Locking the spin states of anionic Fe(L)₂ units in coordination polymers through alkali metal ion incorporation

Discrete Fe^II spin crossover (SCO) compounds have garnered significant interest over the past few decades. However, the assembly of these units into coordination polymers via the incorporation of a second metal ion and investigation of how these ions influence their magnetic properties is extremely limited. Herein, three new coordination polymers, namely, [Li₂Fe(L)₂(H₂O)₄]·4H₂O (1), [Na₂Fe(L)₂(μ₂-H₂O)₄]·2H₂O (2), and [K₂Fe(L)₂(μ₂-CH₃OH)₂] (3), were synthesized based on the anionic spin crossover (SCO) [Fe(L)₂]^2- (H₂L = pyridine-2,6-bistetrazolate) secondary building unit. Single crystal X-ray analyses reveal a structural evolution from a 1D chain for 1 to a 2D layer for 2 and finally to a 3D framework for 3, driven by the increasing ionic radii of the alkali metal ions (Li⁺ → Na⁺ → K⁺). These spin-inactive alkali metal ions dramatically affect the coordination geometry of the Fe^II centers, ultimately exerting a strong influence on the spin state. In compound 1, the Fe^II ions reside in a nearly ideal octahedral environment, locking it in a low-spin (LS) state. In contrast, the highly distorted coordination geometries around the Fe^II ions in 2 and 3 stabilize the high-spin (HS) state of the [Fe(L)₂]^2- units, as confirmed by octahedral distortion parameter analysis of compounds 1-3 and magnetic susceptibility measurements for compounds 1-2. In DMSO solution, all three compounds undergo similar SCO behavior with nearly identical transition temperatures, ascribed to the fact that the solution-phase magnetism is governed by the intrinsic ligand field. This work clearly demonstrates that the spin state of a mononuclear SCO-active unit in the solid state can be effectively and predictably controlled through the strategic introduction of a second coordination metal ion located outside the SCO core, providing a novel design strategy for functional molecular materials with tailored magnetic properties.