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 ((^sqONO)Cu^II(L)<i>_n</i>, <i>n</i>: 1 or 2). The complexes were obtained following a one-pot synthetic protocol by mixing Cu^II, 3,5-di-<i>tert</i>-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 (^sqONO)Cu^II(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, [(^catONO)Cu^II(L)]^-, (^sqONO)Cu^II(L), and [(^bqONO)Cu^II(L)]⁺, suggest that the redox events are ligand-based. DFT computations also formulated the complexes as Cu^II species with the ONO ligand in different oxidation states. For the Cu^II-iminosemiquinone complexes, we calculated small energetic differences between their singlet and triplet states (<i>S</i> = 0 vs <i>S</i> = 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.