Catalytic Properties of Low Oxidation State Iron Complexes in Cross-Coupling Reactions: Anthracene Iron(−I) Complexes as Competent Catalysts

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.

Low-oxidation state cobalt-magnesium complexes: ion-pairing and reactivity

Magnesium cobaltates (Arnacnac)MgCo(COD)2 (1-3) were synthesised by reacting (Arnacnac)MgI(OEt2) with K[Co(η4-COD)2] (COD = 1,5-cyclooctadiene) [Arnacnac = CH(ArNCMe)2; Ar = 2,4,6-Me3-C6H2 (Mes), 2,6-Et2-C6H3 (Dep), 2,6-iPr2-C6H3Mes (Dipp)]. Compounds 1-3 form contact ion-pairs in toluene, while solvent separated ion-pairs are formed in THF. The effect of ion-pairing on the reactivity is illustrated by reaction of 2 with tert-butylphosphaalkyne, which affords distinct 1,3-diphosphacyclobutadiene complexes. The heteroleptic sandwich complex [(Depnacnac)MgCo(P2C2tBu2)]2 (4) is selectively formed in toluene, while the homoleptic bis(1,3-diphosphacyclobutadiene) complex [(Depnacnac)Mg(THF)3][Co(P2C2tBu2)2] (5) is obtained in THF. Complex 4 is a precursor to further unusual phosphaorganometallic compounds. Substitution of the labile COD ligand in 4 by white phosphorus (P4) enabled the synthesis of the phosphorus-rich sandwich compound [(Depnacnac)MgCoP4(P2C2tBu2)]2 (6). The heterobimetallic complex (Cp*NiP2C2tBu2)Co(COD) (7) was isolated after treatment of 4 with Cp*Ni(acac) (Cp* = C5Me5, acac = acetylacetonate).

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.

Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations

The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr₂ with LiEt₃ BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol % cat., 2-10 bar H₂ , 20-80 °C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism.

Synthesis, electronic structure and redox properties of the diruthenium sandwich complexes [Cp*Ru(μ-C10H8)RuCp*]x (x = 0, 1+; Cp* = C5Me5; C10H8 = naphthalene)

Strong electronic coupling was observed between the metal centers in a naphthalene-bridged diruthenium complex.

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

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

Iron-Catalyzed Cross-Coupling of Alkenyl Acetates

Stable C-O linkages are generally unreactive in cross-coupling reactions which mostly employ more electrophilic halides or activated esters (triflates, tosylates). Acetates are cheap and easily accessible electrophiles but have not been used in cross-couplings because the strong C-O bond and high propensity to engage in unwanted acetylation and deprotonation. Reported herein is a selective iron-catalyzed cross-coupling of diverse alkenyl acetates, and it operates under mild reaction conditions (0 °C, 2 h) with a ligand-free catalyst (1-2 mol%).

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.

A tetradentate metalloligand: synthesis and coordination behaviour of a 2-pyridyl-substituted cyclobutadiene iron complex

A novel cyclobutadiene iron complex with four 2-pyridyl-substitutents acts as a bis(bidentate) chelate ligand toward Zn²⁺ cations.

Unraveling the electronic structures of low-valent naphthalene and anthracene iron complexes: X-ray, spectroscopic, and density functional theory studies

Naphthalene and anthracene transition metalates are potent reagents, but their electronic structures have remained poorly explored. A study of four Cp*-substituted iron complexes (Cp* = pentamethylcyclopentadienyl) now gives rare insight into the bonding features of such species. The highly oxygen- and water-sensitive compounds [K(18-crown-6){Cp*Fe(η(4)-C(10)H(8))}] (K1), [K(18-crown-6){Cp*Fe(η(4)-C(14)H(10))}] (K2), [Cp*Fe(η(4)-C(10)H(8))] (1), and [Cp*Fe(η(4)-C(14)H(10))] (2) were synthesized and characterized by NMR, UV-vis, and (57)Fe Mössbauer spectroscopy. The paramagnetic complexes 1 and 2 were additionally characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. The molecular structures of complexes K1, K2, and 2 were determined by single-crystal X-ray crystallography. Cyclic voltammetry of 1 and 2 and spectroelectrochemical experiments revealed the redox properties of these complexes, which are reversibly reduced to the monoanions [Cp*Fe(η(4)-C(10)H(8))](-) (1(-)) and [Cp*Fe(η(4)-C(14)H(10))](-) (2(-)) and reversibly oxidized to the cations [Cp*Fe(η(6)-C(10)H(8))](+) (1(+)) and [Cp*Fe(η(6)-C(14)H(10))](+) (2(+)). Reduced orbital charges and spin densities of the naphthalene complexes 1(-/0/+) and the anthracene derivatives 2(-/0/+) were obtained by density functional theory (DFT) methods. Analysis of these data suggests that the electronic structures of the anions 1(-) and 2(-) are best represented by low-spin Fe(II) ions coordinated by anionic Cp* and dianionic naphthalene and anthracene ligands. The electronic structures of the neutral complexes 1 and 2 may be described by a superposition of two resonance configurations which, on the one hand, involve a low-spin Fe(I) ion coordinated by the neutral naphthalene or anthracene ligand L, and, on the other hand, a low-spin Fe(II) ion coordinated to a ligand radical L(•-). Our study thus reveals the redox noninnocent character of the naphthalene and anthracene ligands, which effectively stabilize the iron atoms in a low formal, but significantly higher spectroscopic oxidation state.