Lariat ethers complexation

Lariat ethers of structure 8 were found to be selective toward Li ion and the lariat crown ether-Li+ complexes are more stable than the corresponding complexes with Na or K+, in methanol. Nevertheless, experiments conducted in aqueous solution showed that Na+ had a better complexation ability than the other two alkali metal cations. Hence, selective complexation of lariat crown ethers with cations changes with the solvent system this may be due in part to the difference in solvation between solvent and cation (Figure 9 f. ... 

Another control experiment was run to further confirm the significance of the cation-pi interaction in these bibracchial lariat ether model complexes. In this case, a diaza-18-crown-6 derivative was prepared in which a 2-phenylethyl pi-donor sidearm was attached to one nitrogen and a 2-methoxyethyl sigma donor was attached to the other <2002CC1808>. The structure is illustrated as 14, above. The solid-state structure of the 14 KI complex showed the typical apical solvation of the ring bound cation. In this case, however, one apex was solvated in the pi-fashion (benzene) and the other by the oxygen sigma donor. 

A new class of non-fluorinated barium P-ketoiminate complexes that contain appended ether "lariats has been reported by Schulz et al. [99, 138]. Six complexes with the general formula Ba[(H3C),3CC0CHC(NC2H40R )R"]2 R = (C2H40) L (n = 0, 1 L = Me and = 2 L = Et), and R = Me or Bu have been fully characterized by standard analytical techniques including, in three cases, single crystal X-ray... 

Liu, Y. Han, B.-H. Li, Y.-M. Chen, R.-T. Ouchi, M. Inoue, Y. Molecular design of crown ethers. 13. Complexation thermodynamics of light lanthanoid nitrates with aza-16-crown-5 lariat in acetonitrile Enhanced selectivity for Nd ". J. Phys. Chem. 1996. 100 (43). 17361-17364. 

Our belief that dynamics and flexibility are both retained in A-pivot lariat ether complexes derives from two sources. First, we have studied the relaxation times (Ti) for a variety of A-pivot lariat ethers using C-NMR [6]. The C-NMR results show essentially no change in the mobility of detectable nuclei whether or not the lariat ether is complexed by sodium or potassium cations. Second, ultrasound relaxation time studies conducted collaboratively with Petrucci and Eyring [13] have confirmed high binding rates for a two-step complexation mechanism. 

Due to their ability to form complexes selectively with metal cations and thereby solubilize such cations in liquid media of low dielectric constant, macrocyclic multidentate ligands have been widely investigated as carriers in metal ion transport processes for the past two decades (2-P). Such macrocycles include crown ethers, lariat ethers (crown ethers with a side arm which contains potential binding sites) (JO), calixarenes, and some cryptands. Studies of structural variations within a given... 

Inclusions of Other Grown Analogues. A variety of crown analogues and hybrid modifications (24—28) with other topological features (lariat ethers (31,32), octopus molecules (33), spherands (eg, (12) (34), torands (35)) including chiral derivatives (36) have been prepared and demonstrated to show particular inclusion properties such as chiroselective inclusion (Fig. 4) (37) or formation of extremely stable complexes (K ">(LR) for (12)... 

Although the lariat ethers (29-31) were conceived on principles related to biological activity, they are interesting candidates for study as either free phase transfer catalysts, or as polymer-bound catalysts. In the latter case, the sidearm could serve both a complexing function and as a mechanical link between macroring and polymer. Polymeric phase transfer catalyst systems have been prepared... 

Intramolecularity in lariat ether complexation was demonstrated in three ways. First, when solution cation binding constants were determined. Kg was shown to be independent of cation and macrocycle concentrations (within certain limits). The efficacy of complexation of a single cation by two macrorings [(ML2) complex formation] should be concentration dependent. Second, ammonium cation binding constants were determined for the series (see above) of monoaza-15-crown-5 and -18-crown-6 compounds having (CH2CH20)jiClH3 (n 0 to 8) sidearms (31). 

The complexation ability of crown-ethers has been improved by the introduction of secondary donor sites covalently bonded to the macrocyclic ring through a flexible arm, e.g. "lariat ethers" (27). It is also known that in particular conditions crown-ethers can make 2 1 sandwich complexes with the cation (8). 

We now proceed to more complicated ionophores in order to testify the validity of this extrathermodynamic relationship and its hypothetical interpretation as an attempt to understand the nature of supramolecular interactions more generally and deeply. The thermodynamic parameters are plotted in Figures 16-19 for long glymes, (pseudo)cyclic ionophore antibiotics, lariat ethers with donating side-arm(s), and bis(crown ethers), whose structural changes upon complexation are schematically illustrated in Figure 20. 

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. 

A plethora of crown ether- or cryptand type molecules have been reported. Some of them are depicted below to show their diversity and versatility. Lariat azaether containing cyclen macrocycle 208 [36] and azaethers involving triazole 209 [37], furane or pyrrole containing macrocycles which can complex two copper 210 [38] or one barium cations 211 [39], spiro-linked crown ethers 212 [40] andcagecompounds 213 [41] which, in addition to two alkali cations, could... 

Additional examples of type d (Scheme 5.1) bifunctional reactants are provided by the alkaline-earth metal ion complexes of lariat ethers 8-10, bearing a sulfhydryl side arm, instead ofthe phenolic hydroxyl of a calixcrown [23,24]. Here the acyl-receiving and acyl-releasing unit, like in papain and ficin, is a sulfhydryl group. 

There has been considerable interest shown in the design and synthesis of functionalized crown ethers, and in whether the appended functional group can then further complex the cation. Examples include the so-called lariat ethers, which may be functionalized on either N or C bridgeheads in the ligand frameworks (89).427... 

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. 

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