Organoiron chemistry

Organoiron chemistry in chemistry is the study of organometallic compounds containing a carbon to iron chemical bond ^{[1] [2]}. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. Iron adopts oxidation states from Fe(-II) through to Fe(IV). Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals.^[3] Organoiron compounds feature a wide range of ligands that support the Fe-C bond; like other organometallic chemistry, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl. But even hard ligands are employed such as amines.

Iron carbonyls

The binary carbonyls and their anions

Important iron carbonyls are the three neutral binary carbonyls, iron pentacarbonyl, diiron nonacarbonyl, and triiron dodecacarbonyl. One or more carbonyl ligand can be replaced by a variety of other ligands (dienes, phosphines) and in this way other organoiron compounds are accessed.

Iron carbonyls have been used in stoichiometric carbonylation reactions, e.g. for the conversion of alkyl bromides to aldehydes. Disodium tetracarbonylferrate, "Collman's Reagent," can be alkylated followed by carbonylation 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)₄(C(O)R)

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.^[4]

(Diene)Fe(CO)₃ derivatives

Iron diene complexes are usually prepared from Fe(CO)₅ or Fe₂(CO)₉. Common dienes are cyclohexadiene, norbornadiene, cyclooctadiene and cyclobutadiene. In the complex with butadiene the diene is in a cis-conformation. Iron carbonyls are used as such as a protective group for dienes in hydrogenations and Diels-Alder reactions. Cyclobutadieneiron tricarbonyl is prepared from 3,4-dichlorocyclobutene and Fe₂(CO)₉. The all-phenyl substituted derivative is synthesized from diphenylacetylene and Fe(CO)₅.

Dienes, many derived from Birch reduction of aromatics, 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 olefin isomerisations of 1,5-cyclooctadiene to 1,3-cyclooctadiene. Cyclohexadiene complexes undergo hydride abstraction to give dienyl derivatives, which add nucleophiles.^[5]

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 complementarily to Fe₂(CO)₉.

Cyclopentadienyl derivatives, including ferrocenes

Ferrocene and its derivatives

The rapid growth of organometallic chemistry can be traced to the discovery of ferrocene, a very stable that foreshadowed the synthesis of many related sandwich compounds. Ferrocene is formed by reaction of sodium cyclopentadienyl with iron chloride:

2 NaC₅H₅ + FeCl₂ → Fe(C₅H₅)₂ + 2 NaCl

Ferrocene displays diverse reactivity localized on the cyclopentadienyl ligands, including Friedel-Crafts reactions and lithation. In terms of applications, ligands such as 1,1'-bis(diphenylphosphino)ferrocene are useful in catalysis. Treatment of ferrocene with aluminium trichloride and benzene gives the cation [CpFe(C₆H₆)]⁺. Oxidation of ferrocene gives the 17e species ferrocenium.

Fp₂ and its derivatives

Fe(CO)₅ reacts with dicyclopentadiene to give the cyclopentadienyl complex cyclopentadienyliron dicarbonyl dimer ([FeCp(CO)₂]₂). Reduction of this species with sodium gives "NaFp" (Fp = FeCp(CO)₂]^-), a potent nucleophile. The derivative [FpCH₂S(CH₃)₂]⁺ has been used in cyclopropanations. ^[6] Pyrolysis of Fp2 gives the cuboidal cluster [FeCp(CO)]₄.

Cyclooctatetraene derivatives

The compound Fe(COT)₂ (COT = Cyclooctatetraene) is well known ^[7], Fe₃(COT)₃ was described in 2009 as the reaction product of Fe(COT)₂ with a catalytic amount of an persistent carbene. It can be regarded as an organic version of triiron dodecacarbonyl ^[8]

Phosphine- and amine-Fe(II) complexes

As for other organometallic compounds, organoiron(II) complexes in the absence of Cp ligands are commonly complemented by tertiary diphosphines and to a lesser extent amine/imine ligands. Complexes of the type FeX₂(diphosphine)₂ figure prominently in this area, provided early examples of C-H bond activation, dihydrogen complexes, and dinitrogen complexes. Complexes derived from Schiff bases are active catalysts for olefin polymerization.^[9]

Organoiron compounds in organic synthesis and homogeneous catalysis

Because of its low cost and low toxicity of its salts, iron is often employed as a stoichiometric reagent. Iron role as a catalyst in organic reactions is overshadowed by the related chemistries of cobalt and nickel. Some main categories are:

• Addition reactions for example the Aldol reaction and Michael reaction^[10]
• Substitution notably coupling reactions. Ferric chloride is a well known catalyst in the Friedel–Crafts reaction. Compounds of the type [(η³-allyl)Fe(CO)⁴⁺X^- are allyl cation synthons in allylic substitution ^{[11] [12] [13]}. Likewise the compound Ph(CO₂)Fe⁺(η2-vinyl(OEt))BF₄^- is a masked vinyl cation^[14]. Disodium tetracarbonylferrate can be regarded a CO dianion synthon.^[15]
• Cycloadditions, for example cyclopropanation using CpFe(CO)₂CH₂S⁺(CH₃)₂BF₄^{- [16]}. Also Ene reaction ^[17]
• Hydrogenation and reduction
• isomerization reactions and rearrangement reactions
• Cross-coupling reactions. Iron compounds such as Fe(acac)₃ catalyze a wide range of cross-coupling reactions with one substrate an aryl or alkyl Grignard and the other substrate an aryl, [alkenyl]] (vinyl), or acyl organohalide. In the related Kumada coupling the catalysts are based on palladium and nickel.

Biochemistry

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.

See also

• Chemical bonds of carbon with other elements in the periodic table:

References