Multicopper Models for the Laccase Active Site: Effect of Nuclearity on Electrocatalytic Oxygen Reduction

Cu complexes of 2,2′-dipicolylamine
(DPA) were prepared and tested as electrocatalysts for the oxygen
reduction reaction (ORR). To study the effect of multinuclearity on
the ORR, two Cu–DPA units were connected with a flexible linker,
and a third metal-binding pocket was installed in the ligand framework.
ORR onset potentials and the diffusion-limited current densities of
di- and tricopper complexes of DPA derivatives were found to be comparable
to those of the simpler Cu–DPA system. Electrochemical analyses,
crystallographic data, and metal-substitution studies suggested that
Cu complexes of DPA derivatives reacted with O2 via a binuclear
intermolecular pathway but that the Cu center in the third binding
site did not participate in the ORR process. This study highlights
the viability of Cu–DPA complexes to mimic the T3-site of laccase,
and serves as a guide for designing future laccase models.