Björn Åkermark

Chemical and Light‐Driven Oxidation of Water Catalyzed by an Efficient Dinuclear Ruthenium Complex

Chemical and light-driven oxidation of water catalyzed by an efficient dinuclear ruthenium complex

Photosensitized water oxidation by use of a bioinspired manganese catalyst

In an artificial version of photosynthesis, sunlight and water are used to produce fuels. Our research focuses on the bottleneck in this process, the photooxidation of water. In the course of developing a water oxidation catalyst, a number of metal complexes have been synthesised, characterised, and studied for catalytic activity. Three of them are dinuclear complexes (Ru, Co and Cu) of 2,6-bis[(2-hydroxybenzyl)-(2-pyridylmethyl)aminomethyl]-4-methylphenol (H3bbpmp). The fourth is a dimeric Ru complex with a ligand containing imidazole and phenol motifs. Additionally, a dinuclear Mn complex with a ligand that contains benzimidazoles and carboxylates coordinating to the metal atoms was also developed. This Mn complex was then covalently linked to [Ru(bpy)3]2+-type photosensitisers, resulting in three different bimetallic dyads. Finally, a dinuclear Fe complex containing the same ligand as the dinuclear Mn complex was synthesised.The potential of the three H3bbpmp complexes as catalysts for oxidation of organic compounds was investigated and it was found that the Ru complex catalyses the oxidation of alcohols to the corresponding ketone or aldehyde using (diacetoxyiodo)benzene as oxidant. The Co complex functions as an electron transfer mediator in a coupled catalytic system for allylic oxidation using oxygen gas. The oxidation of 3,5-di-tert-butylcatechol to the corresponding ortho-quinone with oxygen gas using the copper complex proved that it can be considered as a model of catecholase. The dimeric Ru complex and the dinuclear Mn and Fe complexes proved to catalyse water oxidation when employing stoichiometric amounts of the oxidant [Ru(bpy)3](PF6)3. Furthermore, using [Ru(bpy)2(deeb)](PF6)2 as photosensitiser together with Na2S2O8 as sacrificial electron acceptor in aqueous phosphate buffer at pH = 7.2, photochemical water oxidation was demonstrated. The bimetallic dyads however, did not show catalytic activity for the oxidation of water.

Artificial Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation

Human society faces a fundamental challenge as energy consumption is projected to increase due to population and economic growth as fossil fuel resources decrease. Therefore the transition to alternative and sustainable energy sources is of the utmost importance. The conversion of solar energy into chemical energy, by splitting H2O to generate molecular O2 and H2, could contribute to solving the global energy problem. Developing such a system will require the combination of several complicated processes, such as light-harvesting, charge separation, electron transfer, H2O oxidation, and reduction of the generated protons. The primary processes of charge separation and catalysis, which occur in the natural photosynthetic machinery, provide us with an excellent blueprint for the design of such systems. This Account describes our efforts to construct supramolecular assemblies capable of carrying out photoinduced electron transfer and to develop artificial water oxidation catalysts (WOCs). Early work in our group focused on linking a ruthenium chromophore to a manganese-based oxidation catalyst. When we incorporated a tyrosine unit into these supramolecular assemblies, we could observe fast intramolecular electron transfer from the manganese centers, via the tyrosine moiety, to the photooxidized ruthenium center, which clearly resembles the processes occurring in the natural system. Although we demonstrated multi-electron transfer in our artificial systems, the bottleneck proved to be the stability of the WOCs. Researchers have developed a number of WOCs, but the majority can only catalyze H2O oxidation in the presence of strong oxidants such as Ce(IV), which is difficult to generate photochemically. By contrast, illumination of ruthenium(II) photosensitizers in the presence of a sacrificial acceptor generates [Ru(bpy)3](3+)-type oxidants. Their oxidation potentials are significantly lower than that of Ce(IV), but our group recently showed that incorporating negatively charged groups into the ligand backbone could decrease the oxidation potential of the catalysts and, at the same time, decrease the potential for H2O oxidation. This permitted us to develop both ruthenium- and manganese-based WOCs that can operate under neutral conditions, driven by the mild oxidant [Ru(bpy)3](3+). Many hurdles to the development of viable systems for the production of solar fuels remain. However, the combination of important features from the natural photosynthetic machinery and novel artificial components adds insights into the complicated catalytic processes that are involved in splitting H2O.

