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

The title complex ((PCP)-P-tBu)IrH(Cl) (1; (PCP)-P-tBu = kappa(3)-2,6-(CH2PtBu2)(2)C6H3) appeared to be moderately active in NHMe2 center dot 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 ((PCP)-P-tBu)IrHCl(eta(1)-BH3 center dot NHMe2) (3) stabilized by a NH center dot center dot center dot 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, ((PCP)-P-tBu)IrH2(eta(1)-BH3 center dot NHMe2) complex (6). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine-borane and tert-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2-5 mol % of 1) the hydride complex ((PCP)-P-tBu)IrH(mu(2)-H2BH2) (5) 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 B-11 NMR); dimerization of this species gives the final product [H2BNMe2](2) and ((PCP)-P-tBu)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 ((PCP)-P-tBu)IrH(mu(2)-H2BH2) (5) is proposed. The results obtained allow us to suggest the mechanism of catalytic DMAB dehydrocoupling that could be generalized to other processes.