Room-Temperature Gas-Phase CO₂-to-C₃ Coupling by a 4f-Aromatic Cluster

Room-temperature conversion of CO₂ into value-added multi-carbon products (C₃ or C₃₊ species) remains a major challenge due to poor selectivity and limited C-C coupling pathways. Using mass spectrometry, photoelectron imaging spectroscopy, and density functional calculations, we identify the 4f-metalla-aromatic anion PrB₂C₂ ^-, which-despite lacking a C─C bond-exhibits σ and π double aromaticity involving a 4f atom. We show that PrB₂C₂ ^- reacts with CO₂ at room temperature to generate C₃B₂O₂ ^- with a C─C─C backbone. The C₃-chain formation occurs in three stages involving two distinct C-C coupling steps enabled by flexible Pr-X bonding (X = B, C, O) and Pr-centered electron shuttling. The unique structure of PrB₂C₂ ^- directs CO₂ activation toward C─C─C coupling rather than CO release. These findings deepen the understanding of f-block-mediated small-molecule activation and provide a new and tailored route for producing products with C─C bonds via CO₂ conversion.