Metal structural diversity

Similar structural diversity has been established for the heavier alkali metals also but it is unnecessary to deal with this in detail. The sUTictural chemistry of the organometallic compounds in particular, and of related complexes, has been well reviewed. 

Ionophores constitute a large collection of structurally diverse substances that share the ability to complex cations and to assist in the translocation of cations through a lipophilic interface.1 Using numerous Lewis-basic heteroatoms, an ionophore organizes itself around a cationic species such as an inorganic metal ion. This arrangement maximizes favorable ion-dipole interactions, while simultaneously exposing a relatively hydrophobic (lipophilic) exterior. 

Abstract An overview on the microwave-enhanced synthesis and decoration of the 2(lH)-pyrazinone system is presented. Scaffold decoration using microwave-enhanced transition-metal-catalyzed reactions for generating structural diversity, as well as the conversion of the 2(lH)-pyrazinone skeleton applying Diels-Alder reactions to generate novel heterocyclic moieties are discussed. The transfer of the solution phase to polymer-supported chemistry (SPOS) is also described in detail. 

Directed metallation continues to be developed as a convenient method for regiospecific substitution of pyridines. A mild and general procedure for the preparation of structurally diverse 4-alkyl-2-aminopyridines 37 involves the lithiation/alkylation of aminopyridine derivative 36 <96JOC(61)4810>. 

In this work positron annihilation lifetime spectroscopy (PALS) was used to investigate structural diversity inside zeolite precursor matrix caused by the presence of alkali cations Na, K, Rb and Cs. PALS is an established and well-proven method for structural investigations of various materials, extensively used for metals and alloys, semiconductors and porous materials [3, 4]. In the investigations of zeolites PALS has been mostly used for their void structure and size study [5, 6, 7, 8], also in correlation to... 

We consider the four structurally diverse classes of organometals I-IV, in which the configuration and coordination about the metal centers vary systematically from octahedral, square planar, tetrahedral to linear, respectively. 

A feature of the metal-ion chemistry of these large ring macrocycles is thus the structural diversity which may occur from one system to the next. This diversity can result directly from small changes in the structure of the cyclic ligand and is also aided by the inherent flexibility of the large rings involved. It is clearly also influenced by the nature of the other ligands available for complex formation. 

While the alkoxymetallation process has typically been affected by highly electrophilic metal salts, high-valent metal species generated by an oxidative addition have also been used to activate alkynes through the formation of 7r-complexes. In such cases, the metal-carbon emerging from the attack of an oxygen nucleophile may enter a reaction manifold that leads to an additional C-G bond formation rather than a simple protic quench. This approach, pioneered by Arcadi and Cacci, has proved to be a powerful strategy for the synthesis of structurally diverse substituted... 

In the metal aluminophosphate (MeAPO) family the framework composition contains metal, aluminum and phosphorus [27]. The metal (Me) species include the divalent forms of Co, Fe, Mg, Mn and Zn and trivalent Fe. As in the case of SAPO, the MeAPOs exhibit both structural diversity and even more extensive composihonal variation. Seventeen microporous structures have been reported, 11 of these never before observed in zeoUtes. Structure types crystallized in the MeAPO family include framework topologies related to the zeolites, for example, -34 (CHA) and -35 (LEV), and to the AIPO4S, e.g., -5 and -11, as well as novel structures, e.g., -36 (O.Snm pore) and -39 (0.4nm pore). The MeAPOs represent the first demonstrated incorporation of divalent elements into microporous frameworks. 

Polyphosphazenes and cyclophosphazenes are almost unique as carrier molecules for transition metals because of the wide range of binding sites that can be incorporated into the phosphazene structure. The substitutive mode of synthesis described earlier allows a structural diversity that is not found, for example, in polystyrene, polyphenylene oxide, or other organic carrier polymers. 

MM methods, originally developed for organic compounds, have been modified to describe metal complexes [13-17] where polarization interactions must be considered and where a variety of geometrical configuration is observed. This structural diversity reflecting the existence of multiple local energy minima is due to ligand-dependent effects observed in complexes of... 

One can in principle combine different exchange reactions in the same system in order to further increase the structural diversity accessed by the library. However, as this compounds the problem of selectivity (i.e., one now has two or more reactions that must exclusively involve one pair of functional groups), there are very few examples thus far of the practical implementation of this concept. An early, highly intriguing example was described by Lehn and coworkers in 2001 [68]. In this system, imine exchange (acyl hydrazone formation) and reversible metal coordination were employed in library generation. 

Additionally, nucleic acid bases have been used in the dynamic assembly of mixed-metal, mixed-pyrimidine metallacalix[n]arenes [47]. In this approach, Lippert and coworkers investigated the dynamic assembly of metallacalixarenes based on platinum (Pt ), palladium (Pd°), uracil, and cytosine assemblies with mixed amines. These combinations form cyclic metallacalix[n]arenes structures with n = A and = 8. Of the metallacalix[4]arenes, compounds were formed with five, six, and eight bonded metals, and a variety of nucleobase coimecfivities (UCUC and UCCU). The dynamic nature of this assembly allows access to novel and structurally diverse set of nucleobase metallacalixarenes. 

Structural diversity is achieved through the use of nonbonded pairs of electrons on the ligand of both type II complexes to coordinate additional metal atoms. The S—S distances of known complexes range from 1.98 to 2.15 A. Most S—S distances are intermediate between the distance of 1.89 A for Sj ( Zg ) (104) and 2.13 A for ( Zg+) in Na2S2 (50). The main S—S distances show no clear systematic trend with structural type (cf. Table II). 

The lactone concept is not restricted to the simple model biaryl synthesis presented here. It has been successfully expanded to a broad series of structurally diverse biaryl substrates (e.g., lactones with additional stereocenters and functional groups, configurationally stable lactones, seven-membered lactones, and again configurationally unstable biaryl hydroxy aldehydes ), to different activation modes in the ring-opening step (e.g., use of metallated nucleophiles, carbonyl activation by Lewis acids, (Ti -complexation, etc.), and for various strategies of stereoselection (e.g., external vs. internal asymmetric induction). ... 

If the anion of 2-(2 -hydroxyphenyl)-5(4/l/)-oxazolone 336 is used as a ligand, bis-chelate complexes 337 of copper(II), nickel(II), and zinc(II) have been prepared from the corresponding metal acetates. Alternatively, 336 and 2-(2 -aminophenyl)-5(4//)-oxazolone 340 can act as ligands with metals including palladium(II), platinum(II), ruthenium(II), nickel(II), and copper(II) to produce a variety of structurally diverse complexes 338, 339, and 341 as shown in Schemes 7.109 and 7.110. ° ... 

The structural diversity associated with bicyclic 5-5 fused heterocyclic systems containing two heteroatoms in each ring has been noted previously in both CHEC(1984) <1984CHEC(6)1027> and CHEC-II(1996) <1996CHEC-11(7)115>. In recent years this diversity has also been reflected in the number of applications that have been found for these compounds which include organic metals, molecular clips and receptors and molecular magnets. These fields will be examined in due course through this chapter. 

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