Programming heterometallic 4f–4f' helicates under thermodynamic control: the circle is complete

Three non-symmetrical segmental ligand strands L4 can be wrapped around a linear sequence of oneZn2+ and two trivalent lanthanide cations Ln3+ to give quantitatively directional [ZnLn2(L4)3]8+ triplestrandedhelicates in the solid state and in solution. NMR speciations in CD3CN show negligible decomplexationat a millimolar concentration and the latter helicate can be thus safely considered as a preorganizedC3-symmetrical HHH-[(L43Zn)(LnA)(2−n)(LnB)n]8+ platform in which the thermodynamic propertiesof (i) lanthanide permutation between the central N9 and the terminal N6O3 binding sites and (ii)exchange processes between homo- and heterolanthanide helicates are easy to access (Ln = La, Eu, Lu).Deviations from statistical distributions could be programmed by exploiting specific site recognition andintermetallic pair interactions. Considering the challenging La3+: Eu3+ ionic pair, for which the sizes of thetwo cations differ by only 8%, a remarkable excess (70%) of the heterolanthanide is produced, togetherwith a preference for the formation of the isomer where the largest lanthanum cation lies in the centralN9 site ([(La)(Eu)] : [(Eu)(La)] = 9 : 1). This rare design and its rational programming pave the way for thepreparation of directional light-converters and/or molecular Q-bits at the (supra)molecular level.