Networking a hollow cage via guest coordination

Abstract

A hollow coordination cage with a highly symmetric cavity was successfully self-assembled to form a 2D-network having a less symmetric cavity via networking of disordered guests with a metal connector.

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The hollow of self-assembled cages and capsules provides an isolated micro environment where new properties of molecules are often exhibited through host–guest complexation.¹ Crystallization of the host–guest complexes prepared in solution is an important issue for not only the detailed crystallographic structure analysis but also the induction of new physical properties through the anisotropic orientation of the guest molecules in a crystalline state. When a host compound has a highly symmetric structure, however, it is often inevitable to observe disordered guests in the crystal structure because there are no strong intermolecular interactions between the guests in the void of the host. The site disorder problem hampers the accurate structure analysis of the guests as well as the potential applications of the complexes to highly anisotropic solid materials. Here we report the ordering of the guest orientation in a highly symmetrical host via guest networking. When a guest molecule possesses appropriate coordination sites, a guest network is generated by simply adding a connector (typically a metal ion) on crystallization, preventing the guest disorder problem (Scheme 1).

Scheme 1 Guest ordering by networking of a hollow cage via guest coordination.

Self-assembled M₆L₄ hollow cage 1 has T_d symmetry. The guest disorder problem is often encountered in the crystallographic analysis of its host–guest complexes. To prevent the guest disorder, we examined the guest networking according to the strategy of Scheme 1. We chose 2,4,6-tris(4-pyridyl)triazine (2) as a coordinative guest on the basis of the knowledge that large planar aromatic compounds such as pyrene and perylene provide crystalline host–guest complexes with 1.² The advantage of 2 as the coordinative guest is that this planar guest efficiently stacks on one of the ligands of 1, still leaving a void capable of binding the second guest.³ In this study, Cp′Mn(CO)₃ (3, Cp′ = methylcyclopentadienyl) was used as the second guest because of our interest in its photo-reactivity in the cavity.⁴ Noteworthy is that the non-coordinative second guest 3 is located at a fixed position of the cavity thanks to ordering the first guest (2) via its networking. The pairwise selective recognition of 2 and 3 in the cavity of 1 was effectively carried out in an aqueous solution. Suspending 2 (0.06 mmol) and 3 (0.05 mmol) in a D₂O solution of 1 (0.03 mmol, 2 mL) at 373 K for 1.5 h resulted in the formation of 1⊃(2·3) in a 58% NMR yield. Homo-recognized complex 1⊃(3)₄ was also formed in a 5% NMR yield, while another homo-recognized complex, 1⊃(2)_n, was not detected (Fig. 1).⁵ For 1⊃(2·3) complex, eight signals were displayed in the NMR for the triazine ligand involved in the framework of 1. This observation indicates C_3v symmetry for the 1⊃(2·3) complex. Namely, guest 2 is stacked on one of the four triazine ligands of 1 and the remaining void is occupied by 3.

Fig. 1 NMR spectrum of 1⊃(2·3) in D₂O.

Single crystals of 1⊃(2·3) were obtained by the slow evaporation of the solvent. The molecular structure of 1⊃(2·3) was obtained by X-ray analysis.‡ However, most of the crystal structures revealed guest-disordering where guest 3 was randomly stacked on the four equivalent triazine ligands of 1.

To prevent the guest disorder, we carried out the crystallization of 1⊃(2·3) in the presence of a metal ion connector (Co(NO₃)₂). When an aqueous solution (0.5 mL) of Co(NO₃)₂ (100 mmol) was slowly layered onto an aqueous solution of 1⊃(2·3) (0.005 mmol, 0.5 mL) and kept at 296 K over 4 weeks, we successfully obtained a hexagonal single crystal of {[(Co(H₂O)₄)_1.5(1⊃(2·3))]·[NO₃]₁₅·x(H₂O)}_n (4), whose crystal structure is shown in Fig. 2. The coordinative ligands 2 encapsulated in the cavity of 1 form 2D-honeycombed layers (Fig. 2a) which are stacked along the c-axis to constitute a 1D-channel containing a number of water and nitrate ions.⁶ The co-enclathrated guest 3 is located only at one corner of the cage because three other corners are occupied by ligand 2. Although guest 3 is still disordered at two positions in the corner (Fig. 3), site disorder at the four corners of the tetrahedral cage is prevented by our guest-networking strategy.⁷

Fig. 2 Molecular structure composed of [(Co(H₂O)₄)_1.5(1⊃(2·3))]¹⁵⁺: a) top view, b) side view, c) 2D-network structure consisting of 2 and Co(H₂O)₄ in 4.

Fig. 3 Disordered guest structure in the cage component of [(Co(H₂O)₄)_1.5(1⊃(2·3))]¹⁵⁺. Occupancy factors: a) 46%, b) 34%.

We investigated several combinations of transition metal carbonyl complexes with planar aromatic guests such as triphenyltriazine (5) and triphenylene (6) for selective bimolecular encapsulation. Each combination of 1 with 5 and 6 resulted in bimolecular encapsulation. As expected, the crystallographic analyses of 1⊃(2·5) and 1⊃(2·6) showed severely disordered structures of the guests (Figs. S9 and S10), emphasizing that, if a guest in the cage is hardly influenced by the external environment, the guest orientation can be hardly controlled in the cage in the solid state.

