Iron bimetallic clusters

An explanation for this increase in selectivity with the addition of aluminum could be related to the better dispersion of iron metallic clusters, which could be anchored to the acidic sites on the mesoporous support, as observed by Lim et al [13] for bimetallic systems in MCM41. 

Iron-only hydrogenase, dithiolate-bridged compounds as biomimetic models, 6, 239 Iron oxide films, synthesis, 12, 51 Iron-palladium nanoparticles, preparation, 12, 74 Iron-platinum bimetallic clusters, with isocyanide clustes,... 

In studies on bulk PtFe, IrFe, and PtlrFe alloys, it was observed (61,62) that the magnetic fields at the iron nuclei were much smaller for IrFe than for PtFe. These studies showed also that the magnetic fields for PtlrFe alloys were intermediate between those for PtFe and IrFe alloys. The same trend is found with samples B, C, D in Table 4.3, which is consistent with the view that the iron atoms in sample D are incorporated in bimetallic clusters of platinum and iridium. 

A number of iron-platinum bimetallic clusters with isocyanide ligands have been studied those, such as 129, bridged by a dppm ligand have proved to be versatile (Scheme 30). " Both charged, 129, and uncharged, 130, complexes are possible, depending on the reaction route taken Fe-Pt bond lengths are typically 2.80 A. 

A detailed study of iron-promoted rhodium clusters was conducted by Schii-nemann et al. [235] using TPR, FTIR, TEM and Mossbauer spectroscopy. NaY was exchanged with Fe ions in FeS04 solutions, then rhodium was exchanged from [Rh(NH3)5Cl]Cl2 solutions. The co-exchanged zeolite was calcined from room temperature to 500 °C with a ramp of 0.5 °C min. After reduction at 500°C most of the iron remained in ionic form, but bimetallic clusters with a low Fe content were also formed. Treatment in NaOH of the reduced zeolite followed by calcination and reduction maximized the Fe content that attained ca. 50% according to ferromagnetic resonance data. The Rh-Fe clusters were disrupted in a CO atmosphere with formation of rhodium and iron carbonyls. 

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

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. 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

In summary, the behavior of the RuFe/Si02 system, as revealed by Mossbauer spectroscopic studies, parallels that reported for a number of supported bimetallic catalysts containing iron. The isomer shifts for bulk-like iron in the clusters, the trends in the Mossbauer spectra with the percent metal exposed, and the reversible oxidation-reduction behavior of the iron at room temperature are all consistent with Ru-Fe... 

However, iron complexes may also display a beneficial effect. Thus, a promoting effect on hydroformylation was observed with Si02-supported Rh-Fe " bimetallic carbonyl clusters (Scheme 1.61) [12]. Based on Mossbauer spectroscopy, it was proposed that iron assists during the insertion reaction of CO into the Rh-C bond. Likewise, the hydrogenation of the intermediary alkoxy rhodium species to produce the alcohols may benefit from this bi-site interaction. 

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