Spin state modulation and kinetic control of thermal contraction in a [Fe₂Co₂] discrete Prussian blue analogue

Stimuli-responsive switchable molecules represent an important category of magnetic materials with significant potential for functional devices. However, engineering complexes with controlled switchability remains challenging due to their sensitivity to lattice interactions. Herein, we report a [Fe₂Co₂] square complex [FeTp(CN)₃]₂[Co{(F5-Ph)Py}₂]₂·2ClO₄·4CH₃OH·2H₂O {1·4CH₃OH·2H₂O; (F5-Ph)Py = (Z)-N'-(perfluorophenyl) picolinimidamide and Tp = hydrotris(1-pyrazolyl)borate}, tailored with hydrogen bonding (HB) donor and acceptor moieties for effective lattice interactions. The alteration in HB interactions in crystal phases obtained via single-crystal-to-single-crystal (SC-SC) transformation (i.e., 1·4CH₃OH·2H₂O ↔ 1·2H₂O ↔ 1) led to a remarkable change in magnetic properties. Complexes 1·4CH₃OH·2H₂O and 1 possess [Fe^II _LS(μ-CN)Co^III _LS] and [Fe^III _LS(μ-CN)Co^II _HS] configurations, respectively. Meanwhile, 1·2H₂O demonstrates multi-responsive (thermo-, photo- and pressure) reversible two-step electron transfer coupled spin transition (ETCST) accompanied by thermal contraction and expansion. The complete diamagnetic [Fe^II _LS(μ-CN)Co^III _LS] configuration in 1·2H₂O was obtained at an intermediate temperature accompanied by thermal contraction, an unusual behaviour observed for the first time in cyanide-bridged systems. Additionally, 1·2H₂O displayed temperature-induced excited spin state trapping (TIESST) of ∼70% of the paramagnetic [Fe^III _LS(μ-CN)Co^II _HS] configuration at low temperatures. The isothermal relaxation of the thermally trapped paramagnetic state shows a much faster and complete conversion to a diamagnetic state at ∼140 K, compared to the relaxation observed at other temperatures (100-190 K), corroborating the observed unique magnetic behaviour. Hence, this result provides valuable insight into the strategic design of complexes with enhanced and controlled switchability for potential applications such as actuators and sensors.