Complexes of structural analogues of terpyridyl with iron and zinc; the x-ray crystal structure of bis[2,6-bis(benzimidazol2-yl)pyridine]iron(II) trifluoromethylsulphonate bis-ethanol solvate

Complexes of iron(II) and zinc(II) with a series of tridentate ligands of the type [2,6-bis(benz-X-azol-2-yl)pyridine] (X = imid, thi, ox) have been prepared. The crystal structure of [Fe(2,6-bis(benzimidazol-2-yl)pyridine)2](CF₃SO₃)₂·2C₂H₅OH shows the iron(II) to be low spin with a meridional octahedral coordination by two tridentate ligands. ⁵⁷Fe Mössbauer spectra of the complexes [FeL₂]²⁺ show strong dependence of ligand strength on the non-coordinated heteroatom, X, of the ligand: benzimidazole ligands give low spin complexes, benzoxazoles give high spin complexes and benzthiazoles show spin-crossover properties. Derivatization of the imidazole nitrogen has only a small effect on the ligand strength. The complexes [FeLX₂] (X = anion) are all high spin. Partial dissociation is observed in solvents with a strong affinity for iron(II); the iron(II) complexes show metal-to-ligand charge transfer transitions whose intensity depends strongly on the spin state of the iron