External Photosensitizer Free Function-Integrated Cu(II)-Complex Catalyzed Photo Driven CO2 Reduction

Developing a function-integrated catalyst from earth-abundant elements, capable of efficient light harvesting and electron transfer, is crucial for enhancing the efficacy of CO2 transformation, a critical step in environmental cleanup and advancing clean energy prospects. Traditional approaches relying on external photosensitizers, comprising of 4d/5d metal complexes, often face challenges in intermolecular electron transfer, and attachment of photosensitizing arms to the catalyst increases dependency on intramolecular electron transfer kinetics, underscoring the need for a more integrated solution. We report a new Cu(II) complex, K[CuNDPA] (1[K(18-crown-6)]), bearing a dipyrrin amide-based trianionic tetradentate ligand, NDPA (H3L), which is capable of harnessing light energy, despite having a paramagnetic Cu(II) center, without any external photosensitizer and photocatalytically reducing CO2 to CO in acetonitrile: water (19:1 v/v) with a TON as high as 1132, a TOF of 566 h-1 and a selectivity of 99%. This complex also shows hemilability in the presence of water, which not only plays a role in a proton relay mechanism but also helps stabilize a crucial Cu(I)-NDPA intermediate. The hemilability was justified by the formation of N3O (2) and N2O2 (3) coordinated congeners of the N4 bound complex 1. The overall mechanism was further investigated via spectroscopic techniques like EPR, UV-vis, and Spectroelectrochemistry, culminating in the justification of a single electron-reduced Cu(I)NDPA species as a proposed intermediate. In the next step, the binding of CO2 to Cu(I) complex and subsequent electron transfer to form Cu(II)-CO2•‾ was indirectly probed by a radical trapping experiment via the addition of p-methoxy-2,6-ditertbutylphenol that led to the formation of a phenoxyl radical. This work opens new strategies for designing earth-abundant robust molecular catalysts that can function as photocatalysts without the aid of any external photosensitizers.