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, high temperature superconductors, and switchable molecular magnetism.
In a recent study published in Angew. Chem. Int. Ed., Prof. SHENG Tianlu and his colleagues from Prof. WU Xintao's group at Fujian Institute of Research on the Structure of Matter of Chinese Academy of Sciences synthesized and characterized a pair of cyanido/isocyanido-bridged isomeric compounds, [RuIII-NC-RuII-CN-RuII]3+ and [RuII-CN-RuIII-NC-RuII]3+.
Through the single-crystal X-ray diffraction structure analyses, and infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy, researchers found that the two terminal Ru sites of [RuIII-NC-RuII-CN-RuII]3+ are both equivalent, and no distinction between RuIII and RuII is possible.
They then revealed that the intervalence charge transfer (IVCT) band of [RuIII-NC-RuII-CN-RuII]3+ is solvent-independent, suggesting [RuIII-NC-RuII-CN-RuII]3+ is a class III mixed valence compound.
Upon changing the orientation of the bridging cyanide (CN) group from CN to isocyanide (NC), the IVCT band of [RuII-CN-RuIII-NC-RuII]3+ is also solvent-independent, but the EPR spectra and the X-ray structural data are temperature-dependent, suggesting that [RuII-CN-RuIII-NC-RuII]3+ behaves like a class II–III cyanido-bridged mixed valence species.
From [RuIII-NC-RuII-CN-RuII]3+ to [RuII-CN-RuIII-NC-RuII]3+, the central RuII center in the former is to mediate electron communication between the two terminal ruthenium centers, whereas the central RuIII center in the later plays the role of the electron acceptor. It seems that for [RuII-CN-RuIII-NC-RuII]3+ the electron delocalization throughout Ru-Ru-Ru is divided evenly into two electron delocalization parts from the central ruthenium center.
This study is the first example of the transition from class II-III to class III mixed valence compounds by cyanido-/isocyanido-bridge Isomerism. The results are of great significance in designing molecular conducting devices.
