Supramolecular Assembly with Calix[6]arene and Copper Ions − Formation of a Novel Tetranuclear Core Exhibiting Unusual Redox Properties and Catecholase Activity

The supramolecular biomimetic chemistry based on calix[6]arene N‐ligands has been further explored. A tris(imidazole)CuI complex was treated with 1 mol‐equiv. of cuprous ion under dioxygen to produce a tetranuclear cupric species. X‐ray structural determination of this novel Cu4 complex revealed that the self‐inclusion of an imidazolyl coordinating arm into the hydrophobic calixarene cavity provides the base of coordination for a {ClImCu(OH)2CuIm2}2 assembly. The Cu4 core is maintained in solution and is stable even in a coordinating solvent such as acetonitrile. Magnetic susceptibility measurements evidenced a strong antiferromagnetic coupling in each Cu(OH)2Cu subunit with J = −408 cm−1. The complex displayed catecholase activity in the presence of 3,5‐di‐tert‐butylcatechol behaving as a four‐electron hole with, however, a sluggish CuI4 ⇄ CuII4 regeneration through O2 autooxidation. Finally, electrochemical studies revealed two oxidative reversible processes that successively gave rise to a {CuIICuIII}{CuII2} and a {CuIICuIII}2 mixed‐valence species that could be characterized by UV/Vis and EPR spectroscopy. The overall structure and behavior of this tetranuclear complex is reminiscent of multicopper enzymes. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)