Influence of Halogen Atoms on Spin-Crossover Properties of 1,2,4-Triazole-Based 1D Iron(II) Polymers

The introduce of halogen atoms (F, Cl, Br, I) to the triazole-based ligand N-benzyl-4-amino-1, 2, 4-triazole (L) have shown a significant effect on the spin-crossover (SCO) properties of the corresponding one-dimensional (1D) coordination Fe (II) polymers with chemical formula [Fe(4-X-L)(3)](BF4)(2) (X= F, Cl, Br, and I). Compared with the [FeL3](BF4)(2), the electronegative halogen atoms have increased the pi-acceptor character of these ligands, which thus improved the ligand field strength and in-duced the occurrence of SCO for the Fe (II) polymers at higher temperatures. The SCO transition temperatures T-1/2 have changed from 200 K to 249 (I), 250 (Br), 252 (Cl) and 275 (F) K, respectively. More importantly, the UV-Vis absorption spectrum and density functional theory (DFT) calculations indicated that the transition temperatures and the width of thermal hysteresis loops have shown a positive correlation with the electronegativity of the halogen atoms.