Investigation of the thermal properties of a series of copper selenide cluster molecules

The thermal properties of a series of the six copper selenide cluster molecules [Cu26Se13(PEt2R)(14)] (R = Ph, Et), [Cu44Se22(PEt2Ph)(18)], [Cu70Se35(PEt2R)(23)] (R = Ph, Et) and [Cu140Se70(PEt3)(34)] have been investigated along with a characterization of their thermolysis products. For all cluster molecules the phosphine ligand shells are cleaved at temperatures between 60 degrees C and 200 degrees C depending on the experimental conditions (Helium gas flow or vacuum), the type of phosphine ligand and the size of the cluster molecules. The residues of the thermal treatment to 150 degrees C were found to be nanostructured Cu2Se with crystallite sizes of approximately 12-16 nm which means that the 1-2.5 nm sized cluster cores of the precursor cluster molecules simultaneously grow during this process. A mixture of processes and factors including the strength of the Cu-P bond, the boiling point of the phosphine ligand as well as the thermal stability of the copper selenide clusters against formation of the bulk material determine the shape of the TGA curves. We found some indications in the TGA along with differential scanning calorimetry (DSC), which suggest that the cleavage of the phosphine ligands is probably mostly determined by the tendency of the metastable clusters to form the bulk material meaning their thermodynamic stability and not by the strength of the Cu-P bond. The data for the series of PEt3 ligated clusters reveal that this stability is dependent on the size of the cluster molecules.