Cyclopentadienyliron dicarbonyl dimer

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2), often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "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. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).[1]

Cyclopentadienyliron dicarbonyl dimer
Names
Other names
Bis(cyclopentadienyl)tetracarbonyl-diiron,
Di(cyclopentadienyl)tetracarbonyl-diiron,
Bis(dicarbonylcyclopentadienyliron)
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.032.057
Properties
C14H10Fe2O4
Molar mass 353.925 g/mol
Appearance Dark purple crystals
Density 1.77 g/cm3, solid
Melting point 194 °C (381 °F; 467 K)
Boiling point decomposition
insoluble
Solubility in other solvents benzene, THF, chlorocarbons
Structure
distorted octahedral
3.1 D (benzene solution)
Hazards
Main hazards CO source
R-phrases (outdated) 20/22
S-phrases (outdated) 36/37
Related compounds
Related compounds
 Fe(C5H5)2
Fe(CO)5
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references

Structure

In solution, Cp2Fe2(CO)4 can be considered a dimeric half-sandwich complex. It exists in three isomeric forms: cis, trans, and an unbridged, open form. These isomeric forms are distinguished by the position of the ligands. The cis and trans isomers differ in the relative position of C5H5 (Cp) ligands. The cis and trans isomers have the formulation [(η5-C5H5)Fe(CO)(μ-CO)]2, that is, two CO ligands are terminal whereas the other two CO ligands bridge between the iron atoms. The cis and trans isomers interconvert via the open isomer, which has no bridging ligands between iron atoms. Instead, it is formulated as (η5-C5H5)(OC)2Fe–Fe(CO)2(η5-C5H5) — the metals are held together by a Fe–Fe bond. At equilibrium, the cis and trans isomers are predominant.

In addition, the terminal and bridging carbonyls are known to undergo exchange: the trans isomer can undergo bridging-terminal CO ligand exchange through the open isomer, or through a twisting motion without going through the open form. In contrast, the bridging and terminal CO ligands of the cis isomer can only exchange via the open isomer.[2]

In solution, the cis, trans, and open isomers interconvert rapidly at room temperature, making the molecular structure fluxional. The fluxional process for cyclopentadienyliron dicarbonyl dimer is faster than the NMR time scale, so that only an averaged, single Cp signal is observed in the 1H NMR spectrum at 25 °C. Likewise, the 13C NMR spectrum exhibits one sharp CO signal above –10 °C, while the Cp signal sharpens to one peak above 60 °C. NMR studies indicate that the cis isomer is slightly more abundant than the trans isomer at room temperature, while the amount of the open form is small.[2] The fluxional process is not fast enough to produce averaging in the IR spectrum. Thus, three absorptions are seen for each isomer. The bridging CO ligands appear at ~1780 cm−1 whereas the terminal CO ligands are observed at ~1980 cm−1.[3]

The solid-state molecular structure of both cis and trans isomers have been analyzed by X-ray and neutron diffraction. The Fe–Fe separation and the Fe–C bond lengths are the same 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 the trans isomer reflecting different Cp orbital populations.[4] Although older textbooks show the two iron atoms bonded to each other, theoretical analyses indicate the absence of a direct Fe–Fe bond.[5] Others argue that there are several definitions for what constitutes a "bond" and, through several lines of reasoning, makes the case that there is a bond between the iron atoms.[6]

The averaged structure of these isomers of Cp2Fe2(CO)4 results in a dipole moment of 3.1 D in benzene.[7]

Synthesis

Cp2Fe2(CO)4 was first prepared in 1955 at Harvard by Geoffrey Wilkinson using the same method employed today: the reaction of iron pentacarbonyl and dicyclopentadiene.[6][8]

2 Fe(CO)5 + C10H12 → (η5-C5H5)2Fe2(CO)4 + 6 CO + H2

In this preparation, dicyclopentadiene cracks to give cyclopentadiene, which reacts with Fe(CO)5 with loss of CO. Thereafter, the pathways for the photochemical and thermal routes differ subtly but both entail formation of a hydride intermediate.[4] The method is used in the teaching laboratory.[3]

Reactions

Although of no major commercial value, Fp2 is a workhorse in organometallic chemistry because it is inexpensive and FpX derivatives are rugged (X = halide, organyl).

