Oxide- and Zeolite-Supported Isostructural Ir(C2H4)2 Complexes: Molecular-Level Observations of Electronic Effects of Supports as Ligands

Abstract

Zeolite Hβ- and γ-Al(2)O(3)-supported mononuclear iridium complexes were synthesized by the reaction of Ir(C(2)H(4))(2)(acac) (acac is acetylacetonate) with each of the supports. The characterization of the surface species by extended X-ray absorption fine structure (EXAFS) and infrared (IR) spectroscopies demonstrated the removal of acac ligands during chemisorption, leading to the formation of essentially isostructural Ir(C(2)H(4))(2) complexes anchored to each support by two Ir-O(support) bonds. Atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM) images confirm the spectra, showing only isolated Ir atoms on the supports with no evidence of iridium clusters. These samples, together with previously reported Ir(C(2)H(4))(2) complexes on zeolite HY, zeolite HSSZ-53, and MgO supports, constitute a family of isostructural supported iridium complexes. Treatment with CO led to the replacement of the ethylene ligands on iridium with CO ligands, and the ν(CO) frequencies of these complexes and white line intensities in the X-ray absorption spectra at the Ir L(III) edge show that the electron density on iridium increases in the following order on these supports: zeolite HY < zeolite Hβ < zeolite HSSZ-53 ≪ γ-Al(2)O(3) < MgO. The IR spectra of the iridium carbonyl complexes treated in flowing C(2)H(4) show that the CO ligands were replaced by C(2)H(4), with the average number of C(2)H(4) groups per Ir atom increasing as the amount of iridium was increasingly electron-deficient. In contrast to the typical supported catalysts incorporating metal clusters or particles that are highly nonuniform, the samples reported here, incorporating uniform isostructural iridium complexes, provide unprecedented opportunities for a molecular-level understanding of how supports affect the electronic properties, reactivities, and catalytic properties of supported metal species.