An Ionophore for High Lithium Loading and Selective Capture from

May 15, 2019 - Chem.201958117209-7219 ... and develop a better multinucleating extractant with high lithium loading capacity which is rare in the lite...
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Article Cite This: Inorg. Chem. 2019, 58, 7209−7219

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An Ionophore for High Lithium Loading and Selective Capture from Brine Hardipsinh Gohil,† Sobhan Chatterjee,† Sanjay Yadav,† Eringathodi Suresh,‡ and Alok Ranjan Paital*,† †

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Salt and Marine Chemicals Division & Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Salt & Marine Chemicals Research Institute, G.B. Marg, Bhavnagar-364002, Gujarat, India ‡ Analytical and Environmental Science Division and Centralized Instrument Facility, CSIR-Central Salt & Marine Chemicals Research Institute, G.B. Marg, Bhavnagar-364002, Gujarat, India S Supporting Information *

ABSTRACT: The continuous demand and uneven dispersal of natural mineral resources of lithium with a low recycling rate of lithium commodities have forced researchers to look for alternative resources like geothermal brine, brackish brines, and sea brines. But selective lithium-ion extraction and even lithium-ion binding from these aqueous systems is a recognized challenge due to very high hydration energy and the coexistence of other like metal ions but appealing due to economic benefits. Therefore, the designed synthesis of synthetic ionophores with high lithium selectivity is crucial as they can work on dilute conditions without removal of interfering metal ions. However, most of the lithium selective ionophores known in the literature are mononucleated, and no emphasis is given on designing multinucleating ionophore systems to improve the lithium loading capacity which will open up unexplored paths toward the development of a more sustainable and economical extraction process. Herein, we describe a rare fluorogenic macrocyclic ionophore with two binding pockets for selective lithium recognition and extraction among various major alkali and alkaline earth metal ions of oceanic presence through both solid-state and solution studies. Under solid-liquid extraction conditions, this receptor shows a high lithium loading capacity of 135% with LiClO4 and 69.16% with LiCl salt with exclusive selectivity. Under liquid-liquid extraction conditions, this ionophore shows a loading capacity of 27% with 1 M LiCl and 48.57% with 1 M LiClO4 source phase concentration. This new ionophore, therefore, inspiring further to modify and develop a better multinucleating extractant with high lithium loading capacity which is rare in the literature.

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INTRODUCTION Apart from pharmaceuticals and materials applications, lithium in the form of lithium-ion batteries is the center of attraction for the next-generation clean energy technologies. The continuous demand and the uneven dispersal of natural mineral resources1 in the Earth’s crust have driven policies at various levels for recycling of lithium resources. Therefore, an environmentally benign method for selective lithium extraction and purification has drawn attraction, although it is a global challenge with potential strategic and commercial gains. Though recycling of lithium ions from batteries2 is encouraged for lithium resource security3 and environmental protection,4 the low global rate of recycling has forced researchers to look for alternative resources like geothermal brines, brackish brines, and sea brines.5−8 That is why the lithium extraction from seawater is a concept that periodically reappears in times of energy crisis because of its high demand and depletion of their natural mineral resources. But the low concentration of lithium ions (∼0.2 ppm) in seawater or salt flats (80−1500 ppm) in the presence of higher amounts of other cations such © 2019 American Chemical Society

as sodium, potassium, magnesium, and calcium ions makes it an extremely challenging task.9 In recent times, many efforts have been dedicated for developing methods for lithium extractions, but they have suffered from one drawback to another in the context of seawater or salt flats due to low concentration, huge hydration energy, and cohabitation of other like metal cations.10−13 Therefore, the development of new lithium selective ionophores which can capture lithium ion selectively from aqueous systems is crucial considering the relevance to various brine systems, and they can work on dilute conditions without removal of interfering metal ions. Although ionophores have been considered early on, the synthetic ionophores with high affinity and selectivity for lithium ion is a challenging task. In this regard, considerable efforts have been given on crown ether derivatives,14−19 spherands,20 hemispherands,21,22 metallamacrocycles,23−26 and various lithium Received: January 15, 2019 Published: May 15, 2019 7209

