Characterization and Reactivity Studies of Dinuclear Iridium Hydride Complexes Prepared from Iridium Catalysts with N,P and C,N Ligands Under Hydrogenation Conditions

The dinuclear iridium hydride complexes [IrH(CH3CN)(L1)(μ-H)]2(BArF)2 (7; L1 = (S)-2-(2-((diphenylphosphanyl)oxy)propan-2-yl)-4-isopropyl-4,5-dihydrooxazole, BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), [IrH(CH2Cl2)(L1)(μ-H)]2(BArF)2 (8), [IrH(L2)(μ-H)]2(BArF)2 (9a; L2 = (S)-1-[2-(2-adamantan-2-yl-4,5-dihydrooxazol-4-yl)-ethyl]-3-(2,6-diisopropylphenyl)-1,2-dihydroimidazol-2-ylidene), and [IrH(L3)(μ-H)]2(BArF)2 (9b; L3 = (S)-1-[2-(2-tert-butyl-4,5-dihydrooxazol-4-yl)-ethyl]-3-(2,6-diisopropylphenyl)-1,2-dihydroimidazol-2-ylidene) were prepared from the corresponding mononuclear [Ir(COD)(L)]BArF precursors by treatment with H2 and characterized by 2D NMR spectroscopy and X-ray diffraction. Conversion to a trinuclear iridium hydride complex, which is usually observed for N,P iridium hydride complexes, is inhibited by addition of 0.5 equiv of [H(OEt2)2]BArF or acetonitrile. Reactions with acetonitrile or 6,6′-bi-2-picoline afforded the mononuclear iridium dihydride complexes [Ir(H)2(CH3CN)2(L1)]BArF (5), [Ir(H)2(CH3CN)2(L3)]BArF (10), or [Ir(H)2(6,6′-bi-2-picoline)(L3)]BArF (11). The CH3CN complexes 7 and 10 are inactive as hydrogenation catalysts. In contrast, the coordinatively unsaturated dinuclear complexes 9a and 9b are active catalysts for the hydrogenation of (E)-1,2-diphenyl-1-propene at 50 bar hydrogen pressure.