Steering the host network from cage to channel by ÏÂ·Â·Â·Ï interaction

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Steering the host network from cage to channel by #···# interaction among the guest molecules DURGAM SHARADA, Viswanadha G. Saraswatula, and Binoy K. Saha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00487 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Crystal Growth & Design

Steering the host network from cage to channel by π···π interaction among the guest molecules Durgam Sharada, Viswanadha G. Saraswatula, and Binoy K. Saha* Department of Chemistry, Pondicherry University, Puducherry 605 014, India, E-mail: [email protected]. KEYWORDS. Channel structure · charge transfer complex · π⋅⋅⋅π stacking · halogen…halogen interactions

ABSTRACT. 2,4,6-tris(4-bromophenoxy)-1,3,5-triazine (BrPOT) preferably forms cage type of structure with guest molecule hexamethylbenzene (HMB) or hexafluorobenzene (HFB) individually. However, when both the guests are present simultaneously, strong π···π stacking interaction between the π electron rich HMB and π electron deficient HFB guest molecules forces the host molecules to assemble in a channel structure to include the π···π stacked guest molecules inside the channel. Picric acid (PA) and 1,3,5-trinitrobenzene (TNB) forms charge transfer complexes with HMB inside the channel and the nature of these charge transfer complexes are different from that of the binary complexes of HMB-PA or HMB-TNB. All the three guest molecules, HMB, PA and TNB, have been simultaneously included in the channel lattice of BrPOT to produce a quaternary complex.

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π···π interaction is one of the most discussed interactions studied in the solid state chemistry.1-8 The nature of these interactions is highly complex. These interactions are studied not only for mere basic understanding, but also for its utility in materials science, especially in the field of sensors.9 These interactions also play important roles in several biological systems.10,11 It has been realized that electron deficient π systems form stronger π···π interaction with the electron rich π systems compared to the other probable combinations.12,13 π···π interaction with an estimated energy of −5.38 kcal/mol between hexafluorobenzene and benzene is one of those classic examples. 14,15 The melting point of this complex (297 K) has been found to be higher than the melting point of the individual components (benzene, 279 K and hexafluorobenzene, 278 K). It has also been shown that thermal expansion along the π···π interaction between the electron-rich and electron-deficient compounds is smaller than that between two electron-rich compounds.16 There is a large number of fascinating studies on charge transfer complexes between the electron-rich and electron-deficient compounds reported in the literature.17-19 Host-guest chemistry is another fascinating branch of supramolecular chemistry.20-26 These are multicomponent systems where one or more molecules/ions, generally the larger ones, form the three dimensional lattice, called host, and the other set of molecule/s or ion/s, generally the smaller ones, called guest, occupy the cavities generated in the host network. There are plenty of applications of the host-guest systems, especially in materials science and drug delivery field.27-32 Due to utilitarian reasons, the channel type of cavities has received more attention compared to the other types of cavities.33 Most of the studies have focused on host-host or hostguest interactions.22,23,27,31,34,35 There are reports where due to the presence of specific host-host interactions a particular host network is formed and different types of guest occupy the same cavity without changing the host network36,37 or due to different host-guest interactions the host network changes upon changing the guest molecules.24,35 There are also reports where host network changes just to accommodate different sized and shaped guest molecule in the lattice even though there is no specific interaction, other than vdW type, between the host and guest molecules.38,39 However, the study where guest-guest interaction dictates the host molecules to adopt a particular network, which can facilitate the guest-guest interaction, is rather rare. Here we have clearly demonstrated such an example where the guest-guest interaction is not just an


