Mixed dysprosium-lanthanide nitride clusterfullerenes DyM2N@C80-Ih and Dy2MN@C80-Ih (M = Gd, Er, Tm, and Lu): synthesis, molecular structure, and quantum motion of the endohedral nitrogen atom

Abstract

Systematic exploration of the synthesis of mixed-metal Dy–M nitride clusterfullerenes (NCFs, M = Gd, Er, Tm, Lu) is performed, and the impact of the second metal on the relative yield is evaluated. We demonstrate that the ionic radius of the metal appears to be the main factor allowing explanation of the relative yields in Dy–M mixed-metal systems with M = Sc, Lu, Er, and Gd. At the same time, Dy–Tm NCFs show anomalously low yields, which is not consistent with the relatively small ionic radius of Tm³⁺ but can be explained by the high third ionization potential of Tm. Complete separation of Dy–Gd and Dy–Er, as well as partial separation of Dy–Lu M₃N@C₈₀ nitride clusterfullerenes, is accomplished by recycling HPLC. The molecular structures of DyGd₂N@C₈₀ and DyEr₂N@C₈₀ are analyzed by means of single-crystal X-ray diffraction. A remarkable ordering of mixed-metal nitride clusters is found despite similar size and electronic properties of the metals. Possible pyramidalization of the nitride clusters in these and other nitride clusterfullerenes is critically analyzed with the help of DFT calculations and reconstruction of the nitrogen inversion barrier in M₃N@C₈₀ molecules is performed. Although a double-well potential with a pyramidal cluster structure is found to be common for most of them, the small size of the inversion barrier often leads to an apparent planar structure of the cluster. This situation is found for those M₃N@C₈₀ molecules in which the energy of the lowest vibrational level exceeds that of the inversion barrier, including Dy₃N@C₈₀ and DyEr₂N@C₈₀. The genuine pyramidal structure can be observed by X-ray diffraction only when the lowest vibrational level is below the inversion barrier, such as those found in Gd₃N@C₈₀ and DyGd₂N@C₈₀. The quantum nature of molecular vibrations becomes especially apparent when the size of the inversion barrier is comparable to the energy of the lowest vibrational levels.