Cooperativity of Redox-active Ligand and Cu: Synthesis, Characterization and Reactivity of Mononuclear Cu Complexes

Cu plays a key role in many natural processes. The ability of Cu to switch between two oxidation states (Cu^I and Cu^II) is utilized in nature to perform organic transformation. Some Cudependent metalloenzymes use redox-active (non-innocent) ligands as cofactors in the active sites for multi-electron reactions. For instance, galactose oxidase carries out aerobic alcohol oxidation to aldehyde using tyrosine radical cofactor. The utilization of redox-active ligands provides the additional electrons that exceed the inherent one-electron redox process of bioavailable Cu^I /Cu^II states. Bound by redox-active ligands, Cu complexes can undergo multiply oxidation states and show richer reactivity. 

On one hand, inspired by the active site of lytic polysaccharide monooxygenases (LPMOs), we have prepared a family of Cu complexes bearing tridentate redox-active ligands, including Cu-OH complexes with 3 oxidation states. The effect of anion and ligand arms on the structure of these mononuclear complexes has been analyzed by X-ray crystallography. The different oxidations states of these complexes have been characterized by CV, UV-vis, EPR and XAS. The hydrogen atom transfer reactivity and thermochemistry of the ‘high-oxidation-state’ species was also studied. 

On the other hand, Cu complexes bearing bidentate redox-active ligands have been prepared and characterized by XRD, NMR, CV UV-vis and EPR. The ‘high-valent’ complex can abstract 4 H atoms and be regenerated using oxygen, acting as electron-coupled-proton buffer (ECPB) to perform coupled/decoupled oxidation of organic substrate. Thermochemistry and reactivity of the Cu-based ECPB have been studied in detail. Meanwhile, ligand variation of these Cu-based ECPB were applied to tuning their thermochemistry.