Crown ethers complexes with alkaline earth metals

It was found that the linking of styryl dye fragment to benzocrown ether results in novel photochromic compounds CESD (Crown Ether Styryl Dyes) possessing interesting physico-chemical properties (Scheme 1) [13], The dyes are intensively colored and show significant hypsochromic shifts upon complexation with alkaline earth metal cations in acetonitrile solution. Reversible photochemical reaction E,Z-isomerization is observed for both dyes and their complexes. 

Crown ethers can also form complexes with alkaline earth cations and some divalent transition metal cations. In general, the log2sTs values in water for these complexes are greater than those of alkali metal cations. However, when monovalent and divalent cations... 

Alfimov, M.V., Vedernikov, A.I., Gromov, S.P., Fedorov, Yu.V., Fedorova, O.A., Churakov, AV., Kuz mina, L.G., Howard, J.A.K., Bossmann, S., Braun, A, Woemer, M., Sears, D.F., Saltiel, J. (1999) Synthesis, structure and ion selective complexation of trans and cis isomers of photochromic dithia-18-crown-6 ethers, J. Am. Chem. Soc., 121, 4992-5000 b) Stanislavskii, O.B., Ushakov, E.N., Gromov, S.P., Fedorova, O.A, Alfimov, M.V. (1996) Crown-containing styryl dyes. 14. The influence of N-substitute length on the complex formation of betainic chromogenic 15-crown-5-ether with alkaline earth metal cations, Russ. Chem. Bull, 45, 564-572. 

Ushakov, E.N., Gromov, S.P., Fedorova, O.A., Pershina, Y.V., Alfimov, M.V., Barigelletti, F., Flamigni, L., Balzani, V. (1999) Sandwich-Type Complexes of Alkaline-Earth Metal Cations with a Bisstyryl Dye Containing Two Crown Ether Units, J. Phys. Chem.A, 103, 11188-11193. 

In this particular case the binding of an alkaline or alkaline earth metal ion to the crown ether decreases the stability of the host with u-allose (negative allosterism), since the conformation of the crown-ether changes due to the metal ion complexation [264-268]. 

An important class of alkali and alkaline earth metal amides are Mulvey s inverse crown complexes (also discussed in Chapter 2, dealing with sodium and potassium amides), in which cationic homo- or heterometallic macrocycles are hosts to anionic guest moieties.The term inverse crown indicates that the Lewis acidic/Lewis basic sites are reversed or exchanged in comparison to conventional crown ether complexes. Scheme 3.9 illustrates the range of recently published alkali and alkaline earth metal amide inverse crown complexes (for related Zn species see Chapter 7 on group 12 amides). 

Earlier work in this field has been thoroughly reviewed [1,2]. However, to illustrate in a sensible and logical way the evolution from simple metal ion promotion of acyl transfer in supramolecular complexes to supramolecular catalysts capable of turnover catalysis, an account of earlier work is appropriate. The following sections present a brief overview of our earlier observations related to the influence of alkaline-earth metal ions and their complexes with crown ethers on the alcoholysis of esters and of activated amides under basic conditions. 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

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. 

Very similar results were obtained from the CV studies of ( )-38 and ( )-39, but the observed anodic shifts of the first redox couples upon complexation with K+ were smaller (50 mV for ( )-38 and 40 mV for ( )-39). The reduction of the anodic shift from 90 mV (in ( )-37) to 40 mV (in ( )-38) can be explained by an increasing average distance between the cation bound to the crown ether and the fullerene surface, as the addition pattern changes from trans-1, to trans-2, and to trans-3 [55], Additionally, the effects of different alkali- and alkaline-earth-metal ion salts on the redox properties of ( )-37 were investigated. As expected, all electrochemical data clearly demonstrate a much larger interaction between crown-ether-bound cations with the negatively charged than with the neutral fullerene core [55],... 

Spironaphthoxazine with a crown ether, 8, also shows photochromic behavior that is sensitive to the type of alkali and alkaline earth metal ions [14]. Other derivatives of spiropyran and spiro oxazine, 9-12, which can chelate metal ions were synthesized, and the effects of complexation on their photochromic behavior have been investigated [15-17]. 

These are a subclass of cyclic polydentate ligands that complex alkali and alkaline earth metal ions particularly well. Two examples are shown as 1-XX and 1-XXI. Since systematic names for these are unwieldy, a handy notation in which 1-XX and 1-XXI are called, respectively, 15-crown-5 (even more compactly, 15-C-5) and dibenzo-18-crown-6. Crown ethers with as many as ten oxygen atoms are known and several are commercially available. For more details and references see Section 11-14. 

In the crown ethers (18) the interactions between the ligand and metal ion are considered to be more electrostatic in nature, rather than the covalent binding observed for the transition metal complexes of the aza, thia, and phospha macrocycles. The thermodynamic properties of these macrocycles have been extensively studied, with numerous reviews covering complexation, selectivity, and structural aspects, some with extensive tables of thermodynamic data. Considerable efforts have been made to correlate the interrelationship between cavity size of the macrocycles and stability of alkali and alkaline earth metal complexes. From X-ray and CPK models, cavity radii are determined as 0.86-0.92A for 15-crown-5 (64), 1.34-1.43 A for 18-crown-6 (65), and about 1.7 A for 21-crown-7 (66). For complex formation between the alkali metal ions and 18-crown-6, the maximum stability... 

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