We report herein
a series of neutraltrans-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 =N1,N3-bis((1,5-dimethyl-1H-1,2,3-triazol-4-yl)methylene)propane-1,3-diamine/-2,2-dimethylpropane-1,3-diamine,1/2andN1,N3-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 (ΔT1/2)/hysteresis loop width (ΔThyst) in the range 190–252/5–14 K, while
the photo-generated metastable high-spin (HS) phases are characterized
byTLIESSTtemperatures in the range 44–59
K. Single crystal analysis shows that except1, all compounds
experience reversible symmetry breaking coupled with the thermal SCO.
Furthermore,4experiences 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,and4) shows a correlation
between the cooperativity and the amplitude of changes in the molecule–molecule
interactions in the lattice at the SCO transition.