Isolation and characterization of a tetramethyliron(III) ferrate: an intermediate in the reduction pathway of ferric salts with MeMgBr

One Ligand to Bind them All: S~C~S2- Carbon- and Sulfur-Based Gem-Dianion as Structuring Ligand for Iron Polymetallic Assemblies

Numerous synthetic models of the FeMo-co cluster of nitrogenases have been proposed to find the simplest structure with relevant reactivity. Indeed, such structures are able to perform multi-electrons reduction processes, such as the conversion of N2 to ammonia, and of CO2 into methane and alkenes. The most challenging parameter to imitate is indeed the central carbide ligand, which is believed to maintain the integrity of iron sulfide assembly during the course of catalytic cycles. The study proposes the use of bis(diphenylthiophosphinoyl)methanediide (SCS)2- as an ideal platform for the synthesis of bi- and tetra-metallic iron complexes, in which the iron-carbon interaction is maintained upon structural diversification and redox state changes.

The molecular-level effect of alkoxide additives in iron-catalyzed Kumada cross-coupling with simple ferric salts

The molecular-level role of alkoxide salts, used as alternative additive to N-methylpyrrolidone in iron-catalyzed alkyl-alkenyl/aryl cross-coupling reactions, is investigated. Detailed spectroscopic studies reveal that alkoxides promote the formation of homoleptic organoferrates such as [FeMe3]-, providing an alternative to toxic NMP to access these reactive intermediates.

Microscopic Reactivity of Phenylferrate Ions toward Organyl Halides

Despite its practical importance, organoiron chemistry remains poorly understood due to its mechanistic complexity. Here, we focus on the oxidative addition of organyl halides to phenylferrate anions in the gas phase. By mass-selecting individual phenylferrate anions, we can determine the effect of the oxidation state, the ligation, and the nuclearity of the iron complex on its reactions with a series of organyl halides RX. We find that Ph₂ Fe(I)^- and other low-valent ferrates are more reactive than Ph₃ Fe(II)^- ; Ph₄ Fe(III)^- is inert. The coordination of a PPh₃ ligand or the presence of a second iron center lower the reactivity. Besides direct cross-coupling reactions resulting in the formation of RPh, we also observe the abstraction of halogen atoms. This reaction channel shows the readiness of organoiron species to undergo radical-type processes. Complementary DFT calculations afford further insight and rationalize the high reactivity of the Ph₂ Fe(I)^- complex by the exothermicity of the oxidative addition and the low barriers associated with this reaction step. At the same time, they point to the importance of changes of the spin state in the reactions of Ph₃ Fe(II)^- .

NHC Effects on Reduction Dynamics in Iron-Catalyzed Organic Transformations*

The high abundance, low toxicity and rich redox chemistry of iron has resulted in a surge of iron-catalyzed organic transformations over the last two decades. Within this area, N-heterocyclic carbene (NHC) ligands have been widely utilized to achieve high yields across reactions including cross-coupling and C-H alkylation, amongst others. Central to the development of iron-NHC catalytic methods is the understanding of iron speciation and the propensity of these species to undergo reduction events, as low-valent iron species can be advantageous or undesirable from one system to the next. This study highlights the importance of the identity of the NHC on iron speciation upon reaction with EtMgBr, where reactions with SIMes and IMes NHCs were shown to undergo β-hydride elimination more readily than those with SIPr and IPr NHCs. This insight is vital to developing new iron-NHC catalyzed transformations as understanding how to control this reduction by simply changing the NHC is central to improving the reactivity in iron-NHC catalysis.

