Ice-like Cyclic Water Hexamer Trapped within a Halide Encapsulated

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Ice-like cyclic-water hexamer trapped within halide encapsulated hexameric neutral receptor core: First crystallographic evidence of water cluster confined within receptor-anion capsular assembly Utsab Manna, Senjuti Halder, and Gopal Das Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01693 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Ice-like cyclic-water hexamer trapped within halide encapsulated hexameric neutral receptor core: First crystallographic evidence of water cluster confined within receptor-anion capsular assembly Utsab Manna, Senjuti Halder and Gopal Das* Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India, Fax: +91-361-2582349; Tel: +91-361-2582313; E-mail: gdas@ iitg.ernet.in. Supporting Information Details of experimental, characterization data, additional X-ray figures, table ABSTRACT: Over the past decades, the tren [tris(2aminoethyl)amine] skeleton has materialized as one of the supreme anion binding building block demonstrating strong interplay among topology, complementarity, cooperativity and coordination. However, anion recognition by modest tren-based unsubstituted aromatic urea has been underexplored, mainly due to the deficiency of π-acidic or electronwithdrawing aryl terminals. The report establishes an infrequent hexameric neutral receptor-anion-water molecular self-assembly, where a conformationally flexible C3vsymmetric halide (F /Cl ) encapsulated electron-rich naphthyl group containing N-bridged tripodal urea receptor effectively entraps a chair-shaped ice-like neutral cyclic-water hexamer witin the hexameric cavity of neutral receptorhalide host-guest assembly.

1. Introduction: Water has undoubtedly acknowledged as more scientific, technological interest and played vital role in almost all branches of natural sciences despite its most abundance in earth than any other substance. Because primarily, it is a major chemical constituent of our planet’s surface as well as its necessity for the genesis of life and secondly, it unveils an interesting array of uncommon properties in pure form and 1 as a solvent. Although the stereochemical role of water in proteins is well recognized, however the comprehensive posil tiona evidence for nature of the water molecules such as their ability to form clusters of various size and shape is still 2 preliminary. The study of water clusters, stabilized by hydrogen bonding interactions, their fluctuations and their rearrangement dynamics is significant to understand the 3-7 structures and characteristics of liquid water and ice. This diversity of noncovalent hydrogen bonding interactions is capable of guiding the self-assembly processes in chemical systems such as solution chemistry, cloud and ice formation, and also very important from the perspectives of protein crystallization, stabilization and functioning of biomolecules as it can impose a delicate balance among several possible 8-11 conformations of enzymes essential for their functions. In past decades, intensive experimental and theoretical investi-

gations on water clusters have found for understanding the 12-14 Among the water clusanomalous behavior of bulk water. 15-18 ters, the water hexamer is particularly interesting as it is dominant form as well as building block in ice Ih, besides it 19-20 appears to be pertinent to liquid and bulk water as well. The structural possibilities linked with the water hexamer is quite extensive and theoretical calculations have shown almost isoenergetic several hexameric isomers such as ring, 21-22 23-27 book, bag, cage, and prism topologies. Thus far, chair, 28 29-31 boat, and planar forms have been characterized, trapped by hydrogen bonding in metal based organic, inorganic crystalline host lattices was reported by Moorthy et. al, Custelcean et. al., Bharadwaj et. al., Ye et. al., Gao et. al, Skoulika et.al., Fujita et. al., as these types of crystalline hosts may offer an environment for stabilizing a higher energy water23-31 hexamer. However stabilization of cyclic water hexamer inside the encapsulated anion induced organic supramolecular assembly is still unfamiliar in literature, where anions in such structures act as anchors for holding the water clusters. Moreover crystal engineering of halide/hydraed-halide induced neutral synthetic receptors or selective halide sensing by artificial fluorescent receptors have also attracted a lot of interest due to the critical roles of halides in living organisms as well as in a range of environmental, biological and medical 32-41 purposes . In continuing our pursuit on recognition and

Scheme 1 Molecular structure of L (left) and pictorial representation showing chair shaped cyclic water hexamer trapping inside the halide (F-/Cl-) encapsulated host-guest assembled hexameric cavity (right).

