Oxides with tetranuclear clusters

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... 

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 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 use of nitrite (NO ) as a source of metal nitrosyls is now well established as discussed in Section II,A3. One study of the reaction of PPN(N02) with carbonyl clusters of the Co triad revealed an alternative synthesis of [Co6N(CO)i5] and [Rh6N(CO)i5] "(i5). As shown in Eqs. (48) and (49), both the tetranuclear and hexanuclear starting materials for Rh lead to the same nitrido cluster. The reasonable yields for Rh by either route suggest that the product is somewhat of a thermodynamic sink. Consistent with this idea, Eq. (50) shows the use of gaseous nitric oxide itself to form the Rh nitrido cluster directly (J). 

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). 

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... 

Under various conditions Fe8(CO)i2 reacts with a wide range of nitrogen and oxygen bases to form tetra-, tri-, di-, or mononuclear carbonyl anions (153). The tetranuclear cluster is formed in the reaction of Fe3(CO)i2 with pyridine (195), pyridine iV -oxide and dimethyl sulfoxide (187) Fe3(CO)ff results from reaction with ethylenediamine... 

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