Ligand-Induced Distortions and Magneto-Structural Correlations in a Family of Dinuclear Spin Crossover Compounds with Bipyridyl-Like Bridging Ligands

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  • Dedicated to the memory of Professor Olivier Kahn on the occasion of his 70th birthday

Abstract

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-pyridyl)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 13, 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).

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