N-heterocyclic carbenes as activating ligands in hydrogenation catalysts: Synthetic and catalytic studies

Abstract

Novel hydridochlorocarbonyl ruthenium complexes containing N-heterocyclic carbene (NHC) ligands were prepared in order to explore the effect of the NHC ligand class in hydrogenation chemistry. Routes were developed to RuHCl(CO)(NHC)(PPh 3) (NHC = IMes, H2IMes) from RuHCl(CO)(PPh3) 3, and the hydrogenation activities of these complexes were screened against that of the known complexes RuHCl(CO)(PCy3)2 and RuHCl(CO)(IMes)(PCy3). The combination of the donor ability of the NHC ligand, in conjunction with the lability of the PPh3 group, increased activity for the challenging hydrogenation of sterically hindered trans internal olefins, but also led to competing isomerization of terminal olefins and polymerization of strained cycloolefins. In the course of this work, high yield, efficient new routes were also developed to the known complexes RuHCl(CO)(PCy3)2 and RuHCl(CO)(NHC)(PCy3) from RuHCl(CO)(PPh3)3. The lability of the phosphine groups in the latter precursor complex enabled milder reaction conditions than had been possible in the literature methods. In an effort to gain access to carbonyl-free analogues, synthesis of a ruthenium hydride complex containing a cyclometalated IMes ligand was undertaken. However, the product obtained using the reported method was a novel complex, formulated as RuHCl(IMes)2 on the basis of extensive NMR and MALDI-MS analysis.