition of Hg2+ ions caused a downfield chemical shift of several protons of both the pyridine and imidazothione rings, thereby indicating a strong interaction of the Hg2+ ions with both rings of the compound, presumably via the pyridine nitrogen atom and the CS bond of the fused heterocycle. In addition, the disappearance of the Hi proton in the mercuryC

DOI: 10.1021/acs.inorgchem.5b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Mass spectrometric (MS) studies in the positive mode of electrospray ionization mass spectrometry (ESI-MS) indicated fragmentation of the polymer with mass-to-charge (m/z) peaks at 1259, 942, 760, and 501 corresponding to the fragments shown in Figure S9A−D in the Supporting Information, although the matrix-assisted laser desorption ionization experiments failed to produce any sensible data. The simulated MS for the ESI-MS peaks were in well agreement with the species that were expected from the fragmentation of a polymer containing Hg2+ and 1 in a 2:1 ratio. The simulated MS data matched well with the isotopic distribution of the fragments (see the Supporting Information, Figure S9A−D), and the overlapping region of various fragments provided clues for the structure of the polymer. The binding interaction between Hg2+ and the imidazolethione group has also been observed by Fourier transform infrared (FT-IR) spectroscopy (see the Supporting Information, Figure S14). The IR spectra revealed a large shift of the CS stretch from 1406 to 1552 cm−1,41 indicating the a strong interaction between Hg2+ and the thione group. The molecular structures of 1 and 1a are shown in Figures 6A and 7.

bound form prompted us to speculate the formation of an organomercury bond at the carbon-i. This was also consistent with the 1:2 association of compound 1 with Hg2+ observed in the Job’s plot experiment. It is worth noting that, analogous to the ith position of 1, there are reports of organomercury bond formation in the 2-vinyl and 2-arylpyridine systems.32,33 Recently, Singh and co-workers have also demonstrated the formation of cyclometalation with an N-benzyl derivative.34 At this point, we wondered whether there was formation of any −(metal−ligand• metal)n− type polymeric structure of the metal ion with the ligand32−39 involving coordination of the sulfur atom and the nitrogen center on one end and a C−Hg bond formation on the other end of compound 1 because the carboxylates and sulfur coordinate with mercuric ions efficiently.27 Thus, to explore the existence of the large macromolecular −(metal−ligand• metal)n− type polymeric structure, dynamic light scattering (DLS) studies were carried out. The DLS studies were performed with samples containing Hg2+ (10−5 M) and compound 1 (10−5 M) in aqueous media [99:1 (v/v) water/DMSO] at 25 °C. Measurements using mercuric acetate in the absence of the ligand and compound 1 in the absence of Hg2+ salt were also carried out as controls under the same conditions. Consistent with what was envisioned, the DLS studies revealed the formation of large macromolecular entities. However, to our discontent, the particle sizes obtained with various batches of the samples varied widely. Some of the samples that were left for longer time also indicated turbidity, and the samples that were left for a few days formed precipitates. The size dependence of the particles with time was therefore systematically studied by DLS. The DLS data from the mixture of Hg2+ (1 × 10−5 M) and compound 1 (5 × 10−6 M) in aqueous media [99:1 (v/v) water/DMSO] at 25 °C recorded at 0, 0.5, 1, 5, 10, 24, 48, 72, 120, and 180 h after mixing of the two solutions clearly displayed the growth of larger particles with time. In the absence of Hg2+ salt, the DLS data of the ligand displayed small aggregates of the organic molecule in water, having an average diameter of only 1.7 nm (Figure 5, front trace).40 Upon the addition of Hg2+ salt, the small molecular entities rapidly grew to form macromolecular particles with an average diameter of 42 nm in 0.5 h, and after 120 h, the particle size reached a saturation value with a Z-average hydrodynamic diameter on the order of ∼1 μm.

Figure 6. SCXRD structures of (A) compound 1 and (B) the repeating unit of the metal−organometallic framework 1a. The hydrogen atoms in the structures have been omitted for clarity.

