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A Progressive Cation triggered Anion Binding by Electron-rich Scaffold: Case study of a Neutral Tripodal Naphthyl Thiourea Receptor Utsab Manna, and Gopal Das Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00259 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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Crystal Growth & Design

A Progressive Cation triggered Anion Binding by Electron-rich Scaffold: Case study of a Neutral Tripodal Naphthyl Thiourea Receptor Utsab Manna, and Gopal Das* Department of Chemistry, Indian Institute of Technology Guwahati, Assam-781039, Fax: +91-361-2582349; Tel: +91-361-2582313; E-mail: [email protected].

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: A neutral tripodal electron-rich anion receptor L, decorated from tren building block has been deliberately synthesized from acetonitrile for thorough investigation of host-guest binding aptitude in solid state especially. The naphthyl based tris-thiourea scaffold, without the presence of any π-acidic or electron-withdrawing aryl terminals has been established as a potential system that can efficiently bind anions of varied dimensionality consistently triggered by the size of counter-cation, as regularly observed from SC-X-ray analysis, and well supported by solution state NMR binding studies. The newness of this report is the systematic and consistent solid state anion binding within the inner tripodal cleft of highly electron-rich naphthyl terminals. Although the receptor L was first synthesized in nineteenth century and no solid state binding studies have been reported till now. However, numerous tren based urea/thiourea ligands with several terminal functionalizations having more acidic tripodal hydrophobic pocket was extensively reported as anion receptors in solid state over the last decades. In contrary, even within the less hydrophobic and very small inner tripodal cleft, the naphthyl thiourea receptor L still effectively entraps spherical fluoride (1a, 1b), relatively larger spherical chloride (2a, 2b) and bromide (3a, 3b), planar nitrate (4) and tetrahedral sulphate (5) anions via 1:1 host-guest complexation mode, regularly triggered by n-TBA/TEA countercations.


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1. Introduction The tren [tris(2-aminoethyl)amine] skeleton consisting of three pendant primary amine groups with a tertiary amine center is the archetypal tripodal ligand of interest in coordination chemistry and has developed as one of the superlative anion binding building block over the past years. The observations in natural anion binding proteins, which are potential specific binding pocket containing receptors for active transport systems in cells,1-3 have encouraged the researchers to develop several neutral receptors. Tren-based acyclic receptors with size and shape-selective organic frameworks with multi-armed amine,4-5 amide,6-14 urea/thiourea15-33 hydrogen bonding functionalities have been frequently established to coordinate with targeted anions via formation of monomeric and dimeric capsular assemblies.34-37 These molecular capsules/pseudocapsules have some fascinating coordinating features, because they are capable of creating specific microcavity that encapsulates the guest from the bulk of the solvent system.38-41 Hence, trenbased acyclic receptors having convergent hydrogen-bonding functionalities with one or more electron-withdrawing and π-acidic terminals in most of the cases, have been extensively utilized in solid state anion receptor chemistry from last decades.15-33 In this context, recognition of anions of varied dimensionality within one of the simplest tren-based unsubstituted urea/thiourea analogues has been underexplored especially in solid state, mainly due to the deficiency of πacidic or electron-withdrawing aryl terminals. Although, the tren-based unsubstituted urea/thiourea receptors containing phenyl or naphthyl terminals were reported in last decades by Moran et. al.15 and Wu et. al.16-17 based on similar kinds of H2PO4- and HSO4- oxyanion binding studies in solution state only, however no structural data were available to support those receptor-oxyanion complementarity till date. As these urea/thiourea functionalized tripodal scaffolds offer a structurally pre-organized and flexible core, they can be effectively capable of anion binding due to their favorable conformation for multiple hydrogen bonds with different anions that favor the formation of a stable host-guest complex.18-33 Among spherical anions, fluoride recognition and sensing is an area of massive research interest due to its smallest size and highest electronegativity among halides. Besides, excess of fluoride ions in human body beyond its toxic level can cause osteosarcoma, and an extra amount of fluoride in drinking water is accountable for dental as well as skeletal fluorosis.42 The relatively larger spherical chloride ions are essential due to their substantial role in biological routes like signal transduction as well as the carriage of organic solutes through cell membranes. The bromide ions almost similar sized 3 ACS Paragon Plus Environment


Scheme 1. Molecular architecture and schematic diagram of highly electron-rich tren-based thiourea receptor L and host-guest complexes.

with chloride are also the essential cofactor for the assembly of collagen IV scaffolds in tissue development.42-43 On the other hand, among the oxyanions, the planar nitrate is one of the vital componentts in all living systems as it is the most readily assimilated form of nitrogen by plants.44 The encapsulation of larger tetrahedral sulphate oxyanion is very much anticipated inside the unimolecular capsular assembly, as it is present as contaminant in drinking water as well as acts as a pollutant in nuclear and radioactive waste, that hampered the vitrification process.45,29 Thus far, numerous tripodal oxyurea/thiourea receptors consisting of one or more terminal aryl substitutions have been widely reported by R. Custelcean et. al., P. Ghosh et. al., C. Janiak et. al., P. A. Gale et. al., A. Das et. al. and from our group also,18-33 which can recognize anionic guest in either 1:1, 2:2 or 2:1 host-guest complexation mode via topological complementarity in solid state, based on the solution state anion binding studies of first trenbased urea/thiourea receptors reported by Morán et. al. in 1995.15 In our ongoing effort in the field of anion-receptor chemistry to encapsulate the hydrated/solvated/cation-triggered anionic guests into the host capsules,46-56 herein we report the cation-triggered systematic encapsulation of smaller (fluoride) to larger halides (chloride, bromide) as well as planar (planar) and tetrahedral (sulphate) oxyanions inside the tris-naphthyl thiourea receptor cavity via 1:1 unimolecular host-guest complexation mode. Most importantly, this report delivers the first anion binding crystallographic evidence of one of the simplest tren-based unsubstituted highly electron-rich thiourea derivative L, which was first synthesized by S. Wu et. al. in 1999,17 that 4 ACS Paragon Plus Environment

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demonstrated the only solution state oxyanion binding. The solid state crystal structure of halides and oxyanions with L reveals that the halides are encapsulated within the monomeric capsular/pseudocapsular assembly of the receptor, largely depending upon the size of the nTBA/TEA counteractions with optimal N–H⋯X and N–H⋯S hydrogen bonding interactions, whereas in contrary to the very common 2:1 host-guest reported oxyanion-complexes of trenbased receptors in literature, the napthyl tris-thiourea ligand L efficiently encapsulates planar nitrate as well as tetrahedral sulphate anion within the cation-sealed unimolecular pseudocapsular assembly.

2. Results and Discussion 2.1. Design principles of tripodal receptors and Structural aspects of anion binding: In principle, for a particular receptor molecule to bind with the anionic guests of particular size and geometry, it should hold preorganized directional hydrogen-bond donors especially, tailored on a proper platform/framework e.g. tripodal amine tren. The tripodal naphthyl thiourea ligand L are synthesized in room temperature by reaction between tren and naphthyl isothiocyanate in a 1 : 3 molar ratio from dry acetonitrile. The highly electron-rich receptor L possesses wellorganized tripodal skeleton with three thiourea functions suitable for anion encapsulation via their ideal hydrogen bonding coordination. However, exactly similar receptor synthesized from dried CHCl3 was reported, demonstrating UV-Vis solution state oxyanion binding by S. Wu et. al. in 1999,17 at the beginning of tren-based anion-receptor chemistry. Unfortunately, despite the substantial progress in the field of solid state anion-recognition chemistry in twentieth century, no solid state anion binding studies with L still available to support those solution-state binding data. Alike the electron-withdrawing π-acidic terminal containing numerous reported tripodal receptors,15-33 the receptor L also offer six -NH hydrogen bonds, which can trap anions of diverse geometry including spherical (F¯/Cl¯/Br¯), trigonal planar (NO3¯) and tetrahedral (SO42-) anions in plausible host-guest stoichiometry. In general, thiourea -NH protons are more acidic than that of oxyurea (pKa of 21.1 for thiourea and 26.9 for urea in DMSO),57 and hence the thiourea containing ligands are expected to form more stable anion complexes by stronger hydrogen bonds than their urea analogues. However, deprotonation of -NH protons may occur in presence of strongly basic anions such as, F- and HCO3-, if they are acidic enough because of



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highly electron-withdrawing substituents into the receptor frame. Efforts are made to examine the binding resemblances and divergences of highly electron-rich naphthyl thiouraea receptor L Table 1: Significant observation upon anion binding of naphthyl tris-thiourea receptor L:

Anions by size

Anion-salt added

Coordination environment of anion

Spherical fluoride (F-)

n-TBAF

Unimolcular 1:1 L-F- neutral capsular assembly Unimolcular 1:1 L-F- neutral capsular cation-sealed assembly Unimolecular 1:1 L-Cl- neutral pseudocapsular assembly Unimolecular 1:1 L-Cl- neutral pseudocapsular assembly Unimolecular 1:1 L-Br- neutral pseudocapsular assembly Unimolecular 1:1 L-Br- neutral pseudocapsular assembly Unimolecular 1:1 L-NO3- neutral pseudo-capsular assembly

TEAF Spherical chloride (Cl-)

n-TBACl TEACl

Spherical bromide (Br-)

n-TBABr TEABr

Planar nitrate (NO3-)

