Alternative Route Triggering Multistep Spin Crossover with Hysteresis in an Iron(II) Family Mediated by Flexible...

Multistep spin crossover (SCO) compounds have attracted much attention, since they can be great candidates for high-density multinary memory devices. The introduction of substituents, such as methyl (Me), chloro (Cl), bromo (Br), and methoxy (MeO) groups, at para positions to the phenyl-substituted tripodal N₆ ligand-coordinated SCO Fe^II material, [FeL^Ph](NTf₂)₂ [where L^Ph = tris(2-{[(1-phenyl-1H-1,2,3-triazol-4-yl)methylidene]amino}ethyl)amine and NTf₂ = bis(trifluoromethanesulfonyl)imide], affords a new family of solvent-free Fe^II complexes: [FeL^4-R-Ph](NTf₂)₂ {where L^4-R-Ph = tris[2-({[1-(4-R-phenyl)-1H-1,2,3-triazol-4-yl]methylidene}amino)ethyl]amine, where R = Me (1), Cl (2), Br (3), and MeO (4)}. 1 shows temperature invariant high-spin (HS) state, whereas the others show spin transitions with different characteristics, such as half-SCO (4), two-step SCO (3), and unusual three-step SCO with hysteresis (2). Mössbauer and X-ray absorption fine structure (XAFS) spectroscopic studies of them support the magnetic susceptibilities results. Density functional theory calculations indicate that the electronic effect of different substituents on magnetic properties is negligible in this Fe^II family. Single-crystal X-ray diffraction studies reveal that 1-4 has a similar packing arrangement with three-dimensional supramolecular network via intermolecular π-π and CH···π interactions between complex cations, and CH···X (X = O, N, and F) hydrogen bonding interactions between cations and inherently frustrated NTf₂ anions. Variable-temperature structural studies unveil a variety of stepped SCO behaviors of 2-4 and deactivation of SCO in 1 are governed by the regulation of ordering of NTf₂ counteranions through the subtle modification of terminal substituents of complex cations. Quantitative light-induced excited spin-state trapping (LIESST) effect was observed for 2-4 via green light irradiation (532 nm) at 10 K. This study opens up a new way for systematic control of magnetic response from no SCO to half-, two-step, and finally three-step SCO with hysteresis by precise tuning of the ordering of flexible NTf₂ anions included in the supramolecular network with potentially SCO-active complex cations.