Cyclobutadieneiron tricarbonyl is an organoiron compound with the formula Fe(C<sub>4</sub>H<sub>4</sub>)(CO)<sub>3</sub>. 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.
Cyclobutadieneiron tricarbonyl was first prepared in 1965 by Pettit from 3,4-dichlorocyclobutene and diiron nonacarbonyl:
The compound is an example of a piano stool complex. The C-C distances are 1.426 ÃÂ .
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:
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:
Several years before Petit's work, (C<sub>4</sub>Ph<sub>4</sub>)Fe(CO)<sub>3</sub> had been prepared from the reaction of iron carbonyl and diphenylacetylene.
(Butadiene)iron tricarbonyl is isoelectronic with cyclobutadieneiron tricarbonyl.
In 1956, Longuet-Higgins and Orgel predicted the existence of transition-metal cyclobutadiene complexes, in which the degenerate e<sub>g</sub> orbital of cyclobutadiene has the correct symmetry for ÃÂ interaction with the d<sub>xz</sub> and d<sub>yz</sub> orbitals of the proper metal. The compound was synthesized three years after the prediction This is a case of theory before experiment.