Enroute to other things I ran across an old Gulf R&D patent, US 3294685, titled “Organic compositions containing a metallo cyclopentadienyl”. Sifting through the description my eye caught the interesting content below:

July 1941. A test spray was prepared by dissolving 2.5 grams (3.2 percent) of iron dicyclopentadienyl in ml. of a typical household insecticide base oil. The tests made with this solution employed a dosage of IO-second discharge. An equilibrium :period of 15 seconds followed by an exposure period of 70 seconds, during which the mist was permitted to settle on adult house flies confined in a screen-covered dish, was employed in the tests. The results of the tests showed that of the flies which had been contacted with the base oil containing 3.2 percent by weight of iron dicyclopentadienyl, 53.6 percent were dead after 24 hours. Of theflies which were contacted with the base oil alone, only 13.0 percent were dead after 24 hours. Check flies which were confined for 24 hours without having been contacted with either the base oil or the base oil containing iron dicyclopentadienyl had a death rate of only 0.4 percent. The better than fifty percent mortality of the flies treated with the base oil containing iron dicyclopentadienyl is indicative of the insecticidal properties of naphthas containing a small amount of iron dicyclopentadienyl. Naturally, the amount of metallo cyclopentadienyl used in insecticidal compositions-will vary with the particular compound employed and also depends upon the particular insects for which the spray is intended. The amount of iron dicyclopentadienyl employed in insecticidal compositions intended for use on flies is between about 1.0 and 10.0 percent by weight.

Ya know, a greater than 50 % kill rate seems to be getting a bit sporty for the flies. The ol’ boys at Gulf were studying the suitability of a variety of ferrocene analogs for fuel additive application. What lead them to go from octane enhancement and smoke control to killing flies is not revealed in the patent.

Notice the nomenclature in the patent language. The word ferrocene is not mentioned. Looking at the timeline we see that the Gulf ‘685 patent was filed April 21, 1952, not long after the publication of this curious iron cyclopentadienyl compound by two groups, Kealy & Pauson on 12/15/51, and Miller, Tebboth, and Tremaine on 1/1/52. Though Pauson and Keely published first, an examination of the papers show that Miller, Tebboth, and Tremaine were first to submit- July 11, 1951 vs August 4, 1951 for Pauson and Kealy.

The day before Gulf filed the patent application, April 20, 1952, a groundbreaking paper by Wilkinson, Rosenblum, Whiting, and Woodward was published on the proposed structure of iron bis-cyclopentadienyl. It is reported that the name ferrocene was invented by Mark Whiting, a student of R.B. Woodward and coauthor of the 1952 paper in JACS. The name derives from the ferrous ion and the aromatic (“benzene”) nature of the cyclopentadienyl ligands.

The curious structure was proposed largely on the strength of a single C-H IR band at 3.25 μ. Since all of the C-H bonds appeared to be equivalent, the only structure compatible with the formula, charges and symmetry was the famous η5 (eta five) sandwich structure. Later the word metallocene finds use for this class of substances.

There is disagreement as to some of the details outlined above. An excellent article by Pierre Lazlo and Roald Hoffmann navigates some of the narrower channels in the history of ferrocene. It is well worth the read. Lazlo & Hoffmann suggest that Woodward is thought to have conceived the sandwich structure.

Ferrocene and derivatives would soon prove useful in many areas. A more obscure application is found in the field of rocket propellant additives and function as burn rate stabilizers. In fact, certain ferrocene derivatives appear on the US Munitions List, 22 CFR 121.1, Category V, (f)(4) Ferrocene Derivatives. A good overview of ferrocene and other metallocenes can be found in Wikipedia.

Circling back to the beginning of this piece, the patent application for Gulf ‘685 was filed 4/21/52, only 4 months after the publication on 12/15/51 of the Pauson & Kealy paper and two weeks later the Miller, et al., paper on 1/1/52. In the 4 months between Pauson & Kealy and the Gulf patent filing, two independent groups had published papers reporting the preparation of iron dicyclopentadienyl by different methods, a Harvard group had postulated a structure for the compound using IR data and a novel bonding type, and the Gulf R&D group had produced various analogues for testing as fuel additives. In this short time interval, the first organo-iron compound was taken from a literature source through industrial R&D and a patent application. As a premium, Gulf even determined that it had insecticidal properties. Much happened in a short time.


Wilkinson, Rosenblum, Whiting, and Woodward J. Am. Chem. Soc., 1952, 74 (8), pp 2125–2126. DOI: 10.1021/ja01128a527

Kealy and Pauson, Nature, 168, 1039 (1951). Received Aug. 7, 1951.  DOI: 10.1038/1681039b0

Miller, Tebboth, and Tremaine J. Chem. Soc., 1952,0, 632-635. Received July 11, 1951. DOI: 10.1039/JR9520000632

Laszlo P., Hoffmann R. ACIEE, 2000 Jan; 39(1):123-124.  DOI: 10.1002/(SICI)1521-3773(20000103)39:1<123::AID-ANIE123>3.0.CO;2-Z