Organoiron chemistry

{{Short description|Chemistry of iron compounds containing a carbon-to-iron chemical bond}} '''Organoiron chemistry''' is the chemistry of iron compounds containing a carbon-to-iron chemical bond.<ref>''Synthesis of Organometallic Compounds: A Practical Guide'' Sanshiro Komiya Ed. S. Komiya, M. Hurano 1997</ref><ref>{{cite journal | doi = 10.1021/cr040664h | volume=104 | title=Iron-Catalyzed Reactions in Organic Synthesis | year=2004 | journal=Chemical Reviews | pages=6217–6254 | last1 = Bolm | first1 = Carsten| issue=12 | pmid=15584700 }}</ref> 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.<ref>{{cite journal | last1 = Enthaler | first1 = S. | last2 = Junge | first2 = K. | last3 = Beller | first3 = M. | year = 2008 | title = Sustainable Metal Catalysis with Iron: From Rust to a Rising Star? | doi = 10.1002/anie.200800012 | pmid = 18412184 | journal = Angew. Chem. Int. Ed. | volume = 47| issue = 18 | pages = 3317–3321 }}</ref> 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)== [[File:Sample of iron pentacarbonyl.jpg|thumb|132px|Iron pentacarbonyl.]]

===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:<ref>{{cite journal|title=Nucleophilic Acylation with Disodium Tetracarbonylferrate: Methyl 7-oxoheptanoate and Methyl 7-oxoöctanoate|journal=Organic Syntheses|volume=59|page=102|year=1979|doi=10.15227/orgsyn.059.0102|first1=Richard G.|last1=Finke|first2=Thomas N.|last2=Sorrell}}</ref> :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.<ref>{{cite journal | author = Brunet J. J.| year = 1990 | title = Tetracarbonylhydridoferrates, MHFe(CO)₄: Versatile Tools in Organic Synthesis and Catalysis | journal = Chem. Rev. | volume = 90 | issue = 1041–1059| page = 1041 | doi = 10.1021/cr00104a006 }}</ref>

===Alkene-Fe(0)-CO derivatives=== [[File:(Butadiene)iron-tricarbonyl-3D-balls.png|thumb|(Butadiene)iron tricarbonyl]]

===Monoalkenes=== Iron pentacarbonyl reacts photochemically with alkenes to give Fe(CO)₄(alkene).<ref name=alkene>{{cite journal|title=(+)-(1R,2S,3R)-Tetracarbonyl[(1-3η)-1-(Phenylsulfonyl)- But-2-en-1-yl]iron(1+) Tetrafluoroborate|journal=Org. Synth.|year=2002|volume=78|pages=189 |author=D. Enders |author2=B. Jandeleit |author3=S. von Berg|doi=10.15227/orgsyn.078.0189}}</ref>

===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,<ref>{{cite journal|title=Cyclohexadieneiron Tricarbonyl|first1=Anthony J.|last1=Pearson|first2=Huikai|last2=Sun|year=2008 |doi=10.1002/047084289X.rn00791|journal=E-EROS Encyclopedia of Reagents for Organic Synthesis|isbn= 978-0471936237 }}</ref> 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.<ref>{{cite journal|first1=A. J. |last1=Birch|first2=K. B.|last2=Chamberlain|doi=10.15227/orgsyn.057.0107|title=Tricarbonyl&#91;(2,3,4,5-η)-2,4-cyclohexadien-1-one&#93;iron and Tricarbonyl&#91;(1,2,3,4,5-η)-2-methoxy-2,4-cyclohexadien-1-yl&#93;iron(1+) Hexafluorophosphate(1−) From Anisole|journal=Organic Syntheses|volume=57|pages=107|year=1977}}</ref><ref>{{cite journal|title=Alkylation of Dimedone With A Tricarbonyl(Diene)iron Complex: Tricarbonyl[2-[(2,3,4,5-η)-4-methoxy-2,4-cyclohexadien-1-yl]-5,5-dimethyl-1,3-cyclohexanedione]iron|journal=Org. Synth.|year=1977|volume=57|page=16|doi= 10.15227/orgsyn.057.0016|first1=A. J. |last1=Birch|first2=K. B.|last2=Chamberlain}}</ref>

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.<ref>{{cite book|title=Organic Syntheses via Metal Carbonyls Volume 1|chapter=Cyclic Polymerization of Acetylenes by Metal Carbonyl Compounds |author=C. Hoogzand |author2=W. Hubel|editor1=Wender, I. |editor2=Pino, P.|publisher=Wiley|isbn=0-471-93367-8|year=1968|url-access=registration|url=https://archive.org/details/organicsyntheses02wend}}</ref>

