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.^[1]

Cyclobutadieneiron tricarbonyl

Identifiers
CAS Number	12078-17-0 N
3D model (JSmol)	Interactive image
ChemSpider	9096597
InChI InChI=1S/C4H4.3CO.Fe/c1-2-4-3-1;3*1-2;/h1-4H;;;; Key: PHCIVYPTULNGTJ-UHFFFAOYSA-N
SMILES c1ccc1.[Fe]([C]#[O])([C]#[O])[C]#[O]
Properties
Chemical formula	C7H4FeO3
Molar mass	191.951 g·mol−1
Appearance	pale yellow oil
Boiling point	47 °C (117 °F; 320 K) 3 mm
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Preparation and structure

Cyclobutadieneiron tricarbonyl was first prepared in 1965 by Pettit from 3,4-dichlorocyclobutene and diiron nonacarbonyl:^[2][3]

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.^[4]

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.^[5]

Cyclobutadieneiron tricarbonyl displays aromaticity as evidenced by some of its reactions, which can be classified as electrophilic aromatic substitution:^[6]

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:

Related compounds

Several years before Petit's work, (C₄Ph₄)Fe(CO)₃ had been prepared from the reaction of iron carbonyl and diphenylacetylene.^[7]

(Butadiene)iron tricarbonyl is isoelectronic with cyclobutadieneiron tricarbonyl.

History

In 1956, Longuet-Higgins and Orgel^[8] predicted the existence of transition-metal cyclobutadiene complexes, in which the degenerate e_g orbital of cyclobutadiene has the correct symmetry for π interaction with the d_xz and d_yz orbitals of the proper metal. The compound was synthesized three years after the prediction^[9] This is a case of theory before experiment.^[10]