ZHAO Bo ZHANG Hong-Ling TANG Wei

YUAN Wei-Guan XIONG Fang

JING Lin-Hai QIN Da-Bin②

(Key Laboratory of Chemical Synthesis and Pollution Control of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, China)

1 INTRODUCTION

The co-crystal strategy because of their intriguing supramolecular patterns and applications in the fields of photochemistry, biomedicine and pharmaceutics has become a growing interest subject in recent years[1-2]. Generally, a co-crystal is a multicomponent assembly by non-covalent interactions,such as hydrogen bonding, ion pairing, donor-acceptor interaction, π···π stacking, C–H··π interactions and C–X··π interactions[3-8]. Imidazolium possesses pre-organized structures via H-bond donors which are acid protons situated in the imidazolium ring[9].A few years ago, Alcalde et al. reported the first example of dicationic heterophanes based on imidazolium units as molecular recognition motifs for anions[10], and imidazolium based receptors have been extensively investigated in recent years. There are many exciting results obtained by applying a series of imidazoliums to coordinate with anionic,like X-, TfO-, TFSI-, and so on[11-13]. Recently, Yoon and Kim et. al have synthesized imiodazolium as a triphosphate anion receptor[14]. To the best of our knowledge, it is a circumstantial evidence that imiodazlium is used as an acceptor. Only a limited crystal structures of co-crystal based on imiodazolium are disclosed[15]. Nevertheless, there have been few reports on co-crystals which were assembled by imiodazolium and neutral aromatic ligands.

Hence, we studied the L ligand and neutral molecule (Bpy, Ben)engendering L(Bpy)0.5(co-crystal 1)and L(Ben)(co-crystal 2). The structure of cocrystal 1 indicates the presence of L and Bpy in the stoichiometric ratio 2:1. Co-crystal 2 can be easily observed by the stoichiometric ratio of 1:1.

2 EXPERIMENTAL

2. 1 Materials

All reagents were gained from commercial suppliers and used without any further purification.Solvents were freshly distilled using a standard procedure. The synthesis[16]of L was recorded in supporting information. All experiments were performed in dried glass service under a nitrogen atmosphere. The IR spectra were recorded using KBr pellets in the range of 4000～400 cm-1on a Nicolet 6700 FTIR. NMR spectra were recorded on Bruker Advance Ш 400 (1H NMR, 400 MHz;13C NMR, 100 MHz, respectively). Deuterated DMSO-d6and acetonitrile-d3were used as the locking solvents. Chemical shifts were given in ppm using TMS as an internal standard. Thermogravimetric analysis (TGA)was carried out on a Netzsch STA 449 F3 thermal analyzer from room temperature to 500 ℃ at a heating rate 10 /min℃ under nitrogen atmosphere. UV-vis spectra were performed on a Shimadzu UV-2550 ultraviolet-visible spectrometer.

2. 2 Synthesis

2. 2. 1 L(Bpy)0.5(1)

L (0.3 mmol, 204 mg)and 4,4-bipyridine (0.3 mmol, 47 mg)were combined in acetonitrile (15 mL). The reaction was stirred under refluxing for 4 h in an oil bath. The mixture was filtered after the reaction completed. The filtrate was concentrated to 5 mL in vacuum. Addition of Et2O (20 mL)to the filtrate afforded a yellow precipitate. Yield: 82%. IR(KBr, cm-1): 3420 m, 3170 m, 1660 m, 1510 m,1400 w, 1210 m, 1080 m, 845 vs, 557 m.1H NMR(400 MHz, DMSO-d6, 298 K): δ = 7.30 (t, J = 7 Hz,2H), 7.38 (t, J = 7.8 Hz, 2H), 7.84 (d, J = 6.0 Hz,2H), 7.96 (d, J = 9.2 Hz, 2H), 8.08 (d, J = 8.8 Hz,4H), 8.20 (d, J = 8.4 Hz, 4H), 8.61 (d, J = 7.2 Hz,2H), 8.73 (d, J = 5.6 Hz, 2H), 8.84 (s, 2H), 10.39 (s,2H);13C NMR (100 MHz, DMSO-d6, 298 K): δ =112.63, 118.82, 121.75, 123.92, 124.71, 125.84,126.32, 129.23, 130.30, 135.41, 140.60, 151.03.

