Cyclobutadieneiron tricarbonyl is an

organoiron compound Organoiron chemistry is the chemistry of iron compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrat ...

with the formula Fe(C₄H₄)(CO)₃. It is a yellow solid that is soluble in organic solvents. It has been used in

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...

as a precursor for

cyclobutadiene Cyclobutadiene is an organic compound with the formula . It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is ...

, which is an elusive species in the free state.

Preparation and structure

It was first prepared in 1965 by Pettit from 3,4-dichlorocyclobutene and

diiron nonacarbonyl Diiron nonacarbonyl is an organometallic compound with the chemical formula, formula Fe2(CO)9. This metal carbonyl is an important reagent in organometallic chemistry and of occasional use in organic synthesis. It is a more reactive source of Fe ...

: :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 Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an MLn center, where L is a unidentate ligand. Thousands of such complexes are known. Well-known examples in ...

. The C-C distances are 1.426 Å.

Properties

Oxidative decomplexation of cyclobutadiene is achieved by treating the tricarbonyl complex with

ceric ammonium nitrate Ceric ammonium nitrate (CAN) is the inorganic compound with the formula . This orange-red, water-soluble cerium salt is a specialised oxidizing agent in organic synthesis and a standard oxidant in quantitative analysis. Preparation, properties, ...

. The released cyclobutadiene is trapped with a quinone, which functions as a dienophile. Cyclobutadieneiron tricarbonyl displays

aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

as evidenced by some of its reactions, which can be classified as

electrophilic aromatic substitution Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic n ...

: : It undergoes Friedel-Crafts acylation with acetyl chloride and aluminium chloride to give the acyl derivative 2, with

formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...

and

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

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 In organic chemistry, the Mannich reaction is a three-component organic reaction that involves the amino alkylation of an acidic proton next to a carbonyl () functional group by formaldehyde () and a primary or secondary amine () or ammonia (). ...

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 Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula . Under standard conditions Fe( CO)5 is a free-flowing, straw-colored liquid with a pungent odour. Older samples appear darker. This compound is a common precursor to ...

and

diphenylacetylene Diphenylacetylene is the chemical compound C6H5C≡CC6H5. The molecule consists of two phenyl groups attached to a C2 unit. A colorless solid, it is used as a building block in organic synthesis and as a ligand in organometallic chemistry. Prepar ...

.R. P. Dodge, V. Schomaker, "Crystal Structure of Tetraphenylcyclobutadiene Iron Tricarbonyl", Nature 1960, vol. 186, 798-799. (Butadiene)iron tricarbonyl is isoelectronic with cyclobutadieneiron tricarbonyl.

References

{{iron compounds Carbonyl complexes Organoiron compounds Half sandwich compounds Iron(0) compounds Alkene complexes