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. This 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 paraquats or viologens (G(2)+2X(-)) and crown ethers (H). The bis-(hexafluorophosphate) salts of viologens are predominantly ion paired in acetone; the value of the dissociation constant of paraquat bis(hexafluorophosphate) was determined to be 4.64 (+/- 1.86) x 10(-4) M-2. The complex based on dibenzo-24-crown-8 and paraquat bis(hexafluorophosphate) is not ion paired in solution, resulting in concentration dependence of the apparent association constant K-a,K-exp, (= [complex]/ [H][G(2+)2X(-)]) which is well fit by the treatment, according to mode (1), yielding K-ap = 106 (+/-42) M-1. However, the four complexes of two different bis(m-phenylene)-32-crown-10 derivatives and bis(p-phenylene)-34-crown-10 with paraquat derivatives are all ion paired in solution and therefore Ka,exp is not concentration dependent for these systems, mode (2). X-ray crystal structures support these solution-based assessments in that there is clearly ion pairing of the cationic guest with its PF6- counterions in the solid states of the latter four examples in which access of the counterions to the guests is granted by the relatively large cavities of the hosts and dispositions of the guest species within them.