title: Oligonuclear ferrocene amides: mixed-valent peptides and potential redox-switchable foldamers.
abstract: 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).
authors.affiliation: Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg-University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Siebler
authors.forename: Michael
authors.initials: M
authors.lastname: Linseis
authors.forename: Teuta
authors.initials: T
authors.lastname: Gasi
authors.forename: Luca M
authors.initials: LM
authors.lastname: Carrella
authors.forename: Rainer F
authors.initials: RF
authors.lastname: Winter
authors.forename: Christoph
authors.initials: C
authors.lastname: Förster
authors.forename: Katja
authors.initials: K
authors.lastname: Heinze
mesh_terms: D000577:Amides / Q000138:chemical synthesis / Q000737:chemistry*; D005296:Ferrous Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D000075163:Metallocenes; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010455:Peptides / Q000737:chemistry*
chemical_list: D000577:Amides; D005296:Ferrous Compounds; D000075163:Metallocenes; D010455:Peptides; C004998:ferrocene