Tuning Zr₆ Metal-Organic Framework (MOF) Nodes as Catalyst Supports: Site Densities and Electron-Donor Properties Influence Molecular Iridium Complexes as Ethylene Conversion Catalysts

The Zr₆ nodes of the metal-organic frameworks (MOFs) UiO-66 and UiO-67 are metal oxide clusters of atomic precision and can be used as catalyst supports. The bonding sites on these nodes - that is, hydrogen-bonded H₂O/OH groups on UiO-67 and non-hydrogen-bonded terminal OH groups on UiO-66 - were regulated by modulation of the MOF syntheses. Ir(C₂H₄)₂(C₅H₇O₂) complexes reacted with these sites to give site-isolated Ir(C₂H₄)₂ complexes, each anchored to the node by two Ir-O_node bonds. The supported iridium complexes on these sites have been characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and density functional theory calculations. The ethylene ligands on iridium are readily replaced by CO, and the ν_CO frequencies of the resultant complexes and those of comparable complexes reported elsewhere show that the support electron-donor tendencies increase in the order HY zeolite 蠐 UiO-66 < UiO-67 (= NU-1000) < ZrO₂ < MgO. The sharpness of the IR ν_CO bands shows that the degree of uniformity of the support bonding sites decreases in the order ZrO₂ ≈ UiO-67 ≈ NU-1000 < MgO < UiO-66 蠐 HY zeolite. The reactivity of supported Ir(CO)₂ complexes with C₂H₄ to form Ir(C₂H₄)(CO) and Ir(C₂H₄)₂(CO) is influenced by the support electron-donor properties, with the reactivity increasing in the order MgO = ZrO₂ = NU-1000 (not reactive) < UiO-66 < UiO-67 蠐 HY zeolite. Density functional theory calculations characterizing the complexes supported on NU-1000, UiO-66/67, and HY zeolite concur with the use of the calculated ν_CO bands as indicators of electron-donor properties of the supported metal catalysts. Our calculations also show that the reactivity of the supported Ir(CO)₂ complexes with C₂H₄ is correlated with the electron-donor properties of the iridium center. The supported Ir(C₂H₄)₂ samples are precatalysts for ethylene hydrogenation and ethylene dimerization, with the activity for each reaction increasing with increasing electron-withdrawing strength of the support.