Ligand-Support Interactions During Immobilization of Iridium-Pincer Alkane Dehydrogenation Catalysts

Pincer-ligated iridium complexes have proven to be highly effective catalysts for the dehydrogenation and transfer-dehydrogenation of alkanes. Immobilization onto a solid support offers significant potential advantages in the application of such catalysts, particularly with respect to catalyst separation and recycling. The most effective procedure for immobilization onto a γ-alumina support has been shown to be adsorption of the catalyst through basic functional groups on the para-position of the pincer ligand. Here we describe the nature of the interaction between the functional groups and the γ-alumina surface. Pincer ligands with phosphinite functional groups are of particular interest due to their relatively easy synthesis and high catalytic activity. We show that the phosphinite reacts with surface hydroxyls on γ-alumina to cleave off a phosphine oxide and anchor the pincer ligand to the γ-alumina support. An analogous immobilization is possible on silica, which eliminates olefin isomerization side reactions catalyzed by the γ-alumina support.