Preparation and electrochemical properties of SAM of alkanethiols functionalized with 2-aza[3]ferrocenophane on gold electrode

Thiols functionalized with N-aryl[3]azaferrocenophane formulated as HS-(CH2)n-N(CH2Cp)2Fe (1: n = 6, 2: n = 8, 3: n = 10, 4: n = 12) and disulfide obtained by oxidation of 4 (5) were synthesized via three or four steps reactions starting from 1,1′-ferrocenedimethanol, 4-aminophenol, and α,ω-aklanedithiol. Self-assembled monolayers (SAMs) of these thiols and disulfide on gold electrode were prepared by immersing the electrode in MeCN solutions of the compounds. Cyclic voltammograms of the SAM of 1 (n = 6) exhibited reversible redox of the Fe center at E1/2 = 0.26 V (vs. Ag+/Ag) in the presence of Et4NBF4 in MeCN and at E1/2 = 0.40 V (vs. AgCl/Ag) in the presence of NaClO4 in H2O. Addition of HClO4 to the solutions shifted the redox peaks to higher potentials, E1/2 = 0.51 V (vs. Ag+/Ag) in MeCN and E1/2 = 0.48 V (vs. AgCl/Ag) in H2O, respectively, which was ascribed to positive charge of tertiary ammonium group formed by protonation of the amino group of the azaferrocenophane. E1/2 of SAM of 1 in H2O solution varies depending on the anion contained in the electrolyte, NaClO4 (0.40 V), NaBF4 (0.46 V), Na2SO4 (0.53 V), and NaCl (0.55 V). Kinetic data of electron transfer between the Fe center and the gold surface of the SAM of 2–4 were obtained with variable scanning rate. Laviron’s analysis provided tunneling constant β, 0.05 Å−1, suggesting that the structural changes in the SAMs on oxidation/reduction undergoes the insignificant change of the kinetic constants of the electron transfer depending on the range of the spacer length. In the acidic aqueous media, the kinetic parameters indicated that the imbalanced electron transfer between oxidized and reduced states of the Fe center was caused by the protonation of bridged amine group of azaferrocenophane.