Metal-based dendrimers

Keywords. Metal-based dendrimers, electrochemistry, multielectron processes, luminescence, light harvesting. 

Polynuclear complexes based on octahedral building blocks may be structurally not well defined because of stereogenic problems [8]. However, clever synthetic strategies have recently been devised to obtain chirally pure species [61-65]. Synthesis and, of course, photophysical and photochemical studies of stereochemically pure metal-based dendrimers are still in their infancy. 

The first photophysical investigation performed on stereochemically pure metal-based dendrimers having a metal complex as the core is that concerning the tetranuclear species based on a [Ru(tpphz)3]2+ core (tpphz=tetrapyrido[3,2-a 2, 3 -c 3",2"-h 2",3"j]phenazine) [67]. Dendrimer 45 is an example of this family. In this compound, two different types of MLCT excited states, coupled by a medium- and temperature-dependent photoinduced electron transfer, are responsible for the luminescence behavior. However, the properties of all the optical isomers of this family of compounds are very similar. This finding is also in... 

The detailed investigation of the photophysical properties of metal-based dendrimers of high generation has clearly evidenced that5a,e (i) downhill (or even isoergonic)... 

Figure 13. Structural formulas and schematic representation of some components used for the construction of light-harvesting metal-based dendrimers [37, 38],...

Figure 14. Iterative synthetic strategy for metal-based dendrimers [38].

Metal-acetylide complexes including metal-poly(yne) polymers often show unique properties [21-23]. Thus, metal-acetylide dendrimers are of interest because amplification of the functionality due to metal-acetylide units based on three-dimensional assembly with a regular dendritic structure is expected. 

Complexation of gold ions, [Au(I)], with peripheral phosphine groups of a P-based dendrimer was reported by Majoral et al. [185]. Transmission electron microscopy (TEM) was used to analyze the large aggregates formed by the dendritic gold complexes and a direct correlation was observed between the size of the particles and the dendrimer generation number. In a recent report [186], Majoral et al. further demonstrated that up to 48 diphosphino groups could be anchored to the surface of dendrimers and various dendritic metal-complexes... 

There are, of course, metal-containing dendrimers that belong to more than one of the above-mentioned categories. Examples are the heptametallic dendrimer made of a central Fe(Cp)(C6Me6)+ core and coated with 6 ferrocene moieties [ 12], and the heterometallic dendrimers made of an organic core, containing up to 6 Pt(IV)-based organometallic species in the branches, and coated with up to 12 ferrocene units [13]. 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

When the only metal complex of a dendrimer is that constituting the core of the structure (Fig. la), the most interesting problem is whether and, if so, how much the electrochemical properties (potential value, kinetic reversibility) of the metal-based core are modified by the surrounding branches. 

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. 

In dendrimers based on metals as branching centers (Fig. 1 d), the electrochemical behavior is even more complex since (i) each unit of the dendrimer is electro active, (ii) the chemical nature of the metal-based units constituting the dendrimer may be different, (iii) chemically equivalent units can be different from the topological viewpoint, and (iv) the degree of interaction among the moieties depends on their chemical nature and distance. 

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

Beside these catalytically active metallophosphine dendrimers (see above), preliminary studies on the chemical properties of phoshorus-based dendrimers complexed to metals such as platinum, palladium and rhodium have been described by Majoral, Caminade and Chaudret [21], They showed that these macromolecules (see Scheme 13) could be useful for the (in situ) generation of metallodendrimer catalysts. 

Sihcon chemistry also provides a means for preparing dendrimers capped with metal ions [3,65]. For example, ferrocene [78,79], Co +, [80], Ru [81], and [9] have been hnked to the periphery of sihcon-based dendrimers. These materials are prepared by displacing reactive Si-Cl functional groups with any of a variety of nucleophiles, such as amines, alcohols, or Grignard reagents, containing the metal complexes or hgands. 

Our aim has been the construction of dendrimers that incorporate in their building blocks specific pieces of information such as the capability to absorb and emit visible light and to reversibly exchange electrons.To pursue this aim, we have designed a synthetic strategy to build up dendrimers based on luminescent and redox-active transition metal complexes. Species containing 4, 6, iP 10,2W7 28 gjjj 222930 metal-based units have already been obtained. We will see... 

C. Organotransition Metal Silicon-Based Dendrimers Exhibiting... 

Scheme 5. Silane-based dendrimers containing at the periphery metal-silicon o-bonds.

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