Alkali Metal Adducts of an Iron(0) Complex and Their Synergistic FLP-Type Activation of Aliphatic C-X Bonds

We report the formation and full characterization of weak adducts between Li⁺ and Na⁺ cations and a neutral iron(0) complex, [Fe(CO)₃(PMe₃)₂] (1), supported by weakly coordinating [BAr^F₂₀] anions, [1·M][BAr^F₂₀] (M = Li, Na). The adducts are found to synergistically activate aliphatic C-X bonds (X = F, Cl, Br, I, OMs, OTf), leading to the formation of iron(II) organyl compounds of the type [FeR(CO)₃(PMe₃)₂][BAr^F₂₀], of which several were isolated and fully characterized. Stoichiometric reactions with the resulting iron(II) organyl compounds show that this system can be utilized for homocoupling and cross-coupling reactions and the formation of new C-E bonds (E = C, H, O, N, S). Further, we utilize [1·M][BAr^F₂₀] as a catalyst in a simple hydrodehalogenation reaction under mild conditions to showcase its potential use in catalytic reactions. Finally, the mechanism of activation is probed using DFT and kinetic experiments that reveal that the alkali metal and iron(0) center cooperate to cleave C-X via a mechanism closely related to intramolecular FLP activation.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Catalytic hydrogen atom transfer to alkenes: a roadmap for metal hydrides and radicals

Hydrogen atom transfer from metal hydrides to alkenes appears to underlie widely used catalytic methods – the mechanistic implications are fascinating.

Hydrogen atom transfer from a metal hydride (MHAT) has emerged as a powerful, if puzzling, technique in chemical synthesis. In catalytic MHAT reactions, earth-abundant metal complexes generate stabilized and unstabilized carbon-centered radicals from alkenes of various substitution patterns with robust chemoselectivity. This perspective combines organic and inorganic perspectives to outline challenges and opportunities, and to propose working models to assist further developments. We attempt to demystify the putative intermediates, the basic elementary steps, and the energetic implications, especially for cage pair formation, collapse and separation. Distinctions between catalysts with strong-field (SF) and weak-field (WF) ligand environments may explain some differences in reactivity and selectivity, and provide an organizing principle for kinetics that transcends the typical thermodynamic analysis. This blueprint should aid practitioners who hope to enter and expand this exciting area of chemistry.

Intra- and intermolecular Fe-catalyzed dicarbofunctionalization of vinyl cyclopropanes

Design and implementation of the first (asymmetric) Fe-catalyzed intra- and intermolecular difunctionalization of vinyl cyclopropanes (VCPs) with alkyl halides and aryl Grignard reagents has been realized via a mechanistically driven approach. Mechanistic studies support the diffusion of alkyl radical intermediates out of the solvent cage to participate in an intra- or intermolecular radical cascade with a range of VCPs followed by re-entering the Fe radical cross-coupling cycle to undergo (stereo)selective C(sp2)-C(sp3) bond formation. This work provides a proof-of-concept of the use of vinyl cyclopropanes as synthetically useful 1,5-synthons in Fe-catalyzed conjunctive cross-couplings with alkyl halides and aryl/vinyl Grignard reagents. Overall, we provide new design principles for Fe-mediated radical processes and underscore the potential of using combined computations and experiments to accelerate the development of challenging transformations.

An ab initio multireference study of reductive eliminations from organoferrates(iii) in the gas-phase: it is all about the spin state

The reductive elimination reaction from organoferrates(iii) of the composition [FeR₃R′]⁻ is studied by state-of-the-art multireference electronic structure calculations.

Anticancer Molecule Discovery via C2-Substituent Promoted Oxidative Coupling of Indole and Enolate

C2, C3-disubstituted indole is one of the most frequently encountered motifs in bioactive alkaloids and medicinal chemistry. Thus, developing novel, concise, and efficient access to it is highly desired in drug discovery. Herein, we present such an approach to this scaffold by direct oxidative coupling of C2-substituted indoles and enolates. Compared with indole bearing no C2-substituent, higher yields (up to 96%) were obtained for C2-substituted indoles in most cases. Mechanistic studies showed the reaction went through a Fe-chelated radical-anion oxidative coupling procedure promoted by C2-substituent on indole by two means: (1) stabilizing C2-radical intermediate during the reaction; (2) reducing indole homocoupling. This approach serves as a synthetic useful tool to quickly build up bioactive small molecule library of C2, C3-disubstituted indoles, and several products showed promising anticancer activities. Besides, indomethacin and its analogs were conveniently prepared in three-step sequence efficiently, indicating the potential application of our approach in medicinal chemistry.