In summary, we have demonstrated that guest disorder in 1⊃(2·3) along the four equivalent C_3v axes of hollow cage 1 is prevented by the networking coordinative guest 2via complexation with Co(NO₃)₂ connectors. The crystallographic observation of the second guest 3 is made easier and more accurate by the networking. Accordingly, the solution host–guest chemistry of the highly symmetric cage is easily translated into a solid state chemistry which can be directly subjected to crystallographic analysis. An in situ crystallographic study on the photo-reactivity of 3 in the cavity of 1 is under way.

Notes and references

1. (a) J. Rebek, Jr., Angew. Chem., Int. Ed., 2005, 44, 2068–2078; (b) F. Hof, S. L. Craig, C. Nuckolls and J. Rebek, Jr., Angew. Chem., Int. Ed., 2002, 41, 1488–1508; (c) D. Fiedler, D. H. Leung, R. G. Bergman and K. N. Raymond, Acc. Chem. Res., 2005, 38, 351–360; (d) D. J. Cram and J. M. Cram, Container Molecules and Their Guests, R. Soc. Chem., Cambridge, UK, 1994; (e) R. Warmuth and J. Yoon, Acc. Chem. Res., 2001, 34, 95–105; (f) R. M. McKinlay, G. W. V. Cave and J. L. Atwood, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 5944–5948; (g) S. Seidel, M. Russell and P. J. Stang, Acc. Chem. Res., 2002, 35, 972–983; (h) M. Yoshizawa, M. Tamura and M. Fujita, Science, 2006, 312, 251–254; (i) M. Fujita, M. Tominaga, A. Hori and B. Therrien, Acc. Chem. Res., 2005, 38, 369–378.
2. M. Yoshizawa, M. Tamura and M. Fujita, J. Am. Chem. Soc., 2004, 126, 6846–6847.
3. Prior to this work, there are some examples of successful networking having a channel such as polyrotaxane. However, there is no network system having a cavity for guest encapsulation. (a) K. A. Udachin, L. D. Wilson and J. A. Ripmeester, J. Am. Chem. Soc., 2000, 122, 12375; (b) K.-M. Park, D. Whang, E. Lee, J. Heo and K. Kim, Chem.–Eur. J., 2002, 8, 498–508; (c) K. Kim, Chem. Soc. Rev., 2002, 31, 96; (d) E. Lee, J. Heo and K. Kim, Angew. Chem., Int. Ed., 2000, 39, 2699–2701; (e) E. Lee, J. Kim, J. Heo, D. Whang and K. Kim, Angew. Chem., Int. Ed., 2001, 40, 399–402.
4. (a) M. Kawano, Y. Kobayashi, T. Ozeki and M. Fujita, J. Am. Chem. Soc., 2006, 128, 6558–6559; (b) K. Takaoka, M. Kawano, T. Ozeki and M. Fujita, Chem. Commun., 2006, 1625–1627.
5. Temperature-dependent NMR indicates the presence of two types of encapsulated compounds, 1⊃(2·3) and 1⊃(3)₄ (Fig. S6).
6. Interestingly, the single crystal of 4 is considerably more stable than those of 1⊃(2·3) and 1⊃(3)₄, because of the networking of the cages. Therefore, the single crystal is more suitable for in situ observation of photoreaction by X-ray analysis.
7. One is tilted over 2 while another is perpendicular to 2 (Fig. 3).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/b612562j

‡ Crystallographic analyses of 1⊃(2·3), 1⊃(2·5), 1⊃(2·6), and {[(Co(H₂O)₄)_1.5(1⊃(2·3))]·[NO₃]₁₅·x(H₂O)}_n (4): Crystal data for 1⊃(2·3): monoclinic, C2/c, a = 46.536(2), b = 27.0972(2), c = 37.781(2) Å, β = 101.487(1)°, V = 46687(4) ų, Z = 8, T = 90(2) K, D_calcd = 1.155 g cm⁻³, μ = 0.561 mm⁻¹, Final R₁ (I > 2σ(I)) = 0.1374, wR₂ (all data) = 0.3803, GOF = 1.154.Crystal data for 1⊃(2·5): triclinic, P1̄, a = 24.817(7), b = 24.85(1), c = 24.835(6) Å, α = 64.50(2), β = 65.16(2), γ = 60.19(2)°, V = 11617(6) ų, Z = 2, T = 90(2) K, D_calcd = 1.019 g cm⁻³, μ = 0.564 mm⁻¹, Final R₁ (I > 2σ(I)) = 0.1712, wR₂ (all data) = 0.4301, GOF = 1.026.Crystal data for 1⊃(2·6): tetragonal, I4₁/a, a = 26.084(2), b = 26.084(2), c = 31.490(3) Å, V = 21426(3) ų, Z = 4, T = 90(2) K, D_calcd = 1.398 g cm⁻³, μ = 0.645 mm⁻¹, Final R₁ (I > 2σ(I)) = 0.0643, wR₂ (all data) = 0.1784, GOF = 1.080.Crystal data for 4: monoclinic, C2/c, a = 46.512(12), b = 27.256(7), c = 37.602(10) Å, β = 101.57(2)°, V = 46701(21) ų, Z = 8, T = 90(2) K, D_calcd = 1.195 g cm⁻³, μ = 0.617 mm⁻¹, Final R₁ (I > 2σ(I)) = 0.1647, wR₂ (all data) = 0.3933, GOF = 1.212.All the diffraction data were measured on a Siemens SMART/CCD diffractometer (MoK_α radiation, λ = 0.71073 Å). CCDC 618992–618995 (1⊃(2·3), (4), 1⊃(2·5), and 1⊃(2·6)) contain the supplementary crystallographic data. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b612562j

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