Rotaxane solvation

Bickelhaupt and coworicers have determined the crystal structures of a series of crown ether solvated magnesium compounds. A sequence of these compounds is illustrated as the internally coordinated 13-crown-4-xylylmagnesium chloride (134) and bromide (135), as well as the organometallic, rotaxane (136). Note the similarity between these structures and the corresponding aliphatic dialkylmagnesium rotaxane (83). 

Upon snpramolecnlar complexation, neutral hydrophobic orgauic compounds and hydrophilic metal cations usually show entirely different, often opposite, solvent polarity dependencies. Snch contrasting behavior is very generally observed when one compares the solvophobically versus electrostatically driven complexation for detailed discussion, see, for example, the study by Mizutani et al This difference in solvation property is exploited as a conventional tool for creating a vast variety of supramolecular architectures and devices, including rotaxane-based molecular shuttles and other so-called molecular machines. " ... 

The final step in the construction of the dumbbell-shaped compound 7 involved a Fischer indole synthesis between the protected hydrazine 5 and the ketal 6, which, on treatment with acid Scheme 4), formed the indole ring system and the dumbbellshaped compound 7 in 51% yield. Low temperature NMR spectroscopy on the rotaxane 8.4PF6 in CD3CN showed that in fact the hydroquinol ring was occupied by the tetracationic cyclophane in a ratio of 100 0 This outcome is probably a consequence of steric outweighing electronic factors, leaving the indole to "solvate" the exterior of the tetracationic cyclophane. The quite decisive preference for this rotaxane... 

A similar rotaxane was further synthesized and experiments showed a reverse shuttling behav-ior. 2 This reverse performance was explained by means of n-n interactions that in this case are allowed between the C o and the macrocycle. Indeed, in the previous system, there was an additional amide between the fullerene stopper and the template station, and the solvation of all the amides in DMSO hampered the possibility for the macrocycle to get close to the Cgg. Therefore, this promoted shuttling over the alkyl chain. In the second system, shown in Figure 2.21b, the solvation of the suc-cinamide template released the macrocycle that was attracted by the fullerene via n-n interactions, as demonstrated through cyclic voltammetry experiments. 

PRF-23 and PRF-24 demonstrate a novel way to create PRFs by using permanently interlocked rotaxanes which are connected by a mechanical linkage and not merely held together by relatively weak non-covalent interactions prior to PRF synthesis. This synthetic approach allows for the use of harsher reaction conditions since the linker cannot simply unthread when exposed to competitive solvation during PRF formation. 

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