2. 2. 2 L(Ben)(2)

Replacing 4,4-bipyridine with benzene (0.3 mmol,20 mg)using the same procedure as for 1 obtained 2.Yield: 73%. IR: ν = 3420 m, 1650 w, 1510 m, 1080 m, 787 w, 471 m.1H NMR (400 MHz, DMSO-d6,298 K): δ = 7.12～7.67 (m, 10H), 7.95 (d, J = 8.4 Hz, 2H), 8.07 (d, J = 6.8 Hz, 4H), 8.18 (d, J = 8 Hz,4H), 8.60 (d, J = 5.7 Hz 2H), 8.80 (s, 2H), 10.35 (s,2H);13C NMR (100 MHz, DMSO-d6, 298K): δ =112.64, 118.83, 123.94, 124.71, 125.84, 126.31,128.78, 129.23, 130.31, 135.42, 140.62.

2. 3 X-ray crystallography

X-ray diffraction data for all the crystals were collected at 296(2)K on a Bruker APEX-II CCD diffractometer with Mo-Kα radiation (λ = 0.71073 ?)by using an ω scan mode. The structures were solved by direct methods, and the non-hydrogen atoms were subjected to anisotropic refinement by full-matrix least-squares on F2using SHELX-97 package[17]. Hydrogen atom positions for all structures were located in a difference map. Further data of the crystals are arranged in Table 1. The hydrogen bond parameters are recorded in Table 2.

Table 1. Crystal Data for Co-crystals 1 and 2

?

Table 2. Hydrogen Bond Parameters for Co-crystals 1 and 2

Scheme 1. Molecular components used in this paper

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

3. 1. 1. Structure of co-crystal 1

Crystal of 1 suitable for X-ray crystallography was grown by the layering of diethyl ether into acetonitrile. As depicted in Fig. 1(a), co-crystal 1 crystallizes in a triclinic system presenting space group P1 with two L and one 4,4?-bipyridine in the asymmetric unit. There exist hydrogen bonds, with C··F distance of 3.020～3.372(11)? and C··N distance of 3.378～3.442(10)? (Table 2). The dihedral angle between biphenyl rings is 35.23o.Biphenyl rings are warping, while the matching rings in L are coplanar. The dihedral angles between the biphenyl ring and imidazo[1,5-a]pyridine ring are 14.82o and 57.87o. In co-crystal 1, the structure of L is asymmetrical. The pyridine rings of 4,4?-bipyridine are non-coplanar, and the dihedral angle is 34.61o. 4,4?-bipyridine acts as the N-acceptor. Self-assembly of the molecules produces a C–H··N hydrogen bond which engenders a chainlike motif[18]. As expected, 4,4?-bipyridine bonds to L through C–H··N hydrogen bond, and a chain is formed within bipyridine and imidazolium as shown in Fig. 1(b). The chain was formed more stable,which may be due to the π··π stacking between the benzene rings of biphenyl ring (Cg–Cg = 3.666 ?).The L is involved in π··π stacking between the benzene rings of biphenyl and imidazolium ring of imidazo[1,5-a]pyridine with the Cg–Cg distance of 3.919 ?. A mesh structure was combined together by C–H··π interactions with the C–Cg distance of 2.981 ?, and a 3D structure is eventually constructed by hydrogen bonds, π··π stacking and C–H··π interactions.

Fig. 1. (a)Molecular structure of co-crystal 1 showing the atomic numbering scheme.

