Methanol iron clusters

Iron clusters exhibit facile chemisorption toward methanol, the reaction proceeding with little or no cluster-size selectivity. An interesting feature of this system is that the chemisorption rate constants are nearly identical toward various isotopic sjjecies (CH3OH, CH30D,CD3 0H). If dissociation of a C—H or O—H bond was the initial step, then this should be manifested in an observable kinetic isotope effect. Thus the initial chemisorption step most likely involves the lone-pair orbital localized on the oxygen atom. More extensive studies of the chemistry of the Fe methanol system have been explored using infrared multiple-photon dissociation spectroscopy. These results are discussed in detail in Section Vlll. 

In subsequent years, research in Geoifroy s group supported some of the intermediate steps proposed and added additional details. Among the most relevant discoveries, it was found that the bis imido iron cluster Fe3(CO)9(//3-NPh)2 reacts with methoxide to afford an isolable methoxycarbonyl compound. Reaction of this second cluster with methanol slowly afforded the carbamate (Scheme 2). 

The introduction of iron in the lattice of zeolite topologies such as MFI and CFI is performed via direct synthesis and post-synthetic modification. Dinuclear iron clusters in appropriate zeolites realize oxidation of methane into methanol at room temperature after N2O activation. Both mono and dinuclear iron coordination and lattice coordination is treated with theory and experiment. In FeCIT-5 and Fe-ZSM5 are made by over-exchange via sublimation of iron salts in Al-zeolites. When extra-framework iron oxides are made in zeolites, the extraframework iron oxide clusters can be anchored to zeolites. 

Ford used HP-IR to investigate an acyliron migratory insertion intermediate formed by flash photolysis. Thus, flash photolysis of (7 -Cp)Fe(C0)2C(0)GH3 affords coordinatively unsaturated ( -Gp)Fe(C0)C(0)CH3. Trapping of the latter with CO in the reverse reaction was studied, and the second-order rate constant could be determined for this reaction under the high CO pressures employed. Variable-temperature studies allowed calculation of activation parameters for methyl migration. Iron cluster compounds have been studied for the carbonylation of methanol to methyl formate. Consistent kinetics and a first-order dependence on cluster concentration confirmed the HP-IR results which showed that the cluster remained intact through the catalytic process. 

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

The multiprotein complex methane monooxygenase (MMO) serves meth-anotrophs to convert methane to methanol. It can be either soluble (sMMO) or membrane bound ( particulate , pMMO) and it typically consists of three components, a reductase (MMOR), a component termed protein B (MMOB) and a hydroxylase denoted MMOH. The nature of the metal cofactors in the latter component are reasonably well understood for sMMO as will be discussed in the non-heme iron section. For the pMMO of Methylococcus capsulatus an obligate requirement for copper was shown. As reported in reference 1 a trinuclear Cu(II) cluster was discussed128 but the number and coordination of coppers still is a matter of continuing investigation since then. 

A considerable range of iron carbonyl clusters is now known to contain antimony or bismuth, and later developments have seen this extended to ruthenium and osmium complexes. Whitmire (711) has described the reaction between NaBi03 and methanolic [Fe(CO)5], which affords the anionic triiron cluster [BiFe3(CO)10] (108), which contains a tetrahedral BiFe3 core. Cluster 108 is an example of a closo tetrahedral cluster with three first-row... 

Only two diantimony-iron carbonyl clusters are known. Whitmire (775) has described the synthesis of [Fe3(CO)9 /r3-SbFe(CO)4 2]2 (134) from the reaction between NaSb03 and methanolic [Fe(CO)5j. The structure is similar to that of 132 except that in 132 only one bismuth carries an Fe(CO)4... 

In the effort to modify the catalytic properties of rhodium, Wilson and co-workers prepared Rh/Fe/Si02 and other two-metal-silica combinations (74) methanol yields over a Rh/Fe/Si02 catalyst shown in Table XIV were higher than those over the Rh/MgO, Rh/ZnO, and Rh/LaB6 catalysts prepared from cluster carbonyls, but the selectivity of the latter toward methanol was better than that of bimetallic rhodium-iron catalysts. 

The compound K2 [Rh6(CO)15C] is a yellow powder. It is sensitive to air both in the solid state and in solution and is quite soluble in water, methanol, ethanol, acetone, THF, and acetonitrile. The salts of other cations can be obtained by metathesis, in water for the cesium salt and in methanol for the larger tetra-alkylammonium or phosphonium cations. The tetraethylammonium salt is sparingly soluble in THF, whereas the benzyltrimethylammonium and bis-(triphenylphosphine)imminium salts are soluble. All of these salts are soluble in acetone and acetonitrile. The yellow solution of the potassium salt in THF shows characteristic IR bands at 2040 (vw), 1990 (vs), 1885 (vw), 1845 (s), 1830 (sh, m) 1815 (sh, br) and 1775 (vw, br) cm-1. The IR spectral band shapes depend on solvents and cations. The oxidation of K2 [Rh6(CO)i5C] with iron-(III) ammonium sulfate in water under carbon monoxide leads to the octa-nuclear carbido carbonyl cluster Rhg(CO)i9C,6 whereas under nitrogen RhntCO sQ7 or [H30] [Rhls(CO)28C2]8 is obtained. 

The IRMPD of metal cluster adsorbate complexes provides a unique capability to probe infrared photo physics. The ability to alter the molecular structure by simply varying the cluster size and the number of adsorbates makes it possible to systematically probe the effects of increasing the density of states by increasing the cluster size. Figure 11 shows the iluence dependence of the survival fraction for iron-methanol clusters as a function of the number of iron atoms with one or two adsorbed methanol molecules. As expected, for... 

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