Site-Isolated Molecular Iridium Complex Catalyst Supported in the 1-Dimensional Channels of Zeolite HSSZ-53: Characterization by Spectroscopy and Aberration-Corrected Scanning Transmission Electron Microscopy

Abstract

Zeolite HSSZ-53, which has 1-dimensional channels with 14-ring extra-large pores, was used as a support for a molecular iridium complex synthesized from Ir(C 2 H 4 ) 2 (C 5 H 7 O 2 ) and characterized with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and atomic-resolution aberration-corrected scanning transmission electron microscopy (STEM). The spectra show that Ir(C 2 H 4 ) 2 (C 5 H 7 O 2 ) reacted readily with the bridging OH groups of the zeolite, leading to the removal of C 5 H 7 O 2 ligands and the formation of mononuclear Ir(C 2 H 4 ) 2 complexes bonded to the zeolite by Ir−O bonds at the framework aluminum sites. STEM images confirm the spectra, showing site-isolated iridium centers within the zeolite channels, with no evidence of iridium clusters. The samples constitute a highly uniform, well-defined array of essentially molecular catalytic species in a highly uniform, confined environment, allowing precise investigations of the chemistry of the iridium complex in the absence of solvents. IR spectra show that the supported Ir(C 2 H 4 ) 2 complexes were converted to Ir(C 2 H 5 ) 2 , Ir(CO) 2 , Ir(CO)(C 2 H 4 ), and Ir(CO)(C 2 H 4 ) 2 as various mixtures of H 2 , CO, and C 2 H 4 reacted with the sample. The sample was tested as a catalyst for ethylene hydrogenation and for H−D exchange in the reaction of H 2 + D 2 . The data, combined with results reported for isostructural iridium complexes bonded to zeolite HY and to MgO, demonstrate how the catalytic activity can be tuned by choice of the support, with the support being characterized as a ligand with electron-donating or electronwithdrawing properties. The results demonstrate that the rate of ethylene hydrogenation catalyzed by the supported iridium complexes is limited by H 2 activation when the iridium is electron rich (on the MgO support), whereas the rate-limiting step is C 2 H 4 adsorption when the iridium is electron deficient (on either zeolite support).

Molecular models of site-isolated cobalt, rhodium, and iridium catalysts supported on zeolites: Ligand bond dissociation energies

“…25,27 Most of the optimized dual-ligand Zeo(48-T)Ir(L 1 )(L 2 ) complexes and the optimized mono η 2 ligand Ir(L) complexes were found to form bonds with two O atoms on Al with the distance r(Ir-O) equal to ~ 2.1 Å. This result essentially matches EXAFS and computational results for the zeolite-supported metal complexes, 5,7,8,9,37,38 which indicate that the metal center bonds to 2 terminal O atoms of the acidic Al site with M + replacing a proton. DFT calculations without using ONIOM show that even for the mono ligand complex, Zeo(48-T)Ir(C 2 H 4 ) that the 2O site is slightly more stable than the 3O site.…”

“…The experimental results led to the conclusion that the metal centers become bonded to the zeolite upon removal of an acac ligand by substitution from a proton on the strong Brønsted acidic Al-OH sites of the zeolite. 5,7,8,9 EXAFS and IR spectra of supported metal complexes that incorporate two CO ligands (metal gem-dicarbonyls) confirm that the metalsupport-oxygen bonds form at the zeolite Al sites; the sharp, intense ν CO bands are significantly blue-shifted relative to those of the unsupported organometallic precursors, 5,7,8,9 indicating that the binding sites withdraw electron density from the metals, 21,22 as expected for the strongly acidic Al binding sites.…”

“…Table 2 is a summary of the available calculated and experimental results comparing IR vibrational frequencies of C-H and C≡O bonds for Ir and Rh complexes with various combinations of C 2 H 4 and CO ligands on six different zeolites. 5,7,8,9,37,38,51,52 These metal complexes, including M(C 2 H 4 )(C 2 H 4 ), M(CO)(CO), and M(C 2 H 4 )(CO), are among the bestcharacterized metal complexes on zeolite supports. It is advantageous to use these complexes for comparison because the ν CH and ν CO frequencies of C 2 H 4 and CO ligands are sensitive to the electronic structure of the metal.…”

“…Thoroughly investigated materials in this class include Group 9 metals on zeolite HY, 1,2,3,4 Ir, Rh, and Ru complexes on zeolite Hβ, 5,6 and Ir complexes on zeolite HSSZ-53. 7 The Si/Al atomic ratios of the zeolites were chosen to be high (Si/Al = 30, 18, and 24, for zeolites Y, β, and SSZ-53, 7,8,9 respectively) to allow formation of widely separated supported mononuclear species isolated at the Al sites. The nearly unique, welldefined structures of some of these site-isolated supported species were demonstrated by images of the isolated metal atoms obtained with high-resolution aberration-corrected scanning transmission electron microcopy.…”

Tuning acid–base cooperativity to create next generation silica-supported organocatalysts

“…While heterogeneous catalysts are known to have a distribution of sites, recent work has focused on creating more uniform sites [72,[76][77][78][79]. An interesting method to improve site uniformity is the concept of molecular imprinting that has been utilized extensively and reviewed previously [80][81][82].…”