Sulfate-Incarcerating Nanojars: Solution and Solid-State Studies

Article pubs.acs.org/IC

Sulfate-Incarcerating Nanojars: Solution and Solid-State Studies, Sulfate Extraction from Water, and Anion Exchange with Carbonate Basil M. Ahmed, Christian K. Hartman, and Gellert Mezei* Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, United States S Supporting Information *

ABSTRACT: A series of 9 homologous sulfate-incarcerating nanojars [SO4⊂{Cu(OH)(pz)}n]2− (Cun; n = 27−33; pz = pyrazolate), based on combinations of three [Cu(OH)(pz)]x rings (x = 6−14, except 11)namely, 6 + 12 + 9 (Cu27), 6 + 12 + 10 (Cu28), 8 + 13 + 8 (Cu29), 7 + 13 + 9 (Cu29), 8 + 14 + 8 (Cu30), 7 + 14 + 9 (Cu30), 8 + 14 + 9 (Cu31), 8 + 14 + 10 (Cu32), and 9 + 14 + 10 (Cu33)has been obtained and characterized by electrospray-ionization mass spectrometry (ESI-MS), variable-temperature 1H NMR spectroscopy, and thermogravimetry. The X-ray crystal structure of Cu29 (8 + 13 + 8) is described. Cu32 and Cu33, which are the largest nanojars in this series, are observed for the first time. Despite extensive overlap at a given temperature, monitoring the temperaturedependent variation of paramagnetically shifted pyrazole and OH proton signals in 60 different 1H NMR spectra over a temperature range of 25−150 °C and a chemical shift range from 41 ppm to −59 ppm permits the assignment of individual protons in six different sulfate nanojars in a mixture. As opposed to ESIMS, which only provides the size of nanojars, 1H NMR offers additional information about their detailed composition. Thus, nanojars such as Cu29 (8 + 13 + 8) and Cu29 (7 + 13 + 9) can easily be differentiated in solution. High-temperature solution studies unveil a significant difference in the thermal stability of nanojars of different sizes obtained under kinetic control at ambient temperature, and aid in predicting the structure of the Cu33 nanojar, as well as in explaining the absence of the Cu11 ring from the Cu6−Cu14 series. Anion exchange studies using sulfate and carbonate reveal that, although each anion is thermodynamically preferred by a nanojar of a certain size, the exchange of an already incarcerated anion is hampered by a substantial kinetic barrier. The remarkably strong binding of anions by nanojars allows for the extraction of highly hydrophilic anions, such as sulfate and carbonate, from water into organic solvents, despite their very large hydration energies.

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Extraction of the SO42− anion from aqueous media has been of great interest in recent years, primarily fueled by the need for selective separation of sulfate from nuclear wastes7,8 and seawater used for off-shore drilling.9 Sulfate recognition and separation systems have recently been reviewed10−13 and continue to generate sustained interest.14−28 Most notably, water-soluble sulfate receptors with binding constants (Ka) as high as 8.6 × 109 in highly competitive, pure H2O29,30a value that is 3 orders of magnitude larger than that of the natural sulfate-binding protein (Ka = 8.3 × 106)31have been developed.32 Moreover, selective sulfate separation from water as sulfate salts of organic ligands with extremely low solubility (Ksp = 2.4 × 10−10), on a par with BaSO4 (Ksp = 1.08 × 10−10 at 25 °C),33 has been achieved.34 A few years ago, we reported on the selective binding of the sulfate anion by neutral nanojars in the presence of nitrate and perchlorate anions in large excess, and we provided crystallographic descriptions of two (Bu4N)2[SO4⊂{Cu(μ-OH)(μ-

INTRODUCTION Over the past few years, the peculiar family of copper hydroxide pyrazolate complexes of the formula [anion⊂CuII(μ-OH)(μpz)n] (where pz = pyrazolate anion; n = 27−31), known as nanojars (Scheme 1), has grown from initially crystallographic curiosities1 to a robust anion-extraction system.2−6 Indeed, nanojars are capable of transferring even the most hydrophilic anions, such as CO32−, from water into aliphatic solvents.5 Exclusive to copper and pyrazolate, nanojars self-assemble around anions with large hydration energies and incarcerate them inside a hydrophobic outer shell with unprecedented strength, so that an aqueous Ba2+ solution is unable to precipitate the highly insoluble barium salts of those anions.2,3,5 These unique constructs also display an extraordinary resistance to extreme alkalinities of pH >14, and they can be employed to extract carbonate from a 10 M aqueous NaOH solution.6 Yet, nanojars can easily be disassembled under slightly acidic conditions, leading to the release of the captured anion. The reversible, pH-dependent assembly/disassembly6 of nanojars could therefore be exploited for the extraction and recycling of harmful anions, such as chromate and arsenate.3 © XXXX American Chemical Society