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42-49

sensing of anions , herein we report quite exceptional hydrogen-bonded cyclic ice-like chair-shaped water hexamer built in around halide (F /Cl ) anion encapsulated unusual neutral hexameric self-assembled core. To the best of our knowledge, this work probably describes the first example of anion induced hexameric host-guest assembly, which grabs cyclohexane type cyclic water hexameric cluster (scheme 1). The degree of constructing water clusters that can be imposed by its environment and vice versa becomes one of the key factors to design new host-gest assembly and here both receptor-water and water-water interactions can be important for the stability of overall honey-comb like structure.

2. Results and Discussion: 2.1. Design Aspects of Electron-rich Anion Binding Receptor: The highly electron-rich naphthyl group functionalized tripodal receptor L was first synthesized in good yield by the reaction of triamine with three equivalents of 1-Naphthyl isocyanate in acetonitrile (scheme S1 supporting information). The fluoride and chloride induced water-hexamer complex 1 and 2 respectively of ligand L was obtained as crystalline solid from slow evaporation of aqueous DMF solution of L and respective n-TBAF and n-TBACl salts. The purposeful placement of the anion binding urea functions hanging from the bridgehead nitrogen on a highly organized neutral tripodal scaffold and even more challenging electrondonating naphthyl substitution (absence of any EWG group) is also appropriate for encapsulation of anionic guests via hydrogen bonding coordination. The colorless plate shaped crystals of neutral complex 1 and 2, which grew over a period of 15-20 days, were characterized by single crystal X-ray diffraction, vibrational spectroscopy, X-ray powder diffraction and thermal analysis.

and chair-shaped water hexamer with a particular receptor in ORTEP diagram, (c) cyclic-water hexamer trapping inside the fluoride encapsulated hexameric cavity of L and (d) crystal packing of complex 1 displaying honeycomb shaped architecture as viewed down along the crystallographic a axis.

hexamer trapping induced by encapsulated fluoride (1) or chloride (2) anions in trigonal system, although 1 crystallizes in P 31 c space group with Z = 4 and complex 2 crystallizes in R -3 space group with Z = 6. Theoretical study of free neutral ligand predicts almost no cavity of N-bridged tripodal scaffold, where the orientation of six urea N-H protons is not unidirectional (Figure S32, supporting information). Interestingly, in presence of anionic guests (F /Cl ), the neutral receptor L encapsulates the guest halide inside its C3v- symmetric tripodal cleft by projecting all six urea -NH towards the cavity, as revealed from X-ray analysis of 1 and 2, which is known as the term ‘complementary’ and ‘cooperativity’ in supramolecular chemistry. The asymmetric unit of complex 1 and 2 contain two (in 1) and one (in 2) symmetryindependent one-third parts of the neutral L receptor/s, F /Cl anion/s and n-TBA cation/s along with two/one fully occupied water-oxygen atoms respectively. Each neutral L unit employing its six H-bond donating N-H groups engulf one fluoride/chloride anion by six strong N–H⋯F /N–H⋯Cl interactions via unimolecular capsular assembly formation, where C3-symmetry axis passes through apical N-atom and F /Cl ion respectively in 1 and 2. Interestingly, the H-bond accepting three carbonyl oxygen atoms of a ligand unit projecting

2.2. Comparative X-ray Analysis of Ice-like Cyclic Water Trapped Neutral Anion-Receptor Complexes: The X-ray analysis reveals almost isostructural water-

Figure 2. Partial X-ray pictorial view depicting arrangement of cyclohexane type cyclic water hexamer surrounding the fluoride ion, O⋯O distances in hexamer, distance and angle measurement from fluoride to hexamer in ballstick view (left) and magnified view followed by rotation in spacefill view (right) of complex 1

Figure 1. Partial X-ray structures depicting (a) Space-fill view of fluoride engulfment inside receptor L and position of cyclic water-hexamer around each three receptor arms, (b) H-bonding contacts of encapsulated fluoride

outside the receptor cavity are independently connected by strong O–H⋯O interactions with three differentsymmetryidentical Ow atoms in both compounds (Figure 1, S1 supporting information). Interestingly, note that each Ow atom being trifurcated becomes further H-bonded with either adjacent symmetry-independent (in 1) or symmetryidentical (in 2) water molecules, constructing an ice-like chair-shaped cyclic water hexamer (Figure 1b, S1b supporting information). The X-ray analysis also reveals that one hexameric water cluster which is a simplest supramolecular analogue of cyclohexane, completely stabilized by six O–H⋯O

