Ruthenium molecular complexes immobilized on graphene as active catalysts for the synthesis of carboxylic acids from alcohol dehydrogenation

摘要

Ruthenium complexes containing N-heterocyclic carbene ligands functionalizedwith different polyaromatic groups (pentafluorophenyl, anthracene, and pyrene) areimmobilized onto the surface of reduced graphene oxide. The hybrid materials composed oforganometallic complexes and graphene are obtained in a single-step process. The hybridmaterials are efficient catalysts for the synthesis of carboxylic acids from the dehydrogenationof alcohols in aqueous media. The catalytic materials can be recycled up to ten times withoutsignificant loss of activity. The catalytic activity of the pyrene derivative, Pyr-Ru (3) is enhancedwhen the ruthenium complex is anchored onto the surface of graphene. The carbonaceousmaterial limits the degradation of the ruthenium complex resulting in increased activity andrequiring lower catalyst loadings. The catalytic process of the pyrene hybrid material isheterogeneous in nature due to the strong interaction between the pyrene and graphene. Thecatalytic process of the anthracene and pentafluorophenyl hybrid materials is governed by theso-called ‘boomerang effect’. The ruthenium molecular complex is released from and returnedto the graphene surface during the catalytic reaction. Mechanistic insight has been obtainedexperimentally and theoretically. The energy profile suggests that the rate-determining step is the water nucleophilic attack to a coordinated aldehyde complex to form a gem-diolatecomplex.