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

Abstract

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.