Tuning Zr6 MOF Nodes as Catalyst Supports: Site Densities and Electron-Donor Properties Influence Molecular Iridium Complexes as Ethylene Conversion Catalysts
Yang, D.; Odoh, S. O.; Borycz, J.; Wang, T. C.; Farha, O. K.; Hupp, J. T.;
Cramer, C. J.; Gagliardi, L.; Gates, B. C.
ACS Catal. 2016, 6, 235 (doi:10.1021/acscatal.5b02243).
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–Onode bonds. The supported iridium complexes on these sites have been characterized by infrared (IR) and extended X-ray absorption fine structure spectroscopies and density functional theory calculations. The ethylene ligands on iridium are readily replaced by CO, and the νCO frequencies of the resultant complexes and those of comparable complexes reported elsewhere show that the support electron-donor tendencies increase in the order zeolite HY << UiO-66 < UiO-67 (= NU-1000) < ZrO2 < MgO. The sharpness of the IR νCO bands shows that the degree of uniformity of the support bonding sites decreases in the order ZrO2 ≈ UiO-67 ≈ NU-1000 < MgO < UiO- 66 << 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 << 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 νCO 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.