An electron-transfer [Fe2Co2] square complex exhibiting unprecedented wide room-temperature hysteresis

Switchable molecules with magnetic bistability that can function at standard conditions are highly desirable from the perspective of practical applications. Over the last decade, significant progress has been made in modulating thermally induced electron-transfer-coupled spin transition (ETCST) events between diamagnetic FeIILS(μ-CN)CoIIILS and paramagnetic FeLS(μ-CN)-CoIIHS electron configurations (named LS and HS, respectively, for short) within the Fe/Co mixed-valence cyanometallate complexes. Herein, we prepared a novel [Fe2Co2] square complex [(pzTp)Fe(CN)3Co(bnbpen)]2[NO3]2·4CH3OH·2.5H2O (1·Solv) and its desolvated phase [(pzTp)Fe(CN)3Co(bnbpen)]2[NO3]2 (1), obtained via single-crystal-to-single-crystal (SCSC) transformation. Complex 1·Solv remaining in the diamagnetic state below room temperature exhibits an irreversible ETCST behavior accompanied by desolvation upon heating. Notably, complex 1 displays an unprecedented rate-dependent ETCST behaviour involving with two diamagnetic LS states (a dynamically favoured LS1 state and a thermodynamically favoured LS2 state), between which a net one-way transition from LS1 to LS2 is accessible likely via a structural phase transition (SPT) above 255 K. Consequently, two well separated transition channels (HS ↔ LS1 at around 255 K and HS ↔ LS2 in the temperature range of 317–350 K) are achieved according to either fast (> 20 K/min) or slow rate (< 1 K/min), respectively. At a moderate rate of 3 K/min, complex 1 shows a wide hysteresis of 66 K centered at 328 K, the largest yet reported for discrete Fe/Co cyanometallate complexes. Complex 1 represents the first ETCST example coupled with dynamic SPT, providing a new strategy for manipulating bistability within this fascinating cyanometallate family.