Iridium Complexes with Bidentate Pyridinylidene/ N -Amidate Ligands for Transfer Hydrogenation Catalysis.

A series of iridium pentamethylcyclopentadienyl (Cp*) complexes, [Cp*Ir(κ 2 - R L p / m )Cl], that contain the strongly coordinating bidentate ligands R L p / m were synthesized. The donor groups of the bidentate ligands were an N- amidate and either a para- pyridinylidene remote N -heterocyclic carbene ( R L p ) or a meta- pyridinylidene remote N -heterocyclic carbene ( R L m ). For each type of bidentate ligand, a set of iridium complexes was synthesized, which differed only according to the substituents ( R ) on the phenyl ring associated with the amidate group. The iridium complexes were all fully characterized and molecular structures were obtained by single-crystal X-ray diffraction studies for representative examples. The complexes were found to be good precatalysts in iso -propanol for the transfer hydrogenation of benzaldehyde to give benzyl alcohol. The catalytic activity correlated with the Hammett σ m/p parameters of the phenyl ring substituents, with more electron-donating substituents leading to increased catalytic activity. In all cases, the meta- pyridinylidene complexes, [Cp*Ir(κ 2 - R L m )Cl], performed better than the corresponding para analogues, [Cp*Ir(κ 2 - R L p )Cl].