Theoretical Investigation of a Parallel Catalytic Cycle in CO₂ Hydrogenation by (PNP)IrH₃

The (PNP)­IrH₃ (2,6-bis­(diisopropylphosphinomethyl)­pyridine iridium trihydride) complex by Nozaki is a highly active and selective catalyst for CO₂ hydrogenation to formic acid in aqueous KOH. Previous theoretical investigations found that regeneration of the catalyst is the rate-determining step in this reaction. In the current article we present results from a computational study using density functional theory in order to consider the possibility of sequential insertion of two CO₂ molecules in two Ir–H bonds before the reaction with hydrogen. We found that insertion of a second CO₂ molecule is indeed possible; moreover, this sequential insertion allows formation of a more electrophilic iridium monohydride intermediate, and thereby the process of H₂ cleavage is facilitated. In addition, we considered the influence of ligands coplanar with the PNP ligand on the energy of CO₂ insertion into the (PNP)IrH₂X complex and found that σ- and π-donating ligands promote the reaction