The first seven-coordinated iron(II) complex with a nitrogen donor set hepta-dentate macrocyclic polyamine ligand and different coordination modes with its cobalt(II) analogue

The Fe2+ and Co2+ complexes with a hepta-dentate ligand, 1-methyl-4,7,10-tris(2-pyridylmethyl)-1,4,7,10-tetraazacyclododecane (L), have been synthesized and their crystal structures determined by X-ray diffraction analyses. The two complexes have been found to have different coordination modes. The Fe2+ complex (I) is seven-coordinated with the four cyclen (cyclen=1,4,7,10-tetraazacyclododecane) nitrogen donors and all three pendant pyridine nitrogens, adopting a mono-capped distorted octahedral coordination geometry. To our knowledge, this is the first example of a structurally characterized seven-coordinated Fe2+ complex formed with only one N7 donor set ligand. Whereas the Co2+ complex (II) of L is a normal six-coordination with four cyclen nitrogen donors and two pendant pyridines to give a distorted octahedron in which one pendant pyridine nitrogen remains uncoordinated.