Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product

Abstract

We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as (iPr4)PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer-Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C-H addition to (pincer)Ir to give an allyl intermediate as was previously reported for ((tBu4)PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.

Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates

“…The Ir-pincer catalysts, such as Ir(PCP iPr )(C 2 H 4 ) [PCP iPr = κ 3 -C 6 H 3 -2,6-(CH 2 P i Pr 2 ) 2 ], recently reported by Goldman and co-workers, promote the transfer dehydrogenation, and subsequent double-bond isomerization, of butane, pentane and octane using acceptors such as tert -butylethene, ethene or propene. 45 , 46 These operate at temperatures of 200 °C or above in sealed-tube conditions in which all the alkane is expected to be in the gas phase, and can actually outperform homogeneous systems in terms of activity. Experimental evidence points towards a presumed molecular species as the active catalyst, although the precise details have not been disclosed.…”

A Rhodium–Pentane Sigma‐Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques

“…[13,14] Thef unctionalization by C À Ha ctivation (via intermediate s-complexes) of simple,light, but low-value hydrocarbons, such as butane and pentane,isimportant for their conversion into valuable products that can then enter the chemical feedstock chain. [15] Although s-complexes of light alkanes have been characterized in situ by low temperature NMR and TRIR techniques, [16] published examples that have been structurally characterized are highly disordered (heptane-Fe-porphyrin), [9] or characterized as being mainly electrostatic in nature (K + /alkane). [11] Powder neutron diffraction studies at 4-8 Ks how the binding of ethane or propane with an Fe center in am etal-organic framework.…”

“…[15] Although s-complexes of light alkanes have been characterized in situ by low temperature NMR and TRIR techniques, [16] published examples that have been structurally characterized are highly disordered (heptaneFe-porphyrin), [9] or characterized as being mainly electrostatic in nature (K + /alkane). [11] Powder neutron diffraction studies at 4-8 Ks how the binding of ethane or propane with an Fe center in am etal-organic framework.…”

Batch and Continuous Flow Hydrogenation of Liquid and Gaseous Alkenes Catalyzed by a Silica‐grafted Iridium(III) Hydride

“…Beside the grafting approach, alternative heterogenization methods of Ir pincer complexes have been reported that exploit covalent attachment to a resin, tethering to silica, adsorption onto γ ‐alumina, or even the use of pure solid‐phase catalyst. [10]…”