Rate-Limiting Spin Crossover and Cp Ligand Involvement During Ir(III) Retro-Hydroformylation Catalysis

While catalytic hydroformylation is a well-established reaction, there are only a few reports of homogeneous catalyzed retro-hydroformylation where carbon monoxide and dihydrogen are eliminated from an aldehyde to generate an olefin. Our in-depth assessment of reaction pathways using density functional theory and metadynamics calculations for aldehyde retro-hydroformylation by cyclopentadienyl phosphine-type IrIII catalysts has revealed two surprising and unique aspects of catalysis: (1) Catalytic cycle turnover is determined by the rate of singlet-triplet spin state crossover; and (2) during catalysis, after C–H bond activation, a transient IrIII–H intermediate undergoes intramolecular proton transfer to give a dearomatized η4-Cp-H diene ligand. This IrI intermediate provides the key coordination unsaturation to enable decarbonylation and β-hydride elimination reaction steps. Overall, these mechanistic insights set the stage for the design of novel retro-hydroformylation molecular catalysts.