Iron Precatalyst Research Articles

Ethylene Polymerization by 2-Methyl-8-(benzimidazol)quinolyliron(II) Pre-Catalyst: a DFT Understanding its Chain Propagation and Transfer

The ethylene polymerization mechanism of the 2-methyl-8-(benzimidazol) quinolyliron(II) pre-catalyst is investigated by the DFT calculations, illustrating the possible intermediates with their geometrical and spin configurations. Regarding either methyl or ethyl group bonding on iron cores, the energy barriers for ethylene insertion have been extensively calculated. Within the iron‑methyl species, both resting state and transition state favor the configurations at high-spin state (quintet); whilst the iron-ethyl species prefer the low-spin state. According to the energy barriers, the chain propagation is more favorable than chain transfer for the bidentate iron pre-catalyst, which is well consistent with the experimental observation.

Room temperature hydrophosphination using a simple iron salen pre-catalyst.

Phosphines are fundamentally important to the fine chemicals, pharmaceutical and agrochemical industries. Reported is the first example of alkene hydrophosphination using a designed iron pre-catalyst which yields the anti-Markovnikov products in high yield at room temperature. The phosphine products are excellent pro-ligands for Fe-catalyzed Negishi cross-coupling.

Iron-catalysed alkene hydrogenation and reductive cross-coupling using a bench-stable iron(ii) pre-catalyst

Operationally simple, iron-catalysed hydrogenation and reductive cross-coupling protocols have been developed using a bench-stable iron(II) pre-catalyst. The hydrogenation of 18 alkenes (50–99%) and reductive cross-coupling of vinyl halides with aryl- and alkyl Grignard reagents (8 examples, 18–99%) is reported using 3 mol% pre-catalyst and hydrogen as stoichiometric reductant (1–50 bar).

Iron-Catalyzed, Highly Regioselective Synthesis of α-Aryl Carboxylic Acids from Styrene Derivatives and CO2

The iron-catalyzed hydrocarboxylation of aryl alkenes has been developed using a highly active bench-stable iron(II) precatalyst to give α-aryl carboxylic acids in excellent yields and with near-perfect regioselectivity. Using just 1 mol % FeCl(2), bis(imino)pyridine 6 (1 mol %), CO(2) (atmospheric pressure), and a hydride source (EtMgBr, 1.2 equiv), a range of sterically and electronically differentiated aryl alkenes were transformed to the corresponding α-aryl carboxylic acids (up to 96% isolated yield). The catalyst was found to be equally active with a loading of 0.1 mol %. Preliminary mechanistic investigations show that an iron-catalyzed hydrometalation is followed by transmetalation and reaction with the electrophile (CO(2)).

Light-Induced Enantioselective Hydrogenation Using Chiral Derivatives of Casey’s Iron–Cyclopentadienone Catalyst

We herein report the first example of an asymmetric ketone hydrogenation using chirally modified derivatives of the homogeneous iron(II)–cyclopentadienone–tricarbonyl system, known as Casey’s catalyst. For the synthesis of the chirally modified catalysts, one of three carbonyl ligands was exchanged for a chiral phosphoramidite. To this end, either oxidative decarbonylation using trimethylamine-N-oxide or photolysis was applied. Photolysis was also used to convert the tricarbonyl iron precatalyst (and, analogously, the dicarbonyl phosphoramidite complexes) to the coordinatively unsaturated dicarbonyl (monocarbonyl, respectively) complexes, which are intermediates in the catalytic cycle of ketone hydrogenation. Hydrogen uptake by the latter species affords the “loaded” hydride, as evidenced by 1H NMR spectroscopy. Thus, the preparation of sensitive iron hydrides by the typically low-yielding Hieber reaction could be avoided. Instead, the catalytic cycle is accessed from air-stable carbonyl precursors. In th...

Green Reductive Homocoupling of Bromobenzene

Although transition-metal-catalyzed reactions are important in contemporary organic chemistry, relatively few resources for the second-year organic chemistry curriculum discuss the subject. The inquiry-based experiment described here, an iron-catalyzed preparation of biphenyl from bromobenzene, introduces this topic. The reaction uses an inexpensive and relatively benign iron precatalyst that is air- and moisture-stable, and the experiment can be performed using the equipment found in a typical organic teaching laboratory. The crude product can be analyzed by melting point determination or by gas chromatography. The experiment allows for a general discussion of redox cycles common in many metal-catalyzed reactions, redox processes of organic substrates, and green chemistry. The experiment can be used in organic or inorganic laboratories.