TEANO3

Tetrahedral sulphate (SO42-)

n-TBAHSO4

Unimolcular 1:1 L-SO42- neutral pseudocapsular cation-sealed assembly

Anion coordination number C.N. of fluoride = 6

Average pod distance of L 5.862 Å

C.N. of fluoride = 7

6.625 Å

C.N. of chloride = 7

8.064 Å

C.N. of chloride = 6

7.750 Å

C.N. of bromide = 7

8.093 Å

C.N. of bromide = 6

7.805 Å

C.N. of nitrate = 9

7.816 Å

C.N. of sulphate = 18

10.350 Å

Note: Systematic and consistent halide binding and exceptional oxyanion binding approaches of highly electron-rich unsubstituted neutral naphthyl tripodal thiourea receptor L via unimoleculrar capsular/pseudo-capsular assembly formation triggered by n-TBA/TEA counter-cations.

towards anions of different dimensions (spherical, planar and tetrahedral), especially in the solid state. Traditionally from the perspective of anion coordination chemistry, crystallization has been the fundamental way realizing the structural insights of anion complexes, which are then associated with observed selectivity in solution. Basically, the use thiourea groups in the receptor moiety offer extra advantages, such as enhanced solubility in lower polarity solvents as well as lower tendency to undergo self-association. 2.2. Complete X-ray structural analysis of host-guest complexes: The X-ray structural elucidation of anion bound complexes of naphthyl thiuorea receptor L triggered by either tetrabutylammonium or tetraethylammonium counter-cation have been depicted in the following figures and also have also been formulated in table 1. Despite the absence of any electron-withdrawing π-acidic aromatic terminals, the electron-rich naphthyl tripodal scaffold is still capable to encapsulate anionic guests in solid state via topological complementarity from DMF, DMSO or DMF/DMSO binary solvent mixture. Fortunately, we are able to isolate the single crystals of smaller (fluoride) to larger spherical halide (chloride and bromide) complexes from both of their tetrabutylammonium as well as tetraethylammonium salts. Furthermore, the planar nitrate and larger tetrahedral sulphate complex are also isolated 6 ACS Paragon Plus Environment

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from exceptional unimolecular capsular assembly of relatively smaller naphthyl tripodal scaffold. In this kind of solid state anion binding, the chelate effect may also play a vital role due

Figure 1. X-ray structures (partial) depicting (a) hydrogen bonding coordination environment of adjacent fluoride encapsulated receptors in complex 1a joined by a n-TBA cation, (b) TEA cation sealed fluoride encapsulation by receptor L in complex 1b, (c) spacefill representation of two adjacent fluoride encapsulated assemblies in 1a, (d) spacefill representation of cation sealed fluoride-receptor assembly in 1b, (e) the packing motif of complex 1a and (f) the packing motif of complex 1b, as viewed down along the crystallographic c axis.

to the favorable contributions from both entropy and enthalpy. Noticeably, single crystal of free ligand L have proven to be difficult to attain from different solvent media even after many trials, instead, formation of thick oily liquid settling at the bottom of the crystallization vial have been shown to be observed in most of the cases. The structural elucidation of fluoride (complex 1a, 1b), chloride (complex 2a, 2b), bromide (complex 3a, 3b), nitrate (complex 4) and sulphate (complex 5) complexes of receptor L strongly exposes that the N–H⋯A(anion) interactions are mainly engaged in coordination reinforced by few intermolecular non-covalent interactions from n-TBA/TEA counter-cations, which additionally induces the building of anion-receptor capsular/pseudo-capsular assembly via topological complementarity and assist as the source for efficient crystallization of the underexplored complexes. 2.2.1. Fluoride encapsulated complexes [(n-TBA){(L1)(F)}] (1a) and [(TEA){(L)(F)}] (1b): 7 ACS Paragon Plus Environment


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The unimolecular capsular 1:1 receptor-fluoride complex 1a and 1b were obtained from basic DMF

solution

in

the

presence

of

excess

tetrabutylammonium

(n-TBAF)

and

tetraethylammonium fluoride (TEAF) salts respectively. The X-ray analysis reveals that complex

Figure 2. X-ray structures (partial) depicting (a) hydrogen bonding coordination environment of two face-to-face oriented chloride encapsulated assembly in complex 2a, (b) hydrogen-bonding interactions of two adjacent chloride-water-receptor assemblies in complex 2b, Space-fill representation of chloride encapsulated assemblies in (c) complex 2a, (d) complex 2b and the packing motifs of (e) complex 2a (f) complex 2b, as viewed down along the crystallographic a axis.

1a crystallizes in monoclinic space group P 21/c with Z = 8, whereas complex 1b crystallizes in trigonal space group R 3 c with Z = 6. The asymmetric unit of complex 1a contains two symmetry independent unimolecular capsular receptor units which encapsulate two symmetry independent smallest spherical halides individually inside their tripodal cavities with hydrogen bonds to all six thiourea -NH protons (Figure 1a). The two adjacent symmetry-independent fluoride bound monomeric ligand capsular units, exhibiting ‘conformational isomorphism’, are interconnected by one n-TBA counter-cation unit via one weak C–H⋯O and three weak C–H⋯π contacts (Figure 1a, 1c). Generally kinetic and thermodynamic crystal stability factors control the occurrence of conformational isomorphs and these factors are mostly considered to be the consequences of interrupted crystallization, first demonstrated by Desiraju et al.58 Both the 8 ACS Paragon Plus Environment

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capsular units in complex 1a are almost identical, as the aromatic average naphthyl centroid distances are 5.834 Å and 5.890 Å in the capsular units as well as the identical hexa-cordination of encapsulated fluorides in both conformers. Moreover, two symmetry-identical adjacent

Figure 3. X-ray structures (partial) depicting (a) hydrogen bonding coordination environment of adjacent bromide encapsulated receptors in complex 3a, (b) hydrogen-bonding interactions in adjacent bromidewater-receptor assemblies in complex 3b, Space-fill representation of bromide encapsulated assemblies in (c) complex 3a, (d) complex 3b and the packing motifs in (e) complex 3a (f) complex 3b, as viewed down along the crystallographic a axis.

capsular units conform a R22(14) type cyclic H-bonding network via two Cnaphthyl–H⋯S interactions. On the other hand, the asymmetric unit of complex 1b contains one-third part of a neutral receptor L, single fluoride anion and its corresponding tetraethylammonium counteraction. The X-ray analysis reveals the cation-sealed unimolecular capsular assembly of naphthyl thiourea ligand L, where fluoride anion is encapsulated inside the C3v-symmetric tripodal cavity by six strong thiourea N–H⋯F and one relatively weaker CTEA–H⋯F interactions, exhibiting hepta-coordination of fluoride (Figure 1b, 1d), unlike complex 1a. Interestingly, this cation-sealed 1:1 fluride-receptor assembly of 2b gets extra stability by three weak CTEA– H⋯πnaphthyl interactions within the unimolecular capsular unit. The packing motif of complex 1a reveals almost linear and zigzag coordination polymeric assembly of two independent adjacent fluoride encapsulated conformers (Figure 1e), whereas packing motif of complex 1b exposes a 9 ACS Paragon Plus Environment


compact cation-sealed fluoride encapsulated coordination assembly (Figure 1f) as viewed along the crystallographic c axis. 2.2.2. Chloride complexes [(n-TBA){(L)(Cl)}] (2a) and [(TEA){(L)(Cl)(H2O)}] (2b): The pseudo-capsular 1:1 neutral host-guest complexes 2a and 2b of chloride anion with thiourea receptor L were obtained in the presence of excess n- TBACl and TEACl salts respectively by slow evaporation of basic DMF solution mixtures. Unlike the fluoride complexes 1a and 1b, both the chloride complexes crystallize from the same triclinic space group P- 1 with Z= 2. The asymmetric unit of complex 2a contains one receptor L unit, one chloride anions its corresponding counteraction, whereas complex 2b contains L, Cl-, TEA+ and one additional water molecule of crystallization. Structural elucidation reveals that unlike the fluoride complexes, even the very small tripodal cavity containing naphthyl thiourea ligand L little bit opens up its three arms readily, creating a pseudo-capsular cavity to encapsulate larger chloride ion compared to the reported first generation tren-based thiourea receptors. The crystal structure of complex 2a clearly shows that the pseudo-capsular receptor L unit first encapsulates a single chloride via six thiourea N–H⋯Cl interactions and the same Cl- is again additionally Cnaphthyl– H⋯Cl hydrogen bonded with another adjacent face-to-face oriented identical-symmetry chloride encapsulated pseudocapsular receptor unit (Figure 2a, 2c). Overall, this hepta-coordinated chloride bound 2:2 pseudo-capsular neutral host-guest assemblies get extra stability by three weak CTBA–H⋯Sthiurea and three CTBA–H⋯πnaphthyl interactions (Figure 2a). On the other hand, in complex 2b, the pseudo-capsular L unit traps one chloride anion via hydrogen bonding to the six thiourea groups and the vacant space of 1: 1 pseudo-capsular host-guest assembly is occupied by a water molecule which becomes Owaer–H⋯Sthiurea hydrogen bonded with adjacent receptor unit (Figure 2b, 2d). Interestingly, unlike complex 2a, two adjacent chloride encapsulated host assemblies are oppositely oriented in the crystal system, which gains extra stability by weak CTEA–H⋯Sthiurea and CTEA–H⋯πnaphthyl interactions. The packing motif along the crystallographic a axis reveals the wave-like architecture of face-to-face orientated chloride encapsulated assemblies (Figure 2e) in complex 2a, whereas, linear architecture of 2:2 receptor-chloride-water assemblies (Figure 2f) has been observed in complex 2b. 2.2.3. Bromide complexes [(n-TBA){(L)(Br)}] (3a) and [(TEA){(L)(Br)(H2O)}] (3b): The bromide anion encapsulation inside the pseudo-capsular tripodal cavity of receptor L in the presence of both tetrabutylammonium (n-TBA+) as well as tetraethylammonium (TEA+) salts 10 ACS Paragon Plus Environment

