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 sp(2) 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 mu-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 Fe-III(dpa) complex shows two reversible one-electron oxidation waves, the Fe-III(ppq) complex shows a clear two-electron oxidation associated with the process H2O-(FeFeIII)-Fe-III -> H2O-(FeFeIV)-Fe-IV -> O=(FeFeIII)-Fe-V. Subsequent disproportionation to an Fe=O species is suggested. When the Fe-III(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 Fe-III(dpa) complex also evolves oxygen with TOF = 842 h(-1).