Interactions in hydrogen-bonded macrocycles

Abstract

Hydrogen bonded macrocycles are a particular type of supramolecular structures built from several monomers which are bonded by hydrogen bonds. These macrocycles can be composed by different numbers of building blocks creating cyclic structures of different sizes. The dimension of these macrocycles will rely on the geometrical requirements of the self-assembly imposed by the nature of the monomers involved. Since the synthesis of hydrogen bonded macrocycles is based on non-covalent interactions, its self-assembly is ruled by a thermodynamic controlled equilibrium that will determinate if the monomers self-assemble into the ring closed structure or the open oligomeric one. So, for the macrocycle to be synthetised, the factors ruling this thermodynamic equilibrium must be studied. For this equilibrium to be shifted into the macrocycle architecture, certain conditions regarding the structure of the monomers and the characteristics of their binding sites must be met. If these characteristics are satisfied, the overall stability of the cyclic structure will be higher than the open form and therefore, the self-assembly will be in the macrocycle architecture. The stabilizer factor of the self-assembly into the ring closed structure is known as chelate cooperativity, which can be mathematically defined by the effective molarity (EM), a value quantifying how favourable the intramolecular interaction is over the intermolecular one, or in other words, how favourable the self-assembly into the cyclic structure is over the open oligomeric one. So, the key on obtaining these hydrogen bonded macrocycles relies on molecular preorganization by using building blocks capable of enhancing EM values because its monomeric structure and binding sites characteristics can display large chelate cooperativities.