Received: August 6, 2016

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DOI: 10.1021/acs.inorgchem.6b01909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Information and ref 4 for examples) (see Figure 1A). If the reaction is carried out using copper(II) sulfate, pyrazole,

Scheme 1. Schematic Representation of the Cu29 Nanojar [SO4⊂{Cu(μ-OH)(μ-pz)}8+13+8]2− Compounda

a For the sake of clarity, only one of the two Cu8 rings is shown; see Figure 6 for full structure.

pz)}n] (n = 28 and 31) nanojars.2 More recently, we provided a mass spectrometric characterization of nanojar solutions, and we described two other sulfate nanojars (n = 27 and 29).4 Herein, we report the discovery of yet three other novel sulfate nanojar species, with n = 30, 32, and 33; the latter two represent the largest nanojars obtained to date. The effect of temperature, the amount of Bu4N+ counterion, and the amount of different Pb2+ salts on nanojar yield and size distribution is described. For the first time, we provide a detailed, variabletemperature 1H NMR characterization of six sulfate-incarcerating nanojars in solution, and we discuss the varying effect of the size of the copper pyrazolate ring on the position and temperature dependence of their paramagnetically shifted proton signals in the corresponding NMR spectra. The X-ray crystal structure of (Bu4N)2[SO4⊂{Cu(μ-OH)(μ-pz)}8+13+8] is reported. Thermostability studies in solution and the solid state, including thermogravimetric analysis, are presented. Lastly, anion exchange studies, carbonate vs sulfate binding by nanojars, and sulfate extraction from water into organic solvents are discussed.

Figure 1. ESI-MS(−) spectra (in CH3CN) of the sulfate-incarcerating nanojars: (A) [SO4⊂{Cu(OH)(pz)}n]2− (n = 27−33) from Cu(NO3)2, pyrazole, Bu4NOH and (Bu4N)2SO4 (molar ratio of 29:29:58:1) in THF; (B) from CuSO4, pyrazole, NaOH, and Bu4NOH (molar ratio of 29:29:56:2) in THF, (C) the fraction crystallized out of toluene, and (D) the fraction soluble in toluene.

NaOH, and Bu4NOH (molar ratio of 29:29:56:2) in THF, a slightly different distribution of nanojars is obtained (method (b)), with larger amounts of Cu28SO4 and negligible amounts of Cu30SO4 and Cu33SO4 (Figure 1B). The different outcome in the latter case is attributed to the insolubility of the copper(II) sulfate and NaOH reagents in THF, which also results in a longer time (three days) being needed to complete the reaction. Sulfate-incarcerating nanojars of different sizes have different solubility in toluene. Although a mixture of nanojars is initially completely soluble in toluene, the smaller nanojars slowly crystallize out of the solution. Thus, when (Bu4N)2[SO4⊂{Cu(OH)(pz)}n] (obtained using method (b)) is dissolved in toluene, a clear, deep-blue solution is obtained first. Within minutes, a dark-blue powder starts settling out, which after filtration and rinsing with toluene is identified by ESI-MS(−) as Cu28SO4, with small amounts of Cu27SO4 (Figure 1C). The filtrate, in turn, contains the larger Cu29SO4 and Cu31SO4 nanojars, with small amounts of Cu27SO4, Cu28SO4, and Cu32SO4 (Figure 1D). In addition, a sample of Cu31SO4 is also obtained by using Pb(NO3)2 as an additive (see the section entitled “Effect of Lead Salts on Sulfate Nanojar Size and Purity” below). Effect of the Amount of Bu4NOH on the Size Distribution and Yield of Sulfate−Nanojars. As discussed above, replacing most of the Bu4NOH base by NaOH (just enough Bu4NOH to provide two Bu4N+ counterions per nanojar) leads to a different nanojar distribution. To further

