Organoiron chemistry


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 tetracarbonylferrate. Iron adopts oxidation states from Fe through to Fe. Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals. Organoiron compounds feature a wide range of ligands that support the Fe-C bond; as with other organometals, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl, but hard ligands such as amines are employed as well.

Iron(0) and more reduced states

Carbonyl complexes

Important iron carbonyls are the three neutral binary carbonyls, iron pentacarbonyl, diiron nonacarbonyl, and triiron dodecacarbonyl. One or more carbonyl ligands in these compounds can be replaced by a variety of other ligands including alkenes and phosphines. An iron complex, disodium tetracarbonylferrate, also known as "Collman's Reagent," is prepared by reducing iron pentacarbonyl with metallic sodium. The highly nucleophilic anionic reagent can be alkylated and carbonylated to give the acyl derivatives that undergo protonolysis to afford aldehydes:
Similar iron acyls can be accessed by treating iron pentacarbonyl with organolithium compounds:
In this case, the carbanion attacks a CO ligand. In a complementary reaction, Collman's reagent can be used to convert acyl chlorides to aldehydes. Similar reactions can be achieved with − salts.

Alkene-Fe(0)-CO derivatives

Monoalkenes

Iron pentacarbonyl reacts photochemically with alkenes to give Fe4.

Diene-Fe(0)-CO derivatives

Iron diene complexes are usually prepared from Fe5 or Fe29. Derivatives are known for common dienes like cyclohexadiene, norbornadiene and cyclooctadiene, but even cyclobutadiene can be stabilized. In the complex with butadiene, the diene adopts a cis-conformation. Iron carbonyls are used as a protective group for dienes in hydrogenations and Diels-Alder reactions. Cyclobutadieneiron tricarbonyl is prepared from 3,4-dichlorocyclobutene and Fe29.
Cyclohexadienes, many derived from Birch reduction of aromatic compounds, form derivatives Fe3. The affinity of the Fe3 unit for conjugated dienes is manifested in the ability of iron carbonyls catalyse the isomerisations of 1,5-cyclooctadiene to 1,3-cyclooctadiene. Cyclohexadiene complexes undergo hydride abstraction to give cyclohexadienyl cations, which add nucleophiles. Hydride abstraction from cyclohexadiene iron complexes gives ferrous derivatives.
The enone complex iron tricarbonyl serves as a source of the Fe3 subunit and is employed to prepare other derivatives. It is used similarly to Fe29.

Alkyne-Fe(0)-CO derivatives

Alkynes react with iron carbonyls to give a large variety of derivatives. Derivatives include ferroles, Fe3, Fe3 and many others.

Tri- and polyene Fe(0) complexes

Stable iron-containing complexes with and without CO ligands are known for a wide variety of polyunsaturated hydrocarbons, e.g. cycloheptatriene, azulene, and bullvalene. In the case of cyclooctatetraene, derivatives include Fe2, Fe33, and several mixed COT-carbonyls 3 and Fe2.
is an Fe complex lacking CO ligands.

Iron(I) and iron(II)

As Fe is a common oxidation state for Fe, many organoiron compounds are known. Fe compounds often feature Fe-Fe bonds, but exceptions occur, such as −.

Ferrocene and its derivatives

The rapid growth of organometallic chemistry in the 20th century can be traced to the discovery of ferrocene, a very stable compound which foreshadowed the synthesis of many related sandwich compounds. Ferrocene is formed by reaction of sodium cyclopentadienide with iron chloride:
Ferrocene displays diverse reactivity localized on the cyclopentadienyl ligands, including Friedel–Crafts reactions and lithation. Some electrophilic functionalization reactions, however, proceed via initial attack at the Fe center to give the bent + species. For instance, HF:PF5 and Hg2, give isolable or spectroscopically observable complexes +PF6– and Cp2Fe+–Hg–2, respectively.
Ferrocene is also a structurally unusual scaffold as illustrated by the popularity of ligands such as 1,1'-bisferrocene, which are useful in catalysis. Treatment of ferrocene with aluminium trichloride and benzene gives the cation +. Oxidation of ferrocene gives the blue 17e species ferrocenium. Derivatives of fullerene can also act as a highly substituted cyclopentadienyl ligand.

Fp2, Fp−, and Fp+ and derivatives

Fe5 reacts with cyclopentadiene to give the cyclopentadienyliron dicarbonyl dimer, called Fp2. Pyrolysis of Fp2 gives the cuboidal cluster 4.
Reduction of Fp2 with sodium gives "NaFp", containing a potent nucleophile and precursor to many derivatives of the type CpFe2R. The derivative + has been used in cyclopropanations. The complex CpFe+acyl.

Alkyl, allyl, and aryl compounds

The simple peralkyl and peraryl complexes of iron are less numerous than are the Cp and CO derivatives. One example is tetramesityldiiron.
is a rare example of a neutral per-organo complex of iron.
Compounds of the type +X− are allyl cation synthons in allylic substitution. In contrast, compounds of the type possessing η1-allyl groups are analogous to main group allylmetal species and react with carbon electrophiles to give allylation products with SE2′ selectivity. Similarly, allenyl dicarbonyl complexes exhibit reactivity analogous to main group allenylmetal species and serve as nucleophilic propargyl synthons.

Sulfur and phosphorus derivatives

Complexes of the type Fe226 and Fe226 form, usually by the reaction of thiols and secondary phosphines with iron carbonyls. The thiolates can also be obtained from the tetrahedrane Fe2S26.

Iron(III)

Some organoiron compounds are prepared by oxidation of organoiron compounds. A long-known example being ferrocenium +. Organoiron porphyrin complexes are numerous.

Iron(IV)

In Fe4, Fe is stabilized by an alkyl ligand that resists beta-hydride elimination. Two-electron oxidation of decamethylferrocene gives the dication 2+, which forms a carbonyl complex, 2.

Organoiron compounds in organic synthesis and homogeneous catalysis

In industrial catalysis, iron complexes are seldom used in contrast to cobalt and nickel. Because of low cost and low toxicity of its salts, iron is attractive as a stoichiometric reagent. Some areas of investigation include:
In the area of bioorganometallic chemistry, organoiron species are found at the active sites of the three hydrogenase enzymes as well as carbon monoxide dehydrogenase.