Cryptand group 16 metals

Introduction.—The ability of certain molecules, such as the macrocyclic crown ethers, e.g. 18-crown-6 (28), and the macrobicyclic cryptands, e.g. [2,2,2] cryptand (29), to form complexes with metal and ammonium cations has been extensively investigated in recent years. Since the original discovery by Pedersen, in 1967, of the crown group,some reviews and many papers have appeared on the syntheses and complexing properties of different classes of ligands, but it is not the intention here to go into detail concerning these aspects. Laboratory syntheses of the polyether class are dependent on the Williamson ether synthesis (Equation 11), but methods for production of commonly used compounds, such as (28), have been improved, and many representatives of both the crown and cryptand groups are now commercially available. 

Synthesis by high-dilution techniques requires slow admixture of reagents ( 8-24 hrs) or very large volumes of solvents 100 1/mmol). Fast reactions can also be carried out in suitable flow cells (J.L. Dye, 1973). High dilution conditions have been used in the dilactam formation from l,8-diamino-3,6-dioxaoctane and 3,6-dioxaoctanedioyl dichloride in benzene. The amide groups were reduced with lithium aluminum hydride, and a second cyclization with the same dichloride was then carried out. The new bicyclic compound was reduced with diborane. This ligand envelops metal ions completely and is therefore called a cryptand (B. Dietrich, 1969). 

This ligand forms stable complexes known as cryptands (and it is often abbreviated as crypt in writing formulas) with cations of the group IA metals. More recently, the ligand known as cryptand has been used instead of ethylenediamine. This ligand has the molecular structure... 

Conductivity monitoring is most valuable for studying reactions which have very small spectral changes but which are accompanied by pH changes. The interaction of group 1 and 2 metal ions with cryptands and diaza-crown ethers has been studied by flow/conductivity methods. Conductivity monitoring has been linked to reactions which may follow pulse radiolysis, for example, in examining the... 

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... 

Solid-liquid phase-transfer catalyst.1 The reagent represents a new class of catalysts, acyclic cryptands or tridents. It is singled out of a group as the best compromise of efficiency/price/toxicity. It solubilizes salts of alkali metals as well as of transition metals such as RuC13 and PdCl2, probably because of the flexibility of the molecule. In addition the trident is sensitive to the nature of the anion, but anionic activation is less than that obtained with cryptands. 

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... 

Positively charged or neutral electron-deficient groups may serve as interaction sites for anion binding. Ammonium and guanidinium units, which form +N-H" X bonds, have mainly been used, but neutral polar hydrogen bonds (e.g., with -NHCO- or -COOH functions), electron-deficient centers (boron, tin, mercury, [3.6, 3.7] as well as perfluoro crown ethers and cryptands [3.8], etc.), or metal-ion centres in complexes also interact with anions. 

Thus K+ is bound selectively compared to Na+ by 18-crown-6 and [2.2.2]cryptand, while the reverse is true for dicyclohexyl-16-crown-5 and [2.2.1]cryptand. These ligands are shown in Figure 3. The cation Na+ misfits in 18-crown-6 due to its smaller size. The cryptands form complexes of greater stability than do the crowns, and also show greater selectivity as they have a three-dimensional cavity. These ligands have a flexible structure that allows the stepwise replacement of aqua groups on the metal ion. 

Schiff base macrocycles and phthalocyanines are readily prepared and pre-date crown ethers and cryptands but are more suitable for binding transition metals or softer main group ions. 

The application of new bis-substituted dtc ligands containing various polyether and amide spacer groups has lead to the constmction of a wide range of metal-directed assembled bimetallic macrocycles and cryptands (Scheme 5).48 Cryptands can be assembled through careful choice of a suitable octahedral stereochemical directing metal centre such as iron(III) and cobalt(III). 

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