Tunability in Heterobimetallic Complexes Featuring an Acyclic "Tiara" Polyether Motif

Both cyclic "crown" and acyclic "tiara" polyethers have been recognized as useful for the binding of metal cations and enabling the assembly of multimetallic complexes. However, the properties of heterobimetallic complexes built upon acyclic polyethers have received less attention than they deserve. Here, the synthesis and characterization of a family of eight redox-active heterobimetallic complexes that pair a nickel center with secondary redox-inactive cations (K⁺, Na⁺, Li⁺, Sr²⁺, Ca²⁺, Zn²⁺, La³⁺, and Lu³⁺) bound in acyclic polyether "tiara" moieties are reported. Structural studies with X-ray diffraction analysis were carried out on the monometallic nickel precursor complex to the heterobimetallics and the adducts with K⁺, Li⁺, Sr²⁺, Zn²⁺, and Lu³⁺; the results confirm the binding of secondary cations in the tiara site and demonstrate that the tiara moiety is more conformationally flexible than the analogous 18-crown-6-like moiety of a closely related macrocyclic "crown" ligand. Spectroscopic and electrochemical studies show, however, that the stability and cation-driven tunability of the tiara-based heterobimetallic species are quite similar to those previously measured for crown-based species. Consequently, the tiara motif appears to be at least as equally useful for constructing tunable multimetallic species as the more commonly encountered crown motif; a comprehensive set of titration data collected in an acetonitrile solution support this conclusion as well. Because the use of acyclic tiaras avoids the need for tedious and/or time-intensive syntheses of macrocyclic structures, these findings suggest that tiara motifs could be broadly advantageous in the design of ligands to support multimetallic chemistry.