Dynamic lanthanide exchange between quadruple-stranded cages: the effect of ionic radius differences on kinetics and thermodynamics

Seven different [Ln2L4]2− (Ln = La, Nd, Eu, Tb, Er, Tm and Lu) lanthanide-based quadruple-stranded helicates are here reported and transmetalation among the pre-assembled cages is studied. Combining two homonuclear helicates, [LnA2L4]2− and [LnB2L4]2−, leads to the formation of a mixture of homo- and heteronuclear systems due to ion exchange. This dynamic behaviour is studied by electrospray ionization mass spectrometry (ESI-MS) both qualitatively and quantitatively, allowing one to gain information on the thermodynamics and on the kinetics of the process. The rate of the Ln ion exchange is strongly correlated with the difference in the lanthanide effective ionic radius (ΔEIR). Upon increasing the ΔEIR, the kinetic constants grow exponentially: from the minimum to the maximum value of the ΔEIR, the kinetic constants of the forward and backward reactions increase by three orders of magnitude. In contrast, the equilibrium constant is the same for all the [LnA2L4]2−/[LnB2L4]2− couples, showing that the transmetalation is mainly entropy driven towards a statistical mixture and not affected by the ΔEIR.