Figure 7. SCXRD structures of two strands of the 1D framework 1a. The colors of the atoms follow the same coding as the previous figure.

The 1D polymeric structure is stable under a wide range of pH values from 4 to 7 (see the Supporting Information, Figure S5). However, the structure collapsed almost instantly upon the addition of sodium sulfide. This was manifested in the restoration of the fluorescence signal of compound 1 after the addition of sodium sulfide (see the Supporting Information, Figure S4). We wondered whether the formation of the polymeric structure can be exploited for the uptake and subsequent

Figure 5. Time course of the particle size distribution monitored by DLS studies from 0 to 180 h after the addition of Hg2+ (1 × 10−5 M) to compound 1 (5 × 10−6 M) in aqueous media [99:1 (v/v) water/ DMSO] at 25 °C. D

DOI: 10.1021/acs.inorgchem.5b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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the spectroscopy experiments. The solutions of metal ions were prepared from Ba(NO3)2·4H2O, LiClO4·3H2O, NaClO4, Ni(ClO4), Fe(ClO4)3·xH2O, Mn(ClO4)2, KClO4, Al(NO3)3·9H2O, Fe(ClO4)2· xH2O, Co(ClO4)2·6H2O, Zn(ClO4)2·6H2O, Cd(NO3)2, AgNO3, Hg(OCOCH3)2, Pb(ClO4)2, Ca(ClO4)2·4H2O, and Cu(ClO4)2· 6H2O in H2O. IR spectroscopic data were obtained with a Bruker Alpha Platinum-ATR instrument. UV−vis spectra were recorded with a Cary 60 UV−vis spectrophotometer. Fluorescence measurements were carried out with a Horiba Jobin Yvon fluorometer (Fluoromax-3, Xe-150 W, 250−900 nm). MS data were obtained from an Acquity ultraperformance liquid chromatography (LC)−MS and Bruker maxis impact. pH data were recorded with a Sartorius Basic Meter PB-11 calibrated at pH 4, 7, and 10. Reactions were monitored by thin-layer chromatography (TLC) using Merck plates (TLC Silica Gel 60 F254). Developed TLC plates were visualized with UV light (254 nm). Silica gel (100−200 mesh, Merck) was used for column chromatography. Yields refer to the chromatographically and spectroscopically pure compounds. Synthesis of Compound 1. Compound 1 was synthesized following the standard general method of thiocarbamate synthesis.19−22 Dipicolylamine (0.1 g, 0.50 mmol) in dioxane/water (4:1, v/v) was stirred for 5 min at 0 °C. This was followed by the addition of liquid NH4OH (0.04 mL, 1.0 mmol) to the mixture. After 15 min, carbon disulfide (0.60 mL, 10.0 mmol) was added to the mixture, and the solution was allowed to attain room temperature (25 °C). The reaction mixture was stirred overnight (16 h), and the progress of the reaction was monitored by TLC. After the disappearance of the spot for the starting materials (checked by TLC) and generation of a new spot for the product, the solution was concentrated under reduced pressure. The greenish solid thus obtained was dissolved in dichloromethane (30 mL) and washed with water. The organic layer was dried over anhydrous Na2SO4, and the volatiles were removed under reduced pressure, which in column chromatography [9:1 (v/v) CH2Cl2/MeOH] yielded compound 1 as a greenish solid (0.09 g, 73%), which was subsequently crystallized from a mixture of 1:1 (v/v) CH2Cl2/hexane. Mp: 95.5 °C. The SCXRD data were solved, and the structure of the compound was obtained (CCDC 1062918). 1H NMR (400 MHz, CDCl3): δ 8.57 (d, J = 5.2 Hz, 1H, ArH), 8.28 (d, J = 7.6 Hz, 1H, ArH), 7.66 (td, J = 7.6 and 1.6 Hz, 1H, ArH), 7.44 (d, J = 8.0 Hz, 1H, ArH), 7.22 (q, J = 4.8 Hz, 1H, ArH), 7.17 (s, 1H, ArCH−), 7.11 (d, J = 9.2 Hz, 1H, ArH), 6.73 (q, J = 6.0 Hz, 1H, ArH), 6.53 (td, J = 7.2 and 1.2 Hz, 1H, ArH), 5.65 (s, 2H, ArCH2−). 13C NMR (100 MHz, CDCl3): δ 155.0, 153.6, 149.6, 137.2, 127.6, 125.4, 123.5, 123.2, 122.4, 117.3, 112.3 106.9 53.1. FT-IR (film, cm−1): 3099, 2923, 2855, 1643, 1591, 1471, 1406, 1348, 1305, 750. ESI-MS. Calcd for C13H11N3S [M + H+]: m/z 242.0752. Found: m/z 242.0727. Anal. Calcd for C13H11N3S: C, 64.70; H, 4.59; N, 17.41. Found: C, 64.88; H, 4.47; N, 17.28. Characterization Data of Polymer 1a. 1H NMR (400 MHz, CDCl3): δ 8.58 (s, 1H, ArH), 8.24 (d, J = 5.3 Hz, 1H, ArH), 7.81 (t, J = 3.8 Hz, 1H, ArH), 7.77 (d, J = 5.3 Hz, 1H, ArH), 7.58 (d, J = 9.1 Hz, 1H, ArH), 7.35 (t, J = 6.1 Hz, 1H, ArH), 6.99 (t, J = 9.1 Hz, 1H, ArH), 6.91 (t, J = 6.8 Hz, 1H, ArH), 5.78 (s, 2H, ArCH2−), 1.96 (s, 12H, −COCHa), 1.76 (s, 9H, −COCHb). 13C NMR (125 MHz, CDCl3): δ 176.4, 154.1, 150.3, 150.2, 138.8, 135.7, 125.1, 124.8, 124.6, 124.3, 124.2, 124.1, 123.0, 54.8, 29.8, 22.7. The SCXRD data were solved, and the structure of the polymer was obtained (CCDC 1062919).

