Tunability\nin Heterobimetallic Complexes Featuring\nan Acyclic “Tiara” Polyether Motif

Both cyclic “crown” and acyclic “tiara”\npolyethers have been recognized as useful for the binding of metal\ncations and enabling the assembly of multimetallic complexes. However,\nthe properties of heterobimetallic complexes built upon acyclic polyethers\nhave received less attention than they deserve. Here, the synthesis\nand characterization of a family of eight redox-active heterobimetallic\ncomplexes that pair a nickel center with secondary redox-inactive\ncations (K<sup>+</sup>, Na<sup>+</sup>, Li<sup>+</sup>, Sr<sup>2+</sup>, Ca<sup>2+</sup>, Zn<sup>2+</sup>, La<sup>3+</sup>, and Lu<sup>3+</sup>) bound in acyclic polyether “tiara” moieties are reported.\nStructural studies with X-ray diffraction analysis were carried out\non the monometallic nickel precursor complex to the heterobimetallics\nand the adducts with K<sup>+</sup>, Li<sup>+</sup>, Sr<sup>2+</sup>, Zn<sup>2+</sup>, and Lu<sup>3+</sup>; the results confirm the binding\nof secondary cations in the tiara site and demonstrate that the tiara\nmoiety is more conformationally flexible than the analogous 18-crown-6-like\nmoiety of a closely related macrocyclic “crown” ligand.\nSpectroscopic and electrochemical studies show, however, that the\nstability and cation-driven tunability of the tiara-based heterobimetallic\nspecies are quite similar to those previously measured for crown-based\nspecies. Consequently, the tiara motif appears to be at least as equally\nuseful for constructing tunable multimetallic species as the more\ncommonly encountered crown motif; a comprehensive set of titration\ndata collected in an acetonitrile solution support this conclusion\nas well. Because the use of acyclic tiaras avoids the need for tedious\nand/or time-intensive syntheses of macrocyclic structures, these findings\nsuggest that tiara motifs could be broadly advantageous in the design\nof ligands to support multimetallic chemistry.