The Effect of β-Hydrogen Atoms on Iron Speciation in Cross-Couplings with Simple Iron Salts and Alkyl Grignard Reagents

The effects of β-hydrogen-containing alkyl Grignard reagents in simple ferric salt cross-couplings have been elucidated. The reaction of FeCl3 with EtMgBr in THF leads to the formation of the cluster species [Fe8 Et12 ]2- , a rare example of a structurally characterized metal complex with bridging ethyl ligands. Analogous reactions in the presence of NMP, a key additive for effective cross-coupling with simple ferric salts and β-hydrogen-containing alkyl nucleophiles, result in the formation of [FeEt3 ]- . Reactivity studies demonstrate the effectiveness of [FeEt3 ]- in rapidly and selectively forming the cross-coupled product upon reaction with electrophiles. The identification of iron-ate species with EtMgBr analogous to those previously observed with MeMgBr is a critical insight, indicating that analogous iron species can be operative in catalysis for these two classes of alkyl nucleophiles.

Development and Evolution of Mechanistic Understanding in Iron-Catalyzed Cross-Coupling

Since the pioneering work of Kochi in the 1970s, iron has attracted great interest for cross-coupling catalysis due to its low cost and toxicity as well as its potential for novel reactivity compared to analogous reactions with precious metals like palladium. Today there are numerous iron-based cross-coupling methodologies available, including challenging alkyl-alkyl and enantioselective methods. Furthermore, cross-couplings with simple ferric salts and additives like NMP and TMEDA ( N-methylpyrrolidone and tetramethylethylenediamine) continue to attract interest in pharmaceutical applications. Despite the tremendous advances in iron cross-coupling methodologies, in situ formed and reactive iron species and the underlying mechanisms of catalysis remain poorly understood in many cases, inhibiting mechanism-driven methodology development in this field. This lack of mechanism-driven development has been due, in part, to the challenges of applying traditional characterization methods such as nuclear magnetic resonance (NMR) spectroscopy to iron chemistry due to the multitude of paramagnetic species that can form in situ. The application of a broad array of inorganic spectroscopic methods (e.g., electron paramagnetic resonance, ⁵⁷Fe Mössbauer, and magnetic circular dichroism) removes this barrier and has revolutionized our ability to evaluate iron speciation. In conjunction with inorganic syntheses of unstable organoiron intermediates and combined inorganic spectroscopy/gas chromatography studies to evaluate in situ iron reactivity, this approach has dramatically evolved our understanding of in situ iron speciation, reactivity, and mechanisms in iron-catalyzed cross-coupling over the past 5 years. This Account focuses on the key advances made in obtaining mechanistic insight in iron-catalyzed carbon-carbon cross-couplings using simple ferric salts, iron-bisphosphines, and iron- N-heterocyclic carbenes (NHCs). Our studies of ferric salt catalysis have resulted in the isolation of an unprecedented iron-methyl cluster, allowing us to identify a novel reaction pathway and solve a decades-old mystery in iron chemistry. NMP has also been identified as a key to accessing more stable intermediates in reactions containing nucleophiles with and without β-hydrogens. In iron-bisphosphine chemistry, we have identified several series of transmetalated iron(II)-bisphosphine complexes containing mesityl, phenyl, and alkynyl nucleophile-derived ligands, where mesityl systems were found to be unreliable analogues to phenyls. Finally, in iron-NHC cross-coupling, unique chelation effects were observed in cases where nucleophile-derived ligands contained coordinating functional groups. As with the bisphosphine case, high-spin iron(II) complexes were shown to be reactive and selective in cross-coupling. Overall, these studies have demonstrated key aspects of iron cross-coupling and the utility of detailed speciation and mechanistic studies for the rational improvement and development of iron cross-coupling methods.

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.

Mössbauer and mass spectrometry support for iron(ii) catalysts in enantioselective C–H activation

A combination of electrospray-ionization mass spectrometry and Mössbauer spectroscopy was used to investigate the species generated in situ in highly enantioselective Fe/NHC-catalyzed C–H alkylations.

