Self-assembly of boron-based supramolecular structures

Abstract

This work describes the synthesis and characterization of boronic acid-based supramolecular structures. Macrocycles, dendritic structures, polymers, rotaxanes, and cages were assembled using four types of reversible reactions. The key point of the strategy is the parallel utilization of two –or more– of these reactions. Initially, aryl and alkylboronic acids were condensed with dihydroxypyridine ligands to give tetrameric or pentameric macrocycles, in which four or five boronate esters are connected by dative B-N bonds. These macrocycles were then used as scaffolds for the assembly of more complex structures from the multicomponent reaction of formyl functionalized boronic acids, with dihydroxypyridine ligands and primary amines. Dendritic structures having a tetrameric or pentameric macrocyclic core and four, five, eight, or ten amine-derived groups in their periphery were obtained. Three-component reactions were further used to prepare boronate ester polymers from aryl boronic acids, 1,2,4,5-tetrahydroxybenzene and either 1,2-di(4-pyridyl)ethylene or 4,4'-dipyridine. Crystallographic analyses show that the bis(dioxaborole) units are connected by dipyridyl linkers via dative B-N interactions. A computational study provides evidence that the polymers are strongly colored due to efficient intrastrand charge transfer excitations from the tetraoxobenzene to the dipyridyl linker. This latter property was used to assemble the first boron-based rotaxanes from 1,2-di(4-pyridyl)ethylene, catechol, 3,5-bis(trifluoromethyl)phenylboronic acid and 1,5-dinaphto-38-crown-10 or bis-p-phenylene-34-crown-10. In the last part of this work, boronate ester condensations were combined with imine condensations to build organic macrocycles and cages. The former interaction was also used together with metal-ligand interactions to prepare rhenium-based macrocycles. Finally, a nanometer-sized macrocycle was obtained in one step from four chemically distinct building blocks via the simultaneous utilization of the three reversible reactions.