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

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/K_eq = 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.