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RESULTS AND DISCUSSION Synthesis and Fractionation of a (Bu4N)2[SO4⊂{Cu(OH)(pz)}n] (n = 27−33) Nanojar Mixture. Upon mixing a solution of copper(II) nitrate and pyrazole in tetrahydrofuran (THF) with a solution of Bu4NOH and (Bu4N)2SO4 in THF (29:29:58:1 molar ratio), a deep-blue, clear solution is instantly obtained, which produces a blue precipitate in quantitative yield upon the addition of excess water (method (a)). Electrospray ionization mass spectrometry, ESI-MS(−), identifies the product as a mixture of sulfate-incarcerating nanojars, [SO4⊂{Cu(OH)(pz)}n]2− (CunSO4, n = 27−33; n = 27, m/z 2040.0; n = 28, m/z 2113.4; n = 29, m/z 2187.9; n = 30, m/z 2261.4; n = 31, m/z 2334.9; n = 32, m/z 2409.9; n = 33, m/z 2483.3; m/z values represent the mono-isotopic mass of the most abundant peak within the isotope pattern; all observed values match calculated values (see Figure S1 in the Supporting B

DOI: 10.1021/acs.inorgchem.6b01909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry assess whether the amount of available Bu4N+ counterions has an effect on the size distribution of the resulting nanojars, a pair of experiments was carried out, wherein Bu4NOH was used either in substoichiometric amount, or in excess. Based on ESIMS(−) analysis of the nanojar mixtures obtained under otherwise identical reaction conditions, it becomes evident that, in the latter case, the Cu27, Cu28, and Cu29 nanojars are considerably more abundant, relative to the Cu31 nanojar, than in the former case (see Figure S2 in the Supporting Information). Regardless of their size, all nanojars require two singly charged Bu 4 N + counterions. However, whereas (Bu4N)2[SO4⊂{Cu(OH)(pz)}31] requires 2.0 equiv of Bu4N+ for 31 equiv of copper, (Bu4N)2[SO4⊂{Cu(OH)(pz)}27] requires 2.3 equiv of Bu4N+ for 31 equiv of copper. If Bu4NOH is completely left out of the reaction (only NaOH is used as a base), the resulting sulfate nanojar mixture is impurified with significant amounts of carbonate nanojars, CunCO3 (see Figure S3A in the Supporting Information), and the yield decreases from ∼81% to ∼21%. Although even highpurity, fresh, commercially available NaOH and Bu4NOH contain small amounts of carbonate (up to 0.5% in NaOH, unspecified amount in Bu4NOH), the sulfate nanojar mixture is either free of carbonate nanojars (when obtained via method (b)) or contains negligible amounts of carbonate nanojars, when obtained via method (a). Furthermore, if NaOH is replaced by KOH (which contains up to 2% carbonate) in method (b), an approximately equal mixture of the carbonateand sulfate-incarcerating nanojars is obtained (see Figure 2A).

