Iridium(I) complexes with hemilabile N-heterocyclic carbenes: Efficient and versatile transfer hydrogenation catalysts

16 páginas, 5 figuras, 3 tablas, 3 esquemas.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-ylmethyl, 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)(MeIm(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, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(OiPr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-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 β-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.Financial support from Ministerio de Ciencia e Innovación (MICINN/FEDER) is gratefully acknowledged (Projects: CTQ2010-15221, CSD2006-0015, and CSD2009-00050). J.F. T. thanks the Spanish MICINN for a predoctoral fellowship, and S.W. thanks the Eramus program (Univ. RWTH-Aachen, Germany).Peer reviewe