Computational Mechanistic Study on Cp*Ir Complex-Mediated Acceptorless Alcohol Dehydrogenation: Bifunctional Hydrogen Transfer vs β-H Elimination

There has been an increasing interest in developing efficient AAD (acceptorless alcohol dehydrogenation) catalysts, because of their potential applications in atom economic synthesis, H-2 production from biomass or its fermentation products (mainly alcohols), and the development of organic hydride hydrogen storage systems. Using B3LYP DFT calculations with solvation effects accounted by the SMD solvent model, we have investigated the catalytic mechanism of a novel Ir catalyst (2cat) in the dehydrogenation of 1-phenylethanol (3ol). This study allows us not only to detail the beta-H elimination (BETAHE) pathway proposed by the experimentalists but also to characterize a new pathway called the ligand rotation-promoted hydrogen transfer (LRPHT) pathway. Combining the predicted energetics and experimental results/observations, we confirmed that the LRPHT pathway is more favorable than the BETAHE pathway in 3ol/2cat. According to the favorable LRPHT pathway, we show that the facile ligand rotation between the 18e 2cat complex and the 16e bifunctional reactive species 7bif is responsible for the novelty of the catalyst. The bifunctional reactivity of the species makes the hydrogen transfer feasible for dehydrogenation. The facile ligand rotation is also the reason that the dehydrogenation could be run under neutral conditions, because this activation mode does not require acidic/basic reaction conditions or acid/base promoters to activate the catalyst. Unveiling these characteristics of the new catalyst could aid the advancement of the experimental idea from the perspective of activating catalysts to generate a bifunctional active site via "ligand rotation". We also studied the formation mechanism of the experimentally identified complexes, according to which various experimental observations were rationalized.