Thermal and photoinduced spin-crossover of mononuclear Fe II complexes based on bppCHO ligand.

Two new iron(II) complexes [Fe(bppCHO) 2 ](ClO 4 ) 2 (1·ClO4) and [Fe(bppCHO) 2 ](BF 4 ) 2 ·H 2 O (1·BF4) were synthesized by using 2,6-di(1 H -pyrazol-1-yl)isonicotinaldehyde (bppCHO) as the ligand. The structures and spin states of the complexes were characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. Complex 1·ClO4 exhibits light-induced excited spin state trapping (LIESST) up to 53 K and two-step spin-crossover (SCO) over room temperature, in which the first step shows a thermal hysteresis of 26 K, whereas 1·BF4 exhibits a gradual SCO behavior. A magnetostructural study reveals that the difference in the SCO property is related to the supramolecular interactions and crystal packing effect.