N‑Benzyl Substituted N‑Heterocyclic Carbene Complexes of Iridium(I): Assessment in Transfer Hydrogenation Catalyst

Iridium­(I)
complexes of N-heterocyclic carbenes (NHCs) (1a–3a) were obtained by transmetalation reactions from the corresponding
Ag­(I)-NHC complexes. These complexes have been fully characterized
by 1H, 13C, heteronuclear multiple-bond correlation
NMR spectroscopies, and elemental analysis. X-ray diffraction studies
on single crystals of 1a and 2a confirm
the square planar geometry at the metal center. [IrCl­(CO)2(NHC)] complexes 1b–3b were also
synthesized to compare σ-donor/π-acceptor strength of
NHC ligands. Transfer hydrogenation (TH) reactions of various ketones
and imines have been studied using complexes 1a–3a as precatalysts. N-Benzyl substituted NHC complexes of
Ir­(I) proved to be highly efficient precatalysts in the reduction
of aromatic and aliphatic ketones to afford the corresponding alcohol
products with turnover frequencies values up to 24 000 h–1.