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.