TETRAFERROCENYLETHYLENE, A CHIRAL, ORGANOMETALLIC PROPELLER - SYNTHESIS, STRUCTURE, AND ELECTROCHEMISTRY

Tetraferrocenylethylene is synthesized from diferrocenyl ketone by three different reductive carbon-carbon bond-forming methodologies: (a) an ultrasound-promoted McMurry reaction with low-valent titanium, (b) a modified Clemmensen reduction with zinc and trimethylchlorosilane, and (c) an aluminum-assisted oxygen-tellurium exchange in diferrocenyl ketone and subsequent thermolysis. Mechanistically, the first two methods involve carbenoid intermediates, whereas the third method consists of a twofold extrusion process from a preformed cyclic dimer of diferrocenyl telluroketone. Tetraferrocenylethylene shows spectral properties which are in accord with a sterically highly congested conformation. Noteworthy features include the very low C=C stretching vibration of 1474 cm(-1) in the Raman spectrum, indicative of an elongated and weak C-C double bond, and the magnetic inequivalence of the H-1 and C-13 NMR signals of the hydrogens and carbons of the substituted cyclopentadienyl rings, indicative of a frozen molecular propeller conformation. An X-ray single-crystal structure analysis shows tetraferrocenylethylene to be a chiral, strongly twisted, and sterically congested olefin. The bond length of 138.1 pm of the central double bond and the angles of twisting and torsion are close in value to those of the most distorted olefins known. The helical chirality stems from the uniform twisting of the four alternatingly arranged ferrocenyl substituents. Electrochemically, tetraferrocenylethylene can be oxidized to the tetracation in accord with the number of ferrocenyl units. The donor ability of tetraferrocenylethylene compared to ferrocene itself is strongly enhanced with Delta E(1/2)(1) = -220 mV.