An Ideal C3-Symmetric Sulfate Complex: Molecular Recognition of

Sep 18, 2017 - The anion-binding properties of two tripodal-based hexaureas appended with the m-nitrophenyl (1) and pentafluorophenyl (2) groups have ...
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An Ideal C3‑Symmetric Sulfate Complex: Molecular Recognition of Oxoanions by m‑Nitrophenyl- and Pentafluorophenyl-Functionalized Hexaurea Receptors Bobby Portis,† Ali Mirchi,† Maryam Emami Khansari,† Avijit Pramanik,† Corey R. Johnson,† Douglas R. Powell,‡ Jerzy Leszczynski,*,† and Md. Alamgir Hossain*,† †

Department of Chemistry and Biochemistry, Jackson State University, Jackson, Mississippi 39217, United States Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States

‡

S Supporting Information *

ABSTRACT: The anion-binding properties of two tripodal-based hexaureas appended with the m-nitrophenyl (1) and pentafluorophenyl (2) groups have been studied both experimentally and theoretically, showing strong affinities for sulfate over other inorganic oxoanions such as hydrogen sulfate, dihydrogen phosphate, bicarbonate, nitrate, and perchlorate. The structural analysis of the sulfate complex with 1 reveals that the receptor organizes all urea-binding sites toward the cavity at precise orientations around a tetrahedral sulfate anion to form an ideal C3-symmetric sulfate complex that is stabilized by 12 hydrogen-bonding interactions. The receptor and the encapsulated sulfate are located on the threefold axis passing through the bridgehead nitrogen of 1 and the sulfur atom of the anionic guest. The high-level density functional theory calculations support the crystallographic results, demonstrating that the C3-symmetric conformation of the sulfate complex is achieved due to the complementary NH···O between the receptor and sulfate.

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eas,35−37 pyrroles,38−43 and indoles44−47 have been reported, which provide complementary hydrogen-bonding interactions for sulfate anions. Among them, urea-/thiourea-based receptors have been the focus of considerable interests because of the presence of two directional hydrogen-bond donors in a single unit and their potential applications in quantitative extractions48,49 and transmembrane transport50 of sulfate. Previous theoretical calculations by Hay et al. predicted that a sulfate anion can bind up to six urea groups to provide the optimal saturation with 12 H-bonds.51 This prediction was later confirmed experimentally by Custelcean et al., showing a coordinatively saturated sulfate complex formed by two C3symmetric tripodal trisurea ligands bridged with silver(I) ions.27 Increasing the binding sites with the urea groups to a trenbased ligand, Wu et al. synthesized hexaurea ligands appended with p-nitrophenyl or ferrocenyl groups that were shown to encapsulate tetrahedral sulfates, while each oxygen atom of the guest anion is coordinated with three H-bonds, thus achieving the saturation of the coordination sphere of sulfates with six urea groups.52,53 However, to the best of our knowledge, a sulfate complex bound to a synthetic receptor with a perfect C3 symmetry has not been reported so far. In an effort to synthesize C3-symmetric urea-based receptors with a higher order of binding sites, we recently synthesized a

INTRODUCTION Molecular recognition of sulfate is important because of its significant roles in biological and environmental processes.1−14 In nature, sulfate is found within the hydrophobic pocket of a sulfate-binding protein (SBP) derived from Salmonella typhimurium15 and DNA helicase RepA.16 The structure of the sulfate complex of the SBP reported by Pflugrath and Quiocho in 1985 reveals that the sulfate is encapsulated within the protein’s cleft through a total of seven hydrogen-bonding interactions, where three oxygen atoms are bound with six NH···O interactions and the forth oxygen atom is held with one OH···O bond.15 The structural identification of the sulfate complex of DNA helicase RepA reported by Xu et al. suggests that the sulfate is encapsulated via six hydrogen-bonding interactions occupying the six ATPase active sites of RepA and one OH···O interaction with a water molecule.16 Similar binding arrangement is also observed in a urea-based synthetic receptor complexed with hydrogen sulfate, showing six NH···O bonds and one OH···O hydrogen bond.17 In the context of environmental viewpoints, sulfate is an inorganic contaminant in soil and water and is associated with acid rain that may cause health-related problems after a long-term exposure.18,19 In industry, sulfate is known to interfere with the vitrification process during nuclear waste management;20,21 thus, the separation of sulfate from the nitrate-rich waste mixtures is critical before the clean-up process.21 Inspired from nature’s rule, several types of neutral receptors including amides,22−24 thioamides,25,26 ureas,27−34 thiour© 2017 American Chemical Society

