Heterolytic cleavage of the B–H bond in H3B·L (L = THF, NMe2H) by an electrophilic Ir(iii) pincer complex [Ir(H)(PMe3)(tBu4POCOP)][BAr F4]

Abstract

Six- and five-coordinate iridium complexes, [Ir(H)Cl(PMe₃)(^tBu4POCOP)] (2) (^tBu4POCOP = κ³-C₆H₃-2,6-(OP^tBu₂)₂) and [Ir(H)(PMe₃)(^tBu4POCOP)][BAr^F₄] (3), respectively, have been synthesized and characterized. The reactivity of the electrophilic 16-electron iridium(III) complex 3 with H₂, H₃B·THF, H₃B·NMe₂H, and H₃B·NMe₃ was investigated. The reaction of complex 3 with H₂ resulted in the formation of trans-[Ir(H)(η²-H₂)(PMe₃)(^tBu4POCOP)][BAr^F₄] complex 5. Complex 3 was found to activate the B–H bonds in H₃B·THF and H₃B·NMe₂H (DMAB) in a heterolytic fashion. The reaction of complex 3 with H₃B·THF at 298 K afforded trans-[Ir(H)₂PMe₃(^tBu4POCOP)] complex 6 and a boronium compound, [H₂B(THF)₂][BAr^F₄]. Monitoring the reaction from 179 to 298 K by NMR spectroscopy revealed the formation of a σ-borane intermediate, trans-[Ir(H)(η¹-HBH₂·THF)PMe₃(^tBu4POCOP)][BAr^F₄] (3a-Int), en route to the final products observed at 298 K. The formation of this intermediate species was also investigated by density functional theory (DFT) calculations. The reaction of complex 3 with H₃B·NMe₂H at 298 K yielded a mixture of complexes 3 and 5 and a cyclic diborazane [H₂BNMe₂]₂. The reaction was found to proceed via intermediates, a σ-borane complex, trans-[Ir(H)(η¹-HBH₂·NMe₂H)PMe₃(^tBu4POCOP)][BAr^F₄] (3b), complex 6, [(NHMe₂)₂BH₂][BAr^F₄], and H₂BNMe₂. Complex 3 exhibited no reactivity with H₃B·NMe₃.