Aromatic Substituent Effects on the Spin Crossover

Spin crossover (SCO) compounds are promising for applications in data storage, sensors, and molecular electronics due to their ability to reversibly switch state between high-spin (HS) and low-spin (LS) in response to changes in temperature, pressure, or light, accompanied by variations in electronic structure, magnetism, conductivity, and color.1 The spin transition behavior is mainly governed by ligand field strength and cooperative interactions, necessitating systematic studies to understand the structure–SCO property relationship.2 Aryl groups, characterized by their unique π-conjugation, steric effects, and fluorescence properties, play a crucial role in modulating the structure, electronic behavior and photophysical properties of complexes. In this work, we introduce phenyl (phen), 1-naphthyl (1-nap), 2-naphthyl (2-nap), and anthryl (an) groups into the model spin crossover (SCO) complex Fe(H2Bpz2)2(bipy) (H2Bpz2 = dihydrobis(1-pyrazolyl)borate, bipy = 2,2′-bipyridine) in the C5 and C5ʹ position of the bipy ligand connected by C=N imine bond to afford four Fe(II) complexes Fe(H2Bpz2)2(bipy-R) (R = phen, 1; R = 1-nap, 2; R = 2-nap, 3; R = an, 4) and investigated their SCO properties (Figure 1). Complexes 1 and 3 undergo one-step spin transition with ~57%/50% completeness and T1/2 = 139 K/152 K. Complexes 2 and 4 exhibit two-steps gradual SCO with ~45%/30% completeness (T1/2 = 242 K/289 K) for the first step and ~29%/50% completeness (T1/2 = 137 K/176 K) for the second step revealed by magnetic susceptibility measurements. Unique crystal stacking patterns, rich π··· π interactions raised by aromatic substituent and their influence on SCO behavior in the series complexes were discussed.