Received: August 1, 2017 Accepted: August 28, 2017 Published: September 18, 2017 5840

DOI: 10.1021/acsomega.7b01115 ACS Omega 2017, 2, 5840−5849

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the presence of an excess n-tetrabutylammonium sulfate. Attempts to isolate X-ray quality crystals of other anion complexes with the receptors were unsuccessful. Solution Binding Studies. Binding interactions between the receptors and inorganic oxoanions were investigated using 1 H NMR experiments in DMSO-d6 at room temperature. The anions included in the titration studies were SO42−, HSO4−, HCO3−, H2PO4−, ClO4−, and NO3− in the form of their tetrabutylammonium (TBA) salts. The receptors, owing to their C3-symmetric conformations, exhibited four distinct NH resonances. To make an initial evaluation of the receptor− anion interactions, 1 equiv of each oxoanion was added separately to the receptors (Figure 1a,b). As shown in Figure 1a, the free receptor 1 displays urea−NH resonances at 9.65 (H1), 8.18 (H2), 7.98 (H3), and 6.55 ppm (H4). These peaks were significantly shifted downfield because of the addition of 1 equiv of SO42−, HSO4−, or H2PO4−. For HCO3−, a new set of NH signals appeared downfield along with the peaks of the free ligand. This observation suggests a slow exchange reaction on the NMR time scale. Similar shift changes were also observed for 2 upon the addition of 1 equiv of anions in DMSO-d6 (Figure 1b). On the other hand, the addition of ClO4− or NO3− ions showed a negligible change in the NH chemical shifts of the receptors, thus indicating weaker interactions of the receptor with these anions. For a quantitative assessment of the host−guest interactions, the receptors were titrated with varying amounts of anionic guests in DMSO-d6. The titration curves, as obtained from the changes in the chemical shifts or integrated intensities, were fitted to a 1:1 stoichiometry, as evaluated by nonlinear regression analysis55 and supported by Job plot analysis. The addition of H2PO4− to the receptors also resulted in the downfield shifts of the NH signals. However, the binding constants could not be determined due to the broadening of peaks. Inspecting the observed binding data, as summarized in Table 1, suggests that both receptors display a similar binding trend, whereas 1, as compared to 2, shows comparatively stronger affinity for the anions. It might be due to the aromatic interactions between the peripheral substituents in 1 as supported by the X-ray structure as well as the theoretical calculations, thereby increasing the overall anionbinding ability of the host.

pentafluoro-substituted hexaurea receptor 2 that was found to encapsulate one carbonate anion; however, we were unsuccessful in obtaining the crystals of the sulfate complex with this receptor.54 To examine the influence of attached groups on the binding strength and selectivity, we have been further interested in functionalizing the core cavity and synthesized the m-nitrophenyl-substituted hexaurea receptor 1. Herein, we report that the receptor 1 assembles the urea groups at accurate positions around a tetrahedral sulfate anion, thus forming an ideal C3-symmetric sulfate complex from the interactions of all NH-binding sites with the anion and NH···π interactions. It is shown by NMR studies that the receptors exhibit similar selectivities for sulfate over other oxoanions. Theoretical calculations based on the density functional theory (DFT) have been carried out to understand and support the binding insights on our experimental results.

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RESULTS AND DISCUSSION Synthesis. The hexafunctional urea receptors (Chart 1) were synthesized from the reaction of tris(2-aminophenyl)urea Chart 1. Chemical Structures of the Receptors 1 and 2

with 3 equiv of m-nitrophenyl isocyanate (for 1) or pentafluorophenyl isocyanate (for 2) in a toluene−tetrahydrofuran (THF) mixture to give about 90% yield. The crystals of a sulfate complex [1(SO4)](n-Bu4N)2 were grown from slow evaporation of a dimethyl sulfoxide (DMSO) solution of 1 in

Figure 1. Partial 1H NMR spectra of 1 (a) and 2 (b) in the presence of 1 equiv of different anions in DMSO-d6 ([receptor]0 = 2 mM), showing changes in the NH chemical shifts in DMSO-d6 (see, Chart 1 for the assignment of NH peaks). Overlapped NH peaks are not marked. 5841

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Table 1. Binding Constants (log K) of the Receptors with Anionsa anions

1

2

SO42− HSO4− HCO3− H2PO4− NO3− ClO4−

5.78(3), 5.85(3)b 3.51(2) 3.28(2) d , 3.08(2)b NO3− > ClO4− (see Supporting Information, Table S1). These calculated data are in agreement with the experimental titration results, showing the highest binding energies for dinegatively charged sulfates with both receptors, whereas the m-nitro analogue 1 exhibits stronger anion-binding affinity than its pentafluoro analogue 2.

