Dimeric cryptand

The dimeric cryptand 19, containing four 2,2 -bipyridine subunits, was synthesized from the ferrocene ft/s-acyl chloride with the corresponding azamacrocycle this redox-active luminescent cryptand was ased to complex cations <97CC2195>. 

The reduction of the stannyl radical (t-Bu2MeSi)3Sn with alkali metals produces a variety of structural modifications depending on the solvent used (Scheme 2.55). Thus, in nonpolar heptane, a dimeric stannyllithium species [58c Li ]2 (E = Sn) was formed, whereas in more polar benzene, the monomeric pyramidal structure 58c [Ti -Li (C6H5)] was produced. In the latter compound the Li+ ion was covalently bonded to the anionic Sn atom being at the same time n -coordinated to the benzene ring. A similar monomeric pyramidal CIP 58c [Li (thf)2] was prepared by reduction in polar THE the addition of [2.2.2]cryptand to this compound resulted in the isolation of the free stannyl anion 58c K+([2.2.2]cryptand), in which the ion lacked its bonding to the Sn atom. ... 

The structure of mixed aggregates involving ester enolates is also of major interest to macromolecular chemists, since ionic additives are often introduced in the polymerization medium. The more stable arrangement between lithium 2-methoxyethoxide and MIB lithium enolate was thus calculated (at the DFT level) to be a 5 1 hexagonal complex with similar O—Li lateral coordinations212. The same team has recently extended this study to complexes formed between the same enolate in THF and a-ligands such as TMEDA, DME, 12-crown-4 and cryptand-2,1,1213. Only in the case of the latter ligand could a separate ion pair [(MIB-Li-MIB),2 THF]-, Li(2,l,l)+ be found as stable, still at the DFT level, as the THF solvated dimer [(MIB-Li)2,4 THF]. 

Fig. 4.19. (Top) Dimeric structure of a Pr(III) cryptate with (2.2.1) in which the two monomers are connected through two /r-OH links (redrawn from J. Rebizant et al., J. Incl. Phenom. Mol. Recogn. Chem. 5, 505, 1987. (Bottom) Structure of the [Eu(C104 )(2.2.2)]2+ cation showing the coordination of a perchlorate anion between two arms of the cryptand receptor. Redrawn after M. Ciampolini et al., J. Chem. Soc., Dalton Trans. 974, 1979.

ESTMS of 22-2 and also 22-3 showed the expected cryptates, but also peaks corresponding to cryptand dimers <2004CC2670> Table 2 summarizes the reported data. Further exploration with X-ray crystallography of the cryptates showed strong hydrogen bonding and n-n stacking between the substituted pyridyl units. 

In contrast to the crown ether 28, the cryptand K211 (33) is very beneficial to the control of MMA polymerization initiated by diphenyhnethyllithium in THF at —78 °C. As a result, PMMA with a very low polydispersity index (1.01) was obtained" . The effect of this ligand on the unimeric model, e.g. MiBLi, was studied by NMR spectroscopy. The equilibrium commonly observed in THF between tetramers and dimers is actually shifted towards a monomeric complexed species" . Both the E/Z molar ratio (90/10 instead of 0/100) and the chain tacticity (Table 5) are affected by the cryptand K211 (33) whatever the solvent used ". ... 

The first synthesis of molecules of this kind was reported independently by Vogtle [52] and Hall [51] and both involved condensation of 34 with a range of diazamacrocycles 37 under high dilution conditions to form the corresponding amide cryptands 38 together with dimeric analogues 39 and some polymers. The compounds were easily separated by column chromatography on alumina and were fully characterised by mass spectrometry, elemental analysis, and multinuclear NMR... 

The small cryptand 11 (Fig. 11) was obtained with difficulty, because the major compound obtained was a macrotricyclic tetraamide resulting from a dimerization reaction. It was observed that stable proton cryptates of 11 can be obtained. The [1.1.1] bicycle binds one or two protons inside its intramolecular cavity. The diprotonated cryptate 11, 2H. has a high resistance to deprotonation. The monoprotonated cryptate may be obtained with difficulty but this latter species cannot be fully depro-tonated by base to afford the free cryptand. Smaller analogues of 11 containing only carbon atoms in the three chains were also described. They present similar behavior toward the proton. Another class of cryptands able to strongly bind the proton was developed more recently. 

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