Coupling the Spin State Switching and CO2 Adsorption Operating at Different Temperatures in a Breathing Metal-Organic Framework.

Gas-responsive magnetic response arising from spin-crossover (SCO) phenomena is rare yet highly attractive for molecular sensing and spintronics. However, it remains unclear how gas uptake influences SCO behaviour when the two phenomena occur in separate temperature regimes. Herein, we report a strong influence of CO2 adsorption on the SCO behaviour of a Hofmann clathrate, [FeII(epac)2(PtII(CN)4)]·nEtOH (1·nEtOH, n ≤ 2), despite the two effects occurring at different temperatures. Complex (1·2EtOH) exhibits highly cooperative SCO with a hysteresis width ∼65 K and preserves the framework topology upon desolvation (1). However, hysteresis reduces to 30 K in the guest-free phase (1), underscoring the critical role of lattice solvent in mediating cooperativity. Complex (1) selectively adsorbs CO2 through a gate-opening mechanism at temperatures substantially higher than the SCO region. Despite this mismatch in operating temperatures, increasing CO2 loading progressively stabilizes the low-spin state and shifts the transition temperature (Teq) by ∼20 K. These findings support a delayed guest-mediated coupling mechanism where CO2 first enters the framework at elevated temperatures and subsequently modifies spin-state energetics only after immobilization within the host lattice at lower temperatures. This study establishes that gas adsorption and SCO need not occur within overlapping temperatures to remain strongly coupled.