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

The reaction of 2,3-diferrocenylcyclopropenone (1) with the electrophiles H[BF4] and [Et3O][BF4], respectively, produced the corresponding cyclopropenylium cations [Fc(2)C(3)OH][BF4] (2) and [Fc(2)C(3)OEt] [BF4(]) (3) (Fc = Fe(eta(5)-C5H4)(eta(5)-C5H5)). Reacting 3 with HNEt2 afforded the NE(t)2-functionalized species [Fc(2)C(3)NEt(2)][BF4] (4). However, when 2 was reacted with nucleophiles such as FcLi a ring opening of the C-3 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-alpha,beta-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 C-3 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-alpha, beta-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-alpha, beta-unsaturated ketones. However, the cyclopropenylium cations 2 and 3 show no long time stability under oxidative conditions. (C) 2017 Elsevier B.V. All rights reserved.