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