Article pubs.acs.org/crystal
Positional Isomeric Effect in Nitrophenyl Functionalized Tripodal Urea Receptors toward Binding and Encapsulation of Anions Romen Chutia, Sandeep Kumar Dey, and Gopal Das* Department of Chemistry, Indian Institute of Technology Guwahati, Assam-781039, India S Supporting Information *
ABSTRACT: A set of three positional isomers of nitro-phenyl functionalized tris(urea) scaffold has been extensively studied as the anion coordinating host. The para-isomer, L1, has been structurally authenticated to self-assemble into a dimeric (pseudo)molecular capsule in the presence of oxoanions of varied dimensionality (carbonate and terephthalate anions). Whereas the meta-isomer, L2, has been found to self-assemble into a dimeric molecular capsule in presence of inorganic oxoanions such as hydrogen phosphate and adopts a flat trigonal planar like geometry in the presence of the terephthalate anion where each receptor side arm is in interaction with a carboxylate group of the anion. However, successful crystallization of the ortho-isomer, L3, in the presence of different oxoanions was not fruitful presumably due to the steric effect provided by the nitro group at the ortho-position, which hinders the facile inclusion and coordination of an anion due to electrostatic factor, as confirmed by 2D NOESY NMR analysis of the free receptor. Qualitative and quantitative 1H NMR experimental results showed that the ortho-isomer binds with oxoanions rather feebly as reflected from the apparent binding constant values of HCO3− and H2PO42− as compared to the meta- and para-isomers which can coordinate to these anions very strongly via encapsulation, as confirmed by 2D NOESY NMR analysis.
1. INTRODUCTION Anion coordination by hydrogen bonding scaffolds has emerged into a prominent and active field of research within the realm of supramolecular chemistry.1 Although Nature exemplifies how proteins can selectively and efficiently bind anions by precise noncovalent interactions, the development of artificial receptors for the selective recognition of anions still remains a challenging task, with hydrogen bonds being fundamental in determining binding selectivity via topological complementarity.2 This observation in natural systems has inspired researchers to develop numerous abiotic receptors that employ hydrogen bonds offered by specific binding sites from amide,3 urea/thiourea,4 pyrrole/indole,5 and imidazolium6 functionalities for the recognition and binding of anionic guests on suitable frameworks. Anions generally have very high free energies of solvation (e.g., ΔGhydration of CO32− and H2PO4− are −1315 and −465 kJ mol−1) that must be compensated by the host for effective anion recognition.7 Thus, it is the design and synthesis of sophisticated threedimensional hosts that is essential to fully encapsulate the target anion(s) by creating a highly specific hydrophobic anion binding pocket/cavity. In SBP (sulfate binding protein) from S. typhimurium and PBP (phosphate binding protein) from E. coli, the anion binding site is located in a deep hydrophobic pocket between the two lobes of protein, where the anions can be bound in a fully dehydrated form, efficiently shielded from the hydrophilic environment.2b,c Whereas the binding of anionic guests within preorganized macrocyclic systems such as © 2013 American Chemical Society
cryptands is relatively straightforward to understand, the binding processes of flexible podand receptors remain more elusive.1,2 Since the beginning of anion receptor chemistry, hydrogen bonding tripodal scaffolds has widely been employed for anion binding and encryption featuring receptor cavities, which can contract or expand in response to the incoming anionic guest species.8 The binding ability of tripodal receptors for anionic guests varies with the appended peripheral functions, since both the position and the nature of functional groups modify the hydrogen bonding capability. Theoretical investigation by Hay et al. showed that the position of electron withdrawing substituents on the peripheral aryl moiety have significant consequences on the stability of anion complexes.9 Acyclic receptors with multi-armed hydrogen bonding functionality have been shown to coordinate with targeted anionic species via formation of (pseudo)capsular assemblies.10 When two tripodal molecules with interior anion binding elements create a dimeric capsular assembly, there is a possibility to satisfy the higher coordination numbers required for the binding of multicharged oxoanions such as CO32−, SO42−, and PO43−. Hydrogen bond donating tripodal receptors have shown a number of interesting properties, e.g., anion recognition in water,11 encapsulation of anion−water clusters within dimeric Received: October 29, 2012 Revised: December 24, 2012 Published: January 4, 2013 883
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capsular assembly,10 fixation of aerial carbon dioxide (CO2) as carbonate (CO32−),12 selective salt extraction from water,13 and transmembrane anion transportation.14 Given our interest in the field of supramolecular chemistry of anions, herein we demonstrate the effect of positional isomerism in anion coordination by a set of three nitro-phenyl functionalized tripodal urea scaffolds, L1−L3 (Scheme 1).
