Spin crossover in tetranuclear Fe(II) complexes, {[(tpma)Fe(μ-CN)]4}X4 (X = ClO4(-), BF4(-)).

Two Fe(II) complexes, {[(tpma)Fe(μ-CN)]4}X4 (X = ClO4– (1a), BF4– (1b); tpma = tris(2-pyridylmethyl)amine), were prepared by reacting the {Fe(tpma)}2+ building block with (Bu4N)CN. The crystal structures of 1a and 1b feature a tetranuclear cation composed of cyanide-bridged Fe(II) ions, each capped with a tetradentate tpma ligand. The Fe4(μ-CN)4 core of the complex is strongly distorted, assuming a butterfly-like geometry. Both complexes exhibit gradual temperature-driven spin crossover (SCO) associated with the high-spin (HS) ↔ low-spin (LS) transition at two out of four metal centers. The evolution of HS and LS Fe(II) ions with temperature was followed by a combination of X-ray crystallography, magnetic measurements, and Mossbauer spectroscopy. Only the Fe(II) ions surrounded by six N atoms undergo the SCO. A comparison of the temperature-dependent SCO curves for the samples stored under solvent and the dried samples shows that the former exhibit a much more abrupt SCO. This finding was interpreted in t...