Mössbauer spectroscopy as a probe of magnetization dynamics in the linear iron(I) and iron(II) complexes...

The iron-57 Mössbauer spectra of the linear, two-coordinate complexes, [K(crypt-222)][Fe(C(SiMe3)3)2], 1, and Fe(C(SiMe3)3)2, 2, were measured between 5 and 295 K under zero applied direct current (dc) field. These spectra were analyzed with a relaxation profile that models the relaxation of the hyperfine field associated with the inversion of the iron cation spin. Because of the lifetime of the measurement (10(-8) to 10(-9) s), iron-57 Mössbauer spectroscopy yielded the magnetization dynamics of 1 and 2 on a significantly faster time scale than was previously possible with alternating current (ac) magnetometry. From the modeling of the Mössbauer spectral profiles, Arrhenius plots between 5 and 295 K were obtained for both 1 and 2. The high-temperature regimes revealed Orbach relaxation processes with U(eff) = 246(3) and 178(9) cm(-1) for 1 and 2, respectively, effective relaxation barriers which are in agreement with magnetic measurements and supporting ab initio calculations. In 1, two distinct high-temperature regimes of magnetic relaxation are observed with mechanisms that correspond to two distinct single-excitation Orbach processes within the ground-state spin-orbit coupled manifold of the iron(I) ion. For 2, Mössbauer spectroscopy yields the temperature dependence of the magnetic relaxation in zero applied dc field, a relaxation that could not be observed with zero-field ac magnetometry. The ab initio calculated Mössbauer hyperfine parameters of both 1 and 2 are in excellent agreement with the observed hyperfine parameters.