Cluster oxidative coupling

CO oxidation reaction. The spectral changes in Cluster C are followed hy Cluster B reduction with a rate constant that is similar to the steady-state value. On the other hand, the rate of formation of the characteristic EPR signal for the CO adduct at Cluster A is much slower. Its rate constant matches that for acetyl-CoA synthesis, hut is several orders of magnitude slower than CO oxidation. Therefore, it was proposed that the following steps are involved in CO oxidation (1) CO hinds to Cluster C, (2) EPR spectral changes in Cluster C are accompanied hy oxidation of CO to CO2 hy Cluster C, (3) Cluster C reduces Cluster B, and (4) Cluster B couples to external electron acceptors (133). 

The polyanion [Ge4s] (Fig. 6c) represents a pentamer of [Ge9] and arises from an oxidative coupling of five Ge9 clusters in the reaction of an ethylenedia-mine solution of K4Ge9 and Au(PPh3)Cl. The anion coordinates to four Au" and one cations. In the structure, four Ge9 deltahedra are retained while the fifth has opened up to covalently link the four intact subunits. 

Soluble heptapnicanortricyclane anions [Pny] (Fig. 3a) and trishomocubane-shaped (ufosane-like) anions [Pnn] (Fig. 3d) are very common and known as in the binary solids for Pn = P, As, Sb (Table 2). Oxidative coupling of these monomers leads to the dimers [Pny-Pny]" and [Pnn-Pnn]" for Pn = P and As (Fig. 3b, e), which - as observed for the tetrel element clusters - have an external homoatomic bond, but in this case the structures of the monomeric units are fully retained upon dimerization. A trimeric oxidative coupling product of [Py] is the... 

High surface area hexagonal mesoporous Ge also can be prepared with oxidative self-polymerization chemistry of [Ge9] clusters [48]. This synthetic route does not require external oxidants such as ferrocenium or linking Ge(lV) centers and occurs in the presence of cationic surfactant (iV-eicosane-A ,A -dimethyl-A -(2-hydroxyethyl)ammonium bromide, EDMHEABr) as stmcture-directing agent. The polymerization reaction proceeds through the slow oxidative coupling of (Ge9)-clusters and seems to be accompanied by a two-electron process (2). The electron acceptors in this case appear to be the surfactant molecules or the solvent. 

The industrial synthesis of vinyl acetate [14] via palladium-catalyzed oxidative coupling of acetic acid and ethene using direct 02 reoxidation has already been mentioned (Scheme 3, d). Some NaOAc is required in the reaction medium, and catalysis by Pd clusters, as alternative to Pd(II) salts, was proposed to proceed with altered reaction characteristics [14]. Similarly, the alkenyl ester 37 (Table 5) containing an isolated vinyl group yields the expected enol acetate 38 [55] whereas allylphenol 39 cyclizes to benzofuran 40 with double bond isomerization [56]. 

The enzyme responsible for the photolysis of water in plants is a multisubunit membrane protein (Fig. 5-22 Klein et al., 1991). Four manganese ions, probably as a tetranuclear cluster, are thought to act as a charge accumulating system and as the active site for water oxidation. Both calcium and chloride ions are also required for activity (Babcock, 1987 Ghantokakis and Yocum, 1990). The water oxidation centre (WOC) contains a total of four Mn atoms and causes the oxidative coupling of two water molecules by a currently unknown mechanism. 

The reaction of [Rh7(CO)ig] with trifluorosulfonic acid also results in selective oxidative coupling of two Rh, clusters to give [Rh,4(CO)26] -Even though neither of the protonated species [Rh7(CO)i6H,3 (x = 1, 2) were detected by NMR spectroscopy over a range of temperatures during the course of the reaction (216), the reaction probably follows a similar pathway to that of the hexanuclear dianions. 

In chloroplasts, oxidized thioredoxin is reduced by ferredoxin in a reaction catalyzed by ferredoxin-thioredoxin reductase. This enzyme contains a 4Fe-4S cluster that couples two one-electron oxidations of reduced ferredoxin to the two-electron reduction of thioredoxin. Thus, the activities of the light and dark reactions ofphotosynthesis are coordinated through electron transfer from reduced ferredoxin to thioredoxin and then to component enzymes containing regulatory disulfide bonds (Figure 20.16). We shall return to thioredoxin when we consider the reduction of ribonucleotides (Section 25.3). 

The extensive series of studies on in vitro core formation reported by Harrison and co-workers (32, 63, 82, 140) and others (133) has led to the development of a three-step hypothesis for iron uptake. In the first step, iron entry through the channels, Fe + passes from the outside of the protein through the channels in the apoferritin coat to the interior cavity. The second step, nucleation, involves iron binding to groups on the inner surface of the protein in such a way that a small cluster of coupled Fe ions is formed. The final step, formation of the core, involves the extension of a small nucleating cluster by the addition and oxidation of Fe +. This stage is characterized by an initial catalytic phase, during which the small cluster rapidly expands, followed by a reduced rate of expansion once the core has attained a particular size (—1000-1500 iron atoms per molecule). 

Moreover, because no reversible equilibrium is established, only standard redox potentials may be compared. Among a series of S/S reference couples, the transition between the occurrence and the absence of reaction indicates the upper limit (cluster oxidation) or the lower limit (cluster reduction) of the standard potential value of the cluster to be measured. The critical nuclearity corresponding to this potential is determined by fitting the kinetics calculated by simulation with the experimental signal. 

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