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

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

The reaction between [IrCl(COD)]₂ and dppe in a 1 : 2 ratio was investigated in detail under three different conditions. [IrCl(COD)(dppe)], 1, is formed at room temperature in the absence of base. In the presence of a strong base at room temperature, hydride complexes that retain the carbocyclic ligand in the coordination sphere are generated. In isopropanol, 1 is converted into [IrH(1,2,5,6-η²:η²-COD)(dppe)] (2) on addition of KO^tBu, with k₁₂ = (1.11 ± 0.02) × 10^-4 s^-1, followed by reversible isomerisation to [IrH(1-κ-4,5,6-η³-C₈H₁₂)(dppe)] (3) with k₂₃ = (3.4 ± 0.2) × 10^-4 s^-1 and k₃₂ = (1.1 ± 0.3) × 10^-5 s^-1 to yield an equilibrium 5 : 95 mixture of 2 and 3. However, when no hydride source is present in the strong base (KO^tBu in benzene or toluene), the COD ligand in 1 is deprotonated, followed by β-H elimination of an Ir^I-C₈H₁₁ intermediate, which leads to complex [IrH(1-κ-4,5,6-η³-C₈H₁₀)(dppe)] (4) selectively. This is followed by its reversible isomerisation to 5, which features a different relative orientation of the same ligands (k₄₅ = (3.92 ± 0.11) × 10^-4 s^-1; k_5-4 = (1.39 ± 0.12) × 10^-4 s^-1 in C₆D₆), to yield an equilibrated 32 : 68 mixture of 4 and 5. DFT calculations assisted in the full rationalization of the selectivity and mechanism of the reactions, yielding thermodynamic (equilibrium) and kinetic (isomerization barriers) parameters in excellent agreement with the experimental values. Finally, in the presence of KO^tBu and isopropanol at 80 °C, 1 is transformed selectively to K[IrH₄(dppe)] (6), a salt of an anionic tetrahydride complex of Ir^III. This product is also selectively generated from 2, 3, 4 and 5 and H₂ at room temperature, but only when a strong base is present. These results provide an insight into the catalytic action of [IrCl(COD)(LL)] complexes in the hydrogenation of polar substrates in the presence of a base.