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) (<b>1</b>) 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 <b>1</b> 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)₂S­(CH₂)₂OH (<b>2</b>), Fc­(CH₂)₂S­(CH₂)₂COOH (<b>3</b>), Fc­(CH₂)₂S­(CH₂)­CH­(OH)­(CH₂)­OH (<b>4</b>), Fc­(CH₂)₂S­(CH₂)₃Si­(OMe)₃ (<b>5</b>), bimetallic Fc­(CH₂)₂S­(CH₂)₂O­(CH₂)₂O­(CH₂)₂S­(CH₂)₂Fc (<b>6</b>), and tetrametallic [Fc­(CH₂)₂S­(CH₂)₂COOCH₂]₄C (<b>7</b>) 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)₃S­(CH₂)₂Fc]­O}_<i>n</i>(SiMe₃) (<b>8</b>) 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 <b>8</b> 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 <b>8</b>, 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 <b>8</b> films display nearly ideal redox behavior and exhibit peak potential separations (Δ<i>E</i>_peak) and full width at half-maximum (<i>E</i>_fwhm) values close to those theoretically expected for stable, surface-immobilized electroactive layers