Metals in the Core

Representative examples of dendrimers with metal-containing cores include the materials 8.1-8.6, which were reported in the mid- to late 1990s. For example, first- (8.1), second- (8.2), third- (8.3), and fourth- (8.4) generation zinc porphyrin dendrimers (M = Zn) have been prepared by a convergent synthesis approach [18, 19]. Although the electrochemical and photophysical nature of the metalloporphyr-in core was found to be preserved in the resultant materials, the rate of interfacial electron transfer was found to be reduced due to the separation of the electroactive centers from the electrode surface. Second- (8.5) and third-generation (8.6) 

Mn-containing dendrimers have been prepared and explored as selective oxidation catalysts [20, 21], Improved regioselectivity was observed with various alkene substrates compared to that achieved with the non-dendronized metalloporphyrin. 

Lanthanide-containing dendrimers such as 8.7 have been prepared by a convergent approach that uses polybenzyl ether dendrons and carboxylate anion coordination to the central Er , Tb or Eu cations (Eq. 8.1) [22]. The luminescence of these metallodendrimers has been investigated and was found to be enhanced by both antenna effects involving the polybenzyl ether framework and site-isolation effects that hinder self-quenching processes. 

Another interesting approach to the synthesis of metal-duster core dendrimers involves the displacement of the acetonitrile ligand from the rhenium duster dication [Re6Se6(NCMe)6] with pyridyl-functionalized dendrons. The resultant structures 8.9 possess a hexanudear cluster core and exhibit electronic transitions that are very dependent on the structure of the dendrons [29]. 

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Accumulation of metals in the core sampled in the northern Lagoon, relative to the industrial core, are about 5 times lower for mercury and comparable for the other two metals. Metals reaching this area of the Lagoon are carried by fresh water streams from the basin. A very high number of small productions, whose environmental impact has not yet been adequately evaluated, contribute to this pollution. 

One of the first applications of dendrimers as organometallic hosts was their use as enantioselective catalysts. Indeed, dendrimers that are functionalized with transition metals in the core potentially can mimic the properties of enzymes. Brunner introduced the term dendrizymes for core-functionalized transition metal catalysts which might be used in enantioselective catalysis. The dendrimeric organometallic complex shown in Figure 34 is an example of such a dendrizyme inside which the chiral dendritic branches create a chiral pocket around the transition metal. 

Fig. 2.57. Vertical profiles of heavy metals in the cored sediment of the Dagu Estuary (CSl) (Qin et al., 2006) (With permission from Qin YW)...

The formation of polymers on a conducting electrode surface (TTO, Pt, carbon) occurs irreversibly after the electrochemical oxidation of the above-mentioned groups at around 0.7-1. IV vs. SCE. Redox-active metals in the core of the ligand such as Fe, Co are electrochemically oxidized reversibly before oxidation of the electropol)mierizable group. Oxidation of the ligand is sometime also observed. 

In the above described type of fast reactor or pile it is desirable to dilute the plutonium of the reactive core with natural uranium. This dilution makes it possible to operate the reactor at a higher rate of power production in kilowatts per kilogram of plutonium because the heat 70 conductivity of the plates 19 is increased and the heat production per unit volume of metal in the core may be maintaned at a high value consistent with the ability of the coolant to extract the heat. 

The concentrations of silica and alkali metals in the core of DVDP-1 vary systemically up-section in Fig. 16.17 and generally increase to high levels in the upper 70 m of the core. The lava flows rest on a thick deposit of basanite hyaloclastite which occupies the bottom of the core from 203.1 to about 150 m. The hyaloclastite is overlain by alkali-rich flows of phonotephrite and tephriphonolite from 150 to 125 m. 

In all of the previous examples in this section, at least one of the porphyrin species played both the role of electron or energy donor or acceptor in the device, and the role of the coordinating moiety. A judicious choice of both the metal in the core of the porphyrin and the heteroatom on the coordinating axial base allows the use of nonporphyrinic connectors that will be detailed in the next example. 

While it was shown in some systems that the target activity could be achieved with these catalysts, durability of the catalysts remains a concern. The current research focuses are (i) determining how thick the Pt-rich layer needs to be to provide both sufficient protection (thicker is better) and enhanced activity (thinner is better) and (ii) devising other methods to stabilize the base metal in the cores. 

The reactor, shown in Fig. 14, is designed to operate with a total power level of 5-l/Z mw. It is horizontal only as far as tank and control rods are concerned. The core is cooled by natural convection. Zirconium is considered for base metal in the core instead of aluminum in order to operate with a pressure of 600 psi. 

HIV integrase consists of three distinct domains. The N-terminal domain contains a HHCC motif that coordinates a zinc atom that is required for viral cDNA integration. Three highly conserved amino acids (D,D-35-E) are embedded in the core domain, which form the acidic catalytic triad coordinating one or possibly two divalent metals (Mn + or Mg +). The C-terminal domain (residues 213-288) is responsible for unspecific DNA binding and adopts an overall SH3 fold (Chiu and Davies 2004). The enzyme functions as a multimer and to this end all three domains can form homodimers. 

In summary the simultaneous reduction method usually provides alloyed bimetallic nanoparticles or mixtures of two kinds of monometallic nanoparticles. The bimetallic nanoparticles with core/shell structure also form in the simultaneous reduction when the reduction is carried out under mild conditions. In these cases, however, there is difference in redox potentials between the two kinds of metals. Usually the metal with higher redox potential is first reduced to form core part of the bimetallic nanoparticles, and then the metal with lower redox potential is reduced to form shell part on the core, as shown in Figure 2. The coordination ability may play a role in some extent to form a core/shell structure. Therefore, the simultaneous reduction method cannot provide bimetallic nanoparticles with so-called inverted core/ shell structure in which the metal of the core has lower redox potential. 

A strategy to solve this problem is to separate the core formation process from the reduction of metal ions in the cores as shown in Scheme 1, and use solvent (EG) and simple ions (OH , etc.) as the stabilizers [11]. In the first step of this process, metal salts hydrolyzed in the alkaline solution of EG to give rise to metal hydroxide or oxide colloids, which were then reduced by EG at elevated temperature to produce colloidal metal nanoclusters in the... 

The aqueous solution chemistry of Tc(VII) is dominated by the stability of the TeO anion [4], Nitrido complexes are few and are limited to peroxides and one dimeric nitrido-hydrazido example. The peroxo complexes based on the [TcVIIN(02 )2] core are analogous to the well-known isoelectronic [MoviO(02)2] complexes [117] and are the only examples of nitridoperoxo complexes and rare examples of peroxo complexes of a metal in the +7 oxidation state. 

For the metal in the electrochemical interface, one requires a model for the interaction between metal and electrolyte species. Most important in such a model are the terms which are responsible for establishing the metal-electrolyte distance, so that this distance can be calculated as a function of surface charge density. The most important such term is the repulsive pseudopotential interaction of metal electrons with the cores of solvent species, which affects the distribution of these electrons and how this distribution reacts to charging, as well as the metal-electrolyte distance. Although most calculations have used parameterized simple functional forms for this term, it can now be calculated correctly ab initio. 

This initial hypothesis was later revised, since some researchers (such as Walker et al., 1983) were able to show that, according to the model of inhomogeneous accretion, metallic iron was removed from the Earth s crust in a very early phase and accumulated in the core. These results led to the now generally accepted theory that the young Earth was surrounded by a weakly reducing atmosphere. 

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