Cyclopentadienyliron dicarbonyl dimer

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula (C₅H₅)₂Fe₂(CO)₄, also abbreviated Cp₂Fe₂(CO)₄. It is called Fp₂ or "fip dimer." It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp₂Fe₂(CO)₄ is insoluble in but stable toward water.

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Structure

In solution, Cp₂Fe₂(CO)₄ can be considered a dimeric half sandwich complex. It exists in three isomeric forms: cis, trans, and unbridged. These isomeric forms are distinguished by the position of the ligands. Cis and trans differ in the relative position of C₅H₅ (Cp) ligands. And for both isomers, two CO ligands are terminal whereas the other two CO ligands bridge between the iron atoms. In the unbridged isomer, no ligands bridge between iron atoms - the metals are held together only by the Fe-Fe bond. Cis and trans isomers are the more abundant.

In solution, the three isomers interconvert. The phenomenon of rapidly interconverting structures is called fluxionality. Fluxional process for cyclopentadienyliron dicarbonyl dimer is so fast that only averaged single signal is observed in H NMR spectrum. However, the fluxional process is not fast enough for IR spectrum. Thus, three absorptions are seen for each isomer. The νco bands for bridging CO ligands are around 1780 cm^-1 whereas νco bands for terminal CO ligands are about 1980 cm^-1.^[1]

The solid state of molecular structure of both cis and trans isomers were determined and compared by X-ray and neutron diffraction. Surprisingly, cis and trans have the same metal-metal separation, identical Fe-C bond lengths in the Fe₂C₂ rhomboids, an exactly planar Fe₂C₂ four-membered ring in the trans isomer versus a folded rhomboid in cis with an angle of 164°, and significant distortions in the Cp ring of trans isomer reflecting different Cp orbital populations.^[2]

Synthesis

Cp₂Fe₂(CO)₄ was first isolated as an intermediate in the synthesis of ferrocene from Fe(CO)₅ and dicyclopentadiene and has since been found as a byproduct of many organoiron reactions. Cp₂Fe₂(CO)₄ is synthesized by the reaction of Fe(CO)₅ and dicyclopentadiene.^[1]

2 Fe(CO)₅ + C₁₀H₁₂ → (C₅H₅)₂Fe₂(CO)₄ + 6 CO + H₂

In this preparation, dicyclopentadiene cracks give to cyclopentadiene, which reacts with Fe(CO)₅. Cyclopentadiene reacts with Fe(CO)₅ concomitant with loss of CO. Thereafter, the pathways differ for the photochemical and thermal routes differ subtly but both entail formation of a hydride intermediate.^[2]

Applications

"Fp^-"

Reductive cleavage of the Cp₂Fe₂(CO)₄ produces derivatives formally derived from the cyclopentadienyliron dicarbonyl anion, [CpFe(CO)₂]^- or called Fp^-. Such species are in fact covalent; there is no evidence for the existence of free [CpFe(CO)₂]^- or called Fp^-. A typical reductant is Na, often as an amalgam; otherwise Na/K alloy, alkali metal trialkylborohydrides have been used. [CpFe(CO)₂]Na is a widely studied reagent since it is readily akylated, acylated, or metalated by treatment with an appropriate electrophile.

[CpFe(CO)₂]₂ + Na/Hg → 2 CpFe(CO)₂Na

[CpFe(CO)₂]₂ + 2 KBH(C₂H₅)₃ → 2 CpFe(CO)₂K + H₂ + 2 B(C₂H₅)₃

Treatment of NaFp with an alkyl halide (RX, X = Br, I) produces FeR(C₅H₅)(CO)₂

CpFe(CO)₂K + CH₃I → CpFe(CO)₂CH₃ + KI

FpBr

Bromine oxidatively cleaves the Fe-Fe bond in Fp₂ to give FpBr, CpFe(CO)₂Br.

[CpFe(CO)₂]₂ + Br₂ → 2 CpFe(CO)₂Br

CpFe(CO)₂Br reacts with alkenes to afford cationic alkene-Fp complexes.^[3] The reactions require the addition of a Lewis acid, such as AlBr₃.

