Poison or Promoter? Investigating the Dual-Role of Carbon Monoxide in Pincer-Iridium-Based Alkane Dehydrogenation Systems via Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Abstract

Pincer-ligated iridium complexes of the form [Ir(R4 PCP)L] (R4 PCP = κ 3-C6H3-2,6-(XPR2)2; X = CH2, O; R = tBu, iPr) have previously been shown competent for acceptorless alkane dehydrogenation when supported on silica. It was observed by post-catalysis solid-state NMR that silica-tethered [Ir(C2H4)(≡SiO-tBu4 POCOP)] (3-C2H4) was converted fully to [Ir(CO)(≡SiO-tBu4 POCOP)] (3-CO) at 300 °C. In this work, the characterization of species under dehydrogenation reaction conditions far from equilibrium between butane and butenes (approach to equilibrium Q/Keq = 0.3 at 300 °C) is performed with operando Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) to show the kinetics of species conversion from 3-C2H4 to 3-CO. It is further found that [IrClH(≡SiO-tBu4 POCOP)] (3-HCl), a species considered to be a precatalyst for alkane dehydrogenation, is also fully converted to 3-CO. A mechanism of decomposition is proposed that implicates surface silanol groups, while carbon monoxide acts as a "stabilizer" for the catalyst by promoting their reductive elimination and maintaining the complex in the I oxidation state.

Anchoring an Iridium Pincer Complex in a Hydrophobic Microporous Polymer for Application in Continuous‐Flow Alkane Dehydrogenation

“…Thereby, the advantages of homogeneous as well as heterogeneous catalysis are combined and well-defined materials with uniform active sites of equal activity towards the reactants can be made accessible for new catalytic applications. [14,15] Molecular pincer catalysts have been immobilized on different supports, mainly on silica and metal oxides [2,13,[16][17][18][19][20][21][22][23] as well as metal-organic frameworks [24][25][26][27][28] or even microporous polymer networks, [29] but the resulting materials were mostly investigated for stoichiometric transformation, [30] coupled with metathesis [31] or transfer dehydrogenations, [2,32] whereas direct dehydrogenations remain underexplored. [13,22] Besides the high porosity and chemical robustness, the advantage of using a microporous polymer network (MPN) as support material is the tunable environment of the catalyst, as the polymer backbone can be versatilely functionalized.…”

Regioselective Gas‐Phase n‐Butane Transfer Dehydrogenation via Silica‐Supported Pincer‐Iridium Complexes

“…[2i,5] No leaching occurs under reaction conditions. [5] The summary of the kinetic results is presented in Table 1.…”

“…In all cases, the addition of a hydrogen acceptor depressed the activity in comparison to acceptorless dehydrogenation (Table 1, Figure 3). Given that olefin-bound complexes are not observed during reaction conditions at 300°C, [5] this reduction in activity was likely a consequence of an increased CO concentration in the system, itself arising from the conversion of acceptor olefin used with trace water present. [2i] Although the rate of CO formation was too small to observe or measure, catalyst activity has been demonstrated [5] to be very sensitive to CO pressure and, therefore, the rate may be expected to decrease with increasing olefin content in the gas phase.…”

“…[2i] The stability of such catalysts at elevated temperatures up to 300°C arises from the concurrent generation of CO over these catalysts, which prevents thermal decomposition. [5] However, the selectivity of the system is unremarkable, resulting in an equilibrated mixture of product butenes (ca. 20 % 1-butene) even at very low residence times, possibly due to rapid isomerization over the iridium species.…”

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

“…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. ,− …”