Unlocking the chemistry of the octahydrotriborate anion: robust synthesis, structural elucidation, and unprecedented protonation stability of diamine-substituted derivatives

Abstract

A novel synthetic methodology for the preparation of amino-substituted derivatives of the octahydrotriborate anion has been developed, based on the direct reaction of the [B₃H₈]⁻ anion with amine hydrochlorides. Additionally, the established approach involving the formation of the key [B₃H₇THF] intermediate in toluene, using zirconium(IV) chloride as a halide source, has been significantly optimised. These advanced strategies enabled the synthesis of a comprehensive series of previously inaccessible substituted triborane derivatives with various diamines [B₃H₇Nu], where Nu is ethylenediamine, 1,2-propylenediamine, N,N-dimethylethylenediamine (DMEDA), N,N-diethylethylenediamine (DEEDA), 1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine, 1,12-dodecanediamine, o-phenylenediamine (OPDA) and N-phenyl-1,2-phenylenediamine. The versatility of these compounds was further demonstrated through their conversion to stable protonated salts. The molecular structures of four representative examples—[B₃H₇NH₂C₆H₁₁], [B₃H₇NH₂(C₆H₄)NHC₆H₅], [B₃H₇NH₂C₂H₄NH(C₂H₅)₂]Cl and [B₃H₇NH₂C₃H₆NH(CH₃)₂]I and the monoborane byproduct [BH₂(CyNH₂)₂]Cl—were unambiguously determined by single-crystal X-ray diffraction, providing definitive structural insight into this new class of boron–nitrogen compounds and confirming the coordination mode and supramolecular architecture.