Illuminating the Role of Alcohol Additives in Nickel Photoredox Catalysis via Ground State and Transient Absorption Spectroscopy

Direct excitation of nickel photoredox catalysts has gained popularity over the last several years following the successful activation of nickel by Ir and Ru photosensitizers. The ability to pair these photosensitizers with Ni catalysts facilitated the formation of crucial chemical bonds under relatively mild conditions. However, there are still gaps in our understanding of what influences the performance of these Ni photoredox catalysts. Previous work from our group identified a nickel photocatalyst supported by a tridentate pyridinophane ligand that undergoes wavelength-dependent cross-coupling reactivity. Herein, we report detailed studies of the wavelength dependence of the C-O cross-coupling reactivity, illustrating that the catalytic activity has an optimal wavelength, and excitation on both the blue and red sides of this optimal wavelength lead to a reduction in product yield. We also reveal a crucial catalyst-alcohol pre-irradiation interaction that drastically alters the catalyst’s optical properties. We investigate both wavelength dependency and alcohol dependency via optical transient absorbance spectroscopy and reveal a new substrate binding process responsible for the observed wavelength-dependent cross-coupling catalysis.