Iridium(I) Complexes with Hemilabile N-Heterocyclic Carbenes: Efficient and Versatile Transfer Hydrogenation Catalysts

A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)](+) (R = pyridin-2-ylinethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH3)(cod)(MeImR)](+) (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX( cod) (MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(Melm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h(-1). The complexes [Ir(NCCH3) (cod) (MeImR)](+) (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, alpha,beta-unsaturated ketones, and mimes. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [h(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod) (MeImR)]. The beta-H elimination in the alkoxo complex [Ir(OiPr(cod)(Melm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the beta-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the beta-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity.