Iron Alkyl Complexes Research Articles

Iron(iv) alkyl complexes: electronic structure contributions to Fe–C bond homolysis and migration reactions that form N–C bonds from N2

Silylation of iron-alkyl-dinitrogen complexes gives high-spin iron(iv) products that also have an iron(ii) resonance structure. The stability and reactivity of these compounds depends on the alkyl group, with insertion and homolysis observed.

Alkyl Complexes of Iron(IV) Triphenylcorrole

Alkyl Complexes of Iron(IV) Triphenylcorrole

Synthesis and characterization of a sterically encumbered homoleptic tetraalkyliron(III) ferrate complex

Homoleptic iron-alkyl complexes have been implicated as key intermediates in iron-catalyzed cross-coupling with simple iron salts. Tetraalkyliron(III) ferrate species have been shown to be accessible with either methyl or benzyl ligands, where the former complex is S = 3/2 and distorted square planar while the latter is a S = 5/2 distorted tetrahedral species. In the current study, a new tetraalkyliron(III) complex is synthesized containing modified methylene substituents that incorporate large trimethylsilyl (TMS) groups to further probe steric and electronic ligand effects in tetraalkyliron(III) complexes by increasing the electron-donating ability of the ligand while retaining steric bulk. Detailed structural and DFT studies provide insight into electronic structure and bonding of the four-coordinate trimethylsilylmethyl iron(III) complex compared to the previously reported analogs containing methyl and benzyl ligands.

Synthesis, Characterization, and Reactivity of a High-Spin Iron(II) Hydrido Complex Supported by a PNP Pincer Ligand and Its Application as a Homogenous Catalyst for the Hydrogenation of Alkenes.

This study focused on the synthesis and characterization of a range of low-valent, high-spin iron(II) complexes supported by a carbazole-based PNP pincer-type ligand. The addition of the lithiated ligand (PNP)Li to FeCl2(THF)1.5 yielded the chlorido complex (PNP)FeCl (1), which could be readily converted to the four-coordinate iron(II) alkyl complexes (PNP)FeR [R = CH2SiMe3 (3a), Me (3b), CH2Ph (3c)]. These iron(II) complexes were fully characterized by X-ray analysis and a comprehensive, density-functional-theory-assisted study with complete assignment of their paramagnetic 1H and 13C NMR spectra. Treatment of 1 with KHBEt3 or the addition of molecular hydrogen to (PNP)FeR afforded a high-spin iron(II) PNP hydrido complex, which was identified as the dimer [(PNP)Fe(μ-H)]2 (4) with two bridging hydrido ligands between the iron centers. Exposing complexes 1 and 4 to carbon monoxide led to the corresponding six-coordinate, diamagnetic complexes (PNP)Fe(CO)2Cl (2) and (PNP)Fe(CO)2H (5), of which 2 was present as cis/trans isomers. Furthermore, 4 was found to be an active catalyst for the hydrogenation of alkenes.

Iron achieves noble metal reactivity and selectivity: highly reactive and enantioselective iron complexes as catalysts in the hydrosilylation of ketones.

Chiral iron alkyl and iron alkoxide complexes bearing boxmi pincers as stereodirecting ligands have been employed as catalysts for enantioselective hydrosilylation reactions with unprecedented activity and selectivity (TOF = 240 h(-1) at -40 °C, ee up to 99% for alkyl aryl ketones), which match the performance of previously established noble-metal-based catalysts. This shows the potential of earth-abundant metals such as iron for replacing platinum--metals without any drawbacks for the reaction design.

Elementary Steps of Iron Catalysis: Exploring the Links between Iron Alkyl and Iron Olefin Complexes for their Relevance in CH Activation and CC Bond Formation

AbstractThe alkylation of complexes 2 and 7 with Grignard reagents containing β‐hydrogen atoms is a process of considerable relevance for the understanding of C–H activation as well as C–C bond formation mediated by low‐valent iron species. Specifically, reaction of 2 with EtMgBr under an ethylene atmosphere affords the bis‐ethylene complex 1 which is an active precatalyst for prototype [2+2+2] cycloaddition reactions and a valuable probe for mechanistic studies. This aspect is illustrated by its conversion into the bis‐alkyne complex 6 as an unprecedented representation of a cycloaddition catalyst loaded with two substrates molecules. On the other hand, alkylation of 2 with 1 equivalent of cyclohexylmagnesium bromide furnished the unique iron alkyl species 11 with a 14‐electron count, which has no less than four β‐H atoms but is nevertheless stable at low temperature against β‐hydride elimination. In contrast, the exhaustive alkylation of 1 with cyclohexylmagnesium bromide triggers two consecutive C–H activation reactions mediated by a single iron center. The resulting complex has a diene dihydride character in solution (15), whereas its structure in the solid state is more consistent with an η3‐allyl iron hydride rendition featuring an additional agostic interaction (14). Finally, the preparation of the cyclopentadienyl iron complex 25 illustrates how an iron‐mediated C–H activation cascade can be coaxed to induce a stereoselective CC bond formation. The structures of all relevant new iron complexes in the solid state are presented.

Elementary Steps of Iron Catalysis: Exploring the Links between Iron Alkyl and Iron Olefin Complexes for their Relevance in CH Activation and CC Bond Formation

The alkylation of complexes 2 and 7 with Grignard reagents containing β-hydrogen atoms is a process of considerable relevance for the understanding of C-H activation as well as C-C bond formation mediated by low-valent iron species. Specifically, reaction of 2 with EtMgBr under an ethylene atmosphere affords the bis-ethylene complex 1 which is an active precatalyst for prototype [2+2+2] cycloaddition reactions and a valuable probe for mechanistic studies. This aspect is illustrated by its conversion into the bis-alkyne complex 6 as an unprecedented representation of a cycloaddition catalyst loaded with two substrates molecules. On the other hand, alkylation of 2 with 1 equivalent of cyclohexylmagnesium bromide furnished the unique iron alkyl species 11 with a 14-electron count, which has no less than four β-H atoms but is nevertheless stable at low temperature against β-hydride elimination. In contrast, the exhaustive alkylation of 1 with cyclohexylmagnesium bromide triggers two consecutive C-H activation reactions mediated by a single iron center. The resulting complex has a diene dihydride character in solution (15), whereas its structure in the solid state is more consistent with an η(3) -allyl iron hydride rendition featuring an additional agostic interaction (14). Finally, the preparation of the cyclopentadienyl iron complex 25 illustrates how an iron-mediated C-H activation cascade can be coaxed to induce a stereoselective CC bond formation. The structures of all relevant new iron complexes in the solid state are presented.

Spin Crossover during β-Hydride Elimination in High-Spin Iron(II)– and Cobalt(II)–Alkyl Complexes

It is surprising that rapid β-hydride elimination (βHE) can take place in some high-spin iron(II)– and cobalt(II)–alkyl complexes despite the absence of empty d orbitals. In this study, density functional theory (DFT) is used to analyze the pathways for βHE in alkyl complexes of iron(II) and cobalt(II) supported by β-diketiminate that undergo βHE, and in tris(pyrazolyl)borate (Tp) iron(II)–alkyl complexes that are resistant to βHE. Each reaction pathway includes spin crossover to a transition state with a lower spin and a vacant d orbital; importantly, only the spin crossover accelerated pathway matches experimental rates. The lower spin transition state has a square-planar geometry that is ideal for depopulating one in-plane d orbital that can accept the electrons from the β-hydrogen. The energy of the square-planar transition state is increased by steric bulk around the metal center and by increases in the coordination number at iron, explaining the resistance to βHE in TpFeR. Migratory insertion, the m...

Synthesis and Electronic Structure of Cationic, Neutral, and Anionic Bis(imino)pyridine Iron Alkyl Complexes: Evaluation of Redox Activity in Single-Component Ethylene Polymerization Catalysts

A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been prepared, and their electronic and molecular structures have been established by a combination of X-ray diffraction, Mössbauer spectroscopy, magnetochemistry, and open-shell density functional theory. For the cationic complexes, [((iPr)PDI)Fe-R][BPh(4)] ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = CH(2)SiMe(3), CH(2)CMe(3), or CH(3)), which are known single-component ethylene polymerization catalysts, the data establish high spin ferrous compounds (S(Fe) = 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron reduction to the corresponding neutral alkyls, ((iPr)PDI)Fe(CH(2)SiMe(3)) or ((iPr)PDI)Fe(CH(2)CMe(3)), is chelate-based, resulting in a bis(imino)pyridine radical anion (S(PDI) = 1/2) antiferromagnetically coupled to a high spin ferrous ion (S(Fe) = 2). The neutral neopentyl derivative was reduced by an additional electron and furnished the corresponding anion, [Li(Et(2)O)(3)][((iPr)PDI)Fe(CH(2)CMe(3))N(2)], with concomitant coordination of dinitrogen. The experimental and computational data establish that this S = 0 compound is best described as a low spin ferrous compound (S(Fe) = 0) with a closed-shell singlet bis(imino)pyridine dianion (S(PDI) = 0), demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state change at the iron center from high to low spin. The relevance of the compounds and their electronic structures to olefin polymerization catalysis is also presented.

