Variations in the copper(I)-copper(I) distances in multinuclear clusters with identical coordination geometries. Short metal-metal contacts induced by oligomerization

Abstract

Factors contributing to the variations in the Cu(I)-Cu(I) distances in two clusters with identical ligand and coordination geometries have been analyzed. While the hexamer, 4, exhibits metal-metal distances in the range 2.81-3.25 Á, shorter contacts are found in the corresponding tetramer, 3 (2.60-2.77 Á). EHT calculations reveal relatively little attractive interactions in the corresponding Cu44+ and Cu66+ cores. Introduction of the ligands lowers the reduced overlap populations between the metals further. MNDO calculations with model electrophiles have been carried out to determine the bite angle requirements of the ligands. These are satisfactorily met in the structures of both 3 and 4. The key geometric feature distinguishing 3 and 4 is the Cu-S-Cu angle involving the bridging S~unit. In 4, the corresponding angles are about 90°, while the values in 3 are smaller (70-73°). Wider angles are computed to be energetically favored and are characterized by an open three-center bond and a long Cu-Cu distance. The bridging angles are suggested to be primarily constrained by the mode of oligomerization. Implications of these results for the stability and reactivity of these clusters and for short metal-metal distances in d10 systems in general are discussed.

Route to Cluster Helicates

“…Currently, there is extended controversy over the existence of Cu···Cu interactions in Cu I cores, such as those found in the present case. [23] The distances between the metal centers are close to the sum of the van der Waals radii for two copper atoms (2.80 ) [24] and are in agreement with the proposed existence of Cu···Cu interactions. [25] These interactions would mean that the coordination number of the Cu I ions in 3 should increase from 3 to 6.…”

Route to Cluster Helicates

“…Interestingly, the distances between the “bridged” Cu I ions are shorter (2.672–2.706 Å) than those between the “unbridged” Cu I ions (2.881–2.883 Å). Currently, there is extended controversy over the existence of Cu⋅⋅⋅Cu interactions in Cu I cores, such as those found in the present case 23. The distances between the metal centers are close to the sum of the van der Waals radii for two copper atoms (2.80 Å)24 and are in agreement with the proposed existence of Cu⋅⋅⋅Cu interactions 25.…”

Checking the Route to Cluster Helicates

“…[7] The intermetallic Cu···Cu distances in the [Cu 4 S 4 ] core range from 2.64 to 2.86 Å, and these are consistent with the proposed existence of Cu-Cu interactions. [23][24][25] Consideration of these intermetallic connections would indicate that the coordination number of the copper(I) ions should increase from 3 to 6. The four copper ions of the metallic central core define a distorted tetrahedron.…”