Topics in Catalysis ( IF 2.8 ) Pub Date : 2021-08-19 , DOI: 10.1007/s11244-021-01492-3 Juan Angel de Gracia Triviño 1 , Mårten S. G. Ahlquist 1
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Intermolecular radical coupling (also interaction of two metal centers I2M) is one of the main mechanisms for O–O bond formation in water oxidation catalysts. For Ru(bda)L2 (H2bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = pyridine or similar nitrogen containing heterocyclic ligands) catalysts a significant driving force in water solution is the hydrophobic effects driven by the solvent. The same catalyst has been successfully employed to generate N2 from ammonia, also via I2M, but here the solvent was acetonitrile where hydrophobic effects are absent. We used a classical force field for the key intermediate [RuVIN(bda)(py)2]+ to simulate the dimerization free energy by calculation of the potential mean force, in both water and acetonitrile to understand the differences and similarities. In both solvents the complex dimerizes with similar free energy profiles. In water the complexes are essentially free cations with limited ion paring, while in acetonitrile the ion-pairing is much more significant. This ion-pairing leads to significant screening of the charges, making dimerization possible despite lower solvent polarity that could lead to repulsion between the charged complexes. In water the lower ion pairing is compensated by the hydrophobic effect leading to favorable dimerization despite repulsion of the charges. A hypothetical doubly charged [RuVIIN(bda)py2]2+ was also studied for deeper understanding of the charge effect. Despite the double charge the complexes only dimerized favorably in the lower dielectric solvent acetonitrile, while in water the separated state is more stable. In the doubly charged catalyst the effect of ion-pairing is even more pronounced in acetonitrile where it is fully paired similar to the 1+ complex, while in water the separation of the ions leads to greater repulsion between the two catalysts, which prevents dimerization.
Graphic Abstract
中文翻译：
反离子在 Ru-bda 催化剂分子间自由基偶联中的作用
分子间自由基偶联（也是两个金属中心 I2M 的相互作用）是水氧化催化剂中 O-O 键形成的主要机制之一。对于 Ru(bda)L 2（H 2 bda = 2,2'-bipyridine-6,6'-dicarboxylate，L = 吡啶或类似的含氮杂环配体）催化剂，水溶液中的重要驱动力是由溶剂。相同的催化剂已成功用于从氨生成 N 2，也通过 I2M，但这里的溶剂是乙腈，其中不存在疏水作用。我们使用经典力场作为关键中间体 [Ru VI N(bda)(py) 2 ] +通过计算潜在的平均力来模拟二聚化自由能，以了解水和乙腈中的差异和相似之处。在这两种溶剂中，复合物以相似的自由能分布二聚化。在水中，络合物基本上是游离阳离子，离子配对有限，而在乙腈中，离子配对更为重要。这种离子对导致电荷的显着屏蔽，尽管溶剂极性较低，但可能导致带电复合物之间的排斥，从而使二聚化成为可能。在水中，较低的离子配对被疏水效应补偿，尽管电荷排斥，但导致有利的二聚化。一个假设的双电荷 [Ru VII N(bda)py 2 ] 2+还研究了更深入地了解电荷效应。尽管带双电荷，但配合物仅在较低介电溶剂乙腈中有利地二聚化，而在水中分离状态更稳定。在带双电荷的催化剂中，离子配对的效果在乙腈中更加明显，在乙腈中完全配对类似于 1+ 络合物，而在水中，离子的分离导致两种催化剂之间的排斥力更大，从而阻止了二聚化。
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