Iron catalytically active species

To mimic the square-pyramidal coordination of iron bleomycin, a series of iron (Il)complexes with pyridine-containing macrocycles 4 was synthesized and used for the epoxidation of alkenes with H2O2 (Scheme 4) [35]. These macrocycles bear an aminopropyl pendant arm and in presence of poorly coordinating acids like triflic acid a reversible dissociation of the arm is possible and the catalytic active species is formed. These complexes perform well in alkene epoxidations (66-89% yield with 90-98% selectivity in 5 min at room temperature). Furthermore, recyclable terpyridines 5 lead to highly active Fe -complexes, which show good to excellent results (up to 96% yield) for the epoxidation with oxone at room temperature (Scheme 4) [36]. 

Apart from catalysis with well-defined iron complexes a variety of efficient catalytic transformations using cheap and easily available Fe(+2) or Fe(+3) salts or Fe(0)-carbonyls as precatalysts have been pubhshed. These reactions may on first sight not be catalyzed by ferrate complexes (cf. Sect. 1), but as they are performed under reducing conditions ferrate intermediates as catalytically active species cannot be excluded. Although the exact nature of the low-valent catalytic species remains unclear, some of these interesting transformations are discussed in this section. 

This section provides only a brief insight into iron-catalyzed reactions. Iron complexes as catalytically active species undergo typical steps of transition metal catalysis... 

In contrast to the carbometallation with aryl-Grignard reagents to unfunctionalized alkynes, this reaction does not require a co-catalyst such as copper. Only in one example, applying PhMgBr, was CuBr added in accord with the protocol reported by Hayashi s group. Although the nature of the catalytically active species remains unclear, an alkoxide-directed carbometallation which yields a (vinyl)iron intermediate is proposed. 

The idea of an one-center template mechanism was initially supported by first-order kinetics in iron. Moreover, intermediates 3a, 43 and 44 ands also their transition states in the catalytic cycle (Scheme 8.18) were proved by computational studies [71]. Moreover, mass spectrometric (ESI) [72] and spectroscopic (EXAFS and Raman) studies indicated complex 45 with two equatorial [3-diketonate ligands to be the catalytically active species in solution (Scheme 8.19) [73]. Actually, 4equiv. of FeCl3-6H20 are needed to generate 1 equiv. of complex 45 under reaction conditions ... 

The most prominent reactions catalyzed by low-valent iron species involving radical intermediates are cross-coupling reactions of alkyl halides (recent reviews [32-35]) and atom transfer radical reactions. In cross-coupling reactions the oxidation state of the catalytically active species can vary significantly depending on the reaction conditions very often it is not known exactly. To facilitate a summary, all iron-catalyzed cross-coupling reactions are treated together and involved oxidation states, where known, are mentioned at the example. In contrast, iron-catalyzed Kharasch reactions will be treated at the oxidation state of the iron precursors. 

N. A. Stephenson, A. T. Bell, Effects of methanol on the thermodynamics of iron(III) [tetrakis (pentafluorophenyl)]porphyrin chloride dissociation and the creation of catalytically active species for the epoxidation of cyclooctene, Inorg. Chem. 45 (2006) 5591. 

From 2, it was concluded that the ferryl complex is the catalytically active species. Observation 1 suggested that 80% of the epoxide product in the aerobic reaction is derived from a carbon-based radical, which is quenched by O2 (autoxidation), and this is known to produce epoxide in reactions with cyclooc-tene (325). Methanol (observation 3) is known to quench radicals. The fact that the diols formed are a mixture of cis and trans products (observation 1 this is very unusual in iron-catalyzed olefin oxidations) suggested that the diol results from the capture of OH radicals by the putative carbon-based radical. 

The catalytically active species formed by the treatment of 2,6-bis(imino) pyridine iron(II) chloride complexes with MAO is generally proposed to be a highly reactive monomethylated iron(ll) cation [LFe-Me] (L = 2,6-bis(imino)pyridine ligand) bearing a weakly coordinating counteranion [Me-MAO]. Both monochloride and monoalkyl cationic species are expected to be present in the solution, their relative concentration depending on the MAO/Fe ratio [24]. 

The identification of catalytically active species in FTS is of fundamental importance, as an improved understanding could enable the development of catalysts with increased activity and selectivity. In cobalt- and ruthenium-catalysed FTS, metallic cobalt and ruthenium function as active catalysts. However, in iron-catalysed FTS there are several distinct species generated during the reaction. Due to the lower, or similar, activation energy for iron carbide formation in comparison to carbon monoxide hydrogenation, iron-carbide formation is typically observed in FTS. The formation of several iron-carbide phases have been observed -Fe2C/8 -Fe2.2C (hexagonal... 

As already outlined, mixed metal oxide films and powders play important roles as sensors, catalysts and as supports for catalytically active species. In particular, iron oxide doped silica find application in catalytic processes like Friedel-Crafts alkylation, the conversion of CH4 to HCHO and in the decomposition of methanol and phenol [175]. 

Arata and Hino found that better catalysts could be obtained by calcining Fe(OH)3 at 573 — 873 K. The hydroxide was prepared by hydrolyzing FeCls or Fe(N03)3 9H20. The alkylation reactions were carried out at room temperature with 50 cm of toluene solution (0.5 mol 1 ) of benzyl chloride, t-butyl chloride or acetyl chloride and 0.1 g (for benzylation or t-butylation) or 0.5g (for acetylation) of catalyst. Benzylation and t-butylation was completed within 2 min and 10 min, respectively. For acetylation with acetyl chloride, the reaction was slow, the conversion being 28% after 6 h of reaction. The reaction with acetyl bromide is slighdy faster conversion of 30% was obtained after 4 h.The isomer distribution of alkyltoluenes was 42% ortho, 6% meta and 52% para for benzylation and 3% meta and 97% para for butylation with /-butyl chloride. It was presumed that iron chloride formed on the surface of amorphous iron oxide by its reaction with hydrogen chloride is a catalytically active species for alkylation. 

The above mentioned studies proved that the FeCl -GICs are good precursors to highly active catalysts made by double step reduction followed by HCl treatment. In spite of considerable amount of work performed in this field, the identity of the catalytically active species has not yet been established. The created iron is rather Fe(0) produced by reduction of Fe(II) with potassium and located in the graphite matrix. Such active iron is not readily accessible to the IKl (ref. 10). 

The proposed catalytic cycle is shown in Scheme 31. Hence, FeCl2 is reduced by magnesium and subsequently coordinates both to the 1,3-diene and a-olefin (I III). The oxidative coupling of the coordinated 1,3-diene and a-olefin yields the allyl alkyl iron(II) complex IV. Subsequently, the 7i-a rearrangement takes place (IV V). The syn-p-hydride elimination (Hz) gives the hydride complex VI from which the C-Hz bond in the 1,4-addition product is formed via reductive elimination with regeneration of the active species II to complete the catalytic cycle. Deuteration experiments support this mechanistic scenario (Scheme 32). 

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

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