(734d) 100-Fold Rate Enhancement Caused By Dioxygen Ligands on a Supported, Single-Site Tetrairidium Cluster Catalyst for Ethylene Hydrogenation
The bonding of oxygen to transition-metal centers is of central importance from both a fundamental and technological perspective, in biological and synthetic catalysts. Yet the complexity of these systems usually prevents direct structure-function elucidation. Working with a silica-supported tetrairidium cluster as a simplified model catalyst, we investigated the relationship between oxygen ligands bonded at metal centers of the basal plane of the cluster and the reactivity of a metal atom located at the productive apical site for ethylene hydrogenation. The Ir4 cluster was sterically stabilized with three bulky tert-butyl-calixarene phosphine ligands, which are bound at the basal Ir sites, and as reported previously, it exhibited a low activity for ethylene hydrogenation at 323 K and 1 bar (5% C2H4, 16% H2, balance He). However, after 24 h of O2 treatment, a two order of magnitude increase in activity was observed. The oxygen ligands increased the activity of the apical Ir site for both ethylene hydrogenation and H-D exchange in the reaction of H2 with D2 in this system. In contrast, creation of open sites at the apical Ir atom with a mild oxidant, trimethylamine N-oxide, did not increase the catalytic activity substantially. Thus, we infer that the oxygen bonded to basal plane Ir atoms of the Ir4 tetrahedron served as a ligand that enhanced the catalytic activity of the neighboring productive apical site for ethylene hydrogenation. Reaction orders for ethylene hydrogenation were found to be -0.24 and 0.60 for ethylene and H2, respectively. The near zero and half rate orders for ethylene and H2 are characteristic of a mechanism whereby H2 is able to bond noncompetitively to a surface site where ethylene cannot. IR spectra determined the following characteristics of the terminal carbonyl ligands during a sequence of ethylene hydrogenation catalysis, oxygen treatment, ethylene hydrogenation catalysis, and recarbonylation treatment. A blue shift of approximately 20 cm-1 observed during the first and third steps indicates the formation of a hydride species, evident by an Ir-H band at 2110 cm-1. The terminal carbonyl band returned to its original position in the second and fourth steps, accompanied by the loss of the Ir-H band. Distinct ethyl bands formed during the second ethylene hydrogenation step, after the removal of apical carbonyl ligands. The decarbonylation occurring in the first three steps corresponds to a loss of two terminal and bridging carbonyl ligands from the cluster, and the subsequent recarbonylation was only partial, consistent with bonding of oxygen to the cluster, which was confirmed by EXAFS and Raman spectra, the latter showing the presence of iridium peroxo species with bands at 541 and 713 cm-1. EXAFS and IR spectra both gave evidence of isosbestic points during the oxidation, showing that it was a stoichiometric reaction. The stability of the supported Ir4 clusters after first and second catalysis was confirmed with aberration-corrected STEM imaging. DFT calculations support the irreversible bonding of dioxygen ligands to the basal plane Ir atoms of the cluster and their electronic effects leading to a significant increase in the rate of ethylene hydrogenation as a result of decreasing the energy barrier and increasing the thermodynamic driving force for ethylâ??s reductive elimination to form ethane. This step is inferred to be rate limiting, as supported by IR evidence of ethyl and hydride ligands present concomitantly during the second catalysis step.