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artifact of the host network, rather the π···π interaction among the guest molecules steer the host network from a cage structure to a channel structure. 2,4,6-tris(4-bromophenoxy)-1,3,5-triazine (BrPOT) forms mainly two types of host networks in its several host-guest systems. In one network the host forms a channel type of cavity and in the other network it forms a cage type of cavity40-42. In both the networks the BrPOT host molecules are assembled in a two dimensional hexagonal honeycomb network via type II43,44 Br···Br trimer synthon (Br3 synthon). The nodes of the hexagons are made of either Br3 synthon or triazine ring and these two types of nodes are alternately arranged in the network (Figure 1a). These 2D layers are further stacked one above the other along the “c” axis; however, the pattern of stacking is different in these two types of structures. In the channel structure one layer sits just above the other layer without any offset, but the positions of the two types of nodes are interchanged. Therefore, above the Br3 synthon of the 1st layer there is a triazine ring of the 2nd layer and vice versa. Hence, it can be considered as ab-ab-ab··· type of stacking arrangement (Figure 1b). The channel walls in this structure are made of phenoxy groups. On the other hand, in the cage structure, there are six layers along the “c” axis within a repeating unit. Therefore, the layer arrangement can be designated as abcdef-abcdef··· type (Figure 1b). In this structure, the ab, cd and ef form pairs of layers similar to the ab pair as described in the case of channel structure, but these three pairs of layers are stacked one above another along the “c” axis with some offset. The cd pair moves along the [110] direction with respect to the ab layer by half of the diameter of the hexagon and then ef pair of layers move by equal distance along the same direction with respect to the cd pair. As a result the host network forms cage type of cavities. The side walls of the cages are made of phenoxy groups and the top and bottom of the cages are surrounded by Br3 synthons. In the cage structure, the cavity, occupied by the guest molecule, is surrounded by host molecules from all the directions and hence there is no direct contact between the two neighboring cavities. Moreover, the height of the cavity is around 4.0 Å which is just sufficient to accommodate one aromatic ring in the form of guest molecule. Therefore, guest-guest stacking interaction is also not possible in the cage structure. On the other hand, in the channel structure, the host molecules form a channel which propagates along the “c” axis and the aromatic guest molecules can stack one above the other to fill up the channel. PXRD study


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unambiguously shows that BrPOT forms only cage structure (R3ത) with HFB guest40 (Figure 2a and 2b) whereas it can form either of the channel (P63/m) or the cage structure (R3ത) when the guest molecule is HMB41. We were interested to see whether the π···π stacking between the electron rich and electron poor guest molecules would force the host network to adopt only the channel structure to retain the π⋅⋅⋅π stacking among the guest molecules. We have crystallized BrPOT from EtOAc in the presence of HMB as well as HFB in the solution. Then the possibilities were of having cage structure in which different cages would be occupied by different guests in the same lattice or the two guest molecules form individual cage structures with the BrPOT host as both the guests prefer to form cage structure or HMB forms channel and HFB forms cage structure with BrPOT. Interestingly, PXRD study suggests that we have obtained exclusively channel structure (Figure 2c and 2d), though the guest molecules could not be modeled due to heavy disorder. Presence of HMB and HFB in the channel was confirmed by 1H-NMR and 19F-NMR studies respectively. BrPOT forms channel structure with HMB alone where the host to the guest ratio observed was 1:1.41 However, in the presence of HFB the 1H-NMR shows a 2:1 ratio of BrPOT and HMB in the BrPOT-HMB-HFB complex. Moreover, TG analysis shows a 22.3 % weight loss which corresponds to 2:1:1 ratio of BrPOT:HMB:HFB (Supporting Information, Figure S2). All these studies suggest that HMB and HFB are present in a 1:1 ratio inside the channel of BrPOT host lattice. When both the guests were introduced simultaneously, the π⋅⋅⋅π stacking between the electron rich HMB and electron poor HFB guest molecules steers the host network to form only the channel structure so that π⋅⋅⋅π interaction among the guest molecules is retained in the lattice. We have further exploited this property of forming π⋅⋅⋅π interaction between the electron rich and electron poor aromatic guest molecules inside the channel of the BrPOT host lattice. We have been able to include HMB and picric acid (PA) (BrPOT-HMB-PA) as well as HMB and 1,3,5trinitrobenzene (TNB) (BrPOT-HMB-TNB) in the channel structure of BrPOT host lattice. Because the channel is hexagonal symmetric and the cross section of the channel matches nicely with the diameter of the guest molecules, the guest molecules are forced to stack with a zero to a near zero offset.42 On the other hand, when the guest molecules are co-crystallized without BrPOT host, they stack with a reasonably large offset.16,45 Therefore, π⋅⋅⋅π interaction with a different geometry can be studied in the presence of BrPOT as a host. Recently we have reported


