[en] Crystal structure ; magnetic properties ; Mossbauer spectroscopy
[en] We present a family of novel non-heme trigonal pyramidal iron(II) complexes supported by tris(pyrrolyl-α-methyl)amine ligands of the form [M(solv)n][(tpaR)Fe] (M = Na, R = tert-butyl (1), phenyl (4); M = K, R = mesityl (2), 2,4,6-triisopropylphenyl (3), 2,6-difluorophenyl (5)) and their characterization by X-ray crystallography, cyclic voltammetry, and Mössbauer spectroscopy. Expanding on the initial discovery of slow magnetic relaxation in the recently reported mesityl derivative 2, we report the static and dynamic magnetic properties of a homologous series of high-spin mononuclear iron(II) complexes that exhibit this intriguing behavior. Magnetization experiments reveal large, negative zero-field splitting parameters of D = −48, −40, −36, −26 and −6.2 cm−1 for 1-5, respectively. In the case of 2,6-difluorophenyl 5, high-field EPR experiments provide an independent determination of the zero-field splitting parameters (D = −4.397(9)) that are in reasonable agreement with the magnetization data. Ac susceptibility measurements indicate field-dependent, thermally-activated spin reversal barriers in complexes 1, 2 and 4 of Ueff = 60, 42 and 25 cm−1, respectively. In the case of 1, this value constitutes the highest spin-reversal barrier observed for a mononuclear transition metal complex, a property that has broad implications for the design of molecules that can potentially store and process magnetic information.