Ligand Exchange on and Allylic C–H Activation by Iron(0) Fragments: π-Complexes, Allyliron Species, and Metallacycles

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

Arene Insertion with Pincer-Supported Molybdenum-Hydrides: Determination of Site Selectivity, Relative Rates, and Arene Complex Formation

The synthesis of phosphino(oxazoline)pyridine-supported molybdenum(0) cycloocta-1,5-diene complexes is described. Exposure of these complexes to dihydrogen in the presence of an arene resulted in insertion of the substrate into the molybdenum hydride bond and afforded the corresponding molybdenum cyclohexadienyl hydrides. For mono- and disubstituted arenes, the site selectivity for insertion of the most substituted bond increases with increasing size of the substituent from methyl to ethyl, iso-propyl, and tert-butyl. In contrast, 1,3,5-trisubstituted arenes underwent insertion with exclusive site selectivity. Relative rates of insertion were determined by competition experiments and established faster insertions for electron-rich arenes. Introduction of electron-withdrawing trifluoromethyl groups on the arene resulted in decreased relative rates of insertion and an increased rate for H2 reductive elimination, favoring formation of the corresponding molybdenum η6-arene complex. Studies on the reductive elimination of the cyclohexadienyl ligand with the hydride enabled the synthesis of an enantioenriched cyclohexa-1,3-diene. This study provides new insights into the ligand requirements for catalytic arene hydrogenation and a new strategy for selective arene reduction.

Rh(I) Complexes with Hemilabile Thioether-Functionalized NHC Ligands as Catalysts for [2 + 2 + 2] Cycloaddition of 1,5-Bisallenes and Alkynes

The [2 + 2 + 2] cycloaddition of 1,5-bisallenes and alkynes under the catalysis of Rh(I) with hemilabile thioether-functionalized N-heterocyclic carbene ligands is described. This protocol effectively provides an entry to different trans-5,6-fused bicyclic systems with two exocyclic double bonds in the cyclohexene ring. The process is totally chemoselective with the two internal double bonds of the 1,5-bisallenes being involved in the cycloaddition. The complete mechanism of this transformation as well as the preference for the trans-fusion over the cis-fusion has been rationalized by density functional theory calculations. The reaction follows a typical [2 + 2 + 2] cycloaddition mechanism. The oxidative addition takes place between the alkyne and one of the allenes and it is when the second allene is inserted into the rhodacyclopentene that the trans-fusion is generated. Remarkably, the hemilabile character of the sulfur atom in the N-heterocyclic carbene ligand modulates the electron density in key intermediates, facilitating the overall transformation.

A TMEDA-Iron Adduct Reaction Manifold in Iron-Catalyzed C(sp2 )-C(sp3 ) Cross-Coupling Reactions

Herein, we expand the current molecular-level understanding of one of the most important and effective additives in iron-catalyzed cross-coupling reactions, N,N,N',N'-tetramethylethylenediamine (TMEDA). Focusing on relevant phenyl and ethyl Grignard reagents and slow nucleophile addition protocols commonly used in effective catalytic systems, TMEDA-iron(II)-aryl intermediates are identified via in situ spectroscopy, X-ray crystallography, and detailed reaction studies to be a part of an iron(II)/(III)/(I) reaction cycle where radical recombination with FePhBr(TMEDA) (2_Ph ) results in selective product formation in high yield. These results differ from prior studies with mesityl Grignard reagent, where poor product selectivity and low catalytic performance can be attributed to homoleptic iron-ate species. Overall, this study represents a critical advance in how amine additives such as TMEDA can modulate selectivity and reactivity of organoiron species in cross-coupling.