C-Halide bond cleavage by a two-coordinate iron(i) complex

The two-coordinate iron(i) complex [Fe^I(N(SiMe₃)₂)₂]⁻ is highly efficient in the cleavage of C-halide bonds.

Iron- or Palladium-Catalyzed Reaction Cascades Merging Cycloisomerization and Cross-Coupling Chemistry

A conceptually novel reaction cascade is presented, which allows readily available enynes to be converted into functionalized 1,3-dienes comprising a stereodefined tetrasubstituted alkene unit; such compounds are difficult to make by conventional means. The overall transformation is thought to commence with formation of a metallacyclic intermediate that evolves via cleavage of an unstrained C-X bond in its backbone. This non-canonical cycloisomerization process is followed by a cross-coupling step, such that reductive C-C bond formation regenerates the necessary low-valent metal fragment and hence closes an intricate catalytic cycle. The cascade entails the formation of two new C-C bonds at the expense of the constitutional C-X entity of the substrate: importantly, the extruded group X must not be a heteroelement (X=O, NR), since activated β-C-C bonds can also be broken. This concern was reduced to practice in two largely complementary formats: one procedure relies on the use of alkyl-Grignard reagents in combination with catalytic amounts of Fe(acac)_3, whereas the second method amalgamates cycloisomerization with Suzuki coupling by recourse to arylboronic acids and phosphine-ligated palladium catalysts.

Electronic and Steric Effects on the Reductive Elimination of Anionic Arylferrate(III) Complexes

Arylferrate(III) complexes Ph₃ FeR^- (R=para- and ortho-substituted aryl) are proposed as model systems for the in-depth investigation of reductive eliminations from organoiron(III) species. Electrospray ionization transfers the arylferrate complexes prepared in situ from solution into the gas phase, where mass selection ensures a well-defined population of reactant ions. Upon gas-phase fragmentation, the arylferrate complexes undergo reductive elimination of the cross-coupling product PhR as well as the homo-coupling product Ph₂ . The measured branching ratios between the two competing reaction channels are used to construct a Hammett plot, which shows that electron-donating aryl groups R favor the formation of the cross-coupling product. In this way, the complexes avoid the build-up of too much electron density at the iron center during the reductive elimination. ortho Substitution in R increases the fraction of the homo-coupling product, presumably by hindering the approach between the two aryl groups participating in the reductive elimination. The obtained mechanistic insight substantially advances our understanding of one of the central elementary steps of transition-metal-catalyzed cross-coupling reactions.

Intermediates and Mechanism in Iron-Catalyzed Cross-Coupling

Iron-catalyzed cross-coupling reactions have attracted significant research interest, as they offer numerous favorable features compared with cross-coupling reactions with precious metal catalysis. While this research has contributed to an empirical understanding of iron-catalyzed cross-coupling, the underlying fundamental mechanisms of reaction and structures of catalytically active species have remained poorly defined. The lack of such detail can be attributed to the difficulties associated with studying such iron-catalyzed reactions, where unstable paramagnetic intermediates abound. Recently, the combined application of physical-inorganic spectroscopic methods, concomitant organic product analysis, and air- and temperature-sensitive inorganic synthesis has yielded the most detailed insight currently available on reactivity and mechanism in iron-catalyzed cross-coupling. This Perspective highlights this approach and the limitations of the contributing techniques as well as some of the key features of the catalytic reactions studied and lessons learned.

