Diamagnetic Clusters of Paramagnetic Endometallofullerenes:  A Solid-State MAS NMR Study

Solid powder samples of complexes of the endometallofullerenes (EMF) La@C82 and Y@C82 with hexamethylphosphoramide (HMPA) were studied by magic-angle spinning (MAS) NMR. We have obtained well-resolved 31P NMR spectra and 13C NMR spectra for both La-EMF/HMPA and Y-EMF/HMPA and 139La spectra for the La-EMF/HMPA. The 31P measurements on La-EMF/HMPA and Y-EMF/HMPA have revealed considerable chemical shifts of 31P signals relative to pure HMPA. Two-dimensional exchange 31P experiments revealed that HMPA molecules at different sites in the EMF/HMPA complex do not change positions at a time scale of up to 1 s. Both EMF samples demonstrate a vast chemical shift range for 31P of the bound HMPA molecules. In addition, the La-EMF/HMPA exhibits the enormous spreading of the chemical shifts for 139La. The experimental results suggest that paramagnetic La@C82 and Y@C82 in the solid state form clusters (nanoparticles) in which the exchange coupling of the EMF takes place with quenching of the most electron spins.