Electronic Ligand Modifications on Cobalt Complexes and Their Application toward the Semi-Hydrogenation of Alkynes and Para-Hydrogenation of Alkenes

The effect of the electronic modification of a bis­(carbene) pincer ligand, (^MesCCC^R), on cobalt catalysis has been investigated. The pincer ligand was modified in the para position of the aryl backbone with a tert-butyl and trifluoromethyl moiety to yield the electronic variants that were applied toward the synthesis and characterization of several cobalt complexes, (^MesCCC^R)­Co. The application of the (^MesCCC^R)­Co^I(N₂)­PPh₃ complexes toward the semihydrogenation of alkynes revealed that while the tert-butyl group does not impact reactivity, the loss of electron density at the metal center, by the installation of the CF₃ group, does affect product ratios. Further inspection of the proposed mechanism suggested that the installation of the trifluoromethyl group slows down olefin hydrogenation. This finding was further supported in the application of the (^MesCCC^R)­Co^I-py (py = pyridine) complexes toward the parahydrogenation of ethyl acrylate, which demonstrated that the electron-withdrawing ligand variant produced less polarization.