18C6 complexes

Relative reactivity of (EtOSr) is 19, and that of its 18C6 complex is 150. 

Accordingly, the simplest mechanism that accounts for the enhanced ethanolysis rates in the presence of metal ions is shown in IV, where a metal-bound ethanol molecule, made acidic by metal coordination, serves as a general acid catalyst for C—N bond cleavage. Here again enhanced reactivity is exhibited by the 18C6-complexed metal ethoxide species [13]. 

T2-18C6 complex with Ag+, the entropy effect works against the enthalpy effect so that ArG° is smaller and the complex is weaker than for the corresponding 1,10-A2-18C6 complex. 

Fullerenes C60 and C70 form supramolecular adducts with a variety of molecules, such as crown ethers, ferrocene, calixarene, and hydroquinone. In the solid state, the intermolecular interactions may involve ionic interaction, hydrogen bonding, and van der Waals forces. Figure 14.2.9 shows a part of the structure of [K(18C6)]3-C6o-(C6H5CH3)3, in which Cgg is surrounded by a pair of [K+(18C6)] complexed cations. 

The chemical shift behaviour of Cs in the 18-crown-6 (18C6) complexation with CsBPh4 (68) indicates a two-step reaction whose analysis requires the following processes to be considered ... 

The reaction (I) appears to be responsible for chain initiation in the reaction catalyzed by (Fe(II,III)(acac)2)p (18C6) complexes. 

Figure 6 Zero-pressure extrapolated cross-sections for the primary collision-induced dissociation product of the Na (18c6) complex with Xe in the threshold region as a function of kinetic energy in the center-of mass frame (lower x-axis) and the laboratory frame (upper x-axis). The solid line shows the best fit to the data using the model of Equation (6) convoluted over the neutral and ion kinetic and internal energy broadening for reactants with an internal energy of 0 K. Arrows indicate the threshold values derived from this analysis...

An application of chemical speciations makes the situation clearer. Fig. 7. Indeed, for the stability levels of 18C6 complexes with Cs in [BMIM][N(Tf2)] the 1000-fold excess of 18C6 over Cs+ is needed to provide an absolute [Cs(18C6)2] species domination (93%). When only a 100-fold excess takes place as it happens after 4 hours of power supply, then almost 44% of cesium exist in either [CsN(Tf)2]° or [Cs[N(Tf)2 2] forms ("free" Cs), which are not capable to move to the cathode compartment and the process intensity declines. 

Stability constants, measured in methanol solution, for alkaline earth complexes of a number of ionophores are given in Table XVI (280,289,571-577).8 The values for the complexes of valinomycin and enniatin B lie between the values for the crown ethers 15C5 and 18C6 (cf. Section II.C.5 above), for the middle four entries the values are slightly higher. Stabilities of enniatin B complexes show a modest maximum for Ca2+, and of valinomycin complexes show stabilities increasing up to Ba2+ (281). LogAi values for the Ca2+ complexes of acetate, benzoate, and salicylate are between 4.5 and 4.7 in methanol (578) - the... 

Concerning the synthesis of graft copolymers, Jedlinski et al. have prepared poly(MMA-g-(3BL) copolymers via anionic grafting of 3BL from a modified PMMA backbone [85]. PMMA chains were partially saponified by potassium hydroxide and complexed by 18C6 crown ether so as to act as multifunctional mac-... 

Complexes with Oxygen Macrocycles Figure 15.14 summarizes log K values for the complexes of the 15C5 macrocycle with alkali metal cations, and the 18C6 macrocycle with both alkali metal and alkaline earth metal cations.cc The log K results for these systems were determined from calorimetric measurements. Values are plotted against the ratio of the cation crystal radius to the radius of the hole in the center of the macrocycle.dd... 

A similar trend is observed for the alkaline earth cation complexes with 18C6, with Ba2+ (where the radius ratio is approximately one) forming the strongest complex. 6 We note that the alkaline earth complexes have a log K that is several times as large as for the corresponding alkali metal complexes. This difference is not surprising, since the alkaline earth cations have twice the electronic charge of the alkali metal cations. We will discuss this effect in more detail later. 

The results for reaction (15.37) for the alkali metal cations complexed with 18C6 and 15C5 are shown in Figure 15.15. As with log K, the ArZ° values (ArG°, ArH°, and TAtS°) are plotted against the radius ratio. 

A2-18C6, and of 1,10-T2-18C6. The thermodynamic reasons for this can be understood better by referring to Figure 15.17 where we graph ArG°, ArH°, and TAtS° for the formation of the Sr2+, Ag+, and Hg2+ complexes with the three macrocycles.11 The thermodynamic differences ArZ° for the three cations are plotted on the same scale so that comparisons can easily be made. The alkaline earth cation Sr2+ was chosen for comparison since it has nearly the same ionic radius as Hg2+ and Ag+. 

From Figure 15.17, we see that the Sr2+ complexes of both 18C6 and... 

The Hg2+ complex of 18C6, is again weak, with a small entropy effect working against the enthalpy stabilization. For the Hg2+ complexes with... 

Thermodynamic data are not available in aqueous solution for Sr2+ with 1,10-T2-18C6. Hence, only complexes of the other two macrocycles are compared. 

We note from Figure 15.16 that Pd2+ also forms a very stable complex with 1,10-T2-18C6. Again, the stability of the complex results from both strong enthalpy (AtH° = —82.4 kJ) and entropy (TAtS° = 38.1 kJ) stabilizations, so that ArG° = —120.5 kJ, and hence, log K = 21.1.mm... 

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