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.