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have also been achieved from basic DMF solution mixture, in order to understand the assembly progression with larger homologous spherical anions. Structural analysis reveals that alike the chloride complexes 2a and 2b, the 1:1 bromide-receptor complexes 3a and 3b crystallize from

Figure 4. X-ray structures (partial) of complex 4 depicting (a) hydrogen bonding coordination environment of adjacent nitrate pseudo-encapsulated receptors, (b) space-fill representation of nitratetrapped assemblies and (c) the crystal packing as viewed down along the crystallographic c axis.

the same triclinic space group P- 1 with Z= 2. Interestingly, the X-ray analysis also confirms the isostructural nature of complex 2a and 3a as well as complex 2b and 3b, which clearly indicates the substantial role of corresponding counteraction (either n-TBA+ or TEA+) in host-guest assembly formation. The structural analysis of complex 3a clearly reveals that the nathphyl tristhiourea receptor L in the asymmetric unit first encapsulates a bromide anion by six N–H⋯Br interactions within its pseudo-capsular cavity and then this assembly becomes connected with exactly similar symmetry-identical bromide-bound assembly in face-to-face fashion by two identical Cnaphthyl–H⋯Br interactions (Figure 3a, 3b). Overall, the hepta-coordinated bromidetrapped 2:2 host-guest assemblage of receptor L is further stabilized by six CTBA–H⋯Sthiurea and four CTBA–H⋯πnaphthyl weak interactions (Figure 3a). In contrary, in complex 3b, the two adjacent symmetry-identical hexa-coordinated bromide encapsulated 1:1 assemblies are not present in face-to-face fashion, but they are interconnected by CTEA–H⋯Sthiurea and CTEA– H⋯πnaphthyl interactions (Figure 3d, 3e). Note that, the vacant space of each tripodal pseudocavity in complex 3b is occupied by water molecule in the crystal lattice and two identicalsymmetry intermolecular hydrogen-bonded water molecules along with few CTEA–H⋯Owater interactions helps to gain stability of the self-assembled complex (Figure 3e). The crystal packing shows wave-like helical architecture of bromide-bound assemblies (Figure 3c) in complex 3a and linear architecture of water assisted bromide encapsulated assemblies (Figure 3f)



in complex 3b have been observed as viewed down along the crystallographic a axis, almost similar as in complex 2a and 2b respectively.

Figure 5. X-ray structures (partial) of complex 5 depicting (a) hydrogen bonding coordination environment of adjacent cation-sealed divalent sulphate pseudo-encapsulated receptor, (b) space-fill representation of sulphate sealed environment of receptor and n-TBA cation and (c) the crystal packing as viewed down along the crystallographic b axis.

2.2.4. Nitrate complex [(TEA){(L)(NO3)}] (4): Suitable needle-like crystals of TEANO3 salt with neutral naphtyl tris-thiourea receptor L were obtained from basic DMF/DMSO solvent mixture and the planar nitrate encapsulated pseudocapsular complex crystallizes in the triclinic space group P- 1 with Z= 2. Structural elucidation of complex 4 reveals that one nitrate-bound neutral receptor L in the asymmetric unit assembles with another adjacent identical-symmetry 1:1 receptor-nitrate pseudo-capsular unit by intermolecular Creceptor–H⋯Sthiurea receptor-receptor interactions (Figure 4a, 4b). The crystal structure clearly shows that a nitrate anion is coordinated by nine hydrogen bonds from three thiourea groups (Figure 4a) of the receptor and notably, the receptor around the nitrate ion is not C3v-symmetric, exhibiting two different coordination behaviors. Two thiourea groups from the ligand unit bind three edges of the planar anion (through eight-membered H-bonded rings), while all of the three thiourea groups from same ligand chelate two of the three vertices of the nitrate ion (six-membered H-bonded rings) (Figure 4c). Thus, the O1 atom of nitrate form six hydrogen bonds, but O2 atom accepts two and O3 only one hydrogen bond (Figure 4c). Additionally, the nitrate anion trapped pseudo-capsular host assembly gets extra stability by several CTEA– H⋯Sthiurea and CTEA–H⋯πnaphthyl interactions (Figure 4a). The packing motif of complex 4 along the crystallographic c axis reveals the formation of linear coordination architecture of face-faceface oriented 2:2 receptor-nitrate host-guest assemblies (Figure 4d). 12 ACS Paragon Plus Environment

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2.2.5. Sulphate complex [(TBA){(L)(SO4)}] (5): The colorless single crystals of complex 5 suitable for crystallographic analysis of was achieved from mixed DMF/DMSO solvent mixture of naphthyl tris-thiourea receptor L and excess of tetrabutylammonium hydrogensulphate (n-TBAHSO4). The X-ray analysis reveals that the cation-sealed pseudo-encapsulated sulphate-receptor complex crystallizes from monoclinic system of space group C c with Z = 4 and the asymmetric unit contains one neutral thiourea receptor L unit, one divalent sulphate anion and its corresponding two monovalent n-TBA+ counteraction. Interestingly, unlike the numerous receported 2:2 divalent sulphate encapsulated tripodal urea/thiourea receptors in literature, herein the relatively smaller tripodal cavity containing thiourea scaffold L expands its three arms in such a way that it encapsulates a large tetrahedral sulphate anion in 1:1 host-guest fashion, inside the receptor pseudo-cavity completely sealed by two tetrabutylammonium counter-cations (Figure 5a, 5b). The crystal structure of complex 5 clearly shows that a sulphate anion is coordinated by a total of 18 hydrogen bonding interactions (Figure 5c) among which nine H-bonds are donated from three thiourea –NH groups of receptor, two are donated from o-aryl –CH of receptor and the rest seven are donated from two n-TBA+ countercations via ion-pair type interactions (Figure 5a, 5c). The O1 and O4 oxygen atoms of sulphate anion accept four H-bonds each, while the O2 and O3 each receive five Hbonds (Figure 5c), which exhibits appropriate consistency with electronic structure calculations by Hay et. al.59 signifying that each oxygen atom of an oxoanion could be involved in a maximum of three hydrogen bonds. Note that, the divalent sulphate anion was not present in the solution mixture prior to crystallization and it comes from deprotonation of monovalent hydrogensulphate by hydrogen-bonding activated proton transfer. In literature, this kind of solution-state deprotonation of protonated anionic species viz., H2PO4-, HCO3- and HSO4- is common previously reported by ghosh et. al. and from our group23,27,47,50 , and this occurs due to the multiple H-bonding interactions between anion and receptor, which lowers the pKa of the bound guest to the extent that it is deprotonated by the free guest species in solution as recommended by gale et. al.60 The packing motif of complex 5 reveals the formation of a linear framework of cation-sealed pseudo-capsular neutral receptor-sulphate assemblies (Figure 5d) as viewed down along the crystallographic b axis.



2.3. Comparative anion binding analysis of electron-rich naphthyl tripodal scaffold: It is exceptional and significant to mention from the X-ray analysis of anion complexes that, depending not only on the anion size but also on the involved corresponding counter-cation species, the highly electron-rich naphthyl tri-thiourea neutral receptor L consistently expand or compress its three arms to encapsulate smaller to larger halides as well as planar and tetrahedral oxyanions unusually in 1:1 complexation mode in solid state. The crystal structures of halide

Figure 6. X-ray structures ( partial) depicting the average pod distance among the three terminal naphthyl rings of tripodal thiourea receptor L in the anion complexes (a) 1a, (b) 1b, (c) 2a, (d) 2b, (e) 3a, (f) 3b, (g) 4 and (h) 5.

complexes clearly shows that, as we go from the smallest fluoride to relatively larger chlorides or bromides, the average naphthyl pod distance gradually increases. Interestingly, note that the average pod distance increase from fluoride complexes (1a, 1b) to chloride complexes (2a, 2b) are apparently larger (~ 1.66 Å) than from chloride to bromide (~ 0.04 Å) complexes (3a, 3b) (Figure 6). This halide binding phenomena of receptor L heavily supports that the ionic radius of smallest halide (F- = 1.33 Å) is relatively lower than the ionic radius of chloride (Cl- = 1.81 Å) and bromide (Br- = 1.96 Å) ion of comparable size. Subsequently, the corresponding counteraction of halides as well as the anion-coordinaton number also regularly affects halide binding consistency of receptor L. The average naphthyl tripod distances in TEA+ cation sealed complex 1b becomes little bit larger (~ 0.76 Å) than fully fluoride encapsulated complex 1a, as the coordination number of F- are 7 and 6 respectively in complex 1b and 1a (Figure 6). Whereas, the average naphthyl tripod distances in TBA+ cation triggered hepta-coordinated 14 ACS Paragon Plus Environment

