Structure, magnetism and photomagnetism of mixed-ligand tris(pyrazolyl)methane iron(ii) spin crossover compounds

Abstract

A range of bis-facial tridentate chelate complexes of type [Fe((R-pz)₃CH)((3,5-Me₂pz)₃CH)](BF₄)₂ has been characterised that contain two different tris-pyrazolylmethane ligands, with variations in R being H (complex 1 crystallised as polymorphs 1a and 1b) and 4-Me (2), as well as R = H with a CH₂OH arm off the methane carbon (3). A tris(pyridyl)methane analogue is also described (4). The tris(3,5-dimethylpyrazolyl)methane co-ligand (3,5-Me₂pz), and the BF₄⁻ counterion, are constant throughout. The spin-crossover properties of these Fe(II) d⁶ compounds have been probed in detail by variable temperature magnetic, Mössbauer spectral and crystallographic methods. The effects of distortions from octahedral symmetry around the Fe(II) centres, of crystal solvate molecules (1.5 MeCN in 2 and 2 MeCN in 3) and of supramolecular/crystal packing, are discussed. In the case of 1, subtle twisting of pyrazole rings occurs, as a function of temperature, that has a greater effect upon the relative positions of the Fe(II) chelate molecules in polymorph 1b than in polymorph 1a; this is thought to drive the cooperativity differences observed in the magnetism of the polymorphs. Comparisons are also made between 1 to 4 and their homoleptic, parent [Fe(L)₂] ²⁺ materials. The complexes were screened for the LIESST (light induced excited spin state trapping) effect by measurements of diffuse absorption spectra on the surface of powder samples, at different temperatures. One example, 2, showed a 2-step thermal spin crossover transition and it was probed in detail for its photomagnetic features. The T(LIESST) and T_1/2 values for 2 did not obey an empirical relationship, T(LIESST) = 150 − 0.3T_1/2 followed by many Fe(II)(N-donor)₆ crossover compounds of the bis-tridentate (meridional) type, and possible reasons for this are discussed.