"Two-Story" Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O₂ Binding

Area of Science:

• Supramolecular Chemistry
• Coordination Chemistry
• Organometallic Chemistry

Background:

• Calixarenes are versatile macrocyclic hosts with tunable properties.
• Tren-capped calixarenes offer unique coordination environments for metal ions.
• Understanding metal complex reactivity is crucial for catalysis and materials science.

Purpose of the Study:

• Synthesize and characterize a novel "two-story" calix[6]arene-based ligand.
• Explore the coordination chemistry of this ligand with metal ions like Zn(II) and Cu(II).
• Investigate the electrochemical properties and reactivity of the resulting metal complexes, particularly their interaction with oxygen.

Main Methods:

• Synthesis of a new "two-story" calix[6]arene ligand with a tren cap and amido spacers.
• X-ray diffraction studies to determine the solid-state structures of metal complexes.
• Cyclic voltammetry to probe the electrochemical behavior of copper complexes.
• UV-Vis spectroscopy to characterize reaction intermediates, including superoxide species.

Main Results:

• A six-coordinate Zn(II) complex and a five-coordinate Cu(II) complex were successfully synthesized.
• Dicationic complexes showed limited response to neutral donors but were reactive towards anions and bases.
• Reversible Cu(II)/Cu(I) redox couple observed at negative potentials, attributed to amido arm coordination.
• Formation of an end-on superoxide Cu(II) species upon reaction of the Cu(I) complex with O2.
• Superoxide intermediate proposed to mediate unusual four-electron oxidative pathways.

Conclusions:

• The synthesized "two-story" calixarene ligand provides a unique platform for metal coordination.
• The amido arms play a critical role in stabilizing different metal redox states and influencing reactivity.
• The formation of a superoxide intermediate highlights the potential of these complexes in oxidation catalysis.
• This work expands the scope of calixarene-based ligands for exploring complex coordination chemistry and reactivity.