There is an upsurge of interest in contemporary theoretical and experimental research to unravel the structural details of water clusters. [1±8] The clusters are believed to be the perfect means for characterizing the structural changes and bonding mechanisms in passing from isolated molecules to a macroscopic collection, namely, bulk water, whose anomalous properties still remain mysteries. [1,2,9] A variety of modern theoretical and experimental methods have been employed to characterize the clusters. Both experiment and theory strongly suggest that the water trimer, tetramer, and pentamer have cyclic and quasiplanar minimum-energy structures. The larger clusters are expected to have 3D geometries, with the hexamer behaving as the transition from 2D to 3D structures. [1,2] The X-ray structural characterization of a decamer [10] and an octamer water cluster (cubic [11] and cyclic [12] forms) have been reported recently. Theoretical calculations for the hexamer have revealed the existence of several energy minima corresponding to ™ring∫, ™book∫, ™bag∫, ™cage∫, and ™prism∫ topologies within 0.7 kcal mol À1 . [13±15] The lowest energy conformer is the ™cage∫ followed by the ™book∫ and ™prism∫ structures, which are less than 0.1 and 0.2 kcal mol À1 higher in energy, respectively. [13±15] The ™ring∫ and ™bag∫ structures have been shown to be 0.5 and 0.7 kcal mol À1 higher in energy than the ™cage∫. To date, ™chair∫ [16,17] and ™boat∫ [18] cyclic hexamers included in host lattices have been characterized by X-ray crystallographic analysis. While Saykally and co-workers characterized the cage form of the hexamer, [15] Nauta and Miller reported the detection of a ™quasiplanar∫ hexamer in a helium droplet. [19] Herein, we report the first Xray structural characterization of the high-energy cyclic form of the water hexamer trapped in an organic supramolecular complex with the bimesityl dicarboxylic acid 4. The planar ring constitutes the basic structural motif of the high-pressure Ice II modification, [20] and is also one of the prominent morphologies found in the computer simulation of liquid water. [21] COMMUNICATIONS

A metal-organic framework (MOF), NH(2)-MIL-101(Al), which acts as a bifunctional, site-isolated Lewis acid-Brønsted base heterogeneous catalyst, catalyzes a tandem Meinwald rearrangement-Knoevenagel condensation reaction with remarkable substrate selectivity.

The X-ray crystal packing analyses of the sterically encumbered halogen-substituted benzene carboxylic acids 1-4 reveal a novel and unprecedented crystal packing in that the association of the carboxyl groups through O-H...O bonds results in the generation of a helix along the 41-screw axis. Such an organization of the acids is shown convincingly to be a result of the close packing, which exploits the weaker X...X and C-H...X interactions in conjunction with the stronger O-H...O hydrogen bonds. In contrast, the chloro- and bromo-substituted durene carboxylic acids 6 and 7 exhibit a pattern that is akin to tape/ribbon involving the centrosymmetric-dimer motif and X...X short intermolecular interactions. The structural investigations demonstrate the ability of the weaker interactions in modifying the supposedly "robust" centrosymmetric-dimer motif of the carboxyl groups in a decisive manner.

Crystal tuning: Organic molecules can be xenophobic, preferring to crystallize with their own kind. Though useful for purification, this precludes the tuning of crystal properties by doping or mixing. Nanoporous steroids provide an exception, as their channels can accept a variety of termini (hexagons and spheres). The steroids can be cocrystallized in any ratio to give a wide range of chiral, potentially porous crystalline materials.

Cucurbit[7]uril (CB[7]), an uncharged and water-soluble macrocyclic host, binds protonated amino saccharides (D-glucosamine, D-galactosamine, D-mannosamine and 6-amino-6-deoxy-D-glucose) with excellent affinity (Ka =10(3) to 10(4)  M(-1) ). The host-guest complexation was confirmed by NMR spectroscopy, isothermal titration calorimetry (ITC), and MALDI-TOF mass spectral analyses. NMR analyses revealed that the amino saccharides, except D-mannosamine, are bound as α-anomers within the CB[7] cavity. ITC analyses reveal that CB[7] has excellent affinity for binding amino saccharides in water. The maximum affinity was observed for D-galactosamine hydrochloride (Ka =1.6×10(4)  M(-1) ). Such a strong affinity for any saccharide in water using a synthetic receptor is unprecedented, as is the supramolecular stabilization of an α-anomer by the host.

