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, [Fe II L(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-1 H -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-1 H -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 (Δ T 1/2 )/hysteresis loop width (Δ T 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.