Evolution of O2 in a Seven-Coordinate RuIV Dimer Complex with a [HOHOH]- Bridge: A Computational Study

Evolution of O-2 in a Seven-Coordinate Ru-IV Dimer Complex with a [HOHOH] (-) Bridge : A Computational Study

Oxygen as Oxidant in Palladium-Catalyzed Inter- and Intramolecular Coupling Reactions

Carbazoloquinones, carbazole, dibenzofuran, and related compounds (see scheme) have been synthesized from the respective noncyclic aromatic compounds with catalytic amounts of palladium and oxygen as the single oxidant. Intermolecular couplings of aromatic systems can also be performed with this catalytic system.

Visible light-driven water oxidation catalyzed by a highly efficient dinuclear ruthenium complex

Visible light-driven water oxidation has been achieved by the dinuclear ruthenium complex 1 with a high turnover number of 1270 in a homogeneous system in the presence of a Ru polypyridine complex photosensitizer.

Photo-induced oxidation of a dinuclear Mn(2)(II,II) complex to the Mn(2)(III,IV) state by inter- and intramolecular electron transfer to Ru(III)tris-bipyridine.

To model the structural and functional parts of the water oxidizing complex in Photosystem II, a dimeric manganese(II,II) complex (1) was linked to a ruthenium(II)tris-bipyridine (Ru(II)(bpy)(3)) complex via a substituted L-tyrosine, to form the trinuclear complex 2 [J. Inorg. Biochem. 78 (2000) 15]. Flash photolysis of 1 and Ru(II)(bpy)(3) in aqueous solution, in the presence of an electron acceptor, resulted in the stepwise extraction of three electrons by Ru(III)(bpy)(3) from the Mn(2)(II,II) dimer, which then attained the Mn(2)(III,IV) oxidation state. In a similar experiment with compound 2, the dinuclear Mn complex reduced the photo-oxidized Ru moiety via intramolecular electron transfer on each photochemical event. From EPR it was seen that 2 also reached the Mn(2)(III,IV) state. Our data indicate that oxidation from the Mn(2)(II,II) state proceeds stepwise via intermediate formation of Mn(2)(II,III) and Mn(2)(III,III). In the presence of water, cyclic voltammetry showed an additional anodic peak beyond Mn(2)(II,III/III,III) oxidation which was significantly lower than in neat acetonitrile. Assuming that this peak is due to oxidation to Mn(2)(III,IV), this suggests that water is essential for the formation of the Mn(2)(III,IV) oxidation state. Compound 2 is a structural mimic of the water oxidizing complex, in that it links a Mn complex via a tyrosine to a highly oxidizing photosensitizer. Complex 2 also mimics mechanistic aspects of Photosystem II, in that the electron transfer to the photosensitizer is fast and results in several electron extractions from the Mn moiety.

Palladium-promoted addition of amines to isolated double bonds☆

Abstract Palladium(II) complexes have been found to promote amination of terminal olefins, to give, after reduction, high yields of amines. Internal olefins may also be aminated, but the yields are moderate to low. Cyclohexene and cyclooctene react very sluggishly. In order to avoid formation of bis(amine)palladium complexes, which is a major side reaction, the temperature has to be kept low. Preliminary mechanistic studies show that three moles of amine are required per mole of palladium to ensure good yields.

Water Oxidation by Single-Site Ruthenium Complexes : Using Ligands as Redox and Proton Transfer Mediators

Water Oxidation by Single-Site Ruthenium Complexes : Using Ligands as Redox and Proton Transfer Mediators

Catalytic oxidative aromatic cyclizations with palladium

Using tert-butyl hydroperoxide as oxidant, facile palladium-catalyzed cyclizations of arylaminoquiones have been performed.