Cooperative small-molecule activation toward sustainable catalysis

Abstract

In this research, a series of copper complexes containing polypyridyl ligands were designed to study the influence from ligands, nuclearity, solvents and secondary coordination on reactivity toward small-molecule activation. This structure-reactivity relationship was used in investigation for oxygen reduction reactions (ORR), and applied in detection of ascorbic acid (AsH₂). Firstly, copper (II) complexes containing polypyridyl derivatives ligands (i.e., Cu(dpa), Cu(adpa) and Cu₂ (addpa)) were synthesized and fully characterized by elemental analysis, mass spectrometry and X-ray crystallography. Their electrochemical behaviors were studied by cyclic voltammetry. The Cu(I) complexes which are active species for ORR were generated using AsH₂ as a reducing agent. Redox states of the metal were examined by UV-Vis, NMR and EPR. It was found that copper complexes with various ligand topologies exhibited different reactivity toward O₂. Anthracence moiety in Cu(adpa) and Cu₂ (addpa) played a vital role in facilitating reduction of Cu(II) as well as stability enhancement of Cu(I). The dinuclear complex, Cu₂ (addpa) showed significantly higher ORR activity than that of the mononuclear analogue. The product of ORR was found to be H₂O₂, indicating 2e, 2H⁺ reduction process. Being stable and inactive towards ORR, Cu(adpa) was further investigated as a fluorescence sensor for AsH₂. Reaction of [Cu(adpa)]²⁺ with AsH₂ in CH3CN resulted in turn-on fluorescence due to [Cu(adpa)]⁺ formation.However, when the same reaction was carried out in aqueous solution, the Cu(I) species was gradually oxidized to Cu(II) which hampered the accurate measurement. Notably, addition of Zn(II) in combination with acetate anions helped to stabilize the Cu(I) complex and allowed an accurate detection of ascorbic acids in vitamin C tablets. Secondary coordination sphere modulation of the copper center was proposed to account for this stability enhancement. Overall, the findings obtained from this research have been shown to be applicable in various fields. Also, this can be further used in design for efficient catalysts as well as molecular sensors in the future.