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

Pincer-ligated iridium complexes of the form [Ir-((PCP)-P-R4)L ]((PCP)-P-R4 = kappa(3)-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 degrees C or below. In this work, silica-supported [Ir(C2H4)-(p-(Bu2PO)-Bu-t-(POCOP)-P-tBu4)] (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 degrees C, 100 degrees 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 P-31 MAS NMR and ATR IR are used to demonstrate that the backbone pincer ligand remains intact and coordinated at 340 degrees C. The complex is fully converted to [Ir(CO)(p-(Bu2PO)-Bu-t-(POCOP)-P-tBu4)] (3/SiO2) above 300 degrees C. 3/SiO2 is observed to be catalytically active at the higher temperatures tested, and reaction rates are comparable to those of 1/SiO2 center dot 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.