Synthesis and characterization of para-nitro substituted 2,6-bis(phenylimino)pyridyl Fe(II) and Co(II) complexes and their ethylene polymerization properties

Four iron(II) and cobalt(II) complexes ligated by 2,6-bis(4-nitro-2,6-R 2-phenylimino)pyridines, LMCl 2 ( 1: R = Me, M = Fe; 2: R = iPr, M = Fe; 3: R = Me, M = Co; 4: R = iPr, M = Co) have been synthesized and fully characterized, and their catalytic ethylene polymerization properties have been investigated. Among these complexes, the iron(II) pre-catalyst bearing the ortho-isopropyl groups (complex 2) exhibited higher activities and produced higher molecular weight polymers than the other complexes in the presence of methylaluminoxane (MAO). A comparison of 2 with the reference non-nitro-substituted catalyst (2,6-bis(2,6-diisopropylphenylimino)pyridyl)FeCl 2 (FeCat 5) revealed a modest increase of the catalytic activity and longer lifetime upon substitution of the para-positions with nitro groups (activity up to 6.0 × 10 3 kg mol −1 h −1 bar −1 for 2 and 4.8 × 10 3 kg mol −1 h −1 bar −1 for 5), converting ethylene to highly linear polyethylenes with a unimodal molecular weight distribution around 456.4 kg mol −1. However, the iron(II) pre-catalyst 1 on changing from ortho-isopropyl to methyl groups displayed much lower activities (over an order of magnitude) than 2 under mild conditions. As expected, the cobalt analogues showed relatively low polymerization activities.

Enantiopure Pyridine Bis(oxazoline) “Pybox” and Bis(oxazoline) “Box” Iron Dialkyl Complexes: Comparison to Bis(imino)pyridine Compounds and Application to Catalytic Hydrosilylation of Ketones

The chemistry of enantiopure pyridine bis(oxazoline) iron compounds has been explored. The electronic properties of this ligand class have been evaluated relative to aryl- and alkyl-substituted bis(imino)pyridines using cyclic voltammetry and CO stretching frequencies of iron dicarbonyl compounds. Attempts to prepare a pyridine bis(oxazoline) iron dinitrogen complex by sodium amalgam reduction of the corresponding dichloride precursor yielded a catalytically inactive S = 1 bis(chelate) iron complex. Dialkyl derivatives of the general formula (RPybox)Fe(CH2SiMe3)2 were prepared either by pyridine displacement from (py)2Fe(CH2SiMe3)2 or by direct dialkylation of the appropriate iron dichloride. Two examples of the S = 2 complexes were characterized by single-crystal X-ray diffraction. Each enantiopure Pybox iron dialkyl was evaluated in the catalytic hydrosilylation of various ketones. Efficient turnover was observed at 23 °C with 0.3% of the iron precatalyst using PhSiH3 as the stoichiometric reductant. Fo...

Thermal behavior and pyrolysis products of modified organo-layered silicates as intermediates for in situ polymerization

A systematic study on organo-layered silicate intermediates used for preparing in situ polyethylene nanocomposites was performed by pyrolysis–gas chromatography/mass spectrometry and thermogravimetric analysis. The type and composition of the pyrolysis products gave useful information about mechanism of thermal degradation. The combination of pyrolysis and thermal decomposition data allowed us to describe the evolution of the organoclay structure after the reactive pretreatment steps with alkylaluminoxane cocatalyst and zirconocene or bis(imino)pyridine iron precatalyst, respectively. A proof of the formation of heterogeneous organoclay-immobilized catalyst was obtained.

Characterization of compositional heterogeneity in the polyethylene prepared with bis(imino)pyridyl iron(II) precatalyst and triethylaluminum

AbstractAnalytical tools including solvent gradient elution fractionation (SGEF), GPC, 13C NMR, and differential scanning calorimetry (DSC) are integrated for the characterization of compositional heterogeneity in the polyethylene (PE) prepared with the LFeCl2/AlEt3 catalytic system. The results indicate that at least two different kinds of catalytic species are present in ethylene polymerization. One active species generating branched PE gives low molecular weight; another kind of active species gives high molecular weight PE with high linear structure. The amount of branch decreases with increasing the molecular weights, and the small proportion of the branched PE shows low molecular weight with vinyl‐terminated end group, indicating that the branched PE is generated from the catalytic species giving low activity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Studies on the activation and polymerization mechanism of ethylene polymerization catalyzed by bis(imino)pyridyl iron(II) precatalyst with alkylaluminum

Different alkylaluminums (triethyl-, tri- iso-butyl-, n-trihexylaluminum) were tested as activator for the iron complex LFeCl 2 ( 1) (L = 2,6-bis[(2,4,6-trimethylphenylimino)ethyl]pyridine) in ethylene polymerization. Various polymerization temperatures and the molar ratios of Al/Fe prepared polyethylene (PE) with different molecular weights, molecular weight distributions, polymerization activities and polyethylene structures. Through the analysis of in situ UV–vis absorption spectra, ethylene polymerization results and prepared polyethylene structures, the distinctive behaviors of the catalytic system could result from a new applicable mechanism involving two different kinds of catalytic species LFeRCl (a) and LFeR 2 (b) (R = Et, i-Bu or He), the ratio of which varied with changing Al/Fe mole ratio when alkylaluminums were used as activators. The catalytic species (a) with very high activity mainly produced linear PE with high molecular weight in the bimodal PE. The catalytic species (b) with low activity mainly produced branched PE of low molecular weight in the bimodal PE.