Fp(alkene)⁺

Salts of [Fp(isobutene)]⁺ are widely employed for the preparation of Fp-alkene complexes by alkene exchange. The exchange process is facilitated by the loss of gaseous isobutene.

Alkene-Fp complexes can also be prepared from Fp anion indirectly. Thus, hydride abstraction from Fpalkyl compounds using the triphenylmethyl cation affords [Fp(isobutene)]⁺ complexes.

CpFe(CO)₂Na + RCH₂CH₂I → FpCH₂CH₂R + NaI

FpCH₂CH₂R + Ph₃CBF₄ →

Reaction of NaFp with an epoxide followed by acid-promoted dehydration also affords such alkene complexes.

[CpFe(CO)₂]^- or Fp anion is a good alkene protecting group. Fp(alkene)⁺ are stable with respect to bromination, hydrogenation, and acetoxymercuration, but the alkene is easily released with sodium iodide in acetone or by warming with acetonitrile.^[3]

However, the coordinated alkene is strongly activated toward nucleophile addition, leading to number of carbon-carbon bond formation. Many nucleophile addition show regioselectivity, usually occurring at the more substituted carbon. This is due to the greater positive charge density at this position. However, the regiocontrol is not always good enough to be considered in the organic synthesis. The addition of the nucleophile is completely stereoselective, anti to the Fp group.

Fp-based cyclopropanation reagents

Fp-based reagents are useful for cyclopropanations.^[4] The key reagent is prepared from FpNa and has a good shelf-life, in contrast to typical Simmons-Smith intermediates and diazoalkanes.

CpFe(CO)₂Na + ClCH₂SCH₃ → CpFe(CO)₂CH₂SCH₃ + NaCl

CpFe(CO)₂CH₂SCH₃ + CH₃I + NaBF₄ → [CpFe(CO)₂CH₂S(CH₃)₂]BF₄ + NaI

One of the advantage to [FeCH₂S(CH₃)₂]BF₄ is that its use does not require specialized conditions.

CpFe(CO)₂(CH₂S⁺(CH₃)₂) BF₄^- + (Ph)₂C=CH₂ → 1,1-diphenylcyclopropane

Ferric chloride is added to destroy any byproduct.

Other specialized reactions

Under photochemical conditions, Fp₂ reduces the C-C bond in 1-benzyl-1,4-dihydronicotinamide dimer, (BNA)₂.^[5]

[CpFe(CO)₂]₂ + (BNA)₂ + hv(λ=350nm) → 2[CpFe(CO)₂]^- + 2BNA⁺

References

• T. C. T. Chang, M. Rosenblum, N. Simms “Vinylation of Enolates with a Vinyl Cation Equivalent: trans-3-Methyl-2-Vinylcyclohexanone”

Organic Syntheses, Collective Volume 8, page 479 (synthesis of Fp(H2C=CHOEt) BF4).

1. ^ ^{a b} Girolami, G.; Rauchfuss, T.; Angelici, R. (1999). Synthesis and Technique in Inorganic Chemistry, 3rd Edition, Sausalito: University Science Books, 171-180. ISBN 978-0-935702-48-4. 
2. ^ ^{a b} Sir Geoffrey Wilkinson (ed.) (1982). "Comprehensive Organometallic Chemistry, Volume 4": 513-613. New York: Pergamon Press.
3. ^ ^{a b} Pearson, A. J. (1994). Iron Compounds in Organic Synthesis. San Diego: Academic Press, 22-35. ISBN = 978-0-12-548270-7. 
4. ^ M. N. Mattson, E. J. O'Connor, P. Helquist “Cyclopropanation Using an Iron-Containing Methylene Transfer Reagent: 1,1-Diphenylcyclopropane” Organic Syntheses Collective Volume 9, page 372.
5. ^ S. Fukuzumi, K. Ohkubo, M. Fujitsuka, O. Ito, M. C. Teichmann, E. Maisonhaute and C. Amatore (2001). "Photochemical Generation of Cyclopentadienyliron Dicarbonyl Anion by a Nicotinamide Adenine Dinucleotide Dimer Analogue". Inorg. Chem. 40 (6): 1213-1219. doi:10.1021/ic0009627.