Cryptands lariat ethers

Although the principal application for 9 has been in the synthesis of cryptands (see Chap. 8), this material has also served as precursor to a number of nitrogen based lariat ethers , sometimes referred to as crown complexanes . Binding constants for such compounds have been measured for a few examples in a few cases , but... 

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. 

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. 

Figure 3.18 Podands 3.11 and 3.40, corands 3.41-3.43 and cryptands 3.23 related to [18] crown-6. Compounds 3.41 and 3.42 are members of a hybrid corand-podand family termed lariat ethers. Lariat ethers with two podand arms such as 3.42 are termed BiBLE s (bi-bracchial lariat ethers).

Compare and contrast the properties of the following classes of host molecule. Include in your answer information on selectivity, solubility and binding kinetics (a) naturally occurring ionophores (b) podands (c) corands (e) lariat ethers (f) cryptands (g) calixarenes and (h) spherands. You may find it helpful to present the information in the form of a comparative table. 

The three Schiff-base bibracchial lariat ethers H2l7a-c derivatives have been designed for their ability to form a cryptand-like cavity upon reaction with Lnm ions, the size of which can be tuned by varying the number of -CH2-CH2-O- units in the macrocycle. The two 2-salicylaldiminobenzyl pendant arms fold in such a way that jt-jt interactions between aromatic rings result in the formation of a cryptand-like cavity, as demonstrated by the X-ray structure of the Cem complex with 17b depicted on fig. 25 (Gonzales-Lorenzo et al., 2003). The triplet state of the di-anionic receptor is located at 18750 cm-1 (0-phonon component measured on the Gd111 complex) so that sensitization of the NIR luminescence is not optimum nevertheless, Ndm emission could be detected. 

Fig. 4.38. Stability constants in water at 298 K and = 0.1 M, for lanthanide complexes with the (2.2.1) cryptand and the bibracchial lariat ethers (2.1)DA and (2.2)DA. From data reported by J.-C.G. Biinzli, in Handbook on the Physics and Chemistry of Rare Earths, eds K.A. Gschneidner Jr., L. Eyring, Vol. 9, Ch. 60, North Holland,...

Ion transport, especially cation transport, was one of the early focal points in macrocyclic chemistry, revolving primarily around the crown ethers and cryptands. Later efforts have been to provide switches to control the rates of cation transport. Two examples of the types of switches that have been developed include photo switches using cryptands,and electrochemical switches using anthraquinone-derived lariat ethers. 

Gonzlez-Lorenzo, M., Platas-lglesias, C., Avecilla, E, et al. (2003) A Schiff-base bibracchial lariat ether forming a cryptand-like cavity for lanthanide ions. Inorganic Chemistry, 42, 6946. 

Crown Ethers and Related Hosts - The First Class of Artificial Hosts Crown ethers are macrocyclic polyethers with crown-like shapes. Various cations are selectively bound to the crown ether, depending on the size of the macrocyclic ring. More precise recognition can be accomphshed using modified crown ethers such as lariat ethers and cryptands. 

Figure 2 Examples of synthons for supramolecular assemblies (a) podand, (b) crown ether, (c) lariat ether, (d) cyclotriveratrylene, (e) resorcinarene, (f) calixarene, (g) cryptand...

Cation-Binding Enhancement by Reduced Lariat Ethers and Cryptands... 

---