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thiourea receptors
Inorganica Chimica Acta 486 (2019) 576–581
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Research paper
Fixation of atmospheric CO2 and recognition of anions/hydrated anions: Differential binding mode in protonated vs. neutral tripodal urea/thiourea receptors
T
Santanu Kayal, Utsab Manna, Gopal Das
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Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India
ARTICLE INFO
ABSTRACT
Keywords: Tripodal receptor Anion recognition Aerial CO2 fixation Oxyanion encapsulated capsule Anion-water network
Tris-(2-aminoethyl) amine (Tren) based tripodal thiourea (L1 and L2) and urea (L3) receptors are reported. L1 efficiently fix atmospheric CO2 as CO32– in presence of the hydroxide ion. Both L1 and L2 encapsulate SO42− anion by forming 2:1 host–guest capsular assemblies through hydrogen-bond activated proton transfer. Both L1 and L2 show strong anion binding in their neutral form through strong non-covalent interactions. On contrary, tris-urea ligand L3 recognizes SiF62− and Br− trapped cyclic/acyclic anion-water network assemblies outside the protonated receptor cavities in presence of corresponding inorganic acids HF and HBr respectively.
1. Introduction Recognition of anions and hydrated anions has attracted a great deal of attention due to their remarkable impact in biological, chemical, pharmaceutical, and environmental research [1–7]. An immense attention has mainly been raised to the understanding of anion-water clusters in hydrophobic environments [8-16] due to their vast importance in chemical and biological interfaces [17-20] as well as to find the insight knowledge of solvation mechanism, ion-mobility in bulk, ion translocation in water-membrane interfaces and electrical phenomena [21,22]. Iodide is essential in many neurological activity and thyroid function. Thyroid hormones play various important role to regulate several metabolic processes [23] resulting in the requirement of iodide in foodstuff and drinks as a part of the nutrition [24]. Bromide is also an essential cofactor in collagen IV scaffolds for tissue development [25]. Among several oxyanions recognition of tetrahedral sulfate anion draw immense interest due to their interaction with water molecules as a result of high charge density [26–31]. Consequently it is present in nuclear waste, radioactive waste and drinking water as contamination [32,33]. The extraction of sulfate from an aqueous medium to an organic medium is challenging task. It also affects the vitrification process in nuclear and radioactive waste [34-36]. So the removal of sulfate anion by synthetic hosts is a primary goal in anion recognition chemistry through host–guest interaction. Besides, sulfate ion is also important from the view point of biology as it is bound to proteins in
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salmonella typhimurium bacteria inside the hydrophobic cavity in neutral environment [37]. In modern days, significant rise of CO2 concentration in the atmosphere is a major environmental issue. It is the outcome of increased fossil fuels by industries, transportation, automobiles, etc. which is contributing in huge extent to make global climate change [38-40]. So fixation of atmospheric CO2 into green chemical by metal–organic framework (MOF), silica gel, activated carbon, micro porous aluminosilicates etc. have been highly implemented [41–43]. However, in the field of supramolecular chemistry efficient fixation and storage of aerial CO2 as carbonate/bicarbonate have been accomplished with artificial hydrogen-bonding scaffolds in the presence of hydroxide and fluoride ions [44,45]. In this regard, Gale et. al. have also shown capturing of CO2 as carbamates (alkylammonium/alkylcarbamate) by hydrogen bonding receptors in the presence of aliphatic amines as CO2 scrubbers bubbled with CO2 [46,47]. Gunnlaugsson et al. has also reported very early CO2 fixation through bicarbonate adduct formation [48]. In natural systems protein show selective binding towards anions by no covalent interactions which leads the researchers to develop synthetic receptors that utilize hydrogen bonds for specific binding sites available from amide, amine, urea, thiourea, pyrrole and indole functionalities to bind anionic guests.[1,4,49-58] In this regard tripodal receptors are a special class of acylic ionophores whose side arms functionalized with proper anion binding groups can selectively bind anions via complementary interaction and providing perfect space for anionic guests. Anion induced capsule and
Corresponding author. E-mail address: [email protected] (G. Das).
https://doi.org/10.1016/j.ica.2018.11.008 Received 20 August 2018; Received in revised form 6 November 2018; Accepted 6 November 2018 Available online 08 November 2018 0020-1693/ © 2018 Elsevier B.V. All rights reserved.
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excess tetrabutylammonium hydrogensulphate salts in separate glass vials. Subsequently, the divalent carbonate encapsulated host–guest complex 1b suitable for X-ray analysis was also achieved from the basic DMF solution of receptor L1 and excess tetrabutylammonium hydroxide salts, although carbonate complexation of receptor L2 were not successful even after many trials from different crystallization condition, instead a thick oily mass were seen to be observed at the bottom of the crystallization vials in most of the cases. Structural elucidation reveals that sulphate complex 1a and carbonate complex 1b were crystallize from the same triclinic space group P -1 with Z = 2, whereas the sulphate complex 2a crystallize in the monoclinic space group P 21/c with Z = 2. The asymmetric unit of DMF solvated complex 1a contains two symmetry-independent L1 receptor conformers, two half-occupied symmetry-identical divalent sulphate anions and their corresponding two fully occupied tetrabutylammonium counter cations. Subsequently, complex 2a contains a single L2 receptor unit, one half-occupied divalent sulphate anion and its corresponding one fully occupied tetrabutylammonium counter cation in the asymmetric unit. On the other hand, the asymmetric unit of complex 1b comprises two symmetryindependent L1 receptor conformers, one fully occupied divalent carbonate anion and its corresponding two tetrabutylammonium counter cations. The X-ray analyses of complexes 1a, 1b and 2a clearly reveal that the divalent tetrahedral sulphate anion or divalent planar carbonate anion is fully encapsulated (Fig. 2d, 2e, 2f in space-fill model) inside the neutral dimeric capsular cavity of either receptor L1 or L2 in 2:1 host–guest fashion with the aid of anion-receptor hydrogen-bonding interactions. Note that the four oxygen atoms of each sulphate anion present in the unit cell of either complex 1a and 2a are disordered over eight positions with half-occupancies each. In complex 1a, each symmetry-independent tetrahedral sulphate anions are encapsulated within the dimeric capsule of two face-to-face oriented symmetry-identical L1 receptor units by a total of eighteen (S8 containing sulphate, Fig. 2a) and twenty (S7 containing sulphate) hydrogen-bonding interactions, which are the combination of strong N−Hthiourea···Osulphate, weak C−Hoaryl···Osulphate or Anionsulphate··· π aryl connections. Likewise, the tetrahedral sulphate anion of complex 2a are also encapsulated inside the two face-to-face oriented symmetry-identical L2 receptor dimer by a total of seventeen hydrogen-bonding interactions (Fig. 2c) that are the combination of strong N−Hthiourea···Osulphate and weak C−Ho-aryl···Osulphate interactions. Subsequently, two symmetry-independent L1 receptor conformers in complex 1b also orient in a face-to-face fashion encapsulating a divalent carbonate anion (obtained via hydroxide anion induced aerial CO2 fixation) within the dimeric receptor capsule with the assistance of total seventeen strong N−Hthiourea···Ocarbonate hydrogen-bonding interactions (Fig. 2b). The X-ray analyses also clearly state that each oxyanion in either of the complexes exhibit optimum coordination number, where each oxyanion oxygen atom accepts at least three hydrogen bonds. Also note that, these kinds of strongly basic F−/OH− anion induced aerial CO2 fixation as carbonate/bicarbonate at the air solvent interface [59,63,72,73] and subsequently the divalent sulphate recognition via hydrogen-bonding activated proton transfer reaction from mono-valent HSO4− anion are also reported in literature [74,75].
