Iron Pincer Complexes Research Articles

Mechanistic Studies of Alkyne Hydroboration by a Well-Defined Iron Pincer Complex: Direct Comparison of Metal-Hydride and Metal-Boryl Reactivity.

Iron-hydride and iron-boryl complexes supported by a pyrrole-based pincer ligand, tBuPNP (PNP = anion of 2,5-bis(di-tert-butylphosphinomethyl)pyrrole), were employed for a detailed mechanistic study on the hydroboration of internal alkynes. Several novel complexes were isolated and fully characterized, including iron-vinyl and iron-boryl species, which represent likely intermediates in the catalytic hydroboration pathway. In addition, the products of alkyne insertion into the Fe-B bond have been isolated and structurally characterized. Mechanistic studies of the hydroboration reaction favor a pathway involving an active iron-hydride species, [FeH(tBuPNP)], which readily inserts alkyne and undergoes subsequent reaction with hydroborane to generate product. The iron-boryl species, [Fe(BR2)(tBuPNP)] (R2 = pin or cat), was found to be chemically competent, although its use in catalysis entailed an induction period whereby the iron-hydride species was generated. Stoichiometric reactions and kinetic experiments were performed to paint a fuller picture of the mechanism of alkyne hydroboration, including pathways for catalyst deactivation and the influence of substrate bulk on catalytic efficacy.

Applications of iron pincer complexes in hydrosilylation reactions.

Due to its abundance, low cost and low toxicity, the first-row transition metal, iron is widely preferred as a catalyst in organic synthesis. The only drawback of lower selectivity due to high reactivity and low stability of the metal centre is tuned by using pincer ligands of different types. The different iron pincer complexes thus prepared are extensively used in catalyzing different types of organic reactions with great selectivity and functional group tolerance under moderate reaction conditions. In this review, we focus on the applications of iron pincer complexes in hydrosilylation reactions, especially the hydrosilylation of carbonyl derivatives and alkene/alkynes.

Oxidative Addition of Aryl and Alkyl Halides to a Reduced Iron Pincer Complex.

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-Me2MesCNC)Fe(N2)2 (3,5-Me2MesCNC = 2,6-(2,4,6-Me-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe(II) products (3,5-Me2MesCNC)Fe(Ph)(N2)(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl. A linear free energy of relative reaction rates of electronically differentiated aryl bromides (ρ = 1.5) was consistent with a concerted-type pathway. The oxidative addition of alkyl halides such as methyl-, isobutyl-, or neopentyl halides was also rapid at room temperature, but substrates with more accessible β-hydrogen positions (e.g., 1-bromobutane) underwent subsequent β-hydride elimination. Cyclization of an alkyl halide containing a radical clock and epimerization of neohexyl iodide-d2 upon oxidative addition to (3,5-Me2MesCNC)Fe(N2)2 are consistent with radical intermediates during C(sp3)-X bond cleavage. Importantly, while C(sp2)-X and C(sp3)-X oxidative addition produces net two-electron chemistry, the preferred pathway for obtaining the products is concerted and stepwise, respectively.

Chemically robust and readily available quinoline-based PNN iron complexes: application in C-H borylation of arenes.

Iron catalysts have been used for over a century to produce ammonia industrially. However, the use of iron catalysts generally remained quite limited until relatively recently, when the abundance and low toxicity of iron spurred the development of a variety of iron catalysts. Despite the fact that iron catalysts are being developed as alternatives to precious metal catalysts, their reactivities and stabilities are quite different because of their unique electronic structures. In this context, our group previously developed a new family of quinoline-based PNN pincer-type ligands for low- to mid-valent iron catalysts. These chemically robust PNN ligands provide air- and moisture-tolerant iron complexes, which exhibit excellent catalytic performances in the C-H borylation of arenes. This feature article summarises our recent work on PNN iron complexes, including their conception and design, as well as related reports on iron pincer complexes and iron-catalysed C-H borylation reactions.

