Phase Trapping in Multistep Spin Crossover Compound

The dimeric motif is the smallest unit for two interacting spin centers allowing for systematic investigations of cooperative interactions. As spin transition compounds, dinuclear complexes are of particular interest, since they potentially reveal a two-step spin crossover (SCO), switching between the high spin-high spin [HS-HS], the high spin-low spin [HS-LS], and the low spin-low spin [LS-LS] states. Herein, we report the synthesis and characterization of six dinuclear iron(II) complexes [Fe^II₂(μ₂-L)₂](BF₄)₄ (C1), [Fe^II₂(μ₂-L)₂](ClO₄)₄ (C2), [Fe^II₂(μ₂-L)₂](F₃CSO₃)₄ (C3), [Fe^II₂(μ₂-L)₂](BF₄)₄ (C4), [Fe^II₂(μ₂-L)₂](BF₄)₄ (C5), and [Fe^II₂(μ₂-L)₂](BF₄)₄ (C6), based on the 1,3,4-thiadiazole bridging motif. The two novel bis-tridentate ligands (L = 2,5-bis{[(1H-imidazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole and L = 2,5-bis{[(thiazol-2-ylmethyl)-amino]-methyl}-1,3,4-thiadiazole) were employed in the presence of iron(II) salts with the different counterions. Upon varying ligands and counterions, we were able to change the magnetic properties of the complexes from a temperature-independent [HS-HS] spin state over a one-step spin transition toward a two-step SCO. When cooled slowly from room temperature, the two-step SCO goes along with two distinct phase transitions, and in the intermediate mixed [HS-LS] state distinct HS/LS pairs can be identified unambiguously. In contrast, rapid cooling precludes a crystallographically observable phase transition. For the mixed [HS-LS] state Mössbauer spectroscopy confirms a statistical (random) orientation of adjacent [HS-LS]·[HS-LS]·[HS-LS] chains.