Multiparameter Control of Spin-Crossover in Cyanido-Bridged Fe ^II Complexes: Configuration, Solvent, Anion, and Ligand Effects

Spin-crossover (SCO) materials represent a class of switchable molecular systems with promising applications in information storage and sensing devices. Achieving precise control over their transition characteristics, particularly near room temperature, requires a fundamental understanding of how multiple structural factors cooperatively influence the spin state. Herein, we report the synthesis and characterization of five trinuclear cyanido-bridged complexes, cis-[Cp(dppe)RuII(CN)]2[FeII(dpyn)2](OTf)2 (cis-1) and trans-[Cp(dppe)MII(CN)]2[FeII(dpyn)2](R)2 [trans-1 (M = Ru, R = OTf); trans-2 (M = Ru, R = ClO4–); trans-3 (M = Fe, R = OTf); trans-4 (M = Fe, R = ClO4–); Cp = cyclopentadiene; dppe = 1,2-bis(diphenylphosphino)ethane; dpyn = 2,2′-dipyridylamine; and OTf = CF3SO3–], and demonstrate a multiparameter approach to tailor the SCO behavior through geometric isomerism, anion exchange, solvent engineering, and ligand electronic effects. The trans-isomers exhibit substantially higher spin-transition temperatures (T1/2) than their cis-counterparts, with a remarkable 40 K difference observed between trans-1 and cis-1. Intriguingly, heating a solution of cis-1 cleanly converts it to trans-1, which can be isolated as single crystals upon recrystallization. Desolvation leads to unpredictable T1/2 shifts, most strikingly a 70 K increase for trans-2D, pushing its transition to near room temperature (277 K). Systematic modification of terminal ligand electronic properties further enables fine-tuning of T1/2, confirming that enhanced electron-donating capacity weakens the ligand field strength and consequently lowers the transition temperature. This study establishes a comprehensive strategy for controlling molecular spin states through synergistic structural modifications and provides valuable insights for developing functional SCO materials with tailored properties.