Oxide- and Zeolite-Supported
Isostructural Ir(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> Complexes:
Molecular-Level Observations
of Electronic Effects of Supports as Ligands
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Abstract
Zeolite Hβ- and γ-Al<sub>2</sub>O<sub>3</sub>-supported
mononuclear iridium complexes were synthesized by the reaction of
Ir(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>(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<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes anchored to each support by
two Ir–O<sub>support</sub> 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<sub>2</sub>H<sub>4</sub>)<sub>2</sub> 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<sub>CO</sub> frequencies of these complexes and white line intensities
in the X-ray absorption spectra at the Ir L<sub>III</sub> 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<sub>2</sub>O<sub>3</sub> < MgO. The IR spectra
of the iridium carbonyl complexes treated in flowing C<sub>2</sub>H<sub>4</sub> show that the CO ligands were replaced by C<sub>2</sub>H<sub>4</sub>, with the average number of C<sub>2</sub>H<sub>4</sub> 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