DOI: 10.1021/acs.inorgchem.9b00135 Inorg. Chem. 2019, 58, 7209−7219

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Inorganic Chemistry ionophores/receptors27−34 which often require substantial synthetic efforts. A close analysis of the above lithium selective ionophores/ receptors known in the literature reveals that a majority of them are mononucleating ligands of macrocyclic type or acyclic systems with bulky substituent arms to get selectivity and hydrophobicity toward lithium-ion recognition and extractions. But after substantial synthetic efforts, the drawback of low loading capacity (one Li+ per molecule) for such bulky host molecular systems remains there. However, no emphasis is given on designing multinucleating ionophore systems for lithium ions to improve the loading capacity. Though multinucleating organolithium compounds35−37 are known, multinucleating ionophores for lithium ions are very rare in the literature. However, the designed synthesis of such ionophores is challenging and the key factors to be considered are (i) designing of a ligand with appropriate cavity sizes to get a good selectivity for lithium ions over other alkali and alkaline earth metal ions, (ii) moderate binding required to transport (metal binding and stripping) the metal ion, and (iii) appropriate hydrophobicity to overcome the hydration energy of the lithium ion. As we are working on the development of materials for dual functions of metal ion detection and extraction,38−40 we were also interested in developing a similar ligand system for lithium ions. Therefore, we report here a rare fluorogenic macrocyclic ionophore (L) that can selectively bind two equivalents of lithium ions in the presence of other similar alkali and alkaline-earth metal cations of oceanic presence proven through both solid-state and solution studies. This ionophore also shows high lithium loading capacity with very high selectivity under solid-liquid extraction conditions and is easy to recycle. Therefore, this new ionophore is an inspiration to modify further and develop a more sustainable and economical extraction process for lithium ions. To the best of our knowledge, this new macrocyclic ionophore represents a rare example of a lithium-ion selective multinucleating receptor system with a very rare keto bridging group which shows high lithium loading capacity with selectivity.

substitution reaction by taking 1,8-dihydroxyanthraquinone and excess 1,3-dibromopropane with K2CO3 as a base without any solvent. The second nucleophilic substitution reaction of the above product with 1,8-dihydroxyanthraquinone gives the final product ligand L in moderate yield. In the one-step reaction, taking a stoichiometric amount of starting materials gives ligand L in low yield. The synthesis and characterization data for all compounds are available in the Materials and Methods section (Figures S1−S5, Supporting Information). Photophysical Properties. To study the selectivity of the above macrocyclic ionophore L toward lithium ions over other relevant major metal ions of oceanic presence including Li+, Na+, K+, Mg2+, Ca2+, and Sr2+, both UV−vis and fluorescence studies were carried out using metal perchlorates except strontium for which chloride salt was used (Figure 1). Since the designed extractant ionophore L was hydrophobic, all the titrations were carried out in CH3CN as a solvent. As LiCl gives precipitation immediately after the reaction with ligand L in MeCN, only perchlorate salts were considered except SrCl2 for the spectral studies. The electronic absorption spectrum of the macrocyclic ionophore L shows two absorption bands at 373 and 253 nm. Upon addition of Li+ ions, the band at 373 nm undergoes a small bathochromic shift to 386 nm, whereas, with all other relevant metal ions, no changes were observed under similar conditions (Figure 1a). It is to be noted that this kind of small spectral change is difficult to find with lithium ions,41 and therefore, most of the literature does not discuss the photophysical changes. The association constants were determined by fitting the experimental data from the UV−vis absorption titration experiments with two possible binding models (1:1, 1:2) using an open-access program (supramolecular.org).42,43 Out of these two binding models, the 1:2 binding model gave a reasonable fit as evidenced by covfit (covariance of the fit, which is a measure of the quality of fit)44 and the residual plots.45 The association constants were found to be 217 M−1 (K1) and 12404 M−1 (K2), respectively, and the results of the binding models are summarized in Tables S1 and S2. The association constant K2 > K1 indicates a positive cooperative effect, where insertion of a second lithium ion becomes favorable which is also evident from the crystal structures discussed latterly. The 1:2 complexation reaction was also confirmed from the ESI-MS, and crystal structure analysis addressed latterly. Similarly, the fluorescence spectra of L exhibit a strong emission band at 433 nm and a relatively weak band at 600 nm, upon excitation at 375 nm (Figure 1c). The fluorescence emission of L does not show any changes with Na+, K+, Mg2+, Ca2+, and Sr2+ ions, whereas, with Li+ ion, initially, it undergoes fluorescence quenching at 433 nm peak and then undergoes fluorescence enhancement by the appearance of another hump at 460 nm. The band at 600 nm is probably due to an intermolecular excimer formation46 (π−π stacking of aromatics) whose intensity decreases with dilution as shown in Figure S6. It is also observed that, after addition of Li+ ion, the 600 nm peak intensity decreases but does not vanish completely, indicating the existence of π−π stacking interaction after complexation which is evident from the single crystal X-ray structures of the ligand, and it is lithium complexes as discussed later. This molecule does not show sensitive photophysical properties and hence cannot be an ideal candidate for sensory studies but can be used for qualitative selectivity studies. Competitive photophysical studies by taking other relevant metal ions along with lithium

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RESULT AND DISCUSSION The macrocyclic ionophore can be synthesized either by a onestep reaction or by a two-step reaction as shown in Scheme 1. In the two-step reaction, initially 1,8-bis(3-bromopropoxy) anthracene-9,10-dione was synthesized by a nucleophilic Scheme 1. Synthetic Scheme for the Macrocyclic Ionophore L

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DOI: 10.1021/acs.inorgchem.9b00135 Inorg. Chem. 2019, 58, 7209−7219

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Figure 1. (a)Absorption spectra of L (50 μM) in the presence of excess relevant metal ions in MeCN. (b) UV−vis titration with an increasing concentration of Li+ ion. (c) Fluorescence spectra of L (50 μM) with relevant metal ions with an excitation at 375 nm in MeCN. (d) Fluorescence titration with incremental addition of Li+ ion.

