Iridium complexes bearing PCP-type pincer ligands are the most effective catalysts reported to date for the low-temperature (≤ca. 200 °C) dehydrogenation of alkanes. To investigate the activity of formally isoelectronic ruthenium complexes, we have synthesized the neutral 2,7-di-tert-butyl-4,5-bis(diisopropylphosphino)-9,9-dimethylthioxanthene ( iPr xanPSP) pincer ligand and several Ru complexes thereof. The ( iPr xanPSP)Ru complexes catalyze alkane transfer dehydrogenation of the benchmark cyclooctane/t-butylethylene (COA/TBE) couple with turnover frequencies up to ca. 1 s -1 at 150 °C and 0.2 s -1 at 120 °C, the highest rates for alkane dehydrogenation ever reported at such temperatures. Dehydrogenation of n-octane, however, is much less effective. A combination of experiment and DFT calculations allow us to explain why ( iPr xanPSP)Ru is more effective than ( iPr PCP)Ir for dehydrogenation of COA, while the reverse is true for dehydrogenation of n-alkanes. Considering only in-cycle species and simple olefin complexes, the ( iPr xanPSP)Ru fragment is calculated to be much more active than ( iPr PCP)Ir for dehydrogenation of both COA and n-alkanes. However, the resting state in the ( iPr xanPSP)Ru-catalyzed transfer dehydrogenation of n-alkane is a very stable linear-allyl hydride complex, whereas the corresponding cyclooctenyl hydride is much less stable.
All Science Journal Classification (ASJC) codes
- alkane dehydrogenation
- allyl complexes
- catalyst deactivation
- pincer ligand