Changing the Metal

Variations to the system, to explore their influence on reactivity, can be brought about by changing the metal atom, the hetero atom, or the electronic properties of the additional ligands on the metal. 

1 Double cycloaddition. In the presence of an excess of dmad, the increased 1,3-dipolar reactivity of 13, or rather 14, leads to a second cycloaddi- 

2 Olefins as dipolarophiles. The reactivity of 13 is also sufficient to cycloadd olefins such as dimethyl fumarate and maleate. The conservation of the cis- and trans-configuration of the olefins in products formed on N-protona-tion and insertion of CO into the Ru-N bond of the cycloaddition product indicates that the reaction is stereospecific and most likely to be a concerted process. The CO ligands in 13 are thermally labile, for which reason 13 is only stable in solution under an atmosphere of CO. ° It is therefore not surprising that the substitution products in which a CO is replaced by dimethyl fumarate or maleate are formed as side-products. 

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Corrosion control requires a change in either the metal or the environment. The first approach, changing the metal, is expensive. Also, highly alloyed materials, which are resistant to general corrosion, are more prone to failure by localized corrosion mechanisms such as stress corrosion cracking. 

A remarkable change in reaction course is notable when changing the metal from aluminum to titanium for cydoaddition reactions using BINOL as the chiral ligand. When the chiral aluminum(III) catalyst is applied the cydoaddition product is the major product, whereas for the chiral titanium(IV) catalyst, the ene product is the major product. The reason for this significant change in reaction course is not fully understood. Maybe the glyoxylate coordinates to the former Le-... 

The diastereoselectivity of the addition of metalated croty] compounds 1 to the imine moiety has been constantly improved by changing the metal from magnesium over lithium, zinc, aluminum to boron and tin113 (see Table 7). With the latter a high selectivity in favor of the. vj M-product 3 was achieved. The unh-diastereomer 4 results exclusively from addition of boron reagents to A-arylimines 21. 

Zhu and coworkers have also discovered an interesting switch in the synthesis of different heterocyclic scaffolds from the same starting material, simply by changing the metal [161]. 

The discovery of a zirconium-based catalyst able to promote polyolefin depolymerization encourages the search for more electrophilic catalytic systems that could be obtained either by changing the metal center or the inorganic support. 

The overall structure found first in the (nonactin)KNCS complex552 is retained in changing the metal present. The ligand wraps itself around the cation as the seam of a tennis ball (Figure 30) and the cation is thence eight-fold coordinated by the four furanyl and four carbonyl oxygen donor atoms. The periphery of the molecule is lipophilic and the lowered lipophilicity of nonactin relative to tetranactin leads to a diminished transfer of these compounds. 

The short-term application of an increased dose rate of chemical inhibitor enhances the corrosion inhibiting potential of an already passivated metal surface. Nevertheless, in order to change the metal surface from an active state to a passive state, the electrode potential must be raised to a level above that of the passivation potential. Typically this is achieved by the use of chromate, nitrite, and/or orthophosphate in the presence of oxygen. 

The variety of spin density distributions on the same ligand by changing the metal ions depends on the different weights of the two above contributions. Spin density distributions can also change if a and n mechanisms contribute to different extents. 

One of the major reasons for interest in this area is the ease with which the new hybrid materials properties can be varied by changing the metal, metal oxidation state, metal matrix, and polymer. Multiple metal sites are readily available. This allows the metal-containing system to have a high degree of tunability. This is due to the often strong electronic interaction between the metal... 

While molecular assembly has proven to be effective for a photoelectric conversion system, coordination reactions are possibly a simple approach for connecting such functional molecules, as presented in the previous section. We applied the stepwise coordination method to prepare a photoelectric conversion system. Since the molecular wire exhibits redox conduction through the wire,11,13 efficient photo-electron transport through the redox sites in the wire is also expected. In this section, we demonstrate the fabrication of a photoelectric conversion system using ITO electrodes modified with M(tpy)2 (M = Co, Fe, Zn) complex wires with a terminal porphyrin moiety as a photosensitizer. The behavior of photo-electron transfer from porphyrin to ITO through the molecular wire was investigated by changing the metal element in the M(tpy)2 moieties.14... 

Further synthetic utility can be introduced by changing the metal in the above alkylidene complexes from tungsten to molybdenum [52]. Molybdenum which is less oxophilic than tungsten will tolerate monomers containing mildly reactive functionalities such as esters [52a] and nitriles [52b] without appreciable catalyst deactivation during the polymerization reaction. For example, the polymerization of e rfo,endo-5,6-dicarbomethoxynorbornene (DCNBE) with Mo(CHtBu)(NAr) (OtBu)2 (Ar = 2,6-diisopropylphenyI) (35a) has been reported to give the cone-... 

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