Effect of Lead Salts on Sulfate−Nanojar Size and Purity. Interestingly, if PbSO4 is added to the reaction of CuSO4, pyrazole, KOH (containing K2CO3), and Bu4NOH (29:29:29:56:2 molar ratio), a pure sulfate nanojar mixture is obtained, with absolutely no traces of carbonate (Figure 2B). Yet, when excess PbSO4 (100-fold) is stirred with carbonate nanojars, (Bu4N)2[CO3⊂{Cu(μ-OH)(μ-pz)}n] (n = 27, 29− 31) in wet THF (similar to the self-assembly reaction conditions), no reaction occurs and the carbonate nanojars are recovered intact after stirring for 3 days. We conclude that, during the nanojar self-assembly reaction, PbSO4 (Ksp = 2.53 × 10−8 in H2O at 25 °C)33 sequesters all carbonate before it is incarcerated by nanojars, and forms almost one million times less-soluble PbCO3 (Ksp = 7.40 × 10−14 in H2O at 25 °C).33 In the reaction of CuCO3·Cu(OH)2, PbSO4, pyrazole, NaOH, and Bu4NOH (29:29:58:54:4 molar ratio), however, PbSO4 is unable to sequester all carbonate, and a mixture of sulfate- and carbonate-incarcerating nanojars is obtained (see the section entitled “Anion Exchange Studies: Carbonate- vs SulfateBinding by Nanojars” below). This result can be attributed to the much lower solubility of CuCO3·Cu(OH)2 (Ksp = 7.08 × 10−9) compared to Na2CO3 or (Bu4N)2CO3, as well as to the intimate proximity of carbonate to copper in the former compound. A very different result is obtained when Pb(NO3)2 is used instead of PbSO4 during nanojar self-assembly. The reaction of CuSO4, pyrazole, NaOH, Bu4NOH, and Pb(NO3)2 in a 31:31:60:2:31 molar ratio results in only one single nanojar: (Bu4N)2[SO4⊂{Cu(OH)(pz)}31] (see Figure 2C). Here, Pb(NO3)2 plays a dual role: first, it acts as a carbonate scavenger, and second, it provides acidity. Because of hydrolysis, Pb2+(aq) is a weak acid (pKa = 7.6).35,36 Whereas PbSO4 is very poorly soluble (4.4 mg/100 mL H2O at 25 °C)37 and its saturated aqueous solution is practically neutral, Pb(NO3)2 is highly soluble (59.7 g/100 mL H2O at 25 °C)37 and its aqueous solution is acidic. In a slightly acidic medium, the smaller species of [SO4⊂{Cu(OH)(pz)}n]2− (n = 27−30) are not stable and transform to Cu31SO4. Isolation of the product by pouring a THF solution in water leads to the precipitation of Cu31SO4, and small amounts of Cu32SO4. (Bu4N)2[SO4⊂{Cu(OH)(4-Mepz)}31] was obtained similarly, using 4-methylpyrazole instead of pyrazole. Note that, as Pb(NO3)2 dissolves during the reaction, the Pb2+ ions react with the excess sulfate and precipitate as PbSO4, eliminating any free Pb(NO3)2 from the final mixture. When stirred with Pb(NO3)2 in THF, all nanojars, including Cu31SO4, break down completely to the trinuclear complex [Cu3(μ-OH)(μpz)3(NO3)3]− (as shown by ESI-MS). To prove that the observed effect is due to Pb2+, the experiment was repeated with Ba(NO3)2 (the pKa of Ba2+(aq) is equal to 13.4)35,36 instead of Pb(NO3)2: all nanojars were recovered unchanged after 15 days of stirring under identical conditions. PbSO4, even in large excess (100 equiv per nanojar), is also unable to break down any of the nanojars, and, despite its solubility (Ksp = 2.53 × 10−8) being over 5 orders of magnitude higher than that of PbCO3 (Ksp = 7.40 × 10−14), it is unable to exchange carbonate from CunCO3 to sulfate. Variable-Temperature 1H NMR Characterization of Nanojars. The 1H NMR spectrum of the as-synthesized mixture of sulfate-incarcerating nanojars (method (b)) in DMSO-d6 is very complex, as it contains six overlapping sets of peaks corresponding to Cu 6+12+9 SO 4 , Cu 6+12+10 SO 4 , Cu7+13+9SO4, Cu8+13+8SO4, Cu8+14+9SO4, and Cu8+14+10SO4

Figure 2. ESI-MS(−) spectra (in CH3CN) of the reaction product of CuSO4, pyrazole, KOH (with K2CO3), and Bu4NOH (28:28:54:2 molar ratio) in THF (A) in the absence of a lead salt, (B) in the presence of PbSO4, and (C) in the presence of Pb(NO3)2.

If Bu4NOH is left out (only KOH is used as base), more carbonate nanojars than sulfate nanojars are obtained (see Figure S3B in the Supporting Information), and the yield decreases to ∼13%. We conclude that the presence of Bu4N+ is beneficial in forming THF-soluble nanojars, instead of an intractable side-product, assumed to be [Cu(μ-OH)(μ-pz)]∞,2 and that the amount of Bu4NOH employed has an influence on the yield of the reaction, the size distribution of sulfate nanojars, and the amount of carbonate nanojar impurities. C

DOI: 10.1021/acs.inorgchem.6b01909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Variable-temperature 1H NMR spectra of the mixture of six different sulfate-incarcerating nanojars in DMSO-d6, showing pyrazole proton signals in the 21−41 ppm window.

