Oligonuclear Ferrocene Amides: Mixed-Valent Peptides and Potential Redox-Switchable Foldamers

Trinuclear ferrocene tris‐amides were synthesized from an Fmoc‐ or Boc‐protected ferrocene amino acid, and hydrogen‐bonded zigzag conformations were determined by NMR spectroscopy, molecular modelling, and X‐ray diffraction. In these ordered secondary structures orientation of the individual amide dipole moments approximately in the same direction results in a macrodipole moment similar to that of α‐helices composed of α‐amino acids. Unlike ordinary α‐amino acids, the building blocks in these ferrocene amides with defined secondary structure can be sequentially oxidized to mono‐, di‐, and trications. Singly and doubly charged mixed‐valent cations were probed experimentally by Vis/NIR, paramagnetic 1H NMR and Mössbauer spectroscopy and investigated theoretically by DFT calculations. According to the appearance of intervalence charge transfer (IVCT) bands in solution, the ferrocene/ferrocenium amides are described as Robin–Day class II mixed‐valent systems. Mössbauer spectroscopy indicates trapped valences in the solid state. The secondary structure of trinuclear ferrocene tris‐amides remains intact (coiled form) upon oxidation to mono‐ and dications according to DFT calculations, while oxidation to the trication should break the intramolecular hydrogen bonding and unfold the ferrocene peptide (uncoiled form). 1, 2, 3! The building blocks in oligonuclear ferrocene amides with defined secondary structure can be sequentially oxidized. The singly and doubly charged mixed‐valent cations were probed experimentally by VIS/NIR, paramagnetic NMR and Mössbauer spectroscopy as well as by DFT calculations. The third oxidation breaks the hydrogen bonding and unfolds the ferrocene peptide (see picture).