Fine-Tuning of Lewis Acidity: The Case of Borenium Hydride Complexes Derived from Bis(phosphinimino)amide Boron Precursors

Reactions of bis(phosphinimino)amines LH and L′H with Me₂S⋅BH₂Cl afforded chloroborane complexes LBHCl (1) and L′BHCl (2), and the reaction of L′H with BH₃⋅Me₂S gave a dihydridoborane complex L′BH₂(3) (LH=[{(2,4,6-Me₃C₆H₂N)P(Ph₂)}₂N]H and L′H=[{(2,6-iPr₂C₆H₃N)P(Ph₂)}₂N]H). Furthermore, abstraction of a hydride ion from L′BH₂(3) and LBH₂(4) mediated by Lewis acid B(C₆F₅)₃or the weakly coordinating ion pair [Ph₃C][B(C₆F₅)₄] smoothly yielded a series of borenium hydride cations: [L′BH]⁺[HB(C₆F₅)₃]⁻(5), [L′BH]⁺[B(C₆F₅)₄]⁻(6), [LBH]⁺[HB(C₆F₅)₃]⁻(7), and [LBH]⁺[B(C₆F₅)₄]⁻(8). Synthesis of a chloroborenium species [LBCl]⁺[BCl₄]⁻(9) without involvement of a weakly coordinating anion was also demonstrated from a reaction of LBH₂(4) with three equivalents of BCl₃. It is clear from this study that the sterically bulky strong donor bis(phosphinimino)amide ligand plays a crucial role in facilitating the synthesis and stabilization of these three-coordinated cationic species of boron. Therefore, the present synthetic approach is not dependent on the requirement of weakly coordinating anions; even simple BCl₄⁻can act as a counteranion with borenium cations. The high Lewis acidity of the boron atom in complex 8 enables the formation of an adduct with 4-dimethylaminopyridine (DMAP), [LBH⋅(DMAP)]⁺[B(C₆F₅)₄]⁻(10). The solid-state structures of complexes 1, 5, and 9 were investigated by means of single-crystal X-ray structural analysis.