Cyclobutadieneiron tricarbonyl

Cyclobutadieneiron tricarbonyl is an organoiron compound with the formula Fe(C4H4)(CO)3. It is a yellow oil that is soluble in organic solvents. It has been used in organic chemistry as a precursor for cyclobutadiene, which is an elusive species in the free state.

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  case of theory before experiment.