27Al NMR study of the metal cluster compound Al50C120H180

We present an Al-27 NMR study of the metal cluster compound Al50Cp*(12) which is composed of (identical) Al-50 clusters, each surrounded by a Cp* ligand shell, and arranged in a crystalline 3D array (here Cp* = pentamethylcyclopentadienyl = C-5(CH3)(5)). The compound is found to be non-conducting, the nuclear spin-lattice relaxation in the temperature range 100-300 K being predominantly due to reorientational motions of the Cp* rings. These lead to a pronounced maximum in the relaxation rate at T similar to 170 K, corresponding to an activation energy of about 850 K. Data for the related compound Al4Cp*(4), containing very much smaller Al-4 clusters are also presented. A comparison is drawn with the quadrupolar relaxation recently observed for the non-conducting fraction of Ga-84 molecules in the metal cluster compound Ga-84[N(SiMe3)(2)](20)-Li6Br2(thf)(20)center dot 2toluene.