Effects of Ligand Substituents in a Family of Fe(II) Spin Crossover Complexes

Seven different 2,6-di(1H-pyrazol-1-yl)pyridine ligands were synthesized from 4-cyano-2,6dichloropyridine and 4-methyl-2,6-dichloropyridine and complexed with Fe(II) salts. The spin state of the subsequent materials has been investigated in both the solution and solid state. 4-Halopyrazolyl Fe(II) bpp complexes show anomalous behavior in solution where their spin state appears intermediate between that of high spin and low spin, this has been attributed to decomposition of their complexes in solution. In the solid state the [Fe(bpp)2][X]2 salts have shown the ability to undergo thermal spin crossover. Salts that pack in a specific motif known as a terpyridine embrace undergo spin crossover with hysteresis with remarkable consistency whilst those that do not remain low spin .