Selenium-Bridged Copper Clusters

Clusters 14 and IS are topologically related, as both contain a centered icosahedron 

The molecular structure of 16 differs from those of 14 and 15, since a very irregular Sei3 deltahedron forms the selenium substructure. Twelve ofthe copper centers and ten Cu-PCy3 fragments are bonded to the Sei3 polyhedron in such a way that they are again three- or four-coordinate. Unlike the situation in 14 or IS, it is also possible to observe four copper atoms that show a near-linear coordination to two selenium 

The molecular structures of both 22 and 26 display idealized C3 symmetry that is not realized in the crystal (22 Ci symmetry, 26 C2 symmetry). Along the idealized threefold axis, a selenium ligand is found at the top and at the bottom of the 

The stmctural isomers 23 and 24 are obtained via completely different reaction conditions, including the use of phosphine ligands, different molar ratios of CuOAc PR2R, and the exposure of the reaction mixtures to different final reaction temperatures. 

Although only 11 additional copper atoms are observed in compounds 28-31 [17, 

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Selenium-Bridged Copper Clusters The reactions yielding all PR2R -coated selenium-bridged copper clusters that have been isolated and structurally characterized to date, in order of increasing cluster size, are summarized in Scheme 3.4. 

Scheme 3.4 Survey of the synthesis of selenium-bridged copper clusters protected by terminal phosphine ligands.

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). 

Thermal Behavior of Selenium-Bridged Copper Clusters... 

The copper atoms in the vast majority of the clusters can be assigned a formal charge of +1, while the chalcogen ligands are formally viewed as E or RE groups. Some of the selenium-bridged species, however - and nearly all copper telluride clusters - form nonstoichiometric compounds that display mixed valence metal centers in the formal oxidation states 0 and +I or +I and +11. These observations correlate with those made for the binary phases CU2S, Cu2 xSe, and Cu2- Te [38-40]. 

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. 

The selenium atoms find themselves in varying bridging situations, with connectivities ranging from to pi. With the exception of three atoms within the cluster framework, all selenium atoms are situated on the cluster surface. Only 22 peripheral copper atoms are coordinated by PEt3 ligands which, in turn, effectively screen the cluster from the outside environment. 

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