Oxygen Macrocycles

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. 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. 

As we described earlier, the calorimetric determination of log K allows one to also get ATH for reaction (15.37). The values reported by Izatt and his colleagues were obtained in an aqueous solution with an ionic strength of 0.1. Izatt reports that enough measurements were made in more dilute solutions to show that the enthalpy of dilution to the infinitely dilute solution (the standard state) is small and can be ignored. Hence, we will assume that the AXH values reported are the standard state AtH° values. Thus we have available, ArG°, obtained from equation (15.42), and Ar5° obtained from equation (15.43). 

In comparing ArH°, we note that the values are more negative than are the TArS° values. The result is a negative ArG° for reaction (15.37), resulting in a stable complex for all four alkali metals with both macrocycles. We describe the formation of these complexes as energy (or enthalpy) driven. That is, the favorable ArH° more than compensates for the unfavorable TArS° to give a AXG° 0, and hence, a favorable equilibrium constant K. 

88It would be interesting to compare the above with results for Fr+ with 21C7. Presumably, the radius ratio would be too large and log K would decrease to a value less than that for Cs +.  

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Newkome and co-workers have demonstrated the operation of a template effect in the formation of a pyrido-ester-crown. In the reaction shown in Eq. (2.8), they treated 2-clTloronicotinoyl cliloride with either the disodium or dipotassium salt of pentaethylene glycol. TJie two reactions were conducted under identical conditions except for the presence of sodium vs. potassium cations. Since the product is a six-oxygen macrocycle, its formation would be expected to be favored by K" rather than Na" counter ions for the glycolate. In fact, the yields of crown-lactone were 30% and 48% respectively when Na" and K" were the templating cations. 

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