Multielectron metal complex

When the metal complexes constitute the peripheral units (Fig. lb) and/or belong to the branches (Fig. 1 c) of a dendrimer, a number of equivalent metal-based centers are present since dendrimers are usually highly symmetric species by their own nature. The metal-based centers may or may not interact, depending on distance and nature of the connector units. Multielectron redox processes can therefore be observed, whose specific patterns are related to the degree of interaction among the various units. 

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes in homogeneous solutions, because we believe it should be possible to facilitate multielectron transfer processes at the available coordination sites of such cluster species. 

The interest in highly branched polynuclear metal complexes, and more generally in dendritic species, is related not so much to their size, but rather to the presence of different components. An ordered array of different components can in fact generate valuable properties, such as the presence of cavities having different size, surfaces with specific functions, gradients for photoinduced directional energy and electron transfer, and sites for multielectron transfer catalysis. Studies along these directions are underway in our laboratories. 

Synthetic polymers stabilize metal colloids as important catalysts for multi-electron reactions. Polynuclear metal complexes are also efficient catalysts for multielectron processes allowing water photolysis. 

The results in Table 9.12 confirm that transition-metal complexes can facilitate multielectron reduction pathways for 02. Within the group of complexes the iron(II) systems are the most effective. Thus, Fen(bpy)2+ in MeCN catalyzes a two-electron process at a potential that is 0.73 V less negative than the uncatalyzed one-electron process (—0.87 V) ... 

There are essentially two possibilities to accomplish two-electron or multielectron transfer at metal complexes without formation of one-electron transfer intermediates (e.g., radicals). Appropriate metal centers should have available stable oxidation states that differ by at least two emits. T5rpical examples that represent such photoredox reactions are reductive eliminations such as (X = halide, pseudohalide) (5) ... 

In conclusion, photochemical multielectron transfer of metal complexes has been frequently studied with the intension to realize artificial photosynthesis. In the following sections, this is emphasized when appropriate. However, such reactions are also very interesting in their own right and are described below irrespective of particular applications. 

Polynuclear metal complexes are more suited for water oxidation catalyst because of their nature to act as multielectron transfer reagents in addition to the fact that charge delocalization can lead to stabilization of the catalyst rather than decomposition during the process. The trinuclear ruthenium complexes Ru-red and Ru-brown, [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)j] - (Ru "-Ru" -Ru" ) and [(NH3)sRu-0-Ru(NH3)4-0-Ru(NH3)5] + (Ru -Ru" -Ru ), respectively, have been shown to be efficient water oxidation catalysts for oxygen evolution with high turnover numbers When Ru-red was dissolved in an acidic aqueous solution, it underwent one-electron oxidation with the formation of Ru-brown. When Ru-brown was dissolved in a basic solution, the complex underwent reduction to produce Ru-red. The one-electron oxidation and reduction of the Ru-red and Ru-brown has already been well established (Eq. 11) 6 65-6 )... 

Photosynthetic reduction of carbon dioxide is a facile natural process even though the chemistry is complex and multielectron steps are required. Recent work has demonstrated that metal complexes play a crucial role (JL) and it is anticipated that study of the homogeneous solution chemistry of CO and its metal complexes will... 

The reduction of C02 requires electron transfer in one-electron or multielectron steps either from reducing agents, for example, H2, or electrochemically. H2 can also be produced by water splitting either electrochemically or photochemically. For efficient electrochemical reduction of dissolved CO2, electron transfer catalysts (electron relays, mediators), usually transition metal complexes, are required while photochemical systems need also a photosensitizer. The two approaches can be combined to photoelectrochemical systems, as well. 

A number of transition-metal complexes, both in solution and on electrode surfaces, have been shown to be effective in the electrocatalytic reduction of carbon dioxide. All of those complexes significantly decrease the overpotential for reduction of CO2 by up to IV (as compared to a 1-el reduction to the C02 radical), and yield various multielectron reduction products. Known electrocatalysts yield primarily carbon monoxide and formate anion as the major products of the CO2 reduction. Sullivan et al. did detailed mechanistic work on a sales of bipyridine complexes of transition metals, and made several suggestions concerning the design of new electrocatalysts that would be capable of reducing CO2 past the CO and formate step. Their major recommendation is to use as electrocatalysts "electron reservoir" complexes, i.e. compounds capable of storing multiple electrons. 

The synthesis of Cp 3U Cp = p -CsMes carved a new path for the researchers to go one step ahead in the electrochemical studies of organoac-tinide complexes. The reduction reaction involving more than two electrons are not common for metal complexes containing just one metal. However, the synthesis of complexes of the type Cp 3M led to the development of sterically induced reduction (SIR) chemistry in which sterically crowded complexes of redox inactive metals act as reductants [87,88]. Evans et al. showed that the sterically induced reduction couple, U(III)/U(IV), can act as a multielectron reductant [89]. As an example, Cp 3U reacts as a three-electron reductant with 1,3,5,7-C8H8, (Eq. 12) in which one electron arises from U(III) (Eq. 13) and two result from two CsMes /CsMes half reactions (Eq. 14) presumably via SIR. This phenomenon was further corroborated by the stepwise reduction of phenyl halide with Cp 3lJ (Eq. 15) ... 

Multielectron redox reactions at the interface between two immiscible liquids were first investigated by Bell [8]. This approach was later extended to redox and hydrolysis reactions catalyzed by enzymes [2,9-11], photosynthetic pigments [12], metal complexes of porphyrins [13-15], and submitochondrial particles [16], as well as in systems with an extended surface such as microemulsions [12], vesicles and reversed micelles [17]. Enzymes and pigments embedded in a hydrophihc-hydrophobic interface have properties similar to their functional state in a membrane. For instance, certain enzymes can be highly active at the interface, but virtually inactive in a homogeneous medium. The interface between two immiscible liquids with immobilized photosynthetic pigments can also serve as a simple... 

A further, drastic step towards increasing structural complexity can be made by going from mononuclear to polynuclear complexes. These are systems in which two (or more) metal complex subunits are connected by one (or more) bridging ligand(s) (Fig. 1). Because of this additional complexity, the photophysics of polynuclear complexes is expected to be different from, and more interesting than, that of simple mononuclear species. This is true, in principle, also from the photocatalytic point of view, where the additional possibilities of polynuclear systems, in particular with respect to multielectron redox processes and inter-component energy or electron transfer, could find applications. 

Partigianoni, C. M. Chang, I.-J.Y Nocera, D. G. Multielectron Photochemistry of Quadruply Bonded Metal-Metal Complexes, Coord. Chem. Revs. 1990, 97,105-117. 

Multimetallic catalysts, alloy catalysts, intermetallic compounds, fuel cell catalysts, colloidal intermediates, metal complexes, and metal clusters have received considerable attention [12-21] because the metal-metal cooperating bifunctional catalysts which can activate reactants simultaneously showed high catalytic activity and stereoselectivity under mild conditions [19]. In fact, there have been many bifunctional multimetallic catalysts in which multimetallic alloy- and electro-catalysts offer a way to fine-tune the catalytic properties of metals, atomic composition, and microstrucrnres [16-18, 20]. Cooperative multimetallic activation of oxidants via the multielectron transfer is also a common feature in biological oxidation catalysis [14]. Artificial multimetallic complexes with two or more metal atoms that contain... 

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