Extraction as a Tool in Host-Guest Chemistry

It is noteworthy that extraction may be both a research tool and an ultimate goal in host-guest chemistry, the latter case being still rather rare. As a tool, extraction is common since the works of Cram and co-workers [28-33], who introduced and widely used it for the determination of association constants for amine-macrocycle complexation (see also the review by Cram and Trueblood in Ref. 8). This technique may be summarized as follows. 

For the complexation of guest G and host L, the total extraction equilibrium 

Obviously, K s, = K. a/ICi.str and measuring IQ.x allows determination of the association constant. 

Primarily, Cram and co-workers studied the extraction of cationic guests (i.e., protonated amines) G with neutral (macrocyclic) hosts L in the presence of a suitable hydrophobic anion X  

This case has additional convenience in the possibility of using colored anion (commonly picrate), which is useful for monitoring distribution but is inert with respect to complexation. This technique allowed the determination of a large body of association constants, thus providing a basis for further structure-stability correlations, and Cram s picrate method became a de facto standard. (One experienced in metal ion extraction may note that this approach evidently parallels the well-known distribution method for determining chelate stability constants [34. ) 

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