Silver macrocycles

Macrocycles are well known to bind to a variety of metals in several oxidation states and the macrocycle cavity size can be varied to electronically tune redox couples. A number of electrochemical investigations have been conducted on gold and silver macrocycles . 

RS radical (which dimerizes) and a Cr(III)peroxo complex. Further experimental evidence supports this mechanistic scheme. The kinetics of the oxidation of 2-mercaptopyrimidine by a variety of silver macrocycles follows a second-order rate law/ > A rate-pH profile characteristic of an acid-base equilibrium for the thiol was also observed. The oxidation of CUCIO4 by thiuram disulfide (tds) proceeds by a rate-limiting inner-sphere electron tranfer in the [Cu(tds)] complex. ... 

Organocopper and -silver macrocycles are also known and it would be interesting to check their molecular recognition of anions, if any. Thus, the pentameric (jU-mesityl)copper(I) derivative [21] and the tetrameric (/i-mesityl)silver(I) derivative [22] would be interesting candidates as receptors in host-guest chemistry. 

Stable compounds of silver(II) are found with N, O and F as donor atoms macrocycles are, as elsewhere, able to support the higher oxidation state. As a d9 system, Ag2+ imitates Cu2+ in displaying Jahn-Teller distortion. 

Picolinate and pyridine-2,6-carboxylate give stable complexes, with 4- and 6-coordination. Macrocycles like porphyrins afford silver(II) derivatives most remarkable is the reaction of the macrocycle meso-Me6[14]ane (Figure 4.13). 

Figure 4.13 The structure of the silver(II) complex of the macrocycle mesc>-Me6[14]ane.

Many other cyclic and noncyclic organic carriers with remarkable ion selectivities have been used successfiilly as active hosts of various liquid membrane electrodes. These include the 14-crown-4-ether for lithium (30) 16-crown-5 derivatives for sodium bis-benzo-18-crown-6 ether for cesium the ionophore ETH 1001 [(R,R)-AA -bisd l-ethoxycarbonyl)undecyl-A,yVl-4,5-tctramcthyl-3,6-dioxaoctancdiamide] for calcium the natural macrocyclics nonactin and monensin for ammonia and sodium (31), respectively the ionophore ETH 1117 for magnesium calixarene derivatives for sodium (32) and macrocyclic thioethers for mercury and silver (33). 

From dihydrobis(l-imidazolyl)borate or tetrakis(l-imidazolyl)borate and silver salts the interesting mixed tetrametallic macrocycles 24 and 25 (Fig. 8) are prepared in high yields (40-60%) [36]. 

Thereby, the presence of tertiary monophosphines is critical, because both polymeric (26) and cyclic tetrameric (24 and 25) complexes can be obtained depending on the phosphine added to the reaction mixture. Furthermore, with some of the phosphines rapid decomposition and deposition of metallic silver occurs. In the macrocyclic molecules 24 and 25 a pair of silver atoms is bridged by two bis(l-imidazolyl)borate moieties, with a transannular Ag Ag distance of 8.61 A for 24 and 8.89 A for 25. The conformations of 24 and 25 are... 

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special Interest because of their high catalytic activity for dloxygen reduction. The Interaction of the adsorbed macrocycles with the substrate and their orientation are of Importance In understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytlc layers Include visible reflectance spectroscopy surface enhanced and resonant Raman and Mossbauer effect spectroscopy. This paper Is focused principally on the cobalt and Iron phthalocyanlnes on silver and carbon electrode substrates. 

Other transition metals have received much less attention. Complexes of palladium and 2-amino-phenyl-containing formazans have been reported.397 Mercury complexes of tridentate formazans have been studied.398 Silver complexes of tridentate benzothiazolyl-containing formazans have also been studied.399 Recently, alkali and alkaline earth metals have been the subject of many studies. Formazans such as 228 and 229 as well as the macrocyclic 204 have received considerable attention as metal-specific analytical reagents.400-41 1... 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

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... 

Figure 5 Some examples of silver complexes with Schiff base macrocycles.

Several silver(I) complexes of the macrocyclic Schiff base derived from the [2+2] condensation of terephthalaldehyde and 3-azapentane-1,5-diamine or A,A -bis(3-aminopropyl)methylamine have been described.509,510 The reaction of 2,ll-diaza-difluoro-m-[3,3]-cyclophane with 2,6-bis (bromomethyl)pyridine lead to the 3 + 3 addition product, which gives a complex with two silver... 

Macrocyclic thioethers which contain a carborane backbone have also been prepared and the silver(I) complexes obtained. The reaction of AgN03 with the sulfur-containing macrocyclic ligand... 

Figure 16 Some examples of silver(I) complexes with mixed donor macrocyclic ligands.

Silver(I) complexes with macrocyclic ligands that possess O/N/S as donor atoms have been studied extensively 1474-1533 in Figure 16 a selection of some silver complexes with this type of ligands can be seen. 

In contrast, electrocatalysis in a nonaqueous solvent like dichloromethane with soluble palla-dium(II) and silver(II) porphyrins produces mainly oxalate.145 However, demetallation rapidly deactivates the catalysts. In these cases the catalytic processes are interpreted in terms of reduced forms of the macrocyclic ligand, rather than by formation of Pd1 or Ag1 species following metal-centered reduction. 

Chartres, J. D. Groth, A. M. Lindoy, L. F. Meehan, G. V. Metal ion recognition. Selective interaction of silver(I) with tri-linked N2S2-donor macrocycles and their single-ring analogues. J. Chem. Soc.-Dalton Trans. 2002, 371-376. 

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