The First Isolable Low-valent Ferrocene Enabling the Application of Unprecedented Ferrocene Anolyte

Here, we report the first isolable low-valent biscyclopentadienyl iron complexes stabilized by NHC-functionalized ligands (NHC-Cps), which were characterized by electron paramagnetic resonance (EPR) and ⁵⁷Fe Mössbauer spectroscopy. Additional theoretical studies on these formally low-valent ferrocene complexes clearly explain the origin of their thermodynamic stability and the orbital interactions between iron and NHC-Cp. Exploiting the facile Fe(II/I) redox chemistry, we successfully demonstrated that the NHC-Fc compounds can be applied as the first example of ferrocene anolyte for redox-flow batteries. These low-valent species will not only deepen our understanding of the intrinsic chemistry of low-valent ferrocene but have the potential to open the way for the rational design of low-valent metallocene derivatives for various applications.