Scheme 1. Molecular architectures, schematic representation of synthetic pathways of receptors L1, L2 and L3.
pseudocapsule of tripodal receptors have shown a wide range of properties such as encapsulation of hydrated anion cluster,[72] fixation of carbon dioxide as carbonate,[59,63] liquid–liquid extraction of anions from water,[60] selective separation of anions by crystallization [61] and trans membrane anion transportation.[62] The tripodal moiety of tris(2-aminoethyl) amine has been utilized for long time to recognize spherical anions (fluoride, chloride, bromide, Iodide), planar (carbonate, nitrate), tetrahedral anions (sulphate, phosphate, perchlorate, etc.) and octahedral anions (SiF6). In our continuous effort in the field of anion-recognition chemistry to encapsulate anionic/hydrated-anionic guests[63-73] inside the host cavity, herein we report the full encapsulation of divalent sulfate or carbonate anion in 2:1 neutral host–guest fashion as (L1)2-SO42− in complex 1a, (L1)2-CO32− in complex 1b, (L2)2-SO42− in complex 2a and subsequently, anionwater cyclic/acyclic network assembly formation of SiF62− (complex 3) and Br− (complex 4) at outside the protonated tripodal receptor cavities of L3 (Scheme 1). 2. Results and discussion 2.1. Single crystal X-ray structural analysis studies 2.1.1. Structure of the free host molecules L1 and L2 The proper single crystals suitable for X-ray analysis of free tripodal thiourea receptors L1 and L2 were obtained from individual DMF solutions and X-ray elucidation reveals that they crystallize from the monoclinic space group P 21/n and triclinic space group P-1 respectively. The asymmetric unit of each free receptor contains single L1/L2 ligand unit, where the thiourea –NH groups of each tripodal side arm of either receptor L1 or L2 are projected towards three different directions (Fig. 1) in absence of any anionic guests. Structural analyses also clearly states that each receptor unit of either L1 or L2 receptor unit are coordinated with adjacent symmetry-identical receptor units via strong N e H···S hydrogen-bonding interactions. The adjacent identical-symmetry meta-bromophenyl substituted L1 receptor units get extra stability by several weak C e H···S, N e H···π and Br···Br interactions (Fig. 1b). On the other hand, meta-trifluoromethylphenyl functionalized receptor L2 become additionally stabilized by weak C e H··· π interactions with adjacent symmetry-identical ligands present in the unit cell (Fig. 1d).
2.1.3. Hydrated-hexafluorosilicate complex [{(L3H+)2(SiF62−)}(H2O)7] (3) and Hydrated-bromide complex [{(L3H+)(Br−)}(H2O)2] (4) The good quality crystals suitable for single crystal X-ray analyses of hydrated- hexafluorosilicate complex 3 and hydrated-bromide complex 4 of protonated para-iodophenyl functionalized tris-urea receptor were obtained from the individual basic DMF solvent mixture of ligand L3 and hydrofluoric acid (HF) or hydrobromic acid (HBr) respectively. Structural elucidation reveals that both hydrated-complexes 3 and 4 crystallize in the same monoclinic space group P 21/c with Z = 4. However, the asymmetric unit of complex 3 contains two symmetry-independent protonated L3 receptor conformers, one divalent hexafluorosilicate anion (SiF62−) and seven water molecules of
2.1.2. Sulphate complex [(n-TBA)2{(L1)(SO4)}(DMF)] (1a), carbonate complex [(n-TBA)2{(L1)(CO3)}] (1b) and sulphate complex [(nTBA)2{(L2)(SO4)}] (2a) The good quality colourless crystals of divalent sulphate encapsulated complexes 1a and 2a of respective receptors L1 and L2 were achieved from the individual basic DMF solutions of receptors and 577
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Fig. 1. X-ray structure (partial) of free receptors depicting (a) ORTEP plot of receptor L1 at 30% probability level, (b) non-covalent interactions involved around each identical symmetry L1 receptor unit, (c) ORTEP plot of receptor L2 at 30% probability level and (d) non-covalent interactions involved around each identical symmetry L2 receptor unit.
crystallization. Whereas, the complex 4 comprises of single protonated L3 receptor unit, one monovalent bromide anion and two water molecules of crystallization in the asymmetric unit. X-ray structure determination reveal that the urea –NH group of triopodal protonated receptor L3 in both complexes 3 and 4 are projected in three different directions, closely resemble to the free neutral L1 and L2 tris-thiourea receptors (Average terminal aryl centroid distances are 4.95 Å in 3, 4.69 Å in 4, 4.87 Å in free L1 and 5.33 Å in free L2). However, unlike the free L1 and L2 tris-thiourea receptors, the protonated receptor L3 of each complex 3 and 4 are stabilized by two strong intramolecular N−Hapical···Ocarbonyl and N−Hurea···Ocarbonyl interactions, which may prevent them to open up the tripodal side arms to encapsulate the anionic guests. As a consequence, the octahedral SiF62− anion or spherical Br− anion remain in outside the protonated receptor cavity, constructing various types of
cyclic/acyclic hydrogen-bonded network assemblies (Fig. 3a, 3b, 3d, 3e) with the assistance of crystallized water molecules in complexes 3 and 4. Note that in complex 3, the higher coordinating octahedral SiF62− anion outside the receptor cavity gains additional stability by consecutive formation of cyclic construction of beautiful cyclic hydrogen-bonded network via alternative formation of R1010(16) and R66(10) type cyclic anion-water clusters (Fig. 3a). Subsequently, the lower coordinating spherical bromide anions outside the receptor cavity in complex 4 acquire extra stability by consecutive formation of cyclic R44(12) type hydrogen-bonded architecture via C−Hreceptor···Br interactions (Fig. 3d). The packing motifs show the formation of linear anion-water channels along the outer hydrophobic side of the protonated receptor conformers as viewed along the crystallographic c and b axes of complexes 3 (Fig. 3c) and 4 (Fig. 3f) respectively.
Fig. 2. X-ray structures (partial) of 2:1 receptor-oxyanion complexes depicting the array of hydrogen bonding interactions in (a) (L1)2-SO4 capsule, (b) (L1)2-CO3 capsule, (c) (L2)2-SO4; the space-fill representation of sulphate or carbonate encapsulation in (d) complex 1a, (e) complex 1b and (f) complex 2a. 578
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Fig. 3. X-ray structures (partial) of hydrated-anion complexes depicting (a) hexafluorosilicate-water cyclic architecture formation via strong H-bonding interactions outside the receptor cavity in complex 3, (b) coordination environment of hexafluorosilicate anion, (c) packing diagram of complex 3 from c-axis, (d) cyclic receptorbromide architecture formation via strong H-bonding interactions outside the receptor cavity in complex 4, (e) H-bonding environment of protonated receptors with bromide and water and (f) packing diagram of complex 4 from c-axis.
A correlation of NeH⋯A (Anion) angle vs. H⋯A (Anion) distance displays that most of the hydrogen-bonding interactions among receptor and anions in solid state are present in strong hydrogen bonding interaction region of d (H⋯A) ≤ 2.6 Å and d(D⋯A) ≤ 3.3 Å (Table S1, supporting information).
purification. Solvents used for synthesis and crystallization were bought from Merck, India and used as supplied. 1H NMR spectra were recorded on a Varian FT-600 MHz machine and chemical shifts were recorded in parts per million (ppm) on a scale using tetramethylsilane [Si(CH3)4] or a residual solvent peak as a reference. 13C spectra were obtained at 150 MHz. The electrospray ionization mass spectrometry (ESI-MS) spectra of individual free receptors were recorded in methanol. The FTIR spectra of air-dried samples were recorded on a Perkin-ElmerSpectrum on FT-IR spectrometer with KBr disks over the range of 4000–450 cm−1.
