Self-Assembly of Triphenylboroxine and the Phenylboronic Ester of Pentaerythritol with Piperazine, trans-1,4-Diaminocyclohexane, and 4-Aminopyridine

The dinuclear phenylboronic ester derived from pentaerythritol and trinuclear triphenylboroxine were combined with three diamine tectons, that is, 1,4-diazacyclohexane (pz), trans-1,4-diaminocyclohexane (1,4-chda), and 4-aminopyridine (4-apy), to generate supramolecular N→B bound assemblies and to enhance the knowledge concerning the factors governing the formation of such aggregates. From these reactions, three novel complexes of composition {(PhBO)₃(pz)}_n·nDMF (2), {[(PhBO)₃]₂(1,4-chda)}·1,4-chda (3), and {[(PhBO₂)₂(C₅H₈)][4-apy]₂}·CHCl₃·1.25H₂O (4) were achieved and characterized by elemental analysis, IR and NMR spectroscopy, and single-crystal X-ray diffraction analysis. Structural characterization in the solid state revealed that all three products exhibit N→B bonds but have different compositions concerning the B and N tectons to give a 1:1 adduct for 2, a 2:1 adduct for 3, and a 1:2 adduct for 4. In the solid state, compound 2 comprises a 1D coordination polymer, whereas compounds 3 and 4 have discrete molecular structures. Owing to the presence of N-H hydrogen-bonding sites, in all cases overall 2D or 3D hydrogen-bonding networks are formed. In solution, the N→B aggregates are mostly dissociated at room temperature, as shown by ¹¹B NMR spectroscopy. In combination with representative diamines, di- and trinuclear boron tectons 1a and 1b generate either discrete or infinite N→B bound assemblies, which are further connected through hydrogen-bonding interactions.