Calixpyrrole– Fluoride Interactions
103
potential applications for the removal of both species from contaminated sources, although these are selective for fluorides. Data in Table 2 reveal that with a few exceptions, the stability of the complexes is enthalpy controlled and entropy destabilised. As previously stated, the variations observed in the thermodynamics of complexation of these systems as a result of the medium effect are the result of the solvation changes that the reactants (anion and receptor) and the product (complex anion) undergo in moving from one medium to another. This issue will be carefully addressed in the next section. Given that selectivity is one of the main features in supramolecular chemistry, this is now discussed under three main leadings. (i) Selectivity of calix[4]pyrrole for fluoride over other anions. A quantitative assessment of selectivity requires accurate stability constant data. Whenever possible, different techniques should be used to derive stability constant data. In doing so, the scope and limitations of the methodology employed to derive the data must be carefully considered. Availability of stability constant data allows the calculation of the selectivity factor, S, which is calculated from the ratio of stability constants of two ions and a given receptor in a given solvent and temperature as shown below SX1 =X2 ¼
Ksy ðX1 Þðs1 Þ
ð3Þ
Ksy ðX2 Þðs1 Þ
In equation (3), Ksy is the thermodynamic stability constant, as defined elsewhere [31]. Inspection of Table 3 shows that all the calix[4]pyrrole receptors (except 8-aabb, 22 and 24) in non-aqueous solvents at a given temperature are more selective for the fluoride anion relative to other anions (X ¼ Cl , Br , I , HSO 4 and H2PO4 ). Et HC3
CH3
N
Et
Et +
ON
N
HO
O
O
Et CO2H SO3-
O
O
CH3
N H NH
S O NH
HN
NH
CH3
N H HN
NH
NH
S O NH
CH3
N H HN
NH
H N
H N
H N
22
23
24
S