Efficient Thiol–Yne Click Chemistry of Redox-Active Ethynylferrocene

The application of ethynylferrocene, FcCCH (1), as a highly efficient electroactive precursor for the thiol–yne click reaction is presented. For this purpose, a wide range of functionalized thiols, namely 2-mercaptoethanol, 1-thioglycerol, 3-mercaptopropionic acid, 4-aminothiophenol, and benzene-1,3-dithiol as well as tetrathiol pentaerythritol tetrakis­(3-mercaptopropionate), were investigated. This facile thiol–ethynylferrocene radical reaction has resulted in the quantitative formation and isolation of the newly ferrocenyl–vinyl sulfides FcCHCHS­(CH₂)₂OH (2Z and 2E), FcCHCHSCH₂CH­(OH)­CH₂OH (3Z and 3E), FcCHCHS­(CH₂)₂COOH (4Z and 4E), FcC­(CH₂)­S­(1,4-C₆H₄)­NH₂ (5α), FcCHCHS­(1,3-C₆H₄)­SCHCHFc (6), and [FcCHCHS­(CH₂)₂COOCH₂]₄C (7). Thiol–ethynylferrocene reactions have been initiated either by heat, in toluene with AIBN, or by UV light irradiation in THF in the presence of DMPA as photoinitiator. The outcome of the hydrothiolation of ethynylferrocene strongly depends on the thiol structure and on the initiation method employed. A simple mixing of metallocene 1 with the thiol HS­(CH₂)₂OH or HS­(CH₂)₂COOH in a proper ratio, in THF at 20 °C, in a initiator-free thiol–yne reaction, causes hydrothiolation of 1 to occur, allowing for the formation of vinyl sulfides 2Z, 2E and 4Z, 4E in good isolated yields. In contrast to the bis-addition typically observed for thiol–yne reactions, no double hydrothiolation to FcCCH has been observed for any of the thiols under any conditions studied. Electrochemical studies showed that tetrametallic compound 7, containing four sulfur-bridged ferrocenyl–vinyl moieties, behaves as a tetrapodal adsorbate molecule, exhibiting excellent chemisorption properties, and spontaneously forms robustly adsorbed 7 films onto Au or Pt electrode surfaces.