Fe Catalyst System

Both the Co and the Fe systems have very similar chemistry for the 1 1 codimerization reaction. Although they are almost identical in catalytic selectivity, they do differ in other catalytic properties, especially the rate of reaction (66). In practice, the Co system is superior to the Fe system our discussion will therefore focus mainly on the former system. 

Comparison of Catalytic Properties of Some Fe and Co Catalyst Systems 

In the literature there are many reports of the formation of active catalyst for the 1 1 codimerization or synthesis of 1,4-hexadiene employing a large variety of Co or Fe salts, in conjunction with different kinds of ligands and organometallic cocatalysts. There must have been many structures, all of which are active for the codimerization reaction to one degree or another. The scope of the catalyst compositions claimed to be active as the codimerization catalysts is shown in Table XV (69-82). As with the nickel catalyst system discussed earlier, the preferred Co or Fe catalyst system requires the presence of phosphine ligands and an alkylaluminum cocatalyst. The catalytic property can be optimized by structural control of these two components. 

The best phosphines found so far are bisphosphines of the general formula R2P(CH2) PR2, where n = 2 or 3 and R = aryl groups. Examples of some very good ligands are 1,2-bis(diphenylphosphino)ethane (32) and 1,2-bis(diphenylphosphino)propane (33)  

Tridentate phosphines (77) such as 34 as well as some nonphosphorus chelating ligands such as the bisarsines (73) 

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In 2008, Gade and coworkers reported that the asymmetric hydrosilylation of ketones was catalyzed by the Fe complex with a highly modular class of pincer-type ligand (Scheme 22) [71]. This Fe catalyst system showed both moderate to good... 

The dimerization mechanism for either the Co or the Fe catalyst system is depicted in Scheme 9 according to the postulation by Hata (65) and Miyake et al. (66). The reaction sequences involve 1,4-addition of the... 

Linear dimeri2ation and oligomeri2ation of butadiene can be achieved by using a number of catalyst systems based on Pd, Ni (158—161), and Fe (162). 1,7-Octadiene can be obtained selectively when the dimeri2ation is carried out in the presence of a reducing agent such as formic acid (163—165) or H2/CO2 (166). 

Until now examples for catalytic reactions involving ferrates with iron in the oxidation state of -l-3 are very rare. One example is the hexacyanoferrate 8-catalyzed oxidation of trimethoxybenzenes 7 to dimethoxy-p-benzoquinones 9/10 by means of hydrogen peroxide which was published by Matsumoto and Kobayashi in 1985 [2]. Using hexacyanoferrate 8 product 9 was favored while other catalysts like Fe(acac)3 or Fe2(S04)3 favored product 10 (Scheme 2). The oxidation is supposed to proceed via the corresponding phenols which are formed by the attack of OH radicals generated in the Fe/H202 system. 

A good catalyst for this reaction is [FeH(CO)4], produced by the reaction of [Fe(CO)5] with OH-.348 Aldehydes were reduced by CO in 60 40 water 2-ethoxy-ethanol mixtures under mild condition (30-80 °C, 20 bar CO) with a [Rh6(CO)16]/diamine catalyst system.349 In aqueous... 

Catalysts for this codimerization reaction can be derived from prac-tially all the Group VIII transition metal compounds. Their catalytic properties, such as rate, efficiency, yield, selectivity, and stereoselectivity, vary from poor to amazingly good. Some better-known catalyst systems and their product distributions are listed in Table I. As one can see, the major codimerization product under the given condition is the linear 1 1 addition product, 1,4-hexadiene. The formation of this diene and its related C6 products will become the center of our discussions. The catalyst systems that have been investigated rather extensively are derived from Rh, Ni, Co, and Fe. We shall cover these systems in some detail. A lesser-known catalyst system based on Pd will also be briefly discussed. 

The extreme stereoselectivity toward the synthesis of cis-1,4-hexadiene is attributed to the fact that only cisoid-coordinated 1,3-diene can undergo the addition reaction (65, 66). 1,3-Dienes whose cisoid conformations are stoically unfavorable do not react with ethylene under the dimerization conditions. For example, Hata (65) was able to show that, using an Fe-based catalyst system, l-tra/is-3-pentadiene (40) and 2-methyl-1 -trans-3-pentadiene (41) reacted readily with ethylene to form the expected 1 1 addition products, while l-c/s-3-pentadiene (42) and 4-methyl- 1,3-penta-diene (43) failed to interact with ethylene. The explanation is that the cisoid conformations of 40 and 41 are stoically favorable while those for 42 and 43 are not. 

