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 one
Zn2+ and two trivalent lanthanide cations Ln3+ to give quantitatively directional [ZnLn2(L4)3]8+ triplestranded
helicates in the solid state and in solution. NMR speciations in CD3CN show negligible decomplexation
at a millimolar concentration and the latter helicate can be thus safely considered as a preorganized
C3-symmetrical HHH-[(L43Zn)(LnA)(2−n)(LnB)n]8+ platform in which the thermodynamic properties
of (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 and
intermetallic pair interactions. Considering the challenging La3+: Eu3+ ionic pair, for which the sizes of the
two cations differ by only 8%, a remarkable excess (70%) of the heterolanthanide is produced, together
with a preference for the formation of the isomer where the largest lanthanum cation lies in the central
N9 site ([(La)(Eu)] : [(Eu)(La)] = 9 : 1). This rare design and its rational programming pave the way for the
preparation of directional light-converters and/or molecular Q-bits at the (supra)molecular level.