Slow Electronic Relaxation in T52−A11 Spin Equilibria of Tris(2-methyl-1,10-phenanthroline)Iron(II) Complexes: A Mössbauer Effect Study

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Abstract
The 57Fe Mössbauer effect in [FeL3](ClO4)2 (I) and [FeL3](BF4)2 (II) has been studied between 295 and 4.2°K (L=2-methyl-1,10-phenanthroline). At 295°K, compound I exhibits a quadrupole split pair of lines characteristic of a 5T2 ground state, ΔEQ(5T2)=1.03 mm sec−1 and δIS(5T2)=+ 0.95 mm sec−1. At 244°K, line broadening is observed. At even lower temperatures, e.g., at 196°K, the relative intensity of the doublet has decreased and a second doublet typical of a 1A1 ground state has appeared. At 4.2°K, 72.7% of the intensity is due to 1A1, ΔEQ(1A1)=0.57 mm sec−1, and δIS(1A1)=0.38 mm sec−1. Similar behavior is encountered in II. The observations are accounted for by slow electronic relaxation between 5T2 and 1A1 in the temperature range between 230 and 120°K and the assumption of permanently paramagnetic molecules which become apparent below 77°K. A change of the relaxation at about 244°K is likely. From the temperature dependence of ΔEQ(5T2), the axial field splitting is δ=− 640 cm−1 in I and δ=− 580 cm−1 in II. In conjunction with magnetic data it is demonstrated that the T52−A11 energy separation ε varies with temperature: ε=300 cm−1 at 294°K and ε=990 cm−1 at 98°K in I and similarly in II. In an applied field of 50 kG, the sign of Vzz is positive for the 1A1 component in both I and II.
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