Figure 2. ORTEP diagrams of the L (a) and complex [Li2L(ClO4)(OH2)]ClO4 (b) with atom-numbering scheme (40% probability factor for the thermal ellipsoids). Inset showing conformational changes. 7211

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for L, [Li2L(ClO4)(OH2)]ClO4, and [LiL(Cl)] parameters

L

[Li2L(ClO4)(OH2)]ClO4

composition formula weight temperature/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3 Z ρcalc g/cm3 μ/mm−1 2θmax/deg reflns collected/unique F(000) GOF final R indexes [I ≥ 2σ(I)] final R indexes [all data] CCDC No.

C34H24O8 560.53 150(2) monoclinic P21 4.7842(9) 17.149(3) 15.216(3) 90 91.274(3) 90 1248.1(4) 2 1.492 0.107 54.82 4374/3997 584 1.194 R1 = 0.0601, wR2 = 0.1170 R1 = 0.0665, wR2 = 0.1195 1859399

C34H26Cl2Li2O17 791.33 150(2) triclinic P1̅ 7.501(3) 13.235(4) 17.943(6) 78.408(5) 87.103(6) 75.784(6) 1691.6(9) 2 1.554 0.274 55.34 5371/4612 812 1.143 R1 = 0.0804, wR2 = 0.1958 R1 = 0.0912, wR2 = 0.2029 1859400

[LiL(Cl)] C34 H24 Cl Li O8 602.92 100 monoclinic P121/c1 18.7345(6) 16.7872(6) 8.8075(3) 90 102.2410(10) 90 2706.98(16) 4 1.479 0.199 5534/4253 1248 1.04 R1 = 0.0541, wR2 = 0.1203 R1 = 0.0764, wR2 = 0.1352 1887493

Figure 3. (a) Parallel sandwich-type π−π stacking interactions in L. (b) H-bonding interaction depicting the zigzag motif in L. (c) Dimeric association of [Li2L(ClO4)(OH2)]ClO4 units via O−H···O interactions and π−π stacking present within the dimeric units and parallelly displaced adjacent dimeric units. (d) ORTEP diagram of [LiL(Cl)].

(Materials and Methods section). Under similar conditions, the combination of L with the other metal ions does not show any precipitation, and the characterization data show no complex formation discussed latterly. Good quality single crystals for L and the 1:2 stoichiometric complex adduct with lithium perchlorate salt were obtained, whereas the crystals of 1:2 adduct with LiCl obtained after several trials were not suitable for diffraction. The single crystal X-ray analysis of both

to the same solution show bands of control experiments with lithium ions (Figure S7). Crystal Structure Analysis. To evaluate the lithium binding affinity of the macrocyclic ionophore, we carried out reactions of L with lithium salts (LiCl and LiClO4) following a stoichiometric ratio of 1:2 in a chloroform and methanol mixture which shows precipitation with LiCl immediately and partial yellow precipitation with LiClO4 after some time 7212

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Figure 4. FT-IR spectra of L, [Li2L(ClO4)(OH2)]ClO4, L-LiCl (1:2) adduct, and L-mixed salts.

intermolecular hydrogen bonding (O−H···O interaction) with the ketone oxygen of the neighboring molecule to form a center of a symmetric cyclic dimer (Figure 3c, Table S4). Therefore, two distinct π−π stacking interactions within the hydrogen-bonded cyclic dimer and interdimer offset stacking were observed with centroid to centroid distances of 4.09 and 4.03 Å, respectively. These observations also further justify the claim of excimer formation in ligand fluorescence spectra which does not entirely vanish even after the addition of Li+ ion. As the 1:2 L-LiCl adduct failed in crystallization, to understand the binding of LiCl salt as an ion pair or not, a 1:1 reaction adduct was crystallized to give a good quality crystal which shows that LiCl binds as an ion pair in one of the pockets of L to form the [LiL(Cl)] complex (Figure 3d). It is also observed that the macrocyclic ligand having two similar binding pockets shows similar conformational changes around the bridging arms with only one lithium ion binding (1:1) as compared to the 1:2 complexations. Therefore, it is anticipated that the conformational changes occur with first lithium-ion incorporation that could facilitate lithium ion binding at the second site without any additional conformational changes and hence a positive cooperative effect is desirable, which also supports the 1:2 binding model from the UV−vis titration experiments. FT-IR Analysis. The selective binding of L toward the Li+ ion was also examined by FT-IR spectroscopy in the solid state using KBr pellets (Figure 4). The L shows a characteristic carbonyl stretching vibration band at 1671 cm−1 which shifted to a lower wavenumber of 1644 cm−1 in [Li2L(ClO4)(OH2)]ClO4 and 1630 cm−1 in the L-LiCl (1:2) adduct. These shifts to lower wavenumbers confirm coordination of the carbonyl oxygen of ligand L in both the products. [Li2L(ClO4)(OH2)]ClO4 also shows characteristic vibration bands for perchlorate anion at 621 cm−1 for the free perchlorate anion and a split band at 1085 and 1112 cm−1 for the coordinating perchlorate anion.51 In contrast, the reaction mixture obtained from the reaction of L with other alkali and alkaline earth metal ions (mixed salt) of oceanic presence does not show any change in

