Clusters oxidation

One problem we have had to overcome in developing metal-cluster oxidation-reduction photochemistry is the tendency of excited clusters to dissociate into radical fragments (for... 

The mechanistic implications of these facile formal cluster oxidations arachno nido closo by nett loss of Hp at moderate temperatures are considerable. The processes are accompanied by, and presumably assisted by a flexibility of coordination geometry about the Ir atom and also by its ready oxidation. The metal atom can be seen as a potential source of electrons for cluster bonding either by involving its lone pairs of electrons or by switching between Ir-H-B bridging and Ir-H terminal bonding... 

Scheme 2. Formal cluster oxidation reactions arachno - nido — closo. Reactions labelled and follow first-order kinetics with activation parameters AH, 127 and 122 kJ mol 1 and AS1, 35 and 13 J/K 1 mol 1.

ISP contain four basic core structures which have been characterized crystallographically both in model compounds and in ISP (Rao and Holm, 2004). These are (Figure 13.15), respectively, (A) rubedoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom bound to four Cys residues—the iron atom can be in the +2 or +3 valence (B) rhombic two-iron-two-sulfide [Fe2-S2] clusters—typical stable cluster oxidation states are +1 and + 2 (the charges of the coordinating cysteinate residues are not considered) ... 

The minimal functional module in [NiFe] hydrogenases always contains the NiFe(CN)2(CO) site plus the proximal [4Fe-4S] cluster. The active site in [Fe] hydrogenases consists of the Fe-Fe site linked to a [4Fe-4S] cluster. Oxidation of the hydride is either an action of the dinuclear site alone, or a concerted action of this site plus the proximal cluster. 

The dynamics of signal increase at the beginning of relaxation phase reflects the rate of cluster oxidation catalyzed by the gas sensitive substance. The higher is the slope of the face front of relaxation maximum, the faster is the rate of the process. 

In the reactions described above, only the AuPR3 fragment of the gold phosphine precursor has added to the cluster. Oxidative addition of Au(PR3)X [where X = Cl (90,188, 191), Br (90, 188), I (179), SCN (104), NCO (231), C2R (92, 97), etc.] has also been observed to occur, yielding clusters in which the transition metal is in a higher oxidation state. This type of reaction was first investigated by Nyholm and coworkers (90, 232), and some examples are given below ... 

Nilius N, Ernst N, Freund H-J. On energy transfer processes at cluster-oxide interfaces silver on titania. Chem Phys Lett. 2001 349 351-7. 

Considering all of the manganese catalases together, there have been four cluster oxidation levels that are established Mn(II,II), Mn(II,III), Mn(III,III), and Mn(III,IV). The as-isolated enzyme contains a mixture of these states. The Mn(II,II) enzyme can be prepared by the addition of hydroxylamine to the isolated enzyme. If hydrogen peroxide is added to this sample, without removing the hydroxylamine, the enzyme is converted to the Mn(III,IV) form however, if the hydroxylamine is first... 

For many years, strong metal-metal interactions were largely regarded as chemical curiosities of little interest exemplified by some suboxides of the heavier alkali metals and a few 4 or 5d elements, principally Mo, Nb, and Re. However, research since 1978 has shown that the phenomenon is much more widespread than previously thought. A few reviews covering work through about 1985 on Mo and Nb cluster oxides exist. 

Perhaps the best characterized example of a subsite differentiated [4Fe-4S] protein is aconitase, which catalyzes the citrate-isocitrate isomerization in the citric acid cycle (257). Aconitase isolated aerobically is inactive and contains a [3Fe-4S] cluster. Activity is restored by incubation with Fe and this also reconstitutes the [4Fe-4S] cluster. Oxidation of the core results in loss of the fourth iron atom, regenerating the [3Fe-4S] form. Mossbauer studies have demonstrated that only one of the four iron sites is exchanged (258). X-ray studies on both [3Fe-4S] and [4Fe-4S] forms of pig heart aconitase 258a) showed that insertion of iron into [3Fe-4S] occurs isomorphously. The positions of the common atoms in the two forms of the core agree to within 0.1 A, supporting the view of the [3Fe-4S] cluster as an iron-voided cubane. A similar result was obtained for the seven iron ferredoxin from Azo-... 

Because the cluster-derived catalysts on Si02 were prepared in higher metal dispersions regardless of the precursors used, such marked enhancement of CO conversion and higher selectivities toward alcohols on Fe-containing Ir catalysts are likely to be associated with the generation of IrFe adjacent sites, possibly located at the cluster-oxide support interface, active not only for... 

Transition metal catalysts, specifically those composed of iron nanoparticles, are widely employed in industrial chemical production and pollution abatement applications [67], Iron also plays a cracial role in many important biological processes. Iron oxides are economical alternatives to more costly catalysts and show activity for the oxidation of methane [68], conversion of carbon monoxide to carbon dioxide [58], and the transformation of various hydrocarbons [69,70]. In addition, iron oxides have good catalytic lifetimes and are resistant to high concentrations of moisture and CO which often poison other catalysts [71]. Li et al. have observed that nanosized iron oxides are highly active for CO oxidation at low tanperatures [58]. Iron is unique and more active than other catalyst and support materials because it is easily reduced and provides a large number of potential active sites because of its highly disordered and defect rich structure [72, 73]. Previous gas-phase smdies of cationic iron clusters have included determination of the thermochemistry and bond energies of iron cluster oxides and iron carbonyl complexes by Armentrout and co-workers [74, 75], and a classification of the dissociation patterns of small iron oxide cluster cations by Schwarz et al. [76]. 

Griffin JB, Armentrout PB (1997) Guided ion-beam studies of the reactions of Fe < > (n= 1-18) with CO Iron cluster oxide bond energies. J Chem Phys 107 5345... 

It has also been well established that metals in the very finely divide state are unusually reactive. For example, Parmigiani and co-workers [72] have demonstrated that at low oxygen pressure, supported platinum clusters oxidize at room temperature, whereas the bulk metal reacts at the same pressure only around 800 K. Clearly, the active sites, based on small particle sizes and high defect densities, have implications aside from electrochemistry. 

Condensation of (Bu4N)[Mo(CO)4l3] monomers to M04 clusters occurs in refluxing chlorobenzene. Extraction with 1,2-dichloroethane yields (Bu4N)2[Mo4lioCl], with one unpaired electron in this Mo" cluster . Oxidation of the monomeric Mo(II) reagent with I2 as in reaction (k) yields a butterfly-type [Mo4lii] derivative best viewed as a portion of the familiar [MeXs] octahedral unit, which has had two cis M and one X excised . 

In general, the good quality of the substrate is essential to analyze in detail the properties of the supported nanoclusters. In fact, as we will discuss below, the cluster-oxide interaction is such to influence the shape and the electronic structure of the deposited cluster. A deep understanding of the cluster properties needs therefore a similarly profound knowledge of the interface bonding. Furthermore, a precise knowledge of the surface structure is essential to design realistic theoretical models. 

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