Room‐Temperature Gas‐Phase CO ₂ ‐to‐C ₃ Coupling by a 4 <i>f</i> ‐Aromatic Cluster

ABSTRACT Room‐temperature conversion of CO 2 into value‐added multi‐carbon products (C 3 or C 3+ 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 4 f ‐metalla‐aromatic anion PrB 2 C 2 − , which—despite lacking a C─C bond—exhibits σ and π double aromaticity involving a 4 f atom. We show that PrB 2 C 2 − reacts with CO 2 at room temperature to generate C 3 B 2 O 2 − with a C─C─C backbone. The C 3 ‐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 PrB 2 C 2 − directs CO 2 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 CO 2 conversion.