Polymorphism and pressure driven thermal spin crossover phenomenon in [Fe(abpt)2(NCX)2] (X = S, and Se):: Synthesis, structure and magnetic properties

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.