Computational Study of an Iron(II) Polypyridine Electrocatalyst for CO₂ Reduction: Key Roles for Intramolecular Interactions in CO₂ Binding and Proton Transfer

Abstract

A solar-driven conversion of CO 2 into fuels by artificial photosynthesis would not only mitigate the greenhouse effect but also provide an alternative to obtain fuels in a renewable fashion. To this end, the new iron polypyridine catalyst [Fe(bpy NHEt PY2)L 2 ] 2+(L = H 2 O, CH 3 CN) was recently developed for the electrochemical reduction of CO 2 to CO. In this study, we performed density functional theory (DFT) electronic structure calculations to shed light on a possible pathway for CO 2 reduction and the origin of the selectivity between CO 2 versus hydrogen evolution reaction.The metal center remains Lewis acidic throughout the reduction process due to ligand loss and mainly ligand based reduction stabilized by antiferromagnetic coupling to a high spin Fe(II) center. This results in a high barrier for hydride formation but a facile addition and activation of CO 2 via an η 2 coordination and stabilizing hydrogen bonding by the amine group. The second unoccupied equatorial coordination site opens up the possibility for an intramolecular protonation with a coordinated water ligand. This facilitates protonation because not only CO 2 but also the proton source H 2 O is activated and properly aligned for a proton transfer due to the Fe−OH 2 bond; consequently, both protonation steps are facile. The moderate ligand field allows a rapid ligand exchange for a second intramolecular protonation step and facilitates an exergonic CO release. The lower selectivity of the related [Fe(bpy OH PY2)L 2 ] 2+ compound can be related to the more acidic second coordination sphere because it opens up the possibility of an intramolecular proton transfer which has a comparable barrier to CO 2 addition.

Mechanistic Insights into Co and Fe Quaterpyridine-Based CO2 Reduction Catalysts: Metal–Ligand Orbital Interaction as the Key Driving Force for Distinct Pathways

“…The this binding mode was also observed both experimentally for a nickel complex 115 and in a computational study of a similar iron based catalyst by some of us. 71 The η 2 binding mode benefits from a Lewis acidic metal center due to the additional dative bond from the CO 2 -oxygen and is able to stabilize the CO • -2 moiety after a single reduction event. By contrast, iron-porphyrin based catalysts are only able to form a CO 2 adduct after multiple reductions.…”

Consistent inclusion of continuum solvation in energy decomposition analysis: theory and application to molecular CO2 reduction catalysts

“…To date, several iron catalysts with different ligand frameworks have been developed for the reduction of CO 2 . 57,60,[84][85][86] The most promi-nent family is [Fe(II)TPP] 0 (TPP = tetraphenylporphyrin) 87,88 and its derivatives. 51,[89][90][91][92] Mechanistic studies indicate that stabilizing the activated CO 2 adduct intermediate can substantially improve the performance of the catalyst.…”

“…Electronic structure calculations and EDA can help provide vital insights into catalytic pathways by identifying key intermediates and characterizing substrate-catalyst interactions, allowing one to understand the origin of activity or selectivity as well as the cause of any intrinsic limitation of a catalyst. [54][55][56][57] Many CO 2 reduction catalysts operate in aprotic polar solvents [51][52][53]58 , aqueous solutions 59 , or water/organic solvent mixtures 60,61 . In such cases, it is essential to incorporate solvation effects in electronic structure calculations for one to obtain meaningful and reliable energetic results, especially for adducts of activated CO 2 (CO 2…”

Consistent Inclusion of Continuum Solvation in Energy Decomposition Analysis: Theory and Application to Molecular CO2 Reduction Catalysts

“…To date, several iron catalysts with different ligand frameworks have been developed for the reduction of CO 2 . 56,60,[84][85][86] The most prominent family is [Fe(II)TPP] 0 (TPP = tetraphenylporphyrin) 87,88 and its derivatives. 51,[89][90][91][92] Mechanistic studies indicate that stabilizing the activated CO 2 adduct intermediate can substantially improve the performance of the catalyst.…”

“…Electronic structure calculations and energy decomposition analysis can help provide vital insights into catalytic pathways by identifying key intermediates and characterizing substratecatalyst interactions, allowing one to understand the origin of activity or selectivity as well as the cause of any intrinsic limitation of a catalyst. [53][54][55][56] Many CO 2 reduction catalysts operate in non-protic polar solvents, 51,52,57,58 aqueous solutions, 59 or water/organic solvent mixtures. 60,61 In such cases, it is essential to incorporate solvation effects in electronic structure calculations for one to obtain meaningful and reliable energetic results, especially for adducts of the CO 2 anion (CO 2…”