Directed self-assembly of a ring-in-ring complex

A strategic modification to the corner ligands in Pd(II)-containing, electron-poor cyclophanes has profound repercussions for their assemblies with electron-rich aromatic crown ethers in both the solid and solution states; the formation of ring-in-ring complexes can override competing [3]catenane production on masking the hydrogen bond donor capabilities of the corner ligands

Towards the stepwise assembly of molecular Borromean rings. A donor-acceptor ring-in-ring complex

The assembly of molecular Borromean Rings from constitutionally independent rings in a stepwise manner depends on the preparation of robust ``ring-in-ring'' complexes. The pi-electron rich macrocycle bis-1,5-dinaphtho[50] crown-14 (1) is shown to form a donor-acceptor ring-in-ring complex with the pi-electron poor cyclophane cyclobis(paraquat-4,4'-biphenylene) (2(4+)) in solution. In the crystal superstructure of [1 subset of 2]center dot 4PF(6), CII center dot center dot center dot O interactions between the polyether loops of 1 and the bipyridinium units of 2(4+) could disfavor the threading of dialkylammonium components of a third ring

Strong and reversible binding of carbon dioxide in a green metal-organic framework

The efficient capture and storage of gaseous CO(2) is a pressing environmental problem. Although porous metal organic frameworks (MOFs) have been shown to be very effective at adsorbing CO(2) selectively by dint of dipole-quadruple interactions and/or ligation to open metal sites, the gas is not usually trapped covalently. Furthermore, the vast majority of these MOFs are fabricated from nonrenewable materials, often in the presence of harmful solvents, most of which are derived from petrochemical sources. Herein we report the highly selective adsorption of CO(2) by CD-MOF-2, a recently described green MOF consisting of the renewable cyclic oligosaccharide gamma-cyclodextrin and RbOH, by what is believed to be reversible carbon fixation involving carbonate formation and decomposition at room temperature. The process was monitored by solid-state (13)C NMR spectroscopy as well as colorimetrically after a pH indicator was incorporated into CD-MOF-2 to signal the formation of carbonic acid functions within the nanoporous extended framework

Imprinting chemical and responsive micropatterns into metal-organic frameworks

Wet stamping allows metal–organic framework (MOF) crystals to be imprinted with micropatterns of various organic chemicals. Printing the MOFs with photochromic molecules and pH indicators generates stimuli-responsive micropatterns which change their appearance upon contact with specific chemicals (see picture), thus reporting the environmental “status” of the crystal

Chameleonic binding of the dimethyldiazaperopyrenium dication by cucurbit[8]uril

Two are better than one: The diazaperopyrenium dication acts as both a viologen-like electron-poor and an electron-rich guest, resulting in the formation of a 1:2 complex with cucurbit[8]uril. This chameleonic binding facilitates deaggregation of the dications from aqueous solutions, leading to an increase in the fluorescence quantum yield of the diazaperopyrenium dication

Nanoporous carbohydrate metal-organic frameworks

The binding of alkali and alkaline earth metal cations by macrocyclic and diazamacrobicyclic polyethers, composed of ordered arrays of hard oxygen (and nitrogen) donor atoms, underpinned the development of host–guest supramolecular chemistry in the 1970s and 1980s. The arrangement of −OCCO– and −OCCN– chelating units in these preorganized receptors, including, but not limited to, crown ethers and cryptands, is responsible for the very high binding constants observed for their complexes with Group IA and IIA cations. The cyclodextrins (CDs), cyclic oligosaccharides derived microbiologically from starch, also display this −OCCO– bidentate motif on both their primary and secondary faces. The self-assembly, in aqueous alcohol, of infinite networks of extended structures, which have been termed CD-MOFs, wherein γ-cyclodextrin (γ-CD) is linked by coordination to Group IA and IIA metal cations to form metal–organic frameworks (MOFs), is reported. CD-MOF-1 and CD-MOF-2, prepared on the gram-scale from KOH and RbOH, respectively, form body-centered cubic arrangements of (γ-CD)<sub>6</sub> cubes linked by eight-coordinate alkali metal cations. These cubic CD-MOFs are (i) stable to the removal of solvents, (ii) permanently porous, with surface areas of 1200 m<sup>2</sup> g<sup>–1</sup>, and (iii) capable of storing gases and small molecules within their pores. The fact that the −OCCO– moieties of γ-CD are not prearranged in a manner conducive to encapsulating single metal cations has led to our isolating other infinite frameworks, with different topologies, from salts of Na<sup>+</sup>, Cs<sup>+</sup>, and Sr<sup>2+</sup>. This lack of preorganization is expressed emphatically in the case of Cs<sup>+</sup>, where two polymorphs assemble under identical conditions. CD-MOF-3 has the cubic topology observed for CD-MOFs 1 and 2, while CD-MOF-4 displays a channel structure wherein γ-CD tori are perfectly stacked in one dimension in a manner reminiscent of the structures of some γ-CD solvates, but with added crystal stability imparted by metal–ion coordination. These new MOFs demonstrate that the CDs can indeed function as ligands for alkali and alkaline earth metal cations in a manner similar to that found with crown ethers. These inexpensive, green, nanoporous materials exhibit absorption properties which make them realistic candidates for commercial development, not least of all because edible derivatives, fit for human consumption, can be prepared entire