Dynamic lanthanides exchange between quadruple-stranded cages: 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 pre-assembled cages was 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 was studied by electrospray ionization mass spectrometry (ESI-MS) both qualitatively and quantitatively allowing to gain information on the thermodynamics as well as on the kinetics of the process. The rate of the Ln ion exchange is strongly dependent on the difference in lanthanide effective ionic radius (ΔEIR). Upon increasing ΔEIR, the kinetic constants grow exponentially: from the minimum to the maximum value of ΔEIR, the kinetic constants of the forward and backward reactions increase by three orders of magnitude. On the contrary, 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 ΔEIR.