Cyanide-alkene competition in a diiron complex and isolation of a multisite (cyano)alkylidene-alkene species

The μ-(amino)alkylidyne complex [Fe₂Cp₂(CO)₂(μ-CO){μ-CNMe(CH₂CHCH₂)}]CF₃SO₃, [1]CF₃SO₃, reacted with NBu₄CN in dichloromethane affording the μ-(cyano)(amino)alkylidene [Fe₂Cp₂(CO)₂(μ-CO){μ-C(CN)N(Me)(CH₂CHCH₂)}], 2, in 91% yield. Decarbonylation of 2 by using Me₃NO in acetone at room temperature yielded [Fe₂Cp₂(CO)(μ-CO){μ-κ³_C-C(CN)N(Me)(CH₂CHCH₂)}], 3, containing a multidentate alkylidene-alkene ligand occupying both a bridging site and a terminal site, in admixture with the μ-(amino)alkylidyne cyanide product [Fe₂Cp₂(CN)(CO)(μ-CO){μ-CN(Me)(CH₂CHCH₂)}], 4. The reaction of the μ-(amino)alkylidyne imine complex [Fe₂Cp₂(CO)(μ-CO)(NHCPh₂){μ-CN(Me)(CH₂CHCH₂)}]CF₃SO₃, [7]CF₃SO₃, with NBu₄CN gave 3 with an optimized yield of 75% via imine elimination. According to DFT calculations, 3 is less stable than its geometric isomer 4 by 13.4 kcal mol^-1 and quantitative conversion to 4 was achieved by refluxing a THF solution of 3 for 2 hours. No replacement of alkene coordination occurred upon treating 3 with CO or PPh₃. The previously unknown compounds 2, 3, 4 and [7]CF₃SO₃ were fully characterized by analytical and spectroscopic techniques and the structure of 3 was elucidated by single crystal X-ray diffraction.

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.

Nature of the Short Rh-Li Contact between Lithium and the Rhodium ω-Alkenyl Complex [Rh(CH2CMe2CH2CH═CH2)2]. Inorg Chem 2021;60:8790-8801. [PMID: 34097392 DOI: 10.1021/acs.inorgchem.1c00737] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

We describe the preparation of the cis-bis(η¹,η²-2,2-dimethylpent-4-en-1-yl)rhodate(I) anion, cis-[Rh(CH₂CMe₂CH₂CH═CH₂)₂]^-, and the interaction of this species with Li⁺ both in solution and in the solid state. For the lithium(diethyl ether) salt [Li(Et₂O)][Rh(CH₂CMe₂CH₂CH═CH₂)₂], VT-NMR and ¹H{⁷Li} NOE NMR studies in toluene-d₈ show that the Li⁺ cation is in close proximity to the d_z² orbital of rhodium. In the solid-state structure of the lithium(12-crown-4) salt [Li(12-crown-4)₂][Li{Rh(CH₂CMe₂CH₂CH═CH₂)₂}₂], one lithium atom is surrounded by two [Rh(CH₂CMe₂CH₂CH═CH₂)₂]^- anions, and in this assembly there are two unusually short Rh-Li distances of 2.48 Å. DFT calculations, natural energy decomposition, and ETS-NOCV analysis suggest that there is a weak dative interaction between the 4d_z² orbitals on the Rh centers and the 2p_z orbital of the Li⁺ cation. The charge-transfer term between Rh and Li⁺ contributes only about the 1/5 of the total interaction energy, however, and the principal driving force for the proximity of Rh and Li in compounds 1 and 2 is that Li⁺ is electrostatically attracted to negative charges on the dialkylrhodiate anions.

TMEDA in Iron-Catalyzed Hydromagnesiation: Formation of Iron(II)-Alkyl Species for Controlled Reduction to Alkene-Stabilized Iron(0)

N,N,N',N'-Tetramethylethylenediamine (TMEDA) has been one of the most prevalent and successful additives used in iron catalysis, finding application in reactions as diverse as cross-coupling, C-H activation, and borylation. However, the role that TMEDA plays in these reactions remains largely undefined. Herein, studying the iron-catalyzed hydromagnesiation of styrene derivatives using TMEDA has provided molecular-level insight into the role of TMEDA in achieving effective catalysis. The key is the initial formation of TMEDA-iron(II)-alkyl species which undergo a controlled reduction to selectively form catalytically active styrene-stabilized iron(0)-alkyl complexes. While TMEDA is not bound to the catalytically active species, these active iron(0) complexes cannot be accessed in the absence of TMEDA. This mode of action, allowing for controlled reduction and access to iron(0) species, represents a new paradigm for the role of this important reaction additive in iron catalysis.

