Dendrimer oxidation

There is currently considerable interest in processing polymeric composite materials filled with nanosized rigid particles. This class of material called "nanocomposites" describes two-phase materials where one of the phases has at least one dimension lower than 100 nm [13]. Because the building blocks of nanocomposites are of nanoscale, they have an enormous interface area. Due to this there are a lot of interfaces between two intermixed phases compared to usual microcomposites. In addition to this, the mean distance between the particles is also smaller due to their small size which favors filler-filler interactions [14]. Nanomaterials not only include metallic, bimetallic and metal oxide but also polymeric nanoparticles as well as advanced materials like carbon nanotubes and dendrimers. However considering environmetal hazards, research has been focused on various means which form the basis of green nanotechnology. 

Wiesler U-M, WeU T, Mullen K (2001) Nanosized Polyphenylene Dendrimers. 2/2 1 - 40 WUliams RM, Stocking EM, Sanz-Cervera JF (2000) Biosynthesis of Prenylated Alkaloids Derived from Tryptophan. 209 97-173 Wirth T (2000) Introduction and General Aspects. 208 1-5 Wirth T (2003) Introduction and General Aspects. 224 1-4 Wirth T (2003) Oxidations and Rearrangements. 224 185-208... 

More recently, P-cored derivative (116) was prepared from a straightforward combination of a Heck coupling, to afford an intermediate functionalised stil-bene phosphine oxide (114),a Horner-Wittig reaction yielding the phosphine oxide (115), and finally trichlorosilane reduction (Scheme 31) [89]. Using similar strategies, both the valence isoelectronic N- (117) and C- (118) cored dendrimers have been prepared (Scheme 31). 

Dendrimer-protected colloids are capable of adsorbing carbon monoxide while suspended in solution, but upon removal from solution and support on a high surface area metal oxide, CO adsorption was nil presumably due to the collapse of the dendrimer [25]. It is proposed that a similar phenomena occurs on PVP-protected Pt colloids because removal of solvent molecules from the void space in between polymer chains most likely causes them to collapse on each other. Titration of the exposed surface area of colloid solution PVP-protected platinum nanoparticles demonstrated 50% of the total metal surface area was available for reaction, and this exposed area was present as... 

Catalyst Activation Gas phase activation of supported DENs was examined using in-situ FTIR spectroscopy and FTIR spectroscopy of adsorbed CO. For in-situ dendrimer decomposition studies, the spectra were collected under a gas flow composed of 20% 02/He or 20% H2/He. The supported DEN sample was pressed into a self-supporting wafer, loaded into a controlled atmosphere IR cell, and collected as the sample background. The temperature was raised stepwise and spectra were collected at each temperature until little or no change was observed. After oxidation, the sample was reduced in 20% H2/He flow with various time/temperature combinations. The sample was then flushed with He for lhr at the reduction temperature. After cooling under He flow, a background spectrum was collected at room temperature. A 5% CO/He mixture was flowed over the sample for 15 minutes, followed by pure He. IR spectra of CO adsorbed on the catalyst surface were collected after the gas phase CO had been purged from the cell. 

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. 

Mixed-metal dendrimers containing up to 6 Pt(IV)-based organometallic species in the branches and 12 peripheral ferrocene units (8) have recently been synthesized and their electrochemical behavior investigated [13]. As in the previously discussed examples, multi-electron reversible oxidation processes, assigned to the equivalent, non-interacting ferrocene units, have been observed. The authors point out that cyclic voltammetry is a powerful tool to support the structure of the dendrimers containing ferrocene units. 

The first attempt to construct a dendrimer with an electroactive Ru-polypyridi-ne core was based on the reaction of Ru(bpy)2Cl2 with a branched polyether-substituted phenanthroline ligand (11) [27]. In the potential window +2/-2V, this compound shows a one-electron oxidation process and three distinct one-electron reduction processes that, by comparison with the behavior of the... 

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

In larger structures, the number of equivalent units becomes huge. In the docosanuclear dendrimer made of an Os(II)-based core and 21 Ru(II)-based units, a one-electron oxidation process, assigned to the Os(II)-based unit, is followed by a 12-electron process, due to the simultaneous oxidation of the 12 equivalent and non-interacting peripheral Ru(II)-based units [36]. 

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