Iridium Complexes\nwith Bidentate Pyridinylidene/<i>N</i>‑Amidate Ligands\nfor Transfer Hydrogenation Catalysis

A series of iridium pentamethylcyclopentadienyl\n(Cp*)\ncomplexes,\n[Cp*Ir(κ<sup>2</sup>-<sup><i>R</i></sup>L<sub><i>p</i>/<i>m</i></sub>)Cl], that contain the strongly\ncoordinating bidentate ligands <sup><i>R</i></sup>L<sub><i>p</i>/<i>m</i></sub> were synthesized. The\ndonor groups of the bidentate ligands were an <i>N-</i>amidate\nand either a <i>para-</i>pyridinylidene remote <i>N</i>-heterocyclic carbene (<sup><i>R</i></sup>L<sub><i>p</i></sub>) or a <i>meta-</i>pyridinylidene remote <i>N</i>-heterocyclic carbene (<sup><i>R</i></sup>L<sub><i>m</i></sub>). For each type of bidentate ligand, a set\nof iridium complexes was synthesized, which differed only according\nto the substituents (<i>R</i>) on the phenyl ring associated\nwith the amidate group. The iridium complexes were all fully characterized\nand molecular structures were obtained by single-crystal X-ray diffraction\nstudies for representative examples. The complexes were found to be\ngood precatalysts in <i>iso</i>-propanol for the transfer\nhydrogenation of benzaldehyde to give benzyl alcohol. The catalytic\nactivity correlated with the Hammett σ<sup>m/p</sup> parameters\nof the phenyl ring substituents, with more electron-donating substituents\nleading to increased catalytic activity. In all cases, the <i>meta-</i>pyridinylidene complexes, [Cp*Ir(κ<sup>2</sup>-<sup><i>R</i></sup>L<sub><i>m</i></sub>)Cl],\nperformed better than the corresponding <i>para</i> analogues,\n[Cp*Ir(κ<sup>2</sup>-<sup><i>R</i></sup>L<sub><i>p</i></sub>)Cl].