"Fp−" (FpNa and FpK)

Reductive cleavage of [CpFe(CO)2]2 (formally Fe(I)) produces alkali metal derivatives formally derived from the cyclopentadienyliron dicarbonyl anion, [CpFe(CO)2]− or called Fp− (formally Fe(0)), which is assumed to exist as a tight ion pair. A typical reductant is sodium metal or sodium amalgam;[9] NaK alloy, potassium graphite (KC8), and alkali metal trialkylborohydrides have been used. [CpFe(CO)2]Na is a widely studied reagent since it is readily alkylated, acylated, or metalated by treatment with an appropriate electrophile.[10]

[CpFe(CO)2]2 + 2 Na → 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(η5-C5H5)(CO)2

CpFe(CO)2K + CH3I → CpFe(CO)2CH3 + KI

Fp2 can also be cleaved with alkali metals[11] and by electrochemical reduction.[12][13]

FpX (X = Cl, Br, I)

Halogens oxidatively cleave [CpFe(CO)2]2 to give the Fe(II) species FpX (X = Cl, Br, I):

[CpFe(CO)2]2 + X2 → 2 CpFe(CO)2X

One example is cyclopentadienyliron dicarbonyl iodide.

Fp(η2-alkene)+, Fp(η2-alkyne)+ and other "Fp+"

In the presence of halide anion acceptors such as AlBr3 or AgBF4, FpX compounds (X = halide) react with alkenes, alkynes, or neutral, labile ligands (e.g., ethers, nitriles) to afford Fp+ complexes.[14] In another approach, salts of [Fp(isobutene)]+ are readily obtained by reaction of NaFp with methallyl chloride followed by protonolysis. This complex is a convenient and general precursor to other cationic Fp–alkene and –alkyne complexes.[15] The exchange process is facilitated by the loss of gaseous and bulky isobutene.[16] Generally, less substituted alkenes bind more strongly and can displace more hindered alkene ligands. Alkene and alkyne complexes can also be prepared by heating a cationic ether or aqua complex, e.g., [Fp(thf)]+BF4–, with the alkene or alkyne.[17] [FpL]+BF4– complexes can also be prepared by treatment of FpMe with HBF4·Et2O in CH2Cl2 at –78 °C, followed by addition of L.[18]

Alkene–Fp complexes can also be prepared from Fp anion indirectly. Thus, hydride abstraction from Fp–alkyl compounds using triphenylmethyl hexafluorophosphate affords [Fp(α-olefin)]+ complexes.

FpNa + RCH2CH2I → FpCH2CH2R + NaI
FpCH2CH2R + Ph3CPF6 → [Fp(CH2=CHR)+]PF−
6 + Ph3CH

Reaction of NaFp with an epoxide followed by acid-promoted dehydration also affords alkene complexes. 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.[19]

The alkene ligand in these cations is activated toward attack by nucleophiles, opening the way to a number of carbon–carbon bond-forming reactions. Nucleophilic additions usually occur at the more substituted carbon. This regiochemistry is attributed to the greater positive charge density at this position. The regiocontrol is often modest. The addition of the nucleophile is completely stereoselective, occurring anti to the Fp group. Analogous Fp(alkyne)+ complexes are also reported to undergo nucleophilic addition reactions by various carbon, nitrogen, and oxygen nucleophiles.[20]

Fp(alkene)+ and Fp(alkyne)+ π-complexes are also quite acidic at the allylic and propargylic positions, respectively, and can be quantitatively deprotonated with amine bases like Et3N to give neutral Fp–allyl and Fp–allenyl σ-complexes:[15]

Fp(H2C=CHCH2CH3)+BF4– + Et3N → FpCH2CH=CHCH3 + Et3NH+BF4–
FpCH2CH=CHCH3 + E+BF4– → Fp(H2C=CHCH(E)CH3)+BF4–

Fp–allyl and Fp–allenyl react with cationic electrophiles E (e.g., Me3O+, carbocations, oxocarbenium ions) to generate allylic and propargylic functionalization products, respectively.[15] The related complex [Cp*Fe(CO)2(thf)]+[BF4]– has been shown to catalyze propargylic and allylic C–H functionalization by leveraging the deprotonation and electrophilic functionalization processes described above.[21]

Fp-based cyclopropanation reagents

Fp-based reagents have been developed for cyclopropanations.[22] The key reagent is prepared from FpNa and has a good shelf-life, in contrast to typical Simmons-Smith intermediates and diazoalkanes.

FpNa + ClCH2SCH3 → FpCH2SCH3 + NaCl
FpCH2SCH3 + CH3I + NaBF4 → FpCH2S(CH3)2]BF4 + NaI

Use of [FpCH2S(CH3)2]BF4 does not require specialized conditions.

