Structure of Supramolecular Assemblies Formed by α,δ-Tetramethylcucurbit[6]uril and 4-Nitrophenol

A host-guest assembly, [(C40H44N24O12)·(C6H5NO3)8·13(H2O)] (1), based on a partial substituted cucurbituril, α,δ-tetramethylcucurbit[6]uril (TMeQ[6]), and 4-nitrophenol was synthesized and structurally characterized by single-crystal X-ray diffraction. A combination of hydrogen-bonding between the latticed water molecule and the hydroxyl group of 4-nitrophenol, the hydroxyl group of 4-nitrophenol and the carbonyl groups lining the portals in additon, the C-H···π interactions between the 4-nitrophenol molecules could be the driving forces of formation such an exclusion host-guest assembly.


Introduction
The chemistry of cucurbit[n]urils (Q[n]s) has been studied extensively [1] since the first member of the family, cucurbituril, was structurally characterized by Mock and coworkers in 1981 [2]. The cucurbit[n]urils include cucurbituril (Q [6]) and its homologues, cucurbit [n = 5, 7, 8 and 10]urils (Q [5], Q [7], Q [8] and Q [10]) [3][4][5], and a series of fully or partially substituted derivatives and analogues [6][7][8][9][10][11][12]. The members of the cucurbit[n]uril family have common characteristic features, such as a hydrophobic cavity and polar carbonyl groups surrounding the opening portals. In addition, their varying cavity and portal sizes lead to the formation of inclusion or exclusion complexes with different organic or inorganic species. They also present remarkable molecular recognition properties and act as building blocks for supramolecular chemistry [5,[13][14][15][16][17][18]. Up to now, the supramolecular assemblies of metal or cluster aqua complexes involving the Q[n]s have been mainly limited in those with Q [6] or Q [8]. After Mock and Kim's research [19,20], Fedin and coworkers have done major work in this area and their results were summarized in reviews [21,22]. They found that alkali and alkaline earth metal ions are coordinated directly with carbonyl oxygen atoms of Q [6], but transition metal ions prefer to form hydrogen bonds between the polar C=O groups of Q [6].
We previously reported the controlled synthesis of α, δ-tetramethylcucurbit [6]uril (TMeQ [6]) and the preparation of some supramolecular assemblies of metal aqua complexes and host-guest inclusion complexes involving this substituted Q[n]s ( Figure 1) [11]. In this work, we report the crystal structures of an exclusion supramolecular assembly of TMeQ [6] with 4-nitrophenol

Results and Discussion
The X-ray structure of TMeQ [6] shows that it is a macrocyclic cavity with two identical portals surrounded by carbonyl groups. Unlike the normal cucurbit[n=5-10]urils, the α,δ-substituted Q [6] macrocycle is not circular, but ellipsoid. Its structure is easy to deform and includes molecules which could not be included by the normal cucurbit [6]uril [23].

4a 4b
In both 4-nitrophenol assemblies, we did not observed the π-π stacking which normally occurred in the crystal structure of aromatic compounds, but the C-H···π structure [24][25][26]. In the structure, there are eight C-H···π structures and each of them is symmetric (Table 1, Figure 5).

Experimental
General 4-Nitrophenol was of reagent grade and used without further purification. The TMeQ [6] was prepared by procedures reported previously [11]. Elemental analysis was carried out on an EURO EA-3000 element analyzer.

X-ray Crystallography
The crystal data of the substituted cucurbiturils coordinated with metal ions were collected on a SMART ApexII CCD diffractometer with graphite monochromated Mo Kα radiation ( λ = 0.71073 Å) in the ω-φ scan mode. Lorentz polarization and absorption corrections were applied. Structural solution and full matrix least-squares refinement based on F 2 were performed with the SHELXS-97 and SHELXL-97 program package [27], respectively. All the non-hydrogen atoms were refined anisotropically. Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections were incorporated. The crystallographic data, data collection conditions, and refinement parameters for the compound 1 are listed in Table 2.  (6) 17.908 (6) 18.941 (7) 88.290 (5) 83.072 (5) 63.602 (5 Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as Deposition No. CCDC 659590. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: + 44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk).