Impact of Lewis Acids on the Reactivity of a High-Valent Cu(III) Complex

Redox-inactive metal ions functioning as Lewis acids (LA) play a significant role in modulating the redox reactivity of metal-oxygen intermediates such as metal-oxo, metal-superoxo, and metal-peroxo species. In photosystem II (PS-II), the redox-inactive metal ion Ca^II is critical for O₂ activation, although its precise function remains unclear. Inspired by nature's use of redox-inactive metal ions, this study aims to characterize complexes of high-valent Cu(III) bound Lewis acids, 2-M (where M = Zn^II, Eu^III, Yb^III, and Sc^III), through various spectroscopic techniques, including UV-vis and resonance Raman spectroscopic analyses. These experimental findings are further supported by computational studies. Furthermore, the binding of a redox-active Lewis acid like Ce^III was also investigated, where inner-sphere electron transfer between 2 and Ce^III is witnessed. Notably, the electron transfer rate between Ce^III and 2 is greatly influenced by the binding of other Lewis acids. Additionally, we demonstrated the impact of LA on electron transfer (ET) and hydrogen atom transfer (HAT) reactivity. Our findings show that while the introduction of LA decreases the HAT reaction rate, it conversely leads to enhanced electron transfer reactivity.