Control of the spin state by charge and ligand substitution: two-step spin crossover behaviour in a novel neutral iron(II) complex.

The influence of the charge and steric hindrance on the spin state of a series of four monomeric Fe(II) complexes derived from the tridentate ligands 2-(1H-benzoimidazol-2-yl)-1,10-phenanthroline (Hphenbi) and 2-(1H-benzoimidazol-2-yl)-9-methyl-1,10-phenanthroline (Hmphenbi) and their deprotonated forms (phenbi(-), mphenbi(-)) are investigated. The crystal structure and magnetic properties show that [Fe(Hphenbi)2](BF4)2·1.5C6H5NO2·H2O (1) and its neutral form [Fe(phenbi)2](0)·2CHCl3·H2O (2) are low-spin complexes at 400 K due to the strong ligand field imparted by the terpyridine-like ligand. In contrast, the steric hindrance induced by the methyl group in [Fe(Hmphenbi)2](BF4)2 (3) stabilizes the high-spin state of the Fe(II) ion at all temperatures. Application of a hydrostatic pressure of 0.43 GPa shows that3 displays incomplete thermal-induced spin crossover behaviour. However, upon deprotonation of the ligand the resulting neutral complex [Fe(mphenbi)2]·2CHCl3 (4) shows a complete two-step spin crossover behaviour at ambient pressure.