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 -thiocyanate mononuclear spin crossover (SCO) complexes, [FeL(NCS)] (-), 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 = ,-bis((1,5-dimethyl-1-1,2,3-triazol-4-yl)methylene)propane-1,3-diamine/-2,2-dimethylpropane-1,3-diamine, / and ,-bis((1-ethyl/1-propyl-1-1,2,3-triazol-4-yl)methylene)-2,2-dimethylpropane-1,3-diamine, /]. The thermal-induced SCO behavior is characterized by abrupt transitions with an average critical temperature (Δ)/hysteresis loop width (Δ) in the range 190-252/5-14 K, while the photo-generated metastable high-spin (HS) phases are characterized by temperatures in the range 44-59 K. Single crystal analysis shows that except , all compounds experience reversible symmetry breaking coupled with the thermal SCO. Furthermore, 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 (, and ) shows a correlation between the cooperativity and the amplitude of changes in the molecule-molecule interactions in the lattice at the SCO transition.