1-Cyano-1′-ethynyl-ferrocene: Synthesis and reaction chemistry

Abstract

Abstract Several consecutive synthetic methodologies for the preparation of Fe(η5-C5H4C N)(η5-C5H4C CH) (3) are described. Ferrocene Fe(η5-C5H4C N)(η5-C5H4C(O)Me) (1) reacts under typical Vilsmeier conditions to give as the main product Fe(η5-C5H4C N)(η5-C5H4CCl CH2) (2) and in minor yield Fe(η5-C5H4C N)(η5-C5H4CCl CHC(O)H) (4). Compound 2 could be directly converted to 3 by addition of KOtBu. The title compound is also accessible by the gradual reaction of 4 with NaOH to give Fe(η5-C5H4C(O)NH2)(η5-C5H4C CH) (5), which upon treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and P(O)Cl(OEt)2 produced 3. Organometallic 3 could be homo-coupled to [Fe(η5-C5H4C N)(η5-C5H4C C)]2 (7) in an Eglinton coupling upon addition of [Cu(OAc)2]. With [CuI] and NEt2H in dichloromethane, compound Fe(η5-C5H4C N)(η5-C5H4C CCH2NEt2) (6) was produced by copper-catalyzed three-component coupling. The structures of 3–5 in the solid state were determined by single crystal X-ray structure analysis. While the X-ray structures of 3 and 4 show no peculiarities, the structure of 5 possesses a network structure due to hydrogen bridge bond formation. The electrochemical behavior of 3, 6 and 7 was studied by cyclic voltammetry. It could be shown that 3 possesses a reversible redox event at 550 mV (ΔE = 65 mV), while in homo-coupled 7 two consecutive redox processes at 524 and 680 mV were found indicating that the ferrocenyl units in 7 can be oxidized in a stepwise manner to 7+ and 72+, respectively. The redox separation ΔE with 156 mV implies a possible electron transfer in the mixed-valent species 7+, which was confirmed by spectroelectrochemical studies. From these studies, 7 could be classified as a weakly coupled class II system according to Robin and Day.