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

<p>Pincer-ligated iridium complexes of the <a>form [Ir(<sup>R4</sup>PCP)L] (<sup>R4</sup>PCP = κ<sup>3</sup>-C<sub>6</sub>H<sub>3</sub>-2,6-(XPR<sub>2</sub>)<sub>2</sub>; X = CH<sub>2</sub>, O; R = <i>t</i>Bu, <i>i</i>Pr) </a>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 <a>[Ir(C<sub>2</sub>H<sub>4</sub>)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] </a>(<b>3-C<sub>2</sub>H<sub>4</sub></b>) was converted fully to [Ir(CO)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] (<b>3-CO</b>) at 300 °C. In this work, the characterization of species under dehydrogenation reaction conditions far from equilibrium between butane and butenes (approach to equilibrium <i>Q</i>/<i>K</i><sub>eq</sub> = 0.3 at 300 °C) is performed with <i>operando </i>Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) to show the kinetics of species conversion from <b>3-C<sub>2</sub>H<sub>4</sub></b> to <b>3-CO</b>. It is further found that [IrClH(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] (<b>3-HCl</b>), a species considered to be a precatalyst for alkane dehydrogenation, is also fully converted to <b>3-CO</b>. 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. </p>