Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation

The tetradentate ligand, 2-(pyrid-2′-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp2 center separating the two bipyridine-like subunits. Thus, the four pyridine rings of ppq present a neutral, square planar host that is well suited to first-row transition metals. When reacted with FeCl3, a μ-oxo-bridged dimer is formed having a water bound to an axial metal site. A similar metal-binding environment is presented by a bis-phenanthroline amine (dpa) which forms a 1:1 complex with FeCl3. Both structures are verified by X-ray analysis. While the FeIII(dpa) complex shows two reversible one-electron oxidation waves, the FeIII(ppq) complex shows a clear two-electron oxidation associated with the process H2O–FeIIIFeIII → H2O–FeIVFeIV → OFeVFeIII. Subsequent disproportionation to an FeO species is suggested. When the FeIII(ppq) complex is exposed to a large excess of the sacrificial electron-acceptor ceric ammonium nitrate at pH 1, copious amounts of oxygen are evolved immediately with a turnover frequency (TOF) = 7920 h–1. Under the same conditions the mononuclear FeIII(dpa) complex also evolves oxygen with TOF = 842 h−1.