(175c) Dealuminated Y Zeolite-Supported Mononuclear Iridium Complexes with Reactive Ethylene Ligands: Synthesis, Characterization, and Catalysis of Ethylene Hydrogenation

Highly dealuminated Y zeolite-supported mononuclear iridium complexes with reactive ethylene ligands were synthesized by chemisorption of Ir(C₂H₄)₂(C₅H₇O₂). The supported species were characterized by infrared (IR), ¹³C magic angle spinning nuclear magnetic resonance (MAS NMR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. IR and NMR spectra showed that iridium was bonded to the zeolite after dissociation of the acetylacetonate ligands, while the ethylene ligands remained bonded to the iridium. This result was confirmed by the EXAFS data, which showed the formation of site-isolated mononuclear iridium species with no evidence of Ir-Ir contributions. The EXAFS data show that each Ir atom was bonded on average to four carbon atoms at an Ir-C distance of 2.10 Å (consistent with the presence of two ethylene ligands per Ir atom) and to two oxygen atoms of the zeolite, at an Ir-O distance of 2.18 Å. Treatment of the sample with CO led to removal of the ethylene ligands and the formation of site-isolated mononuclear iridium gem dicarbonyls. This process was followed by transient quick-EXAFS and IR characterization of the surface species and by mass spectrometric analysis of the evolved gas-phase products (ethylene and ethane). The sharpness of the u_CO bands in the IR spectrum characterizing the supported gem dicarbonyl species suggests a high degree of uniformity of the metal complexes in the crystalline support. When the supported iridium ethylene complex in a flow reactor at room temperature was exposed to a stream of H₂, ethane formation was observed by mass spectrometry, and the removal of ethylene ligands was followed by EXAFS and IR measurements. Correspondingly, the supported mononuclear iridium complex was found to be catalytically active for ethylene hydrogenation at room temperature.