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

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Mechanistic Insights into Co and Fe Quaterpyridine-Based CO2 Reduction Catalysts: Metal–Ligand Orbital Interaction as the Key Driving Force for Distinct Pathways
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2021-01-05 , DOI: 10.1021/jacs.0c09380
Matthias Loipersberger ₁ , Delmar G. A. Cabral ₁ , Daniel B. K. Chu ₂ , Martin Head-Gordon _{1,

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Affiliation

Both [CoII(qpy)(H2O)2]2+ and [FeII(qpy)(H2O)2]2+ (with qpy = 2,2':6',2″:6'',2‴-quaterpyridine) are efficient homogeneous electrocatalysts and photoelectrocatalysts for the reduction of CO2 to CO. The Co catalyst is more efficient in the electrochemical reduction, while the Fe catalyst is an excellent photoelectrocatalyst ( ACS Catal. 2018, 8, 3411-3417). This work uses density functional theory to shed light on the contrasting catalytic pathways. While both catalysts experience primarily ligand-based reductions, the second reduction in the Co catalyst is delocalized onto the metal via a metal-ligand bonding interaction, causing a spin transition and a distorted ligand framework. This orbital interaction explains the experimentally observed mild reduction potential and slow kinetics of the second reduction. The decreased hardness and doubly occupied dz2-orbital facilitate a σ-bond with the CO2-π* in an η1-κC binding mode. CO2 binding is only possible after two reductions resulting in an EEC mechanism (E = electron transfer, C = chemical reaction), and the second protonation is rate-limiting. In contrast, the Fe catalyst maintains a Lewis acidic metal center throughout the reduction process because the metal orbitals do not strongly mix with the qpy-π* orbitals. This allows binding of the activated CO2 in an η2-binding mode. This interaction stabilizes the activated CO2 via a π-type interaction of a Fe-t2g orbital and the CO2-π* and a dative bond of the oxygen lone pair. This facilitates CO2 binding to a singly reduced catalyst resulting in an ECE mechanism. The barrier for CO2 addition and the second protonation are higher than those for the Co catalyst and rate-limiting.

中文翻译：

基于 Co 和 Fe 的四吡啶基 CO2 还原催化剂的机理洞察：金属-配体轨道相互作用是不同途径的关键驱动力

[CoII(qpy)(H2O)2]2+ 和 [FeII(qpy)(H2O)2]2+（qpy = 2,2':6',2″:6'',2‴-quaterpyridine）是用于将 CO2 还原为 CO 的高效均相电催化剂和光电催化剂。Co 催化剂在电化学还原中更有效，而 Fe 催化剂是一种优异的光电催化剂（ACS Catal. 2018, 8, 3411-3417）。这项工作使用密度泛函理论来阐明对比催化途径。虽然这两种催化剂主要经历基于配体的还原，但 Co 催化剂的第二次还原通过金属-配体键合相互作用离域到金属上，导致自旋跃迁和扭曲的配体框架。这种轨道相互作用解释了实验观察到的轻度还原电位和第二次还原的缓慢动力学。降低的硬度和双重占据的 dz2 轨道有助于在 η1-κC 结合模式下与 CO2-π* 形成 σ 键。CO2 结合只有在两次还原导致 EEC 机制（E = 电子转移，C = 化学反应）后才有可能，第二次质子化是限速的。相比之下，Fe 催化剂在整个还原过程中保持路易斯酸性金属中心，因为金属轨道不会与 qpy-π* 轨道强烈混合。这允许以 η2 结合模式结合活化的 CO2。这种相互作用通过 Fe-t2g 轨道和 CO2-π* 的 π 型相互作用以及氧孤对键的配键来稳定活化的 CO2。这有助于 CO2 与单一还原的催化剂结合，从而产生 ECE 机制。

更新日期：2021-01-05

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