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)Ru II (CN)] 2 [Fe II (dpyn) 2 ](OTf) 2 ( cis -1 ) and trans -[Cp(dppe)M II (CN)] 2 [Fe II (dpyn) 2 ](R) 2 [ trans -1 (M = Ru, R = OTf); trans -2 (M = Ru, R = ClO 4 – ); trans -3 (M = Fe, R = OTf); trans -4 (M = Fe, R = ClO 4 – ); Cp = cyclopentadiene; dppe = 1,2-bis(diphenylphosphino)ethane; dpyn = 2,2′-dipyridylamine; and OTf = CF 3 SO 3 – ], 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 ( T 1/2 ) 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 T 1/2 shifts, most strikingly a 70 K increase for trans -2 D, pushing its transition to near room temperature (277 K). Systematic modification of terminal ligand electronic properties further enables fine-tuning of T 1/2, 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.