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chloride (2a) and bromide (3a) complexes becomes fraction larger than TEA+ cation triggered hexa-coordinated chloride (2b) and bromide (3b) complexes i.e. ~ 0.03 Å and ~ 0.05 Å respectively. Similarly, in planar nitrate oxyanion encapsulated TEA+ cation triggered complex 4, the average naphthyl tripod distance also becomes fraction larger (~ 0.01 Å) than TEA+ counter-cation involved bromide complex 3b, because the dimension as well as the coordination number of planar nitrate is greater than bromide anion. Finally, in complex 5, the receptor orients in a TBA+ cation sealed bowl-shaped pseudocapsular fashion to encapsulate larger coordinating tetrahedral sulphate, hence the average terminal naphthyl pod distance becomes 10.350 Å (Figure 6), which is very larger compared to nitrate as well the halide complexes. This discrepancy in average tripod distances is attributed for the greater dimension, anion coordination number as well as for the bigger TBA+ counteraction involved in complex formation. A relationship of N–H⋯A angle vs. H⋯A distance in all anion complexes depending on the anion size reveals that most of the receptor thiorea -N–H hydrogen-bonding interactions with corresponding halide and oxyanions in solid state are in the region of strong hydrogen-bonding i.e. d (H⋯A) ≤ 2.6 Å and d(D⋯A) ≤3.3 Å. The N–H⋯A angles vs. H⋯A distances scatter plot (Figure S17, Supporting information) for individual anion complexes also validates that most of the non-covalent interactions unveiling strong hydrogen-bonding character. Hydrogen bonding data table and crystallographic refinement data table of all the anion complexes 1a, 1b, 2a, 2b, 3a, 3b, 4 and 5 is tabulated in table S1 and table 2. 2.4. Solution-state anion binding observation by NMR Spectroscopy: In the field of supramolecular anion recognition chemistry it is now well recognized that the performance of receptors is really quite different in dilute or very dilute solution from their behavior in the solid state. Hence, to investigate the host-guest binding mode in solution, herein we have carried out 1H-NMR spectroscopic studies of all anion complexes as well as titration experiments in DMSO-d6 at room temperature in .qualitative as well as quantitative way. The most significant changes have been observed for the thiourea -NH protons (–NHa and –NHb) of L in the preliminary studies, signifying that the –NH functions provide suitable sites of interaction between the receptor and anion in solution. However, in 1999 S. Wu et.al4c reported exactly the same ligand as an optically changeable receptor for anion, where they demonstrated only solution state complexation of H2PO4- and HSO4- oxyanion by UV-Visible titration and 1H15 ACS Paragon Plus Environment


NMR results and further they proposed 1:1 host-guest complexation from titration experiments, although till now no solid state anion binding support was provided with this potential anion binding platform. Interestingly, the divalent sulphate encapsulated complex 5 in this report supports the 1:1 host-guest binding in solution proposed by S. Wu et. al.17 Subsequently, we have also performed qualitative as well as quantitative NMR experiments with individual F-, Cl-, Br-, NO3- and SO42- salts in solution as evidenced from the solid state and Figure 7 represents the maximum observable shift of thiourea –NH protons in anion complexes in solution compared to

Figure 7. Expanded partial 1H NMR comparative stacked spectra in solution phase of tris-thiourea receptor L with halides and oxyanions as observed from solid state, displaying the observable downfield shifts of thiourea -NHa and -NHb proton resonances of molecular receptor L upon anion complexation.

free ligand L. The 1H-NMR spectra free receptor attained from dry acetonitrile medium shows thiourea –NH peaks at 7.35 and 9.67 ppm, which is very close to the thiourea –NH peaks at 7.40 and 9.70 ppm respectively reported by Wu and coworkers. The 1H-NMR spectra of fluoride complex 1a (-NH peaks at 9.38 and 11.36 ppm) and sulphate complex 5 (-NH peaks at 10.38 and 11.24 ppm) individually shows large downfield shift of thiourea hydrogens compared to –NH peaks free receptor L indicating the rapid formation of hydrogen bonds with fluoride and sulphate in solution. On the other hand, the chloride (2a), bromide (3b) and nitrate (4) complexes show negligible downfield shift of thiourea –NH protons, suggesting that the interaction of these anions are energetically unfavorable in solution state, which is not very uncommon case in literature. Subsequently, following the qualitative studies., we have performed 1H-NMR titration 16 ACS Paragon Plus Environment

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analysis of free ligand with aliquots of standard n-TBAF salt, which shows instant large downfield shift of both thiourea -NH signals (∆δ -NHa = 2.05 ppm and -NHb = 1.72 ppm) after 0.1 eqv F- ion addition (Figure S15, Supporting information), which is very similar with 1HNMR data of isolated crystals of fluoride complex (Figure S6, Supporting information). Similarly, the gradual addition of (n-TBA)2SO4 salts to the solutions of free thiourea ligand L in titration experiments, led to the average downfield shift of thiourea –NH protons (∆δ -NHa = 2.99 ppm and -NHb = 1.50 ppm) followed by huge broadening (Figure S16, Supporting information) respectively, which becomes very similar with the 1H-NMR data of isolated crystals of sulphate complex 5 (Figure S14, ESI). Note that, the considerably larger shift of -NHa (∆δ = 2.99 ppm) relative to -NHb signals (∆δ = 1.50 ppm) in sulphate titration suggests that the divalent sulphate anion is bound more strongly to –NHa rather than -NHb protons of the thiourea receptor in solution, which becomes reliably supported from solid state sulphate binding of receptor, where three -NHa thiourea protons are involved in six H-bond donation and three -NHb protons donates only three H-bonds (Figure 5a) in complex 5. 2.5. Solid state FT-IR studies of anion complexes: The presence of hydrogen bonded halides as well as oxyanions with the thiourea –NH protons of the neutral receptor L in all anion complexes have also been confirmed by solid-state FT-IR analysis. The stretching frequency of thiourea -NH in all complexes show an average notable shift of 60-90 cm-1 with subsequent broadening of the peak in each case comparison to that of free receptor L (ν 3363 cm-1), which is supportive for the existence of strong N–H⋯A hydrogen bonds between L and anion in corresponding complexes. Moreover, the presence of a moderate signal at around 2860-2950 cm-1 has also been observed in each complex, which can be attributed to the C–H stretching frequencies of the TBA/TEA groups. The characteristic intense absorption band at ∼1118 cm-1 and ∼1395 cm-1 are generally used to identify the presence of sulfate and nitrate in individual complexes 5 and 4 respectively. 2.6. Hirshfeld surface analyses: The weak non-covalent interactions involved in the conformational changes of tripodal segments in presence of different dimensions of anions in all complexes can also be visualized by the Hirshfeld surfaces (HSs), which is a useful tool to describe the surface characteristics of molecules. HSs facilitate a unique method of visualizing intermolecular interactions by colourcoding short or long contacts and the intensity of the color signifying the relative strength of the 17 ACS Paragon Plus Environment


interactions. The two dimensional fingerprint plots (FPs) complement those surfaces, quantitatively summarizing the nature and type of intermolecular contacts experienced by the molecules in the crystal and 2D FPs can also be broken down to provide the relative contribution to the Hirshfeld surface area from each type of interactions present, quoted as ‘‘contact contribution’’.67 The contribution of strong N–H⋯A (anion) hydrogen bonds in the above described complexes as well as C–H⋯A (anion), C–H⋯O, C–H⋯π weak interactions can also be visualised by the HSs. The HSs of the halide/oxyanion bound tripodal receptor segment are illustrated in Figure 8a,

Figure 8 Hirshfeld surface analysis displaying (a) the dnorm surfaces of anion bound tripodal segments of complexes 1a, 1b, 2a, 2b, 3a, 3b, 4 and 5, (b) corresponding 2D fingerprint plots with the C⋯H interactions highlighted in color involved in C–H⋯π or C–H⋯O contacts and (c) corresponding 2D fingerprint plots with the H⋯A (anion) interactions highlighted in colour involved in N–H⋯A (anion) contacts.

displaying surfaces that have been mapped over dnorm mostly highlights the intermolecular C– H⋯A (anion) as well as C–H⋯O and C–H⋯π interactions as bright to faint red spots that exist between the adjacent receptor molecules or between receptor and aliphatic -CH donors of neighbouring n-TBA/TEA counter-cations. Interestingly, it is significant to mention that the interaction spots for the H⋯A (anion) contacts were not observed on the receptor surface in most of the halide complexes, although it was quite observable as we go from smaller fluoride to larger spherical halides as well as planar and tetrahedral oxyanion complexes, that can be attributed to the extent of encapsulating anions of different dimensions by most probable conformation of tripodal receptor segment. The contact contribution of H⋯A (anion) interactions


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in fluoride complexes 1a (0.1%) and 1b (0.2%) are lowest among all other complexes (table 2), because fluoride anion is fully encapsulated within the tripodal receptor scaffold where the donor atoms are directed completely towards the cavity forming strong N–H⋯F- hydrogen bonds. Interestingly, Figure 8c represents the 2D FPs for the H⋯A (anion) close contacts with a contact contribution of almost ascending order nicely depending upon the respective counter-cation involved, which can be ascribed for the systematic conformational opening of tripodal thiourea arms in presence of anions of different size. These results from HSs and corresponding 2D FPs of anion complexes fully support the solid state results obtained from X-ray analyses. Note that, almost similar 2D FPs for the C⋯H close contacts with similar contact contributions (Figure 8b, Table 2: Contact contributions (%) from the dnorm surface area of tripodal anion bound receptor segments in complexes Contacts 1a 1b 2a 2b 3a 3b 4 5 H⋯Anion (%)