3. 1. 2 Structure of co-crystal 2

Crystal of 2 suitable for structure determination by X-ray diffraction was obtained by crystallization from acetonitrile/diethyl ether. Single-crystal X-ray diffraction reveals that crystal 2 crystallizes in monoclinic, C2/m space group. According to Table 2,the hydrogen bonds are formed, with C··F distance of 3.171～3.274(7)?. The asymmetric unit 2 consists of one molecule of imidazolium and one molecule of benzene as show in Fig. 2(a). Cocrystal 2 displays a disordered structure because two orientations of imidazo[1,5-a]pyridine ring along its short axis are unresolved. The nitrogen lone pairs would be directed in anti-parallel directions[19]. The dihedral angle between the biphenyl rings is 0.00o, which are also located in the same plane. The dihedral angle between biphenyl and imidazo[1,5-a]pyridine rings is 1.52o. Benzene molecule and biphenyl ring are vertical, and the dihedral angle is 90.00o. Benzene is packaged by L.As a result, theand L alternatively interact to form a 1D supramolecular architecture. Benzene molecule was enclosed by a cavity which was assembled byand L, as indicated in Fig. 2(b).

Fig. 2. (a)Molecular structure of co-crystal 2 showing the atomic numbering scheme.(b)1D supramolecular architecture strengthened by hydrogen bonds

3. 2 Infrared spectra and 1H NMR analysis

The infrared spectrum of L can be observed at 3420 cm-1, which is characteristic of C2-H of the imidazole ring. The corresponding peak in cocrystals 1 and 2 can be found. The band is displayed at 3180 cm-1for the νC-Hof biphenyl in L, whereas the co-crystal 1 reveals the band at 3170 cm-1and the corresponding peak disappears in 2. The band of bending vibration for the C–H of biphenyl is observed at 845 cm-1in L, whereas the band is shifted from 845 to 787 cm-1and shows low transmittance in co-crystal 2. As shown in Fig. 1s.

To acquire experimental support for the proposed binding pattern in solvent, the adduct formation between L and diverse aromatic compounds was investigated by1H NMR spectroscopy.1H NMR spectra for co-crystals 1～2 and L in acetonitrile-d3are shown in Fig. 2s. Imidazo[1,5-a]pyridine protons of L locate at 9.53 (NCHN)and 8.28 ppm (H of pyridine ring). In co-crystal 1, the acid proton situated in the imidazolium ring signal exhibits downfield at 9.54 ppm, whereas the corresponding signals shift upfield in co-crystal 2 and imidazo[1,5-a]pyridine protons occur at 9.51 (NCHN)and 8.26 ppm (H of pyridine ring).

3. 3 Thermogravimetric analysis

To examine the thermal stability of two co-crystals, thermogravimetric analysis was carried out on a Netzsch STA 449 F3 thermal analyzer from room temperature to 500 ℃ under nitrogen atmosphereat a heating rate of 10 ℃/min (Fig. 3s). Co-crystal 1 loses the dipyidyl molecules in the rang of 149～297 ℃ with the observed weight loss of 10.91%(calcd. 10.32%), and then the following sharp weight loss began at 364 ℃ . For co-crystal 2, a weight loss of 9.96% (calcd. 10.32%)from 209 to 354 ℃ indicates the removal of benzene. Decomposition of compound 2 starts at 357 ℃.

3. 4 Ultraviolet absorption

The ultraviolet absorption of co-crystals 1～2 and L at room temperature was investigated in the solid state by UV-vis (Fig. 3). Co-crystals of 1～2 and L exhibit similar absorption spectra. The absorption spectrum is characterized by broad absorption bands of 250～353 nm, which can be assigned to the π-π*transitions[20].

