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

Abstract

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)3 or 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.