Dpp bridging ligands

Fig. 6. Formula of a decanuclear Ru " " metal complex with 2,3-bis (2 -pyridyl) pyrazine (dpp) bridging ligands and 2,2 - bipyridine (bpy) peripheral ligands, and the corresponding scheme (Fig. 2b).

Dendrimer 30 contains ten Ru-based units. The six peripheral Ru ions are easier to oxidize than the inner ones, because the terminal bpy ligands are better electron donors than the 2,3-dpp bridging ligands. As a result, a single six-electron wave is observed on oxidation, indicating the equivalence of the peripheral metal units. Oxidation of inner units was not observed up to -1-2.0 V in acetonitrile, presumably because of the large positive charge accumulated on the periphery [78]. 

To this purpose, multinuclear Ru(II) dendrimers based on 2,3-bis(2 -pyridyl)pyrazine (dpp) bridging ligands offer several advantages over [Ru(bpy)3] -type compounds as photosensitizers in water oxidation, in particular as far as requirements (i) and (iii) are concerned. 

Bis(2-pyridyl)pyrazine (2,3-dpp = (88)), 2,3-bis(2-pyridyl)quinoxaline and 2,3-bis(2-pyridyl) benzoquinoxaline (91) find numerous applieations as bridging ligands in homo- and heterome-tallic complexes. At this point, we consider some dinuelear species complexes eontaining >3 metal centers are covered in Section 5.5.3.1.5. By using the complexes in the series [(bpy)2M-(/u-L)M(bpy)2]" " (M-M = Ru-Ru, Os-Os, Ru-Os L = (88), 2,3-bis(2-pyridyl)quinoxaline or (91))... 

In the past few years we have concentrated our efforts on the N-N bischelating bridging ligands 2,3- and 2,5-bis(2-pyridyl)pyrazine (2,3- and 2,5-dpp), respectively (Figure 1). As we will see later, for the synthesis of dendrimers we have mainly used 2,3-dpp since one of its two chelating sites can be protected by methylation. The methylated form of 2,3-dpp, 2,3-Medpp+, is also shown in Figure 1. 

Recently the divergent iterative approach, welt known in oiganic synthesis, has been successfully extended to prepare polynuclear dendrimer-type complexes containing up to 22 metal centers using 2,3-dpp as bridging ligand. The two... 

Figure 21. Energy-migration patterns in tetranuclear compounds. Empty and full labels indicate and Os, respectively. In the peripheral positions, circles and squares indicate M(bpy)j and M(biq)j components, respectively. For the bridging ligands -W- = 2,3-dpp — 2,5-dpp.

On electrochemical reduction, a series of three waves are present in the cyclo-voltammogram of OsRu3 (Table 5.1). The dpp ligand is easier to reduce than bpy, and it is even more so when it plays the role of a bridging ligand (as it was inferred from the redox studies of the dinuclear species). Therefore, the three reduction waves can be associated to the first one-electron reduction of the three dpp ligands. For potentials... 

The reduction pattern of Ru6 warrants some additional comments it can be considered that the first electron probably enters the inner 2,3-dpp ligand, which should be the easiest site to be reduced however, upon second electron addition to one of the outer bridging ligands, the first electron should move to one of the other outer bridges, for coulombic reasons. This sort of redox-induced electron shift is not rare in this class of metal-polypyridine dendrimers.38... 

In this way, remarkable complexes have been built up, such as the 22-nuclear [Ru (//-2,3-dpp) [Ru(//-2,3-dpp)Ru (u-2,3-dpp)Ru(bpy)2 2]2 3]44+ (11.7). Compound 11.7 is an excellent example of the fact that metal complexes need to share the same bridging ligand in order to interact, thus the 12 peripheral Ru2+ centres, which are not connected to one another in such a way as to allow electronic interaction between them, are all oxidised at the same potential in one single 12-electron process. 

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