Binding constants were obtained by 1H NMR (Varian-400 MHz) titrations of the ligands with tetraethylammonium (TEA)/ntetrabutylammonium (TBA) salts of the respective anions in DMSO-d6 at 298 K. The initial concentration of each receptor solution was 5 mM. Aliquots of anions were added from 50 mM stock solutions of anions (up to 1:10 host/guest stoichiometry). The residual solvent peak in DMSO-d6 (2.500 ppm) was used as an internal reference, and each titration was performed with 15−20 measurements at room temperature. 2.2. Synthesis and Characterization. Receptors L1−L3. Each isomeric receptor was obtained in quantitative yield by the reaction of tris(2-aminoethyl)amine with 3 equiv of the respective nitrophenylisocyanate in tetrahydrofuran (THF)12 and characterized by NMR and FT-IR analyses of the isolated faint yellow solid product (Supporting Information). Complex 2TEA[2L1(CO32−)] (1a). Carbonate-encapsulated complex 1a was obtained as suitable crystals for X-ray diffraction analysis upon slow evaporation of a 5 mL DMSO solution of L1 in presence of excess (TEA)HCO3. The colorless crystals thus obtained were isolated by filtration and dried at room temperature by pressing between the filter papers before characterization by NMR and FT-IR analyses. Isolated yield: 72% based on L1 after 2 weeks of exposure to unmodified atmosphere. 1 H NMR (DMSO-d6, 400 MHz): δ (ppm) 1.11 (t, 24H, TEA CH3), 2.47−2.51 (t, 12H, NCH2), 3.14 (t, 12H, NHCH2), 7.53 (d, 12H, ArH), 7.80 (d, 12H, ArH), 7.96 (s, 6H, ureaNHa), 10.88 (s, 6H, ureaNHb). 13C NMR (100 MHz, DMSO-d6): δ (ppm) 7.05 (×8C, TEACH3), 37.13 (×6C, NCH2), 51.37 (×8C, TEA CH2), 53.34 (×6C, NHCH2), 116.65 (×12C, Ar), 124.5 (×12C, Ar), 139.74 (×6C, Ar), 147.85 (×6C, Ar), 154.84 (×6C, CO), 172.01 (CO32−). FT-IR (υ cm−1): 1108 (CO,CO32−), 1324 (NO2sym.), 1523 (NO2-asym.), 1707 (CO), 2968 (CH), 3327 (N H). Complex 2TBA[2L1(C8H4O42−)] (1b). Terephthalate complex 1b was obtained as suitable crystals for X-ray diffraction analysis from a DMF solution mixture of L1 charged with an excess of a 1:2 solution mixture of terephthalic acid and TBA(OH). A 3 mL DMF solution mixture of terephthalic acid and TBA(OH) (5 equiv. of 1:2 mixture) was stirred for about 15 min before adding to a 5 mL DMF solution of L1. Slow evaporation of the solution at room temperature yielded colorless crystals of 1b within a duration of 2 weeks. Isolated yield: 76% based on L1 after 3−4 weeks of exposure to unmodified atmosphere. 1 H NMR (DMSO-d6, 400 MHz): δ (ppm) 0.78 (t, 24H, TBA CH3), 1.15 (q, 16H, TBACH2), 1.41 (s, 16H, TBACH2), 2.50 (s, 12H, NCH2), 3.02 (t, 16H, TBAN+CH2), 3.11 (t, 12H, NHCH2), 7.38 (s, 6H, ureaNHa), 7.52 (d, 12H, ArH), 7.92 (d, 12H, ArH), 10.364 (s, 6H, ureaNHb). 13C NMR (100 MHz, DMSO-d6): δ (ppm) 13.67 (×8C, TBACH3), 19.43 (×8C, TBACH2), 23.29 (×8C, TBACH2), 37.36 (×6C, NCH2), 53.71 (×6C, NHCH2), 57.80 (×8C, TBAN+CH2), 116.88 (×12C, Ar), 125.18 (×12C, Ar), 128.82 (×4C, Ar), 139.95 (×2C, Ar), 140.23 (×6C, Ar), 147.98 (×6C, Ar), 155.07 (×6C, CO), 172.35 (COO−). FTIR (υ cm−1): 1107 (CO; COO−), 1330 (NO2-sym.), 15021557 (NO2-asym.), 1705 (CO), 2963 (CH), 3321 (NH). Complex TBA[L1(F−)] (1c). Fluoride-encapsulated complex 1c was obtained as crystalline precipitate upon slow evaporation of a 5 mL DMSO solution of L1 in the presence of excess TBAF. Isolated yield: 48% based on L1 after 10 days of exposure to unmodified atmosphere. 1 H NMR (DMSO-d6, 400 MHz): δ (ppm) 0.885 (t, 12H, TBA CH3), 1.26 (q, 8H, TBACH2), 1.50 (t, 8H, TBACH2), 2.49 (s, 6H, NCH2), 3.12 (t, 8H, TBAN+CH2), 3.21 (s, 6H, NHCH2), 7.29 (s, 3H, ureaNHa), 7.64 (d, 6H, ArH), 8.08(d, 6H, ArH), 11.16 (s, 6H, ureaNHb); 19F NMR (DMSO-d6, 400 MHz): δ (ppm) −122.06 (s, bound fluoride). 13C NMR (DMSO-d6, 100 MHz): δ (ppm), 13.45 (×4C, TBACH3), 19.22 (×4C, TBACH2), 23.09 (×4C, TBACH2), 35.58 (×3C, NCH2), 50.513 (×3C, NHCH2), 57.58 (×4C, TBAN+CH2), 116.33 (×6C, Ar), 125.26 (×6C, Ar), 140.00 (×3C Ar), 147.79 (×3C Ar), 154.77 (×3C, C O).
