Iron clusters, reversible oxidation

Bulk electrolyses are used to prepare one-electron reduction or oxidation products. If cyclic voltammetry (CV) reveals reversible redox, the bulk preparation of the reduced (or oxidized) product may be attained. The overall electrode process may be different in controlled-potential electrolysis and in CV because of the time factor (see below). The iron cluster, (h -CjHjFeCO), in nonaqueous electrolytes undergoes a four-membered electron-transfer CV series through three steps. The potentials measured (in CHjCN/O.l M [n-Bu N] [PFJ) are ... 

The evidence for association of the iron atoms with platinum in samples similar to B has been discussed elsewhere in some detail (55). Incorporation of iron atoms into the platinum clusters is indicated by the agreement of the isomer shift for iron in the catalyst with that for iron in a PtFe bulk alloy of similar composition. In addition, it is found that the iron in samples such as B, after reduction at 500°C, exhibits a unique chemical behavior in that it can be reversibly oxidized and reduced at room temperature. This property appears to be characteristic of iron in association with metals such as platinum and palladium (54-57). It is not observed when the iron is present by itself on alumina. 

Sample C, containing iridium and iron, exhibits the same reversible oxidation-reduction behavior, indicating the incorporation of the iron into the iridium clusters. The isomer shift for sample C, however, was not in good agreement with the value of 0.38 mm sec- (58) expected for dilute iron in iridium alloys. This difference may reflect an unusual chemical state for iron which is associated with surface iridium atoms in clusters. A similar situation has recently been reported and discussed for iron-ruthenium catalysts (59). 

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... 

With the exception of the [Fe]-hydrogenases, all other hydrogenases catalyze the reversible oxidation of hydrogen gas. In addition to the [NiFe] and [FeFe]-centers, they contain iron-sulfur clusters as electron-transfer carriers and can use various physiological and non-physiological electron acceptors according to the general reaction ... 

Pyruvate ferredoxin oxidoreductase. Within Clostridia and other strict anaerobes this enzyme catalyzes reversible decarboxylation of pyruvate (Eq. 15-35). The oxidant used by clostridia is the low-potential iron-sulfur ferredoxin.320 3203 Clostridial ferredoxins contain two Fe-S clusters and are therefore two-electron oxidants. Ferredoxin substitutes for NAD+ in Eq. 15-33 but the Gibbs energy decrease is much less (-16.9 vs - 34.9 kj / mol. for oxidation by NAD+). 

A chemistry of cobalt-sulfide-thiolate molecular clusters comparable with that of iron systems has also begun to emerge. Treatment of [Co4( -SPh)6(SPh)4]2- with HS- in acetone affords the octanuclear cluster [Co8(ji4-S)6(SPh)8]4 isolated as its Pr4N+ salt.988 In MeCN solution the complex is red-purple with intense sulfur-core charge transfer bands which obscure the Co" d-d transitions. This behaviour contrasts with that of both mono- and poly-nuclear cobalt"-thiolate complexes, which all display LMCT bands below 440 nm and have well-developed v2 and v3 features. The [Co8(/j4-S6)]4+ core sustains reversible one-electron oxidation and reduction (E]l2 = —0.54, — 1.18 V, MeCN) and chemical reduction with sodium acenaphthylenide in THF gives [Co8(/r4-... 

Doubly bridged trinuclear clusters may be oxidized or reduced, but the stability of the one-electron product depends on the identity of the bridging group and on the terminal ligands bonded to the metals. The iron complex [Fe3(CO)2(/u-S)(/Li-SR)Cp3], the structure of which has not been fully defined, oxidizes reversibly in two one-electron steps. The monocation was synthesized by iodine oxidation of the neutral species but apparently decomposes to [ Fe(CO)( i-SR)Cp 2] (Section III,E) 148). 

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