Rhodium(I) Complexes with Proton-Responsive Anionic Tethered N-Heterocyclic Carbene Ligands: Synthesis and Application in Alkyne Dimerization

A series of rhodium(I) complexes of formula Rh{κ²C,X-(I^DippR)}(cod) and Rh{κ²C,X-(I^DippR)}(CO)₂ {I^DippR = 1-(R)-3-(2,6-diisopropylphenyl)-imidazolin-2-carbene; X = N, O; R = (6-yl-2-pyridone), (CH₂CH₂NC(=O)Ph), (CH₂CH₂COO); (cod = 1,5-cyclooctadiene)} containing anionic-tethered NHC-functionalized ligands, including pyridonato, amidato or carboxylato groups at the N-wingtip has been prepared and their application to alkyne homocoupling has been investigated. The chelate coordination of the NHC-functionalized ligand generates 5, 6 and 7-membered metallacycles for pyridonato, amidato and carboxylato substituents, respectively. Preparation of the mixed bis-NHC derivative Rh{κ²C,O-(I^DippCarbx)}(CO)(IPr) {IPr = 1,3-bis-(2,6-diisopropylphenyl)-imidazolin-2-carbene} is possible due to lability of one of the CO ligands in the bis-CO precursor. Reversible protonation is observed for pyridonato and carboxylato moieties but not for amidato counterpart. Alkyne dimerization catalytic activity is reported for N-based anionic groups whereas carboxylato counterparts are inefficient. A proton transfer mechanism is invoked for the gem-selectivity observed for cod-based catalysts whereas alkyne insertion is the regioselectivity-determining step for CO-containing analogues.