In air a spin crossover active iron(ii) complex of amine/NCBH3− ligands is converted to a low spin complex of imine/CN− ligands

Abstract

Two new mononuclear Fe^II complexes, [FeL₁(NCBH₃)₂] (1) and [FeL₂(CN)₂]·3H₂O (2) (L₁ = N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine, L₂ = N-(2-pyridylmethyl)-N′-(2-pyridylmethylene)-1,2-ethanediamine) were synthesized from the same starting solution under different atmospheric conditions. Complex 1 was isolated under an N₂ atmosphere with an expected molecular structure, namely a tetradentate L₁ ligand and two NCBH₃⁻ co-ligands wrapping an iron(II) ion. It exhibits a gradual spin crossover centered around 355 K, as confirmed by X-ray crystallography, magnetic, DSC and Mössbauer studies. Complex 2 was isolated in the presence of air. One of the secondary amine groups in L₁ undergoes an in situ oxidative dehydrogenation, forming a new monoimine asymmetric ligand L₂. Besides, a CN⁻ co-ligand is also in situ generated from NCBH₃⁻ during the reaction. The strong ligand field strength imposed by CN⁻ and L₂ stabilizes 2 in the LS state. Solvent water molecules in complex 2 are hydrogen bonded into a well-defined 1D water chain. 2 shows a proton conductivity of 8.9 × 10⁻⁵ S cm⁻¹ at 55 °C and 95% relative humidity.