Ruthenium(II) complexes bearing chelating Carboxylate-anchored normal and abnormal Carbenes: Synthesis, characterizations and catalytic applications

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Synopsis : A series of half-sandwich Ru Ⅱ complexes bearing carboxylato-anchored normal and abnormal NHCs (NHCs = N -heterocyclic carbenes) have been synthesized. The abnormal carbene species have exhibited superior catalytic performance compared to their normal carbene counterparts. A protonation-induced substrate binding/activation mechanism has been proposed. Functionalized abnormal/mesoionic NHC (NHC = N -heterocyclic carbene) complexes have been much less investigated in contrast to their normal NHC counterparts. In this work, a series of [C carbene ^O carboxylate ]- and [C carbene ^C’ phenyl ]-type Ru(II) complexes bearing normal and abnormal NHCs have been synthesized. Their spectroscopic features, solid-state structure and aquation reactivity have also been reported. All the as-synthesized complexes have served as precatalysts to catalyze intramolecular or intermolecular carboxylic acid-to-alkyne addition reactions. It was found that the catalytic behavior of abnormal NHC complexes is superior to that of the normal NHC analogs. An anti -Markovnikov addition reactivity, leading to the formation of an E-type enol ester as the major product, was observed. A few stoichiometric experiments have also been carried out to provide preliminary insight into the catalytic mechanism.