AFTER-EFFECTS OF THE CO-57(EC)FE-57 NUCLEAR DECAY IN TRIS(2,2'-BIPYRIDYL)COBALT(II) ENCAPSULATED IN THE SUPERCAGE OF ZEOLITE-Y

Fe-57 emission Mossbauer spectroscopy was used to study the after-effe cts of the Co-57(EC)Fe-57 decay in tris(bipy)cobalt(II) (bipy=2,2'-bip yridyl) encapsulated in the supercage of zeolite Y. The spectra reveal ed two low-spin species, [Fe-57(bipy)(3)](2+) and [Fe-57(bigy)(3)](3+) , as well as a high-spin species of [Fe-57(bipy)(3)](2+). An anomalous ly high spectral contribution of Fe-III was found with an anomalous te mperature dependence, i.e., the amount of Fe-III diminished with the t emperature elevation. A time-delayed Mossbauer experiment was performe d to determine the time scale of this phenomenon, which indicated that the processes responsible for the Fe-III to Fe-II transition was comp leted in the first nanoseconds after the nuclear decay. Another effect of the zeolite encapsulation was the absence of a second high-spin [F e-57(bipy)(3)](2+) species identified in pristine crystalline compound s as an excited ligand field state of Fe2+. This observation is attrib uted to the spatial restriction implied by the supercage. The possible fragmentation origin of the first high-spin [Fe-57(bipy)(3)](2+) spec ies, the complex ion with a damaged ligand, is also discussed. (C) 199 8 American Institute of Physics. [S0021-9606(98)03620-4]