Boron-Nitrogen Dative Bonds in Structural Supramolecular Chemistry

This work describes the use of boron-nitrogen dative bonds in supramolecular chemistry. Molecular tectons have been synthesized for crystal engineering. These tectons are linked via B-N dative bonds to form 1-dimensional polymers and macrocycles. B-N dative bonds were further used for the formation of crystalline organic networks by assembly of a tritopic boronate ester with ditopic N-donor ligands. These networks could be structurally characterized by single crystal X-ray diffraction to reveal 2-dimensional polymers. These structures were observed by PXRD to collapse upon removal of solvent. The reaction of a larger triboronate ester with 4,4'-bipyridine resulted in an organogel which formed with as little as 0.5 wt% of the network in toluene. The orange gel exhibited thermochromic properties, with a Tgel of 60 °C. Association constants for the formation of B-N dative bonds between dioxaborole and pyridyl ligands were examined by 1H NMR and ITC titrations. Ka values in excess of 106 M-1 can be obtained by varying the electronics of the reaction partners. The adducts have been characterized comprehensively by single crystal X-ray diffraction. Finally, the dative B-N bond was used for the synthesis of main-chain supramolecular polymers. Self-complementary monomers were synthesized containing a dialkylaminopyridine and a dioxaborole. These monomers assembled in chloroform to form polymers which were characterized by viscosity and DOSY NMR experiments.

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

Multicomponent Assembly of Macrocycles and Polymers by Coordination of Pyridyl Ligands to 1,4-Bis(benzodioxaborole)benzene

Burçak Içli, Benan Kilbas, Rosario Scopelliti, Kay Severin, Euro Solari

Multicomponent reactions between 1,4-benzenediboronic acid, catechol, and different pyridyl ligands are reported. The condensation of 1,4-benzenediboronic acid with catechol gives 1,4-bis(benzodioxaborole)benzene. Upon crystallization, the ester aggregates with the N-donor ligands through dative BN bonds. Depending on the nature of the pyridyl ligand, molecularly defined macrocycles or polymeric structures are obtained. 1D polymers are formed with 4,4-bipyridine and 1,2-di(4-pyridyl)ethylene, whereas a 2D network is obtained with the tetradentate ligand tetra(4-pyridylphenyl)ethylene. These results highlight the utility of dative BN bonds in structural supramolecular chemistry and crystal engineering.

Wiley-V C H Verlag Gmbh2012

Crystalline and Soft Materials Based on Dative Boron-Nitrogen Bonds

Nicolas Luisier

The thesis describes the utilization of dative boron-nitrogen bond is used for the construction of new materials. In chapter 2.1, the synthesis of new organogels based on bisimidazolyl ligands and diboronate esters is described. Gels with a critical gel concentration as low as 0.02 wt% were obtained. The high tunability of the system allowed the development of a light-responsive gel by means of an azobenzene spacer. Structural characterization was performed by electron microscopy, rheology and X-ray analysis to better understand the aggregation of the polymer components. The association of imidazole and arylboronate esters is investigated with isothermal titration calorimetry and NMR titrations. It is shown that the association constant of imidazole is on average two orders of magnitude higher than the one of pyridyl analogues. In the second part of this chapter, the simultaneous utilization of boronate esters, imine bonds and dative boron-nitrogen bonds was exploited for the fabrication of a four-component organogel in one step. It is demonstrated that each reversible interaction can be addressed selectively by using an appropriate stimulus. In one case, the mechanical resistance of the gel could be significantly increased by a transesterification reaction between two catechols. The synthesis of self-complementary monomers for the formation of supramolecular polymers in solution is described in chapter 2.2. Analysis by DOSY NMR revealed the formation of large assemblies from monomers composed of two imidazolyl end groups and one boronate ester. In the last chapter, the directionality of the boron-nitrogen bond is explored through the de-sign and synthesis of two new tripyridyl ligands and their coordination to diboronate esters. The formation of crystalline polymers is studied by X-ray crystallography. For the first time, polymers composed of chains of macrocycles based on boron-nitrogen bond are observed.

EPFL2016

Crystalline and Soft Materials Based on Dative Boron-Nitrogen Bonds

Nicolas Luisier

EPFL2016