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

Major differences were noted between the systems derived from Fe(CO)c and M(CO) (M = Cr, Mo, and W) with respect to the effect of the base concentration on the reaction rate. Thus in the case of the catalyst system derived from Fe(CO)5 tripling the amount of KOH while keeping constant the amounts of the other reactants had no significant effect on the rate of H2 production (11). However, in the case of the catalyst system derived from W(CO)g the rate of production increased as the amount of base was increased regardless of whether the base was KOH, sodium formate, or triethylamine (12). This increase may be interpreted as a first order dependence on base concentration provided some solution non-ideality is assumed at high base concentrations. Similar observations were made for the base dependence of H2 production in catalyst systems derived from the other metal hexacarbonyls Cr(CO) and Mo(CO) (12). Thus the water gas shift catalyst system derived from Fe(CO)5 has an apparent zero order base dependence whereas the water gas shift catalyst systems derived from M(CO)g (M - Cr, Mo, and W) have an approximate first order base dependence. Any serious mechanistic proposals must accommodate these observations. 

Major differences were also noted between the catalyst systems derived from Fe(CO) and those derived from M(CO) (M = Cr, Mo, and W) with respect to the effect of CO pressure on the reaction rate. In the system derived from Fe(CO)5 the rate of H- production in the early stages of the reaction was independent of the CO loading pressure in the range 10 to 40 atmospheres... 

The activation energies of these water gas shift catalyst systems were determined by rate measurements as a function of temperature. Thus on the basis of rate measurements at the five temperatures 180, 160, 150, 140, and 130°C the activation energy of the catalyst system derived from Fe(C0)5 was estimated at... 

Table I. Comparison of Catalyst Systems Derived from Fe(CO),. and M(C0)6 (M = Cr, Mo, W).

The rate of production using the catalyst system derived from Fe(CO)5 is thus seen to have a first order dependence on Fe(CO)j concentration and to be independent of CO pressure in accord with the experimental observations outlined above. Furthermore, the formate buffer system generated by reaction of CO with the base by equation 2 keeps the OH concentration essentially independent of the amount of base introduced into the system. Therefore the rate of H production using the catalyst system derived from Fe(CO), although having a first order dependence on OH concentration, is essentially independent of the base concentration. 

Asymmetric hydrogenation of nitrones in an iridium catalyst system, prepared from [IrCl(cod)]2, (S)-BINAP, NBu 4 BH4, gives with high enantioselectivity the corresponding A-hydroxylamines which are important biologically active compounds and precursors of amines (480). Further reduction of hydroxylamines to secondary amines or imines can be realized upon treatment with Fe/AcOH (479), or anhydrous titanium trichloride in tetrahydrofuran (THF) at room temperature (481). 

The mononitrosyl complex Fe(TC-5,5)(NO) was suggested to be a logical intermediate in the disproportionation promoted by the Fe(II) system (82b). However, the N02 released during the reaction (Eqs. (39) and (40)), nitrates the aromatic rings of the tropocoronand ligand and renders the resulting complex inactive as a disproportionation catalyst. 

Catalyst systems for the WGS reaction that have recently received significant attention are the cerium oxides, mostly loaded with noble metals, especially platinum 42—46]. Jacobs et al. [44] even claim that it is probable that promoted ceria catalysts with the right development should realize higher CO conversions than the commercial Cu0-Zn0-Al203 catalysts. Ceria doped with transition metals such as Ni, Cu, Fe, and Co are also very interesting catalysts 37,43—471, especially the copper-ceria catalysts that have been found to perform excellently in the WGS reaction, as reported by Li et al. [37], They have found that the copper-ceria catalysts are more stable than other Cu-based LT WGS catalysts and at least as active as the precious metal-ceria catalysts. 

Methane to Methanol and/or Formaldehyde Recent research indicates that a catalyst system in the presence of H2SO4 can convert methane directly into methanol. Homogeneous catalyst systems show promise. Also, heterogeneous Fe-ZSM-5 catalysts are reported to be attractive for this chemistry. Novel plasma reactors to generate hydroxyl radicals are also being investigated. 

These multicomponent catalyst systems have been employed in a variety of aerobic oxidation reactions [27]. For example, use of the Co(salophen) cocatalyst, 1, enables selective allylic acetoxylation of cyclic alkenes (Eq. 6). Cyclo-hexadiene undergoes diacetoxylation under mild conditions with Co(TPP), 2 (Eq. 7), and terminal alkenes are oxidized to the corresponding methyl ketones with Fe(Pc), 3, as the cocatalyst (Eq. 8). 

An HRTEM study of Zn-Cr Fe oxides using surface-profiling methods in the EM has been reported by Briscoe et al (1984, 1985). Chemical kinetic studies of the oxidation of benzene to maleic anhydride over V-Mo-O catalysts (prepared using 3 V205 Mo03), have been described using GC-MS techniques (Lucas et al 1983). However, microstructural information is limited and there are opportunities for EM studies of these catalyst systems. 

Along with enabling technological advances in alkene polymerizations, the application of new ancillary ligands or cocatalysts has become significant in the field of alkene oligomerizations to obtain a-olefms. This research exploits a new phase in the development of known catalyst systems containing Gr, Ni, Pd, Ti metal centers, as well as Fe and Co as newcomers. 

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