Oxidation States, Stability, and Reactivity of Organoferrate Complexes

We have applied a combination of electrospray-ionization mass spectrometry, electrical conductivity measurements, and Mössbauer spectroscopy to identify and characterize the organoferrate species R _nFe _m^- formed upon the transmetalation of iron precursors (Fe(acac)₃, FeCl₃, FeCl₂, Fe(OAc)₂) with Grignard reagents RMgX (R = Me, Et, Bu, Hex, Oct, Dec, Me₃SiCH₂, Bn, Ph, Mes, 3,5-(CF₃)₂-C₆H₃; X = Cl, Br) in tetrahydrofuran. The observed organoferrates show a large variety in their aggregation (1 ≤ m ≤ 8) and oxidation states (I to IV), which are chiefly determined by the nature of their organyl groups R. In numerous cases, the addition of a bidentate amine or phosphine changes the distributions of organoferrates and affects their stability. Besides undergoing efficient intermolecular exchange processes, several of the probed organoferrates react with organyl (pseudo)halides R'X (R' = Et, ⁱPr, Bu, Ph, p-Tol; X = Cl, Br, I, OTf) to afford heteroleptic complexes of the type R₃FeR'^-. Gas-phase fragmentation of most of these complexes results in reductive eliminations of the coupling products RR' (or, alternatively, of R₂). This finding indicates that iron-catalyzed cross-coupling reactions may proceed via such heteroleptic organoferrates R₃FeR'^- as intermediates. Gas-phase fragmentation of other organoferrate complexes leads to β-hydrogen eliminations, the loss of arenes, and the expulsion of organyl radicals. The operation of both one- and two-electron processes is consistent with previous observations and contributes to the formidable complexity of organoiron chemistry.

The N-Methylpyrrolidone (NMP) Effect in Iron-Catalyzed Cross-Coupling with Simple Ferric Salts and MeMgBr

The use of N-methylpyrrolidone (NMP) as a co-solvent in ferric salt catalyzed cross-coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross-coupled product in reactions utilizing alkyl Grignard reagents. Despite the critical importance of NMP, the molecular level effect of NMP on in situ formed and reactive iron species that enables effective catalysis remains undefined. Herein, we report the isolation and characterization of a novel trimethyliron(II) ferrate species, [Mg(NMP)₆ ][FeMe₃ ]₂ (1), which forms as the major iron species in situ in reactions of Fe(acac)₃ and MeMgBr under catalytically relevant conditions where NMP is employed as a co-solvent. Importantly, combined GC analysis and ⁵⁷ Fe Mössbauer spectroscopic studies identified 1 as a highly reactive iron species for the selective formation generating cross-coupled product. These studies demonstrate that NMP does not directly interact with iron as a ligand in catalysis but, alternatively, interacts with the magnesium cations to preferentially stabilize the formation of 1 over [Fe₈ Me₁₂ ]^- cluster generation, which occurs in the absence of NMP.

Elucidating the structure of a high-spin σ-phenyliron(iii) species in a live FeCl3-PhZnCl reaction system

Environmentally benign iron catalysts promote a wide variety of chemical transformations; however, insight into the mechanism and active intermediates is far from satisfactory, and the main difficulties lie in directly "seeing" the active species under "live" catalytic conditions. Herein, an unknown sextet Ph(THF)FeCl₂ species was well-characterized in a live FeCl₃-PhZnCl reaction system for the first time by Raman, in situ IR, electron paramagnetic resonance (EPR), X-ray absorption spectroscopic (XAS) and density functional theory (DFT) calculations. This work provides insight into the structure and reactivity of catalytically relevant σ-aryliron(iii) species, and shall provide useful guidelines for understanding iron chemistry.

Rare Examples of Fe(IV) Alkyl-Imide Migratory Insertions: Impact of Fe-C Covalency in (Me2IPr)Fe(═NAd)R2 (R = neoPe, 1-nor)

The iron(IV) imide complexes, (Me₂IPr)-R₂Fe=NAd (R = ^neoPe (3a), 1-nor (3b)) undergo migratory insertion to iron(II) amides (Me₂IPr)RFe{NR(Ad)} (R = ^neoPe (4a), 1-nor (4b)) without evidence of imidyl or free nitrene character. By increasing the field strength about iron, odd-electron reactivity was circumvented via increased covalency.