Scheme 1. Molecular Structure of Nitrophenyl Appended Tris(urea) Receptors, L1−L3
Structural studies revealed an oxoanion induced self-assembly of the para-isomer (L1) into dimeric (pseudo)molecular capsule as observed in carbonate and terephthalate complexes (1a and 1b), whereas the meta-isomer (L2) can self-assemble into dimeric capsules only in presence of inorganic oxoanions, as observed in hydrogenphosphate complex (2a), and assemble into a 2D sheet-like structure in the presence of terephthalate dianion (2b). In contrast to L1 and L2, structural authentication of the ortho-isomer (L3) in the presence of different oxoanions was not fruitful, and solution-state studies revealed that the receptor binds to oxoanions rather weakly in comparison to the para- and meta-isomers, as reflected from the apparent binding constant values of HCO3− and H2PO42−. However, F− is bound almost equally by the three isomeric receptors. Furthermore, attempted crystallization of L1 in the presence of excess TBAF in DMSO yielded crystalline precipitate of F− bound complex (1c), which was confirmed by 19F and 2D-NOESY NMR analyses of the isolated complex. It is worth mentioning that the meta-isomer (L2) has recently been established as a potential hydrogen bonding scaffold which can efficiently fix atmospheric CO2 as air-stable crystals of CO32− entrapped molecular capsule in DMSO solution induced by the presence of excess TBAF or an equivalent amount of TBAOH.12b
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All reagents and solvents were obtained from commercial sources and used as received without further purification. Tris(2-aminoethyl)amine (tren), nitrophenylisocyanates, and quaternary ammonium salts were purchased from SigmaAldrich and used as received. Solvents for synthesis and crystallization experiments were purchased from Merck-India and used as received. 1 H NMR spectra were recorded on a Varian FT-400 MHz instrument, and chemical shifts were recorded in parts per million (ppm) on the scale using tetramethylsilane (TMS) or residual solvent peak as a reference, and 13C spectra were recorded at 100 MHz on the same instrument. The FT-IR spectra of air-dried samples were recorded on a Perkin-Elmer-Spectrum One FT-IR spectrometer with KBr disks in the range 4000−450 cm−1. 884
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Table 1. Crystallographic Parameters and Refinement Details of the Complexes parameters
1a
1b
2a
2b
formula fw crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z Dc, g cm−3 μ Mo Kα, mm−1 T, K θ max. total reflections indept. reflections observed reflections parameters refined R1, I > 2σ(I) wR2 (all data) GOF (F2)
C71H100N22O21 1597.73 monoclinic C2/c 22.6830(7) 21.3607(7) 17.8940(7) 90.00 105.138(3) 90.00 8369.2(5) 4 1.268 0.095 298(2) 28.65 41828 10505 6075 519 0.0624 0.2082 1.194
C47H68N11O11 963.12 triclinic P1̅ 12.8426(11) 14.3533(12) 14.6146(12) 90.561(4) 103.049(4) 95.773(4) 2609.6(4) 2 1.226 0.089 298(2) 28.38 34227 13012 5253 626 0.0549 0.1546 0.990
C86H132N22O22 1857.11 triclinic P1̅ 17.0817(9) 17.6519(9) 19.7271(10) 70.110(2) 80.692(2) 65.396(2) 5084.1(5) 2 1.213 0.103 298(2) 28.05 69838 24520 21245 1188 0.0735 0.2353 0.955
C87H144N13O19 1676.15 triclinic P1̅ 10.9000(3) 20.4538(4) 24.2322(5) 110.0600(10) 94.7010(10) 98.6720(10) 4964.5(2) 2 1.121 0.079 298(2) 20.27 33240 9551 6406 1079 0.0840 0.3243 1.012
Complex 2TBA[2L2(HPO42−)] (2a). Hydrogen phosphate complex 2a was obtained as suitable crystals for X-ray diffraction analysis from a DMF solution mixture of L2 charged with an excess of a 1:1 solution mixture of TBA(H2PO4) and TBA(OH). The solution mixture was stirred for about 15 min before being allowed to crystallize at room temperature. Isolated yield: 52% based on L2 after 2−3 weeks of exposure to unmodified atmosphere. 1 H NMR (DMSO-d6, 400 MHz): δ (ppm) 0.87 (t, 24H, TBA CH3), 1.24 (q, 16H, TBACH2), 1.49 (q, 16H, TBACH2), 2.52 (s, 12H, NCH2), 3.10 (t, 16H, TBAN+CH2), 3.16 (t, 12H, NHCH2), 7.07 (t, 6H, ArH), 7.40 (d, 6H, ArH), 7.51 (d, 6H, ArH), 7.67 (s, 6H, ureaNHa), 8.15 (s, 6H, ArH), 10.47 (s, 6H, urea NHb). 31P NMR (DMSO-d6, 400 MHz): δ (ppm), 20.98 (s, HPO42−). FT-IR (υ cm−1): 1348 (NO2-sym.), 1525 (NO2 -asym.), 1690 (C O), 2964 (CH), 3394 (NH). Complex 3TBA[L2(C8H4O42−)1.5] (2b). Similar to terephthalate complex 1b, complex 2b was obtained as suitable crystals for X-ray diffraction analysis from a DMF solution mixture of L2 charged with an excess (5 equiv) of a 1:2 solution mixture of terephthalic acid and TBA(OH). Isolated yield: 77% based on L2 after 2 weeks of exposure to unmodified atmosphere. 