Coordination site

What is the scope of Lewis-acid catalysis of Diels-Alder reactions in water An approach of extending the scope by making use of a temporary secondary coordination site is described in Chapter 4. 

This goal might well be achieved by introducing an auxiliary that aids the coordination to the catalyst. After completion of the Diels-Alder reaction and removal of the auxiliary the desired adduct is obtained. This approach is summarised in Scheme 4.6. Some examples in which a temporary additional coordination site has been introduced to aid a catalytic reaction have been reported in the literature and are described in Section 4.2.1. Section 4.2.2 relates an attempt to use (2-pyridyl)hydrazone as coordinating auxiliary for the Lewis-acid catalysed Diels-Alder reaction. 

In a second attempt to extend the scope of Lewis-acid catalysis of Diels-Alder reactions in water, we have used the Mannich reaction to convert a ketone-activated monodentate dienophile into a potentially chelating p-amino ketone. The Mannich reaction seemed ideally suited for the purpose of introducing a second coordination site on a temporary basis. This reaction adds a strongly Lewis-basic amino functionality on a position p to the ketone. Moreover, the Mannich reaction is usually a reversible process, which should allow removal of the auxiliary after the reaction. Furthermore, the reaction is compatible with the use of an aqueous medium. Some Mannich reactions have even been reported to benefit from the use of water ". Finally, Lewis-acid catalysis of Mannich-type reactions in mixtures of organic solvents and water has been reported ". Hence, if both addition of the auxiliary and the subsequent Diels-Alder reaction benefit from Lewis-acid catalysis, the possibility arises of merging these steps into a one-pot procedure. 

In a complexation reaction, the reaction unit is an electron pair. For the metal, the number of reaction units is the number of coordination sites available for binding ligands. For the ligand, the number of reaction units is equivalent to the number of electron pairs that can be donated to the metal. One of the most important analytical complexation reactions is that between the ligand ethylenediaminetetracetic acid (EDTA), which can donate 6 electron pairs and 6 coordinate metal ions, such as Cu thus... 

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. 

The properties of copper(Il) are quite different. Ligands that form strong coordinate bonds bind copper(Il) readily to form complexes in which the copper has coordination numbers of 4 or 6, such as tetraammine copper(Tl) [16828-95-8] [Cu(NH3)4], and hexaaquacopper(Il) [14946-74-8] [Cu(H,0),p+ ( see Coordination compounds). Formation of copper(Il) complexes in aqueous solution depends on the abiUty of the ligands to compete with water for coordination sites. Most copper(Il) complexes are colored and paramagnetic as a result of the unpaired electron in the 2d orbital (see Copper... 

A model involving that variation of the catalyst active perimeter across the knee will first be considered. Afterwards, a model involving the variation of the number of active coordination sites at a constant catalyst surface will be suggested. 

Model based on the variation of the number of active" coordination sites at the catalyst surface. The growth of tubules during the decomposition of acetylene can be explained in three steps, which are the decomposition of acetylene, the initiation reaction and the propagation reaction. This is illustrated in Fig. 14 by the model of a (5,5) tubule growing on a catalyst particle ... 

First, dehydrogenative bonding of acetylene to the catalyst surface will free hydrogen and produce moieties bonded to the catalyst coordination sites. These units are assumed to be the building blocks for the tubules. 

Third, the units are inserted between the catalyst coordination sites and the growing nanotubule (Fig. 14). The last unit introduced will still be bonded to the catalyst coordination sites. From the catalyst surface, a new C2 unit will again displace the previous one, which becomes part of the growing tubule, and so on. 

For the sake of clarity, ten coordination sites are drawn a little further away from the surface of the particle in Fig. 15(a)-(c). These sites are real surface sites and the formal link is shown by a solid line. In this way the different C2 units are easily distinguished in the figure and the formation of six-membered rings is obvious. The planar tubule representations of Fig. 15(a )-(c ) are equivalent to those in Fig. 15(a)-(c), respectively. The former figures allow a better understanding of tubule growth. Arriving C, units are first coordinated to the catalyst coordination... 

Fig. 15. Growth of a (5ii,5n) tubule on the catalyst surface, illustrated by that of the (5,5) tubule. The central grey circle represents the catalyst particle with 10 coordination sites, and the small grey circles represent the other 10 catalyst coordination sites. The normal and bold lines represent single and double bonds, respectively, while coordinative bonds are represented by dotted lines [(a), (b) and (c)] (a ), (b ) and (c ) are the corresponding planar representations.

From Fig. 15, it can be seen that 20 coordination sites of the catalyst are involved in the growth of the (5,5) tubule, and in general, 20n coordination sites will be involved in the growth of the 5n,5n) tubule. It is obvious that the related catalyst particle must... 

Growth mechanism of a (9n,0) tubule, over 24n coordination sites of the catalyst. The growth of a general (9 ,0) tubule on the catalyst surface is illustrated by that of the (9,0) tubule in Fig. 16 which shows the unsaturated end of a (9,0) tubule in a planar representation. At that end, the carbons bearing a vacant bond are coordinatively bonded to the catalyst (grey circles) or to a growing cis-polyacetylene chain (oblique bold lines in Fig. 16). Tlie vacant bonds of the six c/s-polyacetylene chains involved are taken to be coordinatively bonded to the catalyst [Fig. 16(b)]. These polyacetylene chains are continuously extruded from the catalyst particle where they are formed by polymerization of C2 units assisted by the catalyst coordination sites. Note that in order to reduce the number of representations of important steps, Fig. 16(b) includes nine new Cj units with respect to Fig. 16(a). 

The 12 catalyst coordination sites — drawn further away from the surface of the particle (closer to the tubule) — are acting in pairs, each pair being always coordinatively bonded to one carbon of an inserted (F) or of a to-be-inserted (2 ) Cj unit and to two other carbons which are members of two neighbouring cis-polyacetylene chains (3°). It should be emphasized that, as against the (5n,5n) tubule growth, the C2 units extruded from the catalyst particle are positioned in this case parallel to the tubule axis before their insertion. 

Fig. 17. Growth mechanism of a (9/i,0)-(5n,5n) knee involving from 24 to 20 coordination sites of the catalyst, (a)-(g) Planar representation of the successive tubule growing steps (g ) Schlegel diagram representation of the whole knee with the Ci numbering corresponding to that of the individual steps (a)-(g).

The organizational chart further identifies key positions in the project team, including the project director, project manager, site safety and health officer (SSHO), site supervisor, emergency response coordinator, site security, and other specialized positions. 

A somewhat different approach to providing tailored cavities for metal cations was taken by the groups of Cram and Lehn °. Graf and Lehn prepared the spheroidal molecule 21 which has an interesting molecular architecture. The molecule has ten coordination sites within it, six which form an octahedral array and four which are in a tetrahedral arrangement. This remarkable compound is soluble in all solvents from petroleum... 

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