Cyclobutadieneiron tricarbonyl is an
organoiron compound with the formula Fe(C
4H
4)(CO)
3. It is a yellow oil that is soluble in organic solvents. It has been used in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
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:
:C
4H
4Cl
2 + 2 Fe
2(CO)
9 → (C
4H
4)Fe(CO)
3 + 2 Fe(CO)
5 + 5 CO + FeCl
2
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. The released cyclobutadiene is trapped with a
quinone
The quinones are a class of organic compounds that are formally "derived from aromatic compounds benzene.html" ;"title="uch as benzene">uch as benzene or naphthalene] by conversion of an even number of –CH= groups into –C(=O)– groups with ...
, which functions as a
dienophile.
Cyclobutadieneiron tricarbonyl displays
aromaticity
In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The e ...
as evidenced by some of its reactions, which can be classified as
electrophilic aromatic substitution
Electrophilic aromatic substitution (SEAr) is an organic reaction in which an atom that is attached to an aromatic ring, aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitut ...
:
:
It undergoes
Friedel-Crafts acylation with acetyl chloride and aluminium chloride to give the acyl derivative 2, with
formaldehyde
Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...
and
hydrochloric acid
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
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
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage ...
is identical to that of EAS:
:
Related compounds
Several years before Petit's work, (C
4Ph
4)Fe(CO)
3 had been prepared from the reaction of
iron carbonyl 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 Transition metal alkyne complex, ligand in ...
.
(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 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 This is a case of theory before experiment.
References
{{iron compounds
Iron carbonyl complexes
Half sandwich compounds
Iron(0) compounds
Diene complexes