0.1

0.2

3.1

1.9

3.4

3.0

6.0

5.4

H⋯C (%)

15.7

21.2

19.4

20.2

18.7

20.1

20.8

20.3

H⋯N (%)

2.5

2.3

0.9

1.2

0.9

1.1

2.0

0.5

H⋯S (%)

15.8

17.5

12.1

15.3

12.2

14.6

15.8

11.8

H⋯Ow (%)

--

--

--

4.0

--

5.0

--

--

H⋯H (%)

65.7

55.4

61.1

53.5

60.9

52.7

52.4

61.9

table 2) have been observed in n-TBA cation involved chloride (2a) and bromide (3a) complexes, which is partly responsible for their isostructural nature as shown from X-ray studies. Similar kinds 2D FPs are also observed for the C⋯H close contacts of TEA cation involved isostructural chloride (2b) and bromide (3b) complexes. The FPs of all complexes feature spike of several lengths and thickness, and the maximum prominent being the presence of wing-like peripheral spikes for C⋯H contact at the top left and bottom right of each plot (Figure 8b). Figure 8c depicts the fingerprint plots for the H⋯A- (Cl-/Br-/NO3-) close contacts in complexes 2a, 2b, 3a, 3b and 4 display the characteristic sharp ‘’spikes’’ in the upper left and lower right of the plot and show pseudosymmetry on either side of the diagonal where de = di. However, the strong H⋯A- (SO42-) contacts appear as single sharp ‘‘spike’’ in Figure 8c and the two closely symmetric ‘‘spikes’’ for the C⋯H contacts in Figure 8b with contact contributions of 5.4% and 20.3% respectively (table 2) are observed for sulphate pseudo-encapsulated complex 5. 2.7. Density Functional Theory (DFT) study of free receptor: It is worth mentioning that several attempts were also made to crystallize the free receptor L from various solvents in 19 ACS Paragon Plus Environment


different conditions, but being unable to grow good quality crystals, density functional theory (DFT) study was performed for structural elucidation of free tripodal thiourea ligand which reveals the complete non-cooperativity of thiourea –NH protons exhibiting no tripodal cavity at all, unlike the anion complexes (Figure S26, Supporting information). DFT optimizations were carried out using the B3LYP method with 6-31+G(d,p) basis set.

3. Conclusions: In summary, we have expansively established that a particular neutral tripodal scaffold L can effectively entraps anions of different shapes and geometries via conformational change of receptor framework, especially in solid state also, despite consisting of unfavorable highly electron-rich naphthyl functionalization and in the absence of any electron withdrawing π-acidic aryl terminals. Note that, the tris-thiourea receptor L fully encapsulates the smallest halide within its C3-symmetric tripodal cavity, whereas, the neutral ligand little bit opens up their three arms to catch relatively larger spherical chloride, bromide and planar nitrate via pseudo-encapsulation. On the other hand, to grab the bigger tetrahedral sulphate anion, the tripodal receptor moiety stretches its three arms via formation of a more open bowl-shaped cavity in a cation-sealed environment. These kinds of systematic and consistent shape and size selective anion binding of extremely electron-rich tripodal scaffold in 1:1 host-guest complexation mode in solid state also largely corroborates the solution state H2PO4-/HSO4- binding only in 1:1 host-guest binding proposed from UV/Vis absorption spectroscopy reported by S. Wu et. al. in 1999. The stability of all the anion-receptor complexes in solid state intensely governs by the six thiourea –NH hydrogen bond donor sites of L that enables strong binding affinity between host and anion. Successively, the three dimensional flexible molecular structure of L permits the shape as well as preorganization of the receptor to monitor the recognition process and adds directionality to the whole system. These kinds of halide as well as oxyanion binding evidence within highly electron-rich naphthyl tripodal scaffold especially in solid state would be helpful to the supramolecular researchers to realize the neutral host-guest assembly process.


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4. Experimental Section 4.1. Materials and Methods. All the solvents and reagents used as starting material were obtained from commercial sources and used as received without further purification. Tris(2-aminoethyl)amine (tren), 1-Napthyl isothiocyanate, and all the tetraalkylammonium salts were purchased from Sigma-Aldrich and Table 3: Crystallographic Parameters and Refinement Data of anion complexes of neutral receptor L:

Parameters 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.

1a C55 H75 F N8 S 3 963.41 monoclin ic P 21/c 24.648(1 2) 21.278(6) 22.391(8) 90.00 105.744( 4) 90.00

1b C47 H55 F N8 S 3 847.05 trigonal

2a C55 H75 Cl N8 S 3 979.86 triclinic

2b C47 H59 Cl N8 O S 3 883.65 triclinic

3a C55 H75 Br N8 S3 1024.31 triclinic

3b C47 H61 Br N8 O S3 930.12 triclinic

4 C47 H59 N9 O3 S3 894.21 triclinic

5 C71 H111 N9 O4 S4 1282.93 monoclinic

R3c 16.017(1 5) 16.017(1 5) 30.658(3) 90.00 90.00

P -1 10.601(9)

P -1 12.186(7)

P -1 10.660(7)

P -1 12.344(10)

P -1 12.283(7)

Cc 21.512(3)

16.312(8)

12.926(7)

16.306(11)

12.778(11)

12.922(6)

16.070(13)

16.915(9) 93.425(4) 95.816(5)

16.403(9) 90.00 85.660(3)

17.030(10) 93.451(5) 96.068(5)

16.416(13) 84.883(5) 70.394(5)

16.155(8) 85.459(4) 70.109(5)

22.399(2) 90.00 91.554(9)

106.427(6 ) 2779.3(3)

81.205(3)

106.259(6)

80.731(5)

80.961(4)

90.00

2399.6(2)

2813.3(3)

2405.6(4)

2380.3(2)

7740.4(14)

2 1.171 0.224 1052.0 298(2) 25.000 20218

2 1.223 0.253 1265.0 298(2) 27.485 23312

2 1.209 0.884 1024.31 298(2) 24.999 15628

2 1.281 1.028 976.0 298(2) 28.370 39366

2 1.248 0.206 952.0 298(2) 24.999 16221

4 1.101 0.172 2784.0 298(2) 24.999 14499

120.00

11302.1( 8) 8 1.132 0.176 4144.0 298(2) 25.000 47878

6811.4(1 7) 6 1.239 0.210 2699.6 298(2) 24.999 8584

19882

2555

9761

10850

9644

11558

8378

12916

7489

1398

4841

3667

5486

4403

6068

8532

1215

185

628

545

661

545

590

783

0.0900 0.1586 1.084 1818837

0.0692 0.1771 1.100 1818838

0.1010 0.1687 0.853 1818839

0.0801 0.2351 1.118 1818840

0.0757 0.1832 1.026 1818841

0.0644 0.1708 1.068 1818842

0.0783 0.1955 0.947 1818843

0.0759 0.1875 1.090 1818844

used as received. Solvents were purchased from Merck, India, and used as received for synthesis and crystallization experiments. 1H NMR characterization spectra were recorded on a Varian FT600 MHz instrument, and chemical shifts were recorded in parts per million (ppm) on the scale using tetramethylsilane [Si(CH3)4] or a residual solvent peak as a reference and 21 ACS Paragon Plus Environment

13

C spectra of


the receptor were recorded at 150 MHz on the same instrument. The electrospray ionization mass (ESI-MS) spectrum of tris-thiourea ligand was recorded in methanol. The crystals of all the halides and oxyanion complexes of L were washed with a minimum amount of acetonitrile, diethyl ether and dried at room temperature by pressing between filter papers before FT-IR measurements. The FT-IR spectra of air-dried samples of ligand and its anion complexes were recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer with KBr disks in the range 4000-500 cm-1. Binding phenomena of receptor-anion complexation were investigated by 1HNMR (Varian FT-600 MHz) titrations of receptors with tetraethylammonium (TEA)/ tetrabutylammonium (n-TBA) salts of respective anions in DMSO-d6 at 298 K, as observed from solid state. Initial concentrations were [ligand]0 = 10 mM and [anion]0 = 50 mM. Aliquots of anions were added from 50 mM stock solutions of anions. Each titration was performed by 10-12 measurements at room temperature and the residual solvent peak in DMSO-d6 (2.500 ppm) was used as an internal reference. 4.2. Syntheses and Characterization. 4.2.1. Receptor L. The tris-thiourea receptor was synthesized by the reaction of 2.4 g (13 mmol) 1-Naphthyl isothiocyanate and 599 µL Tris(2-aminoethyl)amine (4.0 mmol) in 1:3 equimolar ratio by dissolving them in 25 mL of dry acetonitrile at room temperature in a 100 mL round bottomed flask. Then, the resulting solution mixture was kept under vigorous stirring for 24 hours. The off-white precipitate of tris-thiourea ligand L formed was filtered off and washed several times with acetonitrile, THF to wash out the unreacted starting materials and then dried in vacuum. The white powder of receptor L is characterized by ESI-Mass, NMR, FT-IR analyses. Yield = 85% for L. 1H NMR (600 MHz, DMSO-d6) δ (ppm): 2.56 (bs, 6H, -NCH2), 3.42 (bs, 6H, -NCH2CH2), 7.35 (bs, 3H, -NHa), 7.44-7.45 (d, 3H, ~6.6 Hz, -ArH), 7.49-7.55 (m, 9H, ArH), 7.84-7.87 (m, 6H, -ArH), 7.94-7.95 (d, 3H, ~5.4 Hz, -ArH), 9.67 (s, 3H, -NHb); 13C NMR (150 MHz, DMSO-d6) δ (ppm): 42.8 (×3C, -NCH2), 52.8 (×3C, -NCH2CH2), 123.5 (×3C, Ar-C), 125.9 (×3C, Ar-C), 126.4 (×3C, Ar-C), 126.9 (×3C, Ar-C), 127.0 (×3C, Ar-C), 127.5 (×3C, ArC), 128.8 (×3C, Ar-C), 130.5 (×3C, Ar-C), 134.7 (×3C, Ar-C), 134.9 (×3C, Ar-C), 182.3 (×3C, C=S). FT-IR (KBr): 780 cm-1 vs(C=S, asym), 1527 vs(C=S, sym), 2937 cm-1 vs(C-H), 3051 cm-1 vs(C-H), 3366 cm-1 vs(N-H). ESI-MS: m/z 702.2463 (calculated) and 702.2684 (observed) [L1+H].


