Manganificent potential: A dinuclear manganese catalyst contains metal centers that are coordinated by a central phenolate, with adjoining imidazole and carboxylate groups, which are all important functionalities in the natural oxygen‐evolving center. The complex catalyzes the conversion of water to oxygen in the presence of a single‐electron oxidant [Ru(bpy)3]3+ (see picture, bpy=2,2′‐bipyridine, A=acceptor).

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During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O2 and solar fuels, such as H2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn2(II,III) catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O-O bond formation.

The synthesis of Mn-based catalysts to mimic the structural and catalytic properties of the oxygen-evolving complex in photosystem II is a long-standing goal for researchers. An interesting result in this field came with the synthesis of a Mn complex that enables water oxidation driven by the mild single-electron oxidant [Ru(bpy)3](3+). On the basis of hybrid density functional calculations, we herein propose a water oxidation mechanism for this bioinspired Mn catalyst, where the crucial O-O bond formation proceeds from the formal Mn4(IV,IV,IV,V) state by direct coupling of a Mn(IV)-bound terminal oxyl radical and a di-Mn bridging oxo group, a mechanism quite similar to the presently leading suggestion for the natural system. Of importance here is that the designed ligand is shown to be redox-active and can therefore store redox equivalents during the catalytic transitions, thereby alleviating the redox processes at the Mn centers.

Die Metallzentren eines zweikernigen Mangankatalysators sind an ein zentrales Phenolat, je ein Imidazol sowie an Carboxylate koordiniert – sämtlich wichtige Funktionalitäten des natürlichen Sauerstoff entwickelnden Komplexes. Der Komplex katalysiert die Umwandlung von Wasser in Sauerstoff durch das Einelektronenoxidans [Ru(bpy)3]3+ (siehe Bild: bpy=2,2′‐Bipyridin, A=Akzeptor).

Herein is described the preparation of a dinuclear molecular Ru catalyst for H2O oxidation. The prepared catalyst mediates the photochemical oxidation of H2O with an efficiency comparable to state-of-the-art catalysts.

In this work, we report the preparation and crystal structures of three new oligonuclear complexes, Ru2(bbpmp)(μ‐OAc)3 (4), [Co2(bbpmp)(μ‐OAc)(μ‐OMe)](PF6) (5), [Cu4(Hbbpmp)2(μ‐OAc)(H2O)2](OAc)(PF6)2 (6) {H3bbpmp = 2,6‐bis[(2‐hydroxybenzyl)‐(2‐pyridylmethyl)aminomethyl]‐4‐methyl‐phenol (3)}. The structures of the complexes were determined by single‐crystal X‐ray diffraction. The oxidation states of ruthenium, cobalt and copper in the complexes are +3, +3 and +2, respectively. In 4 and 5, RuIII and CoIII are coordinated to four oxygen and two nitrogen atoms in an octahedral geometry, while in 6, CuII adopts both octahedral (CuN2O4) and square‐pyramidal (CuN2O3) geometry. The potential of the three complexes as oxidation catalysts has been investigated.

Two dinuclear and one mononuclear ruthenium complexes containing neutral polypyridyl ligands have been synthesised as pre-water oxidation catalysts and characterised by (1)H and (13)C NMR spectroscopy and ESI-MS. Their catalytic water oxidation properties in the presence of [Ce(NH(4))(2)(NO(3))(6)] (Ce(IV)) as oxidant at pH 1.0 have been investigated. At low concentrations of Ce(IV) (5 mM), high turnover numbers of up to 4500 have been achieved. An (18)O-labelling experiment established that both O atoms in the evolved O(2) originate from water. Combined electrochemical study and electrospray ionisation mass spectrometric analysis suggest that ligand exchange between coordinated 4-picoline and free water produces Ru aquo species as the real water oxidation catalysts.