Isostructural Ir(C2H4)2 complexes supported on Zr6 based MOFs: determining the electron-donor properties of Zr6 nodes by comparison with those of oxides and zeolites
Dong Yang,a Samuel O. Odoh,b Timothy C. Wang,c Omar K. Farha,c,d Joseph T. Hupp,c Christopher J. Cramer,b Laura Gagliardi,b* Bruce C. Gatesa*
aDepartment of Chemical Engineering & Materials Science, University of California, Davis, California 95616, United States
bDepartment of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, Minne-apolis, Minnesota 55455-0431, United States
cDepartment of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
dDepartment of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
Metal-organic frameworks (MOFs) with Zr6 nodes, exemplified by UiO-66, UiO-67 and NU-1000, are characterized by relatively good thermal stability and are potentially valuable for catalytic applications. The nodes, because of their uniformity, are ”ideal” catalyst supports, and because they are nanostructures, have properties that distinguish them from bulk metal oxides, which are the most widely used catalyst supports. We are investigating the reactivities of these nodes by using organometallic compounds as probe molecules—and catalyst precursors. We used Ir(C2H4)2(acac) (acac is acetylacetonate) as a probe, reacting it with the OH groups on the Zr6 nodes of UiO-66, UiO-67 and NU-1000; the supported species that formed were identified by infrared spectroscopy as mononuclear Ir(C2H4)2 complexes. The binding of these complexes with the nodes were elucidated by EXAFS spectroscopy and Density Function Theory (DFT) calculations. Information about the nodes as electron-donor ligands has been determined in experiments in which the iridium diethylene complexes were converted to iridium carbonyls by reaction with gas-phase CO. The νCO values characterizing the supported species identify them as iridium gem-dicarbonyls, and the data provide the measure of the electron-donor properties of the support relative to those of conventional metal oxide supports. The electron donor strength decreases in the order MgO > ZrO2 (bulk phase) > UiO-67 (=NU-1000) > γ-Al2O3 > UiO-66 >> zeolite HY. The MOF-supported iridium complexes (initially in the form of the diethylene complexes) were tested as catalysts with the simple probe reaction ethylene hydrogenation as well as dimerization. The results show that the electron-donor properties of the supports strongly influence the catalytic activity of each reaction; the activities increase with decreasing electron-donor strength of the support.
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