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[Ir(PCy3)2(H)2(H2B[BOND]NMe2)]+ as a Latent Source of Aminoborane: Probing the Role of Metal in the Dehydrocoupling of H3B⋅NMe2H and Retrodimerisation of [H2BNMe2]2



The IrIII fragment {Ir(PCy3)2(H)2}+ has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H3B⋅NMe2H (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(H3B⋅NMe2H)][BArF4] (2 a), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy3)2(H)2(H2B[BOND]NMe2)][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 H2B[BOND]NMe2 (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 H3B⋅NMe2BH2⋅NMe2H (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 B[BOND]N 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.