1 H NMR (DMSO-d6, 400 MHz): δ (ppm), 0.87 (t, 36H, TBA CH3), 1.27 (q, 24H, TBACH2), 1.49 (s, 24H, TBACH2), 2.54 (s, 6H, NCH2), 3.10 (q, 24H, TBAN+CH2), 3.20 (t, 6H, NHCH2), 7.36 (t, 3H, ArH), 7.61 (d, 3H, ArH), 7.69 (d, 3H, ArH), 7.87 (s, 4H, ArH, terephthalate), 7.96 (s, 3H, ureaNHa), 8.57 (s, 3H, ArH), 10.81 (s, 3H, ureaNHb). 13C NMR (100 MHz, DMSOd6): δ (ppm) 13.46 (×12C, TBACH3), 19.19 (×12C, TBACH2), 23.07 (×12C, TBACH2), 37.22 (×3C, NCH2), 53.56 (×3C, NHCH2), 57.57 (×12C, TBAN+CH2), 111.22 (×3C, Ar), 114.57 (×3C, Ar), 123.44 (×3C, Ar), 128.12 (×6C, Ar), 129.50 (×3C, Ar), 140.53 (×3C,Ar), 142.97 (×3C,Ar), 148.10 (×3C,Ar), 155.75 (×3C, CO), 170.61 (COO−). FT-IR (υ cm−1): 1330 (NO2 -sym.), 1530 (NO2-asym.), 1694 (CO), 2964 (CH). 2.3. X-ray Crystallography. In each case, a crystal of suitable size was selected from the mother liquor and immersed in silicone oil, and it was mounted on the tip of a glass fiber and cemented using epoxy resin. The intensity data were collected using a Bruker SMART APEXII CCD diffractometer, equipped with a fine focus 1.75 kW sealed tube Mo Kα radiation (λ = 0.71073 Å) at 298(3) K, with increasing ω
(width of 0.3° per frame) at a scan speed of 5 s/frame. The SMART software was used for data acquisition. Data integration and reduction were undertaken with SAINT and XPREP15 software. Multiscan empirical absorption corrections were applied to the data using the program SADABS.16 Structures were solved by direct methods using SHELXS-9717 and refined with full-matrix least-squares on F2 using SHELXL-97.18 All non-hydrogen atoms were refined anisotropically; hydrogen atoms attached to all carbon atoms were geometrically fixed; and the positional and temperature factors are refined isotropically. Hydrogen atoms attached with the urea nitrogen atoms were located from the electron Fourier map and refined isotropically. Usually, temperature factors of H-atoms attached to carbon atoms are refined by restraints −1.2 or −1.5 Uiso (C), although the isotropic free refinement is also acceptable. Structural illustrations have been drawn with MERCURY-2.3.19 for Windows. Parameters for data collection and crystallographic refinement details of isolated anion complexes 1a−b and 2a−b are summarized in Table 1.
3. RESULTS AND DISCUSSION Structural information obtained from single crystal X-ray analyses of the anion complexes can provide insight into the binding differences of an anion with the isomeric receptors. From the perspective of anion receptor chemistry, crystallization has traditionally been a route to understand the structural insights of the anion complexes, which are then related to the observed selectivity in solution. 3.1. Single-Crystal X-ray Structural Analysis. 3.1.1. Carbonate Complex of L1, 2TEA[2L1(CO32−)] (1a). Structural elucidation revealed that the CO32−-encapsulated complex 1a crystallizes in the monoclinic system with centrosymmetric space group C2/c. Added HCO3− anion has deprotonated in solution and is bound in the form of CO32− by two inversionsymmetric units of L1 with an array of 14 N−H···O hydrogen bonds to the six urea functions, in the solid-state (Figure 1). A correlation of N−H···O angle vs N−H···O distance shows that 12 out of 14 hydrogen bonds are in the strong hydrogen bonding interaction region of d(H···O) ≤ 2.5 Å and d(D···O) ≤ 3.2 Å (Table 2), which indeed provide high stability to the 885
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Table 2. Hydrogen Bonding Contacts of Anions in Complexes 1a−b and 2a−b complex
D−H···O
d(H···O), Å
d(D···O), Å
⟨D−H···O, deg
1a
N2−H···O10 N3−H···O10 N5−H···O11 N6−H···O11 N8−H···O11 N8−H···O11′ N9−H···O11 N2H···O11 N2H···O10 N3H···O11 N5H···O10 N6H···O10 N8H···O11 N9H···O11 N2H···O21 N3H···O20 N5H···O21 N6H···O19 N8H···O21 N9H···O22 N12H···O22 N13H···O20 N15H···O22 N16H···O22 N18H···O20 N19H···O20 N19H···O19 C5H···O20 C27H···O22 N2H···O14 N3H···O15 N5H···O10 N6H···O11 N8H···O12 N8H···O13 N9H···O13 C14H···O11
2.22(4) 1.99(1) 2.23(2) 2.00(2) 2.63(2) 2.50(2) 1.89(2) 2.51(2) 2.39(2) 1.98(1) 2.14(1) 2.06(1) 2.56(1) 2.01(1) 2.15(3) 2.23(2) 2.04(3) 2.12(3) 1.98(2) 2.31(2) 2.04(3) 2.09(2) 2.31(2) 1.94(3) 2.37(3) 1.98(2) 2.68(3) 2.57(3) 2.59(2) 2.02(4) 1.97(5) 2.05(4) 1.92(4) 2.03(4) 2.65(6) 1.85(4) 2.70(5)
