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

A mononuclear iron(II) compound 1 of the general formula [Fe(L)(2)] (ClO4)(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 ClO4- counteranions within the unit cell. The bond lengths and angles within the coordination polyhedron FeN6 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 Mossbauer 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 = 70K (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.