Transition metal complex ligand site preferences

Polyaza-, polythia-ligands. Recognition of transition metal ions. Replacing the oxygen sites with nitrogen or sulphur yields macrocycles and cryptands that show marked preference for transition metal ions and may also allow highly selective complexation of toxic heavy metals such as cadmium, lead and mercury [2.41-2.44, A.14]. 

The M /nH exchange is initially fast, given the enlargement of the d caused by the presence of die base. Complex species are subsequently formed between the layers of the solid, if the basic groups of the ligand prefer to deprotonate from the Bronsted PO -OH/H O sites in order to coordinate with the Lewis acids, i.e., the transition metal ions. 

Several examples have been given of the reactions of nucleophiles with coordinated organic ligands. Unsaturated hydrocarbons such as ethene, butadiene and benzene are not normally attacked by nucleophiles, but on coordination to a transition metal their susceptibility to such attack is often greatly enhanced, particularly when the complex is cationic. On the other hand many otherwise reactive species such as the cyclohexadienyl cation, protonated benzene (p. 303) can be stabilized by coordination, so that controlled reactions can be performed at ligand sites. In a complex there may be several potential sites at which a nucleophile might attack. Often different nucleophiles prefer different sites. This is illustrated with reference to fy -arenetricarbonylmanganese salts. 

A further property that makes transition metal ions tremendously useful in photochemistry is obviously the possibility of complexing several mmio- or polydentate ligands around a number of metal ions and thus to assemble sophisticated structures where electronic and geometric factors drive the photochemical processes. This principle has been largely applied in artificial photosynthesis for optimizing light collection and make migration of excitation toward a specific site preferred. 

For the combinatorial selection of RNA (or DNA)-transition-metal catalysts, further elements have to be developed and integrated into the scheme (Figure 18.3). In addition to a tethered substrate, a site-specifically attached transition metal ligand needs to be present in each molecule of the hbrary. After loading with the metal, it should allow formation of the catalytically active species, preferably with the reactant tethered to the same RNA molecule. The other reactant carries a purification tag, allowing the selective isolation of only those species in which a reaction had taken place. A further nontrivial requirement is that the attachment of the metal-hgand complex to DNA or RNA does not interfere with the enzymatic copying steps (transcription, reverse transcription (RT), polymerase chain reaction (PCR)). 

In this work, well-defined complexes of biologically important 3d transition metals (Cu(II), Fe(III), Fe(II), and Ni(II)) with either neutral or monodeprotonated anionic adenine or adenosine, synthesized and characterized 5 as described previously, have been used as a model system to study the effects of the interaction of transition metals with purine and purine nucleoside components of nucleic acids on redox properties of the system. The structures of the complexes is simpler than that of nucleic acids and facilitates evaluation of the electrochemical results. The non-phosphorylated monomeric units are suitable model ligands as the use of nucleotides offers complicating factors associated with phosphate due to self-association and self-complexation and preference for the PO4 moiety as the site for complexation. ... 

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