Silver complexes cryptands

The X-ray structure of the monoferrocenyl-silver complex is shown in Fig. 7-71 [195]. The cryptand-like cavity of the peripheral tetraoxa-dizazadecane ring is able to accomodate one Ag ion, but it is thought that a direct Fe-Ag interaction also occurs. The cyclopentadienyl rings of the ferrocene portion are tilted by 10°. 

Similarly, by Schiff-base condensation reactions have been used to generate free cryptands from triamines and dicarbonyls in [2+3] condensation mode. These ligands react with silver(I) compounds to give dinuclear or trinuclear macrocyclic compounds where Ag Ag interactions may be present. Thus, with a small azacryptand a dinuclear complex with a short Ag- Ag distance (55) is found.498 With bigger azacryptand ligands also dinuclear complexes as (56) are achieved but without silver-silver interaction. 65,499-501 A heterobinuclear Ag1—Cu1 cryptate has also been... 

Interestingly Gokel has demonstrated the existence of a direct coordination coupling pathway between this ferrocene cryptand and a silver cation. Complexation studies were carried out with [24] and [25] (as well as other ferrocene cryptand-type species) by X-ray crystallography, FAB mass spectral analysis, nmr and UV/Vis spectroscopy. 

The Ag+ ion is labile. Even with cryptands, which react sluggishly with most labile metal ions, Ag reacts with a rate constant around 10 M s (in dmso). The higher stability of Ag(I) complexes compared with those of the main groups I and II resides in much reduced dissociation rate constants. Dissociation tends to control the stability of most metal cryp-tand complexes. Silver(I) is a useful electron mediator for redox reactions since Ag(I) and Ag(II) are relatively rapid reducers and oxidizers, respectively. Silver(I) thus promotes oxidation by sluggish, if strong, oxidants and catalyses a number of oxidations by S20 in which the rate-determining step is... 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

Results with Sr ] in Mice. While the results with Ag cryp-tate were encouraging, we sought further preliminary evidence of the potential value of labeled cryptates as blood-flow radiopharmaceuticals. There were several reasons for these studies the monovalent silver ion is very polarizable and thus may not be a general model for monovalent cations (5,17). In contrast, divalent cations form stronger inclusive cryptates than monovalent cations of the same ionic radii. On the other hand, the added charge of the divalent ion would require that the cryptand shield more charge if it is to result in an equally lipophilic complex. 

The data obtained for [2.2.2]cryptand in acetonitrile solutions were further investigated.479,480 Silver ions are strongly solvated by acetonitrile and a competition was found to exist between complexation of the ligand and the solvent. This was claimed to be predominantly responsible for the lower stability of Ag[2.2.2]+ in acetonitrile than in water and for the rapid decrease in the stability constant at low mole fraction of acetonitrile (xMccn)> This phenomenon was then studied by determining the rate of formation and dissociation of Ag[2.2.2]+ in acetonitrile-water mixtures.481... 

The first cylindrical macrotricyclic ligands synthesized were (52a) and SSa-d).58 70 Cryptands in which the two monocycles are even farther apart as a result of bridging naphthyl, biphenyl and. related groups have also been reported.188,1 9 The smaller macrocycle (52a) forms complexes with a variety of metal cations, including two silver(I) ions.69,70 190 Crystal data results for the latter complex indicate both Ag+ ions are located slightly out of the plane of the macrocycles (undoubtedly the result of macrocyclic size constraints), but within the central main cavity, with an Ag—Ag distance of 3.88 A.191... 

The larger-ringed macrocycles of (53a-d) form binuclear complexes with alkali metal, alkaline earth, silver(I) and lead(II) cations.68,192 The two 18-membered rings are large enough to allow for cations as large as Rb+ to be incorporated within their cavity, with a net result of increasing the metal-metal separation. Thus, crystal structure data for the disodium complex of (53a) indicate the sodium ions to be 6.40 A apart,193,194 compared to a 3.88 A separation found for the aforementioned disilver complex of (52a). Heteronuclear complex formation has also been observed, e.g. with both Ag+ and Pb2+ incorporated in the same cryptand.192... 

J.C. Mendina, T.T. Goodnow, M.T. Rojas, J.L. Atwood, B.C. Lynn, A.E. Kaifer, G.W. Gokel, Ferrocenyl iron as a Donor Group for Complexed Silver in Eerro-cenyldimethyl[2,2]Cryptand - A Redox-Switched Receptor Effective in Water , J. Am. Chem. Soc., 114,10583 (1992)... 

A host-guest complex, 41, of silver with a bis-ferrocene cryptand has been thoroughly investigated, revealing strong evidence of an interaction between Ag+ and the ferrocene unit, in addition to silver nitrogen (and some silver oxygen) interactions with the macrocycles [85]. 

The mechanism of 1 1 complex formation between palladium(II) and catechol and 4-methylcatechol has been studied in acidic media, and the rate of 1 1 (and 1 2) complex formation between silver(II) and several diols is an order of magnitude higher in basic solution than in acidic. The kinetics of formation and dissociation of the complex between cop-per(II) and cryptand (2,2,1) in aqueous DMSO have been measured and the dissociation rate constant, in particular, found to be strongly dependent upon water concentration. The kinetics of the formation of the zinc(II) and mercury(II) complexes of 2-methyl-2-(2-pyridyl)thiazolidine have been measured, as they have for the metal exchange reaction between Cu " and the nitrilotriacetate complexes of cobalt(II) and lead(II). Two pathways are observed for ligand transfer between Ni(II), Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) and their dithiocarbamate complexes in DMSO the first involves dissociation of the ligand from the complex followed by substitution at the metal ion, while the second involves direct electrophilic attack by the metal ion on the dithiocarbamate complex. As expected, the relative importance of the pathways depends on the stability of the complex and the lability and electrophilic character of the metal ion. 

The bromide is converted to a mixture of g-nitrate and g-glycoside derivatives. The reaction was carried out with silver nitrate and a macrocycle or a cryptand as a catalyst. The reaction was studied with several alcohols. The nitrate derivative formation depends on both the alcohol hindrance and the affinity of the crown ether or the cryptand for Ag. The more bulky the alcohol and the more complexed the silver ion, the greater is the formation of the nitrate. 

The physical state of the silver salt has an influence on the reaction. Our data are obtained with the same batch of finely powdered AgNO. The selectivity was determined from NMR analysis. The results are coherent with the crown ether complexing ability. Crown ethers and are better ligands for silver ion than crown ethers and They form cryptand separated ion pairs which favor nitrate derivative formation. 

Consequently, different factors such as (i) silver ion coordination or solvation by the IL anions and/or cryptand 222 ligand, (ii) junction potential, and (iii) effect of IL nature on the of Fc (discussed below) could simultaneously be operative and responsible for the observed potential variation [20]. The stable lifetime of the [ Agl Ag 222, CH3CN] reference electrode is about 4 weeks and, thereby, similar to that reported by Snook et al. [19], even when the sdver(I) stability is increased by the formation of the Ag+-222 inclusion complex [30]. 

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