0
FERROCENE: A NOVEL ORGANOMETALLIC COMPOUND' MARVIN RAUSCH, MARTIN VOGEL, and HAROLD ROSENBERG Wright Air Development Center, Wright-Patterson AFB, Ohio
ONE of the exciting rewards of research is that occasiollally an experiment will produce a wholly unexpected result, which in turn will open up an entirely new area of research. such was the case in 1951 (I) by lcealy and pausondiscovered ferrocene2 accident (1). These investigators were attempting to oreoare the hvdrocarbon fulvalene (11) bv the 0% d a ~ o u .of cyclo~entadienylmagnesiumm bromide with anhydrous ferric chloride in ether solution. Instead
entire new area of researrh, but to disruss only the chemistry of ferrocene and its derivatives. For further information regarding any of the other metal-cyclopentadienyl derivatives, it is suggested that the reader consult either of two excellent reviews on the subjert (5, 61, and recent publications by G. Wilkinson, et a/.. and E. 0. Fischer, et al. PREPARATION
The original synthesis of ferrocene as described by Kealy and Pauson illustrates a si~nple laboratory method for preparing this compound (I). These investigators suggest that ferrocene is formed, after initial reduction of ferric chloride by the Grignard r e agent, according to the equation: 2CjHIMgBr
of the expected product, an unusually stable orange crystalline compound was isolated for which analysis gave the formula Ii'eCloHlo. This compound was soluble in rommon organic solvents and was remarliably stable towards acid and base. At almost the same time this compound was discovered by Miller, Tebboth, and Tremaine, who isolated it from the reaction of cyclopentadiene vapor with reduced iron a t 300°C. (9). These two nearly simultaneous discoveries were striking in contrast to the repeated failures of earlier investigators to prepare organometallic compounds containing iron (5, 4). Since the initial discovery of ferrorene only five years ago, over 100 technical publications have appeared in the chemical literature concerning metal-cyclopentadienyl compounds. Indeed, a t the present time, cyclopentadiene derivatives of 41 metals and metalloids have been reported. I n addition, many metalcyclopentadienyl compounds containing CO and NO ligands have been reported. I t is not the purpose of this article to review this
' Opiniona expressed are those of the authors and do not news~ m i l yexprem the official opinion of the United States Air Force or Wright Air Development Center. "Ferrocene" is now a. commonly accepted name for bis-(cyclopentadienyl)iron(II). The name was suggested by Professor R. 13. Wood\vard, who first showed that this compound can be considered as a. ucw type of aromatic system. Ferrocene thus emphasizes the analog" to benzene. 268
+ FeCI1
-
Fe(CsH&
+ MgBrz + IllgCI?
The reaction of reduced iron with cyclopentadiene vapor a t 300°C., discovered by Miller, Tehboth, and Tremaine ( 2 ) ,has not been extensively used, since more convenient syntheses have been developed. The reaction of cyclopentadiene with ferrous chloride in the presence of an organic base was initially reported to produce only poor yields of ferrocene (7). 2CsH.
+ FeCI. + 2 Base
-
Fe(CsHs)g
+ 2Base.HCI
By preparing the ferrous chloride in a more reactive form and carrying out the reaction without a solvent, excellent yields of ferrocene have recently been obtained by this method (8). The reaction of cyclopentadienylmagnesium bromide with the ferrous acetylacetonate-pyridine complex in benzene solution has been found to produce an almost quantitative yield of ferrocene (9). By bubbling nitrogen through iron pentacarbonyl and cyclopentadiene separately a t room temperature and heating the mixed vapors a t 250°C., low yields of ferrocene have been obtained (9). If this reaction is carried out a t lower temperatures in the liquid phase. intermediate iron cyclopentadiene-carbonyls can he isolated, which in turn may be converted to ferrocene by thermal decomposition (10, 11). A novel synthesis of ferrocene has recently been reported which involves a replacement reaction (12). Sodium acetate, cyclopentadiene, and mercuric chloride are dissolved in methanol and stirred for several days. The precipitate which is formed is filtered, mixed with iron powder, and the mixture covered wit.h tetrahydrofuran while stirring. Following a rather vigorous reartion, water is added and the aqueous phase exbrarted; 24rr/0-3070 yields of ferrocene result. JOURNAL OF CHEMICAL EDUCATION
Perhaps the most satisfactory laboratory method for preparing ferrocene, as well as many other metalcyclopentadienyl compounds, has been described b y Wilkinson, et ab (8, 13). Ferrous chloride is reacted with cyclopentadienylsodium, using either tetrahydrofuran or 1,2-dimethoxyethane as the solvent.
