Nitrogen silver complexes

Thermodynamics of complex formation of silver with several ligands such amines,368 hindered pyridine bases,369 nitrogen donor solvents,370 and azoles371 have been carried out. Other studies include the secondary-ion mass spectra of nonvolatile silver complexes,372 the relationship between Lewis acid-base behavior in the gas phase and the aqueous solution,373 or the rates of hydride abstraction from amines via reactions with ground-state Ag+.374... 

Table 4. Nitrogen coupling constants A and Q3 for planar copper and silver complexes (data from Rist and Hyde16 in MHz)...

H and 13C NMR spectroscopy have been used to study successive formation of mononuclear and dinuclear complexes with [SJcryptands.483 A heteronudear (Ag+,Pb2+) complex of (77) was described. In the 13C spectra, significant differences were observed for the carbon atoms adjacent to the bridgehead nitrogen atoms, such that the dinuclear silver complex and dinuclear lead complex could be readily distinguished from the heteronudear spedes. The fact... 

In general, the relationship between the Kb values for the bases (measured toward the proton) is approximately linearly related to the K values for the formation of the silver complexes when several nitrogen bases (amines) having similar structure are considered. However, for a base such as 4-cyanopyridine, it is found that the silver complex is much more stable than would be predicted from the basicity of the ligand toward H+. The reason for this is that some multiple bonding in the silver complex is possible as shown here ... 

Examples of the successful application of this method are the use of cop-per(II) acetate for the preparation of neutral complexes [Cu(SR)] (R = aryl, alkyl no base needed) (46), and the use of copper(II) nitrate for the synthesis of anionic species [Cu4(SPh))6]2 and [Cu5(SPh)7]2- (47, 48), as well as of the cationic species [Cu,(SC5H4NH)3]+ (49). The latter complex is interesting in that each thiolate ligand is neutral due to protonation of the pyridine nitrogen atom. Similarly, silver nitrate has been used to prepare several neutral alkyl-thiolato silver complexes (33). The use of copper(I) oxide was reported in the synthesis of a family of copper(I) arenethiolates containing intramolecularly... 

G73>. Similarly l-methyl-4-nitroimidazole chelates with Ru" through an annular nitrogen and a nitro oxygen <88CJC117>. Silver complexes of 4-nitroimidazole are similar <92CJC943>. 

This section includes selected examples of electrochemistry of gold and silver complexes with other ligands and structural types that were not included in previous sections. We have also included a list of references divided into the following areas that can be consulted for additional information sulfur- and nitrogen-containing macrocycles , porphyrins " and clusters . 

In addition, silver-catalyzed asymmetric aza-Diels-Alder reactions provide a useful route to optically active nitrogen-heterocyclic compounds such as piperidines or pyrid-azines. Substituted dihydrobenzofurans can also be enantioselectively prepared through silver-promoted allylation of aldehydes. Other types of silver-mediated cyclizations can also be used in the synthesis of tetrahydrofnrans, tetrahydropyrans, 1,2-dioxetanes, 1,2-dioxolanes, medium-sized lactones, dihydroisoqninolines, and so on. Silver salts can also be used as cocatalysts with other transition metals. Unique activity was observed for these silver-based systems in several cases. Conseqnently, the use of silver can enrich several available heterocyclization methods, and fnrther developments in the application of chiral silver complexes will hopefnlly appear in the near future. 

Spin-spin couplings between nitrogen and silver nuclei, /nAg, and Ag chemical shifts provided useful structural information on silver complexes obtained by Scheele et in the reaction of 3,5-bis[3-(2,4,6-trimethyl-phenyl)imidazolium-1 -ylmethyl] -1 Ff-pyrazole bishexafluorophosphate and 3,5-bis[3-(2,6-diisopropylphenyl)imidazolium-l-ylmethyl]-l.ff-pyrazole bishexafluorophosphate with Ag20. The latter complex is not stable in solution but exists in equilibrium with tetra- and hexanuclear complexes. 

Since both phosphorus and nitrogen compounds are able to coordinate protons as well as silver ion, a question arises as to the stability of the silver complexes in the presence of protons. Data on this point are given on Table IV. 

In summary, it appears that the formation of silver complexes is a function of proton affinity and ultimately of 2a values for both phosphorus and nitrogen compounds. However, the stability of the complex depends on the extent of the substitution. The utility of silver ion for titration of compounds of the phosphine class is potentially much more useful than for the amines. It is probable that silver titrations can be used to differentiate P and N atoms either in mixtures or in a single molecule. 

The stability constants of a great number of mono- and polyolefin complexes of silver nitrate have been determined by gas chromatographic studies. The results of these studies are summarized in Table 6-4. These observations can be utilized for isolation and purification. For example, trans-cycio-octem is separated from the m-isomer by the preferential formation of the more stable fr 2n5-cyclo-octene silver complex. The very strained olefin, cyclo-propene, is quantitatively removed from a mixture of olefins by passing it with nitrogen through an aqueous silver nitrate solution. 

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