Lattice-solvent controlled spin transitions in iron(ii) complexes

Abstract

A series of spin transition (ST) iron(II) compounds of the type [Fe^IIL₂](X)₂·{S}₂ (where L is 4′-(4‴-cyanophenyl)-1,2′:6′1″-bispyrazolylpyridine, X = ClO₄⁻ or BF₄⁻, and S is acetonitrile) was synthesized and magnetically investigated. The effects of the removal of the lattice-solvent molecules and of their different positions relative to the iron(II) cations on the ST process were investigated. Crystallization yields orange block (A·{S}₂) crystals of the composition [Fe^II(L)₂](ClO₄)₂·{S}₂, and two polymorphic compounds of the stoichiometry [Fe^II(L)₂](BF₄)₂·{S}₂ as red coffin (B·{S}₂) and orange block (C·{S}₂) crystals. The Fe–N bond distances of A·{S}₂ (from 1.921(9) to 1.992(3) Å; at 150 K), B·{S}₂ (from 1.943(2) to 2.017(2) Å; at 180 K) and C·{S}₂ (from 1.883(3) to 1.962(3) Å; at 180 K) indicate low spin (LS) states of the respective iron(II) ions. Notably, the observed small difference in the Fe–N distances at 180 K for the two polymorphs B·{S}₂and C·{S}₂ are due to different positions of the acetonitrile molecules in the crystal lattices and illustrate the sensitivity of the spin transition properties on lattice-solvent effects. Variable-temperature single crystal X-ray studies display single-crystal thermochroism (red (LS) ↔ orange (HS)) for A·{S}₂ and B·{S}₂ and ca. 3.6% decrease in the unit cell volume of A·{S}₂ from 4403 ų at 300 K to 4278 ų at 150 K. The temperature dependent magnetic susceptibilities of A·{S}₂ and B·{S}₂ demonstrate systematic increase of the spin transition temperatures (T_1/2) and continuous decreases of the hysteresis loop width (ΔT_1/2) upon slow lattice-solvent exclusion.