Structures and Spin States of Bis(tridentate)-Type Mononuclear and Triple Helicate Dinuclear lron(II) Complexes of Imidazole-4-carbaldehyde azine

Mononuclear [Fe(H2LR)(2)](2+) and dinuclear [Fe(H2LR)(3)](4+) (R = H, 2-Me, 5-Me) complexes containing the new imidazole-4-carbaldehyde azine ligand (H2LH) and its derivatives (H2L2-Me and H2L5-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, H2LR acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H2LR acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate. At room temperature, [Fe-2(H2LH)(3)](CIO4)(4) and [Fe-2(H2L2-Me)(3)](CIO4)(4) were in the high-spin (HS) and low-spin (LS) states, respectively. The results are in accordance with the ligand field strength of H2L2-Me with electron-donating methyl groups being stronger than H2LH, with the order of the ligand field strengths being H2L2-M (e)> H2LH, However, in the mononuclearjFe(H2LH)(2)](CIO4)(2) and [Fe(H2L2-Me)(2)](CIO4)(2) complexes, a different order of Iigand field strengths, H2LH > H2L2-Me, was observed because [Fe(H2LH)(2)] (CIO4)(2) was in the LS state while [Fe(H2L2-Me)(2)](CIO4)(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 H2L2-Me ligand and the other ligand in [Fe(H2L2-Me)(2)](CIO4)(2) is responsible for the observed change in the spin state. Two kinds of crystals, needles and blocks, were isolated for [Fe2(H2LH)(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 up arrow = 190 K and T-c down arrow = 183 K with Delta T= 7 K.