Guest exchange dynamics in an M4L6 tetrahedral host

Guest exchange in an M4L6 supramolecular assembly was previously demonstrated to proceed through a nonrupture mechanism in which guests squeeze through apertures in the host structure and not through larger portals created by partial assembly dissociation. Focusing on the [Ga4L6](12-) assembly {L = 1,5-bis(2',3'-dihydroxybenzamido)naphthalene}, the host-guest kinetic behavior of this supramolecular capsule is defined. Guest self-exchange rates at varied temperatures and pressures were measured to determine activation parameters, revealing negative Delta S-double dagger and positive Delta V-double dagger values {PEt4+: Delta H-double dagger = 74(3) kJ mol(-1), Delta S = -46(6) J mol(-1) K-1, k(298) 0.003 s(-1); NEt4+: Delta H-double dagger = 69(2) kJ mol(-1), Delta S-double dagger = -52(5) J mol(-1) K-1, k(298) = 0.009 s(-1); NMe2Pr2+: Delta H-double dagger 52(2) kJ mol(-1), Delta S-double dagger = -56(7) J mol(-1) K-1, Delta V-double dagger = +13(l) cm(3) mol(-1), k(298) = 4.4 s(-1); NPr4+: Delta H-double dagger = 42(l) kJ mol(-1), Delta S-double dagger = -102(4) J mol(-1) K-1, Delta V-double dagger = +31(2) cm(3) mol(-1), k(298) = 1.4 s(-1)}. In PEt4+ for NEW exchange reactions, egress of the initial guest (G1) is found to be rate determining, with increasing G1 and G2 (the displacing guest) concentrations inhibiting guest exchange. This inhibition is explained by the decreased flexibility of the host imparted by exterior, or exohedral, guest interactions by both the G1 and G2 guests. Blocking the exohedral host sites with high concentrations of the smaller NMe4+ cation (a weak endohedral guest) enhances PEt4+ for NEW guest exchange rates. Finally, guest displacement reactions also demonstrate the sensitivity of guest exchange to thermodynamic endohedral guest binding affinities. When the initial guest (G1) has a weaker affinity for the host, G2 concentration dependence is observed in addition to dependence on the G2 binding strength.