Water oxidation blue dimer catalyst

The first synthetic, non-proteic molecular catalyst capable to oxidize water was reported about 30 years ago [69]. This was the so-called blue dimer, [(bpy)2Ru (H20)(p-0)Ru(H20)G>py)2] - Once electrochemically or chemically activated, the blue dimer undergoes the stepwise loss of four electrcms and four protons, producing an intermediate reactive species that oxidizes water [70,71]. Unfortunately, the blue dimer loses its catalytic efficiency after a few cycles due to the degradation of the organic ligands. However, the blue dimer paved the way to the discovery of other water oxidation catalysts, most of them still based on ruthenium centers [72—78]. In the last few years, molecular catalysts based on iridium centers [79] as well as on cheaper metals such as manganese [80-84], cobalt [85], and iron [86] were also developed. 

One of the most significant attempts to heterogenize a molecular WOC regarded the immobilization of a phosphonate-modified blue dimer onto ITO or nanocrystalline Ti02 and Zr02. The catalyst retains its water oxidation ability on the electrodes surface, as well as its proton-coupled electron transfer (PCET) properties. 

Kuznetsov AE, Geletii YV, fEU CL, Morokuma J, Musaev DG. Dioxygen and water activation processes on mrJti-Ru-subsrituted polyoxometalates comparison with the blue-dimer water oxidation catalyst. J Am Chem Soc. 2009 131 6844-6854. 

The failure of the mononuclear [Ru(bpy)2(py)OH2] to promote oxygen evolution followed by success of the blue dimer and the fact that the OEC itself contains a multi-metal centre, lead researchers to believe that multiple sites were required for catalytic water oxidation. However, the mechanism proposed for the activation of the blue dimer showed that in reality, only one of the metal centres is involved in the oxygen-oxygen bond formation step. This suggestion was later confirmed when Meyer and co-workers showed that single site ruthenium-aqua complexes were capable of oxidizing water in the presence of Ce(iv). One of these catalysts, [Ru(tpy)(bpm)OH2] " (tpy = terpyridine, bpm = 2,2 -bipyrimidine), is shown in Scheme 5.2. 

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