Mechanistic Investigations of Imine Hydrogenation Catalyzed by Cationic Iridium Complexes

Complexes [IrH2(η6‐C6H6)(PiPr3)]BF4 (1) and [IrH2(NCMe)3(PiPr3)]BF4 (2) are catalyst precursors for homogeneous hydrogenation of N‐benzylideneaniline under mild conditions. Precursor 1 generates the resting state [IrH2{η5‐(C6H5)NHCH2Ph}(PiPr3)]BF4 (3), while 2 gives rise to a mixture of [IrH{PhNCH(C6H4)‐κN,C}(NCMe)2(PiPr3)]BF4 (4) and [IrH{PhNCH(C6H4)‐κN,C}(NCMe)(NH2Ph)(PiPr3)]BF4 (5), in which the aniline ligand is derived from hydrolysis of the imine. The less hindered benzophenone imine forms the catalytically inactive, doubly cyclometalated compound [Ir{HNCPh(C6H4)‐κN,C}2(NH2CHPh2)(PiPr3)]BF4 (6). Hydrogenations with precursor 1 are fast and their reaction profiles are strongly dependent on solvent, concentrations, and temperature. Significant induction periods, minimized by addition of the amine hydrogenation product, are commonly observed. The catalytic rate law (THF) is rate = k[1][PhNCHPh]p(H2). The results of selected stoichiometric reactions of potential catalytic intermediates exclude participation of the cyclometalated compounds [IrH{PhNCH(C6H4)‐κN,C}(S)2(PiPr3)]BF4 [S=acetonitrile (4), [D6]acetone (7), [D4]methanol (8)] in catalysis. Reactions between resting state 3 and D2 reveal a selective sequence of deuterium incorporation into the complex which is accelerated by the amine product. Hydrogen bonding among the components of the catalytic reaction was examined by MP2 calculations on model compounds. The calculations allow formulation of an ionic, outer‐sphere, bifunctional hydrogenation mechanism comprising 1) amine‐assisted oxidative addition of H2 to 3, the result of which is equivalent to heterolytic splitting of dihydrogen, 2) replacement of a hydrogen‐bonded amine by imine, and 3) simultaneous Hδ+/Hδ− transfer to the imine substrate from the NH moiety of an arene‐coordinated amine ligand and the metal, respectively. An efficient CN hydrogenation catalyst is generated from the labile precursor [IrH2(η6‐C6H6)(PiPr3)]BF4 in N‐benzylideneaniline hydrogenations by exploiting the coordination and hydrogen‐bonding capabilities of the amine reaction product. The deduced hydrogenation mechanism is ionic, outer‐sphere, and bifunctional, but features a concerted dihydrogen oxidative‐addition step. The picture shows the structures of transition states located by calculations on model compounds.