As electron reservoirs

Astruc has developed the concept of transition metal sandwiches acting as electron reservoir complexes. The characteristic of an electron reservoir is that the reduced form is easily generated and does not decompose to increase stability, the radical center can be sterically protected in the heart of a bulky molecular framework. The [FeCp(arene)] series of complexes, e.g. 7, are prime examples, for which variation of the arene structure modulates the redox potential. 

Numerous transition metals ions form cluster complexes with chalcogenide anions [42-52], Iron and sulphur are unique elements in the sense that no two other elements can generate such a large diversity of cluster structures. This is the consequence of two stable oxidation states of iron ions and strong Fe-S bonds of significantly covalent character [53], Moreover, numerous structures are stable in several oxidation states, so these clusters serve as electron reservoirs in biological systems [51], This is why iron-sulphur proteins usually catalyze redox reactions. 

In a general sense, metal clusters have been regarded as electron reservoirs which can gain or lose electrons at will without molecular disruption. However, few real species possess this property, and the addition or subtraction of more than one electron usually leads to structural changes, and a concomitant increase in reactivity. Detailed studies are being made, therefore, of redox-induced changes in structure and reactivity, studies which promise to provide rational routes to new polymetallic complexes and information about electronic structure. 

POMs are promising catalysts for acid, redox and bifunctional catalysis. In many structures, the transition metal addenda atoms such as Mo or W exist in two oxidation states, which results in different redox properties as determined by polarog-raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has been demonstrated by the preparation and characterization of numerous reduced derivatives [32]. They also exhibit high solubility in polar solvents, which means that they can be used in homogeneous catalysis. The wide range of applications of heteropoly compounds are based on their unique properties which include size, mass, electron and proton transfer (and hence storage) abilities, thermal stability. 

The well-known metal polypyridine [378, 379] and metal-porphyrin [380-382] and phthalocyanine complexes also have extended redox series which make them good candidates as electron-reservoirs. They are thoroughly reviewed and discussed in other chapters of this volume. 

Supramolecules containing metal-polypyridine units, especially the Ru(dpp)-based dendrimers, could be used as electron reservoirs or components of molecular-electronic devices. Supramolecules in which an electroactive M(N,N) group is attached to a receptor capable of molecular recognition (crown ethers, calixarenes, cryptands etc.) can work as electrochemical sensors. Electrochemical recognition of cations as well as anions has been reported [33-35, 257, 263]. 

Strategy II. Redox non-innocent ligands as electron reservoirs... 

Scheme 7 Redox non-innocent bis-iminophenolate ligands as electron reservoir in catalytic C—C bond formation by a Co(lll) complex supported by bis-iminophenolate ligands...

The chemistry of this latter group of complexes has recently attracted much attention, as most of them efficiently reduce CO2 to CO or HCOO. The majority of the mechanistic data on these reductions has come from studies on these complexes. Polypyridyl ligands such as bpy or phen have the unique property of stabilising metals over a wide range of oxidation states whilst, at the same time, also acting as "electron reservoirs" by "storing" electrons (in vacant K orbitals) at potential between - 0.7 and - 1.7 V. 

The complexes [Fe (T] -Cp)(Ti -arene)] in which the arene is CeMee, CeEte or TS.S-t-BusCeHs are stable and do not undergo dimerization or ligand substitution reactions, which allowed to isolate them, study their physical and chemical properties, and use them as electron-reservoir systems with arene = CeMee, since they are strong reductants. 

Four extensive reviews have highlighted the nature of the metal-metal bond and the use of polynuclear species as electron reservoirs, the relationship between clusters and metal surfaces, the mechanisms of cluster rearrangements, and complexes with multiple metal-metal bonds. ... 

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