Order–Disorder, Symmetry Breaking, and Crystallographic Phase Transition in a Series of Bis(trans-thiocyanate)iron(II) Spin Crossover Complexes Based on Tetradentate Ligands Containing 1,2,3-Triazoles

We report herein a series of neutral trans-thiocyanate mononuclear spin crossover (SCO) complexes, [FeIIL­(NCS)2] (1–4), based on tetradentate ligands L obtained by reaction of N-substituted 1,2,3-triazolecarbaldehyde with 1,3-propanediamine or 2,2-dimethyl-1,3-diaminopropane [L = N 1,N 3-bis­((1,5-dimethyl-1H-1,2,3-triazol-4-yl)­methylene)­propane-1,3-diamine/-2,2-dimethylpropane-1,3-diamine, 1/2 and N 1,N 3-bis­((1-ethyl/1-propyl-1H-1,2,3-triazol-4-yl)­methylene)-2,2-dimethylpropane-1,3-diamine, 3/4]. The thermal-induced SCO behavior is characterized by abrupt transitions with an average critical temperature (ΔT 1/2)/hysteresis loop width (ΔT hyst) in the range 190–252/5–14 K, while the photo-generated metastable high-spin (HS) phases are characterized by T LIESST temperatures in the range 44–59 K. Single crystal analysis shows that except 1, all compounds experience reversible symmetry breaking coupled with the thermal SCO. Furthermore, 4 experiences an additional phase transition at ca. 290 K responsible for the coexistence of two HS phases quenched at 10 K through LIESST and TIESST effects. The molecules form hexagonally packed arrays sustained by numerous weak CH···S and C···C/S···C/N···C bonds involving polar coordination cores, while non-polar pendant aliphatic substituents are segregated inside, occupying hexagonal channels. Energy framework analysis of complexes with one step SCO transition (1, 2, and 4) shows a correlation between the cooperativity and the amplitude of changes in the molecule–molecule interactions in the lattice at the SCO transition.