Mechanistic Insights into Transfer Hydrogenation Catalysis by [Ir(cod)(NHC)₂]⁺ Complexes with Functionalized N‑Heterocyclic Carbene Ligands

The synthesis of unbridged biscarbene iridium­(I) [Ir­(cod)­(MeIm∩Z)₂]⁺ complexes having N- or O-functionalized NHC ligands (∩Z = 2-methoxybenzyl, pyridin-2-ylmethyl, quinolin-8-ylmethyl) is described. The molecular structures of the complexes show an antiparallel disposition of the carbene ligands that minimize the steric repulsions between the bulky substituents. However, the complexes were found to be dynamic in solution, due to the restricted rotation about the C­(carbene)–Ir bond that results in two interconverting diasteromers having different dispositions of the functionalized NHC ligands. A rotational barrier of around 80 kJ mol^–1 (298 K) has been determined by 2D EXSY NMR spectroscopy. The iridium­(III) dihydride complex [IrH₂(MeIm∩Z)₂]⁺ (∩Z = pyridin-2-ylmethyl) has been prepared by reaction of the corresponding iridium­(I) complex with molecular hydrogen. These complexes efficiently catalyzed the transfer hydrogenation of cyclohexanone using 2-propanol as a hydrogen source and KOH as base at 80 °C with average TOF values of 117–155 h^–1 at 0.1 mol % iridium catalyst loading. All of the catalyst precursors showed comparable activity independent of both the wingtip type at the NHC ligands and the counterion. Mechanistic studies support the involvement of diene free bis-NHC iridium­(I) intermediates in these catalytic systems. DFT calculations have shown that a MPV-like concerted mechanism (Meerwein–Ponndorf–Verley mechanism), involving the direct hydrogen transfer at the coordination sphere of the iridium center, might compete with the well-established hydrido mechanism. Indirect evidence of a MPV-like mechanism has been found for the catalyst precursor having NHC ligands having with a pyridin-2-ylmethyl wingtip

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-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(O<i>i</i>Pr)(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