Cryptate complexes

Crown and Ciyptate Complexes.—Interest in crown, cryptate, and related complexes of alkali-metal (and alkaline-earth-metal) ions has been maintained during the period of this Report. A new class of ether-ester ligands (4) and (5) has been developed.Whereas (4) forms complexes with Mg, Ca, Sr, and Ba, (5) forms complexes with Ca , Sr , and Ba only. The complex between (4) and Mg is the first reported Mg crown compound although the Mg is probably bound within the cavity, it is possible that it may be bound to the externally directed l,3-dicarboxy-group.  

A number of crystalline complexes of DB12C4 (6), of DB18C6 (7) and its derivatives (8) and (9), and of DB24C8 (10) with some and perchlorates and picrates have been synthesizedIdentification and characterization was effected by elemental analysis, i,r., u.v., n.m.r. spectroscopy, conductivity, and X-ray diffraction analysis. With complexes of (8) and (9), it was not possible to isolate all complexes in the crystalline form indeed, the steric hindrance afforded by the alkyl substituents affects the stability of the complexes and hinders the possibility of isolation.  

The crystal and molecular structures of the complex formed between DB24C8 (10) and two molecules of sodium o-nitrophenolate and of the solvated (MeOH 

Two of the solution structures of these complexes have been 

The crystal structures of Na, K, and Rb tetranactin complexes have been published in full. The data have been considered in detail in a previous Report as a result of abstraction from a preliminary conununication/  

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The stability of cryptate complexes. The cage topology of the cryptands results in them yielding complexes with considerably enhanced stabilities relative to the corresponding crown species. Thus the K+ complex of 2.2.2 is 105 times more stable than the complex of the corresponding diaza-crown derivative - such enhancement has been designated by Lehn to reflect the operation of the cryptate effect this effect may be considered to be a special case of the macrocyclic effect mentioned previously. 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

We began this feasibility project with cryptand [2.2.2] and the monovalent Ag - 110m and divalent Sr-85 cations. These two metal ions have diameters of approximately 0.250 and 0.254 nm, respectively (14). The internal diameter of [2.2.2] has been estimated by CPK space-filling models to be 0.28 nm (3,15). Cryptate complexes form readily upon mixing a solution containing the tracer cation with a solution of cryptand. In order to insure complete complexation we used a molar ratio of ligand to ion of three. 

We envision several potential generator-produced radionuclide labels for cryptates (Table I). Fortunately, early evaluations can be performed more conveniently with longer-lived tracers that are commercially available. The cryptate complexes are conveniently formed from the metal in deionized water and the cryptand dissolved in water or methanol. The complexes form instantly upon... 

Table 8 Log Stability Constants of Some Cryptate Complexes in Water, Log Kl...

The results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

Table 10 Rates and Selectivities of Alkali Metal Cation Transport via Cryptate Complexes...

Ciampolini, M., Dapporto, P., and Nardi, N. (1979) Structure and properties of some lanthanoid(III) perchlorates with the cryptand 4,7,13,16,21,24-hexaoxa-l,10-diazabicyclo[8.8.8]hexacosane, Dalton Trans, 974-977 Hart, F. A., Hursthouse, M. B., Abdul Malik, K. M., and Moorhouse, S. (1978) X-ray crystal structure of a cryptate complex of lanthanum nitrate, Chem. Commun. 549-50. 

In a final example, a Eu(III) cryptate complex has been reported as an example of a pH sensing device [129]. This is an example of a bis bypyridyl-... 

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

In a cryptate complex, the cation is enclosed wholly or partially in a hydrophobic sheath, so that not only are salts of this complexed cation soluble in nonpolar organic solvents but also extractable from aqueous solutions into organic solvents immiscible with water (144). Specific cryptands may be used to selectively complex metals from crude materials or wastes, particularly if they are immobilized on a polymer support (101, 114, 145). 

Graf E, Lehn JM (1975) Synthesis and cryptate complexes of a spheroidal macrotricychc ligand with an octahedrotetrahedral coordination. J Am Chem Soc 97 5022-5024... 

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