Mononuclear Iron Complexes with Tetraazadentate Ligands as Water Oxidation Catalysts

Two iron(II) complexes with tetraazadentate ligands have been synthesised, characterised and evaluated as water oxidation catalysts. The two ligands, N,N-diisopropyl-N,N-bis(2-pyridylmethyl)-1,2-diaminoethane (1), and N-methyl-N-(2-pyridinylmethyl)-2,2-bipyridine-6-methanamine (2), both give iron(II) complexes (1Fe and 2Fe, respectively) with an octahedral coordination geometry with two labile triflate ligands in a cis configuration in the crystal structures. When treated with cerium(IV) as a chemical oxidant in aqueous solution at room temperature the complexes form semi-stable Fe-IV(O) species that can be detected in the UV/Visible region. Both 1Fe and 2Fe can act as catalysts for water oxidation when treated with a large excess of oxidant, but 1Fe is a better catalyst than 2Fe. Possible geometrical factors behind this difference in reactivity are discussed and compared with literature data.