Heteroleptic Fe(II) Complexes with N₄S₂ Coordination as a Platform for Designing Spin-Crossover Materials

Heteroleptic complexes [Fe­(bpte)­(bim)]­X₂ and [Fe­(bpte)­(xbim)]­X₂ (bpte = <i>S</i>,<i>S</i>′-bis­(2-pyridylmethyl)-1,2-thioethane, bim = 2,2′-biimidazole, xbim = 1,1′-(α,α′-<i>o</i>-xylyl)-2,2′-biimidazole, X = ClO₄^–, BF₄^–, OTf^–) were prepared by reacting the corresponding Fe­(II) salts with a 1:1 mixture of the ligands. All mononuclear complexes exhibit temperature-induced spin crossover (SCO) with the onset above room temperature. The SCO is rather gradual, due to low cooperativity of interactions between the cationic complexes, as revealed by crystal structure analyses. These complexes expand the range of the recently discovered Fe­(II) SCO materials with {N₄S₂} coordination environment

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^– (<b>1a</b>), BF₄^– (<b>1b</b>); tpma = tris­(2-pyridylmethyl)­amine), were prepared by reacting the {Fe­(tpma)}²⁺ building block with (Bu₄N)­CN. The crystal structures of <b>1a</b> and <b>1b</b> 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 Mössbauer 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 terms of the increased structural disorder and decreased crystallinity caused by the loss of the interstitial solvent molecules in the dried samples