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

Two new mononuclear FeII complexes, [FeL1(NCBH3)(2)] (1) and [FeL2(CN)(2)]center dot 3H(2)O (2) (L-1 = N, N'-bis(2-pyridylmethyl)-1,2-ethanediamine, L-2 = 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 N-2 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 Mossbauer studies. Complex 2 was isolated in the presence of air. One of the secondary amine groups in L-1 undergoes an in situ oxidative dehydrogenation, forming a new monoimine asymmetric ligand L-2. Besides, a CN-co-ligand is also in situ generated from NCBH3- during the reaction. The strong ligand field strength imposed by CN- and L-2 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 x 10(-5) S cm(-1) at 55 degrees C and 95% relative humidity.