Redox-responsive organometallic foldamers from ferrocene amino acid: solid-phase synthesis, secondary structure and mixed-valence properties

Oligoferrocenes Fmoc-Fca(n)-OMe (n=3-5) are assembled in a stepwise precise manner from Fmoc-protected ferrocene amino acid Fmoc-Fca-OH (H-Fca-OH = 1-amino-1'-ferrocene carboxylic acid; Fmoc = 9-fluorenylmethyloxycarbonyl) via amide bonds on solid supports by sequential Fmoc deprotection, acid activation and coupling steps. The resulting well-defined oligomers form ordered zigzag structures in THF solution with characteristic hydrogen bonding patterns. Electrochemical experiments reveal sequential oxidations of the individual ferrocene units in these peptides giving mixed-valent cations. Optical intervalence electron transfer is detected by intervalence transitions in the near-IR.