Solid state and solution evidence confirms the embedding of an adamantane-like, Ru4O6 fragment by the divacant, gamma-decatungstosilicate ligand. The resulting complex catalyzes water oxidation to oxygen with TON up to 500 and TOF > 450 h-1.

Water is the renewable, bulk chemical that nature uses to enable carbohydrate production from carbon dioxide. The dream goal of energy research is to transpose this incredibly efficient process and make an artificial device whereby the catalytic splitting of water is finalized to give a continuous production of oxygen and hydrogen. Success in this task would guarantee the generation of hydrogen as a carbon-free fuel to satisfy our energy demands at no environmental cost. Here we show that very efficient and stable nanostructured, oxygen-evolving anodes are obtained by the assembly of an oxygen-evolving polyoxometalate cluster (a totally inorganic ruthenium catalyst) with a conducting bed of multiwalled carbon nanotubes. Our bioinspired electrode addresses the one major challenge of artificial photosynthesis, namely efficient water oxidation, which brings us closer to being able to power the planet with carbon-free fuels.

Water oxidation is the crucial stage in the chemical and molecular sequence of photosynthesis, designed by Nature to convert solar light into chemical energy. The artificial “off-leaf” transposition is a major goal of energy research, aiming at the continuous production of hydrogen as a solar fuel, through the photo-catalytic splitting of water. Success in this task primarily depends on the interplay of light-activated multi-electron oxidation and reduction cycles and on the invention of stable and robust water oxidation catalysts, liberating oxygen with fast rates, high quantum yield, and long-term activity. A promising perspective is herein envisaged in the molecular design of functional metal-oxide cores and composite nano-materials

Among molecular water oxidation catalysts (WOCs), those featuring a reactive set of four multi-redox transition metals can leverage an extraordinary interplay of electronic and structural properties. These are of particular interest, owing to their close structural, and possibly functional, relationship to the oxygen evolving complex of natural photosynthesis. In this review, special attention is given to two classes of tetrametallic molecular WOCs: (i) M(4)O(4) cubane-type structures stabilized by simple organic ligands, and (ii) systems in which a tetranuclear metal core is stabilized by coordination of two polyoxometalate (POM) ligands. Recent work in this rapidly evolving field is reviewed, with particular emphasis on photocatalytic aspects. Special attention is given to studies addressing the mechanistic complexity of these systems, sometimes overlooked in the rush for oxygen evolving performance. The complementary role of molecular WOCs and their relationship with bulk oxides and heterogeneous catalysis are discussed.

Isostructural cubane-shaped catalysts [Co(III)(4)(μ-O)(4)(μ-CH(3)COO)(4)(p-NC(5)H(4)X)(4)], 1-X (X = H, Me, t-Bu, OMe, Br, COOMe, CN), enable water oxidation under dark and illuminated conditions, where the primary step of photoinduced electron transfer obeys to Hammett linear free energy relationship behavior. Ligand design and catalyst optimization are instrumental for sustained O(2) productivity with quantum efficiency up to 80% at λ > 400 nm, thus opening a new perspective for in vitro molecular photosynthesis.

Converging UV-vis, EPR, rRaman, and DFT calculations highlight the evolution of [Ru(4)(H(2)O)(4)(mu-O)(4)(mu-OH)(2)(gamma-SiW(10)O(36))(2)](10-), 1, to high-valent intermediates. In analogy with the natural enzyme, five different oxidation states, generated from 1, have been found to power the catalytic cycle for water oxidation. A high electrophilic tetraruthenium(V)-hydroxo species is envisaged as the competent intermediate, undergoing nucleophilic attack by an external water molecule as a key step in the formation of a new O-O bond under catalytic conditions.

The oxygen in Earth's atmosphere is there primarily because of water oxidation performed by photosynthetic organisms using solar light and one specialized protein complex, photosystem II (PSII). High-resolution imaging of the PSII 'core' complex shows the ideal co-localization of multi-chromophore light-harvesting antennas with the functional reaction center. Man-made systems are still far from replicating the complexity of PSII as the majority of PSII-mimetics have been limited to photocatalytic dyads based on a 1:1 ratio of a light absorber, generally a Rupolypyridine complex, with a water oxidation catalyst. Here we report the self-assembly of multi-perylene-bisimide chromophores (PBI) shaped to function by interaction with a polyoxometalate water-oxidation catalyst (Ru 4 POM). The resulting [PBI] 5 Ru 4 POM complex shows: a robust amphiphilic structure and dynamic aggregation into large 2D-paracrystalline domains, a red-shifted light-harvesting efficiency > 40%, and favorable exciton accumulation, with a peak quantum efficiency using 'green' photons (λ> 500 nm). The modularity of the building blocks and the simplicity of the non-covalent chemistry offer opportunities for innovation in artificial photosynthesis.

Photoinduced water oxidation to molecular oxygen takes place in systems made of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as the photosensitizer, [Co4O4(O2CMe)4(py)4] (py = pyridine) as the molecular catalyst and Na2S2O8 as the sacrificial electron acceptor. The photochemical quantum yield of the process reaches the outstanding value of 30% and depends on pH and catalyst concentration. Transient absorption spectroscopy experiments aimed to clarify the first events of the photocatalytic process are also reported.