Boron-Nitrogen Double Tweezers Comprising Arylboronic Esters and Diamines: Self-Assembly in Solution and Adaptability as Hosts for Aromatic Guests in the Solid State

The thermodynamic stability of 1 : 1 and 2 : 1 boron-nitrogen (B -> N) adducts formed between aromatic boronic esters with mono- and diamines was studied in solution by NMR and UV-vis spectroscopy with association energies (Delta G degrees) ranging from -11 to -28 kJ mol(-1). The effect of different substituents in the boronic ester, the nature of the diamine linker, and the effect of the solvent was explored. Stable 2 : 1 B <- N adducts with diamines such as 1,3-diaminopropane were produced in solutions of hydrogen-bonding acceptor solvents (acetonitrile and ethyl acetate), which can be isolated in the solid state as crystalline solvates, whereas the use of noncoordinating solvents such as 1,2-dichloroethane afforded mainly 1 : 1 B <- N adducts. In suitable combinations, aromatic bis-pyridyl diamines produced stable 2 : 1 B <- N adducts that were isolated either as solvent-free solids, solvates, or cocrystals. In these crystalline forms, double-tweezer hosts were observed with an exceptional syn/anti conformational guest-adaptability driven by simultaneous donor-acceptor and C-H...pi interactions in the tweezer cavities, resembling preorganized covalent tweezer hosts. Interestingly, cocrystals with electron-rich guests such as tetrathiafulvalene and pyrene showed non-centrosymmetric crystal lattices with infinite pi-stacked donor-acceptor columns.