A molecular ruthenium catalyst with water-oxidation activity comparable to that of photosystem II

Abstract

Across chemical disciplines, an interest in developing artificial water splitting to O(2) and H(2), 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)] (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline). This speeds up the water oxidation to an unprecedentedly high reaction rate with a turnover frequency of >300 s(-1). This value is, for the first time, moderately comparable with the reaction rate of 100-400 s(-1) of the oxygen-evolving complex of photosystem II in vivo.

Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes

“…31 is analogous to those depicted for various homogeneous catalyst systems. 316,323,324 This is not unexpected given the very dispersed and somewhat tenuous nature of the catalytically active hydrous oxide layer. A common feature of these reaction schemes is that the initial catalytic step involves the deprotonation of a metal coordinated water molecule.…”

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

“…18 A complete Pourbaix diagram is offered in Figure 2B where the zones of predominant species derived from 2 II (H2O) with different electron/proton content are shown. An interesting feature of the Pourbaix diagram for 2 IV (OH) + is the pH 10-14 zone, that has been built by extrapolating the values of the V/IV couple at this pH and by assuming that the redox potential for the Ru V =O/Ru IV =O couple for [Ru IV (bda)(Me-py)2(O) eq ], 3 IV (O) (1.12 V), 15 is the same as for 2 IV (O) (Me-py is 4-methylpyridine). This assumption is based on Lever's approach 22,23,24 where it is shown that the Ru III /Ru II couple for a set of complexes can be calculated based on additive contribution of their respective ligands bonded to the first coordination sphere.…”

“…4 Molecular transition metal complexes constitute an excellent platform to examine these factors since significant information based on an arsenal of spectroscopic, electrochemical and analytical techniques can be used together with the valuable complementary information provided by computational studies. 5,6,7,8,9,10,11,12,13,14 The best water oxidation catalysts reported today are based on seven coordinated Ru complexes containing dianionic ligands such as [2,2'-bipyridine]-6,6'-dicarboxylato (bda 2-) 15,16,17 and [2,2':6',2''-terpyridine]-6,6''-dicarboxylato (tda 2-) (see Figure 1 for drawn structures of these ligands). Particularly impressive is the seven coordinate complex [Ru IV (tda--N 3 O)(py)2(O) eq ], 4 IV (O), (the superscript in roman numbers indicates the formal oxidation state of Ru; py is pyridine; the "eq" superscript means equatorial) that is capable of oxidizing water to dioxygen at maximum turnover frequencies (TOFMAX) of 7,700 s -1 and 50,000 s -1 at pH = 7.0 and pH = 10.0 respectively.…”

A Dynamic View of Proton-Coupled Electron Transfer in Photocatalytic Water Splitting

“…[6][7][8][9][10][11][12] A photoelectrochemical solar fuel cell device combines the functions of light harvesting, charge separation and catalysis. [13][14][15] In the last decade several systems have been proposed employing either metal oxide nanoparticles 8,[16][17][18][19][20][21][22][23][24] or molecular complexes 8,[25][26][27][28] as water oxidation catalyst (WOC). Furthermore, the coupling between the WOC, the chromophore and an electron accepting semiconductor into a photoanode has been achieved through co-absorption of both the catalyst and the chromophore 16,[29][30][31][32] or through dye-WOC supramolecular complexes.…”