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Table 4. Crystal Data and Structure Refinement for the Sulfate Complex of 1 chemical formula formula mass a/Å b/Å c/Å α/deg β/deg γ/deg unit cell volume/Å3 temperature/K space group no. of formula units per unit cell, Z radiation type absorption coefficient, μ/mm−1 no. of reflections measured no. of independent reflections Rint final R1 values (I > 2σ(I)) final R1 values (all data) goodness of fit on F2

CONCLUSIONS

We have designed and synthesized two tripodal-based hexaureas appended with the m-nitrophenyl (1) and pentafluorophenyl (2) groups. Their binding properties have been investigated for inorganic oxoanions such as hydrogen sulfate, dihydrogen phosphate, bicarbonate, nitrate, and perchlorate, showing high binding affinities for the sulfate anion. We have isolated and structurally characterized the sulfate complex of 1 by single-crystal X-ray analysis, confirming the formation of the crystallographically perfect C3-symmetric sulfate complex of 1 in which all six urea groups excellently organize toward the center of the cavity to encapsulate a sulfate anion with 12 NH··· O hydrogen bonds. Both the receptor and the anion are located on the threefold axis passing through the tertiary nitrogen of the receptor and the sulfur atom of the encapsulated sulfate. The computational studies performed by the high-level DFT calculations demonstrate that the C3-symmetric sulfate complex is achieved because of the best complementarity between the receptor and the sulfate anion. Although the complete saturation of the coordination sites of a C3-symmetric sulfate was achieved previously by six urea groups provided by two trisureas27 or single hexaureas52 providing the optimal 12 hydrogen bonds as predicted theoretically,51 however, to the best of our knowledge, a perfect C3-symmetric sulfate complex with a synthetic receptor has not been reported so far. The receptor 1 represents an exceptional example that encapsulates a sulfate anion to form an ideal C3-symmetric sulfate complex.

C48H48N16O12·SO4·2(C16H36N) 1621.99 27.389(2) 27.389(2) 9.7549(7) 90 90 120 6337.3(10) 100(2) P3 3 Mo Kα 0.114 50 226 14 912 0.0399 0.0710 0.1024 1.007

filtration and washed with CH2Cl2. The compound was dried under vacuum to give 1 as a chalky yellow powder. Yield: 1.01 g (89%). mp: 220−222 °C, 1H NMR (500 MHz, DMSO-d6, TSP): δ 9.66 (s, 3H, ArNH), 8.52 (s, 3H, ArH), 8.18 (s, 3H, ArNH), 7.98 (s, 3H, ArNH), 7.86 (d, J = 9.0 Hz, 3H, ArH), 7.68 (d, J = 9.0 Hz, 3H, ArH), 7.56 (m, 6H, ArH), 7.43 (s, 3H, ArH), 7.03 (t, J = 6.1 Hz, 6H, ArH), 6.55 (s, 3H, NH), 3.20 (d, J = 6 Hz, 6H, NHCH2), 2.61 (t, J = 6.1 Hz, 6H, NCH2). 13C NMR (125 MHz, DMSO-d6): 156.7 (ArCO), 153.4 (NHCO), 148.5 (ArC), 141.7 (ArC), 132.3 (ArC), 131.3 (ArC), 130.4 (ArC), 124.7 (ArC), 124.5 (ArC), 116.5 (ArC), 112.4 (NH C H 2 ), 54.4 (NHCH 2 CH 2 ). Anal. Calcd for C48H48N16O12: C, 55.38; H, 4.65; N, 21.53. Found: C, 55.22; H, 4.52; N, 21.56. ESI-MS (+ve) m/z: calcd for C48H49N16O12, 1041.36 [M + H]+; found, 1041.25. The receptor 2 was synthesized following the procedures as described before.54 Synthesis of the Sulfate Complex of 1 ([1·SO4](TBA)2). The sulfate complex of 1 was obtained from slow evaporation of a DMSO solution of 1 (30 mg, 0.029 mmol) in the presence of excess (∼2.0 equiv) n-tetrabutylammonium sulfate in a vial at room temperature in 5 days. Yield: 23 mg (70%). Anal. Calcd for C48H48N16O12: C, 59.24; H, 7.46; N, 15.54. Found: C, 59.28; H, 4.47; N, 15.56. ESI-MS (−ve) m/z: calcd for [C48H48N16O12SO4)/2], 568.16 [(M·SO4)/2]−; found, 568.18. The compound was further characterized by a single-crystal Xray diffraction analysis. NMR Binding Studies. The binding constants of the receptors with different oxoanions (SO42−, HSO4−, H2PO4−, ClO4−, and NO3− in the form of their TBA salts) were obtained by the 1H NMR titrations in DMSO-d6 using a 500 MHz Bruker instrument at room temperature. The initial concentrations of the receptors and anions were 2 and 20 mM, respectively. Sodium salt of 3-(trimethylsilyl) propionic-2,2,3,3d4 (TSP) acid in DMSO was used as an external reference in a capillary tube. Each titration was performed by 12−14 measurements. The association constants (K) were calculated using a 1:1 binding model55 from the changes of chemical shifts of NH for the fast exchange reactions or relative changes in the