2.988(2) 2.807(2) 3.001(3) 2.828(3) 3.355(2) 3.230(2) 2.746(2) 3.255(2) 3.145(2) 2.838(2) 2.927(2) 2.855(2) 3.303(2) 2.851(2) 2.974(4) 3.054(4) 2.814(5) 2.964(5) 2.803(3) 3.150(4) 2.880(5) 2.913(4) 3.078(4) 2.782(4) 3.076(5) 2.816(4) 3.324(5) 3.352(6) 3.404(5) 2.864(6) 2.785(6) 2.866(6) 2.737(6) 2.878(6) 3.346(7) 2.709(5) 3.393(8)
147(1) 158(1) 148(1) 161(1) 143(1) 142(1) 169(1) 144(1) 146(1) 171(1) 151(1) 153(1) 145(1) 165(1) 158(2) 160(2) 148(2) 164(2) 159(3) 164(2) 164(2) 159(2) 148(2) 163(2) 139(3) 162(3) 132(3) 141(3) 146(3) 166(3) 158(3) 157(3) 158(3 167(3) 138(3) 177(3) 132(4)
1b
2a
Figure 1. (a) X-ray structure of 1a depicting the encapsulation of a CO32− anion within a centrosymmetric capsule of L1 and (b) ball-andstick representation (magnified view) depicting the 14 hydrogen bonding contacts on CO32− within the dimeric capsule of L1 in 1a (TEA cations and peripheral nitrophenyl groups are omitted for clarity).
CO32−-encapsulated molecular capsule. The inversion-symmetric molecules are flipped inward toward each other in a face-toface fashion with a distance of 8.00(2) Å between the apical nitrogen atoms (Figure 1) and, thereby, generate a centrosymmetric molecular capsule self-assembled via multiple C−H···O hydrogen bonds between each capsular unit. Such a solutionstate deprotonation of the protonated state of an anion namely, HCO3− and H2PO4−, is common and results because of the formation of multiple hydrogen bonding interactions with the receptor that lower the pKa of the bound guest to the extent that it is deprotonated by the free guest species in solution.8n The presence of carbonate in the isolated crystals has also been confirmed by 13C NMR and FT-IR spectroscopic techniques. 13C NMR of complex 1a (DMSO-d6) showed the resonance for the encapsulated CO32− anion at 172.01 ppm (Supporting Information), which when compared to the free resonance of (TEA)HCO3 (δ = 158.91 ppm) revealed a considerable downfield shift of 13.11 ppm suggesting the solution-state binding of the anion (ESI). Furthermore, a sharp and intense peak at 1108 cm−1 in the FT-IR spectrum of 1a may be attributed to the CO stretching of CO32− anion (ESI). Structural comparison of 1a (2TEA[2L1(CO32−)]) with the reported carbonate-encapsulated complex of the meta-isomer (2TBA[2L2(CO32−)])12b revealed a couple of noteworthy features that differentiate the binding of this anion (CO32−) within the respective self-assembled dimeric capsules. Irrespective of the nature of the countercation (TEA/TBA), both the complexes crystallized in the same space group (C2/c) with 14 hydrogen bonding contacts on the encapsulated CO32− anion. In the carbonate complex of the para-isomer (1a), the anion is coordinated by 14 N−H···O hydrogen bonds, where a
2b
−NH (N8H) group from each receptor molecule of the capsule acts as a bifurcated hydrogen bond donor to carbonate oxygen O11. However, in the carbonate complex of the meta-isomer, the binding occurs with the aid of 12 N−H···O hydrogen bonds (one from each −NH group of the dimeric assembly) and 2 C−H···O hydrogen bonds from an aryl function of each receptor unit (donated from the para-hydrogen with respect to the −NO2 group) of the dimeric assembly having a capsular length of 9.05 Å.12b Thus, substituting the meta-nitrophenyl terminal by its para-analogue in the tripodal urea scaffold resulted in a significant drop of 1.05 Å in the capsular size of the carbonate capsule 1a, which is indeed a consequence of positional isomeric effect. The effect of positional isomerism has also been observed in the solid-state binding of sulfate by L1 and L2 where the former in the presence of sulfate ions resulted in a pseudodimeric capsule partially engulfing a SO42−−3H2O− SO42− adduct, whereas the latter formed a sulfate encapsulated dimeric capsule. Thus, it is the anion that templates the formation of (pseudo)capsular self-assemblies in tripodal scaffolds and not the nature of the quaternary ammonium 886
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Figure 3. (a) X-ray structure of 2a depicting the encapsulation of a HPO42− (spacefilled) anion within the dimeric capsule of L2 and (b) Ball-and-stick representation (magnified view) depicting the 15 hydrogen bonding contacts on HPO42− within the dimeric capsule of L2 (TBA contercations and peripheral nitrophenyl groups are omitted for clarity).
