Synthesis of a series of 1,n′-disubstituted ferrocene derivatives containing disulfides

A series of ferrocene amide cystamine derivatives was prepared from 1′-methoxycarbonylferrocene-1-carboxylic acid in a step-wise fashion, via amide bond formation with a series of Boc-protected diamines, followed by coupling to cystamine. The length of the diamine linker was varied systematically from 2 to 10 methylene groups. All products were characterized spectroscopically. In solution, all ferrocene compounds displayed a quasi-reversible ferrocene/ferrocenium redox couple between 258 and 320 mV versus external Fc/Fc+. The Boc-protected ethylene diamine ferrocene conjugate 7 was characterized by single crystal X-ray crystallography. Intermolecular hydrogen bonding exists between amide groups of adjacent molecules. The presence of the disulfide group allows chemisorption of Fc-conjugates 18–22 onto gold surfaces. Electrochemical measurements allow an evaluation of the electron transfer kinetics using the Butler–Volmer formalism. Electron transfer rate constants were determined to be in the range of 1.871(67)–2.105(258) s−1.