Small-molecule conversions involving multielectron transfer processes enable the conversion of earth-abundant materials into valuable chemicals and are regarded as a solution for environmental and energy shortage problems. In this context, the development of artificial catalysts that promote these reactions is an important research target. In nature, metalloenzymes that contain multinuclear metal complexes as active sites are known to efficiently catalyze reactions under mild conditions. Therefore, using multinuclear metal complexes as artificial catalysts can be an attractive strategy for small-molecule conversions involving multielectron transfer processes. However, multinuclear-metal-complex-based catalysts for these reactions have not been well established. In this Account, we describe our recent advances in the development of multinuclear metal complexes as catalysts for small-molecule conversion, mainly focusing on water oxidation. As small-molecule conversions involving multielectron transfer processes consists of two essential processes, (1) the transfer of multiple electrons and (2) the formation/cleavage of covalent bond(s), catalysts for these reactions should facilitate both steps. Therefore, we assumed that the assembly of redox-active metal ions and the cooperative effect of neighboring coordinatively unsaturated metal ions can promote these processes. On the basis of this assumption, we employed a pentanuclear metal complex as a molecular scaffold for the catalyst. The scaffold has a pentanuclear structure with quasi-D₃ symmetry and consists of a [M₃(μ₃-X)] core (X = O^2– or OH^–) wrapped by two [M(μ-bpp)₃] units (Hbpp = 3,5-bis(2-pyridyl)pyrazole). The metal ions in the triangular core are coordinatively unsaturated, whereas the metal ions at the apical positions are coordinatively saturated. In other words, the pentanuclear scaffold possesses multiple redox-active centers and coordinatively unsaturated sites. It should also be noted that the electron transfer ability of the complex changes dramatically depending on the identity of the constituent metal ions. The iron derivative of the pentanuclear scaffold was found to serve as an electrocatalyst for water oxidation (2H₂O → O₂ + 4e^– + 4H⁺) with a high reaction rate and excellent robustness. The substitution of metal ions in the pentanuclear scaffold to cobalt ions resulted in the development of a catalyst for CO₂ reduction. Furthermore, we investigated the effect of substituents on the ligands of the pentanuclear iron complex and succeeded in precisely manipulating the electron transfer possess. These results clearly demonstrate that the pentanuclear scaffold is an attractive platform for catalysts for small-molecule conversions. Additionally, the intrinsic features of the multinuclear catalytic system, which are totally different from those of conventional mononuclear-metal-complex-based catalysts, are disclosed. In reactions mediated by multinuclear complexes, the multinuclear core can initially accumulate the charge required for catalysis to reach the catalytically active state. Subsequently, the catalyst in the active state reacts with the substrate, initiating electron transfer to the substrate and rearrangement of covalent bonds in the substrate to afford the product. In such a mechanism, the desired number of electrons can be transferred to the substrates in an on-demand fashion, and the formation of undesired chemical species in the targeted catalysis may be prevented. This feature of multinuclear-metal-complex-based catalysts will achieve demanding small-molecule conversions with a high reaction rate, selectivity, and durability.

中文翻译：

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Pentanuclear支架：小分子转化的分子平台。

涉及多电子转移过程的小分子转化使富含地球的物质转化为有价值的化学物质，被视为解决环境和能源短缺问题的解决方案。在这种情况下，开发促进这些反应的人造催化剂是重要的研究目标。实际上，已知含有多核金属配合物作为活性位点的金属酶可以在温和条件下有效催化反应。因此，对于涉及多电子转移过程的小分子转化，使用多核金属配合物作为人工催化剂可能是一种有吸引力的策略。然而，用于这些反应的基于多核金属配合物的催化剂尚未很好地建立。在这个帐户中，我们描述了多核金属配合物作为小分子转化催化剂的最新进展，主要集中在水的氧化上。由于涉及多电子转移过程的小分子转化包括两个基本过程，（1）多个电子的转移和（2）共价键的形成/裂解，因此这些反应的催化剂应有助于两个步骤。因此，我们假设氧化还原活性金属离子的组装和相邻的配位不饱和金属离子的协同作用可以促进这些过程。基于该假设，我们采用五核金属配合物作为催化剂的分子支架。支架具有五核结构，具有准 主要集中在水的氧化上。由于涉及多电子转移过程的小分子转化包括两个基本过程，（1）多个电子的转移和（2）共价键的形成/裂解，因此这些反应的催化剂应有助于两个步骤。因此，我们假设氧化还原活性金属离子的组装和相邻的配位不饱和金属离子的协同作用可以促进这些过程。基于该假设，我们采用五核金属配合物作为催化剂的分子支架。支架具有五核结构，具有准 主要集中在水的氧化上。由于涉及多电子转移过程的小分子转化包括两个基本过程，（1）多个电子的转移和（2）共价键的形成/裂解，因此这些反应的催化剂应有助于两个步骤。因此，我们假设氧化还原活性金属离子的组装和相邻的配位不饱和金属离子的协同作用可以促进这些过程。基于该假设，我们采用五核金属配合物作为催化剂的分子支架。支架具有五核结构，具有准 这些反应的催化剂应有助于两个步骤。因此，我们假设氧化还原活性金属离子的组装和相邻的配位不饱和金属离子的协同作用可以促进这些过程。基于该假设，我们采用五核金属配合物作为催化剂的分子支架。支架具有五核结构，具有准 这些反应的催化剂应有助于两个步骤。因此，我们假设氧化还原活性金属离子的组装和相邻的配位不饱和金属离子的协同作用可以促进这些过程。基于该假设，我们采用五核金属配合物作为催化剂的分子支架。支架具有五核结构，具有准d ₃对称，并且由[M的₃（μ ₃ -X）]芯（X = O ^2-或OH ^{- ）}由两个缠绕[M（μ-BPP）₃ ]单元（Hbpp = 3,5-双（ 2-吡啶基）吡唑）。三角形核中的金属离子是配位不饱和的，而顶端的金属离子是配位饱和的。换句话说，五核支架具有多个氧化还原活性中心和配位不饱和位点。还应注意，配合物的电子转移能力会根据组成金属离子的身份而急剧变化。发现五核骨架的铁衍生物可作为水氧化的电催化剂（2H ₂O→O ₂ + 4e ^– + 4H ⁺）具有高反应速率和出色的耐用性。将五核支架中的金属离子替换为钴离子导致了CO ₂催化剂的开发减少。此外，我们研究了取代基对五核铁配合物配体的影响，并成功地精确控制了电子转移。这些结果清楚地表明，五核骨架是用于小分子转化的催化剂的有吸引力的平台。另外，公开了多核催化体系的固有特征，其与常规的单核金属络合物基催化剂完全不同。在由多核络合物介导的反应中，多核核最初可以积累催化达到催化活性状态所需的电荷。随后，处于活性状态的催化剂与底物反应，引发电子转移至底物并重新排列底物中的共价键，从而得到产物。以这种机制，可以按需的方式将所需数量的电子转移到基板上，并且可以防止在目标催化中形成不希望的化学物种。多核金属络合物基催化剂的这一特性将以高反应速率，选择性和耐用性实现高要求的小分子转化。