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.