Benzene(cyclopentadienyl)iron Hexafluorophosphate1

[12176-31-7]  · C11H11F6FeP  · Benzene(cyclopentadienyl)iron Hexafluorophosphate  · (MW 344.02) (BF4- salt)

[1277-51-6]  · C11H11BF4Fe  · Benzene(cyclopentadienyl)iron Tetrafluoroborate  · (MW 285.86) (free cation)

[51364-24-0]  · C11H11Fe  · Benzene(cyclopentadienyl)iron Hexafluorophosphate

(electrophilically activated arene complex, undergoes nucleophilic addition,1,2 nucleophilic substitution of aryl halides,1,3,4 oxidations and substitutions at benzylic protons,1,5,6 and photochemical or thermal arene removal3,7)

Physical Data: mp 200 °C (dec).

Solubility: insol H2O, Et2O, THF; sol acetone, MeCN; mod sol CH2Cl2.

Form Supplied in: yellow solid; not commercially available. Preparative information is available.2,8

Handling, Storage, and Precautions: [(C6H6)FeCp]PF6 is air stable. No special storage requirements are indicated. Its toxicity is unreported, but is probably low to moderate.

General Description.

Benzene(cyclopentadienyl)iron hexafluorophosphate is the simplest member of the [(arene)FeCp]+ family, which was first prepared in the late 1950s. Although [CpFeCl(CO)2] was first used as the synthetic precursor, it was soon recognized that ferrocene itself is a more convenient alternative.1,2,8 Aluminum Chloride (or other very strong Lewis acid) is needed to produce the cation. Aluminum powder is often added also to reduce any ferricinium cation generated during the reaction. Such preparative conditions must necessarily limit the range of aromatic substrates which can be directly coordinated to the [CpFe]+ fragment. Nevertheless, certain arenes which cannot be added directly can be produced subsequent to coordination via substitution or oxidation reactions. Several review articles on arene-iron complexes feature thorough discussions of preparative considerations and the range of usable a romatics.1

Nucleophilic Addition Reactions.

The coordination of a metal fragment to an aromatic molecule causes the arene to take on an electrophilic character.9 This is especially true when the metal fragment is cationic. Hence [(C6H6)FeCp]+ undergoes attack by moderate to strong nucleophiles, such as lithium and Grignard reagents (eq 1).2 Similarly, hydride may be added using Sodium Borohydride or Lithium Aluminum Hydride. Some ketones have been shown to react as their enolate anions with [(arene)FeCp]+ in the presence of Potassium Hydroxide.10 Unless it is quite sterically hindered, the arene is the preferred site of attack on [(arene)FeCp]+; the product therefore contains a cyclohexadienyl ring and is a ferrocene analog. The stereochemistry of attack is invariably exo with respect to the metal. Substituent effects are typically rather minor.2 Exceptions include methoxycarbonyl, cyano, nitro, and tosyl substituted benzenes, which show 10-20:1 preferences for ortho attack over meta and para.11 Liberation of the ring occurs with rearomatization; thus eq 1 represents an overall nucleophilic aromatic substitution.

A variety of metal fragments activate trienes (such as arenes) toward electrophilic attack to varying degrees. In Table 1 the relative kinetic activation afforded by various metal fragments (independent of the coordinated triene and the nucleophile) is compared.9 This parameter is the electrophilic transferability number (TE). It may be noted that for a given nucleophile, [(C6H6)Mn(CO)3]+ is expected to react at 1.1 × 104 the rate of [(C6H6)FeCp]+. The lesser electrophilicity of the iron cation renders it unreactive toward neutral donors, such as phosphines, which add to the manganese complex.

Nucleophilic Substitution Reactions.

The halobenzene and nitrobenzene complexes are subject to substitution of X- or NO2- by soft anionic nucleophiles, such as heteroatom donors.3,4 This reaction is conveniently conducted in the presence of Sodium Carbonate (eq 2). Stabilized carbanions behave similarly.12 Hard carbanions, however, usually undergo addition reactions with [(PhCl)FeCp]+. The ease with which substitution reactions occur for [(PhCl)FeCp]+ is similar to that of 2,4-dichloronitrobenzene. Chloride substitution is of course even more facile for [(PhCl)Mn(CO)3]+.13

Reactions at Benzylic Sites.

