Tetranuclear clusters oxides

Many bacterial ferredoxins have two such clusters, each of which can be reduced to a paramagnetic state. In other proteins, 4Fe4S clusters can be oxidized to a paramagnetic state. The tetranuclear clusters in these two types of proteins are similar in structure, but they functionally shuttle between different reduction states. In both cases, each iron atom is additionally coordinated by four cysteinyl sulfur ligands. 

Tetranuclear clusters Pt4(CO)5L4 and Pt4(CO)5L3 have been prepared from the reaction of CO on the trinuclear species.336,347 For tetrametallic compounds examples are found with Pt2Mo2, Pt2Co2 and Pt2M2 (M = Cr, Mo, w).348,349 These latter complexes show an irreversible two-electron reduction leading to the rupture of the metallic core into identified fragments. These complexes are of the type [PtM(jU3-CO)(ji2-CO)2Cp(PPh3)]2, and the first two oxidation waves indicate one-electron processes.349 A tetrametallic carbonyl cluster PtOs3(fi-H)2(CO)io(PR3) has also been characterized.350... 

The Cuz active site consists of four copper ions, arranged in a distorted tetrahedron and coordinated by seven histidine residues and one hydroxide anion. This site was detected in nitrous oxide reductase [16, 17] (Figure 5.1g) and is involved in the reduction of N20 to N2. The copper ions in the tetranuclear cluster are bridged by an inorganic sulfur ion [18], which until recently was believed to be a hydroxide anion. Three copper ions are coordinated by two histidine residues, whereas the fourth is coordinated by only one, thus leaving a binding site for the substrate. 

Recently, a copper-catalyzed synthesis of trimethyl- 1,4-benzoquinone, a key intermediate in the industrial synthesis of vitamin E, has been reported (Eq. 11) [49]. In the proposed mechanism, a tetranuclear cluster [Cu4(a -O)Cl10], isolated from the reaction mixture, deprotonates phenol and oxidizes it to a copper-bound phenolate radical, which reacts with dioxygen. 

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 chemistry of reduced Nb and Ta hahdes is rich in clusters with various structures. The metal atoms assemble with metal metal distances close to those in the metal into triangular and tetranuclear clusters but the dominant structural motif is that of the octahedral M6X12 and NbeIg types. Binary, ternary, and quaternary compounds aU crystallize in that type. The Me clusters are characteristic of the chemistry of the lower oxidation states of Nb and Ta, although not restricted to them. These electron-deficient clusters are based on metal ions with average oxidation numbers between III and I. 

The tetranuclear cluster [Os4H3(CO)j2] (70) is irreversibly oxidized in acetonitrile ( pk = 0.44 V vs Ag/AgNOa), and bulk electrolysis provides [Os4H3(CO)i2(NCMe)] (71) in quantitative yield. The cation, which is the first osmium cluster with a butterfly structure unsupported by bridging ligands, may also be prepared from 70 and [NO] (175). 

Tetranuclear Clusters Although common among carbonyl clusters, far fewer examples of tetranuclear clusters are found among the halides and oxides. One example noted previously is W4(OR)12 which... 

Because the tetranuclear cluster of the OEC is most likely not pho-toactivated in and of itself during turnover, manganese systems that oxidize water without being irradiated first are of interest with respect to understanding the function of this complex enzyme. One system that oxidizes water was prepared and studied by Matsushita and co-workers (452, 453). These complexes are mononuclear Mn L Cb complexes, where L is a bidentate Schiff base comprised of 5-nitrosali-cylaldehyde and an alkylmonoamine, with structures that should be similar to MnIV(salpn)Cl2 (Fig. 33). These researchers showed that... 

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