ChemInform Abstract: Involvement of Iron Alkyl Complexes and Alkyl Radicals in the Kharasch Reactions: Probing the Catalysis Using Iron Phosphine Complexes.

The X-ray crystal structure of trans-[FeBr2(depe)2](depe = Et2PCH2CH2PEt2) has been determined, and its catalysis of the reactions between MgBrEt and RBr (R = Et, Prn, Bun, PhCH2 or H2CCHCH2) to give alkanes and alkenes investigated. Detailed analysis of the products and the time courses of the reactions demonstrate that the hydrido-species trans-[FeH(Br)(depe)2]n+(n= 0 or 1) are not intermediates in the reaction, but are products formed in the termination steps. It is shown that the catalysis can proceed by one of two pathways, depending on the alkyl halide. One pathway is proposed to involve the intermediacy of iron(I) species, and the other of iron(III) complexes. Both catalytic cycles contain the same fundamental reaction: the abstraction of a β-hydrogen atom from a [FeEt(Br)(depe)2]n+ species by alkyl radicals to give the alkane and ethylene. The X-ray crystal structure of trans-[FeH(Br)(depe)2]BPh4 is reported.

Bis(imino)pyridine Iron Alkyls Containing β-Hydrogens: Synthesis, Evaluation of Kinetic Stability, and Decomposition Pathways Involving Chelate Participation

Bis(imino)pyridine iron alkyl complexes bearing beta-hydrogens, ((iPr)PDI)FeR (((iPr)PDI = 2,6-(2,6-(i)Pr2-C6H3N=CMe)2C5H3N; R = Et, (n)Bu, (i)Bu, CH2 (cyclo)C5H 9; 1-R), were synthesized either by direct alkylation of ((iPr)PDI)FeCl (1-Cl) with the appropriate Grignard reagent or more typically by oxidative addition of the appropriate alkyl bromide to the iron bis(dinitrogen) complex, ((iPr)PDI)Fe(N2)2 (1-(N2)2). In the latter method, the formal oxidative addition reaction produced ((iPr)PDI)FeBr (1-Br), along with the desired iron alkyl, 1-R. Elucidation of the electronic structure of 1-Br and related 1-R derivatives by magnetic measurements, structural studies and NMR spectroscopy established high spin ferrous compounds antiferromagnetically coupled to chelate radical anions. Thus, the formal oxidative process is bis(imino)pyridine ligand-based (one electron is formally removed from each chelate, not the iron) during oxidative addition. The kinetic stability of each 1-R compound was assayed in benzene-d6 solution and found to produce a mixture of the corresponding alkane and alkene. The kinetic stability of the iron alkyl complexes was inversely correlated with the number of beta-hydrogens present. For example, the iron ethyl complex, 1-Et, underwent clean loss of ethane over the course of three hours, whereas the corresponding 1-(i)Bu compound had a half-life of over 12 h under identical conditions. The mechanism of the decomposition was studied with a series of deuterium labeling experiments and support a pathway involving initial beta-hydrogen elimination followed by cyclometalation of an isopropyl methyl group, demonstrating an overall transfer hydrogenation pathway. The relevance of such pathways to chain transfer in bis(imino)pyridine iron catalyzed olefin polymerization reactions is also presented.

Reversible Beta-Hydrogen Elimination of Three-Coordinate Iron(II) Alkyl Complexes: Mechanistic and Thermodynamic Studies

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionORIGINAL ARTICLEThis notice is a correctionReversible Beta-Hydrogen Elimination of Three-Coordinate Iron(II) Alkyl Complexes: Mechanistic and Thermodynamic StudiesJavier Vela, Sridhar Vaddadi, Thomas R. Cundari, Jeremy M. Smith, Elizabeth A. Gregory, Rene J. Lachicotte, Christine J. Flaschenriem, and Patrick L. HollandCite this: Organometallics 2005, 24, 22, 5494Publication Date (Web):September 20, 2005Publication History Published online20 September 2005Published inissue 1 October 2005https://doi.org/10.1021/om050771hCopyright © 2005 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views571Altmetric-Citations1LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (37 KB) Get e-Alerts Get e-Alerts

Reversible Beta-Hydrogen Elimination of Three-Coordinate Iron(II) Alkyl Complexes: Mechanistic and Thermodynamic Studies

