Design and synthesis of polymetallic complexes based on meso-calix[4]pyrrole: Platforms for multielectron chemistry

A first series of highly metalated calix[4]pyrrole derivatives has been obtained by treating calix[4]pyrrole with [RuCp*(CH₃CN)₃]PF₆ in THF under reflux for 4 h to give the corresponding tetrakisruthenocene-like product and then with Cu(OTf)₂ in the presence of triethylamine to give the corresponding copper(II) complex. This latter product contains five metal centers supported by a core ligand consisting of 32 non-hydrogen atoms. Electrochemical and ESR spectroscopic analyses provide support for the notion that both complexes are capable of transferring four electrons reversibly at a potential of 0.95 V vs SCE. Related studies involving the copper complex reveal the presence of an additional reversible reduction at -0.34 V vs SCE that is ascribed to a Cu(II)/Cu(I) couple. These findings make these systems of potential interest as simple-to-prepare electron reservoirs and as possible models for enzyme active sites.