Figure 2. (a) X-ray structure of 1b depicting the seven hydrogen bonding contacts on each −COO− residue of terephthalate dianion and intramolecular π···π stacking in each receptor molecule of the dimeric capsule of L2 and (b) spacefill representation depicting the pseudoencapsulation of a terephthalate dianion (TBA countercations are omitted for clarity).
The presence of terephthalate in the bulk isolated crystals of 1b has further been confirmed by 1H and 13C NMR (DMSOd6) analyses. 1H NMR showed an appreciable downfield shift of the urea −NH resonances with an average Δδ shift of 0.98 ppm relative to L1, and in 13C NMR, the resonance for the bound −COO − functions occur at 172.35 ppm (Supporting Information) indicating the existence of N−H···O hydrogen bonds with the terephthalate dianion in solution as well. 3.1.3. Hydrogenphosphate Complex of L 2 , 2TBA[2L2(HPO42−)] (2a). Complex 2a crystallizes in the triclinic system with P1̅ space group. The dihydrogen phosphate anion has deprotonated in presence of TBA(OH) and is bound in the form of HPO42− by two units of receptor L2. One ligand is coordinating in the axial mode and the other in facial mode with an array of 13 N−H···O hydrogen bonds to the six urea functions and 2 weak C−H···O hydrogen bonds (Figure 3). A correlation of N−H···O angle vs. N−H···O distance shows that 10 out of 13 N−H···O bonds are in the strong hydrogen bonding interaction region of d(H···O) ≤ 2.5 Å and d(N···O) ≤ 3.2 Å (Table 2). The L2 molecules are flipped inward toward each other in a face-to-face fashion with a distance of 9.859(5) Å between the apical nitrogen atoms and thereby, generate a rigid dimeric capsule assembled by π···π interactions of the phenyl rings (C1g···C4g = 3.817 Å). The presence of HPO42− in the isolated crystals has also been confirmed by 31P NMR spectroscopy. 31P NMR of complex 2a (DMSO-d6) showed the resonance for the encapsulated HPO42− anion at 20.98 ppm (Supporting Information), which when compared to the free resonance of (TBA)H2PO4
cations (TEA/TBA), which indeed have an overall effect in the crystal packing motif. 3.1.2. Terephthalate Complex of L1, 2TBA[2L1(C8H4O42−)] (1b). Structural elucidation revealed the formation of the 2:1 host−guest complex in 1b that crystallizes in the triclinic system with P1̅ space group. Each carboxylate group of a terephthalate dianion is coordinated to a receptor molecule by seven N−H···O hydrogen bonds to the three urea functions (Figure 2a). A correlation of N−H···O angle vs N−H···O distance shows that five out of seven hydrogen bonds are in the strong hydrogen bonding interaction region of d(H···O) ≤ 2.5 Å and d(D···O) ≤ 3.2 Å (Table 2). An anion is bound more strongly to the −NHb protons of the receptor with an average hydrogen bond distance of 2.848 Å, whereas the −NHa protons interact with an average hydrogen bond distance of 3.157 Å. Additionally, two tripodal side arms of a receptor molecule are involved in π−π interaction with a distance of 3.824 Å between the centroids of each aromatic (C2g···C3g). Two inversionsymmetric receptor molecules are flipped inward toward each other in a face-to-face fashion with a distance of 12.25(2) Å between the apical nitrogen atoms (Figure 2b) and, thereby, generate a pseudomolecular capsule engulfing a terephthalate dianion. Additional stability to the crystals of 1b is provided by intermolecular π-stacking interactions and C−H···O interactions between the adjacent anion bound receptor units. 887
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Figure 4. (a) X-ray structure of 2b depicting the double hydrogen bonding contacts on −COO− functions of terephthalate dianion and (b) 2D sheet like structure formed by the extension of hydrogen bonding interactions in 2b (TBA cations are omitted for clarity).
Figure 5. (a) Comparison of the 1H NMR spectra of L1 with (i) F− complex 1c, (ii) CO32− complex 1a, and (iii) L1 in the presence of equiv H2PO4−. (b) Comparison of the 1H NMR spectra of L3 with (i) L3 in the presence of equiv F−, (ii) L3 in the presence of equiv HCO3−, and (iii) L3 in the presence of equiv H2PO4−.
(δ = 2.50 ppm) revealed a considerable downfield shift of 18.5 ppm, suggesting the solution-state binding of the anion. 3.1.4. Terephthalate complex of L2, 3TBA[L2(C8H4O42−)1.5] (2b). Terephthalate complex, 2b crystallizes in the triclinic P1̅ space group with three lattice water molecules as the solvent of crystallization. Structural elucidation revealed the formation of a 1:1.5 host−guest complex, where each tripodal side arm of a receptor molecule is coordinated to a carboxylate function of three individual terephthalate dianions (Figure 4a). Each carboxylate group is coordinated to a urea function via double hydrogen bonding interactions, and each terephthalate dianion is sandwiched between the side arms of two receptor molecules generating an extended 2D sheet-like structure (Figure 4b). The carboxylate groups are bound with an active and equal participation from both the aliphatic −NHa and aromatic −NHb protons with an average donor-to-acceptor distance of 2.806 Å (Table 2). Unlike the terephthalate complex of L1 (1b), the receptor molecule in 2b could not include the terephthalate guest within the tripodal cavity, presumably due to the steric hindrance provided by the meta-nitro substitution of the peripheral aryl functions, and thereby adopts an extended trigonal planar-like geometry by interacting with multiple anionic residues. The inclusion of three H2O molecules in the crystal lattice provide added stabilization to the complex by forming hydrogen bonds with the urea oxygen atoms and with a carboxylate oxygen (O14) of a terephthalate dianion. Furthermore, the H2O molecules are weakly hydrogen bonded to the TBA cations and thereby strengthen the interactions between the cation−anion layers. The presence of terephthalate in 2b has also been confirmed by 1H and 13C NMR (DMSO-d6) analyses of the isolated
Figure 6. 31P NMR spectra of (a) 2a (HPO42− complex of L2), (b) L1 in the presence of TBA(H2PO4), and (c) L3 in the presence of TBA(H2PO4), as compared to the free resonance of TBA(H2PO4).