High-spin organometallic complexes have not received extensive mechanistic study, despite their potential importance as unsaturated intermediates in catalytic transformations. We have found that, with a suitably bulky bidentate ligand, three-coordinate, high-spin alkyl complexes of iron(II) are stable. They undergo isomerization and exchange reactions of the alkyl group through β-hydride elimination and reinsertion, and the β-hydride elimination step is rate-limiting. The alkyl complexes transfer a β-hydrogen atom to CC, CN, and CO double bonds and undergo deprotonation by Bronsted acids. The reversible β-hydride elimination reactions can be used to explore relative M−C bond energies. Competition experiments and density functional calculations demonstrate an enthalpic preference for alkyl isomers with iron bound to the terminal carbon of the alkyl fragment. This preference arises from steric and electronic effects. The steric preference could be overcome with a phenyl substituent, which steers iron to the...

Synthesis, characterization and reactivity of iron–olefin complexes of the type [CpFe(CO) 2(η 2-CH 2CHR)] +X − (Cp=η 5-C 5H 5; R=CH 3 to n-C 14H 29; X=PF 6 or BF 4)

A series of iron–olefin complexes of the type [CpFe(CO) 2(η 2-CH 2CHR)] + (R=CH 3 to n-C 14H 29) have been prepared using various synthesis methods. In addition, the complexes [Cp*Fe(CO) 2(η 2-CH 2CHR)] + (Cp*=η 5-C 5(CH 3) 5; R=CH 3, n-C 3H 7 and n-C 14H 29) and [CpRu(CO) 2(η 2-CH 2CHR)] + (R=CH 3, n-C 3H 7 and n-C 14H 29) have been synthesized by reacting the iron–alkyl complexes Cp*Fe(CO) 2(CH 2CH 2R) or ruthenium–alkyl complexes CpRu(CO) 2(CH 2CH 2R) with Ph 3CPF 6. A number of these complexes are new and have been fully characterized by analytical and spectroscopic methods. The reaction of [CpFe(CO) 2(η 2-CH 2CHR)] + with the isopropoxide ion gave the new ether derivatives [CpFe(CO) 2(CH 2CH{OCH(CH 3) 2}R)] (R=CH 3, n-C 2H 5, n-C 4H 9 and n-C 13H 27).

Effect of anions on the isocyanide insertion reaction in cationic alkyl complexes of iron(II): kinetic, thermodynamic and solution interionic structural studies

The equilibrium and kinetics of isocyanide insertion of complexes [Fe(PMe 3) 2(CO) 2(CH 3)(CNR)] +X − ( 1) [CNR= tert-butylisocyanide; X −=I − ( 1a), BPh 4 − ( 1b); BF 4 − ( 1c)] which afford complexes [Fe(PMe 3) 2(CO) 2(η 2-C(CH 3)NC(CH 3) 3)] +X − ( 2) were investigated in dichloromethane, methanol, acetone and nitromethane. The results indicate that the reaction proceeds via an associative mechanism with the preliminary formation of contact ion pairs. The structure of the contact ion pair in solution was studied by 1H-NOESY and 19F{ 1H}-HOESY NMR spectroscopy. The results indicate that structure is independent of the nature of the solvent and of the counterion. In complexes 1b,c the counterion is located between the CO and the isocyanide ligands; in complexes 2b,c the counterion is located near the dihaptoiminoacyl ligand. The reaction rate increases with the charge density and the coordinating power of the anions. The effect of the solvent can be also explained on the basis of its coordinating power.

Carbonylation reaction of alkyl complexes of iron and molecular structure of cis,trans-[Fe(CO) 2(PMe 3) 2(CH 3)I] and trans,trans-[Fe(CO) 2(PMe 3) 2(CH 3)I

The carbonylation reaction of cis,trans[Fe(CO) 2(PMe 3) 2(CH 3)I] ( Mc) to cis,trans-[Fe(CO) 2(PMe 3) 2(COCH 3)I] ( Ac) and the isomerization of Ac to trans,trans-[Fe(CO) 2(PMe 3) 2(COCH 3)I] ( At) were studied in polar and apolar solvents. The results suggest that the first step of the carbonylation is the formation of the ionic intermediate [Fe(CO) 3(PMe 3) 2(CH 3)]I ( Im); complexes Ac and At are both formed from the intermediate In at different rates due to the differential cooperative effect of the anion. Further information was obtained by a structural study of complexes Mc and trans,trans-[Fe(CO) 2(PMe 3) 2(CH 3)I] ( Mt), using single crystal X-ray diffraction. The structure models were refined to R = 0.034 for 2270 independent reflections for Mc and R = 0.056 for 2381 independent reflections for Mt. Crystal data: monoclinic, space group P2 1/ c for both molecules: a = 13.656(2), b = 9.032(3), c = 13.880(6) A ̊ , β = 106.78(3)° for Mc; a = 13.818(3), b = 8.957(2), c = 13.826(3) A ̊ , β = 105.21(3)° for Mt. Both complexes present octahedral coordination of the ligands around the metal centre.

