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. 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(–II) and Iron(0)

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(–II) complex, disodium tetracarbonylferrate (Na2[Fe(CO)4]), 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:

LiFe(CO)4(C(O)R) + H+ → RCHO (+ iron containing products) Similar iron acyls can be accessed by treating iron pentacarbonyl with organolithium compounds:

ArLi + Fe(CO)5 → LiFe(CO)4C(O)Ar 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 [HFe(CO)4]− salts.

Alkene-Fe(0)-CO derivatives

Monoalkenes Iron pentacarbonyl reacts photochemically with alkenes to give Fe(CO)4(alkene).

Diene-Fe(0)-CO derivatives Iron diene complexes are usually prepared from Fe(CO)5 or Fe2(CO)9. 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 potential protective groups for dienes, shielding them from hydrogenations and Diels-Alder reactions. Cyclobutadieneiron tricarbonyl is prepared from 3,4-dichlorocyclobutene and Fe2(CO)9. Cyclohexadienes, many derived from Birch reduction of aromatic compounds, form derivatives (diene)Fe(CO)3. The affinity of the Fe(CO)3 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(0) complexes gives ferrous derivatives. The enone complex (benzylideneacetone)iron tricarbonyl serves as a source of the Fe(CO)3 subunit and is employed to prepare other derivatives. It is used similarly to Fe2(CO)9.

Alkyne-Fe(0)-CO derivatives Alkynes react with iron carbonyls to give a large variety of derivatives. Derivatives include ferroles (Fe2(C4R4)(CO)6), (p-quinone)Fe(CO)3, (cyclobutadiene)Fe(CO)3 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 (COT), derivatives include Fe(COT)2, Fe3(COT)3, and several mixed COT-carbonyls (e.g. Fe(COT)(CO)3 and Fe2(COT)(CO)6).

Iron(I) and iron(II) As Fe(II) is a common oxidation state for Fe, many organoiron(II) compounds are known. Fe(I) compounds often feature Fe-Fe bonds, but exceptions occur, such as [Fe(anthracene)2]−.

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(II) chloride:

2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl 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 [Cp2Fe–Z]+ species (which are formally Fe(IV)). For instance, HF:PF5 and Hg(OTFA)2, give isolable or spectroscopically observable complexes [Cp2Fe–H]+PF6– and Cp2Fe+–Hg–(OTFA)2, respectively. Ferrocene is also a structurally unusual scaffold as illustrated by the popularity of ligands such as 1,1'-bis(diphenylphosphino)ferrocene, which are useful in catalysis. Treatment of ferrocene with aluminium trichloride and benzene gives the cation [CpFe(C6H6)]+. Further iron arene complexes are also possible. 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 Fe(CO)5 reacts with cyclopentadiene to give the dinuclear Fe(I) species cyclopentadienyliron dicarbonyl dimer ([FeCp(CO)2]2), often abbreviated as Fp2. Pyrolysis of Fp2 gives the cuboidal cluster [FeCp(CO)]4. Very hindered substituted cyclopentadienyl ligands can give isolable monomeric Fe(I) species. For example, Cpi-Pr5Fe(CO)2 (Cpi-Pr5 = i-Pr5C5) has been characterized crystallographically. Reduction of Fp2 with sodium gives "NaFp", containing a potent nucleophile and precursor to many derivatives of the type CpFe(CO)2R. The derivative [FpCH2S(CH3)2]+ has been used in cyclopropanations. The Fp+ fragment is Lewis acidic and readily forms complexes with ethers, amines, pyridine, etc., as well as alkenes and alkynes in the η2 coordination mode. The complex Fp+(η2-vinyl ether)]+ is a masked vinyl cation. Recently, a methane complex, [Fp(CH4)]+[Al(OC(CF3)3)4]–, was prepared and characterized spectroscopically, using a perfluoroalkoxyaluminate as a non-coordinating counterion and 1,1,1,3,3,3-hexafluoropropane as a non-coordinating solvent. Fp-R compounds are prochiral, and studies have exploited the chiral derivatives CpFe(PPh3)(CO)acyl.

Alkyl, allyl, and aryl compounds

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