ligand L (Figure 2a) and perchlorate salt complex (Figure 2b) reveals that two lithium ions are encapsulated at the two coordinating pockets of L in a distorted square pyramidal geometry to form the [Li2L(ClO4)(OH2)]ClO4 complex (Figure 2b and Table 1). The two ethereal oxygens [Li−O range: 1.990(7)−2.023(8) Å] and two bridging keto carbonyl oxygens [Li−O range: 1.971(7)−2.005(8) Å] of each compartment provide the square base, whereas perchlorate oxygen O9 [Li−O9 = 1.981(8) Å] and water oxygen O17 [Li−O19 = 1.896(8) Å] bind Li1 and Li2 axially to complete the fifth coordination sites of a square pyramid. One lithium perchlorate salt is bound as an ion pair, whereas another is separated by the coordinating water molecule. Both the lithium ions are above the square base, giving rise to a distorted square pyramidal geometry as evident from the very low structural index parameter τ (0.07) for both lithium ions.47 It is observed that the bridging arms of the ligand undergo conformational changes to facilitate lithium-ion incorporation, showing a binding affinity toward the Li ions. This is not the case where just the metal ion fits in the cavity without any ligand reorganization. Overall, marginal changes in bond lengths are observed after the complexation with the lithium ions (Table S3). Bond lengths of the keto groups in both L and lithium complex did not show any drastic change but displayed a shorter bond length compared to the enol form of anthraquinone.48 As far as we know, these kinds of keto oxygen bridged metal complexes are very rare49 and never observed in two anthraquinone motifs in the macrocyclic ligand L coined the ideal pocket in favor of the keto bridging mediated 1:2 complexation as compared to mononuclear complexation with one anthraquinone motif per macrocycle.48 The packing viewed down c-axis for the ligand crystals revealed the anthraquinone rings make an offset stacking with a centroid to centroid distance of the different rings of anthraquinone (3.54−3.85 Å) and between the whole anthraquinone moiety (4.78 Å) falls well within the range of the stacking limit (Figure 3a,b).50 In the case of the Li complex, the coordinated water molecule is involved in strong 7213

DOI: 10.1021/acs.inorgchem.9b00135 Inorg. Chem. 2019, 58, 7209−7219

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Inorganic Chemistry the ligand spectrum. These results validate a selective lithiumion binding of L in the solid state. NMR Studies. The formation of the complex [Li2L(ClO4)(OH2)]ClO4 in solution was also evaluated by 7Li NMR titration by using coaxial NMR tubes52 where a standard solution of LiClO4 (0.1 M) in CDCl3/CH3OD (3:2, v/v) was taken in the inner tube as an external chemical shift reference standard and samples were taken in the outer tube in the same solvent system. It has been observed that, without L (Li+:L = 2:0; outer sample tube having 0.001 M lithium salt), a single magnetic resonance peak was observed for the reference and the sample which was set to 0 ppm for free 7Li+ ions (Figure 5a). After addition of the first equivalent of the ligand (Li+:L =

firm upfield shifting was observed with respect to the reference peak, which may be attributed to the shielding effect of the coordinating donor atom electron density of L around the lithium metal ion. Though the exact cause of the deshielding effect for the little downfield shifting is not very clear, it is anticipated that binding of the carbonyl group may be responsible for this small deshielding effect. However, based on these 7Li NMR observations which show an increase in intensity of peaks with the increasing concentration of L and saturation of chemical shifts near the stoichiometric ratio of 2:1 for Li+:L and indicates two lithium-ion binding events by the ligand. Similarly, the 1H NMR measurements of ionophore L (0.012 M), in the presence of various relevant metal ions (MClO4, 0.025 M) in CDCl3/CD3OD (3:2, v/v) solution, were performed to know the selectivity. Comparison of the spectra reveals that, upon exposure to Li+ only, all of the related ionophore (L) proton resonances underwent a small downfield shift (Figure 6, Figure S8, Table S5), whereas, with

Figure 5. (a) 7Li NMR titration spectra of LiClO4 with L in CDCl3CD3OD (3:2) with a constant lithium-ion concentration (0.001 M). (b) 7Li NMR titration spectra of LiCl with L in CDCl3-CD3OD (3:2) with a constant lithium-ion concentration (0.001 M). LiClO4 (0.1 M) in CDCl3-CD3OD solution was used as an external chemical shift reference standard in the coaxial tube with the lithium signal set at 0 ppm.

