Chapter IV Ion Pairing in Fast-Exchange Host-Guest Systems : Concentration Dependence of Apparent Association Constants for Complexes of Neutral Hosts and Divalent Guest Salts with Monovalent Counterions

An equilibrium treatment of complexation of neutral hosts with dicationic guests having univalent counterions includes two possible modes: (1) dissociation of the ion pair prior to interaction of the free dication with the host to produce a complex that is not ion paired and (2) direct complexation of the ion pair to produce an ion paired complex. The treatment is easily modified for complexation of neutral guests by dianionic hosts, or divalent hosts by neutral guests. The treatment was tested by a study of fast exchange host-guest systems based on viologens (G2X) and crown ethers (H). The bis(hexafluorophosphate) salts of viologens are predominantly ion paired in acetone; the value of the dissociating constant of paraquat bis(hexafluorophosphate) was determined to be 7.72 (± 0.77) × 10 M. The complex based on dibenzo-24-crown-8 and paraquat bis(hexafluorophosphate) is