Lipophilicity alkali metal complexes

The steric demand and high lipophilicity of the [CH(SiMe3)2] and the [C(SiMe3)3] ligands have allowed the preparation and structural characterization of a significant number of alkali metal complexes. Examples of such compounds are contained in several comprehensive reviews, the latest published in 2001.9,20,47 9... 

Pieraccini, S., Gottarelli, G., Mariani, P., Masiero, S., Saturni, L., Spada, G. P., Columnar lyomesophases formed in hydrocarbon solvents by chiral lipophilic guanosine-alkali metal complexes. Chirality 200, 13, 7-12. 

A series of well-defined macrocyclic polyethers (crown ethers) have been synthesized by Pedersen (1, 2), who also demonstrated the capability of com-plexing and lipophilizing alkali metal ions. The KMn04-18-crown-6 complex (1) is soluble in benzene (3). Potassium metal is solubilized by a dinaph-thalenated cyclic polyether to form the interesting K -encapsulated anion radical (2) (4). 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

For this puq)ose, the photoswitchable bis(crown ether)s 88 and 89 as well as the reference compound 90 have been synthesized. Compounds 88 and 89 are highly lipophilic derivatives of azobis(benzo-15-crown-5). The parent azobis crown ether was originally developed by Shinkai and its photoresponsive changes in complexation, extraction, and transport properties thoroughly examined. Compared to 87, more distinct structural difference between the cis and trans isomers can be expected for 88 and 89 because in the latter compounds the 15-crown-5 rings are directly attached to the azobenzene group. The photoequilibrium concentrations of the cis and trans forms and the photoinduced changes in the complexation constants for alkali metal ions are summarized in Table 7. 

Cobaltacarboranes also have been used in sensor technology. The sandwich complex [Co(BgC2Hn)2] can be used as a lipophilic coreceptor instead of tetraarylborate ion additives in ion selective electrodes, producing electrodes that are more selective for specific alkali metal ions. ... 

Cram and co-workers described complexation of [2.2]paracyclophane or para-phenylene unit containing crown compounds with primary ammonium salts, diammonium salts and alkali metal salts [23, 24]. Compound 5 makes complexes with tert-butylammonium thiocyanate (1 2 stoichiometry) and hexa-methylenediammonium or decamethylenediammonium hexafluorophosphate (1 1 stoichiometry). Cyclophanes 5-7 (Structure 2) solubilized 2 molar equivalent of tert-butylammonium tetraphenylborate in CHCI3 due to the resulting lipophilic complexes. 

Liquid-membrane electrodes include classical ion-exchange, liquid ion-exchange, and electroneutral ionophore-based liquid monbrane electrodes. Of particular interest are systems where the ion-exchanging compounds are dissolved macrocyclic compounds that have a strong selectivity to alkali metals. The stability of the formed complexes in nonpolar solvents far exceeds that found in water and allows for the fabrication of membrane-free micropipettes where the nonpolar/water interface is the membrane. Unfortunately, this leads to higher resistance than that exhibited by crystalline micropipettes and requires the addition of lipophilic salt to the nonpolar solvent to decrease the pipette resistance. 

As stated at the outset, the organometallic ligand can transport alkali metal cations across phospholipid membranes. Since the molecular surface of the 1 1 complex is polar at one end and apolar at the other, it will be reasonable to assume that transfer of the cations through lipophilic media is more likely to occur in the form of polymeric cation-ligand aggregates than in the form of monomeric entities. The observed structures of the dinuclear Li and the trinuclear Na" " complexes may represent in fact two possible modes of such an aggregation during the process of transport. 

Comparison of the data for the lipophilic diazacrown ethers in Table 2 with that for the analogous crown ether compounds in Table 1 shows that replacement of two oxygen atoms in the macrocyclic ring with NH or NC2H5 units drastically reduces the alkali metal perchlorate transport. Presumably this results from diminished complexation of the hard alkali metal cations by the macrocyclic ligands when two hard oxygen donor sites are replaced by two nitrogen atoms. 

Alkali metal ions are involved in numerous important biological processes, such as transmission of nerve impulses, nervous control of secretion and muscle functions, protein synthesis, and enzymatic regeneration of metabolism. Crown compounds, being complex formers, are naturally apt to interfere in such processes. For instance, it was found that lipophilic cryptands of structural formula... 

---