===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,<ref>D. H. Gerlach, R. A. Schunn, Inorg. Synth. volume 15, 2 ('''1974''') {{doi|10.1002/9780470132463.ch1}}</ref> Fe₃(COT)₃,<ref>{{cite journal | author = Lavallo Vincent, Grubbs Robert H | year = 2009 | title = Carbenes As Catalysts for Transformations of Organometallic Iron Complexes | journal = Science | volume = 326 | issue = 5952| pages = 559–562 | doi = 10.1126/science.1178919 | pmid = 19900894 | bibcode = 2009Sci...326..559L | pmc = 2841742}}</ref> and several mixed COT-carbonyls (e.g. Fe(COT)(CO)₃ and Fe₂(COT)(CO)₆). [[image:Fe(cot)2.svg|thumb|upright=0.6|Bis(cyclooctatetraene)iron is an Fe(0) complex lacking CO ligands.]]

==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]⁻.<ref>{{cite journal|last=Ellis|first=J. E.|title=The Chatt reaction: conventional routes to homoleptic arenemetalates of d-block elements|journal=Dalton Transactions|year=2019|volume=48|issue=26|pages=9538–9563|doi=10.1039/C8DT05029E|pmid=30724934|s2cid=73436073 }}</ref> :right|400px|cyclopentadienyliron dicarbonyl dimer

===Ferrocene and its derivatives=== {{main|ferrocene}} 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 {{awrap|[Cp₂Fe–H]⁺PF₆^–}} and {{awrap|Cp₂Fe⁺–Hg^–(OTFA)₂}}, respectively.<ref>{{Cite journal|last=Astruc|first=Didier|date=2017|title=Why is Ferrocene so Exceptional?|journal=European Journal of Inorganic Chemistry|language=en|volume=2017|issue=1|pages=6–29|doi=10.1002/ejic.201600983|issn=1099-0682}}</ref><ref>{{Cite journal|last1=Malischewski|first1=Moritz|last2=Seppelt|first2=Konrad|last3=Sutter|first3=Jörg|last4=Heinemann|first4=Frank W.|last5=Dittrich|first5=Birger|last6=Meyer|first6=Karsten|date=2017|title=Protonation of Ferrocene: A Low-Temperature X-ray Diffraction Study of [Cp2FeH](PF6) Reveals an Iron-Bound Hydrido Ligand|journal=Angewandte Chemie International Edition|language=en|volume=56|issue=43|pages=13372–13376|doi=10.1002/anie.201704854|issn=1521-3773|pmid=28834022}}</ref><ref>{{Cite journal|last=Cunningham|first=Allan F.|date=1997-03-01|title=Mechanism of Mercuration of Ferrocene: General Treatment of Electrophilic Substitution of Ferrocene Derivatives|journal=Organometallics|volume=16|issue=6|pages=1114–1122|doi=10.1021/om960815+|issn=0276-7333}}</ref>

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.<ref>Petr Stepnicka "Ferrocenes: Ligands, Materials and Biomolecules" J. Wiley, Hoboken, 2008. {{ISBN|0-470-03585-4}}</ref> 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, Cp^i-Pr5Fe(CO)₂ (Cp^i-Pr5 = i-Pr₅C₅) has been characterized crystallographically.<ref>{{Cite journal|last1=Sitzmann|first1=Helmut|last2=Dezember|first2=Thomas|last3=Kaim|first3=Wolfgang|last4=Baumann|first4=Frank|last5=Stalke|first5=Dietmar|last6=Kärcher|first6=Joerg|last7=Dormann|first7=Elmar|last8=Winter|first8=Hubert|last9=Wachter|first9=Christoph|last10=Kelemen|first10=Marc|date=1996|title=Synthesis and Characterization of the Stable Dicarbonyl(cyclopentadienyl)iron Radical [(C5R5)Fe(CO)2] (R  CHMe2)|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.199628721|journal=Angewandte Chemie International Edition in English|volume=35|issue=23–24|pages=2872–2875|doi=10.1002/anie.199628721|issn=1521-3773|url-access=subscription}}</ref>