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Crystal Growth & Design interactions with six different fluoride (in 1)/chloride (in 2) encapsulated ligand moieties (Figure 1c, S1c supporting information). Structural elucidation of cyclic centerosymmetric water hexamer exposes that the average O⋯O distances are 2.773 and 2.732 Å in complex 1 and 2 respectively

Table 1. Hydrogen bond data table of complexes 1 and 2: Complex 1

2

tylammonium counter-cations becomes also supportive to gain extra stability of water hexamer trapped hexameric host-guest assemblies (Figure S1c, S1f supporting information). -

-

-

-

A correlation of N−H···F /Cl angle vs. N−H···F /Cl distance -

-

N6-H6N···F2

0.86

2.03

2.839(6)

N2-H2N···Cl1

0.86

2.54

3.330(4)

153

(Table 1) and the scatter plot of N−H··· F /Cl angles vs. H···A distances (Figure S30, Supporting Information) of individual anion complexes shows that all the receptor N−H hydrogenbonding interactions with corresponding anion in solid state are in the strong H-bonding region of d(H···A) ≤ 2.6 Å and d(D···A) ≤ 3.3 Å. Crystal parameters and refinement data of anion complexes of receptor L are tabulated in Table 2.

N3-H3N···Cl1

0.86

2.48

3.301(4)

160

2.3. Comparative FT-IR Analysis of complexes:

D−H···A

d(D···H)/ Å

d(H···A)/Å

d(D···A)/ Å

2σ(I)

0.0887

0.0944

wR2, I > 2σ(I)

0.2235

0.2529

GOF (F2)

1.115

1.178

CCDC No.

1574568

1574569

reflec-

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4.5. UV-Vis and Fluorescence spectroscopic binding studies: −1

−1

Stock solutions of various anions (1 × 10 mol L ) were pre−3 −1 pared in acetonitrile. A stock solution of L (5 × 10 mol L ) was prepared in DMSO. The solution of L was then diluted to −6 −1 −6 -1 10 × 10 mol L and 20 × 10 mol L with acetonitrile for the fluorescence experiments and UV-Visible experiments respectively. In the UV-Visible selectivity experiment, the test samples were prepared by placing appropriate amounts of the stock solutions of the respective anions into a quartz optical cell of 1 cm path length filled with 1.0 mL of probe −6 −1 solution (20 × 10 mol L ). In the fluorescence selectivity experiment, the test samples were prepared by placing appropriate amounts of the stock solutions of the respective −6 −1 anions into 2.0 mL of the probe solution (10 × 10 mol L ). For UV-Visible and fluorescence titration experiments two different sets of fluoride ion standard solutions having 1 mM and 5 mM concentrations were prepared by diluting the ear−1 −1 lier prepared stock solutions (1 × 10 mol L ) in acetonitrile medium. Quartz optical cells of 1 cm path length were filled with 1.0 mL and 2.0 mL solutions of L for UV-Visible and fluorescence titration experiments respectively, to which the (1 mM and 5 mM) F ion stock solutions were gradually added using a micropipette. For fluorescence measurements, excitation was provided at 320 nm, and emission was acquired from 383 nm. Spectral data were recorded within 2 minutes after addition of the ions. -

The apparent binding constant for the formation of a L–F complex is calculated using the B–H (Benesi-Hildebrand) 61-62 method (eqn. 1) on the basis of change in intensity at 383 nm,

located on a difference Fourier map and refined. In other cases, the hydrogen atoms are geometrically fixed. 59 PLATON/SQUEEZE was performed to refine the host framework in complex 1 excluding the highly disordered tetrabutylammonium counter-cation electron densities. These calculations amount to 356 electrons in the unit cell where Z = 4 and may be attributed to two disordered n-TBA cations, 1 further strongly supported by the detailed integrated H NMR spectra of complex 1 (Fig S9).

4.4. NMR titration studies: Binding stoichiometries and binding constants were ob1 tained and calculated from H NMR (Varian 60 MHz) titrations of receptors with tetrabutylammonium (n-TBA) salt of chloride in DMSO-d6 at 298 K. Initial concentrations were [ligand]0 = 10 mM, and [anion]0 = 50 mM. 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. The association constant log K was calculated by fitting urea NHb signals using the WINEQNMR 60 program.

