Catalyst Activation by Loss of Cyclopentadienyl Ligands
in Hydrogen Transfer Catalysis with Cp*Ir<sup>III</sup> Complexes
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Abstract
The activity of the two related complexes
[Cp*Ir(IMe)<sub>2</sub>X]BF<sub>4</sub> (X = Cl (<b>1</b>),
H (<b>2</b>)) in
transfer hydrogenation from isopropyl alcohol to acetophenone was
investigated. The results suggest that the commonly accepted monohydride
mechanism for transfer hydrogenation mediated by cyclopentadienyl
iridium species does not apply to chloride <b>1</b>. We have
found evidence that, although the two monodentate NHC ligands are
retained in the coordination sphere, the Cp* ligand is completely
released under mild conditions in a precatalytic activation step.
Synthesis of modified versions of the initial precatalyst <b>1</b> with different cyclopentadienyl and NHC ligands demonstrated that
increasing the steric pressure around the iridium center facilitates
precatalyst activation and thus enhances the catalytic performance.
Study of five new iridium(III) complexes bearing mono- or diphosphines
helped us monitor Cp* ligand loss under mild conditions. An unusual
P–C bond cleavage was also noted in a 1,2-bis(dimethylphosphino)methane
(dmpm) ligand. On the basis of these findings, a novel catalyst activation
mechanism is proposed for [(η<sup>5</sup>-C<sub>5</sub>R<sub>5</sub>)Ir] transfer hydrogenation based on the lability of the cyclopentadienyl
ligand