Locking the Spin States of Anionic Fe(L)2 Units in Coordination Polymers through Alkali Metal Ions Incorporation

Abstract

Discrete FeII spin crossover (SCO) compounds have garnered significant interest over the past decades. However, assembling these units into coordination polymers via the incorporation of a second metal ion and investigating 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 FeII centers, ultimately exerting a strong influence on the spin state. In compound 1, the FeII ions reside in a nearly ideal octahedral environment, locking it in a low-spin (LS) state. In contrast, the highly distorted coordination geometries around FeII ions in 2 and 3 stabilize the high-spin (HS) state of [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, ascribing 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 second coordination metal ion located outside the SCO core, providing a novel design strategy for functional molecular materials with tailored magnetic properties.