Thiol-Ene Chemistry of Vinylferrocene: A Simple and Versatile Access Route to Novel Electroactive Sulfur- and Ferrocene-Containing Model Compounds and Polysiloxanes

The potential of vinylferrocene Fc(CH=CH2) (1) as redox-active ene functional precursor for thiol-ene radical reaction has been successfully investigated for the first time. To this end, we initially studied thiol-ene model reactions of 1 with a variety of functionalized monothiols, including 2-mercaptoethanol, 1-thioglycerol, 3-mercaptopropionic acid, (3-mercaptopropyl)-trimethoxysilane, 2-aminoethanethiol hydrochloride, dithiol 2,2'-(ethylenedioxy)diethanethiol, and the tetrafunctional thiol pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). These reactions afforded the newly ferrocenyl-hydrothiolated small-molecule models Fc(CH2)(2)S(CH2)(2)OH (2), Fc(CH2)(2)S(CH2)(2)COOH (3), Fc(CH2)(2)S(CH2)CH(OH)(CH2)OH (4), Fc(CH2)(2)S(CH2)(3)Si(OMe)(3) (5), bimetallic Fc(CH2)(2)S(CH2)(2)O(CH2)(2)O(CH2)(2)S(CH2)(2)Fc (6), and tetrametallic [Fc(CH2)(2)S(CH2)(2)COOCH2](4)C (7) in relatively high yields. Furthermore, the hydrothiolation of the C=C bond of vinylferrocene was successfully extended to the S-H-polyfunctionalized poly[(3-mercaptopropyl)methylsiloxane] (PMMS), which provided access to a new redox-active polysiloxane (Me3SiO){MeSi[(CH2)(3)S(CH2)(2)Fc]O}(n)(SiMe3) (8) containing covalently attached electroactive ferrocenyl-thioether side groups. The thiol-vinylferrocene reactions have been initiated either thermally, in toluene solution with AIBN, or by UV light irradiation in THF in the presence of 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator. The outcomes of the hydrothiolation of vinylferrocene strongly depend on the thiol structure and on the experimental conditions. Thermogravimetric analysis (TGA) of 8 established that the incorporation of pendant thioether-ferrocenyl units within the Si-O-Si backbone significantly improves the thermal stability of polymethylsiloxane in comparison to the S-H-polyfunctionalized PMMS precursor. Electrochemical studies revealed that polysiloxane 8, containing sulfur-bridged ferrocenyl moieties attached along the main chain, exhibits unusual and excellent adsorption properties and spontaneously forms robust films on Pt and Au electrodes. The well-defined and persistent voltammetric waves of the surface-adsorbed 8 films display nearly ideal redox behavior and exhibit peak potential separations (Delta E-peak) and full width at half-maximum (E-fwhm) values close to those theoretically expected for stable, surface-immobilized electroactive layers.