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

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journal contribution
posted on 19.02.2016, 00:04 authored by Stefan Gruber, Markus Neuburger, Andreas Pfaltz
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.