A Molecular Ruthenium Catalyst With Water-Oxidation Activity Comparable To That Of Photosystem Ii

ARTICLES
PUBLISHED ONLINE: 25 MARCH 2012 | DOI: 10.1038/NCHEM.1301

A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II
Lele Duan1, Fernando Bozoglian2, Sukanta Mandal2, Beverly Stewart3, Timofei Privalov3 *, Antoni Llobet2,4 * and Licheng Sun1,5 *
Across chemical disciplines, an interest in developing artificial water splitting to O2 and H2 ,driven by sunlight, has been motivated by the need for practical and environmentally friendly power generation without the consumption of fossil fuels. The central issue in light-driven water splitting is the efficiency of the water oxidation, which in the best-known catalysts falls short of the desired level by approximately two orders of magnitude. Here, we show that it is possible to close that‘two orders of magnitude’ gap with a rationally designed molecular catalyst [Ru(bda)(isoq)2] (H2bda 5 2,2′ -bipyridine6,6′ -dicarboxylic acid; isoq 5 isoquinoline). This speeds up the water oxidation to an unprecedentedly high reaction rate with a turnover frequency of >300 s21. This value is, for the first time, moderately comparable with the reaction rate of 100–400 s21 of the oxygen-evolvingcomplex of photosystem II in vivo.

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plitting water into O2 and H2 using sunlight is a clean and sustainable proposition for addressing many of the energy and environmental problems encountered in our society, because solar energy and water are generally plentiful, and hydrogen 4Hþ þ 4e 2 þ O2), as one is a clean fuel. Water oxidation (2H2O half of the reaction for water splitting, provideselectrons and protons for the second half reaction (hydrogen production), which is catalysed by a proton reduction catalyst. It is the water oxidation step that has long been considered the bottleneck of the water-splitting process, so the invention of highly active and rugged water oxidation catalysts (WOCs) is a key step in the development of light-driven water splitting1–4. In the naturallyoccurring photosystem II (PSII), water oxidation occurs at a [Mn4CaO5] cluster embedded in a complex protein environment5 known as the oxygen-evolving complex (OEC), which produces oxygen at rates estimated at 100–400 s21 (ref. 6). Because replication of the OEC is extremely difficult, a relatively simple metal-containing complex is currently sought that will combine adequate catalytic activity withchemical stability. Indeed, there have been notable developments in relation to mono- and di-nuclear ruthenium- (refs 7–9), iridium- (refs 10–12), manganese- (refs 8,13), cobalt- (refs 14–16) and iron-based (refs 17,18) WOCs, as well as an improved understanding of their O–O bond formation mechanisms (such as water nucleophilic attack of a metal oxo and direct coupling of two metal oxos) since thefirst report of the ‘blue dimer’ by Meyer and co-workers19. Despite these achievements, man-made catalysts are still much slower than the OEC–PSII benchmark. To date, the highest reported turnover frequency (TOF) has yet to exceed 5 s21 (ref. 16), which is clearly inadequate for a practical water-splitting device. To be comparable with in vivo PSII, the reaction rate of catalysed water oxidationshould be at least 100 s21.

We believe we have now successfully narrowed this gap with the rationally designed molecular catalyst presented in this Article. The mononuclear ruthenium complex [Ru(bda)(isoq)2] (1; H2bda ¼ 2,2′ -bipyridine-6,6′ -dicarboxylic acid; isoq ¼ isoquinoline; Fig. 1a) has a much improved catalytic activity (TOF . 300 s21) for water oxidation when using CeIV (Ce(NH4)2(NO3)6)as the oxidant under acidic conditions. The reaction rate corresponding to this TOF equates to the formation of 0.72 l of O2 per milligram of catalyst 1 per minute. The encouragingly high catalytic activity of 1 is combined with a high chemical stability (turnover number, TON ¼ 8,360+91). Complex 1 is the result of the rational development of ruthenium-WOCs (Ru-WOCs) with a general...