Iron cluster patterns

The electrochemical properties of the two types of iron-cluster-derivatized AuNPs resemble that of the monomeric tetrairon cluster, except that they additionally show adsorption due to their large size, and evidence that all the Fe4 clusters are active at about the same potential, thereby indicating that they are sufficiently remote from one another to behave independently. The changes in the cyclic voltammetric pattern caused by the addition of H2P04 and ATP2- oxoanions evidence recognition features that are very different from those obtained with dendritic ferrocene exoreceptors. In particular, the results show that with these iron-cluster-derivatized AuNPs... 

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]. 

Niobium and cobalt clusters exhibit size-sensitive reactions with nitrogen with a reactivity pattern similar to that observed for hydrogen. The reactivity of rhodium clusters (n = 1-12) toward N2 has also been studied. In this case the atoms through the tetramer appear to be inert, with reactivity turning on at Rhj. Maximum reactivity occurs at Rh7, and subsequently drops off by roughly a factor of 2 in going from Rh, to Rh,. Iron clusters appear to be nearly unreactive toward N2. Attempts to induce low-pressure ammonia synthesis on gas-phase iron clusters indicate that hydrogenated iron clusters Fe H are also unreactive toward N2. ... 

Although the concept, where the active sites are seven irons atomic cluster containing C7 atom has been accepted commonly,there have ever been various active sites model, including three iron atomic pattern,six iron atomic pattern etc. Hence, how many iron atoms are being accumulated actually on an active site is still out of the definite conclusion. Besides, irrespective of whether there are different active sites or not, and whether the adsorption forms and reaction rates of ammonia formation on various active sites are the same or not, these also are yet out of the unanimous conclusions. 

D. L. Massart, L. Kaufman and K.H. Esbensen, Hierarchical non-hierarchical clustering strategy and application to classification of iron-meteorites according to their trace element patterns. Anal. Chem., 54 (1982) 911-917. 

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

The Tte of the 3Fe-4S centre in succinate ubiquinone reductase between 4 and 8 K is decreased by interaction with paramagnetic cytochrome b.98 To mitigate the impact of spectral diffusion the relaxation times were measured by a picket-fence sequence with 100 pulses. Analysis of the powder pattern distribution of relaxation times indicated that the anisotropic dipolar interaction dominated over isotropic scalar interaction and a lower limit of 10 A was estimated for the distance between the iron-sulfur cluster and the heme. 

In this chapter we wish to focus on the synthesis, structural chemistry, and reactivity of some iron-group phosphido-bridged carbonyl clusters containing sideways-bound acetylides. The role of the PR2 group here is as a stable, unreactive, but flexible bridging coligand which facilitates exploitation of the unusual reactivity patterns for these /a2-7 2- and /Lt3-rj2-bound hydrocarbyl groups. 

Each major class of oxygenase and monooxygenase enzyme has a vast literature. A general pattern, however, is that most such enzymes have either Fe or Cu as active site metals, often in mrdtinuclear cluster centers. For iron, the Fe(II)/(III) couple is often accessed with Fe(IV) present in certain cases. For copper, the typical states are Cu(I)/(II) with Cu(III) also present in some cases. Since O2 is a 4e oxidant, while the low-valent metals are 1 or 2e reductants, a cluster site can help attain compatibility of oxidant and reductant by allowing the site to take up more than one electron. 

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