Aromatic Substitution Modulated Spin Crossover Behavior in the Model Fe(H2Bpz2)2(bipy) Complex

Spin crossover (SCO) materials are attractive ones for potential applications in high-density information storage, sensors, display devices and molecular electronics, for which 57Fe Mössbauer spectroscopy can play a key role. Fine-tuning of the ligand filed around the central metal ion and supramolecular interactions in the crystal packing contribute to influence the SCO property positively and bring valuable information about magneto-structural correlations, thus increasing general knowledge in the field. The SCO complex Fe(H2Bpz2)2(bipy)1 is a representative example. Modifying the molecular structures can not only adjust the transition temperature T1/2 but also add new physicochemical properties. Our group has recently investigated the effect of alkyl, ester and halogen substitutions on the SCO property of Fe(H2Bpz2)2(bipy) (H2Bpz2 = dihydrobis(1-pyrazolyl)borate; bipy = 2,2′-bipyridine) before.2-4 Here, we introduced different aromatic substitutions, 1-phenylmethanimine (1), 1-(naphthalen-1-yl)methanimine (2), 1-(naphthalen-2-yl)methanimine (3), and 1-(anthracen-9-yl)methanimine (4) in the C5 and C5ʹ position of the bipy ligand and synthesized four H2Bpz2-containing FeII complexes (Figure 1, left). Variable-temperature direct current magnetic susceptibility measurements on polycrystalline samples under 1000 Oe reveal diversiform incomplete SCO behavior (Figure 1, right). Complex 3 shows the most abrupt transition at around T1/2 = 150 K, complex 2 exhibits two steps slow spin transition at around 250 K and 130 K while complex 4 shows the smoothest SCO profile as the change of temperature. Small hysteresis loops were observed in complexes 1-3 at around 70 K, but no hysteresis loop appears at around the transition temperatures. Single crystal structure analysis and 57Fe Mössbauer spectroscopy were used to help explain the magnetic performances.