Catalytic Alkane Transfer Dehydrogenation by PSP-Pincer-Ligated Ruthenium. Deactivation of an Extremely Reactive Fragment by Formation of Allyl Hydride Complexes

Abstract

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 (iPrxanPSP) pincer ligand and several Ru complexes thereof. The (iPrxanPSP)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 (iPrxanPSP)Ru is more effective than (iPrPCP)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 (iPrxanPSP)Ru fragment is calculated to be much more active than (iPrPCP)Ir for dehydrogenation of both COA and n-alkanes. However, the resting state in the (iPrxanPSP)­Ru-catalyzed transfer dehydrogenation of n-alkane is a very stable linear-allyl hydride complex, whereas the corresponding cyclooctenyl hydride is much less stable.

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity

“…Importantly, the ( tBu PPP)Ir unit investigated is not approximately planar like ( R PCP)Ir, nor even approximately symmetrical as typically found with complexes of the isostructural diarylaminebased PNP ligands developed by Ozerov. [53][54] Instead, the pyramidal geometry of the central phosphorus atom results in a bowl-like ligand structure, analogous to the geometry we have recently reported 55 for a (PSP)Ru complex. This in turn results in a highly unsymmetrical positioning of the phosphino-t-butyl groups in which one face of the complex is fairly open while the coordination site trans to the central coordinating P atom is very crowded.…”

Inverse Isotope Effects in Single-Crystal to Single-Crystal Reactivity and the Isolation of a Rhodium Cyclooctane σ-Alkane Complex

“… 32 σ-Alkane complexes of cyclooctane have also been identified as intermediates in alkane dehydrogenation reactions using computational methods. 33 The isolation of [1-COA][BAr F 4 ] thus represents a structurally authenticated example that has a significant lifetime at 293 K. Attempts to characterize [1-COA][BAr F 4 ] or [1-COE][BAr F 4 ] by low temperature solution NMR spectroscopy 8 , 34 , 35 (CD 2 Cl 2 , 183 K) led to the formation of intractable solids.…”

The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand

“…The first observed cyclometallation was reported by Chatt from a Ru(0) intermediate in 1965 [ 29 , 33 ]. In the past few years, the dehydrogenation of alkanes by a Ru(0) catalyst was reported by Goldman [ 34 ] as well as its acceptorless variant with Ru(II) pincer catalysts by Roddick and Huang [ 35 , 36 ].…”