By this method, 85%-90% yields of ferrocene can be obtained. Substituted ferrocenes can be prepared directly by the reaction of ferrous chloride with a substituted cyclopentadienylmagnesium halide or lithium compound. By this method, Pauson has obtained a series of pheuylsubstituted ferrocenes (14). The preparation of his(indenyl)iron(II) from indenyllithium or indenylmagnesium bromide and ferrous chloride represents another example of the direct synthesis of a substituted ferrocene (15, 16). PHYSICAL PROPERTIES AND STRUCTURE
Ferrocene is an orange crystalline solid, which melts a t 173'-174°C. without decomposition (1, 2). The vapor pressure of this compound has been measured up to 400°C.,and a molecular weight of 186 has been calculated for the vapor, indicating it is undissociated and monomeric over the temperature range investigated (17). From these measurements the normal boiling point has been calculated to be 249°C. Ferrocene is readily soluble in common organic solvents such as benzene, ether and ethanol. It is insoluble in and unattacked by water, 10% sodium hydroxide solution, and even boiling concentrated hydrochloric acid (1). The dipole, moment of ferrocene is effectively zero, and it is diamagnetic. The remarkable thermal stability of this compound is indicated by its resistance to pyrolysis a t 470°C. in a nitrogen atmosphere (18). The heat of combustion of ferrocene has been det,ermined, and the heat of formation calculated. The energy of the iron ho carbon bonding plus the resonance energy can be calculated to be 113 kcal. (19). The unusual stability of the compound is in accord with this high value. The ultraviolet spectrum of ferrocene in ethanol or hexane shows maxima a t 325 m.11 and 404 m.p. The iufrared spectrum is strikingly simple. There is a single sharp absorption peak a t 3 . 2 7 ~in the carbonhydrogen stretching region, suggesting that all the carhou to hydrogen bonds in the molecule are equivalent. There are only four other strong absorption bands, a t 12.15~,9.95p, 9 . 0 0 ~ ,and 7 . 0 5 ~ . I t is interesting to note that the bauds a t 9 . 9 5 ~aud 9.0011 are retained in all monosubstituted derivatives of ferrocene but disappear when both rings are substituted. This observation has proved very useful in structural determinations (l4,ZO). Ferrocene has been postulated to possess a pentagonal antiprismatic structural configuration, as illustrated in structure I. Soon after the discovery of ferrocene, several investigators suggested this structure in preference to the structures originally proposed VOLUME 34, NO. 6, JUNE, 1957
by Kealy and Pauson, structures I I I and IV ( 1 ) .
