Zeolite- and MgO-Supported Molecular Iridium Complexes: Support and Ligand Effects in Catalysis of Ethene Hydrogenation and H–D Exchange in the Conversion of H2 + D2

Abstract

Zeolite- and MgO-supported mononuclear iridium diethene complexes were formed by the reaction of Ir(C2H4)2(acac) (acac = acetylacetonate, C5H7O2–) with each support. Changes in the ligand environment of the supported iridium complexes were characterized by infrared, X-ray absorption near edge structure, and extended X-ray absorption fine structure spectroscopies as various mixtures of H2, C2H4, and CO flowed over the samples. In contrast to the nonuniform metal complexes anchored to metal oxides, our zeolite-supported metal complexes were highly uniform, allowing precise determinations of the chemistry, including the role of the support as a macroligand. Zeolite- and MgO-supported Ir(C2H4)2 complexes are each rapidly converted to Ir(CO)2 upon contact with a pulse of CO, and the νCO frequencies indicate that the iridium is more electron-deficient when the support is the zeolite. The Ir(CO)2 complex supported on MgO was highly stable in the presence of various combinations of CO, C2H4, and helium. In contrast, the zeolite-supported Ir(CO)2 complex was found to be highly reactive, forming Ir(CO)3, Ir(CO)(C2H4), Ir(CO)2(C2H4), and Ir(CO)(C2H4)2. The π-bonded ethene ligands of the zeolite-supported Ir(C2H4)2 in H2 were facilely converted to σ-bonded ethyl when treated. However, the stability of the ethene ligands was markedly increased when the support was changed to MgO or when a CO ligand was simultaneously bonded to the iridium. The rates of catalytic ethene hydrogenation and H2/D2 exchange in the presence of a catalyst initially consisting of Ir(C2H4)2 on the zeolite were found to be more than an order of magnitude higher than when MgO was the support. The iridium complexes containing one or more CO ligands were found to be inactive for H2/D2 exchange reactions when the support was MgO, but they were moderately active when it was the zeolite. The effects of the MgO and zeolite supports on reactivity and catalytic activity are attributed to their differences as ligands donating or withdrawing electrons, respectively.