Pyridine-Based N-Heterocyclic Carbene Hydride Complexes of Iridium via C−H Activation.

A series of iridium(III) hydride complexes of chelating pyridine-based N-heterocyclic carbenes (py-NHCs) and remote N-heterocyclic carbenes (remote py-NHCs) were synthesized as stable products from the C−H oxidative addition reactions of pyridine-tethered pyridinium halides and [Ir(COD)2]+. X-ray crystal structures of representative iridium py-NHC and remote py-NHC complexes were determined and in all cases mutual cisorientations of the halide and hydride ligands were observed in the solid state. In the solution phase, the cisisomer was observed as the only product for each iridium(III) py-NHC complex, whereas both cisand transisomers have been observed as equilibration mixtures for iridium(III) remote py-NHC complexes. No β-insertion of the hydride into a CC bond of the COD unit of iridium(III) py-NHC hydride complexes was observed, and theoretical studies indicate that this step is thermodynamically unfavorable. Iridium(III) py-NHC hydride complexes have shown high catalytic activity in transfer hydrogenation of ketone and enones. Computational studies of the donor capacity of those ligands indicate that py-NHCs are more donating than imidazole-derived NHCs. [ABSTRACT FROM AUTHOR]