多核过渡金属团簇结构及其物理化学性质的理论研究

多核过渡金属团簇由于金属-金属相互作用会产生协同效应，通常呈现出单核团簇所不具备的性质；加上金属-配体相互作用带来的稳定化效果，多核过渡金属团簇大多具有结构更灵活、化学成键更复杂和物理化学性质更丰富的特点，这使其应用前景十分广阔。不仅如此，多核过渡金属团簇在生物酶体系中也广泛存在，众多生命活动过程所涉及的酶核心结构中都含有多核过渡金属团簇。近些年来，多核过渡金属团簇因其独特的电子结构和良好的性能在催化、荧光材料、磁性材料等领域均有着广泛应用。对于功能性团簇的发现、对团簇形成机制和稳定化来源的理解，以及对团簇构效关系的研究和探索目前都备受瞩目。
本论文采用密度泛函理论研究了四种多核过渡金属团簇体系，从功能性钯团簇，到仿生固氮多核铁硫簇，再到光系统II 的析氧络合物，我们从理论角度分析了这些团簇的结构特点和化学成键特征、稳定性来源以及构效关系。本文主要成果总结如下：
一、研究了类戊搭烯钯团簇Pd₈(PPh)₂(PPh₃)₂(S–Adm)₆ 内金属-金属、金属-配体相互作用的本质和团簇稳定机制，将该团簇与传统过渡金属半夹心化合物对比，揭示了其独特的几何结构和配位模式。理论计算表明两个毗邻Pd₅五元环内各符合Hückel 芳香性规则，赋予了该团簇显著的稳定性。
二、构筑了仿生固氮模型化合物LFe(μ–η²:η²-bdt)FeL，通过外围配体L的调控改变中心铁原子价态，研究金属价态对双氮化合物吸附的具体影响。揭示了还原性低价铁中心的存在对氮气吸附的必要性，为仿生固氮领域功能性团簇的设计和固氮酶机理的研究提供了理论依据。
三、提供了与铁钼辅因子结构极为类似的W₂Fe₆CS₆(Tp∗)₂(SPh)₃簇合物，并对该团簇的电子结构和化学成键进行了深入研究，理论计算表明该团簇基态结构呈现反铁磁低自旋的特点，丰富了对仿生固氮簇合物结构的认识。
四、研究了自然界光氧合中心CaMn₄O₅团簇静息态结构的稳定性和异构化过程，该团簇内部具有金属芳香性，为新型仿生金属酶团簇的设计尤其是体系稳定性提供新的思路。

Due to the metal-metal interactions, multinuclear transition-metal clusters usually exhibit unique physicochemical properties distinguished from mononuclear transition-metal complexes. Coupled with the stabilization effect arising from the cooperation be-tween metals and ligands, multinuclear transition-metal clusters have the characteristics of flexible structures, complicated chemical bonding, and abundant properties, which of-fer the possibility of more practical applications. Moreover, multinuclear transition-metal clusters exist widely in various enzymes, in which the kernel structure of numerous bio-logical proteins consists of transition-metal elements. In the past decades, multinuclear transition-metal clusters have presented wide applications in catalysis, biological pro-cesses and photochemistry, due to their unique geometric and electronic structures and excellent physicochemical properties. The discovery and synthesis of clusters with char-acteristic structures and unique physicochemical properties, the in-depth understanding of their formation mechanisms and intrinsic stabilization, and the essential exploration of their structure-property relationships are currently attracting much attention.
Our works aim to provide theoretical insights to study the multinuclear transition-metal clusters. From palladium cluster with atomically precise structure, to biomimetic iron-sulfur clusters and oxygen-evolving complex of photosystem II, we investigate their geometric and electronic structures, physicochemical properties, and structure-property correlations. Four main achievements are summarized as the following:
(1) Theoretical investigation on the electronic structures, chemical bonding and optical properties of a pentalene-like Pd₈(PPh)₂(PPh₃)₂(S–Adm)₆ nanocluster. The chemical bonding analysis reveals the essential origin of the stability and the coordination behavior in this cluster. Three Pd-P 6c-2e bonds can be found in each five-membered palladium ring, which follows Hückel 4n+2 rule and gives rise to intrinsic stability.
(2) Theoretical study on the effect of oxidation states of metal sites on the adsorption of dinitrogen species. By regulating the oxidation state of iron atoms via ligand exchange in LFe(μ–η²:η²-bdt)FeL cluster, we emphasize the critical role of low-valent iron sites on nitrogen fix and activation. This study provides a unique perspective to demonstrate the importance of constructing low-valent metal centers in nitrogenase mimic. (3) Theoretical study on the electronic structures and chemical bonding of a W₂Fe₆CS₆(Tp∗)₂(SPh)₃ cluster relevant to FeMoco. Quantum chemical calculations in-dicate that the ground state of this [Fe₆C] cluster is similar to FeMoco, in which the entire iron-sulfur cluster exhibits antiferromagnetic and low-spin characteristics. Our work provides in-depth exploration of FeMoco analogs, and enriches the understanding of chemical bonding in heteroleptic iron–sulfur clusters.
(4) Theoretical investigation on the electronic structures of CaMn4O₅ cluster as well as its isomerization process. We have proposed that the Mn₃O₃ unit in the cubane-like CaMn4O₅ cluster are aromatic, supported by electronic structure analysis. It enriches the concept of aromaticity in transition-metal clusters and provides a unique perspective for the intrinsic stability of nature’s metalloenzymes.

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