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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

preprint
revised on 02.06.2020, 09:43 and posted on 03.06.2020, 11:09 by Boris Sheludko, Cristina Castro, Alan Goldman, Fuat E. Celik

Pincer-ligated iridium complexes of the form [Ir(R4PCP)L] (R4PCP = κ3-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-tBu4POCOP)] (3-C2H4) was converted fully to [Ir(CO)(≡SiO-tBu4POCOP)] (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-tBu4POCOP)] (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.

Funding

NSF CHE-1205189

History

Email Address of Submitting Author

fuat.celik@rutgers.edu

Institution

Rutgers, The State University of New Jersey

Country

USA

ORCID For Submitting Author

0000-0002-5891-6375

Declaration of Conflict of Interest

no conflict of interest

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