3. Conclusions In summary, we have synthesized and developed two flexible tristhiourea receptors L1 and L2 which show strong oxyanion encapsulation abilities their neutral form. Subsequently, in presence of inorganic acids, the apical nitrogen atom of a tripodal tris-urea receptor L3 was easily protonated to form cyclic/acyclic anion-water assemblies outside the receptor cavities. Structural determination approves that both receptors L1 and L2 can readily form divalent sulphate encapsulated 2:1 dimeric host–guest capsular assemblies by H-bonding activated proton transfer from monovalent hydrogensulphate anion. Furthermore, the tris-thioura ligand L1 also produces planar divalent carbonate encapsulated dimeric capsular assemblies by hydroxide anion induced aerial CO2 fixation in solid state. On the other hand, the iodo-pheneyl substituted protonated tris-urea receptor form divalent hexafluorosilicate and monovalent bromide trapped anion-water assembled linear channel outside the receptor cavity in presence of inorganic acids HF and HBr respectively. Hence, the structural analysis clearly unveils the formation of both neutral and protonated anionic assemblies by similar kinds of tris-thiourea and tris-oxyurea receptors containing hydrophobic environment would be useful to the supramolecular researchers to develop the new classes of host–guest assemblages in neutral as well as in protonated form.
4.2. Syntheses and characterization 4.2.1. Receptors L1, L2 and L3 The tripodal receptors were attained in quantitative yield by the reaction of Tren [tris(2-aminoethyl)-amine] (2.0 mmol) with 3.0 equiv of the 3-Bromophenyl isothiocyanate (6.0 mmol), 3-(Trifluoromethyl) phenyl isothiocyanate (6.0 mmol) and 4-Iodophenylisocyanate (6.0 mmol) respectively in individual solution of dry acetonitrile solvent medium. The resulting reaction mixtures in separate 250 mL round-bottomed flask were stirred for overnight at room temperature. Then, the excess acetonitrile was reduced in vacuo and the resulting solid precipitates in each case of ligands L1, L2 and L3 were obtained, filtered off and washed with dry acetonitrile, dry THF and dichloromethane to wash out the unreacted reagents and then characterized by NMR and FT-IR analyses (Supporting Information). Yield = 90%, 80% and 80% for respective ligands L1, L2 and L3. L1: 1H NMR (600 MHz, DMSO‑d6) δ (ppm): 2.738–2.760 (t, 6H, 6.6 Hz, –NCH2), 3.601 (t, 6H, –NCH2CH2), 7.251 (3H, Ar-H), 7.260 (3H, Ar-H), 7.339 (3H, Ar-H), 7.791 (s, 3H, Ar-H), 7.820 (s, 3H, -NHb), 9.739 (s, 3H, -NHa). 13C NMR (150 MHz, DMSO‑d6) δ (ppm): 42.279 (×3C, –NCH2CH2), 52.335 (×3C, –NCH2), 121.602 (×3C, Ar-C), 121.883 (×3C, Ar-C), 125.394 (×3C, Ar-C), 126.887 (×3C, Ar-C), 130.916(×3C, Ar-C). 141.468 (×3C, Ar-C), 180.590 (×3C, thiocarbonyl carbon). FT-IR spectra (KBr pellet): 3230 cm−1 vs(NeH), 3063 cm−1 vs(CeH), 2847 cm−1 vs(CeH), 1566 cm−1 vs(C]S, sym), 702 cm−1 vs(C]S, asym). ESI-MS: m/z 789.9381 [L+H] L2: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 2.766–2.787(t, 6H ∼ 6.3 Hz, –NCH2) 3.629 (t, 6H, –NCH2CH2), 7.386–7.399(d, 3H,
4. Experimental section 4.1. Materials and methods All the reagents and solvents were purchased from commercial sources and used without any purification. Tren [tris(2-aminoethyl)amine], 3-Bromophenyl isothiocyanate, 3-(Trifluoromethyl)phenyl isothiocyanate, 4-Iodophenylisocyanate and tetrabutylammonium salts were purchased from Sigma-Aldrich while common inorganic acids were purchased from local vendors, India and used without further 579
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Table 1 Crystallographic Parameters and Refinement Data of free receptors and anion-complexes. Parameters
L1
L2
1a
1b
2a
3
4
Formula Fw Crystal system Space group a/Å b/Å c/Å α/o β/o γ/o V/Å3 Z Dc/g cm−3 μ Mo Kα/mm−1 F000 T/K θ max. Total no. of reflections Independent reflections Observed reflections Parameters refined R1, I > 2σ(I) wR2, I > 2σ(I) GOF (F2) CCDC No.
C27H30Br3N7S3 788.46 monoclinic P 21/n 20.089(9) 7.610(3) 20.360(7) 90.00 92.984(4) 90.00 3108.4(2) 4 1.685 4.126 1576.0 298(2) 24.997 14,239 5461 3858 361 0.0441 0.0911 1.036 1,861,244
C30H30F9N7S3 755.79 triclinic P -1 7.921(6) 12.808(16) 18.054(19) 70.542(10) 81.001(7) 89.924(8) 1703.2(3) 2 1.474 0.301 776.0 298(2) 24.998 11,757 5994 2252 497 0.0733 0.1434 0.937 1,861,245
'C89H139Br6N17O5S7 2230.99 triclinic P -1 13.828(8) 14.675(8) 27.249(15) 97.691(4) 99.437(5) 95.313(4) 5368.1(5) 2 1.380 2.434 2308.0 298(2) 24.999 40,787 18,807 7001 1167 0.0880 0.1537 1.069 1,861,246
C87H132Br6N16O5S6 2121.85 triclinic P -1 15.853(5) 17.215(8) 21.885(7) 81.151(3) 70.266(3) 65.393(4) 5110.5(4) 2 1.379 2.531 2192.0 298(2) 28.853 45,013 23,180 7277 1059 0.0956 0.1611 1.196 1,861,247
C92H132F18N16O4S7 2092.56 monoclinic P 21/c 14.979(5) 14.038(4) 26.796(12) 90.00 98.610(3) 90.00 5570.8(4) 2 1.247 0.224 2538.0 298(2) 24.999 27,265 9788 5213 641 0.0914 0.1661 0.932 1,861,248
C54H62F6I6N14O13Si 2018.67 monoclinic P 21/c 28.824(9) 13.789(5) 18.252(8) 90.00 99.094(3) 90.00 7163.4(4) 4 1.872 2.700 3896.0 298(2) 25.000 33,069 12,594 7423 856 0.0544 0.1559 1.058 1,861,249
C27H31BrI3N7O5 994.19 monoclinic P 21/c 26.676(14) 15.158(11) 8.490(5) 90.00 91.839(5) 90.00 3431.0(4) 4 1.925 3.943 1904.0 298(2) 25.000 14,568 6019 3632 392 0.0463 0.1248 0.865 1,861,250
7.8 Hz, Ar-H), 7.494–7.520 (t, 3H, 7.8 Hz, Ar-H), 7.660–7.673 (d, 3H, 7.8 Hz, Ar-H), 7.881(s, 3H, -NHb), 7.954(s, 3H, Ar-H), 9.869(s, 3H, -NHa). 13C NMR (150 MHz, DMSO‑d6) δ(ppm): 42.221 (×3C, 52.308(×3C, –NCH2), 119.140(×3C, Ar-C), –NCH2CH2), 120.450(×3C, Ar-C), 123.616(×3C, Ar-C), 125.419(×3C, Ar-C), 126.583(×3C, Ar-C). 130.044 (×3C, -CF3), 140.808 (×3C, Ar-C), 180.834(×3C, thiocarbonyl carbon). FT-IR (υ cm−1): 3340 cm−1 vs (NeH), 3257 cm−1 vs(CeH), 3062 cm−1 vs (CeH), 1577 cm−1 vs(C] S), 702 cm−1 vs(C]S, asym). ESI-MS: m/z 756.1802 [L+H]. L3: 1H NMR spectrum of tris-urea receptor L3 (600 MHz, DMSO‑d6) δ (ppm): 2.568–2.590 (t, J = 6.6 Hz, 6H, –NCH2), 3.155–3.187 (q, J = 6.6 Hz, 6H, –NCH2CH2), 6.175–6.193 (t, J = 5.4 Hz, 3H, –NHa), 7.330–7.345 (d, 6H, J = 9.0 Hz, -ArH), 7.348–7.363 (d, 6H, J = 9.6 Hz, – ArH), 8.668 (s, 3H, -NHb). ESI-MS: m/z 881.9675 [L+H].
