Axial coordination of pyridyl moieties to CoPc (either exogenous or within poly-4-vinylpyridine polymer) dramatically increases the complex’s activity for the CO₂ reduction reaction (CO₂RR). It has been hypothesized that axial coordination to the Co active site leads to an increase in the Co d_z² orbital energy, which increases the complex’s nucleophilicity and facilitates CO₂ coordination compared to the parent CoPc complex. The magnitude of the energy increase in the Co d_z² orbital should depend on the σ-donor strength of the axial ligand─a stronger σ-donating ligand (L) will increase the overall CO₂RR activity of axially coordinated CoPc(L) and vice versa. To test this, we have studied a series of CoPc(L) complexes where the σ-donor strength of L is varied. We show that CoPc(L) reduces CO₂ with an increased activity as the σ-donor ability of L is increased. These observed electrochemical activity trends are correlated with computationally derived CO₂ binding energy and charge transfer terms as a function of σ-donor strength. The findings of this study support our hypothesis that the increased CO₂RR activity observed upon axial coordination to CoPc is due to the increased energy of the d_z² orbital, and highlight an important design consideration for macrocyclic MN₄-based electrocatalysts.

中文翻译：

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通过调节轴向配位配体的 σ-供体强度提高钴酞菁的 CO2 还原活性

吡啶基部分与 CoPc（外源性的或聚 4-乙烯基吡啶聚合物内的）的轴向配位显着增加了复合物对 CO ₂还原反应 (CO ₂ RR)的活性。据推测，与 Co 活性位点的轴向配位导致 Co d _{z ²}轨道能量增加，与母体 CoPc 配合物相比，这增加了配合物的亲核性并促进了 CO ₂配位。Co d _{z ²}轨道能量增加的幅度应取决于轴向配体的σ-供体强度——更强的σ-供体配体（L）将增加整体CO ₂轴向协调的 CoPc(L) 的 RR 活动，反之亦然。为了测试这一点，我们研究了一系列 CoPc(L) 复合物，其中 L 的 σ 供体强度是变化的。我们表明，随着L的 σ 供体能力增加，CoPc(L) 减少 CO ₂并增加活性。这些观察到的电化学活性趋势与计算得出的 CO ₂结合能和电荷转移项相关，作为σ-供体强度的函数。该研究的结果支持我们的假设，即在与 CoPc 轴向配位时观察到的CO ₂ RR 活性增加是由于 d _{z ²}轨道的能量增加，并突出了基于大环 MN ₄的电催化剂的重要设计考虑。