Figure 6. Partial 1H NMR spectra (500 MHz, 3:2 CDCl3/CD3OD, 298 K) of L in the presence of various metal ions showing the shifting of ionophore proton resonances with Li+ ions only with TMS as an internal standard set at 0 ppm.

other metal ions, no shift was observed with respect to the TMS peak set to 0 ppm. These results also validate a selective lithium-ion binding of L in the solution. Mass Analysis. To further confirm the Li+ binding in solution, the mass spectral analysis of all compounds was carried out (Figures S9−S14). The acetonitrile solution of the LiCl (1:2) reaction adduct shows three peaks at m/z 609.52 for [Li2(L)(Cl)]+, 287.26 for [Li2(L)]2+, and 567.50 for [Li(L)]+ (Figure S11), whereas the 7Li NMR titration solution with LiCl shows an additional peak at m/z 651.13 for [Li2(L)(Cl2) + Li+]+ (Figure 7 and Figure S12). On the basis of these observations, a 1:2 complex product [Li2L(Cl)2] similar to perchlorate salt is anticipated. Similarly, the mass spectrum of the [Li2L(ClO4)(OH2)]ClO4 solution shows a peak at m/z 673.13 for [Li2(L)(ClO4)]+ and at 567.16 for [Li(L)]+ (Figure S13). The mass spectrum analysis from the reaction mixture of ligand and salt mixtures shows peaks similar to that of the ligand (Figure S14) which supports no binding affinity toward other metal ions of oceanic presence.

2:0.25), a new magnetic resonance peak was observed for the sample at −0.33 ppm with respect to the reference peak at 0 ppm. For the subsequent experiments, a net upfield shifting was observed with respect to the reference peak and a small downfield shift was observed from −0.33 to −0.22 with varying stoichiometric ratio of the ligand. Similar 7 Li NMR observations were also made with LiCl, where a single resonance peak was observed without L, and a new peak was appeared at −0.36 ppm with the first equivalent addition of L (Li+:L = 2:0.25) with respect to the reference peak at 0 ppm (Figure 5b). For the subsequent experiments, with varying the stoichiometric ratio of ligand, a small downfield shift was observed from −0.36 to −0.28 ppm but with a net upfield shift with respect to the reference peak at 0 ppm. Initially, both the reference and sample (0.001 M Li+ salt) show a single resonance peak at 0 ppm, but with addition of L, the consistent 7214

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nitric acid before the analysis by ICP-OES measurements. Control experiments with the solvent alone extractions were done to know the uptake of salts which was subtracted to calculate the ligand assisted metal-ion uptake. In the presence of lithium salt alone, the ligand loading percentage was estimated to be 70.06% and 67.38% for perchlorate and chloride salts, respectively (Tables S6 and S7). The loading percentage here is defined as the molar percentage of ligand binding to the lithium ions during the exaction experiment. The higher loading percentage with perchlorate salts might be due to the higher hydrophobicity of perchlorate anion as compared to chloride anion. In the case of competitive experiments with a 50 times molar excess of salt mixtures, the loading level of LiCl was found to be 69.16%, whereas, with lower salt concentrations (5 times molar excess), the loading level of LiCl drastically falls to be only 4.16%. In contrast, the loading levels of other relevant salts were found to be less than 1% in both the concentrations except calcium, which shows a mere 4.09% with a higher concentration of salt mixtures. These findings demonstrate the selectivity of the receptor toward lithium ions and supports both solid-state and solution studies. To understand the counterion effect on the extraction process, SLE was repeated at two other different time scales (1 and 6 h) as shown in Figure 8b and tabulated in Table S8. It is observed that the lithium extraction proceeds faster with LiCl but remains almost constant with the time, whereas, with LiClO4, its extraction increases with time and a maximum loading percentage of 135% was observed after 6 h. Since the ligand can bind two lithium ions, the receptor bound percentage reduced to half (67.5%). Liquid-Liquid Extractions (LLE). To know whether the macrocyclic ionophore L is capable of extracting LiCl in liquidliquid extractions (LLE) from the brine system, similar extraction experiments as SLE were carried out. A nitrobenzene solution of L (5 mL of 5 mM) was exposed to the 5 mL aqueous solution of LiCl with varying concentrations of 1 M, 0.5 M, and 10 ppm level for 3 h in a vibrating mixture. The organic phase after the extraction was back-extracted with 0.2 M sulfuric acid aqueous solution, followed by dilution with 2% nitric acid and analyzed by ICP-OES measurements to know the lithium-ion concentration. Control experiments with the

Figure 7. ESI-MS spectrum (partial) of the solution from the 7Li NMR titration experiment with LiCl showing [Li2L(Cl)2] complex formation.

Solid-Liquid Extractions (SLE). Before going to extraction experiments, to examine the reversible nature of the lithiumion binding process, qualitative absorption studies were done with the complex [Li2L(ClO4)(OH2)]ClO4 which is vital for the metal-ion stripping process. It has been observed that, upon biphasic treatment of 0.2 M H2SO4 aqueous solution with the complex solution in chloroform, followed by agitation for a few minutes, the 393 nm peak for the complex reverts back to 376 nm observed for the ligand due to the decomplexation reaction (Figure 8). On the basis of the above findings to evaluate the extraction ability of the macrocyclic ligand L toward lithium ions, initially, solid-liquid extractions (SLEs) were carried out with neat LiClO4, LiCl and competitive experiments in the presence of other interfering metal ions of oceanic presence. These experiments were done by adding nitrobenzene solutions of ligand L to the solid salt mixtures for 3 h in a vibrating mixture. In each case, the separated organic phase was back-extracted with a 0.2 M sulfuric acid aqueous solution and then further diluted with 2%

