Heteroatom-Bridged ortho-Biferrocenes: Stereoselective Synthesis, Structural Features, and Electrochemical Properties

A versatile synthetic protocol is described for the synthesis of heteroatom-bridged biferrocene derivatives, including the symmetric species 2-SnSn and the planar-chiral species 2-SnSi; 2-BSi, and 2-SnP. Treatment of (pR,S-s)-2-lithio-1-(p-tolylsufinyl)-ferrocene with 0.5 equiv of Me2SnCl2 afforded the tin-bridged biferrocene 1-Sn. A second Sn, Si, or P bridge was then incorporated by substitution of the sulfinyl group in the ortho-position with a dimethylstannyl, dimethylsilyl, or tert-butylphosphino group, respectively, togive the doubly bridged biferrocenes 2-SnSn, 2-SnSi, and 2-SnP. The dimethylstannyl moiety of 2-SnSi was subsequently replaced with a borane bridge via a two-step transmetalation procedure comprising treatment with HgCl2, followed by PhBCl2. The formation of ferrocene-fused six-membered heterocycles was confirmed by multinuclear NMR spectroscopy and high resolution MS analyses. The stereochemical configuration of the chiral biferrocenes 2-SnSi, 2-SnP, and 2-BSi was studied by single-crystal X-ray diffraction, chiral HPLC, and optical rotation measurements. The redox characteristics and absorption properties were investigated as well. The longest wavelength absorption experienced a bathochromic shift and an increase in intensity for 2-BSi relative to 2-SnSn and 2-SnSi, indicative of significant charge transfer character. The cyclic voltammograms of 2-SnSn and 2-SnSi displayed two separate one-electron oxidations as expected for the presence of two ferrocene units in close proximity to one another. The incorporation of boron in 2-BSi resulted in an anodic shift of both oxidation waves and an enlarged peak potential separation. The chemical oxidation of 2-BSi was carried out with [Ag(CH2Cl2)]{Al[OC(CF3)(3)](4)}, and the reaction was monitored by NMR. spectroscopy. Attempts to crystallographically characterize the corresponding doubly oxidized species proved unsuccessful.