Structures and spin states of mono- and dinuclear iron(II) complexes of imidazole-4-carbaldehyde azine and its derivatives

Abstract

Mononuclear [Fe(H 2L R) 2]X 2 (R = H, 2-Me, 5-Me, 2-Et-5-Me; X = ClO 4, BF 4) and dinuclear [Fe 2(H 2L R) 3]X 4 complexes containing imidazole-4-carbaldehyde azine (H 2L H) and its derivatives prepared by condensation of 4-formylimidazole, 2-methyl- or 5-methyl-4-formylimidazole, or 2-ethyl-4-methyl-5-formylimidazole, with hydrazine in a 2:1 mole ratio in methanol, were prepared and their magnetostructural relationships were studied. In the mononuclear complexes, H 2L R acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H 2L R acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate structure. At room temperature, [Fe 2(H 2L H) 3](ClO 4) 4 and [Fe 2(H 2L 2-Me) 3](ClO 4) 4 were in the high-spin (HS) and low-spin (LS) states, respectively. The results are in accordance with the ligand field strength of H 2L 2-Me with electron-donating methyl groups being stronger than H 2L H, with the order of the ligand field strengths being H 2L 2-Me > H 2L H. However, in the mononuclear [Fe(H 2L H) 2](ClO 4) 2 and [Fe(H 2L 2-Me) 2](ClO 4) 2 complexes, a different order of ligand field strengths, H 2L H > H 2L 2-Me, was observed because [Fe(H 2L H) 2](ClO 4) 2 was in the LS state while [Fe(H 2L 2-Me) 2](ClO 4) 2 was in the HS state at room temperature. X-ray structural studies revealed that the interligand steric repulsion between a methyl group of an H 2L 2-Me ligand and the other ligand in [Fe(H 2L 2-Me) 2](ClO 4) 2 is responsible for the observed change in the spin state. The same is true for [Fe(H 2L 2-Et-5-Me) 2](ClO 4) 2, while [Fe(H 2L 5-Me) 2](ClO 4) 2 does not involve such a steric congestion and stays in the LS state over the temperature range 5–300 K. Two kinds of crystals (polymorphs) were isolated for [Fe 2(H 2L H) 3](BF 4) 4 and [Fe 2(H 2L 2-Et-5-Me) 3](ClO 4) 4, and they exhibited different magnetic behaviors.