Sulfur-Bridged Copper Clusters

2 Sulfui Bridged Copper Clusters The reactions yielding the phosphine-coated sulfur-bridged copper clusters that have been isolated and structurally 

In contrast to the formal condensation, 2 x CU12 CU20 — 4 Cu, that leads to the sharing of a CU4 rectangular face in 6, a conjunction can be observed via two CU3 

S black spheres P white spheres. Phosphine ligands that are formally lost during the dimerization-condensation are marked by a cross. 

In the molecular structure of 8, the start of the transition is observed, from the so-called small copper sulfide clusters to the middle-sized species. Both cluster sizes possess spherical Cu S—P frameworks with copper centers near the Sn polyhedral edges, but the middle-sized examples show some new structural features coordination numbers other than four for sulfur atoms, and copper centers that bind to more than two sulfur neighbors. 

For naked clusters, the stabilization energies increase continuously with the cluster size, which proves the investigated molecules to be thermodynamically unstable species in vacuo, and thus must be stabilized by ligands. Moreover, the increment of the stabilization energy per monomer unit decreases with increasing cluster size as expected, since it should theoretically converge with infinite cluster size - that is, solid CU2S. 

---

Scheme 3.3 Survey of the synthesis of sulfur-bridged copper clusters protected by terminal...

In the case of selenium-bridged copper clusters, it was found that there is a structural transition from molecular spherical structures towards cutouts of the bulk structure when the clusters reach 70 copper atoms. An analogous structural transformation has not yet been found in the case of sulfur-bridged silver clusters. For etample, the dication [Ag7oS2o(SPh)28(dppm)2o] in 94 shows a sheD-like sulfur substructure that consists of an inner Sg and an outer S40 polyhedron (Figure 3.73). 

Fig. 12. Schematic diagram of metal binding by human CCS. hCCS domains 1, 2, and 3 are labeled with roman numerals. Cysteine residues are designated as S. The disulfide bond in domain 2 is indicated by S-S. (a) Cobalt binding to hCCS. Electronic absorption spectra indicate that two equivalents of Co(II) bind per hCCS monomer, one through three or four cysteine residues in a tetrahedral geometry, and one with a geometry similar to that found in the zinc site of SODl (see text) (Zhu et al., 2000). (b) Copper binding to hCCS. XAS indicates that two Cu(I) ions bind per hCCS monomer in a sulfur-only liganding environment, with an additional heavy atom scatterer peak suggesting the presence of a /t2-bridged dicopper cluster (Eisses et al., 2000).

The acetogenic bacterium Moorella thermoacetica contains a cofactor comprising iron, sulfur, copper and nickel in the enzyme carbon monoxide dehyd-rogenase/acetyl-CoA, and Drennan et al. proposed a new role for Cu in biology on the basis of their structure determination at 2.2 A resolution.81 The Cu bridges the Fe4S4 cluster and Ni centre via three p2-S atoms, and hence the reported EPR... 

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. 

The crystals that are obtained from the cluster formation reactions are intensely colored. In fact, the intensity of the color increases when going from sulfur- to selenium- to tellurium-bridged compounds (see below), as might be expected for an increase in the covalent or (semi-) metallic binding properties. Small copper sulfide and selenide clusters form light red, orange, or purple crystals, but with increasing cluster size the color varies from dark red to reddish-black to (finally) black with a metallic sheen. The optical spectra of some copper selenide cluster compounds have been studied by means of solid-state UV-visible spectroscopy. 

Tetranuclear copper complexes have tetrahedral, square-planar, or butterfly structures. The compound Cu4 (Pr 0)2PS2 4 has a distorted tetrahedral skeleton of the metal atoms with one sulfur atom symmetrically bonded to two copper atoms, while the other sulfur atom forms a bond with the third copper atom. Copper and silver form many clusters containing alkyl and aryl ligands (See Chapter 4). The complex Cu4(CH2SiMe3)4, like Cu4 N(SiMe3)2 4, has a square-planar structure with bridging alkyl ligands. In the alkyl compound, in contrast to the amide one, there are Cu —Cu bonds. 

---