Templation in Noncovalent Synthesis of Hydrogen-Bonded Rosettes

Abstract

In this chapter, hydrogen-bonded assemblies based on the rosette motif are used to describe some examples of templation in noncovalent synthesis.

After a brief description of the synthesis and characterization of these assemblies, the guest-templated selection and amplification of the strongest binding receptor in dynamic libraries is explained. The equilibrating mixtures of the rosette structures (dynamic combinatorial libraries) allow for the target-driven generation of the active constituents of the library. A template effect for the formation and amplification of the strongest hydrogen-bonded receptor is obtained when a guest molecule is added to a library of potential hydrogen-bonded receptors that are under thermodynamic control.

Additionally, templation for the control of the chirality in these supramolecular systems is described at three different levels:

1. Amplification of chirality (“Sergeant and Soldiers” principle), where the achiral building blocks of the assemblies “follow” the templated helicity induced by the chiral components even when chiral molecules are present in very small fractions.

2. Enantioselective noncovalent synthesis (memory of supramolecular chirality), where use of a chiral template interacts stereoselectively to give preferentially one of the two possible enantiomeric forms (P or M-helix). After the template is replaced by an achiral analog the induced chirality is preserved allowing the synthesis of enantiomerically enriched self-assembled double rosette assemblies.

3. Diastereomeric and enantiomeric noncovalent synthesis of double and tetrarosettes by guest emplation, where chiral guest molecules can be used as templates to induce the formation of one specific helicity of the double and tetrarosette assemblies.

Furthermore, the concept of templated synthesis by hydrogen-bonded rosette assemblies is also illustrated with the templated synthesis of covalent cyclic calix[4]arene dimelamine trimers. The synthesis of this trimer is impossible without the template role provided by one of the building blocks of the assembly. The templated synthesis by a double rosette of noncovalent cyclic hydrogen-bonded trimers is also described.

The role of gold and graphite surfaces as template for the formation of hydrogen-bonded nanostructures is also revised. The topology of the structure that is formed by noncovalent interactions on the surfaces is determined by the noncovalent interactions between the surface template and the substrates. Specifically, the growth of individual nanometer-sized hydrogen-bonded assemblies on gold monolayers is templated through an exchange reaction between a double rosette in solution and a single calix[4]arene dimelamine embedded into hexanethiol self-assembled monolayers (SAMs).

On the other hand, first- and second-order template effects using graphite surfaces as templates are shown. The formation of linear rod-like structures on a graphite surface was observed by TM-AFM after deposition of double rosette on the graphite template (first-order template effect). Also, double rosettes having gold atoms coordinated to phosphane groups form nanorod domains after deposition on HOPG template. These metal-containing nanorod arrays might constitute a viable route for the templated (second-order template effect) bottom-up fabrication of, for example, conducting nanowires.