Figure 8. (a) UV−vis absorption spectral changes of the [Li2L(ClO4)(OH2)]ClO4 with acid treatment (aq.) showing decomplexation reaction in CHCl3. (b) Comparison of loading percentage of L (20 mM) with time in solid-liquid extractions using nitrobenzene as a solvent. 7215

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Inorganic Chemistry

solvent was removed by rotary evaporator. Then water was added, and the organic layer was extracted with chloroform, washed with water, dried with Na2SO4, and concentrated to give the crude product, which was purified by column chromatography using chloroform−n-hexane to give a pure yellow colored product (1.95 g, 42%). Mw (C34H24O8) = 560.53. mp 210−214 °C. 1H NMR (CDCl3, 500 MHz) δ ppm/TMS: 1H NMR (CDCl3, 500 MHz) δ ppm/TMS: 7.86 (d, 4H, ArH), 7.56 (t, 4H, ArH), 7.31 (d, 4H, ArH), 4.43 (t, 8H, OCH2), 2.44 (m, 4H, CH2). 13C NMR (CDCl3): 183.55 (CO),182.12 (CO), 157.99 (CO), 134.93, 134.03, 125.29, 121.64, 120.57, 70.47 (OCH2), 28.90 (CH2). HRMS (ESI+): m/z calculated for C34H25O8+: 561.1549 [M + H]+; found: 561.1549. Elemental analysis for C34H24O8, calculated: C, 72.85; H, 4.32; Found: C, 72.80; H, 4.40. IR (KBr, cm−1):1671(vs), 1584(vs), 1454(w), 1317(vs), 1050(w), 749(s). Note: This ligand can be synthesized in a one-step reaction by taking 1,8-dihydroxyanthraquinone (2 g, 8.3 mmol), 1,3-dibromopropane (0.85 mL, 8.3 mmol), and K2CO3 (4 g, 28.9 mmol) in dry DMF and refluxing for 7 days. Yield (12%, 0.56 g). Synthesis of [Li2L(ClO4)(OH2)]ClO4. To the chloroform solution of ligand L (0.3 g, 0.53 mmol taken in 10 mL of chloroform) was dropwise added LiClO4 (0.12 g, 1.12 mmol taken in 2 mL of methanol), and the mixture was stirred for 1 h. Slowly, some yellow precipitation was observed. Then the solvent was removed under vacuum and the precipitate was washed with water and diethyl ether and then dried in a desiccator. Finally, the compound was recrystallized from hot acetonitrile to give block crystalline material. Yield = (0.31 g, 74%). Mw = 791.33. MS: (ESI+) m/z; 673.13: [C34H24Cl2Li3O16]+ for [Li2(L)(CLO4)]+; 567.16: [C34H24LiO8]+ for [Li(L)]+. IR (KBr, cm−1): 3432(vs), 1644(s), 1112(s), 1085(s), 621(s). Elemental analysis for C34H26Cl2Li2O17, calculated: C, 51.61; H, 3.31; Found: C, 51.58; H, 3.36. Complexation Reaction with LiCl. The reaction of the macrocyclic ligand with LiCl was performed similarly as the above where, instead of lithium perchlorate, LiCl was taken with a stoichiometric ratio of 1:2. Yellow colored precipitation was observed immediately after 10−15 min and stirred for 1 h and filtered. The precipitate was washed with water and diethyl ether and then dried in a desiccator and characterized to be [Li2(L)(Cl2)]. MS: (ESI+) m/z; 609.52: [C34H24ClLi2O8]+ for [Li2(L)(Cl)]+, 287.26: [C34H24Li2O8]2+ for [Li2(L)]2+; 567.50: [C34H24LiO8]+ for [Li(L)]+. IR (KBr, cm−1): 1630(vs), 1255(s), 748(s). Elemental analysis for C34H24Cl2Li2O8, calculated: C, 63.28; H, 3.75; Found: C, 63.22; H, 3.78. A similar reaction with a stoichiometric ratio of 1:1 (L:LiCl) gives complex product [Li(L)(Cl)] which was characterized by X-ray crystallography. Elemental analysis for C34H24ClLiO8, calculated: C, 67.73; H, 4.01; Found: C, 67.69; H, 4.04. Complexation Reaction with All Metal Ions Excluding Lithium Ion. To the chloroform solution of ligand L (0.3 g, 0.53 mmol taken in 10 mL of chloroform) was dropwise added a methanolic solution of a salt mixture of NaClO4 (0.12 g, 1.12 mmol), KClO4 (0.15 g, 1.12 mmol), Ca(ClO4)2 (0.26 g, 1.12 mmol), Mg(ClO4)2 (0.25 g, 1.12 mmol), and SrCl2 (0.17 g, 1.12 mmol), and the resultant mixture was stirred for 1 h. No precipitation was observed, and the solvent was removed under vacuum. The solid was washed with water, and diethyl ether, and then dried in a desiccator. This material was characterized to be ligand L, confirmed from the mass spectrum, NMR, and FT-IR. 7 Li NMR Titration Experiments. In a typical experiment, the concentration of the appropriate lithium salts (LiClO4 or LiCl) was kept constant, while the concentration of the ligand L was varied. In this experiment, LiClO4 (0.1 M) was taken in a CDCl3/CH3OD (3:2) solvent mixture as an internal chemical shift reference standard in the coaxial inner tube. For the first sample (Li+:L = 2:0), both inner and outer sample tubes were having only lithium salt solutions (0.001 M, outer tube). Stock solutions of both L and metal salts were prepared separately in a CDCl3/CH3OD (3:2) solvent mixture. For subsequent experiments, eight NMR tubes were taken with sample solutions of fixed metal ion concentration (0.001 M) but with varying

