Formation of Boronate Ester Polymers with Efficient Intrastrand Charge Transfer Transitions by Three-Component Reactions

The three-component reaction of aryl boronic acids with 1,2,4,5-tetrahydroxybenzene and 1,2-bis(4-pyridyl)ethylene or 4,4-bipyridine leads to the formation of dark-purple boronate ester polymers. Crystallographic analyses show that the polymer strands have a zig-zag geometry, and the bis(dioxaborole) units are connected by dipyridyl linkers through dative B-N interactions. Upon dissolution of the polymers in hot chloroform, most of the B-N connections are broken, which indicates that polymer formation is a reversible process. A computational study provides evidence that the strong color of the polymers is due to efficient intrastrand charge-transfer excitations from the tetraoxobenzene to the dipyridyl linker.

Self-assembly of boron-based supramolecular structures

Nicolas Christinat

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.

EPFL2008

Multicomponent Assembly of Boronic Acid-Based Macrocycles, Cages, and Polymers

Burçak Içli

This work describes the synthesis and characterization of boronic acid-based supramolecular structures. Different reversible interactions such as boronate ester formation, imine condensation and dative B–N bond formation are used to assemble macrocycles, cages and polymers. Multicomponent condensation reactions of different diamines with tetraols and formylphenylboronic acids give macrocycles via formation of imine bonds and boronate esters. The [4+2+2] condensation products are generally preferred but larger macrocycles are observed for some combinations of staring materials. The larger macrocycles are in dynamic equilibrium with the smaller [4+2+2] products. Utilization of a triamine instead of the diamine results in the formation of molecular cages. The syntheses of these cages require the formation of 18 covalent bonds between 11 building blocks, and, interestingly, can be performed in a ball mill. Multicomponent reactions of diboronic acids, catechol derivatives, and different pyridyl ligands are described. The condensation of diboronic acids with catechols gives dioxaboroles. Upon crystallization, the ester aggregates with the N-donor ligands via dative B−N bonds. Depending on the nature of the pyridyl ligand, molecularly defined macrocycles, cages or polymeric structures are obtained. One-dimensional polymers are formed with 4,4-bipyridine and 1,2-di(4-pyridyl)ethylene, whereas a two-dimensional network is obtained with the tetradentate ligand tetra(4-pyridylphenyl)ethylene. A tripyridyl linker results in the formation trigonal prismatic cages. The size of the cages can be varied by changing the diboronic acid building block. The cages are able to encapsulate polyaromatic molecules such as triphenylene or coronene. The results highlight the potential of dative B−N bonds in structural supramolecular chemistry and crystal engineering.

EPFL2012

Self-assembly of boron-based supramolecular structures

Nicolas Christinat

EPFL2008