Fe(II) Complexes Incorporating Anthracene-2-sulfonate Anion: Unraveling Photodimerization Potential and Spin Crossover Behavior

Integrating photoactive anions into spin crossover systems offers a promising route toward light-controlled modulation of spin states, enhancing the utility of these materials in photospin information storage. In this study, we report the synthesis and characterization of a novel Fe(II) salt, [Fe(H 2 O) 6 ](2-As) 2, incorporating anthracene-2-sulfonate (2-As – ) as the counteranion, which holds the potential to undergo solid-state photodimerization under favorable packing conditions. Utilizing this salt as a precursor, three mononuclear Fe(II) complexes were successfully synthesized: [Fe(2-pic) 2 (2-As) 2 ] ( 1, 2-pic = pyridin-2-ylmethanamine), [Fe(2,6-hdp) 2 ](2-As) 2 ( 2, 2,6-hdp = 2,6-bis(( E )-1-hydrazineylideneethyl) pyridine), and [Fe(3-bpp) 2 ](2-As) 2 ·2MeOH·H 2 O ( 3, 3-bpp = 2,6-di(pyrazol-3-yl) pyridine). Magnetic susceptibility measurements and 57 Fe Mössbauer spectroscopy reveal that complexes 1 and 2 remain in high-spin and low-spin states, respectively, over the 4–400 K range. Complex 3 exhibits a low-spin state in the solvated form but undergoes a gradual and incomplete spin crossover behavior with a small thermal hysteresis centered at ∼290 K upon desolvation. Single-crystal X-ray diffraction indicates that the 2-As – anion adopts a face-to-edge stacking mode rather than the face-to-face arrangement required for solid-state photoreactivity. However, the extensive hydrogen bonding observed around the sulfonate groups suggests a viable crystal-engineering strategy to modulate anthracene stacking and enable future photoinduced spin-state switching.