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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Abstract
The
dinuclear iridium hydride complexes [IrH(CH<sub>3</sub>CN)(L1)(μ-H)]<sub>2</sub>(BAr<sub>F</sub>)<sub>2</sub> (<b>7</b>; L1 =
(<i>S</i>)-2-(2-((diphenylphosphanyl)oxy)propan-2-yl)-4-isopropyl-4,5-dihydrooxazole,
BAr<sub>F</sub> = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate),
[IrH(CH<sub>2</sub>Cl<sub>2</sub>)(L1)(μ-H)]<sub>2</sub>(BAr<sub>F</sub>)<sub>2</sub> (<b>8</b>), [IrH(L2)(μ-H)]<sub>2</sub>(BAr<sub>F</sub>)<sub>2</sub> (<b>9a</b>; L2 =
(<i>S</i>)-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)]<sub>2</sub>(BAr<sub>F</sub>)<sub>2</sub> (<b>9b</b>; L3 = (<i>S</i>)-1-[2-(2-<i>tert</i>-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)]BAr<sub>F</sub> precursors by treatment with H<sub>2</sub> 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(OEt<sub>2</sub>)<sub>2</sub>]BAr<sub>F</sub> or acetonitrile. Reactions
with acetonitrile or 6,6′-bi-2-picoline afforded the mononuclear
iridium dihydride complexes [Ir(H)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>(L1)]BAr<sub>F</sub> (<b>5</b>), [Ir(H)<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>(L3)]BAr<sub>F</sub> (<b>10</b>), or [Ir(H)<sub>2</sub>(6,6′-bi-2-picoline)(L3)]BAr<sub>F</sub> (<b>11</b>). The CH<sub>3</sub>CN complexes <b>7</b> and <b>10</b> are inactive as hydrogenation catalysts. In
contrast, the coordinatively unsaturated dinuclear complexes <b>9a</b> and <b>9b</b> are active catalysts for the hydrogenation
of (<i>E</i>)-1,2-diphenyl-1-propene at 50 bar hydrogen
pressure
