Complexation alkaline earth ions

Simplest examples are prepared by the cyclic oligomerization of ethylene oxide. They act as complexing agents which solubilize alkali metal ions in non-polar solvents, complex alkaline earth cations, transition metal cations and ammonium cations, e.g. 12—crown —4 is specific for the lithium cation. Used in phase-transfer chemistry. ... 

Washing and cleaning agents containing salts of maleic acid—furan copolymers (106) form complexes with alkaline-earth ions. These cleaning compositions do not contain phosphoms or nitrogen and find use in metal, foodstuff, and machine dishwashing products. 

The second ligand type consists of a large group of cyclic compounds incorporating numbers of ether functions as donors. Structure (22) illustrates a typical example. Such crown polyethers usually show strong complexing ability towards alkali and alkaline earth ions but their tendency to coordinate to transition metal ions is less than for the above... 

Following the original paper, reports of the synthesis of new crowns and crown-like molecules proliferated. A typical property of these systems is their ability to form stable complexes with the alkali metal and alkaline earth ions. Prior to the synthesis of the crowns, the coordination chemistry of the above ions with organic ligands had received very little attention. A further impetus to the study of such complexes was the recognition of the important role of Na+, K+, Mg2+ and Ca2+ ions in biological systems. 

Both these cages form complexes with most of the alkali and alkaline earth ions. In particular, the caesium complex of (240) is especially stable compared to other complexes of this ion. 

Diaza-18-crown-6 substituted with 5-chloro-8-hydroxyquinoline exhibits very interesting complexing properties. For instance, M-6 is very selective for Ba2+ over other alkaline earth cations and for K+ over Na+ in methanol. Unfortunately, investigation of fluorogenic effects with other cations has not been reported. On the other hand, the fluorescence intensity of M-7 was shown to increase by a factor of 1000 in the presence of Mg2+ (in a mixture of methanol/water 1 1 v/v) whereas other alkaline earth ions have no effect. 

Scheme 5.2 outlines our design of nucleophilic transacylation catalysts based on crown-complexed alkaline-earth metal ions. By virtue of the acidity-enhancing effect ofthe complexed metal ion, dissociation ofthe proton-ionizable function XH should take place under moderately basic conditions. The metal ion assists acyl transfer from a reactant ester to the catalyst and its subsequent transfer from the acylated catalyst to an external nucleophile (solvent), thus restoring the active form of the... 

Our design of bimetallic catalysts based on crown-complexed alkaline-earth metal ions, for use in reactions of ester and activated amides endowed with a distal carboxylate anchoring group, is based on the mechanistic hypothesis outlined in Scheme 5.3. Such hypothesis critically rests on the finding that in EtOH solution... 

Thioether analogs of the crowns have been known since the 1930s,103 but metal ion complexes of these ligands have not been investigated with the intensity found for the crowns and polyaza macrocycles. As anticipated from the soft nature of the heteroatom, the sulfur macrocycles show a preference to bind transition metals rather than alkali and alkaline earth ions. Studies have... 

Many of these complexes are more stable than those formed with the corresponding open-chain ligands. This macrocyclic effect is determined by enthalpic as well as by entropic contributions. Still higher stability constants are obtained when side chains with additional donor groups are attached to the macrocycle thus, tetraacetic acid 22 forms complexes with most metal ions, including alkaline earth ions and transition metal ions these complexes show stabilities among the highest known. 

In contrast to the behavior of 68, an efficient chemosensor for Mg2+,123 compound 107 has been shown to be a potential chemosensor for Hg2+.157 Ligand 107 forms stable 1 1 complexes with Hg 2+, Cu2+, Cd2+, Zn2+, Ni2+, and Mg2+ in methanol-water (1 1 vol vol). Much lower association constants (log Ka < 2.5) are observed with the other alkaline earth ions, whereas no complexation is detected (log < 1.5) for alkali metal ions. Complexes with Cu2+ and Ni2+ are not luminescent as... 

The stability of complexes of rare earth ions is often compared with that of the alkaline earth ions 11). Since complexes such as Mg(0104)2 3 OMPA and Ca(0104)2 3 OMPA are quite stable (14), we decided to extend our studies to the reactions of OMPA with rare earth ions. 

Fig. 1 includes three categories of metal ions. In the first group, the water molecules of the inner coordination sphere are so labile that substitution takes place at almost every encounter. The overall rate for the complex formation is thus diffusion controlled. It is not possible to separate one single substitution step from the overall process. The rate constants in this group (to which most of the alkali and some of the alkaline earth ions belong) are therefore only in a very trivial sense characteristic of the nature of the metal ion. 

Murexide forms chelates with all the alkali and alkaline earth ions, but the strength of the complexes varies considerably. The chelation involves the N-bridge as well as the neighbouring oxygens. All complexes are distinguished by strong spectral shifts as compared to the free anion. 

Fig. 12. Absorption spectra for the complexes of alkaline earth ions with murexide...

This section not intended to be a comprehensive survey of the numerous studies on complex formation of alkaline earth ions, but rather to... 

Several different types of Feg clusters have been reported, which were classified as planar, twisted boat, chair, parallel triangles, octahedral and fused clusters [55]. The [Fe6(/r2-OMe)i2(dbm)6] ring, where Hdbm is dibenzoylmethane, is a neutral species, but in the solid state it crystallizes with NaCl to give 18 (Figure 8). In fact, the sodium ion is trapped in the center of the iron ring, which acts like a crown ether complexing the alkaline earth ion [53]. 

It would be interesting to compare the formation constant data of the divalent lanthanide ions with the isoelectronic trivalent ones. Unfortunately, there is a paucity of data for the divalent ions. The Eu(II) ion has a 4/7 configuration and the formation constant measurements by Eckardt and Holleck (39) for the EDTA and DCTA complexes show that the Eu(II)-complex has a much lower value for the formation constant than that for either the Eu(III) or the Gd(III) complexes. A lower log K, value for the Eu(II) complex compared to the Gd(III) complex is expected on the basis of the larger size (20) of the Eu(II) ion, and the values for the Eu(II) complexes compare well with those of the alkaline earth ions (log K1EDTA Mg 8.69, Ca 10.59,... 

See Schmidbaur. H. Classen. H. G. Helbig. J. /tngeir. C/ir/n. Ini. Ed. Engl. 1990,29. 1090-1103 for hiolngically important complexes of alkaline earth and alkali metal ions. Coordination chemistry or alkali metal nnd alkaline earth ions is also discussed in Poonia. N. S. Bajuj, A. V. C/tem. Rev. 1979. 79. 389-445. 

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