Podands alkali metal complexes

Figure 7 Stability constants (log Ks) of various podand alkali metal complexes as a function of ionic radius (a) stepwise loss of terminal groups (b) ligands with stiffened chains (c) variation of terminal group (reproduced with...

The podand 52a as well as the macrocycle 54 show the ability to complex sodium ions with their methoxy functions. The macrocycle exceeds the open-chained compound with regard to the complexation constant by a factor of six. While 52a complexes only Na , 54 shows increasing complex constants for K and Cs with a distinct selKtivity for the larger cesium ion. The order of magnitude of the constants of the alkali-metal complexes corr ponds to those of the crown ethers [51]. 

The interaction of oxygen-containing acyclic ligands with alkali and alkaline earth metal cations has provided a burgeoning area of interest. In historic terms, this was preceded by the advent of crown ethers and the accompanying almost retrospective look at their acyclic precedents. This section is sub-divided into five parts simple chelates, metal complexes as ligands, podands, polypodands and sugars. 

Many polypodands having terminal donor groups are also available, e.g. (39)-(41).33 These also exhibit cation complexation and phase transfer properties. KMn04 and aqueous alkali metal picrates are much more readily taken into organic phases in the presence of these podands than with dibenzo-18-crown-6.239 The KSCN complex of (39) exhibits a novel coordination geometry as all 10 donor atoms participate in coordination of the metal cation, and in order to do this the three arms wrap around the cation in a propeller-like fashion.240... 

Kobuke et al. [40] demonstrated that the podand 28 only forms weak complexes with alkali metal ions. However, when a chiral macrocyclic crown ether structure is formed by complexation with boric acid, alkali metal ions are bound much more strongly (Scheme 10). Stiffening, provided by covalent B-O bonds, improves the preorganization of the ligands that is required for the complexation of metal ions. 

Pseudocrown ethers, whose structures are maintained by coordination bonds instead of covalent bonds like typical crown ethers, are among the most suitable candidates for allosteric regulation of ion binding. A linear podand 2 possessing bipyridine moieties at the ends of the polyether chain was converted easily to the corresponding pseudocrown ether quantitatively by complexation with Cu+ (Scheme 1.1). The pseudocrown ether shows a positive allosteric effect on alkali metal ion selectivity in ion transport. The drastic conformational change from a linear to cyclic structure results in a significant macrocyclic effect favorable for ion selectivity. 

However, the affinity for metal cations is also regulated by the character of the donor atoms contained in the podand framework. " While the all-oxygen podands, in keeping with the hard nature of the oxygen donor atoms, favor the formation of complexes with alkali and alkaline earth metal cations incorporation of nitrogen and sulfur donor atoms into the podand framework is attractive, as it alters the binding tendency of all-oxygen podands so that they may more readily complex cations of the transition metals, such as Ag, Cd, Hg, Co, or Several other... 

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