TBA-CH2) 2.659 (6H, –NCH2), 3.129–3.157(t, 8.4 Hz, 8H, TBA-CH2), 3.544(6H, –NCH2CH2), 7.155(3H, Ar-H), 7.181(3H, Ar-H), 7.337(3H, Ar-H), 7.883(3H, Ar-H), 8.379(s, 3H, -NHb), 10.845(s, 3H, -NHa). 13C NMR (150 MHz, DMSO‑d6) δ(ppm): 13.992 (×4C, TBA-CH3), 19.688(×4C, TBA-CH2), 23.512(×4C, TBA-CH2), 42.097(×3C, –NCH2CH2), 52.608(×3C, –NCH2), 57.942(×4C, TBA-CH2), 121.314(×3C, Ar-C), 121.680(×3C, Ar-C), 125.267(×3C, Ar-C), 126.390(×3C, Ar-C), 130.491(×3C, Ar-C), 142.062(×3C, Ar-C), 168.974(×1C, Carbonate-C]O), 180.653(×3C, thiocarbonyl carbon). FT-IR spectra (KBr pellet): 3230 cm−1 vs(NeH), 3060 cm−1 vs(CeH), 2957 cm−1 vs(CeH), 1546 cm−1 vs(CeO, carbonate), 1477 cm−1 vs (C]S, sym), 708 cm−1 vs(C]S, asym). 2a: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 0.916–0.940 (t, 7.2 Hz, 12H, TBA-CH3), 1.268–1.329 (m, 8H, TBA-CH2), 1.529–1.582 (m, 8H, TBA-CH2) 2.633 (6H, –NCH2), 3.139–3.167(t, 8.4 Hz, 8H, TBA-CH2), 3.341 (solvent-H, H2O), 3.550 (6H, –NCH2CH2), 7.309–7.320 (d, 6.6 Hz, 3H, Ar-H), 7.401–7.426 (t, 7.5 Hz, 3H, Ar-H), 7.763–7.774 (d, 6.6 Hz, 3H, Ar-H), 8.116 (s, 3H, Ar-H), 8.936 (s, 3H, –NHb), 10.681(s, 3H, –NHa). FT-IR spectra (KBr pellet): 3283 cm−1 vs(NeH), 2959 cm−1 vs(CeH), 2929 cm−1 vs(CeH), 1548 cm−1 vs(C]S, sym), 705 cm−1 vs (C]S, asym).
4.2.2. Sulphate complex [(n-TBA)2{(L1)(SO4)}(DMF)] (1a) and carbonate complex [(n-TBA)2{(L1)(CO3)}] (1b) and sulphate complex [(n-TBA)2{(L2)(SO4)}] (2a) The sulphate and carbonate encapsulated complexes of either L1 or L2 of tris-thiourea receptors were achieved by charging excess of tetrabutylammonium hydrogensulphate (10 eqv.) or tetrabutylammonium hydroxide (10 eqv.) into the 5 mL DMF solutions of either L1 (100 mg) or L2 (100 mg) in three separate small glass vials. The resulting solutions after hydrogensulphate or hydroxide salts addition were stirred for about 30 min and were left open to atmosphere for slow vaporization at room temperature. The good quality crystals of sulphate encapsulated complexes 1a, 2a and carbonate encapsulated complex 1b suitable for single crystal X-ray analysis were attained within 10–15 days. Yield = 70%, 85% and 70% for respective complexes 1a, 1b and 2a. 1a: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 0.912–0.936(t, 7.2 Hz, 12H, TBA-CH3), 1.264–1.325(m, 8H, TBA-CH2), 1.523–1.576(m, 8H, TBA-CH2) 2.599 (6H, –NCH2), 3.132–3.160(t, 8.4 Hz, 8H, TBA-CH2), 3.502 (6H, –NCH2CH2), 7.145 (3H, Ar-H), 7.163(3H, Ar-H), 7.465(3H, Ar-H), 7.950(3H, Ar-H), 8.821(s, 3H, -NHb), 10.476(s, 3H, -NHa). FT-IR spectra (KBr pellet): 3277 cm−1 vs(NeH), 2962 cm−1 vs(CeH), 2920 cm−1 vs(CeH), 1540 cm−1 vs(C]S, sym), 718 cm−1 vs(C]S, asym). 1b: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 0.913–0.937(t, 7.2 Hz, 12H, TBA-CH3), 1.265–1.326(m, 8H, TBA-CH2), 1.523–1.576(m, 8H,
4.2.3. Hydrated-hexafluorosilicate complex [{(L3H+)2(SiF62−)}(H2O)7] (3) and Hydrated-bromide complex [{(L3H+)(Br−)}(H2O)2] (4) The hydrated hexafluorosilicate and bromide complexes of protonated tris-oxyurea receptor L3 were achieved from the basic DMF solution mixtures of L1 (100 mg) and inorganic acids HF or HBr respectively from separate small glass vials. After the addition of HF or HBr, the resulting solutions was stirred for about 1 h and kept in open air for slow evaporation at room temperature. The good quality crystals of complex 3 and 4 appropriate for single crystal X-ray analysis were obtained within 20–25 days. Yield: 70% of 3 and 65% of 4 based on L3. 4.3. Crystallographic refinement details The crystallographic refinement details of for free receptors and all neutral as well as protonated anion complexes 1a, 1b, 2a, 3 and 4 are summarized in Table 1 and all the data have been deposited to the CCDC. A crystal of appropriate size was carefully chosen in each case from the mother liquor and immersed in silicone oil and then it was 580
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mounted on the tip of a glass fiber, cemented using epoxy resin. The Xray crystallographic intensity data were collected using Supernova single source at offset, Eos diffractometer using Mo-Kα radiation (λ = 0.71073 Å) equipped with CCD area detector and corresponding data refinement and cell reduction were carried out by CrysAlisPro [76]. The data integration and reduction were undertaken with SAINT and XPREP [77] software and multi-scan empirical absorption corrections were applied to the data using the program SADABS [78]. All the crystal structures were solved by direct methods using SHELXTL-2014 and were refined on F2 by the full-matrix least-squares technique using the SHELXL-2014 program package [79]. Graphics for structural determination are generated using MERCURY 2.3 [80] for Windows. It is important to mention that, in the case of protonated anion complexes 3 and 4, we could not add hydrogen atoms to water molecules even after many trials, because the OeH bond of water molecules was very large and could not be refined. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms attached to all carbon atoms were geometrically fixed, the positional and temperature factors are refined isotropically. The hydrogen atoms are located on a difference Fourier map and refined, wherever it is possible and in other cases, the hydrogen atoms are geometrically fixed.