Fig. 3. UV-vis spectra of L, co-crystal 1 and co-crystal 2 performed in the solid state at room temperature

4 CONCLUSION

In conclusion, we designed a novel strategy that two co-crystals from two-component are assembled based on rigid imidazolium-aromatic ligand synthon.Two new co-crystals and L were characterized by various physicochemical techniques. X-ray diffraction structures of co-crystals 1 and 2 revealed that L interacted with aromatic compound via non-covalent interactions. 1 is a co-crystal between L and 4,4?-bipyridine, which is formed by hydrogen bond and π··π stacking. Co-crystal 2 is constituted between L and benzene through supramolecular interaction such as C–H··F hydrogen bond. The structures of two co-crystals have showed that the new rigid imidazolium L effectively recognizes the neutral molecule through non-covalent interactions.

(1)Fri??i?, T. Supramolecular concepts and new techniques in mechanochemistry: cocrystals, cages, rotaxanes, open metal-organic frameworks.Chem. Soc. Rev. 2012, 41, 3493–3510.

(2)Arenas-García, J. I.; Herrera-Ruiz, D.; Mondragón-Vásquez, K.; Morales-Rojas, H.; H?pfl, H. Modification of the supramolecular hydrogen-bonding patterns of acetazolamide in the presence of different cocrystal formers: 3:1, 2:1, 1:1, and 1:2 cocrystals from screening with the structural isomers of hydroxybenzoic acids, aminobenzoic acids, hydroxybenzamides, aminobenzamides, nicotinic acids, nicotinamides, and 2,3-dihydroxybenzoic acids. Cryst. Growth Des. 2012, 12, 811–824.

(3)(a)Wang, L.; Zhao, L.; Liu, M.; Liu, F. Q.; Xiao, Q.; Hu, Z. Q. Three-dimensional supramolecular architecture based on 4,4?-methylene-bis(benzenamine)and aromatic carboxylic acid guests: synthons cooperation, robust motifs and structural diversity. Sci. China Chem. 2012, 55, 2523–2531.

(b)Takahashi, O.; Kohno, Y.; Nishio, M. Relevance of weak hydrogen bonds in the conformation of organic compounds and bioconjugates:evidence from recent experimental data and high-level ab initio MO calculations. Chem. Rev. 2010, 110, 6049–6076.

(4)(a)Janiak, C. A critical account on π-π stacking in metal complexes with aromatic nitrogen-containing ligands. J. Chem. Soc., Dalton Trans. 2000,21, 3885–3896.

(b)Chen, Z. J.; Lohr, A.; Saha-M?ller, C. R.; Würthner, F. Self-assembled π-stacks of functional dyes in solution: structural and thermodynamic features. Chem. Soc. Rev. 2009, 38, 564–584.

(5)Weiss, H. C.; Bl?ser, D.; Boese, R.; Doughan, B. M.; Haley, M. M. C–H··π interactions in ethynylbenzenes: the crystal structures of ethynylbenzene and 1,3,5-triethynylbenzene, and a redetermination of the structure of 1,4-diethynylbenzene.Chem. Commun. 1997, 18, 1703–1704.

(6)(a)Schneider, H. J. Binding mechanisms in supramolecular complexes. Angew. Chem. Int. Ed. 2009, 48, 3924–3977.

(b)Schottel, B. L.; Chifotides, H. T.; Dunbar, K. R. Anion-π interactions. Chem. Soc. Rev. 2008, 37, 68–83.

(7)Koch, W.; Frenking, G.; Gauss, J.; Cremer, D. Donor-acceptor interaction and the peculiar structures of dications.J. Am. Chem. Soc. 1986, 108, 5808–5817.

(8)Shen, Q. J.; Pang, X.; Zhao, X. R.; Gao, H. Y.; Sun, H. L.; Jin, W. J. Phosphorescent cocrystals constructed by 1,4-diiodotetrafluorobenzene and polyaromatic hydrocarbons based on C–I··π halogen bonding and other assisting weak interactions. CrystEngComm. 2012, 14, 5027–5034.(9)Dupont, J. On the solid, liquid and solution structural organization of imidazolium ionic liquids. J. Braz. Chem. Soc. 2004, 15, 341–350.