A Physical-Inorganic Approach for the Elucidation of Active Iron Species and Mechanism in Iron-Catalyzed Cross-Coupling

Detailed studies of iron speciation and mechanism in iron-catalyzed cross-coupling reactions are critical for providing the necessary fundamental insight to drive new reaction development. However, such insight is challenging to obtain due to the prevalence of mixtures of unstable, paramagnetic organoiron species that can form in this chemistry. A physical-inorganic research approach combining freeze-trapped inorganic spectroscopic studies, organometallic synthesis and GC/kinetic studies provides a powerful method for studying such systems. Mössbauer, EPR and MCD spectroscopy enable the direct investigation of in situ formed iron species and, combined with GC analysis, the direct correlation of reactions of specific iron species to the generation of organic products. This review focuses on a description of the key methods involved in this physical-inorganic approach, as well as examples of its application to investigations of iron-SciOPP catalyzed cross-coupling catalysis.

Intermediates and Reactivity in Iron-Catalyzed Cross-Couplings of Alkynyl Grignards with Alkyl Halides

Iron-catalyzed cross-coupling reactions using alkynyl nucleophiles represent an attractive approach for the incorporation of alkynyl moieties into organic molecules. In the present study, a multitechnique approach combining inorganic spectroscopic methods, inorganic synthesis, and reaction studies is applied to iron-SciOPP catalyzed alkynyl-alkyl cross-couplings, providing the first detailed insight into the effects of variation from sp2- to sp-hybridized nucleophiles on iron speciation and reactivity. Reaction studies demonstrate that reaction of FeBr2(SciOPP) with 1 equiv (triisopropylsilyl)ethynylmagnesium bromide (TIPS-CC-MgBr) leads to a distribution of mono-, bis-, and tris-alkynylated iron(II)-SciOPP species due to rapid alkynyl ligand redistribution. While solvents such as THF promote these complex redistribution pathways, nonpolar solvents such as toluene enable increased stabilization of these iron species and further enabled assessment of their reactivity with electrophile. While the tris-alkynylated iron(II)-SciOPP species was found to be unreactive with the cycloheptyl bromide electrophile over the average turnover time of catalysis, the in situ formed neutral mono- and bis-alkynylated iron(II)-SciOPP complexes are consumed upon reaction with the electrophile with concomitant generation of cross-coupled product at catalytically relevant rates, indicating the ability of one or both of these species to react selectively with the electrophile. The nature of the reaction solvent and Grignard reagent addition rate were found to have broader implications in overall reaction selectivity, reaction rate, and accessibility of off-cycle iron(I)-SciOPP species. Additionally, the effects of steric substitution of the alkynyl Grignard reagent on catalytic performance were investigated. Fundamental insight into iron speciation and reactivity with alkynyl nucleophiles reported herein provides an essential foundation for the continued development of this important class of reactions.

Synthetic, structural and magnetic implications of introducing 2,2′-dipyridylamide to sodium-ferrate complexes

Establishing transamination as an efficient method to access novel heterobimetallic complexes, this study provides magnetic and structural insights of 2,2-dipyridylamide based sodium ferrate complexes.

Iron Catalysis in Organic Synthesis: A Critical Assessment of What It Takes To Make This Base Metal a Multitasking Champion

The current status of homogeneous iron catalysis in organic chemistry is contemplated, as are the reasons why this particular research area only recently starts challenging the enduring dominance of the late and mostly noble metals over the field. Centered in the middle of the d-block and able to support formal oxidation states ranging from -II to +VI, iron catalysts hold the promise of being able to encompass organic synthesis at large. They are expected to serve reductive as well as oxidative regimes, can emulate "noble tasks", but are also able to adopt "early" transition metal character. Since a comprehensive coverage of this multidimensional agenda is beyond the scope of an Outlook anyway, emphasis is laid in this article on the analysis of the factors that perhaps allow one to control the multifarious chemical nature of this earth-abundant metal. The challenges are significant, not least at the analytical frontier; their mastery mandates a mindset that differs from the routines that most organic chemists interested in (noble metal) catalysis tend to cultivate. This aspect notwithstanding, it is safe to predict that homogeneous iron catalysis bears the chance to enable a responsible paradigm for chemical synthesis and a sustained catalyst economy, while potentially providing substantial economic advantages. This promise will spur the systematic and in-depth investigations that it takes to upgrade this research area to strategy-level status in organic chemistry and beyond.