nanojars (Figures 3 and 4). Within each Cux ring (x = 6−14, except 11), all pyrazole and OH groups are magnetically identical, respectively, with the exception of the Cu12-ring, which shows two sets of signals4 for both the pyrazole and the OH protons in the Cu6+12+9SO4 and Cu6+12+10SO4 nanojars. The pyrazole units have two magnetically distinct set of protons: one at the 4-position and one at the 3,5-positions (with 1:2 integrated intensities). Thus, the nanojars described above display 8, 8, 6, 4, 6, and 6 pyrazole proton peaks, respectively, in the 21−41 ppm window, and 4, 4, 4, 3, 3, and 3 OH proton peaks, respectively, in the window between −25 ppm and −59 ppm. At room temperature, the assignment of

each individual peak in this mixture is impossible, because of severe (sometimes complete) overlap. The isolation of three different fractions (Cu27/Cu28, Cu29/Cu31, Cu31/Cu32), combined with variable-temperature measurements over a 25−150 °C range (see Figures S15−S20 in the Supporting Information), permitted the assignment of all 59 peaks (except the one of the OH proton of the Cu14-ring of the Cu8+14+10SO4 nanojar, which is likely overlapped with the corresponding peak of the same size ring of the Cu8+14+9SO4 nanojar). We have previously shown that (i) because of the paramagnetic Cu(II) atoms, the peaks corresponding to pyrazole and OH protons are drastically shifted downfield and upfield, respectively, and (ii) D

DOI: 10.1021/acs.inorgchem.6b01909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Variable-temperature 1H NMR spectra of the mixture of six different sulfate-incarcerating nanojars in DMSO-d6, showing OH proton signals in the window between −25 ppm and −59 ppm.

their chemical shift values are temperature-dependent.4 Fortuitously, the chemical shift of certain peaks displays much larger temperature dependence than those of others. Monitoring the position of each peak in the 60 variabletemperature 1H NMR spectra collected allows the observation of individual peaks that are overlapped at a given temperature, but are clearly separated at a certain different temperature (see Table S1 and Figure S24 in the Supporting Information).

Comparison of the 1H NMR spectra of the Cu6+12+9SO4 nanojar with the corresponding ones of the carbonateencapsulating analogue Cu6+12+9CO3 (ref 4) indicates that, with the exception of the Cu12-ring pyrazole protons, which show a difference of 2σ(I)], 16 060; R(int), 0.0794; data/ parameters/restraints, 23 332/1190/31; goodness-of-fit (on F2): 1.037; R(F) [I > 2σ(I)], 0.0449; Rw(F) [I > 2σ(I)], 0.1099; R(F) (all data), 0.0815; Rw(F) (all data), 0.1311; residual electron density, max/min (e/Å3), 1.973/−1.868.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01909. X-ray crystallographic data for Cu8+13+8SO4 (CIF) ESI-MS, NMR data; thermal ellipsoid plot for Cu8+13+8SO4 (PDF) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation, under Grant No. CHE-1404730. REFERENCES