removal of mercuric ions from aqueous samples. Thus, aqueous solutions containing 0.01 mM of compound 1 were added to Hg2+ solutions containing 0.02 mM of the metal ion in aqueous media [99:1 (v/v) water/DMSO] at 25 °C. The samples were left for 6 days to allow the polymeric framework to precipitate out of the solution. The solid polymers were separated by centrifugation, and the concentration of Hg2+ in the supernatant was determined by fluorimetric methods (see the Supporting Information, Figure S7) and subsequently crosschecked by a colorimetric method via the formation of a dithizonatomercury(II) complex having a characteristic absorption at 492 nm.42 The removal efficiency of a metal ion can be quantified in terms of the distribution coefficient, Kd (the definition and method of determination are given in the Supporting Information).29 According to the literature, a Kd value of 1 × 105 mL/g is considered to be excellent for the purpose of removal of the ion.43 The Kd value with compound 1 for Hg2+ ions via formation of the metal−organometallic polymer was found to be 3.2 × 106 mL/g, which is higher by 1 order of magnitude. The average residual concentration of Hg2+ ions in the supernatant was 0.51 ppm, which corresponds to the removal of 99.1 ± 0.7%. The value of the average residual concentration obtained by the colorimetric method was 0.46 ppm, which corresponds to a slightly higher Kd. Most of the reported methods for the efficient removal of mercuric ions (with >90% removal efficiency) involve supramolecular host− guest chemistry. This work reports an underexplored approach of the highly efficient removal of Hg2+ ions with a self-sacrificial “kamikaze” ligand system involving dual binding modes to the toxic ion through C−H activation and an efficient coordination, leading to the formation of a 1D metal−organometallic polymer. Removal of the mercury ions via more of such ligands are currently under investigation.