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1:1 π⋅⋅⋅π stacked crystal structure of HMB and picric acid (PA).16 In this structure, the offset in π⋅⋅⋅π stacking is very high. We also have made several attempt to grow single crystal of 1:1 cocrystal of HMB and 1,3,5-trinitrobenzene (TNB) from different laboratory solvents to study the charge transfer complex structure, nevertheless none of the batches produced single crystal. Rather, in every case we obtained yellow colored powder material. This co-crystal is also not reported in the CCDC. Here we have successfully used BrPOT channel lattice as a template to form charge transfer complex between HMB and TNB molecules inside the channel. In both the cases (BrPOT-HMB-PA and BrPOT-HMB-TNB) the color of the materials are distinct from the color of HMB-PA and HMB-TNB charge transfer complexes or the physical mixture of BrPOT and HMB-PA or HMB-TNB (Figure 3). Solid state UV-vis spectra of the materials also suggest that the nature of charge transfer complex of the ternary host-guest systems is different from that of the binary co-crystals without the host lattice or the mixtures (Figure 4). We also have investigated the selectivity of the π⋅⋅⋅π interaction inside the channel. Different ratios of BrPOT, HMB and TNB/PA were used (2:1:1, 2:1:2, 2:2:1, 2:1:3, 2:3:1, 2:1:4 and 2:4:1); however, in all the cases the BrPOT:HMB:TNB/PA ratios achieved were very similar (Supporting Information, Figure S3). Though the guest molecules are highly disordered and could not be modeled to study their stacking patterns, this study suggests that the guest molecules are not randomly occupying the channels but preferentially occupying alternate positions along the channel to form the 1:1 π⋅⋅⋅π stacked molecular arrangement. We further explored the possibility of forming quaternary system by crystallizing BrPOT with HMB, PA and TNB in a 2:1:0.5:0.5 ratio from ethyl acetate. NMR study of a single crystal of this combination proved the coexistence of all the three guests in the BrPOT host lattice. In summary, we have investigated the crystal structure of BrPOT-HMB-HFB where BrPOT is host and the HMB and HFB are present as guests in the host lattice. We have shown here that though BrPOT forms cage kind of cavity with HMB and HFB individually, when both the guests are present simultaneously, it forms channel type of cavity. Therefore, the channel structure here is not merely the result of host-guest interactions or size and shape compatibility of the cavity and the guests, but to enable π···π interaction between electron rich HMB and electron poor HFB guests inside the channel. We have further shown that HMB can form charge transfer complexes with TNB or PA inside the channel and the geometries of these charge


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transfer complexes are different from those when these molecules form charge transfer complexes between themselves in the absence of BrPOT host molecule. This study shows that π···π interaction between electron rich and electron poor aromatic guest compounds is capable of steering the host network to adopt a structure where π···π interaction is facilitated. This study further shows that using suitable host-guest pathway it is easily possible to produce a ternary, quaternary or even a higher order component system. ASSOCIATED CONTENT Supporting Information. X-ray crystallography, crystallization, TG plot, crystallographic table and crystallographic data in CIF format for the structures with CCDC 1833946 − 1833948. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected]. Funding Sources B.K.S. thanks Council of Scientific and Industrial Research, India (No. 01(2908)/17/EMR-II) for financial support. D.S. thanks RGNF for a fellowship. ACKNOWLEDGMENT B.K.S. thanks DST-FIST for single crystal X-ray diffractometer facility, CIF, Pondicherry University for TG and NMR facilities and Dr. R. Thakuria, Gauhati University for PXRD. REFERENCES (1) Desiraju, G. R.; Gavezzotti, A. Crystal Structures of Polynuclear Aromatic Hydrocarbons. Classification, Rationalization and Prediction from Molecular Structure. Acta Crystallogr., Sect. B: Struct. Sci. 1989, 45, 473−482.