Wilkinson, et al., suggested the peutagoual autiprismatic structure on the basis of zero d~polemoment, non-polar nature and single C-H absorption band in ferrocene (21). This type of configuration, in which the two cyclopentadienyl rings are symmetrical vith respect to the iron atom, has been appropriately termed a "sandwich" structure by these investigators (22). Conformation of the pentagonal antiprismatic structure of ferrocene has been made by means of X-ray crystallographir data (25, 24, 25). While the structural configuration of frrroccne appears to be certain, the nature of the bonding between the irou atom and the cyclopentadienyl rings is still in dispute (24-51). AROMATIC CHARACTER
Many of the chemical transformat,ious which femocene will undergo result from the "aromatic" character of this compound, first demoustrated by Woodward, et al. (32). These iuvestigators found that ferrocene does not react with maleic anhydride in hoiling benzene and is not hydrogenated when a reduced platinum oxide catalyst is used; such reactions are normally successful with polyolefinic substances. I'urthermore, ferrocene was found to undergo the Friedel-Crafts reaction (see below). These facts, together with the great thermal stability and resistance to acidic reagents, suggested the aromatic character of ferrocene. 1Many other chemical reactions have since been carried out which support this viewpoint. Friedel-Crafts Reaction. One of the most important reactions of ferrocene is its ability to undergo FriedelCrafts acylation. By this method, a variety of ferrocenyl ketones have been prepared (20, 32, 55, 34, 37). Fermcene derivatives in which either one or both cyclopentadienyl rings are acylated can be ohtained, depending on the rat,io of the catalyst, acyl halide and ferrocene used. Fe(ChHs).
Fe(CiHi)t
+
/I
-
RCX
+ 2 d!x
AlCl:,
AlCh
.,
/I
CeHsFeC6H&R
-11
dFe(CsHCR)r
+ HX
+ 21HS
R = Aryl or Alkyl; X = CI or Br
Although aluminum chloride has been the most commonly used catalyst, other catalysts which are effective include hydrogen fluoride, staunic chloride, and boron trifluoride. Acid anhydrides react successfully to yield corresponding acyl derivatives. Attempts to alkylate ferroceue by means of alkyl halidcs aud alcohols have produced low yields of mixed polyalkylated products (38). By a careful chromatographic separation of the reaction products of ferrocene, aluminum chloride, and acetyl chloride, two ieomeric bis-(acetyl)ferrocenes can be isolated in the ratio of about 60: 1. The major product is the symmetrical disuktituted derivative. The infrared spectrum and chemical reactions indicate
that in the minor product both acetyl groups are &ached to one ring (20). Arylation with Diazonium Salts. Another example of the aromatic nature of ferrocene is illustrated by the ease with whirh it is arylated by diazonium salts (37,40,4j, 42).
+
F C ( C ~ H ~ )Aryl-N?+ ~
H
+
+ + H+
CnHaFeCsH~Aryl h ' *
This reaction is especially convenient for the synthesis of monoarylferrocenes, although mixtures containing bis- and polysubstituted derivatives are sometimes obtained in low yields Bis(ary1)ferrocenes can best be prepared starting with the corresponding arylcyclopentadienes (1.4). Arylation can also he accomplished by reacting the ferricinium ion, [Fe(CsHr)J+, with diazonium salts in acid media (@), indicating that the formation of arylferrocenes probably proceeds by a free-radical mechanism. IJsing N-nitrosoacetanilide as the source of phenyl radicals and cyclohexane as the solvent, both mono- and his-(pheny1)ferrocene can be obtained (41). OTHER REACTIONS
Oxidation. Iperrocene is readily oxidized ,under a variety of conditions to the singly charged cation, to which the name "ferricinium ion" has been given (52). Oxidizing agents which have been used include air (in the presence of acid), halogens, ceric sulfate, anthraquinone or sulfur dioxide in hydrogen fluoride, and anodic oxidation (18, 35, 54, 55). Aqueous solutions of the cation are dichroic, appearing red when concentrated and blue when dilute. While acidic solutions of ferricinium salts are stable a t room temperature, the addition of excess base produces immediate decomposition. Reducing agents such as ascorbic acid or zinc dust readily reduce ferricinium ion to ferrocene. The ease of oxidation of ferrocene has prompted its use as a reagent for the reduction of carbonium ions to stable free radicals (56). Metalation. The metalation of ferrocene has been reported by several investigators. The reaction of ferrocene with n-butyllithium in ether produces a mixture of mono- and bis-(1ithio)ferrocene (57, 59).
either ether-alcohol or glacial acetic acid solution to produce a mixture of mono- and his-(chloromercuri) ferrocene (37, 38).
