Selective Self-Assembly of Hexameric Homo- and Heteropolymetallic Lanthanide Wheels: Synthesis, Structure, and Photophysical Studies

A rational approach to the formation of pure heteropolymetallic lanthanide complexes that uses a two-step assembly
strategy and exploits the different size requirements of the two metals included in the final structure is described.
The investigation of the assembly of [LnL2](Otf) (L = 2,2‘:6‘,2‘ ‘-terpyridine-6-carboxylate) complexes into hexametallic
rings hosting an additional hexacoordinated lanthanide cation was crucial for the development of this strategy. The
formation and size of the cyclic assembly are controlled by the ionic radius and by the coordination number of the
lanthanides. The rather high luminescence quantum yield of the heptaeuropium complex (25%) indicates that the
ring structure is well adapted to include highly luminescent lanthanide complexes in nanosized architecture. The
use of a stepwise synthetic strategy leads to the selective assembly of large heteropolymetallic rings. The addition
of a smaller lanthanide ion to the [EuL2](Otf) complex in anhydrous acetonitrile leads selectively to heterometallic
species with the Eu ions located on the peripheral sites and the smaller ion occupying only the central site. The
high selectivity is the result of the different size requirements of the two metal sites present in the cyclic structure.
The heterometallic structure of the isolated [Lu⊂(EuL2)6](Otf)9 complex was confirmed by X-ray diffraction and by
high resolution solid-state photophysical studies. The described synthetic approach allowed us to obtain the first
example of selective assembly of two different lanthanide ions in a large polymetallic structure characterized in
solution and in the solid state and will make the isolation of planned dimetallic combinations presenting different
lanthanide emitters in the peripheral sites possible.