Substitution-Tuned Spin Crossover in a Family of Schiff Base-Like Ligand-Supported Fe^II Complexes

Spin crossover (SCO) complexes have been emerging at the forefront of molecular magnetism because of their unique stimuli-responsive bistability character. Rational design and controllable regulation of key parameters, such as the transition ratio, transition temperature (T1/2), and thermal hysteresis, are essential for their practical applications. These parameters are affected by numerous factors, such as substitutions, counterions, guest molecules, and physical stimuli, where the substitutions can directly influence the coordination ligand field via electronic/steric effects. To access substitution-tuned spin crossover, we synthesized three neutral mononuclear complexes: [FeL1(4-avp)2]·CH2Cl2 (1), [FeL2(4-avp)2]·CH2Cl2 (2), and [FeL3(4-avp)2] (3), utilizing the monodentate ligand 4-(2-(anthracen-9-yl)vinyl)pyridine (4-avp) and N2O22– coordinating Schiff base-like tetradentate ligands L1–L3 with different substituents (L1 bearing a naphthyl; L2 bearing an o-xylene; L3 bearing an o-difluorobenzene). 1 and 2 underwent complete and partial thermally induced spin crossover (1: T1/2 = 158 K; 2: T1/2 = 112 K), respectively, while 3 retained the high-spin state over the measured temperature range. Magneto-structural studies indicate that the substitution of Schiff base-like ligands can influence the molecular packing density and ligand field strength, leading to significant alterations in the spin state and transition temperature of the complexes. This study highlights the impact of substituents on the spin crossover behavior in mononuclear complexes with N2O22– coordinating Schiff base-like ligands, offering valuable insights into the design of molecules with tunable SCO properties.