Hemisphere and Distance-Dependent Steric Analysis of PNN Iron Pincer Complexes Using SambVca 2.1 and Its Influence on Alkene Hydrosilylation

A series of PNN iron pincer complexes was synthesized and structurally characterized by X-ray and computational analysis. Using the optimized structures obtained by density functional theory calculations, the steric parameters of the ligands, based on the percent buried volume (%VBur), were evaluated using SambVca 2.1. Subsequently, the catalytic performance, including product selectivity and alkene hydrosilylation conversion, were examined and the relationship with %VBur values was determined. The product selectivity was found to positively correlate with the %VBur values of the hemisphere, including the location of the reaction site in the inner sphere (<6.0 Å). In contrast, the reaction conversion showed a negative correlation with the steric parameters of the outer sphere. Finally, the steric influence of the inner and outer spheres on catalytic performance was discussed on the basis of the proposed catalytic mechanism.

Hydrogenative Depolymerization of End-of-Life Polycarbonates by an Iron Pincer Complex.

Chemical recycling processes can contribute to a resource‐efficient plastic economy. Herein, a procedure for the iron‐catalyzed hydrogenation of the carbonate function of end‐of‐life polycarbonates under simultaneous depolymerization is presented. The use of a straightforward iron pincer complex leads to high rate of depolymerization of poly(bisphenol A carbonate) and poly(propylene carbonate) yielding the monomers bisphenol A and 1,2‐propanediol, respectively, as products under mild reaction conditions. Furthermore, the iron complex was able to depolymerize polycarbonates containing goods and mixture of plastics containing polycarbonates.

An Iron Pincer Complex in Four Oxidation States.

The synthesis and characterization of a series of homoleptic iron complexes [Fe(benzNHCOCO)2]2-/1-/0/1+ supported by the tridentate bis-aryloxide benzimidazolin-2-ylidene pincer ligand benzNHCOCO2- (II) is presented. While the reaction of 2 equiv of free ligand II with a ferrous iron precursor leads to the isolation of the coordination polymer [Fe(benzNHCOCOK)2]n (1), treatment of II with ferric iron salts allows for the synthesis and isolation of the mononuclear, octahedral bis-pincer compound K[Fe(benzNHCOCO)2] (2) and its crown-ether derivative [K(18c6)(THF)2][Fe(benzNHCOCO)2] (3). Electrochemical studies of 2 suggested stable products upon further one- and two-electron oxidation. Hence, treatment of 2 with 1 equiv of AgPF6 yields the charge-neutral species [Fe(benzNHCOCO)2] (4). Similarly, the cationic complex [Fe(benzNHCOCO)2]PF6 (5) is obtained by addition of 2 equiv of AgPF6. The characterization of complexes 1, 3, and 4 reveals iron-centered reduction and oxidation processes; thus, preserving the dianionic, closed-shell structure of both coordinated benzNHCOCO pincer chelates, II. This implies a stabilization of a highly Lewis acidic iron(IV) center by four phenolate anions rather than charge distribution across the ligand framework with a lower formal oxidation state at iron. Notably, the overall charge-neutral iron(IV) complex undergoes reductive elimination of the pincer ligand, providing a metal-free compound that can be described as a spirocyclic imidazolone ketal (6). In contrast, the ligand-metal bonds in 5, formally an iron(V) complex, are considerably covalent, rendering the assignment of its oxidation state challenging, if not impossible. All compounds are fully characterized, and the complexes' electronic structures were studied with a variety of spectroscopic and computational methods, including single-crystal X-ray diffraction (SC-XRD), X-band electron paramagnetic resonance (EPR), and zero-field 57Fe Mössbauer spectroscopy, variable-field and variable-temperature superconducting quantum interference device (SQUID) magnetization measurements, and multi-reference ab initio (NEVPT2/CASSCF) as well as density functional theory (DFT) studies. Taken altogether, the electronic structure of 5 is best described as an iron(IV) center antiferromagnetically coupled to a ligand-centered radical.