Monorhloromercuriferrocene can he symmetrized uuder a variety of conditione to produce diferrorenylmercury. Aldehyde Condensation. When ferrocene is heated in hydrofluoric acid in the presence of aldehydes, e.g., formaldehyde or henzaldehyde, rondensation products are produced which prohably are hridged derivatives of ferrorene (35, @+).
Miscellan~ousReactions. The reaction of ferrorene and lithium in ethylamine proreeds very rapidly, th? products heing metallic iron and cylopentadiene (45). This reartion offers a convenient mild degradation whirh should be useful in studies ~nvolvingstrurtural isomers of ferrocene derivatives. The reaction of formaldehyde and dimethylamine with ferrocene yields S,X'-(dimethyl)aminomethylferrocene (46). Fe(CsHs)g
-
+ HCHO + HN(CHa)?
C5HaFeCsHCH2N(CHn)n
This tertiary amiue reacts with methyl iodide to produce the quaternary ammonium salt, which can undergo an ortho-substitution rearrangement to produce the 2-methyl derivative (V).
These derivatives can react further to produce suhstit,uted ferrocenes. For example, carbonation and acidification produce a mixture of mono- and bis(carhoxy)ferrocene.
Both mono- and his-(triphenylsi1yl)ferrocene have been obtained in this manner by reaction with triphenylrhlorosilane (39). The reaction of monolithio-ferrocene with a-henzylhydroxylamine produres monoaminoferrocene (@).
Mono- and bis-(acety1)ferrorene may be oxidized by means of sodium hypohalite to mono- and his-(carboxy). ferrocene, apparently without oxidizing the ferrorene nurleus (20, 32).
The mercuration of ferrorene proceeds readily in JOURNAL OF CHEMICAL EDUCATION
Acylferrocenes can be reduced to the corresponding alkyl derivatives by means of the Clemmenson reduction (920, 38, 47). n
In addition, mono- and bis-(acety1)ferrocene can be catalytically hydrogenated to mono- and his-(ethyl) ferrocene. Attempts to halogenate or nitrate ferrocene directly have not been successful since under the conditions necessary for these reactions ferrocene is readily oxidized to the ferricininm ion. Nesmeyanov, et al., have reported that the ferrocenyl halides can be prepared by the reaction of a halogen with either monoor his-(chloromercuri)ferrorene (48).
Curtius rearrangement of the azides of both monoand his-(carboxy)ferrocene in benzyl alcohol solution has been surreesful. The products are benzyl urethanes (920,471. 0
APPLICATIONS
One of the most important applications for which this unique organometallic has been considered is its use as an additive for promoting the smokeless combustion of fuels (&, 49, 50). Low concentrations of ferrocene and its derivatives have a marked effect in reducing carbon formation in home heating oils and in jet fuels. Furthermore, ferrocene has dGmonstrated significant antiknock properties in fuels for spark ignition engines (60). Other potential uses for ferrocene and its derivatives are based on their behavior a s reducing agents, antioxidants, and as excellent organic carriers of iron in high concentration. In view of its catalytic combustion effects, ferrocene has been suggested as a catalyst for various organic reactions. I n the paint industry the metallic derivatives of cvcloventadiene, inclndine . ferrocene, are of potential interest as paint dryers, pigments, and for use in the formation of primer coats for metal surfaces (&, 51). With regard to commercial uses for ferrocene-type materials it must be readily apparent that only minor efforts have been expended thus far. Only a small beginning in the exploration of ferrocene chemistry has been made, and an almost unlimited future appears to lie ahead. VOLUME 34, NO. 6, JUNE, 1951
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FIBCHE;,
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1101 (1955). (49) Chmical Week, Jan. 22, 1956, p. 54. (50) "Dicyclopentadienyliron>' New Products Bullelin, E . I . du Pont de Nemours & Co., May, 1955. (51) "High Purity Dioyolopentadiene and Methyl Cyclopentndime Dimer," Enjay Co., h e . , 1955.
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JOURNAL OF CHEMICAL EDUCATION
