Clusters oxidation-reduction reactions

Apart from reactions involving the oxidative addition of oxygen, the usual oxidation-reduction reactions have also been observed for binuclear technetium clusters with multiple M-M bonds (in this case they must be accompanied by a change in the multiplicity of the Tc-Tc bonds up to their complete dissociation)11 (15,16) [11,80],... 

NADH dehydrogenase and succinate dehydrogenase also contain Fe atoms that are bound by the S atoms of cysteine residues of the protein, in association with additional, inorganic sulfide atoms. Structures of these complexes are shown in figure 10.19. Succinate dehydrogenase has three iron-sulfur centers, one with a [2Fe-2S] cluster, one with [4Fe-4S], and one with a cluster containing 3 Fe atoms and 3 (or possibly 4) sulfides. Iron-sulfur centers undergo one-electron oxidation-reduction reactions. 

Figure 18.13. Iron-Sulfur Clusters. (A) A single iron ion hound hy four cysteine residues. (B) 2Fe-2S cluster with iron ions bridged by sulfide ions. (C) 4Fe-4S cluster. Each of these clusters can undergo oxidation-reduction reactions.

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 induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

For mixed lanthanide-transition metal clusters, Yukawa et al. have synthesized an octahedral [SmNi6] cluster by the reaction of Sm3+ and [Ni(pro)2] in nonaque-ous medium [66-68]. The six [Ni(pro)2] ligands use 12 carboxylate oxygen atoms to coordinate to the Sm3+ ion, which is located at the center of an octahedral cage formed by six nickel atoms. The coordination polyhedron of the central Sm3+ ion may be best described as an icosahedron. The [SmNir, core is stable in solution but the crystal is unstable in air. The cyclic voltammogram shows one reduction step from Sm3+ to Sm2+ and six oxidation steps due to the Ni2+ ions. Later, similar [LaNis] and CjdNif> clusters were also prepared. 

Reduction of the pentaalkyl-1,5-dicarba-closo-pentaboranes(5) 44 with elemental K opens the cluster and subsequent reaction with I2 enforces oxidative fusion of two C2B3 units to give a C4B6 cage (Scheme 3.2-36). The peralkylated hexaboraadamantane derivatives 70 (R = Me, Et X-ray structure analysis for R = Me [89]) rearrange irreversibly into the carboranes 71 with a wido-C4B6 framework [89] (X-ray structural analysis for R = Et [90]). The structure of the nido-C Bf, cluster is fluxional in solution [91]. 

Since this reaction requires no oxidation-reduction chemistry, the finding 15 years ago that aconitase is an Fe-S protein was quite unexpected. Until recently a paradigm for Fe-S proteins has been that they function primarily in reactions requiring electron transfer. Accordingly, the central question in the study of aconitase for the past several years has been What is the function of its Fe-S cluster ... 

For example, opening of the cluster skeleton from closo to nido to arachno will accompany the addition of electron pairs, whether by reduction reactions or by addition of Lewis base molecules that do not dislodge other Lewis bases. Cluster closing is expected to accompany the removal of electron pairs, whether by oxidation or by loss of a substituent together with the electron pair by which it was formally bound to the cluster ... 

Many of the E-M cluster complexes are versatile at undergoing oxidation and/or reduction reactions. As noted in the previous section, protonation of many E-M complexes, especially the anionic ones, may result in oxidation of the compound. A common outcome is simple E-M or M-M bond cleavage. This is illustrated in Eq. (253). It is this process that is responsible for the production of [Bi Co(CO)4 4] upon treatment of Bi[Co(CO)4]3 with Cp2Co as in Eq. (254).64-75 In a few cases, oxidation or reduction proceeds with internal bond breakage or formation as anticipated by the... 

Other biomimetic reactions are based on the catalytic properties of metal ions. Many enzymes require metal ions that function, in one way or another, in oxidation-reduction processes. The wide range of such metal-ion reactions precludes mentioning more than a few in addition to the iron-porphyrin class, and in addition to chlorophyll, a number of enzymes require cobalamin as cofactor ferridoxin and high-potential iron proteins require iron-sulfur clusters, and nitrog-... 

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