(1) Mezei, G.; Baran, P.; Raptis, R. G. Anion Encapsulation by Neutral Supramolecular Assemblies of Cyclic CuII Complexes: A Series of Five Polymerization Isomers, [{cis-CuII(μ-OH)(μ-pz)}n], n = 6, 8, 9, 12, and 14. Angew. Chem., Int. Ed. 2004, 43, 574−577. (2) Fernando, I. R.; Surmann, S. A.; Urech, A. A.; Poulsen, A. M.; Mezei, G. Selective Total Encapsulation of the Sulfate Anion by Neutral Nanojars. Chem. Commun. 2012, 48, 6860−6862. (3) Mezei, G. Incarceration of One or Two Phosphate or Arsenate Species within Nanojars, Capped Nanojars and Nanohelicages: Helical Chirality from Two Closely-Spaced, Head-to-Head PO43− or AsO43− ions. Chem. Commun. 2015, 51, 10341−10344. (4) Ahmed, B. M.; Szymczyna, B. R.; Jianrattanasawat, S.; Surmann, S. A.; Mezei, G. Survival of the Fittest Nanojar: Stepwise Breakdown of Polydisperse Cu27−Cu31 Nanojar Mixtures into Monodisperse Cu27(CO3) and Cu31(SO4) Nanojars. Chem.Eur. J. 2016, 22, 5499−5503. (5) Ahmed, B. M.; Calco, B.; Mezei, G. Tuning the Structure and Solubility of Nanojars by Peripheral Ligand Substitution, Leading to Unprecedented Liquid−Liquid Extraction of the Carbonate Ion from Water into Aliphatic Solvents. Dalton Trans. 2016, 45, 8327−8339. (6) Ahmed, B. M.; Mezei, G. From Ordinary to Extraordinary: Insights into the Formation Mechanism and pH-Dependent Assembly/Disassembly of Nanojars. Inorg. Chem. 2016, 55, 7717− 7728. (7) Moyer, B. A.; Delmau, L. H.; Fowler, C. J.; Ruas, A.; Bostick, D. A.; Sessler, J. L.; Katayev, E.; Pantos, G. D.; Llinares, J. M.; Hossain, M. A.; Kang, S. O.; Bowman-James, K. Supramolecular Chemistry Of Environmentally Relevant Anions. Adv. Inorg. Chem. 2006, 59, 175− 204. (8) Moyer, B. A.; Custelcean, R.; Hay, B. P.; Sessler, J. L.; BowmanJames, K.; Day, V. W.; Kang, S.-O. A Case for Molecular Recognition in Nuclear Separations: Sulfate Separation from Nuclear Wastes. Inorg. Chem. 2013, 52, 3473−3490. (9) Bader, M. S. H. Sulfate Removal Technologies for Oil Fields Seawater Injection Operations. J. Pet. Sci. Eng. 2007, 55, 93−110. (10) Ravikumar, I.; Ghosh, P. Recognition and Separation of Sulfate Anions. Chem. Soc. Rev. 2012, 41, 3077−3098. (11) Custelcean, R. Urea-Functionalized Crystalline Capsules for Recognition and Separation of Tetrahedral Oxoanions. Chem. Commun. 2013, 49, 2173−2182. (12) Dutta, R.; Ghosh, P. Recent Developments in Anion Induced Capsular Self-Assemblies. Chem. Commun. 2014, 50, 10538−10554. (13) Elmes, R. B. P.; Jolliffe, K. A. Anion Recognition by Cyclic Peptides. Chem. Commun. 2015, 51, 4951−4968. (14) Delgado-Pinar, E.; Rotger, C.; Costa, A.; Piña, M. N.; Jiménez, H. R.; Alarcón, J.; García-España, E. Grafted Squaramide Monoamine M

DOI: 10.1021/acs.inorgchem.6b01909 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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[(SO4)2(H2O)4]4− Clusters within Highly Insoluble Imine-Linked BisGuanidinium Crystals. Chem. - Eur. J. 2016, 22, 1997−2003. (35) Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976. (36) Hawkes, S. J. All Positive Ions Give Acid Solutions in Water. J. Chem. Educ. 1996, 73, 516−517. (37) Clever, H. L.; Johnston, F. J. The Solubility of Some Sparingly Soluble Lead Salts: An Evaluation of the Solubility in Water and Aqueous Electrolyte Solution. J. Phys. Chem. Ref. Data 1980, 9, 751− 784. (38) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997. (39) Marcus, Y. Ions in Water and Biophysical Implications: From Chaos to Cosmos; Springer: Dordrecht, The Netherlands, 2012. (40) APEX2 v2014.9-0; Bruker AXS, Inc.: Madison, WI, 2014.