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Acknowledgments This work was supported by CSIR and SERB through grant 01/ 2727/13/EMR-II and SR/S1/OC-62/2011, New Delhi, India. G.D acknowledges the CIF, IIT Guwahati, India and DST-FIST of India for providing instrument facilities. S.K and U.M thank IITG for fellowship. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2018.11.008. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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Research paper
Fixation of atmospheric CO2 and recognition of anions/hydrated anions: Differential binding mode in protonated vs. neutral tripodal urea/thiourea receptors
T
Santanu Kayal, Utsab Manna, Gopal Das
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Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India
ARTICLE INFO
ABSTRACT
Keywords: Tripodal receptor Anion recognition Aerial CO2 fixation Oxyanion encapsulated capsule Anion-water network
Tris-(2-aminoethyl) amine (Tren) based tripodal thiourea (L1 and L2) and urea (L3) receptors are reported. L1 efficiently fix atmospheric CO2 as CO32– in presence of the hydroxide ion. Both L1 and L2 encapsulate SO42− anion by forming 2:1 host–guest capsular assemblies through hydrogen-bond activated proton transfer. Both L1 and L2 show strong anion binding in their neutral form through strong non-covalent interactions. On contrary, tris-urea ligand L3 recognizes SiF62− and Br− trapped cyclic/acyclic anion-water network assemblies outside the protonated receptor cavities in presence of corresponding inorganic acids HF and HBr respectively.
1. Introduction Recognition of anions and hydrated anions has attracted a great deal of attention due to their remarkable impact in biological, chemical, pharmaceutical, and environmental research [1–7]. An immense attention has mainly been raised to the understanding of anion-water clusters in hydrophobic environments [8-16] due to their vast importance in chemical and biological interfaces [17-20] as well as to find the insight knowledge of solvation mechanism, ion-mobility in bulk, ion translocation in water-membrane interfaces and electrical phenomena [21,22]. Iodide is essential in many neurological activity and thyroid function. Thyroid hormones play various important role to regulate several metabolic processes [23] resulting in the requirement of iodide in foodstuff and drinks as a part of the nutrition [24]. Bromide is also an essential cofactor in collagen IV scaffolds for tissue development [25]. Among several oxyanions recognition of tetrahedral sulfate anion draw immense interest due to their interaction with water molecules as a result of high charge density [26–31]. Consequently it is present in nuclear waste, radioactive waste and drinking water as contamination [32,33]. The extraction of sulfate from an aqueous medium to an organic medium is challenging task. It also affects the vitrification process in nuclear and radioactive waste [34-36]. So the removal of sulfate anion by synthetic hosts is a primary goal in anion recognition chemistry through host–guest interaction. Besides, sulfate ion is also important from the view point of biology as it is bound to proteins in
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salmonella typhimurium bacteria inside the hydrophobic cavity in neutral environment [37]. In modern days, significant rise of CO2 concentration in the atmosphere is a major environmental issue. It is the outcome of increased fossil fuels by industries, transportation, automobiles, etc. which is contributing in huge extent to make global climate change [38-40]. So fixation of atmospheric CO2 into green chemical by metal–organic framework (MOF), silica gel, activated carbon, micro porous aluminosilicates etc. have been highly implemented [41–43]. However, in the field of supramolecular chemistry efficient fixation and storage of aerial CO2 as carbonate/bicarbonate have been accomplished with artificial hydrogen-bonding scaffolds in the presence of hydroxide and fluoride ions [44,45]. In this regard, Gale et. al. have also shown capturing of CO2 as carbamates (alkylammonium/alkylcarbamate) by hydrogen bonding receptors in the presence of aliphatic amines as CO2 scrubbers bubbled with CO2 [46,47]. Gunnlaugsson et al. has also reported very early CO2 fixation through bicarbonate adduct formation [48]. In natural systems protein show selective binding towards anions by no covalent interactions which leads the researchers to develop synthetic receptors that utilize hydrogen bonds for specific binding sites available from amide, amine, urea, thiourea, pyrrole and indole functionalities to bind anionic guests.[1,4,49-58] In this regard tripodal receptors are a special class of acylic ionophores whose side arms functionalized with proper anion binding groups can selectively bind anions via complementary interaction and providing perfect space for anionic guests. Anion induced capsule and
Corresponding author. E-mail address: [email protected] (G. Das).
https://doi.org/10.1016/j.ica.2018.11.008 Received 20 August 2018; Received in revised form 6 November 2018; Accepted 6 November 2018 Available online 08 November 2018 0020-1693/ © 2018 Elsevier B.V. All rights reserved.
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excess tetrabutylammonium hydrogensulphate salts in separate glass vials. Subsequently, the divalent carbonate encapsulated host–guest complex 1b suitable for X-ray analysis was also achieved from the basic DMF solution of receptor L1 and excess tetrabutylammonium hydroxide salts, although carbonate complexation of receptor L2 were not successful even after many trials from different crystallization condition, instead a thick oily mass were seen to be observed at the bottom of the crystallization vials in most of the cases. Structural elucidation reveals that sulphate complex 1a and carbonate complex 1b were crystallize from the same triclinic space group P -1 with Z = 2, whereas the sulphate complex 2a crystallize in the monoclinic space group P 21/c with Z = 2. The asymmetric unit of DMF solvated complex 1a contains two symmetry-independent L1 receptor conformers, two half-occupied symmetry-identical divalent sulphate anions and their corresponding two fully occupied tetrabutylammonium counter cations. Subsequently, complex 2a contains a single L2 receptor unit, one half-occupied divalent sulphate anion and its corresponding one fully occupied tetrabutylammonium counter cation in the asymmetric unit. On the other hand, the asymmetric unit of complex 1b comprises two symmetryindependent L1 receptor conformers, one fully occupied divalent carbonate anion and its corresponding two tetrabutylammonium counter cations. The X-ray analyses of complexes 1a, 1b and 2a clearly reveal that the divalent tetrahedral sulphate anion or divalent planar carbonate anion is fully encapsulated (Fig. 2d, 2e, 2f in space-fill model) inside the neutral dimeric capsular cavity of either receptor L1 or L2 in 2:1 host–guest fashion with the aid of anion-receptor hydrogen-bonding interactions. Note that the four oxygen atoms of each sulphate anion present in the unit cell of either complex 1a and 2a are disordered over eight positions with half-occupancies each. In complex 1a, each symmetry-independent tetrahedral sulphate anions are encapsulated within the dimeric capsule of two face-to-face oriented symmetry-identical L1 receptor units by a total of eighteen (S8 containing sulphate, Fig. 2a) and twenty (S7 containing sulphate) hydrogen-bonding interactions, which are the combination of strong N−Hthiourea···Osulphate, weak C−Hoaryl···Osulphate or Anionsulphate··· π aryl connections. Likewise, the tetrahedral sulphate anion of complex 2a are also encapsulated inside the two face-to-face oriented symmetry-identical L2 receptor dimer by a total of seventeen hydrogen-bonding interactions (Fig. 2c) that are the combination of strong N−Hthiourea···Osulphate and weak C−Ho-aryl···Osulphate interactions. Subsequently, two symmetry-independent L1 receptor conformers in complex 1b also orient in a face-to-face fashion encapsulating a divalent carbonate anion (obtained via hydroxide anion induced aerial CO2 fixation) within the dimeric receptor capsule with the assistance of total seventeen strong N−Hthiourea···Ocarbonate hydrogen-bonding interactions (Fig. 2b). The X-ray analyses also clearly state that each oxyanion in either of the complexes exhibit optimum coordination number, where each oxyanion oxygen atom accepts at least three hydrogen bonds. Also note that, these kinds of strongly basic F−/OH− anion induced aerial CO2 fixation as carbonate/bicarbonate at the air solvent interface [59,63,72,73] and subsequently the divalent sulphate recognition via hydrogen-bonding activated proton transfer reaction from mono-valent HSO4− anion are also reported in literature [74,75].
