Reversible and Irreversible Vapor-Induced Guest Molecule Exchange in Spin-Crossover Compounds

Spin-crossover (SCO) complex [Fe(tpa)(NCS)2] (tpa = tri(2-pyridylmethyl)amine) crystallized in two solvate forms, yellow [Fe(tpa)(NCS)2]·X [Fe:X = 1:1; X = n-PrOH (complex is named as n-PrOH), i-PrOH (i-PrOH), CH2Cl2 (CH2Cl2), CHCl3 (CHCl3), MeCN (MeCN)] and red [Fe(tpa)(NCS)2]2·Y [Fe:Y = 2:1; Y = MeOH (MeOH), EtOH (EtOH)], respectively. Between the two forms, interesting solvent-vapor induced in situ reversible and irreversible guest molecule exchanges, [Fe(tpa)(NCS)2]·X ⇌ [Fe(tpa)(NCS)2]2·Y, occurred in the solid state followed by dramatic color changes as well as distinct structural and SCO behavior transformations. Comprehensive studies on structures and SCO behaviors associating guest exchanges have been conducted by X-ray single-crystal diffraction, PXRD, IR, elemental analysis, and magnetic measurements, respectively. This discrete molecular system shows unique solvent-dependent SCO behavior related to the nature of solvent molecules; the distinct color changes during guest exchange originate from the alternations of electronic states of the guest-sensitive FeII centers, providing an effective route to fine-tune and optimize materials’ properties by systematic structural perturbation, or serving for detection of toxic gases, such as CH2Cl2 and CHCl3.