Synthesis,structure, and thermal behavior of [Cu70Se35(PEt2Ph)24]

We investigated the molecular and crystal structure as well as the thermal behavior of [Cu₇₀Se₃₅(PEt₂Ph)₂₄] using single-crystal X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), X-ray powder diffractometry, and thermal gravimetric analysis (TGA). The use of a He-cooled sample head in HRTEM makes possible for the first time the investigation of semiconductor cluster molecules under the extreme conditions of the experiment and, thus, comparison with the results of single-crystal X-ray diffraction. The lattice spacings and packing symmetry of the three-dimensional cluster molecule superlattice found in HRTEM are in good agreement with the ones determined by X-ray structure diffraction. The atomic ordering inside the cluster molecules, which resembles a hexagonal packing of the selenium atoms, could not be resolved in the HRTEM measurements, since upon irradiation with the electron beam the clusters start to move after a few seconds and the images go out of focus. This movement is possibly affected by cleavage processes of the phosphine ligands, as could be observed in thermal gravimetric analysis under comparably mild conditions. Powder diffraction measurements show that this cleavage of ligand molecules is accompanied by the formation of larger copper selenide cluster cores and ends up at high temperatures in the formation of bulk α-Cu₂Se.