Three-Coordinate Iron(0) Complexes with N-Heterocyclic Carbene and Vinyltrimethylsilane Ligation: Synthesis, Characterization, and Ligand Substitution Reactions

Low-coordinate iron(0) species are implicated as intermediates in a range of iron-catalyzed organic transformations. Isolable iron(0) complexes with coordination numbers of less than four, however, are rarely known. In continuing with our interests in three-coordinate iron(0) complexes with N-heterocyclic carbene (NHC) and alkene ligation, we report herein the synthesis and ligand substitution reactivity of three-coordinate iron(0) complexes featuring monodentate alkene ligands, [(NHC)Fe(η²-vtms)₂] (vtms = vinyltrimethylsilane, NHC = 1,3-bis(2',6'-diisopropylphenyl)-imidazol-2-ylidene (IPr), 1; 1,3-bis(2',6'-diisopropylphenyl)-4,5-tetramethylene-imidazol-2-ylidene (cyIPr), 2; 1,3-bis(2',6'-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene (7-IPr), 3). Complexes 1-3 were synthesized from the one-pot reactions of ferrous dihalides with the N-(2,6-diisopropylphenyl)-substituted NHC ligands, vtms, and KC₈. Reactivity study of 1 revealed its facile ligand substitution reactions with terminal aryl alkynes, ketones, isocyanides, and CO, by which iron(0) complexes [(IPr)Fe(η²-HCCAr)] (Ar = Ph, 5; p-CH₃C₆H₄, 6; 3,5-(CF₃)₂C₆H₃, 7), [(IPr)Fe(η²-OCPh₂)₂] (8), [(IPr)Fe(CNR)₄] (R = 2,6-Me₂C₆H₃, 9; Bu^t, 10), and (IPr)Fe(CO)₄ (11) were prepared in good yields. These iron(0) complexes have been characterized by ¹H NMR, solution magnetic susceptibility measurement, single-crystal X-ray diffraction study, ⁵⁷Fe Mössbauer spectroscopy, and elemental analysis. Characterization data and computational studies suggest S = 1 ground-spin states for three-coordinate iron(0) complexes 1-3 and 5-8 and S = 0 ground states for 9-11. Theoretical studies on the three-coordinate complexes 1, 6, and 8 indicated pronounced metal-to-ligand backdonation from occupied Fe 3d orbitals to the π* orbitals of the C═C, C≡C, and C═O moieties of the π ligands. In addition, 1 proved an effective precatalyst for the cyclotrimerization reaction of alkynes.

Iron-Catalyzed Reactions of 2-Pyridone Derivatives: 1,6-Addition and Formal Ring Opening/Cross Coupling

In the presence of simple iron salts, 2‐pyridone derivatives react with Grignard reagents under mild conditions to give the corresponding 1,6‐addition products; if the reaction medium is supplemented with an aprotic dipolar cosolvent after the actual addition step, the intermediates primarily formed succumb to ring opening, giving rise to non‐thermodynamic Z,E‐configured dienoic acid amide derivatives which are difficult to make otherwise. Control experiments as well as the isolation and crystallographic characterization of a (tricarbonyl)iron pyridone complex suggest that the active iron catalyst generated in situ exhibits high affinity to the polarized diene system embedded into the heterocyclic ring system of the substrates, which likely serves as the actual recognition element.

Mechanism of the Bis(imino)pyridine-Iron-Catalyzed Hydromagnesiation of Styrene Derivatives

Iron-catalyzed hydromagnesiation of styrene derivatives offers a rapid and efficient method to generate benzylic Grignard reagents, which can be applied in a range of transformations to provide products of formal hydrofunctionalization. While iron-catalyzed methodologies exist for the hydromagnesiation of terminal alkenes, internal alkynes, and styrene derivatives, the underlying mechanisms of catalysis remain largely undefined. To address this issue and determine the divergent reactivity from established cross-coupling and hydrofunctionalization reactions, a detailed study of the bis(imino)pyridine iron-catalyzed hydromagnesiation of styrene derivatives is reported. Using a combination of kinetic analysis, deuterium labeling, and reactivity studies as well as in situ ⁵⁷Fe Mössbauer spectroscopy, key mechanistic features and species were established. A formally iron(0) ate complex [ ^iPrBIPFe(Et)(CH₂═CH₂)]^- was identified as the principle resting state of the catalyst. Dissociation of ethene forms the catalytically active species which can reversibly coordinate the styrene derivative and mediate a direct and reversible β-hydride transfer, negating the necessity of a discrete iron hydride intermediate. Finally, displacement of the tridentate bis(imino)pyridine ligand over the course of the reaction results in the formation of a tris-styrene-coordinated iron(0) complex, which is also a competent catalyst for hydromagnesiation.