Stereospecific syntheses of functionalised iron alkyl complexes with a stereogenic metal center from an iron formyl precursor

Treatment of the stable formyl complex Fe(C 5Ph 5)(CO(PMe 3)(CHO) ( 1) with an excess of methyl triflate and subsequent addition of NH 4PF 6 produces the secondary carbene complexes [Fe( η 5−C 5Ph 5)(CO)(PMe 3)( = CHOMe)][PF 6] ( 2, 93%). Methyl lithium acts as a dealkylating reagent with 2 to regenerate 1 (81%). Treatment of methoxycarbene complex 2 with LiAlD 4 gives the methoxymethyl complex Fe( η 5−C 5Ph 5)(CO)(PMe 3)(CH 2OMe) ( 3) in 95% yield. Similarly, reaction of 2 with LiAlD 4 gives a 90:10 mixture of Fe( η 5−C 5Ph 5)(CO)(PMe 3)(CHDOMe) ( 3 d 1 , 80%) diastereoisomers. The carbene complex 2 reacts with MeONa to give the iron dimethylacetal complex Fe( η 5−C 5Ph 5)(CO)(PMe 3)(CH(OMe) 2 ( 4, 55%). The cyano(methoxy)methyl complex Fe( η 5−C 5Ph 5(CO)(PMe 3){CH( rmCN)(OMe)} ( 5, 85%) is obtained from the reaction of 2 1 equivalent of KCN. NMR analyses of the crude product revealed that complex 5 was formed as a 85:15 mixture of diastereoisomers. The carbene complex 2 reacts with TMSCCLi to afford a 90:10 mixture of diastereoisomers of the stable methoxy(trimethylsilyethynyl)methyl iron complexes Fe(C 5Ph 5)(CO)(PMe 3) {CH(OMe)(C  CTMS)} ( 6). After extraction and filtration 6 is isolated as a pure diastereoisomer in 63% yield.

Thermodynamic and kinetic studies of migratory carbon monoxide insertion in alkyl complexes of iron(II)

The reaction of cis,trans-Fe(CO) 2 (PMe 3 ) 2 MeX with carbon monoxide to give the corresponding cis,trans- and trans,trans-acetyls has been studied in toluene for X Cl>Br>I and are determined by the withdrawing power of X, while the equilibrium constants of trans,trans-acetyl as compared to cis,trans-acetyl follow the order I>Br>Cl>>CN.

Involvement of iron alkyl complexes and alkyl radicals in the Kharasch reactions: probing the catalysis using iron phosphine complexes

The X-ray crystal structure of trans-[FeBr2(depe)2](depe = Et2PCH2CH2PEt2) has been determined, and its catalysis of the reactions between MgBrEt and RBr (R = Et, Prn, Bun, PhCH2 or H2CCHCH2) to give alkanes and alkenes investigated. Detailed analysis of the products and the time courses of the reactions demonstrate that the hydrido-species trans-[FeH(Br)(depe)2]n+(n= 0 or 1) are not intermediates in the reaction, but are products formed in the termination steps. It is shown that the catalysis can proceed by one of two pathways, depending on the alkyl halide. One pathway is proposed to involve the intermediacy of iron(I) species, and the other of iron(III) complexes. Both catalytic cycles contain the same fundamental reaction: the abstraction of a β-hydrogen atom from a [FeEt(Br)(depe)2]n+ species by alkyl radicals to give the alkane and ethylene. The X-ray crystal structure of trans-[FeH(Br)(depe)2]BPh4 is reported.

Communal Products of Nucleophilic Attack at Iron Carbene and Photolysis of Iron Alkyl Complexes. Crystal Structures of [formula omitted] (L = CO and PPh 3) and [formula omitted

Treatment of the carbene CpFel(CO){ CHS(CH 2 ) 2 S} with LiBEt 3 H or photolysis of the alkyl CpFe{CHS(CH 2 ) 2 S}(CO) 2 results in a mixture of CpFe{η2-CHS(CH 2 ) 2 S}(CO), containing a labile Fe—S bond, and a dimeric species, the tricyclic diferradithiacyclohexane[CpFe{CHS(CH 2 ) 2 S}(CO)] 2 .