Fp(CH2S+(CH3)2) BF−
4 + (Ph)2C=CH2 → 1,1-diphenylcyclopropane + ....

Ferric chloride is added to destroy any byproduct.

Precursors to Fp=CH2+ like FpCH2OMe, which is converted to the iron-carbene upon protonation, have also been used as cyclopropanation reagents.[23]

Photochemical reaction

Fp2 exhibits photochemistry.[24] Upon UV irradiation at 350 nm, it is reduced by 1-benzyl-1,4-dihydronicotinamide dimer, (BNA)2.[25]

[CpFe(CO)2]2 + (BNA)2 → 2[CpFe(CO)2]− + 2BNA+

References
  1. Kelly, William J. (2001). "Bis(dicarbonylcyclopentadienyliron)". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rb139. ISBN 0471936235.
  2. Harris, Daniel C.; Rosenberg, Edward; Roberts, John D. (1974). "Carbon-13 nuclear magnetic resonance spectra and mechanism of bridge–terminal carbonyl exchange in di-µ-carbonyl-bis[carbonyl(η-cyclopentadienyl)iron](Fe–Fe)[{(η-C 5 H 5 )Fe(CO) 2 } 2 ]; cd-di-µ-carbonyl-f-carbonyl-ae-di(η-cyclopentadienyl)-b-(triethyl -phosphite)di-iron(Fe–Fe)[(η-C 5 H 5 ) 2 Fe 2 (CO) 3 P(OEt) 3 ], and some related complexes". J. Chem. Soc., Dalton Trans. (22): 2398–2403. doi:10.1039/DT9740002398. ISSN 0300-9246.
  3. Girolami, G.; Rauchfuss, T.; Angelici, R. (1999). Synthesis and Technique in Inorganic Chemistry (3rd ed.). Sausalito, CA: University Science Books. pp. 171–180. ISBN 978-0-935702-48-4.
  4. Wilkinson, G., ed. (1982). Comprehensive Organometallic Chemistry. 4. New York: Pergamon Press. pp. 513–613. ISBN 978-0-08-025269-8.
  5. Green, Jennifer C.; Green, Malcolm L. H.; Parkin, Gerard (2012). "The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds". Chem. Commun. 2012 (94): 11481–11503. doi:10.1039/c2cc35304k. PMID 23047247.
  6. Labinger, Jay A. (2015-01-01). "Does cyclopentadienyl iron dicarbonyl dimer have a metal–metal bond? Who's asking?". Inorganica Chimica Acta. Metal-Metal Bonded Compounds and Metal Clusters. 424: 14–19. doi:10.1016/j.ica.2014.04.022. ISSN 0020-1693.
  7. Cotton, F. Albert; Yagupsky, G. (1967-01-01). "Tautomeric changes in metal carbonyls. I. .pi.-Cyclopentadienyliron dicarbonyl dimer and .pi.-cyclopentadienyl-ruthenum dicarbonyl dimer". Inorganic Chemistry. 6 (1): 15–20. doi:10.1021/ic50047a005. ISSN 0020-1669.
  8. Piper, T. S.; Cotton, F. A.; Wilkinson, G. (1955). "Cyclopentadienyl–carbon monoxide and related compounds of some transitional metals". J. Inorg. Nucl. Chem. 1 (3): 165–74. doi:10.1016/0022-1902(55)80053-X.
  9. Chang, T. C. T.; Rosenblum, M.; Simms, N. (1988). "Vinylation of Enolates with a Vinyl Cation Equivalent: trans-3-Methyl-2-Vinylcyclohexanone". Organic Syntheses. 66: 95.; Collective Volume, 8, p. 479
  10. King, B. (1970). "Applications of Metal Carbonyl Anions in the Synthesis of Unusual Organometallic Compounds". Acc. Chem. Res. 3 (12): 417–427. doi:10.1021/ar50036a004.
  11. Ellis, J. E.; Flom, E. A. (1975). "The Chemistry of Metal Carbonyl Anions: III. Sodium-Potassium Alloy: An Efficient Reagent for the Production of Metal Carbonyl Anions". J. Organomet. Chem. 99 (2): 263–268. doi:10.1016/S0022-328X(00)88455-7.
  12. Dessy, R. E.; King, R. B.; Waldrop, M. (1966). "Organometallic Electrochemistry. V. The Transition Series". J. Am. Chem. Soc. 88 (22): 5112–5117. doi:10.1021/ja00974a013.