There is an upsurge of interest in the synthesis of coordination polymers in contemporary supramolecular chemistry, as coordination polymerization may lead to materials with controllable functions such as porosity, sensing, nonlinear optical (NLO) activity, and chirality.[1] Crystal engineering based on predesigned organic linkers and metal centers (building blocks) with specific coordination geometries is an important approach in the preparation of coordination materials with desired functions.[2] Based on the knowledge of the structures of the ligands and the coordination geometries of a variety of metal centers, diverse 2D and 3D nets-several of which are analogous to structures of inorganic materials-have been engineered in the field of metal-organic frameworks (MOFs). Thus, the syntheses of 3-connected nets corresponding to the topologies of SrSi 2 /-(10,3)-a, [3] ThSi 2 /(10,3)-b, [4] (12,3), [5] (3,4)-connected nets with the topologies of boracite, [6] Cu 15 Si 4 , [7] (5, 3 4 ), [8] and (3,6)-connected nets corresponding to the topologies of rutile [9] and pyrite [10] have been accomplished. Insofar as the 4-connected nets are concerned, the syntheses of MOFs with unusual topologies [11] and with topologies corresponding to those of diamond, [1c,d, 12] NbO, [13] quartz, [14] and PtS [15] have been reported. Similarly, the (4,8)-and 6-connected metalorganic frameworks with fluorite [16] and cubic [17] topologies, respectively, have been designed and synthesized. In view of the success of such building-block approaches to realize metal-organic frameworks with specific topologies, the multidentate ligands with novel structural features offer the possibility to construct new and unique coordination poly-

Previous work has shown that certain steroidal bis-(N-phenyl)ureas, derived from cholic acid, form crystals in the P61 space group with unusually wide unidimensional pores. A key feature of the nanoporous steroidal urea (NPSU) structure is that groups at either end of the steroid are directed into the channels and may in principle be altered without disturbing the crystal packing. Herein we report an expanded study of this system, which increases the structural variety of NPSUs and also examines their inclusion properties. Nineteen new NPSU crystal structures are described, to add to the six which were previously reported. The materials show wide variations in channel size, shape, and chemical nature. Minimum pore diameters vary from ∼0 up to 13.1 Å, while some of the interior surfaces are markedly corrugated. Several variants possess functional groups positioned in the channels with potential to interact with guest molecules. Inclusion studies were performed using a relatively accessible tris-(N-phenyl)urea. Solvent removal was possible without crystal degradation, and gas adsorption could be demonstrated. Organic molecules ranging from simple aromatics (e.g., aniline and chlorobenzene) to the much larger squalene (Mw = 411) could be adsorbed from the liquid state, while several dyes were taken up from solutions in ether. Some dyes gave dichroic complexes, implying alignment of the chromophores in the NPSU channels. Notably, these complexes were formed by direct adsorption rather than cocrystallization, emphasizing the unusually robust nature of these organic molecular hosts.

Despite the remarkable progress made in the self-assembly of nano- and microscale architectures with well-defined sizes and shapes, a self-organization-based synthesis of hollow toroids has, so far, proved to be elusive. Here, we report the synthesis of polymer microrings made from rectangular, flat and rigid-core monomers with anisotropically predisposed alkene groups, which are crosslinked with each other by dithiol linkers using thiol-ene photopolymerization. The resulting hollow toroidal structures are shape-persistent and mechanically robust in solution. In addition, their size can be tuned by controlling the initial monomer concentrations, an observation that is supported by a theoretical analysis. These hollow microrings can encapsulate guest molecules in the intratoroidal nanospace, and their peripheries can act as templates for circular arrays of metal nanoparticles.