Scheme 1. Molecular architectures, schematic representation of synthetic pathways of receptors L1, L2 and L3.
pseudocapsule of tripodal receptors have shown a wide range of properties such as encapsulation of hydrated anion cluster,[72] fixation of carbon dioxide as carbonate,[59,63] liquid–liquid extraction of anions from water,[60] selective separation of anions by crystallization [61] and trans membrane anion transportation.[62] The tripodal moiety of tris(2-aminoethyl) amine has been utilized for long time to recognize spherical anions (fluoride, chloride, bromide, Iodide), planar (carbonate, nitrate), tetrahedral anions (sulphate, phosphate, perchlorate, etc.) and octahedral anions (SiF6). In our continuous effort in the field of anion-recognition chemistry to encapsulate anionic/hydrated-anionic guests[63-73] inside the host cavity, herein we report the full encapsulation of divalent sulfate or carbonate anion in 2:1 neutral host–guest fashion as (L1)2-SO42− in complex 1a, (L1)2-CO32− in complex 1b, (L2)2-SO42− in complex 2a and subsequently, anionwater cyclic/acyclic network assembly formation of SiF62− (complex 3) and Br− (complex 4) at outside the protonated tripodal receptor cavities of L3 (Scheme 1). 2. Results and discussion 2.1. Single crystal X-ray structural analysis studies 2.1.1. Structure of the free host molecules L1 and L2 The proper single crystals suitable for X-ray analysis of free tripodal thiourea receptors L1 and L2 were obtained from individual DMF solutions and X-ray elucidation reveals that they crystallize from the monoclinic space group P 21/n and triclinic space group P-1 respectively. The asymmetric unit of each free receptor contains single L1/L2 ligand unit, where the thiourea –NH groups of each tripodal side arm of either receptor L1 or L2 are projected towards three different directions (Fig. 1) in absence of any anionic guests. Structural analyses also clearly states that each receptor unit of either L1 or L2 receptor unit are coordinated with adjacent symmetry-identical receptor units via strong N e H···S hydrogen-bonding interactions. The adjacent identical-symmetry meta-bromophenyl substituted L1 receptor units get extra stability by several weak C e H···S, N e H···π and Br···Br interactions (Fig. 1b). On the other hand, meta-trifluoromethylphenyl functionalized receptor L2 become additionally stabilized by weak C e H··· π interactions with adjacent symmetry-identical ligands present in the unit cell (Fig. 1d).
2.1.3. Hydrated-hexafluorosilicate complex [{(L3H+)2(SiF62−)}(H2O)7] (3) and Hydrated-bromide complex [{(L3H+)(Br−)}(H2O)2] (4) The good quality crystals suitable for single crystal X-ray analyses of hydrated- hexafluorosilicate complex 3 and hydrated-bromide complex 4 of protonated para-iodophenyl functionalized tris-urea receptor were obtained from the individual basic DMF solvent mixture of ligand L3 and hydrofluoric acid (HF) or hydrobromic acid (HBr) respectively. Structural elucidation reveals that both hydrated-complexes 3 and 4 crystallize in the same monoclinic space group P 21/c with Z = 4. However, the asymmetric unit of complex 3 contains two symmetry-independent protonated L3 receptor conformers, one divalent hexafluorosilicate anion (SiF62−) and seven water molecules of
2.1.2. Sulphate complex [(n-TBA)2{(L1)(SO4)}(DMF)] (1a), carbonate complex [(n-TBA)2{(L1)(CO3)}] (1b) and sulphate complex [(nTBA)2{(L2)(SO4)}] (2a) The good quality colourless crystals of divalent sulphate encapsulated complexes 1a and 2a of respective receptors L1 and L2 were achieved from the individual basic DMF solutions of receptors and 577
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Fig. 1. X-ray structure (partial) of free receptors depicting (a) ORTEP plot of receptor L1 at 30% probability level, (b) non-covalent interactions involved around each identical symmetry L1 receptor unit, (c) ORTEP plot of receptor L2 at 30% probability level and (d) non-covalent interactions involved around each identical symmetry L2 receptor unit.
crystallization. Whereas, the complex 4 comprises of single protonated L3 receptor unit, one monovalent bromide anion and two water molecules of crystallization in the asymmetric unit. X-ray structure determination reveal that the urea –NH group of triopodal protonated receptor L3 in both complexes 3 and 4 are projected in three different directions, closely resemble to the free neutral L1 and L2 tris-thiourea receptors (Average terminal aryl centroid distances are 4.95 Å in 3, 4.69 Å in 4, 4.87 Å in free L1 and 5.33 Å in free L2). However, unlike the free L1 and L2 tris-thiourea receptors, the protonated receptor L3 of each complex 3 and 4 are stabilized by two strong intramolecular N−Hapical···Ocarbonyl and N−Hurea···Ocarbonyl interactions, which may prevent them to open up the tripodal side arms to encapsulate the anionic guests. As a consequence, the octahedral SiF62− anion or spherical Br− anion remain in outside the protonated receptor cavity, constructing various types of
cyclic/acyclic hydrogen-bonded network assemblies (Fig. 3a, 3b, 3d, 3e) with the assistance of crystallized water molecules in complexes 3 and 4. Note that in complex 3, the higher coordinating octahedral SiF62− anion outside the receptor cavity gains additional stability by consecutive formation of cyclic construction of beautiful cyclic hydrogen-bonded network via alternative formation of R1010(16) and R66(10) type cyclic anion-water clusters (Fig. 3a). Subsequently, the lower coordinating spherical bromide anions outside the receptor cavity in complex 4 acquire extra stability by consecutive formation of cyclic R44(12) type hydrogen-bonded architecture via C−Hreceptor···Br interactions (Fig. 3d). The packing motifs show the formation of linear anion-water channels along the outer hydrophobic side of the protonated receptor conformers as viewed along the crystallographic c and b axes of complexes 3 (Fig. 3c) and 4 (Fig. 3f) respectively.
Fig. 2. X-ray structures (partial) of 2:1 receptor-oxyanion complexes depicting the array of hydrogen bonding interactions in (a) (L1)2-SO4 capsule, (b) (L1)2-CO3 capsule, (c) (L2)2-SO4; the space-fill representation of sulphate or carbonate encapsulation in (d) complex 1a, (e) complex 1b and (f) complex 2a. 578
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Fig. 3. X-ray structures (partial) of hydrated-anion complexes depicting (a) hexafluorosilicate-water cyclic architecture formation via strong H-bonding interactions outside the receptor cavity in complex 3, (b) coordination environment of hexafluorosilicate anion, (c) packing diagram of complex 3 from c-axis, (d) cyclic receptorbromide architecture formation via strong H-bonding interactions outside the receptor cavity in complex 4, (e) H-bonding environment of protonated receptors with bromide and water and (f) packing diagram of complex 4 from c-axis.
A correlation of NeH⋯A (Anion) angle vs. H⋯A (Anion) distance displays that most of the hydrogen-bonding interactions among receptor and anions in solid state are present in strong hydrogen bonding interaction region of d (H⋯A) ≤ 2.6 Å and d(D⋯A) ≤ 3.3 Å (Table S1, supporting information).
purification. Solvents used for synthesis and crystallization were bought from Merck, India and used as supplied. 1H NMR spectra were recorded on a Varian FT-600 MHz machine and chemical shifts were recorded in parts per million (ppm) on a scale using tetramethylsilane [Si(CH3)4] or a residual solvent peak as a reference. 13C spectra were obtained at 150 MHz. The electrospray ionization mass spectrometry (ESI-MS) spectra of individual free receptors were recorded in methanol. The FTIR spectra of air-dried samples were recorded on a Perkin-ElmerSpectrum on FT-IR spectrometer with KBr disks over the range of 4000–450 cm−1.
3. Conclusions In summary, we have synthesized and developed two flexible tristhiourea receptors L1 and L2 which show strong oxyanion encapsulation abilities their neutral form. Subsequently, in presence of inorganic acids, the apical nitrogen atom of a tripodal tris-urea receptor L3 was easily protonated to form cyclic/acyclic anion-water assemblies outside the receptor cavities. Structural determination approves that both receptors L1 and L2 can readily form divalent sulphate encapsulated 2:1 dimeric host–guest capsular assemblies by H-bonding activated proton transfer from monovalent hydrogensulphate anion. Furthermore, the tris-thioura ligand L1 also produces planar divalent carbonate encapsulated dimeric capsular assemblies by hydroxide anion induced aerial CO2 fixation in solid state. On the other hand, the iodo-pheneyl substituted protonated tris-urea receptor form divalent hexafluorosilicate and monovalent bromide trapped anion-water assembled linear channel outside the receptor cavity in presence of inorganic acids HF and HBr respectively. Hence, the structural analysis clearly unveils the formation of both neutral and protonated anionic assemblies by similar kinds of tris-thiourea and tris-oxyurea receptors containing hydrophobic environment would be useful to the supramolecular researchers to develop the new classes of host–guest assemblages in neutral as well as in protonated form.
4.2. Syntheses and characterization 4.2.1. Receptors L1, L2 and L3 The tripodal receptors were attained in quantitative yield by the reaction of Tren [tris(2-aminoethyl)-amine] (2.0 mmol) with 3.0 equiv of the 3-Bromophenyl isothiocyanate (6.0 mmol), 3-(Trifluoromethyl) phenyl isothiocyanate (6.0 mmol) and 4-Iodophenylisocyanate (6.0 mmol) respectively in individual solution of dry acetonitrile solvent medium. The resulting reaction mixtures in separate 250 mL round-bottomed flask were stirred for overnight at room temperature. Then, the excess acetonitrile was reduced in vacuo and the resulting solid precipitates in each case of ligands L1, L2 and L3 were obtained, filtered off and washed with dry acetonitrile, dry THF and dichloromethane to wash out the unreacted reagents and then characterized by NMR and FT-IR analyses (Supporting Information). Yield = 90%, 80% and 80% for respective ligands L1, L2 and L3. L1: 1H NMR (600 MHz, DMSO‑d6) δ (ppm): 2.738–2.760 (t, 6H, 6.6 Hz, –NCH2), 3.601 (t, 6H, –NCH2CH2), 7.251 (3H, Ar-H), 7.260 (3H, Ar-H), 7.339 (3H, Ar-H), 7.791 (s, 3H, Ar-H), 7.820 (s, 3H, -NHb), 9.739 (s, 3H, -NHa). 13C NMR (150 MHz, DMSO‑d6) δ (ppm): 42.279 (×3C, –NCH2CH2), 52.335 (×3C, –NCH2), 121.602 (×3C, Ar-C), 121.883 (×3C, Ar-C), 125.394 (×3C, Ar-C), 126.887 (×3C, Ar-C), 130.916(×3C, Ar-C). 141.468 (×3C, Ar-C), 180.590 (×3C, thiocarbonyl carbon). FT-IR spectra (KBr pellet): 3230 cm−1 vs(NeH), 3063 cm−1 vs(CeH), 2847 cm−1 vs(CeH), 1566 cm−1 vs(C]S, sym), 702 cm−1 vs(C]S, asym). ESI-MS: m/z 789.9381 [L+H] L2: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 2.766–2.787(t, 6H ∼ 6.3 Hz, –NCH2) 3.629 (t, 6H, –NCH2CH2), 7.386–7.399(d, 3H,
4. Experimental section 4.1. Materials and methods All the reagents and solvents were purchased from commercial sources and used without any purification. Tren [tris(2-aminoethyl)amine], 3-Bromophenyl isothiocyanate, 3-(Trifluoromethyl)phenyl isothiocyanate, 4-Iodophenylisocyanate and tetrabutylammonium salts were purchased from Sigma-Aldrich while common inorganic acids were purchased from local vendors, India and used without further 579
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Table 1 Crystallographic Parameters and Refinement Data of free receptors and anion-complexes. Parameters
L1
L2
1a
1b
2a
3
4
Formula Fw Crystal system Space group a/Å b/Å c/Å α/o β/o γ/o V/Å3 Z Dc/g cm−3 μ Mo Kα/mm−1 F000 T/K θ max. Total no. of reflections Independent reflections Observed reflections Parameters refined R1, I > 2σ(I) wR2, I > 2σ(I) GOF (F2) CCDC No.
C27H30Br3N7S3 788.46 monoclinic P 21/n 20.089(9) 7.610(3) 20.360(7) 90.00 92.984(4) 90.00 3108.4(2) 4 1.685 4.126 1576.0 298(2) 24.997 14,239 5461 3858 361 0.0441 0.0911 1.036 1,861,244
C30H30F9N7S3 755.79 triclinic P -1 7.921(6) 12.808(16) 18.054(19) 70.542(10) 81.001(7) 89.924(8) 1703.2(3) 2 1.474 0.301 776.0 298(2) 24.998 11,757 5994 2252 497 0.0733 0.1434 0.937 1,861,245
'C89H139Br6N17O5S7 2230.99 triclinic P -1 13.828(8) 14.675(8) 27.249(15) 97.691(4) 99.437(5) 95.313(4) 5368.1(5) 2 1.380 2.434 2308.0 298(2) 24.999 40,787 18,807 7001 1167 0.0880 0.1537 1.069 1,861,246
C87H132Br6N16O5S6 2121.85 triclinic P -1 15.853(5) 17.215(8) 21.885(7) 81.151(3) 70.266(3) 65.393(4) 5110.5(4) 2 1.379 2.531 2192.0 298(2) 28.853 45,013 23,180 7277 1059 0.0956 0.1611 1.196 1,861,247
C92H132F18N16O4S7 2092.56 monoclinic P 21/c 14.979(5) 14.038(4) 26.796(12) 90.00 98.610(3) 90.00 5570.8(4) 2 1.247 0.224 2538.0 298(2) 24.999 27,265 9788 5213 641 0.0914 0.1661 0.932 1,861,248
C54H62F6I6N14O13Si 2018.67 monoclinic P 21/c 28.824(9) 13.789(5) 18.252(8) 90.00 99.094(3) 90.00 7163.4(4) 4 1.872 2.700 3896.0 298(2) 25.000 33,069 12,594 7423 856 0.0544 0.1559 1.058 1,861,249
C27H31BrI3N7O5 994.19 monoclinic P 21/c 26.676(14) 15.158(11) 8.490(5) 90.00 91.839(5) 90.00 3431.0(4) 4 1.925 3.943 1904.0 298(2) 25.000 14,568 6019 3632 392 0.0463 0.1248 0.865 1,861,250
7.8 Hz, Ar-H), 7.494–7.520 (t, 3H, 7.8 Hz, Ar-H), 7.660–7.673 (d, 3H, 7.8 Hz, Ar-H), 7.881(s, 3H, -NHb), 7.954(s, 3H, Ar-H), 9.869(s, 3H, -NHa). 13C NMR (150 MHz, DMSO‑d6) δ(ppm): 42.221 (×3C, 52.308(×3C, –NCH2), 119.140(×3C, Ar-C), –NCH2CH2), 120.450(×3C, Ar-C), 123.616(×3C, Ar-C), 125.419(×3C, Ar-C), 126.583(×3C, Ar-C). 130.044 (×3C, -CF3), 140.808 (×3C, Ar-C), 180.834(×3C, thiocarbonyl carbon). FT-IR (υ cm−1): 3340 cm−1 vs (NeH), 3257 cm−1 vs(CeH), 3062 cm−1 vs (CeH), 1577 cm−1 vs(C] S), 702 cm−1 vs(C]S, asym). ESI-MS: m/z 756.1802 [L+H]. L3: 1H NMR spectrum of tris-urea receptor L3 (600 MHz, DMSO‑d6) δ (ppm): 2.568–2.590 (t, J = 6.6 Hz, 6H, –NCH2), 3.155–3.187 (q, J = 6.6 Hz, 6H, –NCH2CH2), 6.175–6.193 (t, J = 5.4 Hz, 3H, –NHa), 7.330–7.345 (d, 6H, J = 9.0 Hz, -ArH), 7.348–7.363 (d, 6H, J = 9.6 Hz, – ArH), 8.668 (s, 3H, -NHb). ESI-MS: m/z 881.9675 [L+H].