solvent alone extraction were done to know the uptake of salts which was subtracted to calculate the ligand assisted metal-ion uptake. The extraction results show that the loading percentage of L was estimated to be 25.63% with 1 M LiCl solution which falls to 12.91% with 0.5 M and no extraction was observed with 10 ppm LiCl concentration (Tables S9 and S10). Under similar conditions, the loading percentage of L was estimated to be 48.57% with 1 M LiClO4 solution which falls to 25.92% with 0.5 M, and interestingly, a 2.07% was observed with 10 ppm LiClO4 source phase concentration. Similar competitive extraction experiments in the brine systems with other interfering metal ions of oceanic presence show the lithium loading percentage of 27.06% with 1 M and 7.92% with 0.5 M LiCl concentration. It is also observed that, in the presence of the ligand, the uptake of other metal ions falls below the solvent alone uptake values. These results show the selectivity of the ligand toward the LiCl salt. It should be noted that there are only a few synthetic receptors known in the literature which can extract LiCl from water considering the high enthalpy of hydration. These early results with 1 M LiCl source phase concentration are impressive and competitive with literature reports21 but need further improvement through modifications of the ligand to increase the hydrophobicity and binding affinity to overcome the hydration energy of lithium ions. Overall, the new ligand provides a further scope to develop new multinucleating lithium selective ionophores with high lithium loading capacity for its potential applications as an extractant.

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MATERIALS AND METHODS

All the chemicals were purchased from Sigma-Aldrich. Electronic spectra were recorded using a Shimadzu UV 3101PC spectrophotometer. Mass-spectrometric analyses were performed by using the ESI technique on a Q-TOF Micro TM LC-MS instrument. FT-IR spectra were taken using KBR disks on a PerkinElmer GX spectrophotometer. Elemental analyses (C, H, and N) were measured on a 2400 PerkinElmer elemental analyzer. 1H and 13C NMR spectra were recorded on a Bruker Avance 500 and 600 MHz JEOL FT-NMR spectrometer. An Edinburgh Instruments model Xe-900 was used to measure emission spectra and all the spectra reported after applying the emission corrections. ICP-OES analyses were done using a PerkinElmer instrument (Optima 2000DV). Synthesis of 1,8-Bis(3-bromopropoxy) Anthracene-9,10dione. To a 100 mL round-bottom flask were added 1,3dibromopropane (40 mL, 392 mmol), 1,8-dihydroxyanthraquinone (4 g, 16.6 mmol), and anhydrous potassium carbonate (14 g, 101.3 mmol), and the mixture was stirred at 100 °C under an argon atmosphere. After 72 h, the reaction mixture was cooled, filtered, and washed with hexane (50 mL). The residue was extracted three times with CHCl3, and the solvent was removed under a rotary evaporator to give the crude product which was purified by column chromatography using ethyl acetate−hexane to give a dark yellow colored product (4.8 g, 60%). Mw = 482.16. 1H NMR (CDCl3, 500 MHz) δ ppm/TMS: 7.84 (d, 2H, ArH), 7.62 (t, 2H, ArH), 7.30 (d, 2H, ArH), 4.27 (t, 4H, OCH2), 3.84 (t, 4H, BrCH2), 2.43 (m, 4H, CH2). 13C NMR (CDCl3): 184.07 (CO), 182.31 (CO),158.59, 133.94, 124.65, 119.77, 119.53, 67.05 (OCH2), 32.43, (CH2Br), 29.81 (CH2). Elemental analysis for C20H18Br2O4, calculated: C, 49.82; H, 3.76; Found: C, 49.78; H, 3.81. Synthesis of Macrocyclic Ionophore L. 1,8-Dihydroxyanthraquinone (2 g, 8.3 mmol) was added to 40 mL of dry acetonitrile in a round-bottom flask, followed by anhydrous potassium carbonate (8 g, 57.8 mmol), and refluxed under an argon atmosphere for 3 h. The above mixture cooled down to room temperature, and 1,8-bis(3bromopropoxy) anthracene-9,10-dione (4 g, 8.3 mmol) was added to it and refluxed. After 72 h, the reaction mixture was cooled, and the 7216

