Aromatic and Heterocyclic Ligands

The organic chemistry of aromatic and heteroaromatic compounds is particularly well-understood. The effects of substituents and ring heteroatoms on the rates and sites of attack by nucleophiles and electrophiles are both predictable and capable of rationalisation. What can the co-ordination chemist offer to an area as well-characterised as this Perhaps because the organic chemistry of these systems is so well-behaved, the potential for modification of reactivity by co-ordination is great. 

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Among N-donor ligands, ammonia, aliphatic, aromatic, and heterocyclic amines are widely used as ligands at present (see Sec. 2.2.4.2). Amine complexes are obtained by direct (immediate) interaction of ligands and metal salts, for example (3.8) [8] ... 

Wide synthetic possibilities for modification of coordinated ligands are opened up by the classic reactions of electrophilic and nucleophilic substitution in complexes of aliphatic, aromatic, and heterocyclic compounds [314,359,418 422]. For example, the transformations (3.196) were known long ago [419] ... 

As a result of interaction of 843 and pyridine, the adduct 845 is formed [53], The structures of coordination compounds 844 and 845 were proved by x-ray diffraction. As shown above (Sec. 3.4.3.2), the direct ammonia synthesis [55,56] with participation of various ligands (especially aliphatic, aromatic, and heterocyclic amines, aminoalcohols), elemental metals (or their oxides), and NH4SCN in mostly non-aqueous media, opens definite possibilities for obtaining thiocyanate complexes. In this respect, transformation (4.9) should be mentioned [57] ... 

While UOi+ forms a very large number of complexes with oxygen-donor ligands of all types, particular effort has been devoted to carboxylic acids, from the simplest (formic, acetic, oxalic acids) to polyfunctional, aromatic and heterocyclic acids. One motive for investigating these compounds is the possible role of simple carboxylic acids as reductants [3, 5] of excited UVI, generating U,v which can then reduce Puiv to PuIn which is more readily separable than Pulv from UVI in the treatment of nuclear waste. Another significant role has been proposed for carboxyl-functionalized polymers which show potential in the solid-phase extraction of UV1 from dilute solution [13]. 

The complexes are arranged according to the type of carboxylic acid they contain, from the simplest monobasic aliphatic acids to dibasic, aromatic and heterocyclic acids. With each acid, the full range of its complexes is explored, including those containing additional ligands and those in which the Uiacid ratio varies from that required for electroneutrality. 

Since most Ni1 species with simple N-donor ligands are prone to disproportionation into Ni° and Ni11, relatively few Ni1 complexes with nonmacrocyclic N-donor ligands have been reported. Formation of Ni1 species is in most cases proposed on the basis of electrochemical data, although ligand-centered redox processes have to be considered. The ligands usually contain imine donor atoms or aromatic N-heterocycles, which because of their 7r-acceptor ability favor stabilization of lower oxidation states. 

The adducts Mn(S2PBu12)2(bipy) and Mn PBu Cphen) are octahedral and in solid state form supramolecular architectures through n—n interactions between the aromatic heterocyclic ligands.106 Technetium derivatives, important for radiological imaging, such as " TcN(S2PR2)2 (R = Me, Et, Pr, /-Pr), have been described.107... 

Infrared (and NMR) data on a variety of M(CO) -containing compounds involving phenyl-substituted heterocyclic aromatics as n- and ir-ligands M = Cr,Mo, W, n = 3,4,5... 

Theme 4 is located in a similar position to Theme 3 to the right of Theme 1 between Tm4 and Tm5, but recognizes esters and heterocyclic ring systems. Theme 4 is comparatively rare and principally relates to muscarinic receptor ligands. Ser or Thr residues are situated on Tm4 at position 164 and on Tm5 at position 207, and these stabilize the ester moiety via classical H-bonding. Supporting aromatic residues Trp 168 on Tm4 and Phe212... 

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