Dendrimer cluster

Laboratory procedures are presented for two divergent approaches to covalent structure controlled dendrimer clusters or more specifically - core-shell tecto(dendrimers). The first method, namely (1) the self assembly/covalent bond formation method produces structure controlled saturated shell products (see Scheme 1). The second route, referred to as (2) direct covalent bond formation method , yields partial filled shell structures, as illustrated in Scheme 2. In each case, relatively monodispersed products are obtained. The first method yields precise shell saturated structures [31, 32] whereas the second method gives semi-controlled partially shell filled products [30, 33],... 

The cyclovoltammograms of the dendrimer-clusters 1, 2 and 3 in CH2CI2 (Pt, 0.1 M n-Bu4NPF6) resemble that of the parent cluster [Fe4(/z3-CO)4Cp4] itself [101], the clusters being sufficiently remote from one another in the den-... 

Fig. 31. Two-dimensional projections of various branch-cell defects which may lead to divergence from ideal dendrimer development. Branch defective dendrimer, A intra-dendrimer looping, B inter-dendrimer bridging, C inter-dendrimer looping, D dendrimer cluster (bridged/looped), E...

The assembly of dimers, trimers and other well defined dendrimer clusters have recently been described by us [167, 186], thus demonstrating the viability of GDS derived dendrimers as fundamental, nanoscopic building blocks for covalent constructions. The ability to differentiate sectors of a dendrimer as described by Frechet et al. [102a] and others [2] should allow directionalized assembly of more complex nanostructures. 

Fig. 52. Finite series of nesting spheres representing atoms, repeat units, branch cells, dendrons, dendrimers, dendrimer clusters, and dendrimer macro-lattices (ordered infinite networks) illustrating an abiotic hierarchy analogous to that found in biological system...

Given the simplicity of the above characterization of the reaction product by P NMR [13] and the excellent selectivity of this model reaction when excess [Ru3(CO)i2] was used, the same reaction between the phosphine dendrimers and [Ru3(CO)i2] could be more confidently envisaged. This reaction, catalyzed by 1 % equiv. [Fe Cp( / -C6Me6)] was carried out in THF at 20 °C. The dendrimer-cluster assembly was obtained in 50 % yield. This shows the selectivity and completion of the coordination of each of the 32 phosphino ligands of P-P to a Ru3(CO)h cluster fragment (Scheme 34). 

Finally, the 64-branch phosphine DAB-t e r-G4-[N(CH2PPh2)2]32 analogously reacts with [Ru3(CO)i2] and 1 % [Fe Cp( -C6Me6)] (20°C, THF, 20 min) to give the dark-red 192-Ru dendrimer. Characterization of the purity of these dendrimer-cluster assemblies is conveniently monitored by P NMR. This application should find extension to other metal-carbonyl clusters and other families of phosphine dendrimers. 

Scheme 11.17 Electron-transfer-chain catalyzed ligand substitution of one Ru-coordinated CO by dendritic phosphine termini in Reetz s 32-phosphine dendrimer under ambiant conditions leading to the 32-Ru3(CO)u dendrimer-cluster. The ETC mechanism41-42 proceeds for the introduction of the 32 cluster fragments in the dendrimer for ligation of the first Ru3(CO)n fragment to the dendritic phosphine. Then, this first complex [dendriphosphine.Ru3(CO)n] would undergo the same ETC cycle as [Ru3(CO)12] initially does to generate the bis-cluster complex [dendriphosphine. Ru3(CO)n 2], and so onto Scheme 11.18.

Scheme 11.18 Electron-transfer-chain mechanism for the synthesis of the 96-Ru dendrimer-cluster complex.

Studies of nanochemical systems span many areas, from the study of the interactions of individual atoms and how to manipulate them, how to control chemical reactions at an atomic level, to the study of larger molecular assembhes, such as dendrimers, clusters, and polymers. From studies of assemblies, significant new structures—such as nanotubes, nanowires, three-dimensional molecular assembhes, and lab-on-a-chip devices for separations and biological research—have been developed. 

Morphological observation by SEM showed interlocking of the adhesive in the adherend surface due to microstructures (star-like or spheres) formed by the hyperbranches or dendrimer clusters on the interface. 

A distinct core-shell dimensional enhancement was observed as a function of the sum of the core-shell generation values used in the construction of the series (e.g., G4/G3, polydispersed dendrimer cluster/gel formation observed for 1 1 reaction ratios described in our earlier work [55]. 

Duncan and co-workers also created one of the first covalently linked DDS based on a dendrimer scaffold [52]. Carboxylated (COOH) PAMAM dendrimers were used to attach cisplatin, a common chemotherapeutic in clinical use. The covalent attachment of the drug increases the specific solubility of cisplatin ten-fold, and enables a loading of approximately 25% to be achieved for the PAMAM dendrimer. However, dendrimer clusters form as a consequence of the multiple COOH groups on the dendrimer scaffold and intermolecular reactions involving cisplatin. In vitro evaluation has shown that the covalent conjugation to the dendrimer results in a lower toxicity, and in vivo experiments have shown that the blood clearance rate is lower, with higher intra-tumoral concentrations of platinum than those achieved with the free drug. 

Choi, Y, Thomas, T., Kotlyar, A., Islam, M. T., and Baker, J. R. 2005. Synthesis and functional evaluation of DNA-assembled polyamidoamine dendrimer clusters for cancer cell-specific targeting. Chem Biol 12(1), 35-43. 

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