Selectivity for HCO₂^– over H₂ in the Electrochemical Catalytic Reduction of CO₂ by (POCOP)IrH₂

Abstract

It has been demonstrated experimentally that electrochemical CO2 reduction catalyzed by (POCOP)­IrH2 ([C6H3-2,6-[OP­(tBu)2]2]­IrH2) produces formate without significant H2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from IrIII dihydride explains the selectivity for formate over H2 and is corroborated by reduction potential (irreversible reduction of (POCOP)­Ir­(H)­(NCMe)2 + at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of IrIII(H)­(NCMe)2 +) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution reaction (HER) from both IrIII(H)2 and IrI–H– but are kinetically hindered, posing computed activation barriers above 25 kcal/mol at pH 7. However, with formate or bicarbonate acting as cocatalyst, the barriers are lowered to 18.8 kcal/mol. The preference of (POCOP)Ir to form a dihydride instead of a dihydrogen adduct also disfavors the HER and facilitates catalyst regeneration. In contrast, substituting cobalt for iridium raises the barrier for hydride transfer to CO2 by 12.0 kcal/mol and lowers the required reduction potential to −1.65 V vs NHE. Calculated driving forces for hydride transfer from IrI and IrIII intermediates illustrate different strategies for positioning the hydricity relative to the thermodynamic hydricities of H2/H+ and HCOO–/CO2. The data support an approach of selecting a hydricity that is just thermodynamically able to reduce CO2. The effect of solvation on calculated driving forces for hydride transfer is also discussed.

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

“…This geometry suggests that 6, a rhodium(III) dihydrogen adduct, would not be a stable species in solution. Summarizing, both of our calculated protonation routes with low barriers involve participation of both the hydride and the metal, a scheme seen previously in biological, 73,74 heterogeneous, 71 and inorganic 75,76 systems. Further, both low-barrier pathways calculated to operate via the hydride 3 are lower in energy than protonation via the bridging pathway, TS 2.…”

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

“…This geometry suggests that 6, a rhodium(III) dihydrogen adduct, would not be a stable species in solution. Summarizing, both of our calculated protonation routes with low barriers involve participation of both the hydride and the metal, a scheme seen previously in biological, 73,74 heterogeneous, 71 and inorganic 75,76 systems. Further, both low-barrier pathways calculated to operate via the hydride 3 are lower in energy than protonation via the bridging pathway, TS 2.…”

Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

“…The high FE, which we consider quantitative in view of the work up by vacuum distillation, and therefore the desirable lack of competing H2 evolution is likely due to the literature known water stability of iridium hydrides. [40,43,44] Importantly, in the absence of catalyst 1, no n-octane was formed by electrolysis under otherwise identical conditions as confirmed by a control experiment (SI, Figure S10 and S15). Together, the high activity and faradaic efficiency confirmed the excellent performance of catalyst 1 for electro-hydrogenation, which prompted us to further investigate the underlying catalytic reactivity.…”

Electrocatalytic Transfer Hydrogenation of 1‐Octene with [( tBu PCP)Ir(H)(Cl)] and Water

“…Entering the electrocatalytic cycle for e‐TH with electro‐activated complex 2 , opens up a variety of plausible reaction pathways, which will be discussed and evaluated in the following. Based on the combined electrochemical results, our control experiments, and literature support including a quantum chemical evaluation published for CO 2 reduction on the related POCOP system, [48,50,54] the reaction cascade shown in Scheme 2 is proposed as a likely mode of operation for the electro‐hydrogenation of alkenes with 1 .…”

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

“…This geometry suggests that 6, a rhodium(III) dihydrogen adduct, would not be a stable species in solution. Summarizing, both of our calculated protonation routes with low barriers involve participation of both the hydride and the metal, a scheme seen previously in biological, 73,74 heterogeneous, 71 and inorganic 75,76 systems. Further, both low-barrier pathways calculated to operate via the hydride 3 are lower in energy than protonation via the bridging pathway, TS 2.…”

Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

“…The high FE, which we consider quantitative in view of the work up by vacuum distillation, and therefore the desirable lack of competing H2 evolution is likely due to the literature known water stability of iridium hydrides. [40,43,44] Importantly, in the absence of catalyst 1, no n-octane was formed by electrolysis under otherwise identical conditions as confirmed by a control experiment (SI, Figure S10 and S15). Together, the high activity and faradaic efficiency confirmed the excellent performance of catalyst 1 for electro-hydrogenation, which prompted us to further investigate the underlying catalytic reactivity.…”

Electrocatalytic Transfer Hydrogenation of 1‐Octene with [( tBu PCP)Ir(H)(Cl)] and Water

“…Entering the electrocatalytic cycle for e‐TH with electro‐activated complex 2 , opens up a variety of plausible reaction pathways, which will be discussed and evaluated in the following. Based on the combined electrochemical results, our control experiments, and literature support including a quantum chemical evaluation published for CO 2 reduction on the related POCOP system, [48,50,54] the reaction cascade shown in Scheme 2 is proposed as a likely mode of operation for the electro‐hydrogenation of alkenes with 1 .…”

Role of Ligand Protonation in Dihydrogen Evolution from a Pentamethylcyclopentadienyl Rhodium Catalyst

“…This geometry suggests that 6, a rhodium(III) dihydrogen adduct, would not be a stable species in solution. Summarizing, both of our calculated protonation routes with low barriers involve participation of both the hydride and the metal, a scheme seen previously in biological, 73,74 heterogeneous, 71 and inorganic 75,76 systems. Further, both low-barrier pathways calculated to operate via the hydride 3 are lower in energy than protonation via the bridging pathway, TS 2.…”

Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

“…The high FE, which we consider quantitative in view of the work up by vacuum distillation, and therefore the desirable lack of competing H2 evolution is likely due to the literature known water stability of iridium hydrides. [40,43,44] Importantly, in the absence of catalyst 1, no n-octane was formed by electrolysis under otherwise identical conditions as confirmed by a control experiment (SI, Figure S10 and S15). Together, the high activity and faradaic efficiency confirmed the excellent performance of catalyst 1 for electro-hydrogenation, which prompted us to further investigate the underlying catalytic reactivity.…”

Electrocatalytic Transfer Hydrogenation of 1‐Octene with [( tBu PCP)Ir(H)(Cl)] and Water

“…Entering the electrocatalytic cycle for e‐TH with electro‐activated complex 2 , opens up a variety of plausible reaction pathways, which will be discussed and evaluated in the following. Based on the combined electrochemical results, our control experiments, and literature support including a quantum chemical evaluation published for CO 2 reduction on the related POCOP system, [48,50,54] the reaction cascade shown in Scheme 2 is proposed as a likely mode of operation for the electro‐hydrogenation of alkenes with 1 .…”