二级配位层质子交换位点钴分子络合物催化四电子还原氧为水,ACS Catalysis

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二级配位层质子交换位点钴分子络合物催化四电子还原氧为水
ACS Catalysis ( IF 11.3 ) Pub Date : 2023-04-04 , DOI: 10.1021/acscatal.3c00822
Avijit Das ₁ , Afsar Ali ₁ , Geetika Gupta ₁ , Aakash Santra ₁ , Priya Jain ₁ , Pravin P. Ingole ₁ , Satadal Paul ₂ , Sayantan Paria ₁

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^{控制 4e –} /4H ⁺氧还原相对于 2e ^– /2H ⁺还原的选择性是制造用于燃料电池阴极的高效催化剂的关键挑战。细胞色素 c 氧化酶 (C c O)活性位点上的酪氨酸残基已被证明可以控制氢原子转移反应和 Fe-O-O-Cu 部分的 O-O 键裂解生成水。在由卟啉衍生物支持的几个小分子铁络合物中，研究表明，二级配位层质子化位点的存在在指导氧还原反应 (ORR) 的选择性和速率方面起着重要作用。在这项研究中，我们设计并合成了单核 Co ^III双吡啶双肟配体的复合物( 1 )，其中肟位点可以参与可逆质子交换反应。在缓冲水溶液和含有三氟乙酸的乙腈中研究了1的电催化 ORR作为质子源。我们观察到，在 0.1 M 磷酸盐缓冲溶液 (PBS) 中，1对 4e ^– /4H ⁺在 pH 4 下O ₂的还原具有选择性，并且选择性随着缓冲介质 pH 值的增加而降低，产生约。75% H ₂ O，pH 7。然而，在 0.1 M 醋酸盐缓冲溶液 (ABS) 中，1对 O–O 键的裂解产生 H _{2仍然具有高度选择性}_{O 在 pH 4 和 pH 7 时。在 PBS 和 ABS 中，H 2} O 形成的过电势 (η) （约 0.8–0.65 V）随着 pH 值的增加成比例下降。在乙腈中，1对电催化和化学 ORR 的4e ^– /4H ⁺还原保持高选择性。估计 H _{2的过电势为 760 mV}O在乙腈中产生。动力学分析表明催化剂浓度对 25 °C 下的反应速率具有一级依赖性。然而，在 -40 °C 的乙腈中，过氧桥联双核钴 (III) 络合物的形成被认为是 ORR 途径中的反应中间体。我们推测配体的肟支架作为质子交换位点并协助质子偶联电子转移 (PCET) 反应性以切割酸性缓冲溶液和乙腈中的 O-O 键，理论研究进一步证实了这一点。密度泛函理论 (DFT) 计算表明，乙酸根离子在 pH 7.0 下充当介质，用于将质子从肟支架转移到 Co ^III (OOH) 中间体的远端氧，负责对 4e 的高选择性^– /4H ⁺ O ₂的减少。

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Catalytic Four-Electron Reduction of Oxygen to Water by a Molecular Cobalt Complex Consisting of a Proton Exchanging Site at the Secondary Coordination Sphere

Controlling the selectivity of 4e^–/4H⁺ reduction of oxygen over 2e^–/2H⁺ reduction is a key challenge in making efficient catalysts for fuel cell cathodes. A tyrosine residue poised over the active site of cytochrome c oxidase (CcO) has been demonstrated to control the hydrogen atom transfer reactions and cleavage of the O–O bond of a Fe–O–O–Cu moiety to yield water. In a couple of small-molecule iron complexes supported by porphyrin derivatives, it was shown that the presence of protonation sites at the secondary coordination sphere plays an important role in directing the selectivity and rate of the oxygen reduction reaction (ORR). In this study, we designed and synthesized a mononuclear Co^III complex (1) of a bis-pyridine-bis-oxime ligand where the oxime site can participate in reversible proton exchange reactions. Electrocatalytic ORR of 1 was investigated in aqueous buffer solutions and acetonitrile containing trifluoroacetic acid as the proton source. We observed that in a 0.1 M phosphate buffer solution (PBS), 1 is selective for 4e^–/4H⁺ reduction of O₂ at pH 4, and the selectivity decreases with increasing the buffer medium’s pH, producing ca. 75% H₂O at pH 7. However, in a 0.1 M acetate buffer solution (ABS), 1 remained highly selective for the cleavage of the O–O bond to produce H₂O at pH 4 and pH 7. The overpotential (η) of H₂O formation (ca. 0.8–0.65 V) decreased proportionally with increasing pH in PBS and ABS. In acetonitrile, 1 remained highly selective for 4e^–/4H⁺ reduction for electrocatalytic and chemical ORR. An overpotential of 760 mV was estimated for H₂O production in acetonitrile. Kinetic analysis suggests the first-order dependence of catalyst concentration on the reaction rate at 25 °C. However, the formation of a peroxo-bridged dinuclear cobalt(III) complex was noted as a reaction intermediate in the ORR pathway in acetonitrile at −40 °C. We conjecture that the oxime scaffold of the ligand works as a proton exchanging site and assists in the proton-coupled electron transfer (PCET) reactivity to cleave the O–O bond in the acidic buffer solutions and acetonitrile, further corroborated by theoretical studies. Density functional theory (DFT) calculation suggests that the acetate ion works as a mediator at pH 7.0 for transferring a proton from the oxime scaffold to the distal oxygen of the Co^III(OOH) intermediate, responsible for high selectivity toward 4e^–/4H⁺ reduction of O₂.

更新日期：2023-04-04

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