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4.2 2. Fluoride complexes [(n-TBA){(L1)(F)}] (1a) and [(TEA){(L)(F)}] (1b): Complex 1a and 1b with teir respective tetrabutylammonium and tetraethylammonium salts were obtained as suitable crystals for X-ray diffraction analysis upon slow evaporation of 5 mL basic DMF/DMSO

solutions

of

L

(100

mg,

0.142

mmol)

and

excess

of

tetrabutylammonium/tetraethylammonium fluoride (10 eqv.) salts. The colorless crystals in both cases of fluoride encapsulated complexes were obtained within 10-15 days and isolated by filtration and dried at room temperature by pressing between the filter papers before characterization by NMR as well as FT-IR analyses. Isolated yield: 75% for 1a and 85% for 1b based on L. 1H NMR (600 MHz, DMSO-d6) δ (ppm) of 1b: 1.13-1.16 (t, 12H, ~7.2 Hz, TEACH3), 2.79-2.81 (t, 6H, ~5.4 Hz, -NCH2), 3.17-3.20 (q, 8H, ~7.2 Hz, TEA-CH2), 3.78-3.80 (t, 6H, ~5.4 Hz, -NCH2CH2), 6.11-6.13 (t, 3H, ~7.2 Hz, -ArH), 6.99-7.02 (t, 3H, ~7.8 Hz, -ArH), 7.38-7.41 (t, 3H, ~7.8 Hz, -ArH), 7.62-7.64 (d, 3H, ~7.8 Hz, -ArH), 7.68-7.69 (d, 3H, ~8.4 Hz, ArH), 7.73-7.75 (d, 3H, ~8.4 Hz, -ArH), 7.81-7.82 (d, 3H, ~7.2 Hz, -ArH), 9.36-9.39 (d, 3H, NHa), 11.33-11.39 (d, 3H, -NHb); FT-IR spectra (KBr) of 1a: 783 cm-1 vs(C=S, asym), 1547 vs(C=S, sym), 2992 cm-1 vs(C-H), 3266 cm-1 vs(C-H), 3419 cm-1 vs(N-H). 4.2.3. Chloride complexes [(n-TBA){(L)(Cl)}] (2a) and [(TEA){(L)(Cl)(H2O)}] (2b): The chloride complexes 2a and 2b was attained by charging excess of tetrabutylammonium and tetraetylammonium chloride salts (10 eqv.) respectively into the 5 mL DMF solutions of L (100 mg, 0.142 mmol) in separate small glass vials. The resulting solution mixtures after addition of respective salts were stirred for half an hour and were left open to atmosphere for slow evaporation. The colorless crystals in both cases of chloride trapped complexes of L were obtained within 15-20 days and isolated by filtration and dried at room temperature by pressing between the filter papers before characterization by NMR and FT-IR analyses. Yield: 65% for 2a and 80% for 2b based on L. 1H NMR (600 MHz, DMSO-d6) δ (ppm) of 2a: 0.92-0.95 (t, 12H, ~7.8 Hz, TBA-CH3), 1.27-1.34 (m, 8H, TBA-CH2), 1.54-1.59 (m, 8H, TBA-CH2), 2.64 (bs, 6H, NCH2), 3.14-3.17 (t, 8H, ~8.4 Hz, TBA-CH2), 3.54-3.57 (q, 6H, ~6.6 Hz, -NCH2CH2), 7.41-7.43 (t, 3H, ~6.6 Hz, -ArH), 7.47-7.50 (m, 9H, -ArH), 7.81-7.82 (t, 3H, ~4.8 Hz, -ArH), 7.84 (bs, 3H, -NHa), 7.89-7.90 (d, 3H, ~8.4 Hz, -ArH), 7.92-7.93 (d, 3H, ~8.4 Hz, -ArH), 9.91 (s, 3H, -NHb); FT-IR spectra (KBr): 786 cm-1 vs(C=S, asym), 1540 vs(C=S, sym), 3031 cm-1 vs(C-H), 3239 cm1

vs(C-H), 3467 cm-1 vs(N-H).



Page 24 of 34

4.2.4. Bromide complexes [(n-TBA){(L)(Br)}] (3a) and [(TEA){(L)(Br)(H2O)}] (3b): The chloride pseudo-encapsulated unimolecular complexes 3a and 3b was acquired as proper crystals for X-ray diffraction analysis upon slow evaporation of two separate DMF solution mixtures

of L

(100

mg, 0.142

mmol)

and

excess

of tetrabutylammonium

and

tetraethylammonium bromide (10 eqv.) salts respectively. The colorless crystals of receptorbromide 1:1 complexes in both cases thus obtained were isolated by filtration and dried at room temperature by pressing between the filter papers before characterization by NMR as well as FTIR analyses. Isolated yield: 75-80% for 3a and 3b based on L2. 1H NMR (600 MHz, DMSO-d6) δ (ppm) of 3b: 1.14-1.17 (t, 12H, ~7.2 Hz, TEA-CH3), 2.56 (bs, 6H, -NCH2), 3.18-3.22 (q, 8H, ~7.2 Hz, TEA-CH2), 3.42 (bs, 6H, -NCH2CH2), 7.37 (bs, 3H, -NHa), 7.44-7.45 (d, 3H, ~8.4 Hz, - ArH), 7.49-7.50 (d, 3H, ~7.8 Hz, -ArH), 7.51-7.54 (m, 6H, -ArH), 7.84-7.85 (d, 3H, ~7.8 Hz, ArH), 7.86-7.87 (d, 3H, ~6.6 Hz, -ArH), 7.94-7.96 (t, 3H, ~5.4 Hz, -ArH), 9.68 (s, 3H, -NHb); FT-IR spectra (KBr): 777 cm-1 vs(C=S, asym), 1530 vs(C=S, sym), 2962 cm-1 vs(C-H), 3285 cm1

vs(C-H), 3457 cm-1 vs(N-H).

4.2.5. Nitrate complex [(TEA){(L)(NO3)}] (4): The nitrate trapped monomeric pseudo-capsule of L as complex 4, was attained by charging an excess of tetrabutylammonium nitrate (10 equiv.) into the 5 mL DMF solution of L (100 mg, 0.142 mmol) in a small glass vial. The resulting solution mixture after the excess addition of TEANO3 salt was stirred for about 30 min and kept it to open atmosphere for slow evaporation at room temperature. The colourless crystals of complex 4 was isolated by filtration and dried at room temperature by pressing between the filter papers before characterization before characterization by NMR and FT-IR analyses. Isolated yield: 65-65% based on L. 1H NMR (600 MHz, DMSO-d6) δ (ppm): 1.14-1.17 (t, 12H, ~7.2 Hz, TEA-CH3), 2.56 (bs, 6H, -NCH2), 3.183.21 (q, 8H, ~7.2 Hz, TEA-CH2), 3.41 (bs, 6H, -NCH2CH2), 7.35 (bs, 3H, -NHa), 7.44-7.45 (d, 3H, ~7.2 Hz, -ArH), 7.49-7.50 (d, 3H, ~7.8 Hz, -ArH), 7.51-7.54 (m, 6H, -ArH), 7.84-7.87 (m, 6H, -ArH), 7.94-7.96 (t, 3H, ~4.8 Hz, -ArH), 9.67 (s, 3H, -NHb); FT-IR spectra (KBr): 783 cm1

vs(C=S, asym), 1395 cm-1 vs(N-O), 1540 vs(C=S, sym), 3051 cm-1 vs(C-H), 3310 cm-1 vs(C-H),

3467 cm-1 vs(N-H). 4.2.6. Sulphate complex [(TBA){(L)(SO4)}] (5): The divalent sulphate pseudo-encapsulated complex 5 was attained as suitable crystals for X-ray analysis upon slow evaporation of a 5 mL basic DMF solution of L (100 mg, 0.142 mmol) from 24 ACS Paragon Plus Environment

