A route to mechanically interlocked architectures that requires only a catalytic quantity of template is described. The strategy utilizes the Cu(I)-catalyzed 1,3-cycloaddition of azides with terminal alkynes. Chelating the Cu(I) to an endotopic-binding macrocycle means that the metal atom binds to the alkyne and azide in such a way that the metal-mediated bond-forming reaction occurs through the cavity of the macrocycle, forming a rotaxane. Addition of pyridine to the reaction mixture enables the Cu(I) to turn over during the reaction, permitting substoichiometric amounts of the metal to be used. The yields are very high for a rotaxane-forming reaction (up to 94% with stoichiometric Cu(I); 82% with 20 mol % of Cu(I)), and the procedure is practically simple to do (no requirement for an inert atmosphere nor dried or distilled solvents).

A synthetic approach to rotaxane architectures is described in which metal atoms catalyze covalent bond formation while simultaneously acting as the template for the assembly of the mechanically interlocked structure. This "active-metal" template strategy is exemplified using the Huisgen-Meldal-Fokin Cu(I)-catalyzed 1,3-cycloaddition of azides with terminal alkynes (the CuAAC "click" reaction). Coordination of Cu(I) to an endotopic pyridine-containing macrocycle allows the alkyne and azide to bind to metal atoms in such a way that the metal-mediated bond-forming reaction takes place through the cavity of the macrocycle--or macrocycles--forming a rotaxane. A variety of mono- and bidentate macrocyclic ligands are demonstrated to form [2]rotaxanes in this way, and by adding pyridine, the metal can turn over during the reaction, giving a catalytic active-metal template assembly process. Both the stoichiometric and catalytic versions of the reaction were also used to synthesize more complex two-station molecular shuttles. The dynamics of the translocation of the macrocycle by ligand exchange in these two-station shuttles could be controlled by coordination to different metal ions (rapid shuttling is observed with Cu(I), slow shuttling with Pd(II)). Under active-metal template reaction conditions that feature a high macrocycle:copper ratio, [3]rotaxanes (two macrocycles on a thread containing a single triazole ring) are also produced during the reaction. The latter observation shows that under these conditions the mechanism of the Cu(I)-catalyzed terminal alkyne-azide cycloaddition involves a reactive intermediate that features at least two metal ions.

Full and shortened single-walled and multiple-walled carbon nanotubes were suspended in water to form stable suspensions in the presence of a surfactant. Optical limiting properties of the suspensions were determined for 532-nm pulsed laser irradiation, and the results were comparable with those of carbon black aqueous suspension. Solubilization of the shortened carbon nanotubes was achieved by attaching the nanotubes to highly soluble poly(propionylethylenimine-co-ethylenimine) or by functionalizing the nanotubes with octadecylamine. The soluble carbon nanotube samples formed homogeneous solutions in room-temperature chloroform. Optical limiting properties of these solutions were also determined for 532-nm pulsed laser irradiation, and the results were found to be quite different from those of the carbon nanotube aqueous suspensions. Apparently, the carbon nanotubes exhibit significantly weaker optical limiting responses in homogeneous solutions than in suspensions. Mechanistic implications of the experimental results are discussed.

We report the synthesis of a [2]catenate using a square planar palladium(II) template, together with two isomers of the interlocked structure: a single tetradentate macrocycle that adopts a "figure of eight" conformation to encapsulate the metal and a complex in which the two macrocycles of the catenane are not interlocked. The three isomers can each be selectively formed depending on how the building blocks are assembled and cyclized. Olefin metathesis of both building blocks while they are attached to the metal gives the single large macrocycle in 77% yield. Cyclizing the monodentate unit prior to attaching both ligands to the metal gives the [2]catenate in 78% yield. Preforming the tridentate macrocycle produces a complex in two atropisomeric forms-threaded and nonthreaded-in a 2:3 ratio, which do not interconvert in dichloromethane at room temperature over 7 days. RCM of the nonthreaded atropisomer affords the complex with two noninterlocked macrocyclic ligands; RCM of the threaded atropisomer generates the topologically isomeric [2]catenate. Heating the acyclic atropisomers in acetonitrile provides a mechanism for their interconversion via ligand exchange, allowing the threaded:nonthreaded ratio to be varied from 2:3 to 8:1. All three fully ring-closed complexes were characterized unambiguously by 1H NMR spectroscopy and X-ray crystallography. As far as we are aware, this is the first time such a set of three formal topological and constitutional isomers has been described.

The importance of carbohydrate recognition in biology, and the unusual challenges involved, have lead to great interest in mimicking saccharide-binding proteins such as lectins. In this review, we discuss the design of artificial carbohydrate receptors, focusing on those which work under natural (i.e. aqueous) conditions. The problem is intrinsically difficult because of the similarity between substrate (carbohydrate) and solvent (water) and, accordingly, progress has been slow. However, recent developments suggest that solutions can be found. In particular, the "temple" family of carbohydrate receptors show good affinities and excellent selectivities for certain all-equatorial substrates. One example is selective for O-linked beta-N-acetylglucosamine (GlcNAc, as in the O-GlcNAc protein modification), while another is specific for beta-cellobiosyl and closely related disaccharides. Both show roughly millimolar affinities, matching the strength of some lectin-carbohydrate interactions.

The synthesis of catenanes and rotaxanes using the hard trivalent transition metal ion cobalt(III) as a template is reported. Tridentate dianionic pyridine-2,6-dicarboxamido ligands, each with two terminal alkene groups, coordinate Co(III) in a mutually orthogonal arrangement such that entwined or interlocked molecular architectures are produced by ring-closing olefin metathesis. Double macrocyclization of two such ligands bound to Co(III) afford a non-interlocked "figure-of-eight" complex in 42% yield, the structure determined by X-ray crystallography. Preforming one macrocycle and carrying out a single macrocyclization of the second bis-olefin with both ligands attached to the Co(III) template led to the isomeric [2]catenate in 69% yield. The mechanically interlocked structure was confirmed by X-ray crystallography of both the Co(III) catenate and the metal-free catenand. A Co(III)-template [2]rotaxane was assembled in 61% yield by macrocyclization of the bis-olefin ligand about an appropriate dianionic thread. For both catenanes and rotaxanes, removal of the metal ion via reduction under acidic conditions to the more labile Co(II) gave neutral interlocked molecules with well-defined co-conformations stabilized by intercomponent hydrogen bonding.

Any which way, but not loose: A range of diverse noncovalent binding interactions are observed with a [2]rotaxane (see structure) and a [2]catenane that are not found with similar but not mechanically interlocked fragments. (C (macrocycle) turquoise, other C atoms yellow, O red, N blue, H gray, F green, S orange.)

Stay flexible: Rigid preorganization is not always the best approach to molecular recognition. Unlike previous synthetic lectins, new receptors (see picture) were synthesized that possess conformational freedom which allows hydrophobically driven collapse of the cavity. Nonetheless, they bind their carbohydrate targets in water with ground‐breaking affinities (up to 4500 M−1 for methyl cellobioside, R=Me) and selectivities.