From diferrocenyl-cyclopropenone to diferrocenyl-cyclopropenylium cations and triferrocenylpropenones: An electrochemical study

Abstract

The reaction of 2,3-diferrocenylcyclopropenone (1) with the electrophiles H[BF4] and [Et3O][BF4], respectively, produced the corresponding cyclopropenylium cations [Fc2C3OH][BF4] (2) and [Fc2C3OEt][BF4] (3) (Fc = Fe(η5-C5H4)(η5-C5H5)). Reacting 3 with HNEt2 afforded the NEt2-functionalized species [Fc2C3NEt2][BF4] (4). However, when 2 was reacted with nucleophiles such as FcLi a ring opening of the C3 cycle was observed and E- and Z-1,2,3-triferrocenylpropenones Z-5 and E-5 were formed. Crystallization of a dichloromethane solution containing 3, layered with hexane, gave the BF3 adduct of diferrocenylcyclopropenone 6 at the interphase. The molecular solid state structures of Z-5, E-5 and 6 were determined by single crystal X-ray diffraction. Compound E-5 possesses a helical chirality and crystallizes in an enantiopure form. The electronic properties of the ferrocenyl-substituted cyclopropenylium cations 2–4 and of the triferrocenyl-α,β-unsaturated ketones Z-5 and E-5 were studied by cyclic voltammetry and square wave voltammetry. In comparison to the cyclopropenone derivative 1, the ferrocenyl oxidation processes of the cyclopropenylium cations 2 and 3 are shifted towards higher potentials, which is caused by the lower electron density of the strained C3 rings. Compound 4 decomposes during the electrochemical measurements. Furthermore, it could be shown that E/Z isomerism has only small effects on the electronic properties of the triferrocenyl-α,β-unsaturated ketones. Spectroelectrochemical UV–Vis/NIR measurements carried out on 1 and Z-5 and E-5 confirmed electron-transfer interactions within the 2,3-diferrocenylcyclopropenone and the 1,2,3-triferrocenyl-α,β-unsaturated ketones. However, the cyclopropenylium cations 2 and 3 show no long time stability under oxidative conditions.