The Discovery of Ferrocene, the First Sandwich Compound -
George 6. Kauffman California State University, Fresno, Fresno, CA 93740 The serendipitous discovery in 1951 of ferrocene, the first recognized sandwich compound, opened an entire new area of research, and since that time numerous cyclopentadienyl derivatives of various metals and metalloids have been reported ( I ) .Today such compounds have not only contributed to our theoretical knowledge of chemical bonding hut also have found industrial applications ranging from antiknock additives to polymerization catalysts (2). In 1973 the tremendous importance of the metallocenes was underscored by the award of the Nobel Prize in chemistry jointly to Geoffrey Wilkinsou and Ernst Otto Fischer for their pioneering work on the development of the chemistry of sandwich compounds of transition metals, which has revolutionized organometallic chemistry and has had a significant impact on the broader fields of inorganic, organic, and theoretical chemistry (3). The discovery had its roots in the non-benzenoid aromatic systems (4) studied by Robert D. Haworth at the University of Sheffield in England. Peter L. Pauson, a young doctoral
Although CoC12 is more effectively used for such coupling reactions, Pauson chose FeC13 because it was available in anhydrous form and because it might oxidize dihydrofulvalene directly to fulvalene. Kealy and Pauson (8) obtained an orange solid (m.p. 173-174"C.), soluble in ether, benzene, and methanol, which they first believed was fulvalene, but which the analytical data (C, 64.6; H, 5.6; Fe, 30.1; m.w., 186.5) proved to he an organoiron compound, CloH~oFeor his(cyclopentadieny1)iron(I1). The new con~poundwas remarkably stable, being nnattacked by water, 10% NaOH, and conc. HC1, even on hoiling. Much of it was recovered unchanged from solution in conc. HzS04 after standing a t room temperature overnight. In order to confirm the presence of iron both qualitatively and quantitatively, the compound required prolonged hoiling with HN03 or fuming with HC104. Kealy and Pauson formulated the reaction for its formation as:
75), and after receiving his degree, immediately began postdoctoral research on non-benzenoid aromatics a t Duquesne University in Pittsburgh. In 1950, R. D. Brown of the University of Melbourne had predicted that the hypothetical hydrocarbon fulvalene
should be a non-henzenoid molecule (6), so in order to test Brown's prediction, Pauson assigned the synthesis of this compound to his new research student Thomas J. Kealy. Since Gilman and Lichtenwalter (7) had described the synthesis of biphenyl in almost quantitative yield by treating the Grignard reagent phenylmagnesium bromide with iron, cobalt, or nickel chlorides, Pauson attempted to synthesize dihydrofulvalene by reflnxing cyclopentadienylmagnesium bromide with anhydrous iron(II1) chloride in anhydrous ether and decomposing the reaction mixture with iced ammonium chloride solution:
4
D- -j Fe2+
n after initial reduction of FeC13 to FeC12 by the Grignard reagent. Aware that earlier workers had failed to obtain organoiron compounds (91, Kealy and Pauson attributed the stability of the new compound to "the tendency of the cyclopentadienyl group to become "aromatic" by acquisition of a negative charge, resulting in important contributions from the resonance form (11) and intermediate forms." Actually, the research team of Samuel A. Miller, John A. Tehboth, and John F. Tremaine of the British Oxygen Co., Ltd. had antedated Kealy and Pauson's discovery by reacting reduced iron with cyclopentadiene vapor in a nitrogen atmosphere at 300DC.( 1 0 ) . Their internal report was written in 1948, but they did not submit their article to the Journal of the Chemical Society until 1951 (received July 11,1951almost a month before Kealy and Pauson's note, dated August 7, 1951), and it was not published nntil the following year. Geoffrey Wilkinson believes that workers at Union Carbide were probably the first to prepare the compound-in the 1930's-while cracking cyclopentadiene through hot iron tubes ( l b). William von Eggers Doering first suggested the sandwich
Volume 60
Number 3
March 1983
185
structure in a conversation with Pauson, but neither considered publishing this idea until an X-ray diffraction study of the compound was made (Ic, 11). In September 1951,Pauson had arranged for Prof. J. M. Robertson of the University of Glasgow to carry out such a study, hut Robertson was the Baker lecturer at Cornell University at the time, which led to unforeseen delays in getting the work under way. In a note received hv the Journal of the American Chemical Societv on March 24; 1952, and pubiished on April 20,1952, ~ i l k i ~ s o n , Rosenblum, Whitinx, and Woodward, on the basis of magnetic susceptibility, infrared absorption spectrum, and zipole moment measurements, proposed the now familiar sandwich structure for the compound (12):
Independently of Wilkinson et al., E. 0. Fischer (141, by X-ray diffraction studies, arrived at the antiprismatic structure. The X-ray diffraction studies of Eiland and Pepinsky (15) and Dunitz end Orgel (16) were initiated as a result of the first Wilkinson et al. note (12). Literature Cited (1) la) Rausch, M., VogeLM., and Rosenber~,H., J. CHEM. EollC., 34,263 (1857);Rauseh, M.D.,Cand. J. Chem.,41,1289(1963);(b) Hunt. M. D.,ibid.,37,SfiS (1%60);Rausch, C. B..Educ. Chem.. 14,110 11977);(c) Psuron, P. L..Buorr. Rcu. (London), 9,391 (1855): Purr & A p p l . C h e m . 49,639 11977). (2) G h a n , H., Adu. Ownomat. Cham. 7.1 (1968):Stone,F. G. A,. and West, R. (Editors), "Advances in Organometallic Chemistry: Academic Press,New York. 13661976. (31 Seyferth, D.. and Daviann. A,. S~icncr,182,609 (1973). (4) Ginsburg, D.. "Nun-Benzenoid Aromatic compounaa: Interscience Publishers, New Ynrk, 1959; Breslow, R., Chem Eng. News, 43 (261.90 (June 28,1965). 15) Haworth, R. D., Moore, B. P.,and Pauson, P. L.. J . Chem. Soi,1948,1045. (6) Brown, R. D., Nature. 165,Sfifi 11950). Although fulvalene has never been isolated. phenyl- and halogen~substitutediuivalenes hawe been prepared as well as fuivdene complexes of iron and manganese IRausch, M.D., Kouar, R. F., and Kraihanzel, C. S., J Amer. Chem Soc. 91, 1259 (19601). For an X-ray diffrfraction study of h L l W M. R . m d Wormdd. L,Inoig Cham.,8,1970 v d w e ) d i i r ~(CiaHn)zFe%seeChurchill, ,