Isolation, Characterization, and Reactivity of Fe8Me12(-): Kochi's S = 1/2 Species in Iron-Catalyzed Cross-Couplings with MeMgBr and Ferric Salts

Iron-catalyzed cross-couplings with simple ferric salts have been known since the 1970s, pioneered by Kochi for cross-coupling using alkylmagnesium nucleophiles including MeMgBr. While Kochi observed the formation of a S = 1/2 iron species in reactions of simple ferric salts with MeMgBr proposed to be an iron(I) species, the identity of this species has remained undefined for nearly 40 years. Herein, we report the isolation and characterization of [MgCl(THF)5][Fe8Me12], which combined with EPR and MCD studies is shown to be consistent with Kochi's S = 1/2 species. Reaction studies with β-bromostyrene demonstrate that this species alone displays minimal reactivity but, when combined with additional MeMgBr, leads to rapid and selective formation of cross-coupled product.

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

Recent advances in iron-catalysed cross coupling reactions and their mechanistic underpinning

Advances in iron-catalysed cross coupling from 2010–2015 are critically reviewed.

How low does iron go? Chasing the active species in fe-catalyzed cross-coupling reactions

The catalytic cross-coupling reactions of organic halides or related substrates with organometallic nucleophiles form the cornerstone of many carbon-carbon bond-forming processes. While palladium-based catalysts typically mediate such reactions, there are increasing concerns about the long-term sustainability of palladium in synthesis. This is due to the high cost of palladium, coupled with its low natural abundance, environmentally deleterious extraction (∼6 g of metal are produced per ton of ore), toxicity, and competition for its use from the automotive and consumer electronics sectors. Therefore, there is a growing interest in replacing palladium-based catalysts with those incorporating more earth-abundant elements. With its low cost, high natural abundance, and low toxicity, iron makes a particularly appealing alternative, and accordingly, the development of iron-catalyzed cross-coupling is undergoing explosive growth. However, our understanding of the mechanisms that underpin the iron-based catalytic cycles is still very much in its infancy. Mechanistic insight into catalytic reactions is not only academically important but also allows us to maximize the efficiency of processes or even to develop entirely new transformations. Key to the development of robust mechanistic models for cross-coupling is knowing the lowest oxidation state in the cycle. Once this is established, we can explore subsequent redox processes and build the catalytic manifold. Until we know with confidence what the lowest oxidation state is, any cycles proposed are largely just guesswork. To date, Fe(-II), Fe(-I), Fe(0), Fe(I), and Fe(II) have been proposed as contenders for the lowest-oxidation-state species in the cycle in iron-catalyzed cross-coupling; the aim of this Account is to pull together the various pieces of evidence in support, or otherwise, of each of these suggestions in turn. There currently exists no direct evidence that oxidation states below Fe(0) are active in the catalytic cycle. Meanwhile, the reactivity required of the lowest-oxidation-state species has been observed with model compounds in higher oxidation states, implying that there is no need to invoke such low oxidation states. While subzero-valent complexes do indeed act as effective precatalysts, it is important to recognize that this tells us that they are efficiently converted to an active catalyst but says nothing about the oxidation states of the species in the catalytic cycle. Zero-valent heterogeneous iron nanoparticles can be formed under typical catalytic conditions, but there is no evidence to suggest that homogeneous Fe(0) complexes can be produced under comparable conditions. It seems likely that the zero-valent nanoparticles act as a reservoir for soluble higher-oxidation-state species. Fe(II) complexes can certainly be formed under catalytically relevant conditions, and when bulky nucleophilic coupling partners are exploited, potential intermediates can be isolated. However, the bulky reagents act as poor proxies for most nucleophiles used in cross-coupling, as they give Fe(II) organometallic intermediates that are kinetically stabilized with respect to reductive elimination. When more realistic substrates are exploited, reduction or disproportionation to Fe(I) is widely observed, and while it still has not been conclusively proved, this oxidation state currently represents a likely candidate for the lowest one active in many iron-catalyzed cross-coupling processes.