Thermal and Photoinduced Spin Crossover in a Mononuclear Iron(II) Complex with a Bis(pyrazolyl)pyridine Type of Ligand

Abstract

Abstract A mononuclear iron(II) compound 1 of the general formula [Fe( L ) 2 ](ClO 4 ) 2 { L = 4‐[2,6‐bis(pyrazol‐1‐yl)pyridin‐4‐yl]benzaldehyde} was prepared and structurally characterised. Single‐crystal X‐ray structure analysis revealed the presence of a complex dication [Fe( L ) 2 ] 2+ and two ClO 4 – counteranions within the unit cell. The bond lengths and angles within the coordination polyhedron FeN 6 indicate the low‐spin state of the central iron(II) metal ion at T = 180 K. Magnetic investigations elucidate spin crossover with T 1/2 = 285 K. The experimental magnetic susceptibility data could be satisfactorily fitted with the Curie law in combination with the Ising‐like model. The room‐temperature character of the spin crossover was further studied by variable‐temperature far‐IR, Vis and Mössbauer spectroscopy. Laser irradiation of 1 carried out at 10 K gives rise to a complete low‐spin to high‐spin photoconversion. A subsequent temperature‐dependent investigation revealed the existence of a photoexcited metastable HS state up to T LIESST = 70 K (LIESST = light‐induced excited‐spin‐state trapping) as well as the presence of a light‐induced thermal hysteresis loop with a width of 10 K.