Dinuclear clusters

Metal-metal multiple bonds in dinuclear clusters. F. A. Cotton and R. A. Walton, Struct. Bonding (Berlin), 1985, 62,1 (188). 

Cotton FA, Walton RA (1985) Metal-Metal Multiple Bonds in Dinuclear Clusters. 62 1-49... 

There are hundreds of iron-containing enzymes. In general, the iron can exist as (a) a mononuclear site, in which it is coordinated by a tetrapyrrole structure (hemes) or strictly by amino acid residues that donate oxo, nitrogen, or sulfur ligands (b) a dinuclear site in which the irons are bridged by oxo, nitrogen, or sulfur coordination (c) a trinuclear site as in the 3Fe-4S clusters or (d) a tetranuclear site as in the [4Fe-4S] clusters. 

Reactions of clusters with mononuclear or dinuclear metal complexes frequently provide a method of expanding the metal core nuclearity under controlled conditions. The majority of medium- and high-nuclearity homometallic clusters has been prepared from lower-nuclearity cluster precursors by thermolyses ("heat-it-and-hope ) reactions. This is less true of the heterometallic clusters in this... 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

Dobbek H, L Gremer, R Kiefersauer, R Huber, O Meyer (2002) Catalysis at a dinuclear [CuSMo(=0)OH] cluster in a CO dehydrogenase resolved at 1.1-A resolution. Proc Natl Acad Sci USA 99 15971-15976. 

Density functional theory (D FT) modeling calculations show that a dinuclear gold(I) amidinate complex is less stable than the tetranuclear gold(I) amidinate cluster, [Au4(HNC(H)NH)4]. However, replacing C by Si in the backbone reduces ring strain and makes the energies similar. Figures 1.21 and 1.22 [39]. 

The [FeFe]-hydrogenases have been known since the 1930s [305], and Warburg recognized sulfur-coordinate iron as an essential element of the enzyme [306]. The [FeFe]-hydrogenase from Clostridium pasteurianum accommodates 20 iron atoms organized in one [2Fe-2S] cluster, three [4Fe S] clusters and the so-called H-cluster which is a [4Fe-4S] cluster covalently linked to a dinuclear [FeFe]... 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

The [2Fe-2S] dinuclear cluster itself is a classical example of such a system paramagnet in the reduced (1+) state it is made up of a high-spin ferric ion (S = 5/2) and a high-spin ferrous ion (S = 2), coupled antiparallel into a system spin S = 1/2. Under the simple model of Equations 11.15 and 11.16 the system g-values are (Gibson et al. 1966)... 

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