Ligand-induced distortions and magneto-structural correlations in a family of dinuclear spin crossover compounds with bipyridyl-like bridging ligands
Résumé
The synthesis and characterization of a new series of dinuclear compounds [(Fe(dpia)(NCBH3)(2)}(2)(A)], where dpia = di(2-picolyl)amine, A = 4,4'-bipyridine (1), 1,2-di(4-pyridy1)ethyne (bpac) (2), or 1,2-di(4-pyridyl)ethane (bpen) (3), are reported. Variable-temperature magnetic susceptibility measurements of the three compounds show that they have different magnetic properties. Compound 1 exhibits a relatively cooperative two-step spin crossover (SCO) behavior. A short plateau at 210 K separates two steps and conforms to about 50 % of the complexes undergoing a thermal spin conversion. Compounds 2 and 3 present gradual full one-step spin transitions. The thermodynamic parameters of 1-3, obtained through the fitting of the magnetic data with the regular solution model, are compared with those for similar dinuclear SCO compounds. The single-crystal X-ray structure of 1 was solved at three temperatures, 290 K, 211 K, and 110 K, corresponding to three spin-state isomers, namely [HS-HS], [HS-LS], and [LS-LS]. At the plateau temperature, both HS (high spin) and LS (low spin) sites in the mixed pair are unresolved and only an average Fe-N bond length was obtained. The structural characteristics and magnetic behavior of the new two-step SCO compound 1 are in full agreement with magneto structural correlations for previously reported related dinuclear SCO compounds. This analysis confirms the validity of a recently proposed mechanism responsible for three types of SCO in dinuclear compounds (one-step, two-step, and partial 50% transition).