Synthesis, characterization, and neutron activation of holmium metallofullerenes

Isolation of the first macroscopic quantities of endohedral holmium metallofullerenes (principally Ho@C82, Ho-2@C-82, and Ho-3@C-82 by LD-TOF mass spectrometry) has been accomplished by carbon-are 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(NO3)(3) . xH(2)O. Monoisotopic Ho-165 offers a unique combination of advantages for neutron-activation studies of metallofullerenes, and purified samples containing Ho-165@C-82, Ho-165(2)@C-82, and Ho-165(3)@C-82 have been activated by high-flux neutron irradiation (phi=4 x 10(13) n cm(-2) s(-1)) to generate Ho-166 metallofullerenes, which undergo beta(-) decay to produce stable Er-166. 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.