Crown structure

Bell and Hall have incorporated an organometallic unit into a crown by using the ferrocenyl unit as part of the ring or as a third strand. The unit is incorporated either as the 1,1 -diformylferrocene or the corresponding acid. In the former case, the bis-imine is prepared and reduced to give the saturated crown (see structure 24). In the latter case, the acid is converted into its corresponding chloride and thence into the diamide by reaction with a diamine. Diborane reduction affords the saturated amino-crown. Structure 24 could be prepared by either of these methods but the dialdehyde approach was reported to be poor compared to the amide approach which afforded the product in ca. 60% yield . 

The crown conformation is not necessarily the most stable structure for free (uncomplexed) crown ethers. Examine the lowest energy structure of 18-crown-6, and compare it to the crown structure. Explain why the crown structure is less stable. Use equation (1) to calculate the equilibrium ratio of lowest-energy and crown conformers of 18-crown-6 at room temperature. What causes a shift in conformation in the presence of metal cations ... 

Sulfur is made up of S8 molecules each molecule has a cyclic (crown) structure. Phosphorus contains P< molecules each molecule has a tetrahedral structure. On the basis of molecular size and shape, which would you expect to have the higher melting point ... 

There is evidently a flat potential energy surface for these clusters as different skeletal isomers can actually be isolated in the solid state for Au9[P(C6H40Me)3]8(N03)3 a brown form has a D4d crown structure and a green form has a D2h structure. In solution, however, they share a common structure with identical31P NMR spectra either the solution cluster has a regular arrangement of phosphines, or it is fluxional [186]. 

In a deliberate attempt to trap the carbanion/amide intermediates formed en route to the homoamido inverse crown complexes, [ (TMP)(/z-Bu)(/x-TMP)NaMg(TMEDA) ] 432 (Figure 38) was isolated and characterized.440 This complex spans the superbase and inverse crown structural modalities. 

Matel, L., Bilbao, T. 1989. Extraction of strontium using an irradiated extraction agent of crown structure. J. Radioanal. Nucl. Chem. 137(3) 183-190. 

Fig. 6.5 C3v crown structure of a stable, isolable C6(JH 8 molecule that is produced in high yield by direct hydrogenation of solid C60 (Darwish et al. 1996 Wagberg et al. 2005) and most conveniently by a polyamine hydrogenation at 200 °C in standard glassware (Briggs et al. 2005)...

Variable-temperature NMR spectroscopic studies indicate that the crown structure is retained in solution with the compounds existing as two distinct inter-converting conformers with W and Z shapes in benzene solution. 

Alkali and alkaline-earth metal complexes with inverse crown structures... 

In addition to the traditional iso- and heteropoly anions just discussed, recent work has revealed that others, many of them very large, with crown-type structures can be obtained. Examples of these are the [NaPsW Ono]14- ion, which has a double crown structure with five-fold symmetry, [H7P8W4gOi84]33 , which has a double crown structure with four-fold symmetry and two structurally similar crown species of molybdenum, [Mo36On2]8 , and Mo34Oiio(NO)4(H20)14. ... 

Au9(P(4-MeOC6H4)3)g] +, has a centered crown structure (26). The two compounds have similar electronic spectra and P NMR spectra even at low temperature, indicating that these compounds undergo facile intramolecular skeletal rearrangement in solution. 

The crown structure of cpc/o-octasulfur, Sg, is only one member of an homologous series of S (n = 6-26) rings (Figure 4). Additionally, cyclic sulfur oxides such as SeO and S7O2 and mixed sulfur-selenium systems are known. These latter compounds are of some coimnercial significance, with applications as diverse as antidandruff shampoos, fireworks, and polymerization inhibitors. 

S) The X-ray data, found to fit a crown structure in early work, were later found to fit the boat-chair equally weU. 

Two-dimensional NMR spectroscopy ((double quantum fdtering (DQF), correlation spectroscopy (COSY), hetero-nuclear multiple quantum correlation (HMQC), heteronuclear multiple bond correlation (HMBC)) as well as liquid secondary ionization mass spectrometry (LSI MS) and UV-Vis spectroscopies were used to establish crown structures of TTFs 33 ( =l-3). In the case of the macrocycle 33 ( = 1), two protons of each methylene group of the SCH2CH2O fragments were not identical and gave an AA BB system. This observation was in accordance with the expected low conformational mobility of the polyether bridge in ( )-33 ( = 1) as compared with (Z)-33 ( = 1). The macrocycle ( )-33 ( = 2) behaved similarly to ( )-33 ( =1), whereas the protons under discussion were equivalent in ( )-33 ( = 3) <2001CFJ447>. 

The trimer has a chair structure analogous to cyclohexane and the tetramer has a crown structure analogous to Sj,. In the previous section we saw that cyclopho.s-phazenes could be turned inio linear polymers thermally. The same is true for siloxanes. In order to achieve high molecular weights (the production of silicone rubbers), very pure cyclic tetramer is heated with a trace of base (KOH) ... 

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