Rhodium(I)-NHC Complexes Bearing Bidentate Bis-Heteroatomic Acidato Ligands as gem-Selective Catalysts for Alkyne Dimerization.

: A series of Rh( κ 2 -BHetA)( η 2 -coe)(IPr) complexes bearing 1,3-bis-hetereoatomic acidato ligands (BHetA) including carboxylato (O,O), thioacetato (O,S), amidato (O,N), thioamidato (N, S), and amidinato (N,N), have been prepared by reaction of the dinuclear precursor [Rh( µ -Cl)(IPr)( η 2 -coe)] 2 with the corresponding anionic BHetA species. The Rh I -NHC-BHetA compounds catalyze the dimerization of aryl alkynes showing excellent selectivity for the head-to-tail enynes. Among them, the acetanilidato-based catalyst has shown an outstanding catalytic performance reaching unprecedented TOF levels of 2500 h -1 with complete selectivity for the gem isomer. Investigation of the reaction mechanism supports a non-oxidative pathway in which the BHetA ligand behaves as proton shuttle through intermediate κ 1 -HBHetA species. However, in the presence of pyridine as additive, the identification of the common Rh III H(C≡CPh) 2 (IPr)(py) 2 intermediate gives support for an oxidative route.