1/(I -I0) = 1/{K(Imax - I0)C} + 1/( Imax - I0)

(1)

I0 is the emission intensity of L at λ = 383 nm for F-, I is the observed emission intensity at the particular wavelength in the presence of a certain concentration of the fluoride ion (C), Imax is the maximum emission intensity value that was obtained at λ = 383 nm for F during titration with varying F -1 ion concentration, K is the apparent binding constant (M ) and was determined from the ratio for intercept versus slope of a linear relationship and C is the concentration of the fluoride added during titration. ASSOCIATED CONTENT

Supporting Information Details of additional X-ray Figures giving characterization 1 data for the receptor L and its complexes 1 and 2, H NMR 13 and C NMR spectra, FT-IR spectra, PXRD pattern, TGA 1 curves, ESI-MS of free L, H NMR titration stack plots, UVVis spectra, job’s plot from solution state NMR and fluorescence titration spectra, distance vs angle scatter plots, hydrogen-bonding data, optimized structure, calculated energy and coordinates of optimized geometry of free receptor, hydrogen-bonding table of complexes (PDF) available here. See DOI: 10.1039/x0xx00000x. AUTHOR INFORMATION

Corresponding Author E-mail: gdas@ iitg.ernet.in

Present Addresses

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Crystal Growth & Design Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India, Fax: +91-361-2582349; Tel: +91-361-2582313. ACKNOWLEDGMENT 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 and DST-FIST for providing instrument facilities. U.M. thanks IIT Guwahati for fellowship. REFERENCES (1) Ludwig. R. Water: From Clusters to the Bulk Angew. Chem. Int. Ed. 2001, 40, 1808. (2) Mukhopadhyay, U.; Bernal I. Self-Assembled Hexameric Water Clusters Stabilized by a Cyano-Bridged Trimetallic Complex Cryst. Growth Des 2005, 5, 1687. (3) Infantes, L.; Motherwell, S. Water clusters in organic molecular crystals CrystEngComm 2002, 4, 454. (4) Gillon, A. L.; Feeder, N.; Davey, R. J.; Storey, R. Hydration in Molecular Crystals - A Cambridge Structural Database Analysis Cryst. Growth Des. 2003, 3, 663. (5) Benson, S. W.; Siebert, E. D. A simple two-structure model for liquid water J. Am. Chem. Soc. 1992, 114, 4269. (6) Udachin, K. A.; Ripmeester, J. A. A complex clathrate hydrate structure showing bimodal guest hydration Nature 1999, 397, 420. (7) Ma, B.-Q.; Sun, H.-L.; Gao, S. Formation of two-dimensional supramolecular icelike layer containing (H2O)12 rings Angew. Chem., Int. Ed. 2004, 43, 1374. (8) Barbour, L. J.; Orr, G. W.; Atwood, J. L. An intermolecular (H2O)10 cluster in a solid-state supramolecular complex Nature 1998, 393, 671. (9) Ben-Naim, A. Molecular recognition - viewed through the eyes of the solvent Biophys. Chem. 2002, 101, 309. (10) Quiocho, F. A.; Wilson, D. K.; Vyas, N. K. Substrate specificity and affinity of a protein modulated by bound water molecules Nature 1989, 340, 404. (11) ten Wolde, P. R.; Frenkel, D. Enhancement of protein crystal nucleation by critical density fluctuations Science 1997, 277, 1975. (12) Janiak, C.; Scharamann, T. G. Two-Dimensional Water and Ice Layers: Neutron Diffraction Studies at 278, 263, and 20 K J. Am. Chem. Soc. 2002, 124, 14010; (13) Kim, J.; Majumdar, D.; Lee, H. M.; Kim, K. S. Structures and energetics of the water heptamer: comparison with the water hexamer and octamer J. Chem. Phys., 1999, 110, 9128; (14) Liu, K.; Cruzan, J. D.; Saykally, R. J. Water clusters Science, 1996, 271, 929. (15) Byrne, P.; Lloyd, G. O.; Clarke, N.; Steed, J. W. A “Compartmental” Borromean Weave Coordination Polymer Exhibiting Saturated Hydrogen Bonding to Anions and Water Cluster Inclusion Angew. Chem. Int. Ed. 2008, 47, 5761. (16) Luo, G.-G.; Xiong, H.-B.; Dai, J.-C. Syntheses, Structural Characterization, and Properties of {[Cu(bpp)2(H2O)2](tp)·7H2O} and {[Cu(bpp)2(H2O)](Ip)·7H2O} Complexes. New Examples of the Organic Anionic Template Effect on Induced Assembly of Water Clusters (bpp = 1,3-Bis(4-pyridyl)propane, tp = Terephthalate, ip = Isophthalate) Cryst. Growth Des., 2011, 11, 507. (17) Sun, D.; Yang, C-F.; Xu, H-R; Zhao, H-X.; Wei, Z.-H.; Zhang, N.; Yu, L-J.; Huang, R.-B.; Zheng, L.-S. Synthesis, characterization and property of a mixed-valent AgI/AgII coordination polymer Chem. Commun., 2010, 46, 8168. (18) Ghosh, S. K.; Bharadwaj, P. K Structure of a Discrete Hexadecameric Water Cluster in a Metal-Organic Framework Structure Inorg. Chem. 2004, 43, 6887. (19) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: Oxford, U.K., 1969. (20) Speedy, R. J.; Madura, J. D.; Jorgensen, W. L. Network topology in simulated water J. Phys. Chem. 1987, 91, 909.

