Polymorphism and Pressure Driven Thermal Spin Crossover Phenomenon in [Fe(abpt)2(NCX)2] (X = S, and Se): Synthesis, Structure and Magnetic Properties

[EN] The monomeric compounds [Fe(abpt)(2)(NCX)(2)] (X = S (1), Se (2) and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) have been synthesized and characterized. They crystallize in the monoclinic P2(1)/n space group with a = 11.637(2)Angstrom, b = 9.8021(14)Angstrom, c = 12.9838(12)Angstrom, beta=101.126(14)degrees, and Z=2 for 1, and a=11.601(2)Angstrom, b=9.6666(14)Angstrom, c=12.883(2)Angstrom, beta=101.449(10)degrees, and Z= 2 for 2. The unit cell contains a pair mononuclear [Fe(abpt)2(NCX)21 units related by a center of symmetry. Each iron atom, located at a molecular inversion center, is in a distorted octahedral environment. Four of the six nitrogen atoms coordinated to the Fe(H) ion belong to the pyridine-N(l) and triazole-N(2) rings of two abpt ligands. The remaining trans positions are occupied by two nitrogen atoms, N(3), belonging to the two pseudo-halide ligands. The magnetic susceptibility measurements at ambient pressure have revealed that they are in the high-spin range in the 2 K-300 K temperature range. The pressure study has revealed that compound 1 remains in high-spin as pressure is increased up to 4.4kbar, where an incomplete thermal spin crossover appears at around T-1/2 = 65 K. Quenching experiments at 4.4 kbar have shown that the incomplete character of the conversion is a consequence of slow kinetics. Relatively sharp spin transition takes place at T-1/2 = 106, 152 and 179 K, as pressure attains 5.6, 8.6 and 10.5 kbar, respectively.This work was funded by the European Commission trough TMR-Network Thermal and Optical Switching of Spin States (TOSS) , Contract No. ERB-FMRX-CT98-0199EEC/TMR, the Deutsche Forschungsgemeinschaft, Schwerpunktprogramm Molekularer Magnetismus ,the Fondsder Chemischen Industrie and the Ministerio Español de Ciencia y Tecnología (project BQU 2001-2928).Gaspar, AB.; Muñoz Roca, MDC.; Moliner, N.; Ksenofontov, V.; Levchenko, G.; Gütlich, P.; Real, JA. (2003). Polymorphism and Pressure Driven Thermal Spin Crossover Phenomenon in [Fe(abpt)2(NCX)2] (X = S, and Se): Synthesis, Structure and Magnetic Properties. Monatshefte für Chemie - Chemical Monthly. 134(2):285-294. https://doi.org/10.1007/s00706-002-0508-5285294134