Alkaline earth metal complexes crown ethers

Breccia, P., Cacciapaglia, R.. Mandolini. L. and Scorsini, C. (1998) Alkaline-earth metal complexes of thiol pendant crown ethers as turnover catalysts of ester cleavage. J. Chem. Soc., Perkin Trans., 2, 1257. 

The types of alkali and alkaline earth metal complexes subjected to molecular mechanics modeling fall into four categories crown ethers[U9,486 491], cryp-tands[492 493], spherands[494,495], and other biologically important ligands, such as ionophores and cyclic antibiotics [496 499]. 

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... 

Rate and stability constants have been reported for the formation of alkaline earth metal complexes with the diaza crown ethers (2,2) and 2,2-Me2 in methanol. The values of Ks and kd for the (2,2) complexes are displaced in parallel to the corresponding values for the 2,2-Me2 complexes, and variations of with metal ion radius are reflected in similar (inverse) variations of kd. 

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]. 

The mechanisms of regioselective and stereoselective 2 -E 2-photocycloadditions have been extensively reviewed. The intramolecular 2 -E 2-photocycloaddition of 2-allyl-2-(l//)-naphthalenone (13) on the surface of silica produces all four cycloadducts (14)-(17) (Scheme 4). ° Molecular mechanics have been used to study the regio- and stereo-selectivity of the 2 -E 2-photocycloadditions in complexes containing crown ether styryl dyes and alkaline earth metal cations."... 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

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. 

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. 

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. 

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