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(43) Samanta, S.; Manna, U.; Ray, T.; Das G. An aggregation-induced emission (AIE) active probe for multiple targets: a fluorescent sensor for Zn2+ and Al3+ and a colorimetric sensor for Cu2+ and F- Dalton Trans., 2015, 44, 18902. (44) Manna, U.; Nayak, B.; Hoque, M. 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. (45) 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. (46) 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. (47) Manna, U.; Chutia, R.; Das, G. Entrapment of cyclic fluoride−water and sulfate−water−sulfate cluster within the selfassembled structure of linear meta-phenylenediamine based bis-urea receptors: Positional isomeric effect Cryst. Growth Des. 2016, 16, 2893. (48) 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. (49) 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. (50) König, H. A. Z. A cubic ice modification Kristallogr. 1944, 105, 279. (51) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: Oxford, U.K., 1969. (52) Gruenloh, C. J.; Carney, J. R.; Arrington, C. A.; Zwier, T. S.; Frederick, S. Y.; Jordan, K. D. Infrared spectrum of a molecular ice cube: the S4 and D2d water octamers in benzene-(water)8 Science 1997, 276, 1678. (53) Warnet, Ph.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Näslund, L. A.; Hirsch, T. K.; Ojamäe, L.; Glatzel, P.; Pettersson, L. G. M.; Nilsson, A. The Structure of the First Coordination Shell in Liquid Water Science 2004, 304, 995. (54) Sheldrick, G. M. SAINT and XPREP, 5.1 ed.; Siemens Industrial Automation Inc.: Madison, WI, 1995. (55) Sheldrick G. M. SADABS, empirical absorption Correction Program; University of Göttingen: Göttingen, Germany, 1997. (56) Sheldrick, G. M. Crystal structure refinement with SHELXL Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3. (57) Mercury 2.3 Supplied with Cambridge Structural Database; CCDC: Cambridge, U.K., 20011. (58) Farrugia, L. J. ORTEP-3 for windows - a version of ORTEP-III with a graphical user interface (GUI) J. Appl. Crystallogr., 1997, 30, 565. (59) Van der Sluis P.; Spek, A. L. BYPASS: an effective method for the refinement of crystal structures containing disordered solvent regions Acta Crystallogr., 1990, A46, 194. (60) Hynes, M. J. EQNMR: a computer program for the calculation of stability constants from nuclear magnetic resonance chemical shift data J. Chem. Soc., Dalton Trans. 1993, 311. (61) Benesi, H. A.; Hildebrand, J. H. A spectrophotometric investigation of the interaction of iodine with aromatic hydrocarbons J. Am. Chem. Soc., 1949,71, 2703. (62) Han, F.; Bao, Y.; Yang, Z.; Fyles, T. M.; Zhao, J.; Peng, X.; Fan, J.; Wu, Y.; Sun, S. Simple bisthiocarbonohydrazones as sensitive, selective, colorimetric, and switch-on fluorescent chemosensors for fluoride anions Chem.–Eur. J., 2007, 13, 2880.

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Ice-like cyclic-water hexamer trapped within halide encapsulated hexameric neutral receptor core: First crystallographic evidence of receptor-anion capsule induced cyclic-water confined self-assembly Utsab Manna, Senjuti Halder and Gopal Das

Ice-like cyclic water-hexamer entrapment inside the fluoride/chloride encapsulated neutral hexameric host-guest core is observed.

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