Moessbauer studies of spin crossover in two-dimensional iron complexes

A hexadentate N6 tripodal ligand, H3LMe (tris[2-(((2-methylimidazol-4-yl) methylidene)- amino)ethyl]amine), affords three kinds of 2D assemblies, [FeIIH3LMe][FeIIILMe](X)2 (1), [FeIIH3LMe][FeIILMe]X (2) and [FeIIH3LMe) Cl-X (3). In the present study, we have employed the 57Fe Moessbauer spectroscopy to understand their complicated spin crossover (SCO) behaviors. Comound l (X- = NO3-) consists of extended 2D puckered sheets based on imidazole- imidazo- late hydrogen bonds between the FeII and FeIII components. Moessbauer results showed that l exhibits a successive two step SCO behavior: LS (low spin) FeII-LS FeIII ↔ HS (high spin) FeII-LS FeIII ↔ HS FeII-HS FeIII. The temperature dependence of the Moessbauer line-shape of the FeIII site was characteristic of a 'rapid spin interconversion'. Two types of FeII ions in 2 (X- = ClO4-, PF6-, etc.), which has a 2D sheet structure analogous to that of 1, were not distinguished by the spectroscopy. On the other hand, the derived HS/LS ratio vs, T data indicated that the two types of FeII ions have different spin transition temperatures. The size of X- influences the HS/LS ratio vs. T profile. A steep LS FeII ↔ HS FeII transition was found for 3, similarly to 1 and 2, due to the 2D layer structure constructed by NH ··· Cl- hydrogen bonds. The Moessbauer spectroscopy revealed the variety of SCO properties of 3, including the time- dependent 'frozen-in effect'.