Electrochemically Triggered Co-Conformational Switching in a [2]catenane Comprising a Non-Symmetric Calix[6]arene Wheel and a Two-Station Oriented Macrocycle

Catenanes with desymmetrized ring components can undergo co-conformational rearrangements upon external stimulation and can form the basis for the development of molecular rotary motors. We describe the design, synthesis and properties of a [2]catenane consisting of a macrocycle—the ‘track’ ring—endowed with two distinct recognition sites (a bipyridinium and an ammonium) for a calix[6]arene—the ‘shuttle’ ring. By exploiting the ability of the calixarene to thread appropriate non-symmetric axles with directional selectivity, we assembled an oriented pseudorotaxane and converted it into the corresponding oriented catenane by intramolecular ring closing metathesis. Cyclic voltammetric experiments indicate that the calixarene wheel initially surrounds the bipyridinium site, moves away from it when it is reduced, and returns in the original position upon reoxidation. A comparison with appropriate model compounds shows that the presence of the ammonium station is necessary for the calixarene to leave the reduced bipyridinium site

Covalent capture of oriented calix[6]arene rotaxanes by a metal-free active template approach

We describe the active template effect of a calix[6]arene host towards the alkylation of a complexed pyridylpyridinium guest. The acceleration of the reaction within the cavity is significant and rim-selective, enabling the efficient preparation of rotaxanes with full control of the mutual orientation of their nonsymmetric components

Plugging a bipyridinium axle into multichromophoric calix[6]arene wheels bearing naphthyl units at different rims

Tris-(N-phenylureido)-calix[6]arene derivatives are heteroditopic non-symmetric molecular hosts that can form pseudorotaxane complexes with 4,4'-bipyridinium-type guests. Owing to the unique structural features and recognition properties of the calix[6]arene wheel, these systems are of interest for the design and synthesis of novel molecular devices and machines. We envisaged that the incorporation of photoactive units in the calixarene skeleton could lead to the development of systems whose working modes can be governed and monitored by means of light-activated processes. Here we report on the synthesis, structural characterization, and spectroscopic, photophysical and electrochemical investigation of two calix[6]arene wheels decorated with three naphthyl groups anchored to either the upper or lower rim of the phenylureido calixarene platform. We found that the naphthyl units interact mutually and with the calixarene skeleton in a different fashion in the two compounds, which thus exhibit a markedly distinct photophysical behavior. For both hosts, the inclusion of a 4,4'-bipyridinium guest activates energy- and/or electron-transfer processes that lead to non-trivial luminescence changes

Efficient active-template synthesis of calix[6]arene-based oriented pseudorotaxanes and rotaxanes

A substrate can modify its chemical features, including a change of its reactivity, as a consequence of non-covalent interactions upon inclusion within a molecular host. Since the rise of supramolecular chemistry, this phenomenon has stimulated the ingenuity of scientists to emulate the function of enzymes by designing supramolecular systems in which the energetics and selectivity of reactions can be manipulated through programmed host-guest interactions and/or steric confinement. In this paper we investigate how the engulfment of a positively charged pyridinium-based guest inside the Ï\u80-rich cavity of a tris-(N-phenylureido)calix[6]arene host affects its reactivity towards a SN2 reaction. We found that the alkylation of complexed substrates leads to the formation of pseudorotaxanes and rotaxanes with faster kinetics and higher yields with respect to the standard procedures exploited so far. More importantly, the strategy described here expands the range of efficient synthetic routes for the formation of mechanically interlocked species with a strict control of the mutual orientation of their non-symmetric molecular components

Covalent capture of oriented calix[6]arene rotaxanes by a metal-free active template approach

We describe the active template effect of a calix[6]arene host towards the alkylation of a complexed pyridylpyridinium guest. The acceleration of the reaction within the cavity is significant and rim-selective, enabling the efficient preparation of rotaxanes with full control of the mutual orientation of their nonsymmetric components

Synthesis and recognition properties of calix[4]arene semitubes as ditopic hosts for N-alkylpyridinium ion pairs

A series of calix[4]arene semitubes (4a–c) having alkyl bridging spacers of different lengths and flexibilities on their lower rim was synthesised and characterised in solution and in the solid state by 1H NMR spectroscopy and X-ray crystallography. The binding properties of these novel ditopic hosts toward a series of N-alkylpyridinium ion pairs were investigated in low polar media by 1H NMR and UV-vis spectroscopy and compared with those of the corresponding calix[4]arene monotopic host 5. These studies showed that in low polar solvents, the calix[4]arene semitubes behave as homoditopic hosts giving rise with N-alkylpyridinium ion pairs to supramolecular adducts of 1 : 2 host–guest stoichiometry

Calix[6]arene-Based [3]Rotaxanes as Prototypes for the Template Synthesis of Molecular Capsules

: In this work, the ability of several bis-viologen axles to thread a series of heteroditopic tris(N-phenylureido)calix[6]arene wheels to give interwoven supramolecular complexes to the [3]pseudorotaxane type was studied. The unidirectionality of the threading process inside these nonsymmetric wheels allows the formation of highly preorganised [3]pseudorotaxane and [3]rotaxane species in which the macrocycles phenylureido moieties, functionalised with either ester, carboxylic, or hydroxymethyl groups, are facing each other. As verified by NMR and semiempirical computational studies, these latter compounds possess the correct spatial arrangement of their subcomponents, which could lead, in principle, upon proper bridging reaction, to the realisation of upper-to-upper molecular capsules that are based on calix[6]arene derivatives

Calix[6]arene-Based [3]Rotaxanes as Prototypes for the Template Synthesis of Molecular Capsules

In this work, the ability of several bis-viologen axles to thread a series of heteroditopic tris(N-phenylureido)calix[6]arene wheels to give interwoven supramolecular complexes to the [3]pseudorotaxane type was studied. The unidirectionality of the threading process inside these nonsymmetric wheels allows the formation of highly preorganised [3]pseudorotaxane and [3]rotaxane species in which the macrocycles phenylureido moieties, functionalised with either ester, carboxylic, or hydroxymethyl groups, are facing each other. As verified by NMR and semiempirical computational studies, these latter compounds possess the correct spatial arrangement of their subcomponents, which could lead, in principle, upon proper bridging reaction, to the realisation of upper-to-upper molecular capsules that are based on calix[6]arene derivatives

Synthesis and characterization of constitutionally isomeric oriented calix[6]arene-based rotaxanes

10Oriented rotaxanes based on a calix[6]arene wheel and nonsymmetric bipyridinium axles have been synthesized. The selective formation of orientational isomers was achieved. Spectroelectrochemistry and electron spin resonance measurements were used to investigate the relative direction of motion of the components.reservedmixedZanichelli V.; Ragazzon G.; Arduini A.; Credi A.; Franchi P.; Orlandini G.; Venturi M.; Lucarini M.; Secchi A.; Silvi S.Zanichelli, V.; Ragazzon, G.; Arduini, A.; Credi, A.; Franchi, P.; Orlandini, G.; Venturi, M.; Lucarini, M.; Secchi, A.; Silvi, S