Development of Activator-free Iron Pincer Complexes for Alkene Hydrosilylation and Elucidation of Its Activation Mechanism

Activation of iron dihalide complex for hydrosilylation of alkene was examined along with the disproportionation of hydrosilane. The pre-catalyst prepared in the reaction of activators with iron di...

Iron(II) complexes with N2O2 coordinating Schiff base-like equatorial ligand and 1,2-bis(pyridin-2-ylethynyl)benzene as axial pincer ligand

Three new unique mononuclear iron(II) pincer complexes were synthesized using 1,2-bis(pyridin-2-ylethynyl)benzene as axially coordinating pincer ligand and N2O2 coordinating Schiff base-like equatorial ligands. Magnetic susceptibility measurements reveal that all three complexes remain in the high spin state throughout the entire temperature range investigated. Reasons for this are restraining sterical interactions revealed in the single crystal x-ray structure analysis and extended DFT-computational studies of one of the pincer complexes. Those interactions also lead to the formation of unexpected side products during the synthesis such as a complex with two ethanol molecules as axial ligand, whose x-ray structure was determined.

Redox-Active, Boron-Based Ligands in Iron Complexes with Inverted Hydride Reactivity in Dehydrogenation Catalysis

For a series of PBP-type iron(II) pincer complexes, the central donor group based on tricoordinate boron is demonstrated to be redox-active, formally yielding iron(0) and a boronium-species by reve...

Base‐Free Iron Catalyzed Transfer Hydrogenation of Esters Using EtOH as Hydrogen Source

Herein, we report on the use of the iron pincer complex Iron-MACHO-BH, in the base-free transfer hydrogenation of esters with EtOH as a hydrogen source. More than 20 substrates including aromatic and aliphatic esters and lactones were reduced affording the desired primary alcohols and diols with moderate to excellent isolated yields. It is also possible to reduce polyesters to the diols with this method, enabling a novel way of plastic recycling. Reduction of the renewable substrate methyl levulinate proceeds to form 1,4-pentanediol directly. The yields are largely governed by the equilibrium between the alcohol and the ethyl ester.

Base‐Free Iron Catalyzed Transfer Hydrogenation of Esters Using EtOH as Hydrogen Source

AbstractHerein, we report on the use of the iron pincer complex Iron‐MACHO‐BH, in the base‐free transfer hydrogenation of esters with EtOH as a hydrogen source. More than 20 substrates including aromatic and aliphatic esters and lactones were reduced affording the desired primary alcohols and diols with moderate to excellent isolated yields. It is also possible to reduce polyesters to the diols with this method, enabling a novel way of plastic recycling. Reduction of the renewable substrate methyl levulinate proceeds to form 1,4‐pentanediol directly. The yields are largely governed by the equilibrium between the alcohol and the ethyl ester.

Iron Pincer Complexes as Catalysts in Cross-coupling of Aryl Halides and Phenylboronic Acid

Pincer complexes with iron as active metal center were synthesized to study their catalytic activity in Suzuki-Miyaura coupling reactions. Tridentate pincer ligand was synthesized by the reaction of diphenylchlorophosphine with m-aminophenol and m-phenylenediamine respectively in a 2:1 ratio in the presence of triethylamine as a base and tetrahydrofuran as solvent media. The resultant ligand was complexed with FeSO4 to obtain PCP complexes, C-1 with O and N atoms in the side arms and C-2 with both N atoms in the side arms. The synthesized complexes were examined for their C-C coupling efficiency in cross-coupling between phenyl boronic acid and para substituted halobenzenes. The research study aims to provide an alternative approach to the Pd catalyzed cross coupling methods, an otherwise subjugated method to obtain cross-coupled products. Also the study implores on the effect of variation in the side arm atom relating to the donating ability of the ligand and thereby relatively affecting the coupling yield of the catalysts.

