Ferrocene—how it all began

P.L. Pauson / Journal of Organometallic Chemistry 637–639 (2001) 3–6 5In those days, the Chemical Society published lists ofpapers accepted for publication in its journal. About amonth before our Nature note was due, I read of apaper on ‘Dicyclopentadienyliron’ by Miller, Tebbothand Tremaine of the British Oxygen Co. I wrote to Dr.Miller and told him that we appeared to have the samecompound, and so it proved when he sent me a proofcopy of the J. Chem. Soc. paper. It also showed thatthey had used a radically different route and that theirpaper had been submitted before we ever tried thereaction, but it was to appear in the February 1952issue of J. Chem. Soc., whereas Nature’s faster methodof publication gave us the apparent precedence with a1951 publication date. In an accompanying note Millermentioned that they had had the compound for about3 years. Even this, however, was quite possibly not thefirst preparation. The following tale was told about ayear later by E.O. Brimm of Linde Air Products.Brimm, who had done some work on metal carbonylsand other organometallics, was one of several industrialchemists who, on reading our report, wanted to makesome of the compound. Therefore he enquired of acolleague at Union Carbide (Linde’s parent company)whether they had any cyclopentadiene. The reply thatthey no longer did so was accompanied by the statementthat, some years previously, they had terminatedwork on the cracking of cyclopentadiene because ayellow sludge clogged the iron pipes they used. Theyhad not attempted to isolate or analyse this but hadkept a bottle of it; it was ferrocene. The British Oxygengroup’s synthesis was, in fact, not dissimilar as it involvedpassing cyclopentadiene over a heated, iron-containingammonia synthesis catalyst.As is well-known, the correct structure of dicyclopentadienylironwas put forward in two independent publications:one by E.O. Fischer and W. Pfab, the other byG. Wilkinson, M. Rosenblum, M.C. Whiting, and R.B.Woodward. Both groups had repeated our preparationand made physical measurements that strongly supportedthe ‘Doppelkegel’ or ‘sandwich’ structure butfell short of full proof. Fischer relied chiefly on preliminaryX-ray data indicating that the molecules arecentrosymmetric, while the Harvard group cited thesingle C–H frequency in the IR and the diamagnetism.Only weeks went by before two independent completeX-ray structure determinations were published. 5At Woodward’s suggestion that the compound mightbe aromatic, Whiting and Rosenblum went on to showthat it readily undergoes Friedel–Crafts acylation and,on this basis, suggested the name ‘ferrocene’—with the‘ene’ ending implying aromaticity. We had indeed prepareda novel, non-benzenoid aromatic, but a veryunexpected addition to that class!5By Dunitz and Orgel and by Eiland and Pepinsky.Both Fischer and Wilkinson very soon started addingbis-cyclopentadienyls of other transition metals; TomKealy had tried nickel and cobalt but was thwarted bythe insolubility of their anhydrous halides, which preventedtheir reaction with the ethereal solution of cyclopentadienylmagnesiumbromide. Fischer overcamethis problem by turning to the hexammine complexes ofthese metals and Wilkinson by using the acetylacetonates.In the meantime I was spending the academic year1951–1952 at the University of Chicago as a postdoctoralfellow working on peroxide chemistry with M.S.Kharasch. My next move was to Harvard where I wasappointed to a ‘Du Pont Fellowship’ to work independently.Although my application for this had beenbased on a scheme to synthesise the alkaloid colchicine(a tropolone), Professor Bartlett, as Chairman of thedepartment, made clear that I was free to do whateverI liked. It cannot have been unexpected that by then Iwas anxious to play a part in the work on metalloceneswhich was in full swing in both Woodward’s andWilkinson’s laboratories.En route to Harvard, I visited the Du Pont laboratoriesat Wilmington. There, Dr Victor Weinmayr wasdoing work on the organic chemistry of ferrocene. Ontalking about other metals, he told me that nickelworked and that I should feel free to use this informationprovided that I did not disclose its source.Combining this hint with what I knew already, I thereforeemployed nickel acetyl acetonate in my first experimentsat Harvard and, by the time Wilkinson andCotton both returned from their summer vacations, Iwas able to show them a sample of the beautifulemerald-green crystals of nickelocene. They showedthat it is paramagnetic and we extended our jointstudies to the benzo derivatives of ferrocene and of thecobaltocenium cation, which I prepared from indene bysimilar techniques, leaving them to do all the physicalmeasurements.During this and the following 2 years, Fischer andWilkinson made bis-cyclopentadienyl–metal and cyclopentadienylmetalcarbonylcompounds of most of thetransition metals. It seemed almost inevitable that theywere in constant competition as to who would be firstwith the next, fairly obvious target. 6At the end of my year at Harvard, I returned to theUK to take up a lectureship in organic chemistry atSheffield University. There, I soon had the benefit ofPhD students to do all the hard work for me. Weconcentrated on the organic aspects. The first student(G.D. Broadhead), after showing that ferrocene undergoesacetylation very much faster than anisole, foundthat it would undergo formylation under Vilsmeyer6 This competition effectively ceased in 1955, when Fischer’s success(with W. Hafner) in making bis-benzenechromium led him toconcentrate heavily on arene complexes.