Self-sensitized Cu(ii)-complex catalyzed solar driven CO₂ reduction

Developing a self-sensitized 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 4d/5d metal complexes, involve intermolecular electron transfer, and attachment of photosensitizing arms to the catalyst necessitates intramolecular electron transfer, 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) centre, 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 the 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 such as 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 the Cu(i) complex and subsequent electron transfer to form Cu(ii)-COO^·- was indirectly probed by a radical trapping experiment via the addition of p-methoxy-2,6-di-tert-butylphenol that led to the formation of a phenoxyl radical. This work provides new strategies for designing earth-abundant robust molecular catalysts that can function as photocatalysts without the aid of any external photosensitizers.