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Design, Synthesis, and Evaluation of Self-assembled Receptors

The synthesis and isolation of many of these receptors proved to be quite tedious, however, although occasionally convergent strategies could be used. We therefore looked for an alternative way to build up such structures and decided to make use of self-assembly processes of metal complexes [4] to build up self assembled oligo(BINOL) analogs [5]. (How similar metal complexes can be used to stabilize peptide microstructures is discussed in Chapter 1.3.)

We therefore designed and synthesized two ligands 10 and 11 following the basic idea to employ a convergent modular approach, where elaborated building blocks can be combined in a rather fast and flexible manner. These ligands 10 and 11 were thought to form di- and mononuclear coordination complexes in self-assembly processes with suitable metal ions thereby orienting the BINOL groups in a fashion potentially useful for the molecular recognition of monosaccharide derivatives (Figures 2.1.4 and 2.1.5).

Having accomplished the synthesis (Figure 2.1.6), the next task was to prove the formation of the self-assembled metal coordination complexes. This was achieved by NMR and ESI-MS experiments and elemental analysis and in one instance we were even able to perform X-ray structure analysis (Figures 2.1.7-2.1.9).

To learn more about the effect of the metal ion on the binding of these receptors we also synthesized a covalently linked analog of the [M112]+ complex in which the central metal bis(bipyridine) complex was substituted by a spirobifluorene moiety [6]. Figure 2.1.10 shows the structures of both, which we obtained from molecular modeling studies.

Although we strictly followed our convergent building block approach again, the synthesis of12 (Figure 2.1.11) proved to be considerably more demanding, because its isolation by chromatography turned out to be quite onerous; this again demonstrated the advantages of employing self-assembly processes of smaller molecules to obtain access to larger functional aggregates, rather than synthesizing covalently linked structures.

With the aggregates and the covalent analog available we started to evaluate the potential of these structures to act as receptors for monosaccharides in organic

2.1.3 Design, Synthesis, and Evaluation of Self-assembled Receptors | 113

Fig. 2.1.4. Bis(2,2'-bipyridine)-substituted BINOL 10 and its dimeric and trimeric dinuclear metal complex.

Fig. 2.1.4. Bis(2,2'-bipyridine)-substituted BINOL 10 and its dimeric and trimeric dinuclear metal complex.

Fig. 2.1.5. Bis(BINOL)-substituted 2,2'-bipyridine 11 and its dimeric metal complex.

Fig. 2.1.5. Bis(BINOL)-substituted 2,2'-bipyridine 11 and its dimeric metal complex.

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