Iridium complexes with α-diimines: Catalytic hydrogen transfer from isopropanol to cyclohexanone

Cationic Iridium Complexes Bearing Imidazol-2-ylidene Ligands as Transfer Hydrogenation Catalysts

“…Catalytic Transfer Hydrogenation. Complexes 1 − 5 , together with their precursors [Ir(cod)Cl] 2 and [Ir(cod)(py) 2 ]PF 6 and complexes formed in situ from [Ir(cod)(py) 2 ]PF 6 and diazabutadiene ligands (RNCHCHNR, DAB−R; R = cyclohexyl, DAB-Cy; R = 2,4,6-trimethylphenyl, DAB-Mes; R = adamantyl, DAB-Ad; R = 2,4,6-trimethoxyphenyl, DAB-trimethoxyphenyl) for comparison, were tested for catalytic activity in the hydrogen transfer reduction of cyclohexanone to cyclohexanol, employing 2-propanol as hydrogen donor. The required activation time was 30 min and involved heating a solution of catalyst and base (KOH) in 2-propanol at 80 °C, which was followed by addition of the substrate.…”

IrI(η4-diene) precatalyst activation by strong bases: formation of an anionic IrIII tetrahydride

“…This second resonance disappears upon addition of further dppe to readjust the dppe/Ir stoichiometry to 1 : 1. On the basis of the known behaviour for several other bidentate ligands, for instance α-diimines, 62 bis-and tris-pyrazoles, 63,64 phosphine-thioethers, 42 and also for one diphosphine, 65 this is assigned to the salt [Ir(dppe) (COD)] + [IrCl 2 (COD)] − . Addition of more dppe beyond the 1 : 1 ratio maintained the sharp 31 P{ 1 H} resonance of [IrCl(COD) (dppe)], but also generated an additional resonance at δ 50.0 (in C 6 D 6 ), which we attribute to [IrCl(dppe) 2 ] on the basis of the follow-up reaction in the presence of base (vide infra).…”