Spin Crossover in Iron(II) Complexes of 1-Methyl-2-(Pyridin-2-Yl)imidazole and 1-Methyl-2-(Pyridin-2-Yl)benzimidazole

Abstract

Tris (ligand)iron(II) complexes of 1-methyl-2-(pyridin-2-y1) imidazole and 1-methyl-2-(pyridin-2- y1) benzimidazole are diamagnetic and intensely coloured in solution. Magnetic and electronic and Mossbauer spectral data for the complexes in the solid state show an anomalous temperature-dependence, slight for the imidazole system but strong for the benzimidazole system, which indicates population at elevated temperatures of quintet electronic states. The spin crossover in these systems is displayed to higher temperatures compared to that in the complexes of the unsubstituted ligands. Data from the electronic spectra of the nickel(II) complexes provide a basis for comparison of the behaviour of both the substituted and unsubstituted ligands. These spectra reveal a strong contribution from the 3A_2g → ^lA_g spin-forbidden transition which is expected for ligands which exert a field strength close to the iron(II) crossover value.