Reduction of Fp₂ with sodium gives "NaFp", containing a potent nucleophile and precursor to many derivatives of the type CpFe(CO)₂R.<ref>{{cite journal | author1 = Keith H. Pannell | author2 = Hemant K. Sharma | title = (Cyclopentadienyl)dicarbonylmethyliron ((η⁵-C₅H₅)Fe(CO)₂CH₃, FpMe), a Seminal Transition-Metal Alkyl Complex: Mobility of the Methyl Group | journal =Organometallics | year = 2010 | doi = 10.1021/om1004594 | volume=29 | issue = 21 | pages=4741–4745}}</ref> The derivative [FpCH₂S(CH₃)₂]⁺ has been used in cyclopropanations.<ref name="orgsynth">{{OrgSynth | collvol = 9 | collvolpages = 372 | year = 1998 | title = Cyclopropanation using an Iron-Containing Methylene Transfer Reagent: 1,1-Diphenylcyclopropane | author1 = Matthew N. Mattson | author2 = Edward J. O'Connor | author3 = Paul Helquist | prep = cv9p0372}}</ref> 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.<ref>{{cite journal|journal=Org. Synth.|year=1988|volume=66|page=95|title=Vinylation Of Enolates with a Vinyl Cation Equivalent: trans-3-Methyl-2-vinylcyclohexanone |author=Tony C. T. Chang |author2=Myron Rosenblum |author3=Nancy Simms|doi=10.15227/orgsyn.066.0095|doi-access=free}}</ref> 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.<ref>{{Cite journal |last=Watson |first=James D. |last2=Field |first2=Leslie D. |last3=Ball |first3=Graham E. |date=2022-09-28 |title=[Fp(CH 4 )] + , [η 5 -CpRu(CO) 2 (CH 4 )] + , and [η 5 -CpOs(CO) 2 (CH 4 )] + : A Complete Set of Group 8 Metal–Methane Complexes |url=https://pubs.acs.org/doi/10.1021/jacs.2c07124 |journal=Journal of the American Chemical Society |language=en |volume=144 |issue=38 |pages=17622–17629 |doi=10.1021/jacs.2c07124 |issn=0002-7863|hdl=1959.4/102418 |hdl-access=free }}</ref>

Fp-R compounds are prochiral, and studies have exploited the chiral derivatives CpFe(PPh₃)(CO)acyl.<ref>Karola Rück-Braun "Iron Acyl Complexes" in Transition Metals for Organic Synthesis. Vol. 1. 2nd Ed., M. Beller, C. Bolm, Eds. Wiley-VCH, 2004, Weinheim. {{ISBN|3-527-30613-7}}.</ref>

===Alkyl, allyl, and aryl compounds=== [[File:Fe2mes4.png|thumb|upright=0.9|tetramesityldiiron is a rare example of a neutral per-organo complex of iron]] 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 cation synthons in allylic substitution.<ref name=alkene/> 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 S_E2′ selectivity.<ref>{{Cite journal|last1=Cutler|first1=A.|last2=Ehnholt|first2=D.|last3=Lennon|first3=P.|last4=Nicholas|first4=K.|last5=Marten|first5=David F.|last6=Madhavarao|first6=M.|last7=Raghu|first7=S.|last8=Rosan|first8=A.|last9=Rosenblum|first9=M.|date=1975-05-01|title=Chemistry of dicarbonyl .eta.5-cyclopentadienyliron complexes. General syntheses of monosubstituted .eta.2-olefin complexes and of 1-substituted .eta.1-allyl complexes. Conformational effects on the course of deprotonation of (.eta.2-olefin) cations|journal=Journal of the American Chemical Society|volume=97|issue=11|pages=3149–3157|doi=10.1021/ja00844a038|issn=0002-7863}}</ref> Similarly, allenyl(cyclopentadienyliron) dicarbonyl complexes exhibit reactivity analogous to main group allenylmetal species and serve as nucleophilic propargyl synthons.<ref>{{Cite journal|last1=Wang|first1=Yidong|last2=Zhu|first2=Jin|last3=Durham|first3=Austin C.|last4=Lindberg|first4=Haley|last5=Wang|first5=Yi-Ming|date=2019-12-18|title=α-C–H Functionalization of π-Bonds Using Iron Complexes: Catalytic Hydroxyalkylation of Alkynes and Alkenes|journal=Journal of the American Chemical Society|volume=141|issue=50|pages=19594–19599|doi=10.1021/jacs.9b11716|pmid=31791121|s2cid=208611984 |issn=0002-7863}}</ref>

===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.<ref>King, R. B., "Organosulfur Derivatives of Metal Carbonyls. I. The Isolation of Two Isomeric Products in the Reaction of Triiron Dodecacarbonyl with Dimethyl Disulfide", J. Am. Chem. Soc., 1962, 84, 2460.</ref> The thiolates can also be obtained from the tetrahedrane Fe₂S₂(CO)₆.

