Synthesis, Characterization, and Neutron Activation of Holmium Metallofullerenes

Isolation of the first macroscopic quantities of endohedral holmium metallofullerenes (principally Ho@C 82, Ho 2 @C 82, and Ho 3 @C 82 by LD-TOF mass spectrometry) has been accomplished by carbon-arc and preparative HPLC methodologies. The detailed procedure for production and isolation of the metallofullerenes includes a new technique whereby holmium-impregnated electrodes are prepared simply by soaking porous graphite rods in an ethanolic solution of Ho(NO 3 ) 3 · x H 2 O. Monoisotopic 165 Ho offers a unique combination of advantages for neutron-activation studies of metallofullerenes, and purified samples containing 165 Ho@C 82, 165 Ho 2 @C 82, and 165 Ho 3 @C 82 have been activated by high-flux neutron irradiation (Φ = 4 × 10 13 n cm -2 s -1 ) to generate 166 Ho metallofullerenes, which undergo β - decay to produce stable 166 Er. Chemical workup of the irradiated samples, followed by re-irradiation, has been used to demonstrate that observed decomposition of holmium metallofullerenes is due mainly to “fast” neutron damage rather than to holmium atom nuclear recoil ( E max = 200 eV). This implies that metallofullerene damage can be minimized by using neutron fluxes with the highest possible thermal component. A detailed analysis of the neutron-activation results is presented as a prelude to the possible use of neutron-activated metallofullerenes in nuclear medicine.