Silver complexes pyridines

To minimize the formation of fuhninating silver, these complexes should not be prepared from strongly basic suspensions of silver oxide. Highly explosive fuhninating silver, beheved to consist of either silver nitride or silver imide, may detonate spontaneously when silver oxide is heated with ammonia or when alkaline solutions of a silver—amine complex are stored. Addition of appropriate amounts of HCl to a solution of fuhninating silver renders it harmless. Stable silver complexes are also formed from many ahphatic and aromatic amines, eg, ethylamine, aniline, and pyridine. 

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... 

Several ligands containing pyridine or related groups and sulfur atoms have been synthesized and the silver complexes studied the ligands used are shown in Figure 15. Many of the complexes have a supramolecular structure. 

The complexes of silver(I) ions with substituted pyridines are well known and some representative thermodynamic data for their formation are included in Table 13.81-83 Several attempts have been made to correlate the stability of the silver complexes with the basicity of the substituted pyridines.81,82 In general, a linear relationship between the logarithm of the formation constants and the pKa of the free bases was found to exist. 

For amino- and cyano-pyridines, it has been found that coordination to silver primarily occurred through the pyridyl N atom.82,83 IR spectral evidence has been used to show that this was not the case for substituted 2-amino-85 or 2-cyano-pyridines, however. Some cyano stretching frequencies for silver complexes are collected in Table IS.86,87... 

No structural data are available for silver(H) pyridine salts. Based on the evidence500 of ease of doping at all levels of the silver and copper persulfate complexes into the cadmium spedes, isomorphism was assumed. X-Ray powder photographs have since shown this not to be the case.499... 

Electron spin resonance studies of silver(II) pyridine complexes have proved to be extremely useful in determining the nature of the spedes in solution. Since natural silver has two isotopes, 107Ag and 109Ag, in approximately the same abundance, both of spin / = J, and since their nuclear magnetic moments differ by less than 15%, interpretation of spectra is often considered in terms of a single nucleus. The forms of the hyperfine splitting patterns for IN, cis and trans 2N, 3N and 4N, would be expected to be quite different and hence the number of pyridines can be readily assessed from well-resolved spectra. Spin Hamilton parameters obtained from both solid and frozen solution spectra are collected in Table 64.497 499 501-510... 

After successful application of the silver catalyst shown in olefin aziridination (Section 6.1.1), He and coworkers showed that intramolecular amidation was possible with both hydrocarbon-tethered carbamates and sulfamate esters.24 They found that only the Bu3tpy silver complex could catalyze efficient intramolecular amidation, while other pyridine ligands gave either dramatically lower yields or complicated product mixtures. In an interesting control study, both copper and gold were also tested in this reaction. Both the copper and gold Bu tpy complexes can mediate olefin aziridination, but only silver can catalyze intramolecular C-H amidation, indicating that the silver catalyst forms a more reactive metal nitrene intermediate. 

A useful expansion of the pyrrole-ring annelation methodology was found when 3-alkynyl-6,8-dimethylpyrimido[4,5-< ]pyridazine-5,7(6//,8//)-dione reacts with an amine and silver permanganate pyridine complex. It was postulated that first amination at the C4-position takes place, followed by an intramolecular cyclization into the pyrrolopyridazinopyrimidine derivative (Scheme 35). A similar reaction was reported with 6-alkynyl-l,3-dimethylpyrimido[4,5-Z]pyrazinedione (03MI3). 

Obviously, addition of further AgBF to the mono-silver complex activates coordination to pyridine and formation of the dinuclear silverfl) complex. However, the dinuclear complex is in equilibrium with a trinuclear, triangular complex (see Figure 3.4). 

A rattier interesting phenomenon occurs when pyridine functionaUsed imidazoUum iodide is used to generate the silver(I) complex using the Ag O method. In this case, Wang et al. isolated a trinuclear silver complex consisting of three bis-carbene silver(I) units bonded to the same central iodide anion (see Figure 3.7) where the pyridyl substituents remain pendant [25]. 

Reacting 2 eqniv. of the silver complex of the fnnctionalised carbene with [Ni(PPh3)2Cl2] yields the bis-carbene nickel(II) complex with the two carbenes andpyridines in cis-position and thus pyridine and carbene trans to each other [37], There are no free coordination sites available and the chloride anions are nncoordinated (see Figure 3.12). 

Silver dichromate-pyridine complex (tetrapyridine silver dichromate), Ag2Cr207 4C5H5N, an orange-yellow solid prepared by adding a warm solution of potassium dichromate to a solution of silver nitrate in water and pyridine, oxidizes allylic and benzylic alcohols to aldehydes and acyl-tiins to a-diketones [659]. 

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... 

Methyl-2-pyridin-2-yl-2,4-dihydro-pyrazol-3-one 40 was synthesized by heating ethyl 3-oxobutanoate 5 and (lH-pyridin-2-ylidene)-hydrazine 39 in methanol containing potassium hydroxide (04IC4387) (Scheme 9). Compound 40 was used to synthesize, among others, silver complexes. 

In several papers silver-containing complexes were used as active components of selective stationary phases. Cook and Givand [60], for example, used silver ion complexes with different compounds as stationary phases. The best separation was obtained with the silver complex with pyridine. Complexes with 2,6-dimethylpyridine, quinoline, isoquinoline, 2,2-bipyrryl, etc., did not interact with olefins, which can probably be explained by steric factors [60]. 

Two homologs of silver complexes of tetrazole-functionalized pyridines (193) were reported by Gallardo et a/. as perchlorate salts and a crystal structure was determined from past experience, at least one author of this review regards them as brave to wish to heat a dry perchlorate salt at all. However, they found that the complexes where n = 9 and 14 both showed an SmA phase at temperatures above 150 °C. 

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