Local coordination geometry and spin state in novel Fe(II) complexes with 2,6-bis(pyrazol-3-yl)pyridine-type ligands as controlled by packing forces: structural correlations.

A substituted 2,6-bis(pyrazol-3-yl)pyridine (3-bpp) ligand, H(4)L, created to facilitate intermolecular interactions in the solid, has been used to obtain four novel Fe(II) complexes: Fe(H(4)L)(2)(2)⋅2 CH(3)NO(2)⋅2 H(2)O, Fe(H(4)L)(H(2)LBF(2))⋅5 C(3)H(6)O (H(2)LBF(2) is an in situ modified version of H(4)L), Fe(H(4)L)(2)(2)⋅2 C(3)H(7)OH and Fe(H(4)L)(2)(2)⋅4 C(2)H(5)OH. Changing of spin-inactive components (solvents, anions or distant ligand substituents) causes differences to the coordination geometry of the metal that are key to the magnetic properties. Magnetic measurements show that, contrary to the previously published complex Fe(H(4)L)(2)(2)⋅H(2)O⋅2 CH(3)COCH(3), the newly synthesised compounds remain in the high-spin (HS) state at all temperatures (5-300 K). A member of the known family of Fe(II)/3-bpp complexes, Fe(3-bpp)(2)(2)⋅1.75 CH(3)COCH(3)⋅1.5 Et(2)O, has also been prepared and characterised structurally. In the bulk, this compound exhibits a gradual and incomplete spin transition near 205 K. The single-crystal structure is consistent with it being HS at 250 K and partially low spin at 90 K. Structural analysis of all these compounds reveals that the exact configuration of intermolecular interactions affects dramatically the local geometry at the metal, which ultimately has a strong influence on the magnetic properties. Along this line, the geometry of Fe(II) in all published 3-bpp compounds of known structure has been examined, both by calculating various distortion indices (Σ, Θ, θ and Φ) and by continuous shape measures (CShMs). The results reveal correlations between some of these parameters and indicate that the distortions from octahedral geometry observed on HS systems are mainly due to strains arising from intermolecular interactions. As previously suggested with other related compounds, we observe here that strongly HS-distorted systems have a larger tendency to remain in that state.