Synthesis and Characterization of Copper Complexes Featuring a Redox-Active ONO Ligand in Three Molecular Oxidation States.

In this research article, we report the synthesis, characterization, and reactivity of a family of Cu complexes bearing a tridentate iminosemiquinone ligand and ancillary amine ligands ((ONO)Cu(L), : 1 or 2). The complexes were obtained following a one-pot synthetic protocol by mixing Cu, 3,5-di--butylcatechol, and aqueous ammonia in the presence of an amine base. The Cu complexes were structurally characterized by single-crystal X-ray diffraction analysis (SC-XRD). Cyclic voltammetry measurements showed that the Cu complexes reached three molecular oxidation states in a reversible fashion. The reaction between the Cu-iminosemiquinone complex (ONO)Cu(L) with cobaltocene (1e donor) and ferrocenium (1e acceptor) produced the corresponding reduced and oxidized complexes. Structural and spectroscopic characterization (SC-XRD, UV-vis, and EPR) of the Cu complexes in the three oxidation states, namely, [(ONO)Cu(L)], (ONO)Cu(L), and [(ONO)Cu(L)], suggest that the redox events are ligand-based. DFT computations also formulated the complexes as Cu species with the ONO ligand in different oxidation states. For the Cu-iminosemiquinone complexes, we calculated small energetic differences between their singlet and triplet states ( = 0 vs = 1), which explain their magnetic behavior in solution. Our results provide evidence of how Cu-radical metalloenzymes might tune their electronic structure to modulate their reactivity.