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

Abstract

The (PNP)IrH 3 (2,6-bis(diisopropylphosphinomethyl)pyridine iridium trihydride) complex by Nozaki is a highly active and selective catalyst for CO 2 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 CO 2 molecules in two Ir−H bonds before the reaction with hydrogen. We found that insertion of a second CO 2 molecule is indeed possible; moreover, this sequential insertion allows formation of a more electrophilic iridium monohydride intermediate, and thereby the process of H 2 cleavage is facilitated. In addition, we considered the influence of ligands coplanar with the PNP ligand on the energy of CO 2 insertion into the (PNP)IrH 2 X complex and found that σand π-donating ligands promote the reaction.

Poly(ethyleneimine)‐tethered Ir Complex Catalyst Immobilized in Titanate Nanotubes for Hydrogenation of CO2 to Formic Acid

“…Considering the fact that an aqueous basic medium (NaOH aq.) is required to produce formate, the produced formate ion is displaced by hydrogen to afford a cationic intermediate Ir species (Step 4), which is subsequently deprotonated in the basic reaction medium to regenerate the initial active species (Step 5) . The produced formate is considered to be stabilized either by forming sodium formate (HCOONa) with Na + ions existing in the reaction medium or by adsorption onto the basic sites of PEI and TNT surface, thereby enabling the continuous production of formate over the present catalytic system.…”

Unraveling Metal/Pincer Ligand Effects in the Catalytic Hydrogenation of Carbon Dioxide to Formate

“…To date, the most promising homogeneous hydrogenation pincer catalysts consist of transition metal complexes (e.g., Ir, Rh, Ru, and Fe) incorporating PNP pincer ligands based on a 2,6-dimethylpyridine scaffold where the P and N atoms are directly bound to the metal center. − The connecting atoms of the pincer backbone can be swapped with either N or C, which leads to a host of different ligand families (e.g., PNN, NNN, PCP), − as illustrated in Figure b. Perhaps the most well-known homogeneous catalytic system for the hydrogenation of CO 2 to formate used a Ir III –PNP catalyst (PNP = 2,6-bis­(diiso­propyl­phosphino­methyl)­pyridine) and yielded a maximum turnover number of 3500000. , Subsequent experimental − and theoretical − investigations have also revealed that this, as well as other metal–pincer complexes, should be capable of catalyzing the hydrogenation reaction. Nevertheless, despite considerable experimental and computational work, a comprehensive picture on why certain metal/ligand combinations are superior to others is currently lacking.…”

Simple Ligand Modifications to Modulate the Activity of Ruthenium Catalysts for CO₂ Hydrogenation: <i>Trans</i> Influence of Boryl Ligands and Nature of Ru―H Bond

“…In the present case, our work presented that not only the nature of Ru-H bond correlates with the hydride transferring process, but also the trans ligand of hydride relates to catalytic activity in the whole pathway for the hydrogenation of CO2 to formate. Furthermore, some study 49 demonstrated that the process of CO2 insertion into Ru-H bond via direct hydride abstraction pathway is a sequentially uphill step. Therefore, the whole process of hydrogenation of CO2 to formate should be paid attention and the overall barriers are used to measure the catalytic efficiency of these complexes.…”