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
Lamentations on Chemistry 
11 comments
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July 27, 2017 at 5:59 am
Philip Rakita
As a chemist by training and would-be lawyer, I find this post fascinating. I grew up, chemically speaking, during the era when Metallocenes were all the rage. I met Wilkinson and knew Hoffman.
And, as for the insecticide application, well, you try to write your patents as broadly as you can get away with, never knowing where the commercially successful application will be down the road.
Ferrocene never made it as an insecticide (or a fuel additive either), but a different metallocene, MMT [methylcyclopentadienyl manganese tricarbonyl] was the material of choice to replace tetra ethyl lead as an octane enhancer in gasoline.
Philip
July 27, 2017 at 9:44 am
gaussling
Thanks for mentioning MMT. A good example of large scale use of a metal cyclopentadienyl complex. CpML3 complexes are sometimes referred to as a “piano stool” complexs.
August 4, 2017 at 4:50 pm
Tom
The fly killing experiment might have been included as needed proof of utility, a more serious requirement for patentability in the past. Does the 1941 date imply a much earlier synthesis by Gulf? They probably thought there was a sigma bond connecting the ferrous iron atom to a single carbon atom in each cyclopentadienyl ring. I speculate Gulf made it before WWII, couldn’t find a use for it, and forgot about.
August 6, 2017 at 12:18 pm
gaussling
Hi Tom,
Thanks for the informative comment. I don’t know the reason for the 1941 date. Good question. I just did a cut and paste from the patent into the blog without really thinking about it..
August 10, 2017 at 2:58 pm
Philip Rakita
Yes, that 1941 date is particularly queer. As far as the published history of Ferrocene goes, the first synthesis was in late 1951, It seems to me that there could be two explanations. (1) Gulf really did synthesize ferrocene a decade before anyone else and kept it secret until they pulled out the old research results to embellish their patent or (2) the 1941 is merely a typo for 1951. Inquiring minds want to know. [Oh, and if we keep this thread going, we’ll all end up as co-authors for a neat paper to appear in a future edition of the Journal of the History of Chemistry. 🙂
August 11, 2017 at 7:34 am
gaussling
Always thinkin’ Phil. That’s what I like about you.
August 11, 2017 at 7:39 am
Philip Rakita
I’m serious. The “History of Ferrocene” that appears in Wikipedia is only the skeleton of the story. Putting some flesh on those bones would make entertaining (and informative) reading.
August 13, 2017 at 10:36 am
Tom
I printed out the ‘685 patent and the 1941 date is the tail end of a reference to the insecticide test method. An interesting thing about this patent is the 14 1/2 year delay between the April 1952 application date and issuance of the patent. Public PAIR is down on Sundays but I’ll take a look at the file wrapper.
August 13, 2017 at 11:54 am
Philip Rakita
It will be interesting to see what you find. I remember looking at these old patents at some point in graduate school (1966-70) and ran across an industrial patent that reported the synthesis of Bis(cyclopentadienyl)Cu(II).
Never saw such a compound before or since and I expect it was one (of several?) paper claims. Patents are full of funny stuff.
August 14, 2017 at 12:44 pm
Tom
Public PAIR was back up but unfortunately there was nothing. Patent files this old still must not be scanned by the Patent Office. These files are public records but would require getting a law firm in Alexandria to go physically copy them. With 14.5 years of prosecution this would be a pretty thick file.
The ‘685 patent shows ferrocene as a sigma bonded structure. Claim 1 of this patent concern alkyl substituted ferrocenes, but not the parent molecule. The claim is in the form of an empirical formula and would cover all forms of ferrocene. Even though the Inventors had an incorrect structure for ferrocene, that wouldn’t invalidate a possible patent claim to the compound. Other claims in the patent concern compositions of any ferrocene and with hydrocarbon fuels or with candle wax.
What did invalidate any possible patent claim to the parent ferrocene were two prior references cited in the patent S. A. Miller, J. A. Tebboth, and T. Tremaine, Journal of The Chemical Society, 632, February 1952 and T. J. Kealy and P. L. Pauson, Nature, 168, 1039, 1951. There is also a cryptic mention to making ferrocene directly from iron metal and cyclopentadiene at 300 oC but no literature reference is provided.
I guess I am surprised that ferrocene wasn’t made earlier. It’s not a hard synthesis, the Grignard derived from cyclopentadiene has been known since at least 1938 (Shappirio US 2,134,625), and the potassium salt of cyclopentadienyl anion I think has been known since the turn of the twentieth century.
August 15, 2017 at 5:48 am
Philip Rakita
Tom,
You’ve captured two interesting points here. Those of us who grew to scientific adulthood post 1952 have always taken for granted the special kind of bonding that exists in metal-orgaincs like the pi-cyclopentadiene compounds. Before that time, the classic example of metal-organics was the alkyl zinc materials and they were viewed as having classic sigma-bonds. And nobody really understood the structure of Zeise’s Salt.
The second point is that industrial chemists do follow the current literature assiduously and look to capitalize on any perceived opportunity for a new commercial enterprise. Hence Ferrocene in insecticides or motor fuel or whatever. Getting priority in the patent world is the predominant driver.