B. Bildstein et al., TETRAFERROCENYLETHYLENE, A CHIRAL, ORGANOMETALLIC PROPELLER - SYNTHESIS, STRUCTURE, AND ELECTROCHEMISTRY, Organometallics, 14(9), 1995, pp. 4334-4342
Tetraferrocenylethylene is synthesized from diferrocenyl ketone by thr
ee different reductive carbon-carbon bond-forming methodologies: (a) a
n ultrasound-promoted McMurry reaction with low-valent titanium, (b) a
modified Clemmensen reduction with zinc and trimethylchlorosilane, an
d (c) an aluminum-assisted oxygen-tellurium exchange in diferrocenyl k
etone and subsequent thermolysis. Mechanistically, the first two metho
ds involve carbenoid intermediates, whereas the third method consists
of a twofold extrusion process from a preformed cyclic dimer of diferr
ocenyl telluroketone. Tetraferrocenylethylene shows spectral propertie
s 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 N
MR signals of the hydrogens and carbons of the substituted cyclopentad
ienyl rings, indicative of a frozen molecular propeller conformation.
An X-ray single-crystal structure analysis shows tetraferrocenylethyle
ne 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 distor
ted olefins known. The helical chirality stems from the uniform twisti
ng of the four alternatingly arranged ferrocenyl substituents. Electro
chemically, tetraferrocenylethylene can be oxidized to the tetracation
in accord with the number of ferrocenyl units. The donor ability of t
etraferrocenylethylene compared to ferrocene itself is strongly enhanc
ed with Delta E(1/2)(1) = -220 mV.