Structure–Property Relations in Aryl-Functionalized [Fe(H2Bpz2)2(bipy-R)] Spin Crossover Complexes

Aryl substituents, characterized by unique π-conjugation, steric effects, and optical properties, play a crucial role in modulating the structure, electronic and photophysical properties of coordination complexes. Herein, phenyl, 1-naphthyl and 2-naphthyl groups were introduced at the C5/C5′ positions of the bipyridine ligand into the prototypical spin crossover complex [Fe(H2Bpz2)2(bipy)] (H2Bpz2 = dihydrobis(1-pyrazolyl)borate, bipy = 2,2′-bipyridine) via C═N linkage, affording three Fe(II) complexes [Fe(H2Bpz2)2(bipy-N═C–R)] (R = phenyl (1), 1-naphthyl (2), and 2-naphthyl (3)). Magnetic susceptibility and 57Fe Mössbauer studies reveal gradual and incomplete one-step spin crossover for 1 (∼58% conversion (T1/2 = 140 K) and 3 (52% conversion, T1/2 = 154 K). In contrast, 2 displays a gradual two-step spin crossover, with ∼52% conversion at high temperature (T1/2 = 239 K) and a second ∼22% step at low temperature (T1/2 = 124 K). Variable-temperature single-crystal X-ray diffraction reveals that inequivalent Fe(II) sites with different spin transition activity, symmetry breaking, intermolecular π···π interactions are collectively responsible for the incomplete and stepwise spin transition. Variable-temperature photoluminescence measurements reveal a gradual decrease in fluorescence intensity upon heating for 1 and 3, whereas 2 exhibits fluorescence enhancement within the spin crossover region. These findings highlight the role of aryl substituents in governing spin transition profiles.