Electron Transfer Excited State Can Become New Ground State in Mixed Valence Compound (Chemistry)

The investigation of mixed valence compounds with metal to metal charge transfer may help to understand the ubiquitous electron transfer in chemical, physical and biological systems. Besides, such compounds may have potential applications in molecular electronics. 

In a study published in Angew. Chem. Int. Ed., Prof. SHENG Tianlu and his colleagues from Prof. WU Xintao’s group at the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, have synthesized and characterized a series of isocyanidometal-bridged compounds [FeII-CN-RuIII-NC-RuII]2+, [FeII-CN-RuIII-NC-FeII]3+ and [FeIII-CN-RuIII-NC-FeII]4+.  

The compounds [FeII-CN-RuII-NC-RuII]2+ were formed by the connection of an oxidizable RuII isocyanide unit with two redox FeII fragments. [FeII-CN-RuIII-NC-FeII]3+ and [FeIII-CN-RuIII-NC-FeII]4+ are the one-electron and two-electron oxidation compounds of [FeII-CN-RuII-NC-RuII]2+, and obtained by the reaction of [FeII-CN-RuII-NC-RuII]2+ with one equivalent of Cp2Fe(PF6) and two equivalents of AgPF6 or NOPF6, respectively.

Through the single-crystal X-ray diffraction structure analyses and infrared (IR), 57Fe Mossbauer and UV/Vis/NIR spectroscopy, the researchers found that for the one-electron oxidation compounds in which the energy level of the bridge state (FeII-RuIII-FeII) is lower than that of the mixed valence states (FeIII-RuII-FeII and FeII-RuII-FeIII), the electron transfer excited state becomes more and more stable as methyl substitution on the Cp ligand increases, leading to the simultaneous observations of the bridge-state and mixed-valence states on the IR timescale.  

Upon further one-electron oxidation of the above one-electron oxidation compounds, the bridge state (FeIII-RuII-FeIII) level of the two-electron oxidation compounds increases and is slightly higher than that of the mixed valence states (FeIII-RuIII-FeII and FeII-RuIII-FeIII).  

As the donor effect becomes stronger, the coupling between terminal Fe and bridge Ru centers becomes stronger, resulting in the bridge state becoming more and more stable.  

When the methyl number on the Cp ligand increases to be five, the bridge state (excited state) becomes the new ground state (mixes with the mixed valence state). In this situation, no electron transfer (ET) energy barrier exists between the different states. 

This study might be helpful for better understanding the ET process in nature and for the design of novel electron transfer materials.

Reference: Yu‐Ying Yang, Dr. Xiao‐Quan Zhu, Prof. Jean‐Pierre Launay, Cheng‐Bin Hong, Dr. Shao‐Dong Su, Dr. Yue‐Hong Wen, Prof. Xin‐Tao Wu, Prof. Tian‐Lu Sheng, “The Electron Transfer Process in Mixed Valence Compounds with a Low‐lying Energy Bridge in Different Oxidation States”, Angew. Chem. Int. Ed. 2021, 60, 4804. https://onlinelibrary.wiley.com/doi/10.1002/anie.202014501

Provided by Chinese Academy of Sciences

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