Structural diversity in iron(ii) complexes of 2,6-di(pyrazol-1-yl)pyridine and 2,6-di(3-methylpyrazol-1-yl)pyridine

Abstract

The syntheses, magnetochemistry and crystallography of [Fe(L¹)₂]I_0.5[I₃]_1.5 (1), [Fe(L¹)₂][Co(C₂B₉H₁₁)₂]₂ (2) and [Fe(L²)₂][SbF₆]₂ (3) (L¹ = 2,6-di(pyrazol-1-yl)pyridine; L² = 2,6-di(3-methylpyrazol-1-yl)pyridine) are described. Compounds 1 and 3 are high-spin between 5–300 K. For 1, this reflects a novel variation of an angular Jahn–Teller distortion at the iron centre, which traps the molecule in its high-spin state. No such distortion is present in 3; rather, the high-spin nature of this compound may reflect ligand conformational strain caused by an intermolecular steric contact in the crystal lattice. Compound 2 exhibits a gradual high → low spin transition upon cooling with T_½ = 318 ± 3 K, that is only 50% complete. This reflects the presence of two distinct, equally populated iron environments in the solid. One of these unique iron centres adopts the same angular structural distortion shown by 1 and so is trapped in its high-spin state, while the other, which undergoes the spin-crossover, has a more regular coordination geometry. In contrast with 3, the solvated salts [Fe(L²)₂][BF₄]₂·4CH₃CN and[Fe(L²)₂][ClO₄]₂·(CH₃)₂CO both undergo gradual thermal spin-transitions centred at 175 ± 3 K.