Methane, reaction with iron clusters

The synthetic methods used involve reaction of a cluster anion with [AuCIL], elimination of methane between a cluster hydride and [AuMeL] or addition of LAu+ units to metal-metal bonds. The emphasis here will be on structure and reactions of the complexes. Some examples of mixed gold clusters are given in Table 15, where it can be seen that most work has been on derivatives of clusters of iron, ruthenium and osmium. 

FIGURE 8. Postulated mechanism for MMO. The inner cycle are postulated intermediates in the catalytic cycle (only the binuclear iron cluster of the MMOH component is shown). The outer cycle represents the intermediates detected during a single turnover beginning with diferrous MMOH and ending with diferric MMOH. The rate constants shown are for 4 C and pH 7.7. The rate shown for the substrate reaction RH with Q is that for methane. The alignment of the two cycles shows the postulated structures for the intermediates. 

Platinum-iron on alumina catalysts were characterized by Mbssbauer spectroscopy (Section 4) and their activity tested. Iron in clusters with high Pt Fe ratios, about 5, and fully combined with platinum, was catalytically inert for the CO-H2 synthesis reaction, attributed to a decrease in the electron density of the iron as indicated by the Mbssbauer isomer shift. The direction of electron transfer was opposite to that proposed for alkali-metal promoted iron catalysts. At low Pt Fe ratio, 0.1, ferromagnetic iron as well as Fe " ions and PtFe clusters were produced and dominated the activity/selectivity pattern. Rhodium on silica catalysts produced C2-compounds containing oxygen, specifically acetic acid, acetaldehyde and ethanol, with methane as the other major product. The addition of iron moved the C2-product formation sharply in favour of ethanol and now methanol was also formed. ... 

The power of AES to identify the true nature of the surface of a working catalyst has been demonstrated by Dwyer and Somorjai." They used an apparatus in which the polycrystalline foil could be used as a catalyst for CO/H2 and CO2/H2 reactions at 6 atmospheres pressure. Clean iron at 300 °C gave CH4, 85%, and other C2 to C5 hydrocarbons in small amounts. It rapidly became covered with 1 monolayer of C with a reduction in rate. When multilayers of C had formed, methane alone was produced by H2/CO, but at a further-diminished rate. The H2/CO2 reaction on initially clean Fe produced 97% methane and a marked increase in methanation rate. Both C and O accumulated on the surface during this reaction. The authors point out that in the case of the H2/CO reaction the monolayer carbon may not be the active catalyst. There is one piece of evidence in their work which points to Fe, perhaps in clusters, being the active site. They studied the CO/H2 reaction on pre-oxidized Fe and... 

Coenzymes M and B are involved in the final steps of methane formation involving the reduction of methyl-coenzyme M to methane (Figure 1). This reaction is catalyzed hy methyl-coenzyme M methylreductase, a nickel-dependent oxidoreductase. This enzyme catalyzes the reaction of methyl-coenzyme M with coenzyme B to form methane and a heterodisulfide between coenzymes B and M. The final step in methanogenesis results in the reduction of this heterodisulfide back to coenzymes M and B. This reaction is catalyzed by a heterodisulfide reductase that unexpectedly contains an iron—sulfur cluster. ... 

No details are available on the evolution of the four-iron butterfly cation to methane, but further protonation of the framework and reductive elimination of CH4 seem likely. The four-metal butterfly framework appears to play a significant role in these reactions, particularly in activating carbon monoxide through II —CO formation. Significantly, the proton-induced reduction has been observed with other four- and six-metal carbonyl clusters, but the reaction does not appear to occur with clusters with fewer nuclei (248). By analogy with the findings in the iron system, this minimum metal nucleus number requirement suggests that n —CO may be involved in all of these reactions. 

The use of Me3NO to induce substitution of dppm (bis(diphenylphosphino)-methane) for CO molecules on dinuclear iron complexes led to insertion of CO into C-C bonds of alkyne-derived metallacycles. Similar behavior was observed when [PPNJCl salts were used to favor the formation of alkyne-substituted triruthenium dppm-containing clusters.I This behavior should be compared with the insertion of CO into allenylidene and phosphido-bridging ligands occurring when dppm coordinates to binuclear ruthenium complexes as shown in Fig. 3. This reaction is a nucleophilic attack of the coordinated allenylidene and phosphido groups on a coordinated CO (see Section 2.8.2.2). 

---