crystals. 1H NMR showed a high downfield shift of the urea −NH resonances with an average Δδ shift of 1.64 ppm relative to the free receptor, and in 13C NMR, the resonance of the bound −COO− functions occurs at 170.61 ppm (Supporting Information) indicating the presence of N−H···O hydrogen bonds with the terephthalate dianion in solution as well. Significant differences in the chemical shift values of −NH resonances (in 1H NMR) and −COO− resonances (in 13C NMR) of complexes 1b and 2b may be attributed to the different modes of terephthalate binding with the isomeric receptors L1 and L2 in solution as well. 3.2. Effect of Positional Isomerism in Solution-State Anion Binding. 1H NMR analyses (DMSO-d6) of the isolated anion complexes (1a−c, 2a,b) and of receptors L1 and L3 in 888
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Information). Thus, binding of HCO3− with the para-isomer L1 is comparable to that of the meta-isomer L2.12b On the other hand, titration of the ortho-isomer L3 with TEA(HCO3) showed an average downfield shift of 0.66 ppm for the −NH protons, and the association constant (log K) was calculated to be 1.38 with a 1:1 host−guest stoichiometry in agreement with the Job’s plot analysis (Supporting Information). 1 H NMR analysis of the F− complex 1c showed an appreciable downfield shift of 0.80 and 1.80 ppm for the −NHa and −NHb protons, respectively, suggesting that F− is bound more strongly to the −NHb protons of the receptor (Figure 5a). Titration data of L1 with standard TBAF solution gave the best fit for 1:1 host−guest stoichiometry with an association constant (log K) value of 3.72 (Supporting Information). However, in a qualitative experiment, addition of 1 equiv of TBAF to a solution of L3 in DMSO-d6 resulted in a downfield shift of 1.45 and 1.14 ppm for the −NHa and −NHb protons, respectively (Figure 5b), indicating that −NHa protons are more strongly hydrogen bonded to F− than −NHb protons. Similar to L1 and L2, titration of L3 with TBAF gave the best fit for 1:1 host−guest stoichiometry with an apparent binding constant (log K) value of 3.45 (Supporting Information). Similarly, in the qualitative test for H2PO4− binding, addition of 1 equiv. of the anion to the individual solutions of L1 and L3, resulted in a downfield shift of 1.46 and 0.77 ppm for the −NHa resonance and a shift of 1.22 and 0.65 ppm for the −NHb resonance of the isomeric receptors (Figure 5). Titration of L1 and L3 with TBA(H2PO4) gave the best fit for 1:1 host− guest stoichiometry with apparent binding constant (log K) values of 4.14 and 1.86, respectively. However, 31P NMR analyses of the individual receptorH2PO4− (excess) solutions showed a huge downfield shift of 18.50 ppm for the 31P resonance of bound H2PO4− relative to the free resonance of TBA(H2PO4). The shift is comparable with the 31P resonance of complex 2a, where a HPO42− anion is bound within the dimeric capsular assembly of the meta-isomer, L2 (Figure 6). Recently, employing a tris(thiourea) receptor, we have demonstrated that side-cleft binding of a phosphate moiety resulted in upfield shift of the 31P resonance, whereas encapsulation of the anion resulted in downfield shift of the 31 P resonance relative to the free resonance of TBA(H2PO4).8o Thus, 31P NMR studies establish the encapsulation of a phosphate moiety within the tripodal scaffolds of the isomeric receptors. From the 1H NMR analyses, it can be argued that ortho-nitro substitution at the peripheral aryl function significantly reduces
Figure 7. 2D-NOESY NMR spectra of receptors L1 and L3 in DMSOd6.
presence of equivalent amounts of different anions such as F−, HCO3−, and H2PO4− further demonstrate the effect of positional isomerism in nitrophenyl functionalized urea receptors. 2D-NOESY NMR experiments (DMSO-d6) performed with the isolated anion complexes and receptors L1 and L3 provide insight into the solution-state encapsulation of inorganic anions.20 1 H NMR analysis of the CO32−-encapsulated complex 1a showed an average downfield shift of 1.55 ppm for the −NH protons, indicating a strong solution-state binding of CO32− with the urea functions (Figure 5a). 1H NMR titration data of L1 with aliquots of standard TEA(HCO3) solution gave the best fit for 1:1 host−guest stoichiometry with an apparent binding constant (log K) value of 4.07 (Supporting
Scheme 2. Schematic Representation of the through-Space Interactions between Different Sets of Protons in (a) L1 and (b) L3 As Established from the 2D NOESY NMR Experiments of L1 and L3
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Figure 8. 2D NOESY NMR spectra of isolated complexes (a) 1a, (b) 1c, (c) 1b, and (d) 2b.
