One clean alternative to fossil fuels would be to split water using sunlight. However, to achieve this goal, researchers still need to fully understand and control several key chemical reactions. One of them is the catalytic oxidation of water to molecular oxygen, which also occurs at the oxygen evolving center of photosystem II in green plants and algae. Despite its importance for biology and renewable energy, the mechanism of this reaction is not fully understood. Transition metal water oxidation catalysts in homogeneous media offer a superb platform for researchers to investigate and extract the crucial information to describe the different steps involved in this complex reaction accurately. The mechanistic information extracted at a molecular level allows researchers to understand both the factors that govern this reaction and the ones that derail the system to cause decomposition. As a result, rugged and efficient water oxidation catalysts with potential technological applications can be developed. In this Account, we discuss the current mechanistic understanding of the water oxidation reaction catalyzed by transition metals in the homogeneous phase, based on work developed in our laboratories and complemented by research from other groups. Rather than reviewing all of the catalysts described to date, we focus systematically on the several key elements and their rationale from molecules studied in homogeneous media. We organize these catalysts based on how the crucial oxygen-oxygen bond step takes place, whether via a water nucleophilic attack or via the interaction of two M-O units, rather than based on the nuclearity of the water oxidation catalysts. Furthermore we have used DFT methodology to characterize key intermediates and transition states. The combination of both theory and experiments has allowed us to get a complete view of the water oxidation cycle for the different catalysts studied. Finally, we also describe the various deactivation pathways for these catalysts.
Metal nanoparticles have been used for a long time to catalyze chemical reactions in both heterogeneous and homogeneous phases.[1] The analysis of traditional heterogeneous and homogeneous catalysis requires very different techniques that are difficult to combine for the study of metal nanoparticles, in which distinguishing between colloidal and molecular catalysis is difficult.[2] Thus, many questions concerning the reactivity of metal nanoparticles are still open, particularly the nature of intermediate surface species, knowledge of which is important for the development of new nanocatalysts and new catalytic transformations. Some of us have used solid-state NMR spectroscopy for this purpose recently, [3] and herein we report the combination of this method with desorption techniques for investigating the reactivity of ruthenium nanoparticles.The synthesis of metal nanoparticles by hydrogenation of organometallic precursors in the presence of organic ancillary ligands, such as amines, thiols, or carboxylic acids as stabilizers, has been investigated for over fifteen years by some of us.[4] In particular, essentially monodisperse, very small ruthenium nanoparticles, which display a remarkable surface coordination chemistry, can be obtained using [Ru-(cod)(cot)] as a precursor (cod = 1,5-cyclooctadiene; cot = 1,3,5-cyclooctatriene). This system, and similar ones involving Pd, Pt, or Rh nanoparticles, catalyzes a number of chemical reactions such as olefin hydrogenation, CÀC coupling, and hydrogenation of aromatic hydrocarbons.[5] Some of us have shown independently that palladium nanoparticles stabilized by asymmetric phosphite groups are good enantioselective alkylation catalysts.[6] This result provides strong evidence for the direct coordination of ligands, in this case phosphite groups, to the palladium surface.The coordination of ligands such as CO, [7] amines, [8] and organosilanes, [9] has previously been established by NMR spectroscopy studies in solution or in the solid state. The coordination of hydrogen to metal nanoparticles, however, is especially important. Hydrogen binding to clean metal surfaces has been well established by surface science, and it is generally accepted that one hydrogen atom is adsorbed per surface metal atom. [10] We have recently demonstrated the presence of mobile hydrides, which are in slow exchange with gaseous dihydrogen, on the surface of amine-protected ruthenium nanoparticles using a combination of gas-phase 1 H NMR and solid-state 2 H NMR spectroscopy. [3] Furthermore, other species, such as alkenes or arenes, may adsorb on the surface during a catalytic process or give rise to new reactive intermediates, including alkyl groups and carbenes. The important question which then arises is whether these groups are stable and can be detected spectroscopically, as in organometallic complexes.Herein we describe: 1) the synthesis of a new class of phosphine-protected ruthenium nanoparticles, 2) the characterization of phosphine coordination by NMR spectroscopy techniques, 3) the p...
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