Interlocked Compounds Containing Metals

Another fascinating approach to catenanes via self-assembly involves metal-coordination which templates or directs the assembly of catenane frameworks. After brief discussions on the recent examples of metal-containing catenanes and related topologically interesting molecules (Sections 4.2.1-4.2.3), the focus will be on the self-assembly of Pd(II)-linked catenanes (Sections 4.2.4 and 4.2.5). 

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Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

On a simpler scale the Rybak-Akimova group found that when 4/-(amino-methylene)benzo[18]crown-6 was treated with an acid salt, mass spectrometric evidence indicated that dimers were the most stable species [13]. This was backed up by an X-ray crystal structure of the compound crystallized from methanol which revealed pairs of mutually interlocked crowns. The amine termini had become pro-tonated and the resultant ammonium group, illustrated in Fig. 8.4, formed a complex with a second crown. Although yet to be attempted, it would be intriguing to react the complex with a linear component containing a second amine and a bulky stopper group to generate a metal-free, interlocked pair of rotaxanes with two amine stations . The contraction and extension of the complex could then be controlled as a function of pH. 

Instead of using a metal ion as a clasp , it is also possible to use weak interactions between ring components for catenane interlocking. In the example shown in Fig. 3.25, a C-shaped precursor containing viologen-type cyclophane was complexed to a cyclophane with benzene ring moieties. The cationic bipyridine moiety in the C-shaped component was then sandwiched by electron-rich benzene units. Cyclization of the C-shaped precursor using a dibromo compound resulted in interlocked catenanes. This electronic interaction between two kinds of species results in more efficient catenane formation. 

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