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a glass vial in presence of excess tetrabutylammonium hydrogensulphate (10 eqv.). The colorless crystals of 1:1 cation sealed pseudo-capsular host-guest complex 5 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: 80% based on L. 1H NMR (600 MHz, DMSO-d6) δ (ppm): 0.92-0.94 (t, 12H, ~7.8 Hz, TBA-CH3), 1.27-1.33 (m, 8H, TBA-CH2), 1.53-1.58 (m, 8H, TBA-CH2), 2.64 (bs, 6H, -NCH2), 3.13-3.17 (t, 8H, ~8.4 Hz, TBA-CH2), 3.683.70 (q, 6H, ~4.8 Hz, -NCH2CH2), 7.30-7.33 (t, 3H, ~7.8 Hz, -ArH), 7.38-7.40 (d, 3H, ~7.2 Hz, ArH), 7.41-7.44 (m, 6H, -ArH), 7.73-7.74 (d, 3H, ~8.4 Hz, -ArH), 7.86-7.87 (d, 3H, ~8.4 Hz, ArH), 8.06-8.07 (d, 3H, ~8.4 Hz, -ArH), 10.37-10.39 (t, 3H, ~6.0 Hz, -NHa), 11.24 (s, 3H, NHb); FT-IR spectra (KBr): 777 cm-1 vs(C=S, asym), 1118 cm-1 vs(S-O), 1547 vs(C=S, sym), 2959 cm-1 vs(C-H), 3233 cm-1 vs(C-H), 3426 cm-1 vs(N-H). 4.3. Crystallographic Refinement Details: The crystallographic refinement parameter details of data collection for all anion complexes 1a, 1b, 2a, 2b, 3a, 3b, 4 and 5 of thiourea ligand L are summarized in Table 3 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 it was mounted on the tip of a glass fiber and cemented using epoxy resin. The X-ray 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.61 The data integration and reduction were undertaken with SAINT and XPREP62 software and multi-scan empirical absorption corrections were applied to the data using the program SADABS.63 All the 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.64 Graphics for structural illustrations are generated using MERCURY 2.365 for Windows. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms attached to all the 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. In other cases, the hydrogen atoms are geometrically fixed.



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4.4. Hirshfeld surface analysis: The Hirshfeld surfaces66,68-74 (HSs) and 2D fingerprint plots75-76 (FPs) are constructed based on the electron distribution calculated as the sum of spherical atom electron densities77-78 and obtained from the results of single crystal X-ray diffraction studies. The Hirshfeld surface is unique79 for a given crystal structure and set of spherical atomic electron densities, besides it proposes the possibility of attaining both qualitative as well as quantitative insight into the intermolecular interaction of molecular crystals. The HS surrounding a molecular fragment is well-defined by points where the contribution to the electron density from the molecule of interest is equal to the contribution from all the other molecules and two distances for each point on that isosurface are defined: de, the distance from the point to the nearest nucleus external to the surface, and di, the distance to the nearest nucleus internal to the surface. The normalized contact distance, dnorm is a ratio encompassing the distances of any surface point to the nearest interior (di) and exterior (de) atom and the van der Waals radii of the atoms, which is given by eqn (1) simplifies effective detection of the regions of particular importance to intermolecular interactions.66 dnorm = {(di - rivdw) / rivdw} + {(de – revdw) / revdw}

(1)

The value of dnorm is negative or positive or zero. The negative dnorm value indicates the sum of di and de is shorter than the sum of the relevant vdW radii, considered to be the closest contact and is visualized as red color in the HSs. The white colour denotes intermolecular distances close to vdW contacts with dnorm equal to zero, while contacts longer than the sum of vdW radii with positive dnorm values are coloured with blue. The combination plot of di vs de is the 2D fingerprint plot which recognizes the existence of various types of intermolecular interactions. The HSs are mapped with dnorm, and 2D FPs of all anion-receptor complexes are presented in Figure 8 and contact contributions from the dnorm surface area are summarized in table 2, which were generated using Crystal Explorer 3.1.80

Acknowledgments This work was supported by CSIR and SERB through grant 01/2727/13/EMR-II and SR/S1/OC62/2011, New Delhi, India. CIF IIT Guwahati and DST-FIST for providing instrument facilities. U.M. thanks IIT Guwahati for fellowship.


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Supporting Information Additional crystallographic data characterization data and CIF files for all the anion complexes 1a, 1b, 2a, 2b, 3a, 3b, 4 and 5; ESI-Mass spectra, 1H NMR, 13C NMR spectra, FT-IR spectra, 1H NMR titration stack plots in solution phase, distance vs angle plots, hydrogen-bonding data table. References: 1. Luecke, H.; Quiocho, F. A. High specificity of a phosphate transport protein determined by hydrogen bonds Nature, 1990, 347, 402. 2. He, J. J.; Quiocho, F. A. A nonconservative serine to cysteine mutation in the sulfate-binding protein, a transport receptor Science, 1991, 251, 1479. 3. Anslyn, E. V.; Smith, J.; Kneeland, D. M.; Ariga, K.; Chu, F Strategies for phosphodiester complexation and cleavage Supramol. Chem., 1993, 1, 201. 4 Manna, U.; Nayak, B.; Hoque, N.; Das G. Influence of the cavity dimension on encapsulation of halides within the capsular assembly and side-cleft recognition of a sulfate-water cluster assisted by polyammonium tripodal receptors CrystEngComm, 2016, 18, 5036. 5 Hoque, N.; Manna, U.; Das, G. Encapsulation of fluoride and hydrogen sulfate dimer by polyammonium-functionalised first- and second-generation tripodal: cavity-induced anion encapsulation Supramol. Chem. 2016, 28, 284. 6. Beer, P. D.; Hesek, D.; Hodacova, J.; Stokes, S. E. J. Chem. Soc. Acyclic redox responsive anion receptors containing amide linked cobalticinium moieties Chem. Commun., 1992, 270. 7. Beer, P. D.; Hazlewood, C.; Hesek, D.; Hodacova, J.; Stokes, S. E. 1-Triphenylstannyl-nidopentaborane(9): an example of tin-119-boron-11 coupling in a pyramidal borane J. Chem. Soc. Dalton Trans., 1992, 1327. 8. Beer, P. D.; Chen, Z.; Goulden, A. J.; Graydon, A.; Stokesand, S. E.; Wearb, T. Selective electrochemical recognition of the dihydrogen phosphate anion in the presence of hydrogen sulfate and chloride ions by new neutral ferrocene anion receptors J. Chem. Soc. Chem. Commun., 1993, 1834. 9. Valiyaveettil, S.; Engbersen, J. F. J.; Verboom, W.; Reinhoudt, D. N. Synthesis and Complexation Studies of Neutral Anion Receptors Angew. Chem. Int. Ed., 1993, 32, 900. 10. Ravikumar, I.; Lakshminarayanan, P. S.; Ghosh, P. Anion binding studies of tris(2aminoethyl)amine based amide receptors with nitro functionalized aryl substitutions: A positional isomeric effect Inorg. Chim. Acta., 2010, 363, 2886. 27 ACS Paragon Plus Environment


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22. Akhuli, B.; Ravikumar, I.; Ghosh, P. Acid/base controlled size modulation of capsular phosphates, hydroxide encapsulation, quantitative and clean extraction of sulfate with carbonate capsules of a tripodal urea receptor Chem. Sci., 2012, 3, 1522. 23. Ravikumar, I.; Lakshminarayanan, P. S.; Arunachalam, M.; Suresh, E.; Ghosh, P. Anion complexation of a pentafluorophenyl-substituted tripodal urea receptor in solution and the solid state: selectivity toward phosphate Dalton Trans., 2009, 4160. 24. Ravikumar, I.; Ghosh, P. Efficient fixation of atmospheric CO2 as carbonate in a capsule of a neutral receptor and its release under mild conditions Chem. Commun. 2010, 46, 1082. 25. Busschaert, N.; Wenzel, M.; Light, M. E.; Iglesias-Hernandez, P.; Perez-Tomas, R.; Gale, P. A. J. Am. Chem. Soc., 2011, 133, 14136. 26. Chutia, R.; Dey, S. K.; Das, G. Positional Isomeric Effect in Nitrophenyl Functionalized Tripodal Urea Receptors toward Binding and Encapsulation of Anions Cryst. Growth Des., 2013, 13, 883. 27. Dey, S. K.; Chutia, R.; Das, G. Oxyanion-Encapsulated Caged Supramolecular Frameworks of a Tris(urea) Receptor: Evidence of Hydroxide- and Fluoride-Ion-Induced Fixation of Atmospheric CO2 as a Trapped CO32- Anion Inorg. Chem., 2012, 51, 1727. 28. Chutia, R.; Dey, S. K.; Das, G. Self-Assembly of a Tris(Urea) Receptor as Tetrahedral Cage for the Encapsulation of a Discrete Tetrameric Mixed Phosphate Cluster (H2PO4-.bul.HPO42-)2 Cryst. Growth Des., 2015, 15, 4993. 29. Jose, D. A.; Kumar, D. K.; Ganguly, B.; Das, A. Rugby-Ball-Shaped Sulfate-Water-Sulfate Adduct Encapsulated in a Neutral Molecular Receptor Capsule Inorg. Chem., 2007, 46, 5817. 30. Basu, A.; Das, G. Encapsulation of divalent tetrahedral oxyanions of sulfur within the rigidified dimeric capsular assembly of a tripodal receptor: first crystallographic evidence of thiosulfate encapsulation within neutral receptor capsule Dalton Trans., 2012, 41, 10792. 31. Dey, S. K.; Das, G. Encapsulation of trivalent phosphate anion within a rigidified π-stacked dimeric capsular assembly of tripodal receptor Dalton Trans., 2011, 40, 12048. 32. Dey, S. K.; Basu, A.; Chutia, R.; Das, G. Anion coordinated capsules and pseudocapsules of tripodal amide, urea and thiourea scaffolds RSC Adv., 2016, 6, 26568. 33. Bose, P.; Dutta, R.; Santra, S.; Chowdhury, B.; Ghosh, P. Combined Solution-Phase, SolidPhase and Phase-Interface Anion Binding and Extraction Studies by a Simple Tripodal Thiourea Receptor Eur. J. Inorg. Chem., 2012, 35, 5791. 29 ACS Paragon Plus Environment


