After-effects of the 57Co(EC)57Fe 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-effects of the Co-57(EC)Fe-57 decay in tris(bipy)cobalt(II) (bipy=2,2'-bipyridyl) encapsulated in the supercage of zeolite Y. The spectra revealed 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 anomalously high spectral contribution of Fe-III was found with an anomalous temperature dependence, i.e., the amount of Fe-III diminished with the temperature elevation. A time-delayed Mossbauer experiment was performed to determine the time scale of this phenomenon, which indicated that the processes responsible for the Fe-III to Fe-II transition was completed in the first nanoseconds after the nuclear decay. Another effect of the zeolite encapsulation was the absence of a second high-spin [Fe-57(bipy)(3)](2+) species identified in pristine crystalline compounds as an excited ligand field state of Fe2+. This observation is attributed to the spatial restriction implied by the supercage. The possible fragmentation origin of the first high-spin [Fe-57(bipy)(3)](2+) species, the complex ion with a damaged ligand, is also discussed. (C) 1998 American Institute of Physics. [S0021-9606(98)03620-4]