==Iron(III)== Alkylation of FeCl₃ with methylmagnesium bromide gives [Fe(CH₃)₄]^–, which is thermally labile.<ref>{{cite journal |doi=10.1016/j.poly.2018.10.041|title=Synthesis and Characterization of a Sterically Encumbered Homoleptic Tetraalkyliron(III) Ferrate Complex |year=2019 |last1=Sears |first1=Jeffrey D. |last2=Muñoz |first2=Salvador B. |last3=Cuenca |first3=Maria Camila Aguilera |last4=Brennessel |first4=William W. |last5=Neidig |first5=Michael L. |journal=Polyhedron |volume=158 |pages=91–96 |pmid=31031511 |pmc=6481957 }} and references therein.</ref> Such compounds may be relevant to the mechanism of Fe-catalyzed cross coupling reactions.<ref name=Byers>{{cite journal |doi=10.1039/C5QI00295H|title=Recent Advances in Iron-Catalysed Cross Coupling Reactions and Their Mechanistic Underpinning |year=2016 |last1=Mako |first1=T. L. |last2=Byers |first2=J. A. |journal=Inorganic Chemistry Frontiers |volume=3 |issue=6 |pages=766–790 }}</ref>

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.<ref>{{Cite journal |last=Ogoshi |first=Hisanobu |last2=Sugimoto |first2=Hiroshi |last3=Yoshida |first3=Zen-Ichi |last4=Kobayashi |first4=Hanako |last5=Sakai |first5=Hiroshi |last6=Maeda |first6=Yutaka |date=1982 |title=Syntheses and magnetic properties of aryliron(III) complexes of octaethylporphyrins |url=https://doi.org/10.1016/S0022-328X(00)85854-4 |journal=Journal of Organometallic Chemistry |volume=234 |issue=2 |pages=185–195 |doi=10.1016/s0022-328x(00)85854-4 |issn=0022-328X|url-access=subscription }}</ref><ref>{{Cite journal |last=Lexa |first=Doris |last2=Mispelter |first2=Joel |last3=Saveant |first3=Jean Michel |date=1981 |title=Electroreductive alkylation of iron in porphyrin complexes. Electrochemical and spectral characteristics of .sigma.-alkylironporphyrins |url=https://pubs.acs.org/doi/abs/10.1021/ja00413a004 |journal=Journal of the American Chemical Society |language=en |volume=103 |issue=23 |pages=6806–6812 |doi=10.1021/ja00413a004 |issn=0002-7863|url-access=subscription }}</ref> [[File:CUHXAEFetppPh.png|thumb|Structure of Fe(tetraphenylporphyrin)C₆H₅.<ref>{{cite journal|title=Molecular stereochemistry of low-spin five-coordinate phenyl(meso-tetraphenylporphyrinato)iron(III)|author=Pascal Doppelt|journal=Inorg. Chem.|year=1984|volume=23|issue=24|pages=4009–4011|doi=10.1021/ic00192a033}}</ref>]]

==Iron(IV)== thumb|194px|Fe(4-norbornyl)₄ is a rare example of a low-spin tetrahedral complex. In Fe(norbornyl)₄, Fe(IV) is stabilized by an alkyl ligand that resists beta-hydride elimination.<ref name='Mnor4'>{{cite journal|author = B. K. Bower and H. G. Tennent|title = Transition metal bicyclo[2.2.1]hept-1-yls|journal = J. Am. Chem. Soc.|year = 1972|volume = 94|issue = 7|pages = 2512–2514|doi = 10.1021/ja00762a056}}</ref> 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.<ref>{{Cite journal|last1=Casitas|first1=Alicia|last2=Rees|first2=Julian A.|last3=Goddard|first3=Richard|last4=Bill|first4=Eckhard|last5=DeBeer|first5=Serena|last6=Fürstner|first6=Alois|date=2017-08-14|title=Two Exceptional Homoleptic Iron(IV) Tetraalkyl Complexes|url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201612299|journal=Angewandte Chemie International Edition|language=en|volume=56|issue=34|pages=10108–10113|doi=10.1002/anie.201612299|pmid=28251752 |url-access=subscription|hdl=11858/00-001M-0000-002D-C624-F|hdl-access=free}}</ref>