34. Arunachalam, M.; Ghosh, P. Anion induced capsular self-assemblies Chem. Commun., 2011, 47, 8477. 35. Arunachalam, M.; Ahamed, B. N.; Ghosh, P. Binding of Ammonium Hexafluorophosphate and Cation-Induced Isolation of Unusual Conformers of a Hexapodal Receptor Org. Lett., 2010, 12, 2742. 36. Gale, P. A. Structural and Molecular Recognition Studies with Acyclic Anion Receptors Acc. Chem. Res., 2006, 39, 465. 37. Singh, A. S.; Sun, S.-S. Dynamic self-assembly of molecular capsules via solvent polarity controlled reversible binding of nitrate anions with C3 symmetric tripodal receptors Chem. Commun., 2011, 47, 8563. 38. Yamauchi, Y.; Fujita, M. Self-assembled cage as an endo-template for cyclophane synthesis Chem. Commun., 2010, 46, 5897. 39. Ikemoto, K.; Inokuma, Y.; Fujita, M. The Reaction of Organozinc Compounds with an Aldehyde within a Crystalline Molecular Flask Angew. Chem., Int. Ed., 2010, 49, 5750. 40. Yoshizawa, M.; Klosterman, J. K.; Fujita, M. Functional Molecular Flasks: New Properties and Reactions within Discrete, Self-Assembled Hosts Angew. Chem., Int. Ed., 2009, 48, 3418. 41. Rebek Jr., J. Host–guest chemistry of calixarene capsules Chem. Commun., 2000, 637. 42. Biochemistry of Halogens and Inorganic Halides, ed. Krik, K. L. Plenum Press, New York, 1991. 42. Ayoob, S.; Gupta, A. K. Fluoride in drinking water: a review on the status and stress effects Crit. Rev. Environ. Sci. Technol., 2006, 36, 433. 43. McCall, A. S.; Cummings, C. F.; Bhave, G.; Vanacore, R.; Page-McCaw, A.; Hudson, B. G. Bromine Is an Essential Trace Element for Assembly of Collagen IV Scaffolds in Tissue Development and Architecture Cell, 2014, 157, 1380. 44. Cadenasso, M. L.; Pickett, S. T. A.; Grove, M. J. Integrative approaches to investigating human-natural systems : the Baltimore ecosystem study Natures Sciences Sociétés 2006, 14, 4. 45. Eller, L. R.; Stepien, M.; Fowler, C. J.; Lee, J. T.; Sessler, J. L.; Moyer, B. A. Octamethyloctaundecylcyclo[8]pyrrole: a promising sulfate anion extractant J. Am. Chem. Soc., 2007, 129, 11020. 46. Manna, U.; Halder, S.; Das, G. Ice-like Cyclic Water Hexamer Trapped within a Halide Encapsulated Hexameric Neutral Receptor Core: First Crystallographic Evidence of a Water 30 ACS Paragon Plus Environment

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Cluster Confined within a Receptor-Anion Capsular Assembly Cryst. Growth Des., 2018, DOI: 10.1021/acs.cgd.7b01693. 47. Manna, U.; Kayal, S.; Samanta S.; Das, G. Fixation of atmospheric CO2 as novel carbonate(water)2-carbonate cluster and entrapment of double sulfate within a linear tetrameric barrel of a neutral bis-urea scaffold Dalton Trans., 2017, 46, 10374. 48. Manna, U.; Chutia, R.; Das, G. Entrapment of Cyclic Fluoride-Water and Sulfate-WaterSulfate Cluster Within the Self-Assembled Structure of Linear meta-Phenylenediamine Based Bis-Urea Receptors: Positional Isomeric Effect Cryst. Growth Des., 2016, 16, 2893. 49. Manna, U.; Nayak, B.; Das, G. Dual Guest [(Chloride)3-DMSO] Encapsulated Cation-Sealed Neutral Trimeric Capsular Assembly: Meta-Substituent Directed Halide and Oxyanion Binding Discrepancy of Isomeric Neutral Disubstituted Bis-Urea Receptors Cryst. Growth Des., 2016, 16, 7163. 50. Manna, U.; Das, G. Anion binding consistency by influence of aromatic meta-disubstitution of a simple urea receptor: regular entrapment of hydrated halide and oxyanion clusters CrystEngComm, 2017, 19, 5622. 51. Manna, U.; Kayal, S.; Nayak, B.; Das, G. Systematic size mediated trapping of anions of varied dimensionality within a dimeric capsular assembly of a flexible neutral bis-urea platform Dalton Trans., 2017, 46, 11956. 52. Basu, A.; Das, G. Encapsulation of a discrete cyclic halide water tetramer [X2(H2O)2]2-, X = Cl-/Br- within a dimeric capsular assembly of a tripodal amide receptor Chem. Commun., 2013, 49, 3997. 53. Basu, A.; Chutia, R.; Das, G. Dual modes of binding on hexafluorosilicate anion by a C3v symmetric flexible tripodal amide ligand in solid state CrystEngComm. 2014, 16, 4886. 54. Hoque, M. N.; Das, G. Hydrated anion glued capsular and non-capsular assembly of a tripodal host: solid state recognition of bromide-water [Br5-(H2O)6]5- and iodide-water[I2(H2O)4]2- clusters in cationic tripodal receptor CrystEngComm., 2014, 16, 4447. 55. Basu, A.; Das, G. A C3v Symmetric Tripodal Urea Receptor for Anions and Ion-pairs: Formation of Dimeric Capsular Assemblies of the Receptor during Anion and Ion-pair Coordination J. Org. Chem. 2012, 79, 2647.



56. Gogoi, A; Das, G. Electronic substitution effects on anion coordination of a tripodal thiourea receptor: evidences of deprotonation of oxy-anions in solid and solution Supramol. Chem., 2013, 25, 819. 57. Bordwell, F. G. Equilibrium acidities in dimethyl sulfoxide solution Acc. Chem. Res., 1988, 21, 456. 58. Desiraju, G. R. Crystal engineering: a holistic view Angew. Chem, Int. Ed. 2007, 46, 8342. 59. Hay, B. P.; Firman, T. K.; Moyer, B. A. Structural Design Criteria for Anion Hosts: Strategies for Achieving Anion Shape Recognition through the Complementary Placement of Urea Donor Groups J. Am. Chem. Soc. 2005, 127, 1810. 60. Caltagirone, C.; Hiscock, J. R.; Hursthouse, M. B.; Light, M. E.; Gale, P. A. 1,3Diindolylureas and 1,3-diindolylthioureas: anion complexation studies in solution and the solid state Chem. Eur. J. 2008, 14, 10236. 61. CrysAlisPro, version 1171.33.34d; Oxford Diffraction Ltd. [release 27-02-2009 CrysAlis 171. NET]. 62. SMART, SAINT, and XPREP; Siemens Analytical X-ray Instruments Inc., Madison, WI, 1995. 63. Sheldrick, G. M. SADABS, Program for area detector adsorption correction, Institute for Inorganic Chemistry; University of Göttingen, Germany, 1996. 64. Sheldrick, G. M. Acta Crystallogr., Sect. C: Crystal structure refinement with SHELXL Struct. Chem., 2015, 71, 3. 65. Mercury 2.3 Supplied with Cambridge Structural Database; CCDC: Cambridge, U.K., 20011. 66. Spackman, M. A.; McKinnon, J. J. Fingerprinting intermolecular interactions in molecular crystals CrystEngComm, 2002, 4, 378. 67. Clark, T. E.; Makha, M.; Sobolev, A. N.; Raston, C. L. Mapping Out the Molecular Interplay in Monohalobenzene Inclusion Complexes of p-H-calix[5]arene Using Hirshfeld Surfaces Cryst. Growth Des., 2008, 8, 890. 68. McKinnon, J. J.; Jayatilaka, D.; Spackman, M. A. Towards quantitative analysis of intermolecular interactions with Hirshfeld surfaces Chem. Commun., 2007, 3814. 69. Spackman, M. A.; McKinnon, J. J.; Jayatilaka, D. Electrostatic potentials mapped on Hirshfeld surfaces provide direct insight into intermolecular interactions in crystals CrystEngComm, 2008, 10, 377. 32 ACS Paragon Plus Environment

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For Table of Contents Use Only

A Progressive Cation triggered Anion Binding by Electron-rich Scaffold: Case study of a Neutral Tripodal Naphthyl Thiourea Receptor Utsab Manna, and Gopal Das* Department of Chemistry, Indian Institute of Technology Guwahati, Assam-781039, Fax: +91-361-2582349; Tel: +91-361-2582313; E-mail: [email protected].

Construction of unimolecular halide and oxyanion encapsulated cation-triggered assemblies by highly electron-rich tripodal naphthyl thiourea neutral scaffold is observed.


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Progressive Cation Triggered Anion Binding by ... - ACS Publications

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