Chelating agents, bipyridyls

In 1956 it was found that when pyridine is refluxed with a modified Raney-nickel catalyst, 2,2 -bipyridine (1) is formed in satisfactory yield. The isomeric bipyridines could not be detected, and the product was readily purified. Similar heterocyclic biaryls have been formed in the same way from substituted pyridines and from some related compounds, the yield being dependent on the nature of the compound. The reaction has become the method of choice for the preparation of 2,2 -bipyridine, and it is now used on an industrial scale. Bipyridyls are of particular importance as chelating agents. 

Chelate rings can be formed by pyridines containing a-substituents such as carboxyl or CH = NR. Important bicyclic chelating agents are 2,2 -bipyridyl (55 Y = H), o-phenanthroline (56) and... 

Another pattern of reductive activation has been observed in the Ni hydrogenase of Methanobacterium formicicum (79). In this enzyme, reductive activation is irreversible, requires a strong reductant, and is inhibited by the chelating agent 2,2 -bipyridyl. The conversion of a [3Fe-xS] cluster to a [4Fe-4S] has been suggested. 

In analytical chemistry 2,2 -bipyridyl and its derivatives have many applications they form bidentate chelating agents with metal ions, amongst them iron, ruthenium, copper, and platinum. However, a 6-substituent decreases the chelating ability of 2,2 -bipyridyl by a steric interaction, while 6,6 -disubstitution totally inhibits it. 

Chelate complexes. Chelate rings can be formed by pyridines containing -substituents such as carboxyl or CH=NR. Important bicyclic chelating agents are 2,2-bipyridyl 68 (R = H), o-phenanthroline 69 and 8-hydroxyquinoline 70, which all form bis and tris complexes with many metals <1994CSR327>. This type of complex formation has many analytical applications. Overlap between the d-orbitals of the metal atom and the pyridine -orbitals is believed to increase the stability of many of these complexes. Steric effects can hinder complex formation as in 68 (R = Me). 

In the presence of a chelating agent, Lj [e.g., bipy (2,2 -bipyridyl), 1,10-phen-(1,10-phenantholine), or diphos (l,2-Ph2PCgH4PPh2)], Hg(02CCgF5)2 provides (C6F5)2Hg-L2 on thermolysis. 

Farver and Nord (84) have found that Tl(II)-2,2 -bripyridyl is formed at a dropping mercury electrode when thallium(III)-2,2 -bipyridyl complexes in 0.1 M KNO3 solution are reduced at low thallium concentrations (<1 mM) and pH >5. They suggested that Tl(II) complexes with other chelating agents (N and O donors) could probably also be observed polarographically. 

The procedure described here Is by far the most efficient synthesis of terpyridine. Previous preparations include the dehydrogenation of pyridine with ferric chloride, the Ullman reaction of 2-bromopyridine and 2,6-dibromopyridine, the action of copper on 2-bromopyridine and 6-bromo-2,2 -dipyridyl, the reaction of iodine or ferric chloride with 2,2 -bipyridyl and the reaction of 2,2 -bipyridyl with 2-1ithiopyr1d1ne (40% yield). Terpyridine is a very effective chelating agent. 

It is noteworthy that several quite simple N N chelating agents have anthelmintic properties, e.g. 2,2 -bipyridyl and o-phenanthroline 11,15) (Baldwin, 1948a Dwyer et al, 1952). The much-used vermicide, thiabendazole (6.J2), strongly chelates transition metals between the two nitrogen atoms (Kowala et al, 1971). 

Oxyacids, like citric or tartaric acids, and polyols, like saccharose are also used, mainly as masking agents, in qualitative analysis. The action of some specific reagents, like oc-a -bipyridyl for iron(II) and dimethylglyoxime for nickel(II), is also based on the formation of chelate complexes. In quantitative analysis the formation of chelates is frequently utilized (complexometric titrations). ... 

Various nitrogen compounds have found use in corrosion inhibition, as fire-proofing agents for epoxy resins and flame retardants for polyurethane foams. For these and other uses, the reader is referred to the indices of Chemical Abstracts. One recent report tells of the use of Mn" chelates of phenanthroline and bipyridyl as contraceptive and antivenereal agents. The antibacterial and antifungal properties of these compounds have long been known. ... 

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