Studies of spin crossover in some tris[2-(2′-pyridyl)benzimidazole] iron(II) complexes

Abstract

Four complexes of the bidentate chelating ligand 2-(2′-pyridyl)benzimidazole (pbim) are described: [Fe(pbim)₃]-Br₂; [Fe(pbim)₃][NO₃]₂·OH₂; and [Fe(pbim)₃][ClO₄]₂·xOH₂(x= 1 or 2). The complexes have been characterized by Mössbauer spectra (4.2–300 K), susceptibility measurements (80–310 K), solid-state (80–300 K) and solution visible spectra (298 K), and conductance measurements and i.r. spectra (295 K). All four complexes exhibit ⁵T_2g–¹A_1g spin crossover, the details of which are sensitivie to the nature of the anion and, for the perchlorate derivatives, the number of molecules of water of crystrallization. The latter effect is attributed to hydrogen bonding between water molecules and the imino-hydrogen atom of the ligand. From magnetic-perturb-action Mössbauer spectra, it is deduced that the [Fe(pbim)₃]²⁺ cation has a mer-octahedral configuration in both spin states, indicating substantial inequivalence of the pyridine and imidazole nitrogen atoms in the pbim ligand.