(10)Ermitas, A.; Carmen, A. R.; Santiago, G. G.; Esther, G. R.; Neus, M.; Llu?sa, P. G. Hydrogen bonded driven anion binding by dicationic[14]imidazoliophanes. Chem. Commun. 1999, 3, 295–196.

(11)(a)Suresh, V.; Ahmed, N.; Youn, S.; Kim, K. S. An imidazolium-based fluorescent cyclophane for the selective recognition of iodide. Chem. Asian J. 2012, 7, 658–663.

(b)Zapata, F.; Caballero, A.; White, N. G.; Claridge, T. D. W.; Costa, P. J.; Felix, V.; Beer, P. D. Fluorescent charge-assisted halogen-bonding macrocyclic halo-imidazolium receptors for anion recognition and sensing in aqueous media. J. Am. Chem. Soc. 2012, 134, 11533–11541.

(12)Leclercq, L.; Suisse, I.; Nowogrocki, G.; Agbossou-Niedercorn, F. Halide-free highly-pure imidazolium triflate ionic liquids: preparation and use in palladium-catalysed allylic alkylation. Green Chem. 2007, 9, 1097–1103.

(13)Matsumi, N.; Miyake, M.; Ohno, H. Molten salts bearing anion receptor. Chem. Commun. 2004, 24, 2852–2853.

(14)(a)Kwon, J. Y.; Singh, N. J.; Kim, H. N.; Kim, S. K.; Kim, K. S.; Yoon, J. Y. Fluorescent GTP-sensing in aqueous solution of physiological pH. J.Am. Chem. Soc. 2004, 126, 8892–8893;

(b)Xu, Z.; Singh, N. J.; Lim, J.; Pan, J.; Kim, H. N.; Park, S.; Kim, K. S.; Yoon, J. Unique sandwich stacking of pyrene-adenine-pyrene for selective and ratiometric fluorescent sensing of ATP at physiological pH. J. Am. Chem. Soc. 2009, 131, 15528–15533.

(15)(a)Leclercq, L.; Suisse, I.; Nowogrocki, G.; Agbossou-Niedercorn, F. On the solid state inclusion of tetrabutylammonium cation in the imidazolium/trifluoromethanesulfonate H-bonds network observed in ionic co-crystals. J. Mol. Struct. 2008, 892, 433–437.

(b)Leclercq, L.; Suisse, I.; Roussel, P.; Agbossou-Niedercorn, F. Inclusion of tetrabutylammonium cations in a chiral thiazolium/triflate network:solid state and solution structural investigation. J. Mol. Struct. 2012, 1010, 152–157.

(16)Hutt, J. T.; Aron, Z. D. Efficient, single-step access to imidazo[1,5-a]pyridine N-heterocyclic carbene precursors. Org. Lett. 2011, 13, 5256–5259.(17)Sheldrick, G. M. SHELXS-97 and SHELXL-97, Program for X-ray Crystal Structure solution and Refinement.University of G?tingen, G?tingen, Germany 1997.

(18)Aaker?y, C. B.; Beatty, A. M.; Helfrich, B. A.; Nieuwenhuyzen, M. Do polymorphic compounds make good cocrystallizing agents? A structural case study that demonstrates the importance of synthon flexibility. Cryst. Growth Des. 2003, 3, 159–165.

(19)Hu, L.; Spencer, E. C.; Wang, G.; Yee, G.; Slebodnick, C.; Hanson, B. E. Novel cationic copper coordination frameworks constructed from copper phosphate 8-rings and 4,4?-bisimidazolylbiphenyl as a bridging ligand. Inorg. Chem. Commun. 2008, 11, 982–984.

(20)Deng, Z. P.; Huo, L. H.; Zhao, H.; Gao, S. A co-crystal strategy to tune the supramolecular patterns and luminescent properties: ten well-designed salts assembled by arenedisulfonic acid with diverse diamines. Cryst. Growth Des. 2012, 12, 3342–3355.