Dehydrogenation of Alkanes Using Molecular Catalysts

Fang, Huaquan; Liu, Guixia; Huang, Zheng

doi:10.1002/9781119379256.ch21

Cited by 6 publications

(10 citation statements)

References 91 publications

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“…An example of this of particular interest in our laboratory has been the chemistry of (PCP)Ir complexes as catalysts for alkane dehydrogenation. [28][29][30][31][32][33][34][35][36][37][38][39][40] In the context of this relationship, and in combination with the potential for an anionic phosphido group to engage in MLC behavior, we have begun an investigation of (PPP)Ir complexes in which the central group is formally anionic phosphido. [41][42][43][44][45][46][47][48][49][50][51] Scheme 1.…”

Section: ■ Introductionmentioning

confidence: 99%

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

Gordon

Lease

Emge

et al. 2021

Preprint

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The selective functionalization of alkanes and alkyl groups is a major goal of chemical catalysis. Toward this end, a bulky triphosphine with a central secondary phosphino group, bis(2 di-t-butyl-phosphinophenyl)phosphine (tBuPHPP), has been synthesized. When complexed to iridium it adopts a meridional (“pincer”) configuration. The secondary phosphino H atom can undergo migration to iridium to give an anionic phosphido-based-pincer (tBuPPP) complex. We describe novel metal-ligand cooperativity of the iridium-phosphido unit. Stoichiometric reactions of the (tBuPPP)Ir complexes reflect a distribution of steric bulk around the iridium center in which the coordination site trans to the phosphido group is quite crowded, one coordination site cis to the phosphido is even more crowded, while the remaining site is particularly open. The (tBuPPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of n-alkanes, by about two orders of magnitude. The electronic properties of the iridium center are very similar to that of well-known analogous (RPCP)Ir catalysts. Accordingly, DFT calculations predict that (tBuPPP)Ir and (tBuPCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by (RPCP)Ir proceeds through an intermediate trans-(PCP)IrH2(alkene), (tBuPPP)Ir follows a pathway proceeding via cis-(PPP)IrH2(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. (tBuPPP)Ir and (tBuPCP)Ir, however, have analogous resting states: square planar (pincer)Ir(alkene). Alkene coordination at the crowded trans site is therefore unavoidable in the resting states. Thus the resting state of the (tBuPPP)Ir catalyst is destabilized by the unusual architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.

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Section: ■ Introductionmentioning

confidence: 99%

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

Gordon

Lease

Emge

et al. 2021

Preprint

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“…Additionally, the resulting olefin, cyclooctene (COE), tends to bind to metal centers less strongly than linear olefins and has less tendency to form allyl complexes; for both reasons, it is therefore less likely to inhibit catalysis. COA has therefore long been the standard substrate of choice for studies of alkane dehydrogenation catalysis, at least at the early stages of catalyst development. − , …”

Section: Results and Discussionmentioning

confidence: 99%

“…COA has therefore long been the standard substrate of choice for studies of alkane dehydrogenation catalysis, at least at the early stages of catalyst development. [3][4][5][6][7]55 The free energy of dehydrogenation of COA, to give COE and H 2 , is ΔG°= 15.5 kcal/mol, while ΔG°= 97.2 kcal/mol for the formation of 2H • from H 2 . 33 Therefore, for the formation of COE and two hydrogen atoms from COA, ΔG°= 112.7 kcal/ mol (eqs 1−3).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…The dehydrogenation of alkanes to give olefins offers almost unlimited potential as a route to their catalytic conversion to commodity chemicals or to alkanes of higher fuel value, e.g., through olefin coupling after dehydrogenation. , Over the past decades, great progress has been made toward the use of molecular transition-metal catalysts for alkane dehydrogenation. − They offer highly attractive regioselectivity for the terminal position of n -alkanes in some cases ,− and operating temperatures much lower than required with conventional heterogeneous catalysts. , The most widely studied class of such catalysts has been pincer-ligated iridium complexes. ,− …”

Section: Introductionmentioning

confidence: 99%

“…Such acceptorless dehydrogenation requires significant energy input and the rate of H 2 expulsion is proportional to the low H 2 concentrations. The long reaction times allow for undesirable secondary reactions such as isomerization, resulting in the loss of valuable regioselectivity. − Moreover, the partial pressure of the hydrogen produced is necessarily extremely low, rendering it very difficult to realize its significant potential value as a byproduct.…”

Section: Introductionmentioning

confidence: 99%

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Catalytic Dehydrogenation of Alkanes by PCP–Pincer Iridium Complexes Using Proton and Electron Acceptors

et al. 2021

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Dehydrogenation to give olefins offers the most broadly applicable route to the chemical transformation of alkanes. Transition-metal-based catalysts can selectively dehydrogenate alkanes using either olefinic sacrificial acceptors or a purge mechanism to remove H 2 ; both of these approaches have significant practical limitations. Here, we report the use of pincer-ligated iridium complexes to achieve alkane dehydrogenation by proton-coupled electron transfer, using pairs of oxidants and bases as proton and electron acceptors. Up to 97% yield was achieved with respect to oxidant and base, and up to 15 catalytic turnovers with respect to iridium, using t-butoxide as base coupled with various oxidants, including oxidants with very low reduction potentials. Mechanistic studies indicate that (pincer)IrH 2 complexes react with oxidants and base to give the corresponding cationic (pincer)IrH + complex, which is subsequently deprotonated by a second equivalent of base; this affords (pincer)Ir which is known to dehydrogenate alkanes and thereby regenerates (pincer)IrH 2 .

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Oxidative and Nonoxidative Routes to Alkane Dehydrogenation

Labinger

2018

Alkane Functionalization

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Dehydrogenation of Alkanes Using Molecular Catalysts

Cited by 6 publications

References 91 publications

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

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

Catalytic Dehydrogenation of Alkanes by PCP–Pincer Iridium Complexes Using Proton and Electron Acceptors

Oxidative and Nonoxidative Routes to Alkane Dehydrogenation

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