Electronic and steric modifications of cyclopentadienyl (Cp) ligands have led to a corresponding growth in their applications such as catalysts in the syntheses of small molecules or polymers and as novel materials for pharmaceutics and single-molecular magnets. Previously, we reported the imidazolium-/pyrrolinium-substituted persistent zwitterionic ferrocenate derivatives from the direct functionalization of metallocene using N-heterocyclic carbenes. However, this approach is highly limited to electron-deficient metallocenes. Herein, we present two new redox non-innocent Cp compounds, imidazolium- and pyrrolinium-substituted zwitterionic Cps, which were obtained in high yields and could be handled easily owing to their high stability in air and moisture, unlike other known Cps. The redox non-innocent behavior of the Cp ligands was demonstrated via experimental and computational analyses of their half-sandwich molybdenum and sandwich ruthenium complexes. One-electron reduction of those complexes generates mainly ligand-centered radicals, rather than reducing the metal centers, which was validated by EPR analysis and supported by DFT calculations. These results collectively indicate that the utility of redox non-innocent Cp ligands could preserve the structure of sandwich and half-sandwich complexes.

Redox non-innocent cyclopentadienyl ligands as electron reservoirs for sandwich complexes

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