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(30) Atwood, J. L.; Barbour, L. J.; Jerga, A. Storage of Methane and Freon by Interstitial Van der Waals Confinement. Science. 2002, 296, 2367−2369. (31) Garai, A.; Mukherjee, S.; Ray, S. K.; Biradha, K. Tuning Emission Properties via Aromatic Guest Inclusion in Organic Salts Composed of 4,4′-Dinitro-2,2′,6,6′-tetracarboxybiphenyl and Acridine. Cryst. Growth Des. 2018, 18, 581−586. (32) Chandra, S.; Chowdhury, D. R.; Addicoat, M.; Heine, T.; Paul, A.; Banerjee, R. Molecular Level Control of the Capacitance of Two-Dimensional Covalent Organic Frameworks: Role of Hydrogen Bonding in Energy Storage Materials. Chemistry of Materials. 2017, 29, 2074−2080. (33) Couderc, G.; Hulliger, J. Channel Forming Organic Crystals: Guest Alignment and Properties. Chem.Soc.Rev. 2010, 39, 1545–1554. (34) MacGillivray, L. R.; Reid, J.; Ripmeester, J. A. Conformational Isomerism Leads to Supramolecular Isomerism and Nanoscale Inclusion in 2D Extended Framework Solids Based on C-Methylcalix[4]resorcinarene. Chem. Commun. 2001, 1034–1035. (35) Saraswatula, V. G.; Saha, B. K. Modulation of Thermal Expansion by Guests and Polymorphism in a Hydrogen Bonded Host. Cryst. Growth Des. 2015, 15, 593−601. (36) Saha, B. K.; Nangia, A. Self-Assembled Organic Tubular Host for Van der Waals Guest Inclusion. CrystEngComm. 2006, 8, 440–443. (37) Saha, B. K.; Nangia, A. Ethynyl Group as a Supramolecular Halogen and C≡C−H···C≡C Trimer Synthon in 2,4,6-Tris(4-ethynylphenoxy)-1,3,5-triazine. Cryst. Growth Des. 2007, 7, 393−401. (38) Bhattacharya, S.; Saha, B. K. Isostructurality in the Guest Free Forms and in the Clathrates of 1,3,5-Triethyl-2,4,6-tris(4-halophenoxy)methylbenzenes. Cryst. Growth Des. 2012, 12, 169−178. (39) Bhattacharya, S.; Saha, B. K. Polymorphism Through Desolvation of the Solvates of a Van der Waals Host. Cryst. Growth Des. 2013, 13, 606−613. (40) Saha, B. K.; Jetti, R. K. R.; Reddy, L. S.; Aitipamula, S.; Nangia, A. Halogen TrimerMediated Hexagonal Host Framework of 2,4,6-Tris(4-halophenoxy)-1,3,5-triazine. Supramolecular Isomerism from Hexagonal Channel (X = Cl, Br) to Cage Structure (X = I). Cryst. Growth Des. 2005, 5, 887−899. (41) Saraswatula, V. G.; Bhat, M. A.; Bhattacharya, S.; Saha, B. K. Network and Guest Dependent Thermal Stability and Thermal Expansion in a Trigonal Host. J. Chem. Sci. 2014, 126, 1265–1273. (42) Jetti, R. K. R.; Thallapally, P. K.; Xue, F.; Mak, T. C. W.; Nangia. A. Hexagonal Nanoporous Host Structures Based on 2,4,6-Tris-4-(halo-phenoxy)-1,3,5-triazines (Halo=Chloro, Bromo). Tetrahedron 2000, 56, 6707−6719.


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Figures:

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Figure 1. (a) Two layers of hexagonal network mediated via Br3 synthons. Bottom layer “a” is shown in blue color and the top layer “b” is shown in default colors. (b) Schematic diagram of the hexagonal network, channel structure and cage structure.

(a)

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(c)

(d) Figure 2. Experimental (a) and simulated (b) PXRD pattern of BrPOT-HFB. Experimental (c) and simulated (d) PXRD pattern of BrPOT-HMB-HFB. Relative intensities in (c) and (d) don’t match because the guest molecules could not be modeled and hence the simulated PXRD is generated without the guest molecules.


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Figure 3. Color of the different samples.

(a)


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(b) Figure 4. (a) Sloid state UV-vis spectra of BrPOT, HMB-PA (1:1), physical mixture of BrPOT and HMB-PA complex (2:1:1) and the ternary system, BrPOT-HMB-PA (2:1:1). (b) (a) Sloid state UV-vis spectra of BrPOT, HMB-TNB (1:1), phsical mixture of BrPOT and HMB-TNB complex (2:1:1) and the ternary system, BrPOT-HMB-TNB (2:1:1).


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TOC

The host forms cage type of cavity with hexamethylbenzene and hexafluorobenzene individually, but when both of these guests are present simultaneously, it forms channel type of cavity to facilitate π⋅⋅⋅π interactions among the electron rich and electron deficient guest molecules.


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Steering the host network from cage to channel by ÏÂ·Â·Â·Ï interaction

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Steering the host network from cage to channel by ÏÂ·Â·Â·Ï interaction