TBA-CH2) 2.659 (6H, –NCH2), 3.129–3.157(t, 8.4 Hz, 8H, TBA-CH2), 3.544(6H, –NCH2CH2), 7.155(3H, Ar-H), 7.181(3H, Ar-H), 7.337(3H, Ar-H), 7.883(3H, Ar-H), 8.379(s, 3H, -NHb), 10.845(s, 3H, -NHa). 13C NMR (150 MHz, DMSO‑d6) δ(ppm): 13.992 (×4C, TBA-CH3), 19.688(×4C, TBA-CH2), 23.512(×4C, TBA-CH2), 42.097(×3C, –NCH2CH2), 52.608(×3C, –NCH2), 57.942(×4C, TBA-CH2), 121.314(×3C, Ar-C), 121.680(×3C, Ar-C), 125.267(×3C, Ar-C), 126.390(×3C, Ar-C), 130.491(×3C, Ar-C), 142.062(×3C, Ar-C), 168.974(×1C, Carbonate-C]O), 180.653(×3C, thiocarbonyl carbon). FT-IR spectra (KBr pellet): 3230 cm−1 vs(NeH), 3060 cm−1 vs(CeH), 2957 cm−1 vs(CeH), 1546 cm−1 vs(CeO, carbonate), 1477 cm−1 vs (C]S, sym), 708 cm−1 vs(C]S, asym). 2a: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 0.916–0.940 (t, 7.2 Hz, 12H, TBA-CH3), 1.268–1.329 (m, 8H, TBA-CH2), 1.529–1.582 (m, 8H, TBA-CH2) 2.633 (6H, –NCH2), 3.139–3.167(t, 8.4 Hz, 8H, TBA-CH2), 3.341 (solvent-H, H2O), 3.550 (6H, –NCH2CH2), 7.309–7.320 (d, 6.6 Hz, 3H, Ar-H), 7.401–7.426 (t, 7.5 Hz, 3H, Ar-H), 7.763–7.774 (d, 6.6 Hz, 3H, Ar-H), 8.116 (s, 3H, Ar-H), 8.936 (s, 3H, –NHb), 10.681(s, 3H, –NHa). FT-IR spectra (KBr pellet): 3283 cm−1 vs(NeH), 2959 cm−1 vs(CeH), 2929 cm−1 vs(CeH), 1548 cm−1 vs(C]S, sym), 705 cm−1 vs (C]S, asym).
4.2.2. Sulphate complex [(n-TBA)2{(L1)(SO4)}(DMF)] (1a) and carbonate complex [(n-TBA)2{(L1)(CO3)}] (1b) and sulphate complex [(n-TBA)2{(L2)(SO4)}] (2a) The sulphate and carbonate encapsulated complexes of either L1 or L2 of tris-thiourea receptors were achieved by charging excess of tetrabutylammonium hydrogensulphate (10 eqv.) or tetrabutylammonium hydroxide (10 eqv.) into the 5 mL DMF solutions of either L1 (100 mg) or L2 (100 mg) in three separate small glass vials. The resulting solutions after hydrogensulphate or hydroxide salts addition were stirred for about 30 min and were left open to atmosphere for slow vaporization at room temperature. The good quality crystals of sulphate encapsulated complexes 1a, 2a and carbonate encapsulated complex 1b suitable for single crystal X-ray analysis were attained within 10–15 days. Yield = 70%, 85% and 70% for respective complexes 1a, 1b and 2a. 1a: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 0.912–0.936(t, 7.2 Hz, 12H, TBA-CH3), 1.264–1.325(m, 8H, TBA-CH2), 1.523–1.576(m, 8H, TBA-CH2) 2.599 (6H, –NCH2), 3.132–3.160(t, 8.4 Hz, 8H, TBA-CH2), 3.502 (6H, –NCH2CH2), 7.145 (3H, Ar-H), 7.163(3H, Ar-H), 7.465(3H, Ar-H), 7.950(3H, Ar-H), 8.821(s, 3H, -NHb), 10.476(s, 3H, -NHa). FT-IR spectra (KBr pellet): 3277 cm−1 vs(NeH), 2962 cm−1 vs(CeH), 2920 cm−1 vs(CeH), 1540 cm−1 vs(C]S, sym), 718 cm−1 vs(C]S, asym). 1b: 1H NMR (600 MHz, DMSO‑d6) δ(ppm): 0.913–0.937(t, 7.2 Hz, 12H, TBA-CH3), 1.265–1.326(m, 8H, TBA-CH2), 1.523–1.576(m, 8H,
4.2.3. Hydrated-hexafluorosilicate complex [{(L3H+)2(SiF62−)}(H2O)7] (3) and Hydrated-bromide complex [{(L3H+)(Br−)}(H2O)2] (4) The hydrated hexafluorosilicate and bromide complexes of protonated tris-oxyurea receptor L3 were achieved from the basic DMF solution mixtures of L1 (100 mg) and inorganic acids HF or HBr respectively from separate small glass vials. After the addition of HF or HBr, the resulting solutions was stirred for about 1 h and kept in open air for slow evaporation at room temperature. The good quality crystals of complex 3 and 4 appropriate for single crystal X-ray analysis were obtained within 20–25 days. Yield: 70% of 3 and 65% of 4 based on L3. 4.3. Crystallographic refinement details The crystallographic refinement details of for free receptors and all neutral as well as protonated anion complexes 1a, 1b, 2a, 3 and 4 are summarized in Table 1 and all the data have been deposited to the CCDC. A crystal of appropriate size was carefully chosen in each case from the mother liquor and immersed in silicone oil and then it was 580
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mounted on the tip of a glass fiber, cemented using epoxy resin. The Xray crystallographic intensity data were collected using Supernova single source at offset, Eos diffractometer using Mo-Kα radiation (λ = 0.71073 Å) equipped with CCD area detector and corresponding data refinement and cell reduction were carried out by CrysAlisPro [76]. The data integration and reduction were undertaken with SAINT and XPREP [77] software and multi-scan empirical absorption corrections were applied to the data using the program SADABS [78]. All the crystal structures were solved by direct methods using SHELXTL-2014 and were refined on F2 by the full-matrix least-squares technique using the SHELXL-2014 program package [79]. Graphics for structural determination are generated using MERCURY 2.3 [80] for Windows. It is important to mention that, in the case of protonated anion complexes 3 and 4, we could not add hydrogen atoms to water molecules even after many trials, because the OeH bond of water molecules was very large and could not be refined. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms attached to all carbon atoms were geometrically fixed, the positional and temperature factors are refined isotropically. The hydrogen atoms are located on a difference Fourier map and refined, wherever it is possible and in other cases, the hydrogen atoms are geometrically fixed.
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Acknowledgments This work was supported by CSIR and SERB through grant 01/ 2727/13/EMR-II and SR/S1/OC-62/2011, New Delhi, India. G.D acknowledges the CIF, IIT Guwahati, India and DST-FIST of India for providing instrument facilities. S.K and U.M thank IITG for fellowship. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2018.11.008. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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