Iridium complexes bearing PCP-type pincer ligands have been extensively studied as alkane dehydrogenation catalysts and the catalytic mechanism has been well established. (PCP)Ru complexes have also been reported to be very effective catalysts for alkane dehydrogenation but the mechanism of their operation is not known. Herein we report a combined mechanistic experimental and DFT study of alkane dehydrogenation by (iPrPCP)Ru complexes. DFT calculations indicate that the most favorable cycle proceeds exclusively through paramagnetic (odd-electron) species, although a diamagnetic pathway is not prohibitively high-energy. Diamagnetic species (iPrPCP)Ru(H)(NBD) and (iPrPCP)Ru(H)(COD) are catalyst precursors but their activity requires loss of the corresponding diene. The diamagnetic complex (iPrPCP)Ru(H)(H2)2 has been synthesized and used as a pre-catalyst but all diamagnetic signals in the NMR spectra are quickly lost within the course of the catalytic runs. The resting state under the reaction conditions is calculated to be the paramagnetic species (iPrPCP)Ru(alkene). If only diamagnetic species were considered, (iPrPCP)Ru(H)(alkene) would be calculated to be the resting state. Trapping experiments and spectroscopic studies have been used to determine the actual speciation.

Mechanistic investigations of pincer-ruthenium-catalyzed alkane dehydrogenation

ACS Spring 2022

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