Towards Novel Topologies

Formally compound 16 belongs to the AABC-type of tetra-ureas (compare Section 5.3.2) for which numerous regioisomeric dimers are possible. However, these substituents at the urea residues ensure that selectively only one dimer is formed, in which no overlap of the loops occurs and no penetration of the loop by the bulky group takes place. NMR spectra are in agreement with the formation of a single C2-symmetrical dimer, which is necessarily composed of the same enantiomer of 16. 

In fact, after metathesis and hydrogenation a single reaction product is isolated in 40-55% yield, which cannot be split in its calix[4]arenes by hydrogen bond breaking solvents [56]. Although the structure for several single crystals could not be solved, the NMR and ESI mass spectra serve as an unambiguous structural proof. 

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Over the past few decades, much attention has been focused on polycatenanes, which consist of mechanically interlocked structures that have novel topologies and, not unexpectedly, display somewhat different properties than do commonly used, conventional polymers. The linear polycatenanes (type A in Figure 17.1) are aesthetically perfect, and are expected to possess maximized effects of topologically bonded structures on properties. However, the synthesis of such linear polycatenanes remains one of the most difficult and as-yet unachieved synthetic goals in polymer science. Due to the relatively easy preparation of bifunctionaUzed [2]catenanes, success in the directed synthesis of polycatenanes has been mainly limited to the poly[2]catenanes, which contain essential mechanical hnkages. Nonetheless, some progress has been made recently towards creating polymeric catenanes and polycatenane networks. 

The synthesis of aluminophosphate molecular sieves in 1980s represents a breakthrough in the development of microporous materials. Since then, much of the worldwide synthetic efforts have been directed toward nonsilicate microporous materials. Many novel framework topologies could be found with phosphates, and many other elements could be incorporated into phosphates to produce additional new framework topologies or new compositions. 

The great improvements in yield made it possible to study the specific properties of the knots related to their topology, to resolve the enantiomers, and also to study their coordination chemistry. It also became possible to prepare the first chemical knot composite and to prove its complex and unusual topology. The various complexes of knotted ligands display extraordinary kinetic inertness toward demetalation and, because of the proximity between the two copper ions in the helical core of the knot, novel electronic properties could be evidenced. In particular, the Cu(II)-Cu(II) oxidation state is strongly destabilized, as shown by the extremely high redox potential of the system [ 0.9V vs SCE (standard calomel electrode) in acetonitrile], which makes it almost unique in copper chemistry. 

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