Mechanistic Divergence in Dirhodium Carbene Catalysis: A DFT Prediction of Bis-Carbene Intermediates.

Dirhodium paddlewheel complexes are cornerstone catalysts in organic synthesis, yet the existence and catalytic role of bis-carbene intermediates remain contentious. Through systematic density functional theory (DFT) studies at the M06L/dgdzvp(SMD) level, we demonstrate that axial and bridging ligands critically govern the relative stability of mono- versus bis-carbene species. Central to this process is π-backdonation from the Rh-Rh π* orbital into the vacant p orbital of the carbene moiety, which facilitates charge transfer and separation. This interaction stabilizes triplet intermediates and promotes the formation of bis-carbene adducts. A fundamental electronic structure difference is identified: in mono-carbene species, two radicals are delocalized across the Rh-Rh-C framework, whereas in bis-carbene species, they localize on individual Rh─C bonds. Our findings offer a refined mechanistic understanding of dirhodium catalysis, highlighting ligand-controlled spin-state modulation and intermediate speciation as crucial determinants of selectivity in C-X functionalization and cyclization reactions.