High Activity and Selectivity for Catalytic Alkane–Alkene Transfer (De)hydrogenation by (^tBuPPP)Ir and the Importance of Choice of a Sacrificial Hydrogen Acceptor

Abstract

The triphosphorus-coordinating pincer iridium fragment (tBuPPP)Ir was recently reported to be highly active for the catalytic dehydrogenation of n-alkanes. Dehydrogenation is calculated to be highly regioselective for the terminal position of n-alkanes. The extremely high intermolecular selectivity observed in n-alkane/cycloalkane competition experiments supports the prediction of extremely high regioselectivity for dehydrogenation of n-alkanes. The use of sterically unhindered hydrogen acceptors is key to observing the high activity of the (tBuPPP)Ir fragment. 4,4-Dimethylpent-1-ene (TBP) is found to be particularly convenient for this purpose. With the commonly used hydrogen acceptor 3,3-dimethylbut-1-ene (TBE), (tBuPPP)Ir affords n-alkane dehydrogenation at a rate no different than that obtained with the well-known fragment (iPrPCP)­Ir. However, with the use of TBP as acceptor, (tBuPPP)Ir shows much greater activity for n-alkane transfer dehydrogenation than previously reported catalysts, affording appreciable rates even at 50 °C, an unprecedentedly low temperature for catalytic alkane transfer dehydrogenation. Also critical to the identification of (tBuPPP)Ir as a highly effective catalyst is the use of n-alkane substrate rather than the commonly used “model” dehydrogenation substrate, cyclooctane, with which dehydrogenation rates are much lower than those with n-alkanes.

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru–P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies

“…Most importantly, the site trans to the central phosphorus is also very crowded; this raises the energy of the square-planar resting state ( tBu PPP)­Ir­(olefin). However, the site cis to the central phosphorus and exo to the bowl-shaped pincer–metal unit is very sterically unhindered , which allows for low-energy transition states (TSs) and intermediates in which the steric demand is shifted to that site. , …”

“…However, the site cis to the central phosphorus and exo to the bowl-shaped pincer−metal unit is very sterically unhindered, which allows for low-energy transition states (TSs) and intermediates in which the steric demand is shifted to that site. 1,2 Other than iridium, 3,4 the metal that has probably featured most prominently in alkane dehydrogenation is ruthenium. 5−7 Roddick 8,9 and Huang 10,11 have reported examples of high activity with ruthenium complexes of anionic PCP ligands, and we have reported high activity with a ruthenium complex of a neutral PSP ligand.…”

“…1 The ( tBu PPP)Ir fragment was found to be extremely reactive for alkane-transfer dehydrogenation. 1,2 This was attributable in large part to an unusual architecture. Two sites in an approximately octahedral coordination geometry based on the ( tBu PPP)Ir fragment are very crowded, including the extremely crowded site cis to the central phosphorus and endo to the bowl-shaped pincer−metal unit (Figure 1).…”

Dimeric Rh Complexes Supported by a Bridging Phosphido/Bis(Phosphine) PPP Ligand

“… 9 , 10 Most recently, the Goldman group showed that (PPP tBu )Ir complexes are remarkably active catalysts for transfer alkane dehydrogenation. 11 , 12 The structurally closest PPP relative comes from the Thomas group ( Figure 1 ), where the central phosphide site is also connected to two nitrogens. 13 , 14 The PPP ligands in the complexes in Figure 1 have been known to form both monomeric complexes with a meridionally binding PPP pincer and dimeric complexes with bridging phosphides, presaging the observations in this work.…”

Low-Temperature Dehydrogenation of Cyclooctane by Using Pincer Iridium Complexes Bearing an N,N′-Diarylated PNCNP Ligand

“… − Since then, various iridium complexes with other combinations of ligating motifs ,− and non-iridium transition-metal complexes (Rh, ,− Ru, − Os, and Ti) have been examined, while most of them are less active than the original (PCP)Ir complexes. Recent noteworthy exceptions are the (κ 3 - P , P , P ) pincer iridium complex , and (κ 2 - P , P )­(chloro)­iridium complex, which are effective catalysts for the transfer dehydrogenation of terminal alkanes and 1,1-disubstituted ethanes, respectively.…”