Enabling Two-Electron Pathways with Iron and Cobalt: From Ligand Design to Catalytic Applications

Homogeneous catalysis with Earth-abundant, first-row transition metals, including iron and cobalt, has gained considerable recent attention as a potentially cost-effective and sustainable alternative to more commonly and historically used precious metals. Because fundamental organometallic transformations, such as oxidative addition and reductive elimination, are two-electron processes and essential steps in many important catalytic cycles, controlling redox chemistry-in particular overcoming one-electron chemistry-has been as a central challenge with Earth-abundant metals. This Perspective focuses on approaches to impart sufficiently strong ligand fields to generate electron-rich metal complexes able to promote oxidative addition reactions where the redox changes are exclusively metal-based. Emphasis is placed on how ligand design and exploration of fundamental organometallic chemistry coupled with mechanistic understanding have been used to discover iron catalysts for the hydrogen isotope exchange in pharmaceuticals and cobalt catalysts for C(sp²)-H borylation reactions. A pervasive theme is that first-row metal complexes often promote unique chemistry from their precious-metal counterparts, demonstrating that these elements offer a host of new opportunities for reaction discovery and for more sustainable catalysis.

FeCl2-Mediated Rearrangement of Allylic Alcohols

A mild, one-pot procedure to produce 3-substituted allylic alcohols from α,β-unsaturated ketones is described. The addition of an organolithium nucleophile produces a tertiary allylic alcohol as an intermediate, which undergoes a 1,3-OH-migration assisted by FeCl₂. The proposed mechanism indicates that a syn-facial migration occurs for the major product. Yields as high as 98% for the one-pot reaction are reported.

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.

One-Pot Sequential Kinetic Profiling of a Highly Reactive Manganese Catalyst for Ketone Hydroboration: Leveraging σ-Bond Metathesis via Alkoxide Exchange Steps

A comprehensive experimental and computational mechanistic study of the highly enantioselective hydroboration of ketones catalyzed by a manganese(II) alkyl boxmi pincer complex is reported. The catalyst operates at low catalyst loadings (down to 0.01 mol %) under very mild conditions (typically -40 °C) and facilitates the reduction of both aryl alkyl and dialkyl ketones with excellent selectivity (up to >95%ee). Catalyst activation pathways were investigated, demonstrating that a manganese(II) hydride and a manganese(II) alkoxide species are part of the catalytic cycle and can be generated via σ-bond metathesis of the alkyl precursor with the borane or by alcoholysis. Extensive kinetic experiments based on a "one-pot sequential kinetic profiling" approach under various conditions in combination with kinetic simulations reveal that two catalytic cycles are effective with this earth-abundant base metal catalyst: (i) a minor MnH/borane-mediated insertion cycle, in which the subsequent, product-releasing metathesis step is rate determining ( k _m = 0.076 s^-1), giving a background reaction, which is zeroth order in substrate concentrations, and (ii) a major MnOR/borane-based alkoxide exchange process, leveraging the high-barrier metathesis via the affiliation to an insertion step. The latter features non-integer reaction orders in both reagents due to a combination of an adduct formation step ( k _a = 2.12 M^-1 s^-1, k _-a = 0.49 s^-1) and a substrate insertion step of comparable rates ( k _ai = 3.74 M^-1 s^-1). The kinetic findings are underpinned by high-level density functional theory calculations of the mechanism, control experiments, and kinetic isotope effect/Hammett/Eyring analysis in different concentration regimes. The study highlights the role of a rigorous mechanistic understanding of homogeneous catalytic processes in 3d metals for rational catalyst discovery and optimization.