Cyclobutadieneiron tricarbonyl

Preparation and structure

Cyclobutadieneiron tricarbonyl was first prepared in 1965 by Pettit from 3,4-dichlorocyclobutene and diiron nonacarbonyl: :C4H4Cl2 + 2 Fe2(CO)9 → (C4H4)Fe(CO)3 + 2 Fe(CO)5 + 5 CO + FeCl2 The compound is an example of a piano stool complex. The C-C distances are 1.426 Å.

Properties

Oxidative decomplexation of cyclobutadiene is achieved by treating the tricarbonyl complex with ceric ammonium nitrate. The released cyclobutadiene is trapped with a quinone, which functions as a dienophile.

Cyclobutadieneiron tricarbonyl displays aromaticity as evidenced by some of its reactions, which can be classified as electrophilic aromatic substitution:

:[[Image:CyclobutadieneirontricarbonylReactions.png|600px|Cyclobutadieneiron tricarbonyl Reactions]]

It undergoes Friedel-Crafts acylation with acetyl chloride and aluminium chloride to give the acyl derivative 2, with formaldehyde and hydrochloric acid to the chloromethyl derivative 3, in a Vilsmeier-Haack reaction with N-methylformanilide and phosphorus oxychloride to the formyl 4, and in a Mannich reaction to amine derivative 5.

The reaction mechanism is identical to that of EAS:

:[[Image:Cyclobutadieneirontricarbonylreactionmechanism.png|400px|Cyclobutadieneiron tricarbonyl EAS reactionmechanism]]

Related compounds

Several years before Petit's work, (C4Ph4)Fe(CO)3 had been prepared from the reaction of iron carbonyl and diphenylacetylene.

(Butadiene)iron tricarbonyl is isoelectronic with cyclobutadieneiron tricarbonyl.

History

In 1956, Longuet-Higgins and Orgel predicted the existence of transition-metal cyclobutadiene complexes, in which the degenerate eg orbital of cyclobutadiene has the correct symmetry for π interaction with the dxz and dyz orbitals of the proper metal. The compound was synthesized three years after the prediction This is a case of theory before experiment.

References

References

1. (2003). "(Cyclobutadiene)iron Tricarbonyl. A Case of Theory before Experiment". Organometallics.
2. ''Cyclobutadiene- and Benzocyclobutadiene-Iron Tricarbonyl Complexes'' G. F. Emerson, L. Watts, R. Pettit; [[J. Am. Chem. Soc.]]; '''1965'''; 87(1); 131-133. {{doi. 10.1021/ja01079a032
3. (1970). "Cyclobutadieneiron Tricarbonyl". Organic Syntheses.
4. (1988). "Structure of tricarbonyl(η⁴-cyclobutadienyl)iron(0) at −45 °C". [[Inorganic Chemistry (journal).
5. L. Brener. (1976). "Cyclobutadiene In Synthesis: ''endo''-Tricyclo[4.4.0.02,5]deca-3,8-diene-7,10-dione". Org. Synth..
6. Cyclobutadieneiron Tricarbonyl. A New Aromatic System J. D. Fitzpatrick, L. Watts, G. F. Emerson, R. Pettit [[J. Am. Chem. Soc.]]; '''1965''', vol. 87, 3254-3255 {{doi. 10.1021/ja01092a050
7. R. P. Dodge, V. Schomaker, "Crystal Structure of Tetraphenylcyclobutadiene Iron Tricarbonyl", Nature 1960, vol. 186, 798-799.{{doi. 10.1038/186798b0
8. (1956-01-01). "385. The possible existence of transition-metal complexes of cyclobutadiene". Journal of the Chemical Society (Resumed).
9. (1959-01-21). "Ein Nickel-Komplex des Tetramethyl-Cyclobutadiens". Angewandte Chemie.
10. Seyferth, Dietmar. (2003-01-01). "(Cyclobutadiene)iron TricarbonylA Case of Theory before Experiment". Organometallics.