Supramolecular Control of a Mononuclear Biomimetic Copper(II) Center: Bowl Complexes vs Funnel Complexes

Modeling the mononuclear site of
copper enzymes is important for
a better understanding of the factors controlling the reactivity of
the metal center. A major difficulty stems from the difficult control
of the nuclearity while maintaining free sites open to coordination
of exogenous ligands. A supramolecular approach consists in associating
a hydrophobic cavity to a tripodal ligand that will define the coordination
spheres as well as access to the metal ion. Here, we describe the
synthesis of a bowl CuII complex based on the resorcinarene
scaffold. This study supplements a previous work on CuI coordination. It provides a complete picture of the cavity–copper
system in its two oxidation states. The first XRD structure of such
a bowl complex was obtained, evidencing a 5-coordinate CuII ion with the three imidazole donors bound to the metal (two in the
base of the pyramid, one in the apical position) and with an acetate
anion, completing the base of the pyramid, and deeply included in
the bowl. Solution studies conducted by EPR and UV–vis absorption
spectroscopies as well as cyclic voltammetry highlighted interaction
with coordinating solvents, various carboxylates that can sit either
in the endo or in the exo position depending on their size as well
as possible stabilization of hydroxo species in a mononuclear state.
A comparison of the binding and redox properties of the bowl complex
with funnel complexes based on the calix[6]­arene core further highlights
the importance of supramolecular features defining the first, second,
and third coordination sphere for control of the metal ion.