Above room temperature spin crossover in mononuclear iron(II) complexes featuring pyridyl-benzimidazole bidentate ligands adorned with aliphatic chains.

Two bidentate ligands (L1 = 1-pentyl-2-(pyridin-2-yl)-1H-benzimidazole and L2 = 1-heptyl-2-(pyridin-2-yl)-1H-benzimidazole) were employed for the synthesis of five mononuclear Fe(II) coordination compounds 1–5 containing perchlorate, tetrafluoroborate and triflate counterions. Single-crystal X-ray diffraction analysis confirmed the expected molecular structures of all the reported compounds, revealing a moderately distorted octahedral geometry of {FeN₆} coordination chromophores. All five compounds exhibit thermal spin crossover with T_1/2 temperatures allocated above 400 K. The theoretical calculations supported the experimental magnetic investigation and helped to explain the electronic structures of the reported complexes with respect to the occurrence of thermal spin state switching. In addition, compound 4 was employed for the preparation of Langmuir–Blodgett films and fabrication of molecular films using the method of spontaneous evaporation of the subphase. While the formation of Langmuir–Blodgett films was unsuccessful due to the instability of the compound at the water/air interface, the latter technique allowed the formation of molecular films of 4 with well-defined thickness and homogeneity. [ABSTRACT FROM AUTHOR]