Iron pincer complex and its graphene oxide composite as catalysts for Suzuki coupling reaction

We report the synthesis of Fe-NCN pincer complex as homogenous catalyst and its composite by immobilizing the complex on amino functionalized graphene oxide as a heterogeneous catalyst for Suzuki coupling reactions. Both the complex and the composite were employed in catalyzing the Suzuki-Miyaura cross-coupling reaction between the aryl halide and phenyl boronic acid in acetonitrile solvent media with Cs2CO3 as a base. Effect of substitution over aryl halide was also investigated. Immobilization of the pincer complex had advantageous recovery and reuse of the catalyst as compared to its homogenous analog with no significant decrease in the catalytic efficiency.

Highly Efficient Hydrogenation of Levulinic Acid into γ-Valerolactone using an Iron Pincer Complex.

The search for nonprecious-metal-based catalysts for the synthesis of γ-valerolactone (GVL) through hydrogenation of levulinic acid and its derivatives in an efficient fashion is of great interest and importance, as GVL is an important a sustainable liquid. We herein report a pincer iron complex that can efficiently catalyze the hydrogenation of levulinic acid and methyl levulinate into GVL, achieving a turnover number of up to 23 000 and a turnover frequency of 1917 h-1 . This iron-based catalyst also enabled the formation of GVL from various biomass-derived carbohydrates in aqueous solution, thus paving a new way toward a renewable chemical industry.

Chemoselective Hydrogenation of Aldehydes on an Iron Pincer Complex Catalyst

Chemoselective Hydrogenation of Aldehydes on an Iron Pincer Complex Catalyst

Regioselective deuteration of alcohols in D2O catalysed by homogeneous manganese and iron pincer complexes

Regioselective, base-metal catalyzed deuteration of alcohols in D2O.

Ambireactive (R3 P)2 BH2 Groups Facilitating Temperature-Switchable Bond Activation by an Iron Complex

An iron pincer complex containing a hemi-labile (R3 P)2 BH2 group exhibits temperature-switchable reactivity patterns: a reversible B-H activation concomitant with a P-B bond cleavage is observed at room temperature. Below 4 °C, intra- and intermolecular C-H activation pathways are becoming faster and more dominant. Mechanistic investigations reveal that the lability of the (R3 P)2 BH2 group in combination with the exothermic formation of σ-bonded complexes are responsible for the switchable bond activation. Finally, a protocol for an iron-catalyzed H/D-exchange of organic solvents in the absence of oxidants has been developed.

Synthesis and Electronic Ground-State Properties of Pyrrolyl-Based Iron Pincer Complexes: Revisited.

The pyrrolyl-based iron pincer compounds [(tBuPNP)FeCl] (1), [(tBuPNP)FeN2] (2), and [(tBuPNP)Fe(CO)2] (3) were prepared and structurally characterized. In addition, their electronic ground states were probed by various techniques including solid-state magnetic susceptibility and zero-field 57Fe Mössbauer and X-band electron paramagnetic resonance spectroscopy. While the iron(II) starting material 1 adopts an intermediate-spin (S = 1) state, the iron(I) reduction products 2 and 3 exhibit a low-spin (S = 1/2) ground state. Consistent with an intermediate-spin configuration for 1, the zero-field 57Fe Mössbauer spectrum shows a characteristically large quadrupole splitting (ΔEQ ≈ 3.7 mm s-1), and the solid-state magnetic susceptibility data show pronounced zero-field splitting (|D| ≈ 37 cm-1). The effective magnetic moments observed for the iron(I) species 2 and 3 are larger than expected from the spin-only value and indicate an incompletely quenched orbital angular momentum and the presence of spin-orbit coupling in the ground state. The experimental findings are complemented by density functional theory computations, which are in good agreement with the experimental data. Most notably, these calculations reveal a low-lying (S = 2) excited state for complex 1; furthermore, the computed Mössbauer parameters for all complexes studied herein are in excellent agreement with the experimental findings.

Selective Hydrogenation of Nitriles to Secondary Imines Catalyzed by an Iron Pincer Complex

Selective hydrogenation of nitriles to secondary imines catalyzed by an iron complex, the pincer complex (iPr-PNP)Fe(H)Br(CO), in the presence of catalytic base, is reported. A wide range of (hetero)aromatic and aliphatic nitriles are hydrogenated to the corresponding secondary imines under mild conditions.