  13. Dessy, R. E.; Weissman, P. M.; Pohl, R. L. (1966). "Organometallic Electrochemistry. VI. Electrochemical Scission of Metal-Metal Bonds". J. Am. Chem. Soc. 88 (22): 5117–5121. doi:10.1021/ja00974a014.
  14. Silver, J. (1993). Chemistry of Iron. Dordrecht: Springer Netherlands. ISBN 9789401121408. OCLC 840309324.
  15. Cutler, A.; Ehnholt, D.; Lennon, P.; Nicholas, K.; Marten, David F.; Madhavarao, M.; Raghu, S.; Rosan, A.; Rosenblum, M. (1975-05-01). "Chemistry of dicarbonyl .eta.5-cyclopentadienyliron complexes. General syntheses of monosubstituted .eta.2-olefin complexes and of 1-substituted .eta.1-allyl complexes. Conformational effects on the course of deprotonation of (.eta.2-olefin) cations". Journal of the American Chemical Society. 97 (11): 3149–3157. doi:10.1021/ja00844a038. ISSN 0002-7863.
  16. Mark M. Turnbull (2001). "Dicarbonyl(cyclopentadienyl)(isobutene)iron Tetrafluoroborate". Encyclopedia of Reagents for Organic Synthesis. eEROS. doi:10.1002/047084289X.rd080. ISBN 0471936235.
  17. Iron, Ruthenium and Osmium. Schriver, D. F., Bruce, M. I., Wilkinson, G. Kidlington: Elsevier Science. 1995. ISBN 978-0-08-096396-9. OCLC 953660855.CS1 maint: others (link)
  18. Redlich, Mark D.; Mayer, Michael F.; Hossain, M. Mahmun (2003). "Iron Lewis Acid [(η5-C5H5)Fe(CO)2(THF)]+ Catalyzed Organic Reactions". Aldrichimica Acta. 36: 3–13.
  19. Pearson, A. J. (1994). Iron Compounds in Organic Synthesis. San Diego, CA: Academic Press. pp. 22–35. ISBN 978-0-12-548270-7.
  20. Akita, Munetaka; Kakuta, Satoshi; Sugimoto, Shuichiro; Terada, Masako; Tanaka, Masako; Moro-oka, Yoshihiko (2001-06-01). "Nucleophilic Addition to the η2-Alkyne Ligand in [CpFe(CO)2(η2-R−C⋮C−R)]+. Dependence of the Alkenyl Product Stereochemistry on the Basicity of the Nucleophile". Organometallics. 20 (13): 2736–2750. doi:10.1021/om010095t. ISSN 0276-7333.
  21. Wang, Yidong; Zhu, Jin; Durham, Austin C.; Lindberg, Haley; Wang, Yi-Ming (2019-12-18). "α-C–H Functionalization of π-Bonds Using Iron Complexes: Catalytic Hydroxyalkylation of Alkynes and Alkenes". Journal of the American Chemical Society. 141 (50): 19594–19599. doi:10.1021/jacs.9b11716. ISSN 0002-7863. PMID 31791121.
  22. Mattson, M. N.; O'Connor, E. J.; Helquist, P. (1992). "Cyclopropanation Using an Iron-Containing Methylene Transfer Reagent: 1,1-Diphenylcyclopropane". Organic Syntheses. 70: 177.; Collective Volume, 9, p. 372
  23. Johnson, M.D. (1982), "Mononuclear Iron Compounds with η1-Hydrocarbon Ligands", Comprehensive Organometallic Chemistry, Elsevier, pp. 331–376, doi:10.1016/b978-008046518-0.00049-0, ISBN 978-0-08-046518-0, retrieved 2019-12-11
  24. Wrighton, M. (1974). "Photochemistry of Metal Carbonyls". Chem. Rev. 74 (4): 401–430. doi:10.1021/cr60290a001.
  25. Fukuzumi, S.; Ohkubo, K.; Fujitsuka, M.; Ito, O.; Teichmann, M. C.; Maisonhaute, E.; Amatore, C. (2001). "Photochemical Generation of Cyclopentadienyliron Dicarbonyl Anion by a Nicotinamide Adenine Dinucleotide Dimer Analogue". Inorg. Chem. 40 (6): 1213–1219. doi:10.1021/ic0009627. PMID 11300821.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.