Bulky Calixarene Ligands Stabilize Supported Iridium Pair-Site Catalysts.

Although essentially molecular, noble metal species provide active sites and highly tunable platforms for design of supported catalysts, the susceptibility of the metals to reduction and aggregation and the consequent loss of catalytic activity and selectivity limit opportunities for their application. Here, we demonstrate a new construct to stabilize supported molecular noble metal catalysts, taking advantage of sterically bulky ligands on the metal that serve as surrogate supports and isolate the active sites under conditions involving steady-state catalytic turnover in a reducing environment. The result is demonstrated with an iridium pair-site catalyst incorporating P-bridging calix[4]arene ligands dispersed on siliceous supports, chosen as prototypes because they offer weakly interacting surfaces on which metal aggregation is prone to occur. This catalyst was used for hydrogenation of ethylene in a flow reactor. Atomic-resolution imaging of the Ir centers and spectra of the catalyst before and after use show that the metals resisted aggregation and deactivation, remaining atomically dispersed and accessible for catalysis. This strategy thus allows stabilization of the catalysts even when they are weakly anchored to supports.