American Chemical Society
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Electronic Relaxation Phenomena Following 57Co(EC)57Fe Nuclear Decay in [MnII(terpy)2](ClO4)2·1/2H2O and in the Spin Crossover Complexes [CoII(terpy)2]X2·nH2O (X = Cl and ClO4):  A Mössbauer Emission Spectroscopic Study

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posted on 2001-02-13, 00:00 authored by Hiroki Oshio, Hartmut Spiering, Vadim Ksenofontov, Franz Renz, Philipp Gütlich
The valence states of the nucleogenic 57Fe arising from the nuclear disintegration of radioactive 57Co by electron capture decay, 57Co(EC)57Fe, have been studied by Mössbauer emission spectroscopy (MES) in the 57Co-labeled systems:  [57Co/Co(terpy)2]Cl2·5H2O (1), [57Co/Co(terpy)2](ClO4)2·1/2H2O (2), and [57Co/Mn(terpy)2](ClO4)2· 1/2H2O (3) (terpy = 2,2‘:6‘,2‘ ‘-terpyridine). The compounds 1, 2, and 3 were labeled with ca. 1 mCi of 57Co and were used as the Mössbauer sources at variable temperatures between 300 K and ca. 4 K. [Fe(terpy)2]X2 is a diamagnetic low-spin (LS) complex, independent of the nature of the anion X, while [Co(terpy)2]X2 complexes show gradual spin transition as the temperature is varied. The Co(II) ion in 1 “feels” a somewhat stronger ligand field than that in 2; as a result, 83% of 1 stays in the LS state at 321 K, while in 2 the high-spin (HS) state dominates at 320 K and converts gradually to the LS state with a transition temperature of T1/2 ≈ 180 K. Variable-temperature Mössbauer emission spectra for 1, 2, and 3 showed only LS-57Fe(II) species at 295 K. On lowering the temperature, metastable HS Fe(II) species generated by the 57Co(EC)57Fe process start to grow at ca. 100 K in 1, at ca. 200 K in 2, and at ca. 250 K in 3, reaching maximum values of 0.3 at 20 K in 1, 0.8 at 50 K in 2, and 0.86 at 100 K in 3, respectively. The lifetime of the metastable HS states correlates with the local ligand field strength, and this is in line with the “inverse energy gap law” already successfully applied in LIESST relaxation studies.