Ethers, crown podands

New approaches for development of specific carriers for use in liquid membrane are described (i) computer-aided design of cation-specific carriers and (ii) functionalization of rare earth complexes as anion carriers. A new series of Li(I) and Ag(I) ion-specific carriers are successfully designed using MM2, MNDO and density functional calculations. Computer chemistry provides a rational basis for design and characterization of cation-specific carriers of armed crown ether-and podand-types. Lipophilic lanthanide tris(p-diketonates) are shown to be a new class of membrane carriers. They form 1 1 complexes with anionic guests and mediate transport of amino acid derivatives. Since these complexes exhibit different anion transport properties from those of crown ethers, further applications of rare earth complexes offer promising possibilities in the development of specific anion carriers for liquid membrane systems. 

It is of interest to compare the structures of the podates with those of the coronates, even if the polyethyleneglycols are not exact analogues of the crown ethers. The podands adopt a ring-like conformation with all the O atoms pointing towards the metal ion. The R(III) ion and these O atoms are more or less coplanar two bidentate nitrates are coordinated axially above and below this plane while the third nitrate is either bonded to the metal ion through the open side of the ligand chain or not bonded (podand E06). In this latter complex, the... 

Isomorphous series in nitrato complexes with crown ethers and podands, as determined from X-ray... 

Solvenl-extraction of lanthanide ions by crown ethers and podands. 

Fig. 7. Crown type and analogous receptor molecules of different varieties (1) crown ethers (2) cryptands (3) a podand (4) a spherand and (5) the natural...

Podates AcycHc analogues of crown ethers /coronands and cryptands (podands, eg, (11) (30) are also capable of forming inclusion compounds (podates) with cations and uncharged organic molecules, the latter being endowed with a hydrogen bond fiinctionahty. Podates normally are less stable than coronates and cryptates but have favorable kinetics. 

The terms crown and cryptand have been universally adopted. A number of other terms have enjoyed less widespread recognition as noted above. Recently, Vogtle and Weber have proposed use of the terms crown ether, cryptand and podand according to the following scheme. Their suggested definitions are as follows ... 

As stated above, systematic names of macrocyclic host molecules were absurdly complicated for routine discussions [22]. Therefore Vogtle proposed the name coronand for crown ethers, and that of coronates for their complexes while cryptand complexes were called cryptates . The corresponding noncyclic analogues are podands such as 64 [23] and podates, respectively. The cumbersome name podando-coronands (and correspondingly podando-coronates ) was proposed for lariat ethers [24] having at least one sidearm like 65. Examples of hemispherands 66 [25], cavitands 25 [26] and those of some other hosts are discussed in Chapter 7 in some detail, whilst the exceptional stability of fragile guests 4 [2a] and 67 [27] in the hemicarcerand 5 cavity are discussed in Chapters 1 and Section 7.3. 

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. 

An additional nuance in the nomenclature of these compounds concerns their complexes. The open-chained compounds are often referred to as podands and their complexes as podates. The cyclic ethers may also be called coronands and their complexes are therefore coronates. Complexed cryptands are cryptates. The even more complicated structures known as spherands, cavitands, or carcerands are called spherates, cavitates, or carcerates, respectively, when complexed. The combination of a macrocycle (crown ether or coro-nand) and a sidechain (podand) is typically called a lariat ether. 

Organic acids, amino acids, 241 aminocarboxylates, (tio) crown ethers, (tio) cryptands, podands, phenols, heterocyclic and aromatic compounds, adenosine derivatives... 

Acyclic hosts with pendant binding sites are termed podands. The simplest podands are simply acyclic analogues of the crown ethers such as pentaethyleneglycol dimethylether (3.11), analogous to [18] crown-6, or its di-ol analogue (3.12). 

Figure 3.11 Binding constants (log K) for K+ binding by simple podand, crown and lariat ether compounds in methanol at 25 °C.

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