Tunability in Heterobimetallic Complexes Featuring an Acyclic “Tiara” Polyether Motif

Both cyclic “crown” and acyclic “tiara” polyethers have been recognized as useful for binding of metal cations and enabling 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 (XRD) 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 binding of the 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 homologous macrocyclic 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. As use of acyclic tiaras avoids the need for tedious and/or time-intensive syntheses of macrocyclic structures, these findings suggest tiara motifs could be broadly advantageous in design of ligands to support multimetallic chemistry.