Spin crossover in two 1D Fe(II) polymers with 1,2,4-triazole thiourea building blocks

Abstract

The 1D chain [Fe(Etutrz)3](ClO4)2 ⋅ 1.5H2O (2) (Etutrz = 1-ethyl-3-(4H-1,2,4-triazol-4-yl) thiourea), displays pronounced thermochromism with a purple color at 77 K while the sample is white at 300 K. Investigation of magnetic properties reveal an abrupt spin transition around 227 K. Differential scanning calorimetry studies on cooling display a first order phase transition at around 200 K with an entropy variation of ΔS = 61.3 J mol− 1 K− 1. 57Fe Mossbauer spectroscopy of 2 confirms a complete spin transition with a 100% high-spin population at 300 K (isomer shift δHS = 1.04(1) mm/s, quadrupole splitting ΔEQ = 2.86(2) mm/s). The Fe(II) ions convert to the low-spin state at 78K (δLS = 0.53(2) mm/s). The quadrupole splitting, ΔEQ = 0.29(2) mm/s, confirms the presence of distorted octahedra within the 1D chain. The 1D chain [Fe(Etutrz)3](BF4)2 ⋅2MeOH (1) exhibits a different magnetic behavior with a gradual spin conversion at T1/2 = 221 K, whereas thermochromic properties are maintained.