Mechanism of Dimethylamine–Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex

The title complex (^<i>t</i>BuPCP)­IrH­(Cl) (<b>1</b>; ^<i>t</i>BuPCP = κ³-2,6-(CH₂P<i>t</i>Bu₂)₂C₆H₃) appeared to be moderately active in NHMe₂·BH₃ (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (^tBuPCP)­IrHCl­(η¹-BH₃·NHMe₂) (<b>3</b>) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (^tBuPCP)­IrH₂(η¹-BH₃·NHMe₂) complex (<b>6</b>). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and <i>tert</i>-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of <b>1</b>) the hydride complex (^tBuPCP)­IrH­(μ²-H₂BH₂) (<b>5</b>) was identified, which is not a dormant catalytic species but the steady-state intermediate formed as a result of the B–N bond breaking. DMAB dehydrogenation yields the borazane monomer H₂BNMe₂ (detected by ¹¹B NMR); dimerization of this species gives the final product [H₂BNMe₂]₂ and (^tBuPCP)­IrH₄ as the catalyst resting state. The scenario of B–N bond cleavage in DMAB leading to byproducts of dehydrogenation such as bis­(dimethylamino)­hydroborane and (^tBuPCP)­IrH­(μ²-H₂BH₂) (<b>5</b>) is proposed. The results obtained allow us to suggest the mechanism of catalytic DMAB dehydrocoupling that could be generalized to other processes