2 research outputs found
Mechanistic Insights into Transfer Hydrogenation Catalysis by [Ir(cod)(NHC)<sub>2</sub>]<sup>+</sup> Complexes with Functionalized NāHeterocyclic Carbene Ligands
The synthesis of unbridged biscarbene
iridiumĀ(I) [IrĀ(cod)Ā(MeImā©Z)<sub>2</sub>]<sup>+</sup> complexes
having N- or O-functionalized NHC
ligands (ā©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
CĀ(carbene)āIr bond that results in two interconverting diasteromers
having different dispositions of the functionalized NHC ligands. A
rotational barrier of around 80 kJ mol<sup>ā1</sup> (298 K)
has been determined by 2D EXSY NMR spectroscopy. The iridiumĀ(III)
dihydride complex [IrH<sub>2</sub>(MeImā©Z)<sub>2</sub>]<sup>+</sup> (ā©Z = pyridin-2-ylmethyl) has been prepared by reaction
of the corresponding iridiumĀ(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 Ā°C
with average TOF values of 117ā155 h<sup>ā1</sup> 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 iridiumĀ(I) intermediates in these
catalytic systems. DFT calculations have shown that a MPV-like concerted
mechanism (MeerweināPonndorfāVerley 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)]<sup>+</sup> (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH<sub>3</sub>)(cod)(MeImR)]<sup>+</sup> (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<sup>ā1</sup>. The complexes [Ir(NCCH<sub>3</sub>)(cod)(MeImR)]<sup>+</sup> (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, Ī±,Ī²-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 Ī²-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 Ī²-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 Ī²-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