Presence or Absence of Thermosalient Effect Depending on Terminal Alkyl Moieties in Spin Crossover Iron(II) Molecular Crystals

Herein, we report the presence or absence of thermosalience coupled with spin crossover (SCO) by varying the alkyl substituents in iron(II) molecular crystals, [Fe(LPh,R)2(NCS)2], where R = cyclopropylmethyl (CH2cPr, 1) and isobutyl (iBu, 2) and LPh,CH2cPr = N-(cyclopropylmethyl)-1-(1-phenyl-1H-1,2,3-triazol-4-yl)methanimine and LPh,iBu = N-isobutyl-1-(1-phenyl-1H-1,2,3-triazol-4-yl)methanimine. 1 exhibits an abrupt high-spin (HS) ↔ low-spin (LS) SCO at a transition temperature (Tc) of 203 K, while 2 exhibits a gradual SCO in a wide temperature range centered at 149 K. In addition to the SCO, only 1 exhibits a thermosalient (TS) effect (crystal jumping or moving) near the Tc upon initial cooling. Single-crystal X-ray diffraction (SCXRD) studies at room temperature (296 K) revealed that 1 and 2 are isomorphic; i.e., the complex molecules have the same HS FeN6 octahedral coordination environment with cis-coordinated two NCS ligands and almost identical supramolecular assemblies via intermolecular CH···S and π···π interactions, including the paired neighboring alkyl groups aligned to the crystallographic a-axis. Variable-temperature SCXRD studies revealed that (1) the CH2cPr groups of 1 were disordered at 296–203 K, while the iBu groups of 2 were ordered at 296–100 K, and (2) the disorder-to-order transition of 1 at around Tc was associated with the emergence of anisotropic lattice expansion along the a-axis, exhibiting a TS effect upon cooling.