Ferrocene and Silicon-Containing Oxathiacrown Macrocycles and Linear Oligo-Oxathioethers Obtained via Thiol-Ene Chemistry of a Redox-Active Bifunctional Vinyldisiloxane

The application of 1,3-divinyl-1,3-dimethyl-1,3-diferrocenyldisiloxane [(CH2=CH)FcMeSi](2)O (1) as an efficient electroactive vinylsilane precursor for thiol-ene radical reactions is described. In order to determine if steric or electronic limitations due to the redox-active metallocene moiety might affect the bis(hydrothiolation) of the bifunctional vinylsilane, a model reaction was initially performed between 1 and 2-mercaptoethanol, resulting in the formation of the newly sulfur-containing carbosiloxane 2 in excellent yield. The bis(thiol-ene) reaction of 1 was successfully extended to 2,2'-(ethylenedioxy)diethanethiol, affording a series of novel precisely defined, redox-active oxathiacrown macrocycles (3 and 4(n)) and sulfur-bearing linear oligo-carbosiloxanes (5(n) and 6(n)) with a backbone in which the -S(CH2)(2)-O(CH2)(2)O-(CH2)(2)S- chain and the -Si-O-Si- bond, bearing pendant ferrocenes, are alternately linked by ethylene bridges. These 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, even though UV photoinitiated thiol-ene reactions were found to be the most efficient hydrothiolations. All newly silicon-containing oxathioether-based ferrocenyl compounds 2-6(n) have been thoroughly characterized using a combination of elemental analysis, multinuclear NMR spectroscopy, FT-IR, and MALDI-TOF mass spectrometry to establish their chemical structures and chain-end functionalities. The electrochemical behavior of 2-6(n) has been examined by cyclic and square wave voltammetries, in dichloromethane solution using [PF6](-) and [B(C6F5)(4)](-) as supporting electrolyte anions of different coordinating ability. The sulfur-rich cyclic and linear oligomers 4(n)-6(n) exhibit excellent chemisorption properties and spontaneously form robustly adsorbed electroactive films onto Au or Pt electrode surfaces. The cation complexation ability of diferrocenyl silaoxathiacrown ether 3 has been studied using electrochemical and H-1 NMR spectroscopic techniques. The voltammetric behavior of receptor 3 has proved to be very sensitive to the presence of Hg2+ cation. The ability of macrocycle 3 to bind Hg2+ has also been investigated through electronic structure calculations, being the interaction between the cation and one of the cyclopentadienyl rings the responsible of the different behavior of both redox-active centers.