Tuning the Pyridone Scaffold within a Rhodium-NHC Platform for gem-Specific Alkyne Dimerization via a Ligand-Assisted Proton Shuttle Mechanism

A series of mononuclear square-planar Rh{κ²N,O-BHetA}(η²-coe)(NHC) (BHetA = Bis-Heteroatomic Acidato) complexes have been prepared. Modifications of the pyridonato BHetA-type ligand architecture include 4-Me, 5-Me, 6-Me, 3-Br, 4-Br, 4-OMe, and 5-NO₂ substitutions as well as pyrimidonato, succinimidato, and 2-piperidonato catalysts. Two structural isomers have been observed for the complexes, depending on the stereoelectronic properties of the ligand. The structure–activity relationship has been studied for gem-specific alkyne dimerization via a cooperative ligand-assisted proton shuttle mechanism. Density functional theory calculations have revealed a mechanistic pathway involving the hemilabile coordination of the BHetA ligand, CMD deprotonation, π-alkyne protonation, and reductive elimination. The increase in oxygen basicity imparted by the substituent in the pyridonato ligand is key, the 4-methyl derivative being the most active catalyst. However, a favored iminol–amide tautomerization precludes an increase in catalytic activity for the more basic saturated piperidonato catalyst.