Macrocycles macrocycle equilibrium between

When the neutral macrocycles [64] were dissolved in solvents other than water, equilibria between the neutral forms and betaine structures were also found. In ethanol, the equilibrium between a phenol and a betainic... 

The discussion can be restricted to the first and second reduction processes that are of particular interest in this context. The shift of the bipyridinium-based process is in agreement with the catenane coconformation in which the bipyridinium unit is located inside the cavity of the macrocyclic polyether (Fig. 13.33a) because of the CT interactions established with both the electron donor units of the macrocycle, its reduction is more difficult than in the free tetracationic cyclophane. The shift of the trans-1,2-bis(4-pyridinium)ethylene-based reduction indicates that, once the bipyridinium unit is reduced, the CT interaction that stabilize the initial coconformation are destroyed and, thereby, the tetracationic cyclophane circumrotates through the cavity of the macrocyclic polyether moving the tra ,v-bis(pyridinium)ethylene unit inside, as shown by comparison of its reduction potential with that of a catenane model compound.19 The original equilibrium between the two coconformations associated with catenane 384+ is restored upon oxidation of both units back to their dicationic states. 

Macrocyclization equilibrium data from metathesis reactions of cycloolefins are compared with predictions of a novel, simple RIS scheme for polyalkenes with at least three single bonds between adjacent double bonds. The predictions agree with experiment within the combined experimental errors, supporting the conformational models and confirming that macrocyclization equilibrium has indeed been established in the metathesis reactions. 

A saturation transfer experiment demonstrated that the multiplicity of resonances arises from an equilibrium between different compounds macrocycles where a chloride ion is bound axially to iron and aggregates where the axial position is essentially free and the electronic 7t system gives rise to stacking interactions (see Sect. 4.3). 

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (cf. the thermodynamic template effect in macrocycle synthesis, Section 3.9.1) and vice versa. 

The reversal of shielding effects was not observed in the H NMR spectrum of the dichloride salt of 48c recorded in chloroform-, indicating that the macrocycle unfolded into a planar structure upon protonation [154], Systematic studies revealed that protonation of 48c triggers complicated equilibria, which involve, in addition to the free base, the ring-inverted monocation 49c and two distinct conformations of the dication (50c-l and 50c-2) [164], The equilibrium between the latter two forms was found to depend on the amount of excess acid added and the solvent. 

In the case of the purely aliphatic ligand 2,2,6,6-tetrakis(amino-methyl)-4-azaheptane (12), complex formation with copper appears to proceed in two steps, as elucidated by titration experiments with the fully protonated ligand (12 5 HC1). Three and two protons from (Hr,12) + are sequentially abstracted, and the predominant species after full deprotonation appears to be a dinuclear complex in which two copper(II) ions are coordinated, each in square planar fashion, by the l,3-diaminoprop-2-yl units of two molecules of pentaamine ligand, thus forming a macrocyclic complex of composition [Cu2(12)2]4+ (23). The UV/vis spectral data show an interesting solvent dependence, suggesting an equilibrium between [Cu2(12)2]4 + and two equivalents of mononuclear complex [Cu(12)]2+ under suitable conditions. ESR spectroscopic data are also compatible with the formulation of a dinuclear species. Further addition of base to an aqueous solution of [Cu2(12)2]4+ gives the mononuclear hydroxo complex [(12)Cu(OH)]+, as inferred from the UV/vis spectroscopic data. 

Fig. 6 Dynamic equilibrium between two metallamacrocycle anion receptors. The X-ray structure of the [2 + 2] macrocycle encapsulating CF3SO3- is shown in the lower left corner...

The [2]catenanes 185 and 186 incorporated different 7t-electron-rich macrocyclic components. As a result, their H NMR spectra showed the existence of two translational isomers in solution as shown in Scheme 28. The ratios between the two translational isomers A and B of [2]catenanes 185 (by 181 -[cyclobis(para( uat-/)-xylylene)][PF,3]4) and 186 (by 182-[cyclobis(paraquat-p-xylylene)][PF6]4) are 60 40 and 70 30 at — 30°C, respectively. Increasing the temperature up to +30 °C resulted in an increase of the population of the translational isomers B in 185 and 186 to 55 45 and 30 70, respectively. A temperature dependence of the equilibrium between the translational isomers associated with 185 and 186 was observed. These [2]catenanes can be regarded as temperature-responsive molecular switches. 

CoHMD + homogeneously catalyzes both electroreduction of CO2 and water reduction in water, water-acetonitrile or DMF solutions [8, 9], The CO-to-Fl2 ratio produced is typically less than 1 and strongly depends on the experimental conditions used (i.e., applied potential, amount of water, electrolysis time, etc). The chiral N-H centers of the HMD macrocycle give rise to two isomers, N-rac and N-meso, as shown in Figure 2. The N-rac isomers of both Co HMD + and Co HMD+ predominate in MeCN (>90 %) and water at room temperature. The equilibrium between the N-rac and N-meso cobalt(II) isomers is very slowly established in acidic aqueous and organic media (<2 x 10 s ) by contrast, equilibration of the cobalt(I) isomers is relatively rapid (>2 x 10 s ) [13, 14]. 

In addition to the above thermodynamic conditions for ring formation, the kinetics of the reactions must be considered. Thus, for a reaction to take place the two ends of the polymer chain must be in the correct conformation for sufficient time for the new bond to form. The kinetic factor for cyclization is proportional to Rg, so the net effect of the thermodynamic and kinetic factors is that rings are not favoured between n = 8 and k = 11. Suter (1989) has considered the theoretical approaches of Jacobsen and Stockmayer and compared theoretical and experimental values for macrocyclization equilibrium constants. This has also been performed for Monte Carlo as well as rotational-isomeric-state calculations for the statistical conformations of cyclic esters (decamethylene fiimarates and maleates) and agreement with experimental molar cyclization equilibrium constants found (Heath et al, 2000). 

Scheme 14 Reaction scheme showing the equilibrium between the [2+2] and [3+3] metallo-macrocycles...

The results discussed above, strongly suggest the presence of a dynamic equilibrium between various macrocyclic species in solution. Upon addition of the appropriate anionic template, amplification of one of them is then... 

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