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:[4]
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.[5]
Alkene-Fe(0)-CO derivatives
Monoalkenes
Iron pentacarbonyl reacts photochemically with alkenes to give Fe(CO)4(alkene).[6]
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.[8][9]
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.[10]
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,[11] Fe3(COT)3,[12] 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−.[13]
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.[14][15][16]
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.[17] Treatment of ferrocene with aluminium trichloride and benzene gives the cation [CpFe(C6H6)]+. Oxidation of ferrocene gives the blue 17e species
ferrocenium. Derivatives of
fullerene can also act as a highly substituted cyclopentadienyl ligand.
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.[18]
Reduction of Fp2 with sodium gives "NaFp", containing a potent
nucleophile and precursor to many derivatives of the type CpFe(CO)2R.[19] The derivative [FpCH2S(CH3)2+ has been used in
cyclopropanations.[20] 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.[21] 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.[22]
Fp-R compounds are
prochiral, and studies have exploited the chiral derivatives CpFe(PPh3)(CO)acyl.[23]
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.
Compounds of the type [(η3-allyl)Fe(CO)4+X− are
allyl cationsynthons in
allylic substitution.[6] In contrast, compounds of the type [(η5-C5H5)Fe(CO)2(CH2CH=CHR)] possessing η1-allyl groups are analogous to main group allylmetal species (M = B, Si, Sn, etc.) and react with carbon electrophiles to give allylation products with SE2′ selectivity.[24] Similarly, allenyl(cyclopentadienyliron) dicarbonyl complexes exhibit reactivity analogous to main group allenylmetal species and serve as nucleophilic propargyl synthons.[25]
Sulfur and phosphorus derivatives
Complexes of the type
Fe2(SR)2(CO)6 and Fe2(PR2)2(CO)6 form, usually by the reaction of thiols and secondary phosphines with iron carbonyls.[26] The thiolates can also be obtained from the tetrahedrane
Fe2S2(CO)6.
Some organoiron(III) compounds are prepared by oxidation of organoiron(II) compounds. A long-known example being
ferrocenium [(C5H5)2Fe]+. Organoiron(III) porphyrin complexes, including alkyl and aryl derivatives, are also numerous.[29][30]
Iron(IV)
In Fe(norbornyl)4, Fe(IV) is stabilized by an alkyl ligand that resists
beta-hydride elimination.[32] Surprisingly, FeCy4, which is susceptible to beta-hydride elimination, has also been isolated and crystallographically characterized and is stable at –20 °C. The unexpected stability was attributed to stabilizing dispersive forces as well as conformational effects that disfavor beta-hydride elimination.[33]
Two-electron oxidation of
decamethylferrocene gives the dication [Fe(C5Me5)22+, which forms a carbonyl complex, [Fe(C5Me5)2(CO)](SbF6)2.[34] Ferrocene is also known to undergo protonation at the iron center with HF/AlCl3 or HF/PF5 to give the formally Fe(IV) hydride complex, [Cp2FeH]+[PF6–.[35][36]
Iron(V, VI, VII)
In 2020, Jeremy M. Smith and coworkers reported crystallographically characterized Fe(V) and Fe(VI) bisimido complexes based on a bidentate bis(carbene)borate ligand.[37] By virtue of the supporting ligand architecture, these species constitute organometallic Fe(V) and Fe(VI) complexes.
In 2024,
Karsten Meyer and coworkers reported a crystallographically characterized Fe(VI) nitrido complex, [(TIMMNMes)FeVI(≡N)(F)](PF6)2·CH2Cl2, which bears a tris(N-heterocyclic carbene) ligand (tris[(3-mesityl-imidazol-2-ylidene)methyl]amine). Related Fe(V) complexes were crystallographically characterized in the same study, while an Fe(VII) species that decomposes above –50 °C was characterized by
Mössbauer spectroscopy.[38]
Organoiron compounds in organic synthesis and homogeneous catalysis
In industrial catalysis, iron complexes are seldom used in contrast to
cobalt and
nickel. Because of the low cost and low toxicity of its salts, iron is attractive as a stoichiometric reagent. Some areas of investigation include:
Complexes derived from Schiff bases are active catalysts for olefin polymerization.[39]
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.
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