Dpp ligands

It is instructive to see how the reduction potentials of the three 2,3-dpp ligands of the central core change in going from the mononuclear Ru(2,3-dpp)3 to the tetra-and decanuclear complexes (Figure 15). ... 

Figure 15. Correlation between the reduction potentials of the core 2,3-dpp ligands in various complexes. The black circles indicate that the reduction of the core ligand occurs after reduction of other sites (see text).

Although the building blocks of the metal-polypyridine dendrimers are mononuclear species, the effective models for the high-nuclearity dendrimers are the dinuclear species. This is because the properties of the mononuclear and dinuclear compounds (absorption, luminescence, and redox properties) are significantly different, as a consequence of the bis-chelation of the dpp ligand.20-22 In the high-nuclearity dendrimers, dpp always plays the role of bridge, so the redox properties (and indeed also the spectroscopic properties) of the dendrimers are directly connected to the properties of the dinuclear species. Representative dinuclear species are discussed here. 

Different sequences of reduction processes have also been obtained, when the peripheral bpy ligands are replaced by ligands such as 2,2 -biquinoline (biq), whose reduction potential is intermediate between the first and second reduction potentials of the bridging dpp ligands. Details can be found in the original reference 21. 

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 the case of dendrimer 22, Ru (p-2,3-dpp)Ru (p-2,3-dpp)Ru(bpy)2]2 hl2° 1 (Fig. 6.16a),61 the first oxidation process (+1.53 V versus SCE) involves the exchange of six electrons at the same potential and is attributed to the peripheral Ru-based units which are oxidized at less positive potentials than the internal ones because the bpy ligands are worse electron acceptors than the bridging 2,3-dpp ligands. Furthermore, the six peripheral Ru-based units are not expected to interact with one another, because they are not directly connected. Successive oxidation of the... 

Cu(dpp)2] (24) has a long-lived MLCT excited state, which is luminescent at room temperature. Stabilization of Cu by the dpp ligand is evident from the unusually high oxidation potential of [Cu(dpp)2]. ... 

The majority of the anionic transition-metal complexes discussed above are electrostatically bound to the charged PPy moiety. An interesting alternative can be found in the work of Bidan et al. [25]. These workers synthesized Cu(dpp)2, which contained two entwined dpp ligands covalently attached to pyrrole units. Electropolymerization of the complex yielded an... 

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