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 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 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-Fe(III)Fe(III) → H2O-Fe(IV)Fe(IV) → O═Fe(V)Fe(III). 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).
A new Ru(II)Rh(III) structural motif [(bpy)2Ru(dpp)RhCl(tpy)](4+) with one halide on the Rh(III) center demonstrates light-driven proton reduction ability, establishing that two halide ligands are not mandatory despite all prior systems containing a cis-RhCl2 catalytic site. This new design provides a novel approach to modulate Rh(III) redox behavior and catalytic activity with insight into catalytic intermediates.
A series of Ru(II) complexes that behave as water oxidation catalysts were prepared involving a tetradentate equatorial ligand and two 4-substituted pyridines as the axial ligands. Two of these complexes were derived from 2,9-di-(pyrid-2'-yl)-1,10-phenanthroline (dpp) and examine the effect of incorporating electron-donating amino and bulky t-butyl groups on catalytic activity. A third complex replaced the two distal pyridines with N-methylimidazoles that are more electron-donating than the pyridines of dpp and potentially stabilize higher oxidation states of the metal. The tetradentate ligand 2-(pyrid-2'-yl)-6-(1'',10''-phenanthrol-2''-yl)pyridine (bpy-phen), possessing a bonding cavity similar to dpp, was also prepared. The Ru(II) complex of this ligand does not have two rotatable pyridines in the equatorial plane and thus shows different flexibility from the [Ru(dpp)] complexes. All the complexes showed activity towards water oxidation. Investigation of their catalytic behavior and electrochemical properties suggests that they may follow the same catalytic pathway as the prototype [Ru(dpp)pic2](2+) involving a seven-coordinated [Ru(IV)(O)] intermediate. The influence of coordination geometry on catalytic performance is analyzed and discussed.
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