Nanoparticles as Simple Systems for Sulfate Recognition in Pure Water. Chem. Commun. 2012, 48, 2609−2611. (15) Custelcean, R.; Williams, N. J.; Seipp, C. A. Aqueous Sulfate Separation by Crystallization of Sulfate−Water Clusters. Angew. Chem., Int. Ed. 2015, 54, 10525−10529. (16) Qin, L.; Hartley, A.; Turner, P.; Elmes, R. B. P.; Jolliffe, K. A. Macrocyclic Squaramides: Anion Receptors with High Sulfate Binding Affinity and Selectivity in Aqueous Media. Chem. Sci. 2016, 7, 4563− 4572. (17) Seipp, C. A.; Williams, N. J.; Bryantsev, V. S.; Custelcean, R.; Moyer, B. A. A Conformationally Persistent Pseudo-Bicyclic Guanidinium for Anion Coordination as Stabilized by Dual Intramolecular Hydrogen Bonds. RSC Adv. 2015, 5, 107266−107269. (18) Mateus, P.; Delgado, R.; André, V.; Duarte, M. T. Sulfate Recognition by a Hexaaza Cryptand Receptor. Org. Biomol. Chem. 2015, 13, 834−842. (19) Gogoi, A.; Das, G. Charge-Assisted Complexation of Anions of Different Dimensionality by Benzimidazole-Based Receptors Bearing −OH Functionality. Cryst. Growth Des. 2012, 12, 4012−4021. (20) Dalapati, S.; Alam, M. A.; Jana, S.; Saha, R.; Biswas, S.; Guchhait, N. A Molecular Lock and Key: “Unlocked−Locked” Conformational Switching of a Receptor by Anions. ChemPlusChem 2012, 77, 93−97. (21) Custelcean, R.; Sloop, F. V., Jr.; Rajbanshi, A.; Wan, S.; Moyer, B. A. Sodium Sulfate Separation from Aqueous Alkaline Solutions via Crystalline Urea-Functionalized Capsules: Thermodynamics and Kinetics of Crystallization. Cryst. Growth Des. 2015, 15, 517−522. (22) Ghosh, T. K.; Dutta, R.; Ghosh, P. Unusual Recognition and Separation of Hydrated Metal Sulfates [M2(μ-SO4)2(H2O)n, M = ZnII, CdII, CoII, MnII] by a Ditopic Receptor. Inorg. Chem. 2016, 55, 3640− 3652. (23) Kim, S. K.; Lee, J.; Williams, N. J.; Lynch, V. M.; Hay, B. P.; Moyer, B. A.; Sessler, J. L. Bipyrrole-Strapped Calix[4]pyrroles: Strong Anion Receptors That Extract the Sulfate Anion. J. Am. Chem. Soc. 2014, 136, 15079−15085. (24) Basu, A.; Das, G. A C3v-Symmetric Tripodal Urea Receptor for Anions and Ion Pairs: Formation of Dimeric Capsular Assemblies of the Receptor during Anion and Ion Pair Coordination. J. Org. Chem. 2014, 79, 2647−2656. (25) Wei, M.; Wu, B.; Zhao, L.; Zhang, H.; Li, S.; Zhao, Y.; Yang, X.J. A Bis-Bisurea Receptor with the R,R-Cyclohexane-1,2-Diamino Spacer for Phosphate and Sulfate Ions. Org. Biomol. Chem. 2012, 10, 8758−8761. (26) Sommer, F.; Kubik, S. Anion Binding of a Neutral Bis(cyclopeptide) in Water−Methanol Mixtures Containing up to 95% Water. Org. Biomol. Chem. 2014, 12, 8851−8860. (27) Rhaman, M. M.; Ahmed, L.; Wang, J.; Powell, D. R.; Leszczynski, J.; Hossain, M. A. Encapsulation and Selectivity of Sulfate with a Furan-Based Hexaazamacrocyclic Receptor in Water. Org. Biomol. Chem. 2014, 12, 2045−2048. (28) Young, P. G.; Jolliffe, K. A. Selective Recognition of Sulfate Ions by Tripodal Cyclic Peptides Functionalised with (Thio)urea Binding Sites. Org. Biomol. Chem. 2012, 10, 2664−2672. (29) Kubik, S. Anion Recognition in Water. Chem. Soc. Rev. 2010, 39, 3648−3663. (30) Langton, M. J.; Serpell, C. J.; Beer, P. D. Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective. Angew. Chem., Int. Ed. 2016, 55, 1974−1987. (31) Jacobson, B. L.; Quiocho, F. A. Sulfate-Binding Protein Dislikes Protonated Oxyacids: A Molecular Explanation. J. Mol. Biol. 1988, 204, 783−787. (32) Zhou, H.; Zhao, Y.; Gao, G.; Li, S.; Lan, J.; You, J. Highly Selective Fluorescent Recognition of Sulfate in Water by Two Rigid Tetrakisimidazolium Macrocycles with Peripheral Chains. J. Am. Chem. Soc. 2013, 135, 14908−14911. (33) CRC Handbook of Chemistry and Physics, 82nd Edition; CRC Press: Boca Raton, FL, 2001. (34) Custelcean, R.; Williams, N. J.; Seipp, C. A.; Ivanov, A. S.; Bryantsev, V. S. Aqueous Sulfate Separation by Sequestration of N

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