Multi-functionalized ferrocenes: –Synthesis and characterization –

Ferrocenes with up to four formyl functionalities have been synthesized by selective one- or two-fold ortho-lithiation of 1,1′-di(1,3-dioxan-2-yl)ferrocene followed by the reaction with dimethylformamide. Subsequent deprotection afforded tri- and tetraformyl-ferrocene in good yields. Wittig reactions convert the aldehydes to tri- and tetravinyl-ferrocene. The structures of three formyl-ferrocenes in the solid-state have been determined by single crystal X-ray diffraction. In two of the structures the ferrocene moieties form 1D chains by parallel displaced π-interactions with Cp⋯Cp distances as short as 3.21 A. Electrochemical measurements (CV) demonstrated an irreversible behavior of the electron-poor multi-formyl ferrocenes. Reducing the measurement temperature down to −40 °C and the usage of [NnBu4][B(C6F5)4] prevents side reactions and hence reversible redox-processes could be obtained. It was found that the number of formyl functionalities correlates with the ferrocenyl's redox potential in a strictly linear fashion.