3. CONCLUSION In this work, we have designed a receptor for mercuric ions that efficiently binds mercury through coordination as well as C−H activation mode, forming a C−Hg bond. This monomeric unit containing two mercury ions forms an extended metal− organometallic framework via acetate bridges. The ligand thus acts as a self-sacrificial species for removal of the mercuric ions and is, therefore, called a “kamikaze” system. Formation of the framework has been confirmed by various spectrometric methods and SCXRD structure. Owing to the high affinity of the ligand toward Hg2+ and the stability of the metal− organometallic framework under ambient conditions, this system, at the very least, can foster a novel pathway for the separation of Hg2+ ions from aqueous samples with high affinity. Further improvement of the scaffold might lead to promising ligands for faster separation and removal of Hg2+ ions from contaminated samples. 4. EXPERIMENTAL SECTION Instruments and Reagents. All reactants and reagents were commercially available and were used without further purification unless otherwise indicated. Solvents used were purified and dried by standard methods. The structures of the compounds were determined by 1D and 2D NMR spectrometry and other spectroscopic techniques. 1 H and 13C NMR spectra were recorded with 400 MHz JEOL and 500 MHz Bruker instruments. Chemical shifts are reported as δ values relative to an internal reference of tetramethylsilane (TMS) for 1H NMR and the solvent peak in the case of 13C NMR. The spectroscopic-grade solvents for the spectroscopic experiments were free from any fluorescent impurity. Double-distilled water was used for

DLS Studies. The particle sizes of the aggregates were measured by DLS experiments with a Malvern Zetasizer Nano ZS instrument E

DOI: 10.1021/acs.inorgchem.5b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry equipped with a 4.0 mW He−Ne laser operating at a wavelength of 633 nm. The samples and background were measured at room temperature (25 °C) at a scattering angle of 173°. DLS experiments were carried out with an optically clear solution of compound 1 (10−5 M) in aqueous media [99:1 (v/v) water/DMSO] in the presence and absence of the Hg2+ ion. The solution was prepared from Hg(OAc)2 and 1 and equilibrated for 30 min before the measurements were taken. Determination of Quantum Yields. The quantum yields (Φ) of compounds 1 and 1a were calculated by comparing their integrated fluorescence intensities (excitation at 350 nm) and the absorbance values at 350 nm with those of quinine sulfate. Quinine sulfate (ΦS = 0.54) was dissolved in 0.1 M H2SO4 (refractive index: 1.33), and compounds 1 and 1a were dissolved in water (refractive index: 1.33) in aqueous media [99:1 (v/v) H2O/DMSO]. The integrated fluorescence intensity is the area under the fluorescence curve in the wavelength range from 365 to 680 nm. The relative quantum yield was calculated from the following equation: ΦF = ΦS ·

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2 A S FU ηU 2 AU FS ηS

where ΦS is the quantum yield of the quinine sulfate (standard), A is the absorbance at the excitation wavelength (subscript S for the standard and U for an unknown sample), F is the area under the emission spectra, and η is the refractive index of the solvent.44

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02104. Synthetic scheme, absorption and emission spectra, UV− vis titration, fluorescence response, pH stability, determination of Kd and residual Hg2+, ESI-MS spectra, 1H and 13C NMR spectra, FT-IR spectra, crystallograohic parameters, bond lengths and angles, and 1D polymeric network (PDF) Crystallographic CIF file (CCDC 1062918 and 1062919) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank U.K.−India Education and Research Initiative (UKIERI)-DST for funding (Grant INT/UK/P-98/ 14). S.A.R. is supported by an IISER Ph.D. Fellowship, S.M. by a CSIR Senior Research Fellowship, and B.R. by a DSTINSPIRE Fellowship. We acknowledge an anonymous reviewer for his encouraging comments and helpful suggestions.

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DOI: 10.1021/acs.inorgchem.5b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02104 Inorg. Chem. XXXX, XXX, XXX−XXX