Bridged dendrimers

Figure 37 Bridged dendrimer types (a) linear bridged, (b) radial bridged, (c) macro-cyclic bridged.

An example of this type of dendrimer is the polynuclear species that can be prepared from 2,3-bis(2-pyridyl)pyrazine as bridging ligands, bipyridines as terminal bgands, and either Ru(II) or Os(II) as the metal centre. 

Stang and coworkers also reported the synthesis of platinum-acetylide dendrimers using 1,3,5-triethynylbenzene as the bridging ligand [27]. Their strategy was very similar to the above, and they prepared dendrimers up to the second generation. 

The reaction of the first-generation dendron 29 with the core, which is a trin-uclear platinum complex (30) bridged by 1,3,5-triethynylmesitylene, in a molar ratio of 3 1 resulted in the formation of the first-generation dendrimer (31). Since the reaction proceeded quantitatively and no side products were observed, the first-generation dendrimer was easily isolated by alumina column chromatography. 

When tetranuclear platinum complex (35) bridged by tetra(4-ethynylphenyl)-methane was used as the core (Fig. 6), dendrimers having more platinum atoms than those prepared from trinuclear platinum core 30 in the molecule were obtained up to the third generation. GPC analysis of these dendrimers revealed that the dendrimers with a tetraplatinum core have a similar molecular size to those with the triplatinum core 30. 

PAMAM dendrimers are synthesized in a multistep process. Starting from a multifunctional amine (for example ammonia, ethylenediamine, or tris(2-amino-ethyl)amine) repeated Michael addition of methylacrylate and reaction of the product with ethylenediamine leads to dendrimers of different generation numbers [1,9]. Two methylacrylate monomers are added to each bifunctional ethylenediamine generating a branch at each cycle. Unreacted ethylenediamine has to be completely removed at each step to prevent the initiation of additional dendrimers of lower generation number. Excess methylacrylate has also to be removed. Bridging between two branches of the same or of two different dendrimers by ethylenediamine can also be a problem, and has to be avoided by choosing appropriate reaction conditions. 

Thus, the concept and manifestation of molecular recognition mediated supramolecular self-assembly of small chemical units, be it an ABn type building block or a dendrimer molecule, have proved to be vital factors in bridging the gap between small molecules and novel new-age materials. 

The dendrimer-type tetranuclear Ru(II)-Os(II)3 complex (22, protonated form) shows an interesting electrochemical behavior due to the presence of free basic sites in its bridging ligands [41]. The protonated form shows a 3-1 oxidation pattern due to the simultaneous oxidation of the three Os-based units, followed by the one-electron oxidation of the Ru-based unit. On addition of base, the six chelating moieties (three on the Ru center and one on each Os center) undergo deprotonation. This causes changes in the oxidation potential of the metal ions, with a consequent switching from 3-1 to 1-3 in the oxidation pattern. 

M. Dubber and T. K. Lindhorst, Trehalose-based octopus glycosides for the synthesis of carbohydrate-centered PAMAM dendrimers and thiourea-bridged glycoclusters, Org. Lett., 3 (2001) 4019 1022. 

A similar type of biotin-dendritic multimer also was used to boost sensitivity in DNA microarray detection by 100-fold over that obtainable using traditional avidin-biotin reagent systems (Stears, 2000 Striebel et al., 2004). With this system, a polyvalent biotin dendrimer is able to bind many labeled avidin or streptavidin molecules, which may carry enzymes or fluorescent probes for assay detection. In addition, if the biotinylated dendrimer and the streptavidin detection agent is added at the same time, then at the site of a captured analyte, the biotin-dendrimer conjugates can form huge multi-dendrimer complexes wherein avidin or streptavidin detection reagents bridge between more than one dendrimer. Thus, the use of multivalent biotin-dendrimers can become universal enhancers of DNA hybridization assays or immunoassay procedures. 

Note It is essential to carefully examine the analytical data for the presence of cyclic compounds as a result of side reactions involved. The cyclization occurs as dendrimer amidation versus bridging amidation are kinetically similar. To prevent the intradendrimeric cyclization a large excess (50-fold) of 1,2-ethylenediamine is required. The excesses are removed to an undetectable level by azeotropic techniques. 

In dendrimers based on metal complexes, the metal-containing units are linked together by bridging ligands. The role played by the bridging ligands is extremely... 

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... 

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