Photocatalytic CO2 to CO Conversion by an Elusive Mn(II)-Based Molecular Catalyst

Renewable-driven, sustainable, and facile conversion of CO2 to high-value chemicals is regarded as a crucial step in CO2 management in our ongoing fight against energy issues and mitigating the climate change crisis. In this pursuit, first-row transition metal-based photocatalytic systems emerge as an attractive option for efficient and selective CO2 reduction. However, designing such catalysts is an uphill task, considering their lack of activity in higher oxidation states and their inherent vulnerability in low oxidation states. This study presents a Mn­(II)-based molecular catalyst [Mn­(apap)2Br2, apap = bis-6-amino-2­(phenylazo)­pyridine] for visible light-driven CO2 reduction. This catalyst is rationally designed with the inclusion of a multifunctional apap ligand. The inherent bulkiness of this ligand leads to a distorted octahedral geometry around the Mn­(II) site, while the presence of the peripheral amine functionality in the ligand motif crafts intra- and intermolecular hydrogen bonding networks. The single crystal X-ray diffraction elucidates such minute details of the Mn­(apap)2Br2 complex, allowing us to have a clear molecular visualization. This molecule is tested for photocatalytic CO2 conversion in the presence of a variable photosensitizer (PS) and sacrificial electron donors in organic media. The combination of this catalyst along with an Ir-based PS and sacrificial electron donor 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo­[d]­imidazole (BIH) displayed the optimized photocatalytic CO2 reduction to CO by Mn­(apap)2Br2 (TON ∼ 636) in water-blended dimethylacetamide (DMA) media, where it outperforms competitive H2 production by ∼20:1. The detailed spectroscopic studies indicated a reductive quenching electron transfer as the major course of this photocatalysis. The Mn­(apap)2Br2 complex remains stable during long hours of photocatalysis in the midst of the photodegradation of the PS molecules. The perfect assembly of the redox-active ligand motif, peripheral proton relay, and the subtle distortion around the Mn-center translates into one of the rare examples of an efficient, product-selective, and robust photocatalyst, which primarily alters between Mn­(II) and Mn­(I) states during the catalysis. This catalyst design strategy can be expanded to produce a new genre of durable and effective CO2-activating photocatalysts in our pursuit of reducing the carbon footprint of anthropogenic activities.