Constructing Alkyl Chain Modified FeII Spin Crossover Complexes through Complementary Pair Strategy

Four alkyl chains and cyclohexane modified Fe II complexes [Fe(L n )(L’)](ClO 4 ) 2 ⋅solv (n=1, 2, 3, 4) were synthesized and characterized. X‐ray diffraction study showed that these complexes were constituted by asymmetric mononuclear Fe II entities, incorporating alkyl‐chain‐modified bpp COOH (2,6‐bis(1H‐pyrazol‐1‐yl)isonicotinic acid) ligands and 6,6’’‐2,6‐dimethoxyphenyl‐substituted terpy ligand (L’). Magnetic susceptibility measurements revealed that complexes 1 , 2 , and 3 were in the high spin state across the measured temperature range, whereas complex 4 displayed an incomplete spin crossover phenomenon, with a transition temperature ( T 1/2 ) at 240 K. Investigations into variable‐temperature structure and magneto‐structural correlations revealed that the integration of alkyl‐chain‐modified bipyridyl units fostered the π⋅⋅⋅π intra‐ and intermolecular interactions, contributing to distinct crystal stacking and spatial configurations in complex 4 when compared to its counterparts. These findings underscore the pivotal influence of intramolecular interactions on the Fe II spin states and highlight the significance of designing flexible ligands for modulating spin‐crossover characteristics.