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EXPERIMENTAL SECTION General. All reagents and solvents were purchased from Sigma-Aldrich and were used as received. The synthesized compounds were characterized using the common laboratory techniques as described before.36 Synthesis of the Receptors 1 and 2. The synthesis of 1 was carried out by the reaction of tris(2-aminophenyl)urea54 (0.60 g, 1.09 mmol) with 3 equiv of 3-nitrophenyl isocyanate (0.54 g, 3.38 mmol) in a solution of toluene and THF (2:1, 150 mL). The reaction mixture was refluxed overnight at 100−110 °C under a nitrogen atmosphere and was cooled at room temperature. The precipitate thus formed was collected by 5846

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affinity and selectivity in aqueous media. Chem. Sci. 2016, 7, 4563− 4572. (4) Hossain, M. A.; Kang, S. O.; Kut, J. K.; Day, V. W.; BowmanJames, K. Influence of charge on anion receptivity in amide-based macrocycles. Inorg. Chem. 2012, 51, 4833−4840. (5) Gale, P. A.; García-Garrido, S. E.; Garric, J. Anion receptors based on organic frameworks: highlights from 2005 and 2006. Chem. Soc. Rev. 2008, 37, 151−190. (6) Katayev, E. A.; Ustynyuk, Y. A.; Sessler, J. L. Receptors for tetrahedral oxyanions. Coord. Chem. Rev. 2006, 250, 3004−3037. (7) 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. (8) Haque, S. A.; Saeed, M. A.; Jahan, A.; Wang, J.; Leszczynski, J.; Hossain, M. A. Experimental and Theoretical aspects of anion complexes with a thiophene-based cryptand. Comments Inorg. Chem. 2016, 36, 305−326. (9) Caltagirone, C.; Gale, P. A. Anion receptor chemistry: Highlights from 2007. Chem. Soc. Rev. 2009, 38, 520−563. (10) Mullen, K. M.; Beer, P. D. Sulfate anion templation of macrocycles, capsules, interpenetrated and interlocked structures. Chem. Soc. Rev. 2009, 38, 1701−1713. (11) Steed, J. W. Coordination and organometallic compounds as anion receptors and sensors. Chem. Soc. Rev. 2009, 38, 506−519. (12) 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. (13) Jin, C.; Zhang, M.; Wu, L.; Guan, Y.; Pan, Y.; Jiang, J.; Lin, C.; Wang, L. Squaramide-based tripodal receptors for selective recognition of sulfate anion. Chem. Commun. 2013, 49, 2025−2027. (14) Fowler, C. J.; Haverlock, T. J.; Moyer, B. A.; Shriver, J. A.; Gross, D. E.; Marquez, M.; Sessler, J. L.; Hossain, M. A.; BowmanJames, K. Enhanced anion exchange for selective sulfate extraction: overcoming the hofmeister bias. J. Am. Chem. Soc. 2008, 130, 14386− 14387. (15) Pflugrath, J. W.; Quiocho, F. A. Sulphate sequestered in the sulphate-binding protein of Salmonella typhimurium is bound solely by hydrogen bonds. Nature 1985, 314, 257−260. (16) Xu, H.; Strater, N.; Schroder, W.; Bottcher, C.; Ludwig, K.; Saenger, W. Structure of DNA Helicase RepA in complex with sulfate at 1.95 A resolution implicates structural changes to an ‘open’ form. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2003, 59, 815−822. (17) Pramanik, A.; Thompson, B.; Hayes, T.; Tucker, K.; Powell, D. R.; Bonnesen, P. V.; Ellis, E. D.; Lee, K. S.; Yu, H.; Hossain, M. A. Seven-coordinate anion complex with a tren-based urea: binding discrepancy of hydrogen sulfate in solid and solution states. Org. Biomol. Chem. 2011, 9, 4444−4447. (18) Olomu, A. B.; Vickers, C. R.; Waring, R. H.; Clements, D.; Babbs, C.; Warnes, T. W.; Elias, E. High incidence of poor sulfoxidation in patients with primary biliary cirrhosis. N. Engl. J. Med. 1988, 318, 1089−1092. (19) Amerongen, A. V. N.; Bolscher, J. G. M.; Bloemena, E.; Veerman, E. C. I. Sulfomucins in the human body. Biol. Chem. 1998, 379, 1−26. (20) Ravikumar, I.; Ghosh, P. Recognition and separation of sulfate anions. Chem. Soc. Rev. 2012, 41, 3077−3098. (21) Moyer, B. A.; Singh, R. P. Fundamentals and applications of anion separations; Springer: US, 2012. (22) Bondy, C. R.; Loeb, S. J. Amide based receptors for anions. Coord. Chem. Rev. 2003, 240, 77−99. (23) Hossain, M. A.; Llinares, J. M.; Powell, D.; Bowman-James, K. Multiple hydrogen bond stabilization of a sandwich complex of sulfate between two macrocyclic tetraamides. Inorg. Chem. 2001, 40, 2936− 2937. (24) Kang, S. O.; Begum, R. A.; Bowman-James, K. Amide-based ligands for anion coordination. Angew. Chem., Int. Ed. 2006, 45, 7882− 7894.

intensity of the NH resonance for complexes and free receptors as described previously.54 UV−Vis Binding Studies. The receptor 1 showed an absorption at λmax = 351 nm in DMSO, whereas no absorption was observed for 2 because of the absence of an optically active chromophore. UV−vis titration studies were performed by titrating 1 with different oxoanions as their TBA salts in DMSO at room temperature. The initial concentrations of the receptor and the anions were 1.5 × 10−4 and 1.5 × 10−2 M, respectively. Each titration was performed by 15 measurements in the range of 0−35 equiv of anions, and the binding constant K was calculated by fitting the relative UV−vis absorbance or wavelength with a 1:1 binding model.55 X-ray Crystallography. The single-crystal structure of 1 was analyzed using a diffractometer with a Bruker APEX CCD area detector,66 as described before.67 Details of the crystal data and structure refinement are listed in Table 4. The structure was refined by a full-matrix least-squares method using the SHELXL2013 program.68

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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/acsomega.7b01115. Characterization spectra, NMR and UV−vis titration spectra, calculated binding energies, and Cartesian coordinates for DFT calculations (PDF) Crystallographic information files (CIF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (M.A.H.). ORCID

Md. Alamgir Hossain: 0000-0003-4978-1664 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Science Foundation is acknowledged for a CAREER award (CHE-1056927) to M.A.H. The authors are thankful to the National Science Foundation (NSF/CREST HRD-1547754) for financial support and want to acknowledge the Extreme Science and Engineering Discovery Environment (XSEDE) by the National Science Foundation grant number OCI-1053575 and XSEDE award allocation number DMR110088. The authors are also thankful to the Mississippi Center for Supercomputing Research (MCSR) for providing state-of-the-art high-performance computing facilities for supporting this research. The NMR core facility at Jackson State University was supported by the National Institutes of Health (G12RR013459).

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DOI: 10.1021/acsomega.7b01115 ACS Omega 2017, 2, 5840−5849

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DOI: 10.1021/acsomega.7b01115 ACS Omega 2017, 2, 5840−5849