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-(IDippR)}(cod) and Rh{κ2C,X-(IDippR)}(CO)2 {IDippR = 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-(IDippCarbx)}(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.