ortho-isomer showed two NOE couplings between the different sets of aryl protons (Hc···Hd and He···Hf) in addition to the urea Ha···Hb interaction (Scheme 2b). The lack of throughspace Hb···Hc interaction in L3 implies that the nitro-group is oriented toward the receptor cavity along with the urea −NH protons. Thus, ortho-nitro aromatic substitution to the Nbridged urea scaffold acts as a barricade for the receptor cavity that resists the binding and encapsulation of larger anions, in contrast to the para-isomer. Additionally, 2D-NOESY NMR analyses of the isolated anion complexes (Figure 8) further provide evidence of the subtle interplay of positional isomeric effect toward binding and/or encapsulation of anionic guests. NOESY NMR spectra of the carbonate and fluoride complexes 1a and 1c showed the disappearance of off-diagonal Ha···Hb coupling indicating the encapsulation of the respective anions within the tripodal cavity of receptor L1. However, the terephthalate complex of L1 (1b), showed the existence of off-diagonal Ha···Hb and Hb···Hc couplings as observed in the NOESY NMR spectrum of the free receptor, which suggests that the terephthalate dianion may not be engulfed within the receptor cavity in the solution state. Similar through space NOE couplings have also been observed in the 2D spectrum of the terephthalate complex of L2 (2b), which showed the presence of Ha···Hb and Hb···Hf interactions.
the ability of the tripodal urea scaffold toward binding and encapsulation of planar and tetrahedral oxoanions, as evident from the changes in −NH chemical shift values of the isomeric receptors in the presence of oxoanions such as HCO3− and H2PO4− and their log K values. However, fluoride is bound almost equally by the isomeric receptors. Furthermore, it is interesting to note that, in the 1H NMR spectra of complexes 1a−c and of an equimolar solution of L1 and H2PO4−, there are no observable changes in the positions of the aryl protons relative to the free receptor (L1), indicating the preorganization of the para-isomer that does not undergo any significant solution-state conformational changes upon binding and encapsulation of anions. However, 1H NMR spectra of the ortho-isomer (L3), in the presence of equivalent amounts of F−, HCO3−, and H2PO4−, showed a considerable upfield shift of the aryl protons, suggestive of a solution-state structural reorganization of the receptor side arms that could facilitate the formation of multiple N−H···A− hydrogen bonds to coordinate the anion within the receptor cavity via opening of the −NO2 barricade. The most significant changes in chemical shift have been observed for the more acidic aryl protons −Hc and −Hf, which get shifted upfield with an average Δδ value of 0.72 and 0.40 ppm, respectively (Figure 5b). 2D-NOESY NMR analyses of L1 and L3 in DMSO-d6 (Figure 7) provide evidence of the conformational preorganization of the isomeric receptors. The para-isomer showed a strong NOE coupling between the urea protons (Ha···Hb) and a comparatively weak coupling between the urea proton −Hb and ortho-aryl proton −Hc (Hb···Hc) (Scheme 2a), whereas the
4. CONCLUSION In summary, we have demonstrated the effect of positional isomerism in a set of nitrophenyl functionalized tripodal urea receptors toward solid and solution-state binding of anions of 890
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different dimensions. Structural elucidation revealed that the para-nitro-functionalized receptor L1 has the ability to encapsulate oxoanions of different sizes within the dimeric/ pseudodimeric capsular assembly of the receptor. Similarly, the meta-nitro-functionalized receptor L2 has the capability to encapsulate divalent inorganic oxoanions such as HPO42− within a dimeric capsular assembly of the receptor. However, in the presence of aromatic carboxylates such as terephthalate dianion, the receptor molecule adopts an extended and flat conformation where each receptor side arm is involved in complementary hydrogen bonding interactions with a carboxylate group of the dianion. Structural evidence of anion binding with receptor L3 could not be established, presumably because the nitro group substitution at the ortho-position acts as a barricade that resists the facile inclusion of oxoanions within the tripodal scaffold, as confirmed by 2D NOESY NMR analysis of the free receptor. The auxiliary evidence for the phenomenon comes from the apparent binding constant values of anions such as HCO3− and H2PO4−. Thus, the set of nitrophenyl functionalized tripodal urea scaffold can provide insight into the deliberate choice and incorporation of substituted aryl functions as peripheral groups for the design and synthesis of anion induced capsular self-assemblies of acyclic podand receptors having pendant arms.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Crystallographic files in CIF format, additional crystallographic data, characterization data, and 1H NMR titration plots. This material is available free of charge via the Internet at http:// pubs.acs.org.
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS G.D. gratefully acknowledges Department of Science and Technology (DST) New Delhi, India, for the financial support, DST-FIST for single crystal X-ray diffraction facility, and Central Instrument Facility at IIT Guwahati. R.C. and S.K.D. acknowledge IIT Guwahati, India, for fellowships.
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REFERENCES
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