Continuous-Flow Alkane Dehydrogenation by Supported Pincer-Ligated Iridium Catalysts at Elevated Temperatures

Pincer-ligated iridium complexes of the form [Ir(^R4PCP)L] (^R4PCP = κ³-C₆H₃-2,6-(XPR₂)₂; X = CH₂, O; R = tBu, iPr) are efficient homogeneous alkane dehydrogenation catalysts that have been reported to be highly active at temperatures of 240 °C or below. In this work, silica-supported [Ir(C₂H₄)(p-^tBu₂PO-^tBu4POCOP)] (1/SiO₂) was used to study a model continuous-flow gas-phase acceptorless alkane dehydrogenation system. This particular supported framework is thermally stable at temperatures up to 340 °C, 100 °C above the highest temperature at which analogous homogeneous complexes have been reported to show stable activity, with observed butane dehydrogenation rates of ca. 80 mol_butenes mol_cat. ^-1 h^-1. Solid-state ³¹P MAS NMR and ATR IR are used to demonstrate that the backbone pincer ligand remains intact and coordinated at 340 °C. The complex is fully converted to [Ir(CO)(p-^tBu₂PO-^tBu4POCOP)] (3/SiO₂) above 300 °C. 3/SiO₂ is observed to be catalytically active at the higher temperatures tested, and reaction rates are comparable to those of 1/SiO₂. 3/SiO₂ and 1/SiO₂ act as resting states for the active 14-electron fragment, through dissociation of the CO or olefin ligand, respectively. Given that 3/SiO₂ is air resistant at ambient temperature and is structurally stable and catalytically active at elevated temperatures, it is a suitable candidate as a catalyst for the highly endothermic acceptorless dehydrogenation of alkanes.