Transition metals tetranuclear complexes

III. A Survey of Magnetochemical Data on First-Row d-Block Transition-Metal Tetranuclear Cluster Complexes... 

Ru and Os tetranuclear clusters, 6, 874 in transition metal complex electron counting, 1, 2-3 Eight-membered rings, via ring-closing diene metathesis,... 

Using the same fundamental approach as has been noted for the preparation of dinuclear complexes, larger transition-metal ion-centered dendrimers have been constructed. In addition to the tetranuclear species already noted,1218 mono-, di- and triruthenium centered species have been prepared using the substituted tris(2,2,-bipyridyl) bridging ligand... 

Direct evidence for the formation of a metalla-2-oxetane in a metal mediated oxidation reaction was recently reported by Sharp and coworkers [121]. The oxidation of norbornylene by the tetranuclear platinum(II) p-oxo complex 1 yielded nearly quantitative formation of the platina-2-oxetane 2 shown in Fig. 18. The compound 2 could be isolated and the structure was identified with X-ray structure analysis. However, it is unclear if the reaction of this late transition metal complex takes place via [2-1-2] addition of a metal-oxo moiety across the C=C double bond. The authors write that the formation of the C-0 bond allows considerable speculation on this process. DFT calculations are underway to help differentiate the various possibihties [121]. 

A second effect is to be found in the acceleration of the initial activation of the nitroarene. We could observe this effect on the reactivity of both Ru3(CO)i2 and Ru(CO)s. We have previously mentioned that Han and others could not evidence any accelerating effect of chloride in the reaction between Ru3(CO)i2 and nitrobenzene [157]. However, from several studies, partly conducted by us yide infra), it has become now clear that the initial aetivation of nitroarenes from transition metal complexes always proceeds through an intermediate electron transfer from the complex to the nitro compound. Thus, any modification of Ru3(CO)i2 which increases its oxidability, specifically the introduction of the anionic ligand chloride, should increase its reactivity towards nitroarenes. However, the reaction between Ru3(CO)i2 and unsubstituted nitrobenzene requires a relatively high temperature even in the presence of chloride and at temperatures over 50°C and under a nitrogen atmosphere the initially formed [Ru3(CO)u(Cl)] is rapidly converted to a less reactive tetranuclear cluster, [Ru4(CO)i3(p-Cl)] [179]. 

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