Self-complexation

Equation (2.57) presumes not only that the solution is very dilute, but also that the solute ions do not undergo side reactions, such as hydrolysis or self-complexation in the saturated solution. A striking case for which, in spite of the low solubility, these conditions are not obeyed is mercury(I) chloride (calomel, Hg2Cl2) in water. Its solubility product at 25°C is K,p = 1.43 x 10mof l its solubility is Sqa = 8.4 x 10 mol L, which is not equal to ( fsp/4) l This is due to the hydrolysis of the Hg ) ion and its disproportionation to form dissolved... 

The highest ratio is obtained in the 2,7-dinitro derivative. It is probable that the planarity of the nitro derivatives and the way the molecules are packed in the bulk play an important role in the photoelectric behavior. A self-complex type of structure may be formed, which favors charge transfer in the solid. 

The self-complexed catalyst of ALB and a base was also successfully applied to the above-mentioned three-component coupling reaction of an enone, an aldehyde, and a malonate. A catalytic asymmetric synthesis of 11 -deoxy-PGFla was achieved by utilizing this reaction as the first key step (Scheme 8D.14) [31], Furthermore, a potential key intermediate 25 for the synthesis of PGFla was synthesized in relatively high yield with extremely high enantioselec-tivity (97%) by the kinetic resolution of the three-component coupling between racemic enone 24, aldehyde 22, and methyl dibenzylmalonate 23 [31],... 

Phenoxatellurines display a tendency to self-complexation that demonstrates their ambient character as both strong n-donors and 77-acceptors. The 1 1 adduct of phenoxatellurine with 2-methyI-8-chlorophenoxatellur-ine (27JCS116 28JCS506) and 2 I adduct of 2,8-dinitrophenoxatellurine with 2-nitrophenoxatellurine (27JCS116) have been described. 

The hetero-dimerization behavior of dye-modified -cyclodextrins with native CDs was investigated by means of absorption and induced circular dichroism spectroscopy in aqueous solution [43], Three types of azo dye-modified /i-CDs show different association behavior, depending on the positional difference and the electronic character of substituent connected to the CD unit in the dye moiety. p-Methyl Red-modified fi-CD (1), which has a 4-(dimethylamino)azobenzene moiety connected to the CD unit at the 4 position by an amido linkage, forms an intramolecular self-complex, inserting the dye moiety in its / -CD cavity (Figure 13). 1 also associates with native a-CD by inserting the dye residue into the a-CD cavity. The association constants for such hetero-dimerization are 198 M"1 at pH 1.00 and 305 M 1 at pH 6.59, which are larger than the association constants of 1 for / -CD (43 M 1 at pH 1.00). 

I. Alkorta et al., Chiral recognition in self-complexes of tetrahydroimidazo[4,5-d]imidazole derivatives From dimers to heptamers. J. Phys. Chem. A 110, 2259-2268 (2006)... 

Our next case history takes what we have learnt about donor-acceptor interactions between arene building blocks in interlocked molecules and exploits that knowledge base in a more conventional intramolecular arena. The self-complexing compound 164+ (Figure 10) incorporates [11] a linear polyether thread intercepted by a DNP ring system, which is co-... 

Fig. 20.10. Base pairs found in homopolymer nucleic add self-complexes. The unsymmetrical uridine U-U and 2-thiouridine s2U-s2U base pairs lead to antiparaUel orientation of polynucleotide chains similar to the RNA double helix. In the symmetrical AH+ -AH+ and C-CH+ base pairs, the dyad axis coinddes with the helix axis and gives rise to parallel polynucleotide chains. For the same reason, the four polynucleotide chains in [poly(G)]4 are related by a fourfold rotation axis and are also parallel

One might have expected similar results for retinol (vitamin A), (Fig. 3) however, the retinol needs to be in the colloidal state to give a blue-green product [115,116]. Tetracyanoquinodimethane (TCNQ) cations are produced by both (3-carotene and retinol, with the former showing a stronger interaction [117]. In ethanol the retinol-I2 complex is not as stable as in water, which, according to Slifkin [1], suggests the possibility of two separate processes for electron transfer to iodine to produce I. Self-complexes of retinol in saline solution, probably as cation-anion forms, oxidize rapidly in air [118,119], presumably because the retinol cation is more reactive then the neutral molecule, similar to the autooxidation of catecholamine [120]. 

Figure 19. Electrochemically induced dethreading/rethreading of the self-complexing compound 28 + (MeCN, 298 K) [36],...

Fig. 8. A light-driven movement of an pendant arm which bears an ammonium group and is covalently linked to a crown ether. UV irradiation induces the trans-to-cis rearrangement of the azobenzene fragment. The self-complexing process is favored by the establishing of hydrogen bonding interactions between the ammonium group and the oxygen atoms of the crown. Decomplexation (cis-to-trans re-isomerization) can take place either via irradiation with visible light or thermally...

Fig. 14. Redox-switchable complexation in a self-complexing molecule...

Of interest are the self-complexes formed by /nnitroaniline, quinhydrone, etc, where the same species has both donor and acceptor character in different portions of the molecule. In such a case the ground state of the complex is given by equation (7) with b -- c. The presence of charge transfer forces in these species leads to a closer packing of these molecules in their crystals than would occur in the absence of these forces33 43 78. 

Several X-ray structures of the parent p-substituted calix[5]arenes have been obtained, including 5 as the acetone complex, the tetralin complex, the ethyl acetate complex, and the self complex 51.1.3,3-tetramethyibutyi... 

X-Ray crystallographic structures of a) 5 intermolecularly self-complexed Taken from Gallagher et and (b) the penta-i-... 

---