Isolation and spectroscopic characterization of new endohedral fullerenes in the size gap of C-74 to C-76

New alkaline earth metal endohedral fullerenes Sr@C-74, Sr@C-76-I,II and Ca@C-74, prepared by means of the RF-method, have been isolated using multistep HPLC. The purity was ascertained by anionic LDI TOF mass spectroscopy, considering the isotopic patterns of the compounds. The influence of the incorporated metal on the electronic structure has been studied by VIS-NIR and Raman spectroscopy. Photoexcited triplet-state EPR spectroscopy was used to investigate the structure of these otherwise EPR-silent fullerenes. Displaying the frequency of the cage vs. en-capsulated metal vibrational modes as a function of the square root of the reciprocal masses of the metals clearly separates the M2+@C-n(3)- and the M2+@C-n(2-) families. This seems to be a generally applicable tool for monitoring the metal to fullerene charge transfer.