The [CpFe]+ fragment is stable towards oxidation. This allows the elaboration of coordinated methylbenzenes to benzoic acids (eq 3).5 Conversion to acid derivatives, i.e. esters, acid chlorides, amides, and nitriles, may subsequently be accomplished. In very specialized instances, benzylic carbons can be oxidized to ketones.14 Coordinated aniline can be oxidized to coordinated nitrobenzene using hydrogen peroxide and acid. Benzylic protons are rendered especially labile by the electron-withdrawing power of [CpFe]+.6 Hence ring methyls can be alkylated to produce tentacled benzenes (eq 4).15

Arene Removal Reactions.

The aromatic ligand can be photochemically7 or thermally3 removed. The liberated [CpFe]+ fragment is then trapped with 3 mol of coordinating solvent or donor ligands. Alternatively, electron-rich aromatics may be used, facilitating arene exchange. However, the scope of the latter chemistry has not been explored in any detail to date.

1. (a) Astruc, D. T 1983, 38, 4027. (b) Sutherland, R. G.; Iqbal, M.; Piorko, A. JOM 1986, 302, 307. (c) Astruc, D. Top. Curr. Chem. 1992, 160, 47.
2. (a) Khand, I. U.; Pauson, P. L.; Watts, W. E. JCS(C) 1968, 2257. (b) Khand, I. U.; Pauson, P. L.; Watts, W. E. JCS(C) 1968, 2261. (c) Khand, I. U.; Pauson, P. L.; Watts, W. E. JCS(C) 1969, 116. (d) Khand, I. U.; Pauson, P. L.; Watts, W. E. JCS(C) 1969, 2024.
3. Lee, C. C.; Gill, U. S.; Iqbal, M.; Azogu, C. I.; Sutherland, R. G. JOM 1982, 231, 151.
4. Lee, C. C.; Abd-El-Aziz, A. S.; Chowdhury, R. L.; Piorko, A.; Sutherland, R. G. Synth. React. Inorg. Metal-Org. Chem. 1986, 16, 541.
5. Nesmeyanov, A. N.; Vol'kenau, N. A.; Sirotkina, E. I.; Deryabin, V. V. Dolk. Chem. 1967, 177, 1110.
6. (a) Astruc, D.; Hamon, A. R.; Althoff, G.; Roman, E.; Batail, P.; Michaud, P.; Mariot, J. P.; Varret, F.; Cozak, D. JACS 1979, 101, 5445. (b) Hamon, J. R.; Saillard, J. Y.; LeBeuze, A.; McGlinchey, M.; Astruc, D. JACS 1982, 104, 7549.
7. (a) Gill, T. P.; Mann, K. R. IC 1980, 19, 3007. (b) Gill, T. P.; Mann, K. R. JOM 1981, 216, 65. (c) Gill, T. P.; Mann, K. R. IC 1983, 22, 1986. (d) Schrenk, J. L.; Palazzotto, M. C.; Mann, K. R. IC 1983, 22, 4047.
8. Astruc, D.; Dabard, R. BSF 1975, 2571.
9. Kane-Maguire, L. A. P.; Honig, E. D.; Sweigart, D. A. CRV 1984, 84, 525.
10. (a) Sutherland, R. G.; Chowdhury, R. L.; Piorko, A.; Lee, C. C. CC 1985, 1296. (b) Sutherland, R. G.; Chowdhury, R. L.; Piorko, A.; Lee, C. C. CJC 1986, 64, 2031.
11. (a) McGreer, J. R.; Watts, W. E. JOM 1976, 110, 103. (b) Sutherland, R. G.; Zhang, C. H.; Chowdhury, R. L.; Piorko, A.; Lee, C. C. JOM 1987, 333, 367. (c) Zhang, C. H.; Chowdhury, R. L.; Piorko, A.; Lee, C. C.; Sutherland, R. G. JOM 1988, 346, 67.
12. (a) Moriarty, R. M.; Gill, U. S. OM 1986, 5, 253. (b) Moriarty, R. M. Synth. React. Inorg. Metal-Org. Chem. 1986, 16, 1103.
13. Pauson, P. L.; Segal, J. A. JCS(D) 1975, 1677.
14. Lee, C. C.; Demchuk, K. J.; Gill, U. S.; Sutherland, R. G. JOM 1983, 247, 71.
15. (a) Molines, F.; Astruc, D. AG(E) 1988, 27, 1347. (b) Molines, F.; Astruc, D. CC 1989, 614.

Robert D. Pike

The College of William and Mary, Williamsburg, VA, USA

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