Pyridine complexes with metals

Scrutiny of Chemical Abstracts reveals literally hundreds of structures containing pyridine or its derivatives analyzed by X-ray crystallography, and by far the greater majority are involved in complexes with metallic ions, from which the important piece of information to be gained is the geometry of coordination of the metallic ion. 

Selenium hexafluoride, SeFg, the only clearly defined hexahalide, is formed by reaction of fluorine with molten selenium, It is more reactive than the corresponding sulfur compound, SFs, undergoing slow hydrolysis. Selenium forms tetrahalides with fluorine, chlorine, and bromine, and dihalides with chlorine and bromine. However, other halides can be found in complexes, e.g,. treatment of the pyridine complex of SeF/i in ether solution with HBr yields (py)2SeBrc Selenium tetrafluoride also forms complexes with metal fluorides, giving MSeF complexes with the alkali metals. 

Imidazoles are amphoteric compounds with a basic, pyridine-type nitrogen (they are about 106 times more basic than oxazoles and 104 times more basic than thiazoles173), and (where the NH is unsubstituted) a weakly acidic, pyrrole-type amino nitrogen in the ring. In consequence, imidazoles readily form salts with acids and often form salts (or complexes) with metals. The sparingly soluble silver salts formed by imidazoles have been used by Giesemann et al.174 as intermediates in the synthesis of 1-triphenylmethylimidazoles. Normally, however, the salts formed with acids are more important in isolation and purification procedures. 

In another similar example nanocomposite was formed in a polyurethane matrix. Solvent soluble polyurethane had pyridine groups attached which formed complexes with metal salts. Films were then formed and subjected to a reducing agent in order to produce particulate metal filler. In this case the distribution of the filler which was formed was not uniform because the filler had tendency to aggregate (even though it was chemically attached to the matrix prior to the reduction). The following were factors controlling size and shape of these metal particles ... 

The potential complexity of nucleophilic substitution reactions of haloimidazoles is impressively illustrated by the reaction of 3-bromoimidazo-[l,2-a]pyridine (515) with metal amides. No less than four tele-substitution products, 516, and the debrominated compound 517 were formed and adequately explained without resort to didehydro intermediates. Products indicating... 

Di-(2-pyridyl) pyridine and tetrapyridyl also form complexes with metal ions. Whilst the former functions as a tridentate chelate molecule, it is doubtful if the tetrapyridyl is quadridentate . 

Pyridine-2-aldehyde oximes have attracted attention because of their ability to form chelate complexes with metals -s. Gomplexing with ferrous ions increases the acid strength of the oxime group s (see Table 5.10). Generally, the aldehyde oximes give with methyl iodide the quaternary salts , but from 6-methylpyridine-2-aldehyde oxime, the oxime N-methyl ether results . 

The pyridine-2-carboxylic acids are distinguished by their ability to form chelate complexes with metal ions . The property has been known since Skraup" tested all 19 of the pyridine-carboxylic acids and observed that those with 2-carboxyl groups gave characteristic colours with ferrous ions. The colour is due to an absorption band of low intensity at 4,000 A. 2,6-Dicarboxylic acids absorb at longer wavelengths . 

The dipole moments (p. 126), spectra (p. 132) ionization constants (p. 149) and ability to complex with metals (p. 160) have been discussed. Some pyridine 1-oxides are recorded in Table 6,17. 

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