Structures and spin states of bist (tridentate)-type mononuclear and triple helicate dinuclear Iron(II) complexes of Lmidazole-4-carbaldehyde azine

Mononuclear [Fe(H₂L^R)₂]²⁺ and dinuclear [Fe₂(H₂L^R)₃]⁴⁺ (R = H, 2-Me, 5-Me) complexes containing the new imidazole-4-carbaldehyde azine ligand (H₂L^H) and its derivatives (H₂L ^2-Me and H₂L^5-Me) prepared from the condensation reaction of 4-formylimidazole or 2-methyl- or 5-methyl-4- formylimidazole with hydrazine (2:1) were prepared, and their magnetostructural relationships were studied. In the mononuclear complexes, H₂L ^R acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H₂L^R acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate. At room temperature, [Fe₂(H ₂L^H)₃](CIO₄)₄ and [Fe₂(H₂L^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 H₂L with electron-donating methyl groups being stronger than H ₂L_H, with the order of the ligand field strengths being H₂L^2-Me > H₂L^H. However, in the mononuclear [Fe(H₂L_H)₂](ClO₄) ₂ and [Fe(H₂L^2-Me)₂](ClO ₄)2 complexes, a different order of ligand field strengths, H ₂L^H > H₂L^2-Me, was observed because [Fe(H₂L^H)₂](ClO₄) ₂ was in the LS state while [Fe(H₂L^2-M) ₂](ClO₄)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 H₂L^2-Me ligand and the other ligand in [Fe(H₂L^2-Me)₂](ClO ₄)₂ is responsible for the observed change in the spin state. Two kinds of crystals, needles and blocks, were isolated for [Fe ₂(H₂L^H)₃](BF₄) ₄, and both exhibited a sharp spin transition, [LS-HS] [HS-HS]. The spin transition of the block crystals is more abrupt with a hysteresis, T _c↑ = 190 K and T_c↓ = 183 K withΔT= 7 K.