Synthesis and characterization of tripodal iron(II) complexes prepared from 2-pyridinecarboxaldehyde and 1-methyl-2-imidazolecarboxaldehyde: stabilization of iron(II) cations with N6 donor sets

Reaction in air of iron(III) salts with tripodal ligands formed from the condensation of tris-(2-aminoethyl)amine (tren) with three equivalents of 2-pyridinecarboxaldehyde (py) or 1-methyl-2-imidazolecarboxaldehyde (NCH3Im) yielded exclusively the iron(II) complexes, [Fetren(py)3]X2 (X=ClO4 − or PF6 −) or [Fetren(NCH3Im)3](ClO4)2. The complexes were characterized by EA, IR, UV, Mössbauer, and mass spectroscopy. The structure of [Fetren(py)3](ClO4)2 was determined at 100 and 290 K. The structures are essentially the same and feature an octahedral iron with facial coordination of pyridine and imine nitrogen atoms with average bond distances of 1.9747 and 1.9523 Å, respectively, at 290 K. The short Fe–N bond distances and lack of variation with temperature support a low spin, 1A, assignment for the iron atom. The center nitrogen atom of the tren is essentially planar and is outside of bonding interaction with the iron, 3.45 Å. The low spin assignment is supported by Mössbauer spectroscopy, which reveals the presence of two low spin forms that are not in thermal equilibrium. In contrast, the Mössbauer spectrum of [Fetren(NCH3Im)3](ClO4)2 reveals the presence of both high spin, 5T, and low spin, 1A, forms at room temperature, which on cooling to 77 K simplify to the low spin form.