DOI: 10.1021/acs.inorgchem.9b00135 Inorg. Chem. 2019, 58, 7209−7219

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Inorganic Chemistry stoichiometric ratio of ligand concentration which was equilibrated for 2 h before the acquisition of the NMR spectrum. Each time, the same inner tube having the same standard solution was inserted into the outer sample tube after cleaning the outer surface properly. Solid-Liquid Extraction (SLE) Studies. 2 mL of receptor solution in nitrobenzene (20 mM) was added to the solid LiClO4 or LiCl (50 times molar excess) and salt mixtures (50 or 5 times molar excess) and agitated for 3 h in a vibrating mixture and filtered. Then, 0.5 mL of the resulting filtrate was removed and placed into new vials. Then each of the above nitrobenzene solution containing vials was treated with 1 mL of 0.2 M sulfuric acid. The vials were shaken for 5 min and allowed to stand for 6−8 h. The aqueous phase was removed carefully and diluted with 2% HNO3 to make up the volume up to 5 mL and analyzed by ICP-OES measurements. Control experiments with nitrobenzene alone were carried out in parallel. All reported concentrations are an average of results in duplication after dilution. The loading percentage here is defined as the molar percentage of ligand, containing the lithium ions after the exaction experiment which can be calculated by employing the formula

lithium loading capacity of 135% with LiClO4 and 69.16% with LiCl salt. Under LLE conditions, this ionophore shows a loading capacity of 27% with 1 M LiCl and 48.57% with 1 M LiClO4 source phase concentration. But these early results with this ionophore are competitive with reported ionophores and suggest necessary modification around the peripheral carbonyl groups or the spacer linker between the two anthraquinone motif to enhance the binding efficiency to overcome the hydration energy. Overall, the new ligand provides a further scope to develop new multinucleating lithium selective receptors with high lithium loading capacity for its potential applications as an extractant.

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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.9b00135.

Loading % = no. of moles of metal ion extracted/no. of moles

Figures S1−S14 and Tables S1−S5 (PDF)

of receptor used × 100 Liquid-Liquid Extraction (LLE) Studies. The extraction procedure was followed similar to the SLE method where 5 mL of receptor solution in nitrobenzene (5 mM) was exposed to the 5 mL LiCl aqueous solution (1 or 0.5 M) or 5 mL mixed metal solution (1 M or 0.5 M) and agitated for 3 h in a vibrating mixture. Then, after equilibrating for 3 h, 2 mL of the nitrobenzene parts was removed and placed into new vials. Then each of the above nitrobenzene parts was treated with 5 mL of 0.2 M sulfuric acid. The vials were shaken for 5 min and allowed to stand for 6−8 h. Then 1 mL of the aqueous phase was removed carefully and diluted with 2% HNO3 to make up the volume up to 10 mL and analyzed by ICP-OES measurements. Control experiments with nitrobenzene alone were carried out in parallel. All reported concentrations are an average of results in duplication after dilution. The receptor/ligand loading percentage was calculated by employing the formula

Accession Codes

CCDC 1859399, 1859400, and 1887493 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Eringathodi Suresh: 0000-0002-1934-6832 Alok Ranjan Paital: 0000-0002-9883-2299

Loading % = no. of moles of metal ion extracted/no. of moles

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AUTHOR INFORMATION

of receptor used × 100

Notes

The authors declare no competing financial interest.

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CONCLUSIONS In conclusion, we report here a rare example of a lithium-ion selective mutinucleating ionophore for selective lithium-ion recognition among various dominant alkali and alkaline earth metal ions of oceanic presence by both solid-state (single crystal X-ray crystallography, FT-IR) and solution studies (electronic absorption spectroscopy, fluorescence, 7Li NMR titrations, and mass spectroscopy). All the above analyses demonstrate the selectivity of the macrocyclic ligand toward lithium ions and stoichiometric 1:2 complex formation. We also provide structural evidence toward 1:2 complex formations through a very rare keto bridging group with perchlorate salts. The crystal structure also reveals the existence of π−π stacking interactions in both the ligand and the lithium complex supported by fluorescence excimer emission. Though crystallization of a 1:2 complex [Li2LCl2] with LiCl was unsuccessful, the 7Li NMR and mass spectral analyses confirm the formation of the above with a 1:2 stoichiometry. A 1:1 stoichiometric reaction adduct with LiCl crystallizes well to form the [LiLCl] complex whose structure shows that LiCl binds as an ion pair. This ionophore shows exclusive selectivity toward lithium ions under both solid-liquid and liquid-liquid extraction conditions with high loading capacity. Under SLE conditions, this ionophore shows a high

ACKNOWLEDGMENTS The authors acknowledge the Department of Science and Technology (DST), India (SB/S1/IC-12/2013 and EMR/ 2016/002135), for financial support. The authors also acknowledge the Analytical Division and Centralized Instrumental Facility of CSIR-CSMCRI for materials characterization. H.G. is also thankful to AcSIR for providing the Ph.D. degree. CSIR-CSMCRI communication no. 158/2018.

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REFERENCES

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