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

Abstract

The synthesis of unbridged biscarbene iridium(I) [Ir(cod)(MeIm∩Z) 2 ] + complexes having Nor O-functionalized NHC ligands (∩Z = 2-methoxybenzyl, pyridin-2ylmethyl and 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 disposition 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 [IrH 2 (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 hydrogen source and KOH as base at 80 °C with average TOFs of 117-155 h-1 at 0.1 mol% iridium catalyst loading. All the catalyst precursors showed comparable activity independent both of the wingtip type at the NHC ligands or 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.

An Insight into Transfer Hydrogenation Reactions Catalysed by Iridium(III) Bis‐N‐heterocyclic Carbenes

“…Rhodium complex 6 was tested in order to assess the influence of the metal centre on the catalytic activity. Analogously to what has been reported previously for NHC complexes,3,12,10d 6 is markedly less active than its iridium counterpart 5 , the latter being the most active complex presented in this work.…”

Small “Yaw” Angles, Large “Bite” Angles and an Electron‐Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride‐Transfer Activity

“…The plausible reason for enhanced activity of the conformationally less‐strained (with small yaw and large bite angles) and electron‐rich complexes might be a relatively easy hydride transfer from the in situ generated iridium hydride species to the ketone substrate, probably by means of an outer‐sphere mechanism during catalysis (Figure ). For NHC‐based Cp*Ir catalytic systems, an inner‐sphere MPV (Meerwein–Ponndorf–Verley)‐type concerted mechanism has been reported earlier, but this type of mechanism relies on generating additional vacant coordination site at the metal center by decoordination of the cyclometalated/bidentate NHC wingtip. However, from 1 H NMR spectroscopic analysis, we did not observe any decoordination (or ring‐opening) of the cyclometalated ring of complex 7 when it was heated under reflux in 2‐propanol for 1 h (see Supporting Information).…”

The Golden Age of Transfer Hydrogenation

“…507 A series of new iridium−NHC complexes were synthesized through transmetalation reactions from the in situ prepared silver(I)−NHC complexes by Gulcemal's group (Scheme 53). Good results have been obtained in the reduction of carbonyls in the TH process using the ester-functionalized iridium(I) complex 140 as catalyst showing that >99% conversions and 96−99% yields were achieved, while various carbonyls were hydrogenated by 0.1 mol % of 140 using 2-PrOH as hydrogen source within 1.5−6 h. 508 The TH of carbonyls catalyzed by 141 and 142 was completed in very short reaction times down to 2 min, reaching 12 000 h −1 TOF.…”