Two-electron oxidation of decamethylferrocene gives the dication [Fe(C₅Me₅)₂]²⁺, which forms a carbonyl complex, [Fe(C₅Me₅)₂(CO)](SbF₆)₂.<ref>{{Cite journal|last1=Malischewski|first1=Moritz|last2=Seppelt|first2=Konrad|last3=Sutter|first3=Jörg|last4=Munz|first4=Dominik|last5=Meyer|first5=Karsten|date=2018|title=A Ferrocene-Based Dicationic Iron(IV) Carbonyl Complex|journal=Angewandte Chemie International Edition|language=en|volume=57|issue=44|pages=14597–14601|doi=10.1002/anie.201809464|issn=1521-3773|pmid=30176109|s2cid=52145802 }}</ref> 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]^–.<ref>{{Cite journal |last=Rosenblum |first=Myron |last2=Santer |first2=J. O. |date=1959 |title=PROTONATION OF FERROCENE BY STRONG ACIDS 1 |url=https://pubs.acs.org/doi/abs/10.1021/ja01529a075 |journal=Journal of the American Chemical Society |language=en |volume=81 |issue=20 |pages=5517–5518 |doi=10.1021/ja01529a075 |issn=0002-7863|url-access=subscription }}</ref><ref>{{Cite journal |last=Malischewski |first=Moritz |last2=Seppelt |first2=Konrad |last3=Sutter |first3=Jörg |last4=Heinemann |first4=Frank W. |last5=Dittrich |first5=Birger |last6=Meyer |first6=Karsten |date=2017-10-16 |title=Protonation of Ferrocene: A Low‐Temperature X‐ray Diffraction Study of [Cp 2 FeH](PF 6 ) Reveals an Iron‐Bound Hydrido Ligand |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201704854 |journal=Angewandte Chemie International Edition |language=en |volume=56 |issue=43 |pages=13372–13376 |doi=10.1002/anie.201704854 |issn=1433-7851|url-access=subscription }}</ref>

==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.<ref>{{Cite journal |last=Martinez |first=Jorge L. |last2=Lutz |first2=Sean A. |last3=Yang |first3=Hao |last4=Xie |first4=Jiaze |last5=Telser |first5=Joshua |last6=Hoffman |first6=Brian M. |last7=Carta |first7=Veronica |last8=Pink |first8=Maren |last9=Losovyj |first9=Yaroslav |last10=Smith |first10=Jeremy M. |date=2020-10-16 |title=Structural and spectroscopic characterization of an Fe(VI) bis(imido) complex |url=https://www.science.org/doi/10.1126/science.abd3054 |journal=Science |language=en |volume=370 |issue=6514 |pages=356–359 |doi=10.1126/science.abd3054 |issn=0036-8075|url-access=subscription }}</ref> 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, [(TIMMN^Mes)Fe^VI(≡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.<ref>{{Cite journal |last=Keilwerth |first=Martin |last2=Mao |first2=Weiqing |last3=Malischewski |first3=Moritz |last4=Jannuzzi |first4=Sergio A. V. |last5=Breitwieser |first5=Kevin |last6=Heinemann |first6=Frank W. |last7=Scheurer |first7=Andreas |last8=DeBeer |first8=Serena |last9=Munz |first9=Dominik |last10=Bill |first10=Eckhard |last11=Meyer |first11=Karsten |date=2024-01-30 |title=The synthesis and characterization of an iron(VII) nitrido complex |url=https://www.nature.com/articles/s41557-023-01418-4 |journal=Nature Chemistry |language=en |pages=1–7 |doi=10.1038/s41557-023-01418-4 |issn=1755-4349|doi-access=free |pmc=10997499 }}</ref>

==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: * Hydrogenation and reduction, for example catalyst Knölker complex. * 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. *Complexes derived from Schiff bases are active catalysts for olefin polymerization.<ref>Allan, L. E. N.; Shaver, M. P.; White, A. J. P. and Gibson, V. C., "Correlation of Metal Spin-State in alpha-Diimine Iron Catalysts with Polymerization Mechanism", Inorg. Chem., 2007, 46, 8963-8970.</ref>

==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.

==Further reading== *{{cite book |doi=10.1016/B978-0-12-417101-5.X5001-X |title=The Organic Chemistry of Iron |year=1978 |isbn=978-0-12-417101-5 |editor=E. A. Koerner von Gustorf |editor2=F.-W. Grevels |editor3=I. Fischler |publisher=Academic Press}}

==References== {{Reflist|2}}

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Category:Organoiron compounds