[Ir(PCy₃)₂(H)₂(H₂BNMe₂)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H₃BNMe₂H and Retrodimerisation of [H₂BNMe₂]₂

摘要

The IrIII fragment {Ir(PCy₃)₂(H)₂}+ has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H₃BNMe₂H (A) to ultimately give dimeric aminoborane [H₂BNMe₂]₂ (D). Addition of A to [Ir(PCy₃)₂(H)₂(H₂)₂][BArF₄] (1; ArF=(C₆H₃(CF₃)₂), gives the amine-borane complex [Ir(PCy₃)₂(H)₂(H₃BNMe₂H)][BArF₄] (2?a), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy₃)₂(H)₂(H₂BNMe₂)][BArF₄] (3). DFT calculations have been used to probe the mechanism of dehydrogenation and show a pathway featuring sequential BH activation/H₂ loss/NH activation. Addition of D to 1 results in retrodimerisation of D to afford 3. DFT calculations indicate that this involves metal trapping of the monomer–dimer equilibrium, 2?H₂BNMe₂ ? [H₂BNMe₂]₂. Ruthenium and rhodium analogues also promote this reaction. Addition of MeCN to 3 affords [Ir(PCy₃)₂(H)₂(NCMe)₂][BArF₄] (6) liberating H₂BNMe₂ (B), which then dimerises to give D. This is shown to be a second-order process. It also allows on- and off-metal coupling processes to be probed. Addition of MeCN to 3 followed by A gives D with no amine-borane intermediates observed. Addition of A to 3 results in the formation of significant amounts of oligomeric H₃BNMe₂BH₂NMe₂H (C), which ultimately was converted to D. These results indicate that the metal is involved in both the dehydrogenation of A, to give B, and the oligomerisation reaction to afford C. A mechanism is suggested for this latter process. The reactivity of oligomer C with the Ir complexes is also reported. Addition of excess C to 1 promotes its transformation into D, with 3 observed as the final organometallic product, suggesting a BN bond cleavage mechanism. Complex 6 does not react with C, but in combination with B oligomer C is consumed to eventually give D, suggesting an additional role for free aminoborane in the formation of D from C.