Spectroscopic Characterization of μ-η¹:η¹-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium Clusters

Abstract

Although oxygen is a common ligand in supported-metal catalysts, its coordination has been challenging to elucidate. Using a well-defined Ir-dimer cluster that incorporates a µ-η 1 :η 1-peroxo ligand, we observe a FT-Raman band at 756 cm-1 assigned to the 16 O-16 O stretch, and a greatly enhanced intensity at 788 cm-1. The frequency of this latter band does not change upon 18 O labeling, suggesting it arises due to a change in symmetry accompanying bridging peroxo-ligand incorporation. We also investigate reaction of oxygen with a silica-supported tetrairidium carbonyl cluster protected with bulky electron-donating phosphine ligands (p-tert-butylcalix[4]arene(OPr)3(OCH2PPh2; Ph = phenyl; Pr = propyl), and observe the same Raman band at 788 cm-1 , which is associated with formation of similar bridging peroxo ligands on the tetrairidum frame. IR spectra recorded as the supported cluster was decarbonylated in sequential exposures to (i) H2, (ii) O2, (iii) H2, and (iv) CO are consistent with two bridging peroxo ligands bonded irreversibly per supported tetrairidium cluster, replacing bridging carbonyl ligands, without altering either the cluster frame or bound phosphine ligands. X-ray absorption near edge and infrared spectra recorded as the cluster reacted with O2 include isosbestic points signifying a stoichiometrically simple reaction, and mass spectra of the effluent gas identify CO2 formed by oxidation of one terminal CO ligand per cluster and H2 (not H2O)-evidence that hydride ligands had been present on the cluster. The results demonstrate that O2 reacts with intact ligated metal polyhedra on supports-an inference that pertains broadly to oxidation catalysis on supported noble metals.

Mechanical Control of Rate Processes: Effect of Ligand Steric Bulk on CO Exchange in Trisubstituted Tetrairidium Cluster Catalysts

“…On the basis of mechanical concepts involving the steric bulk of cone calixarene-based ligands with phosphine lower-rim substituents, we have stabilized various cluster catalysts. Specifically, we have demonstrated the steric protecting role of bulky calix[4]­arene phosphine ligands in stabilizing subnanometer gold and tetrairidium clusters with open coordinatively unsaturated sites, which could not be achieved with smaller non-calixarene ligands, − and have most recently used this rigorous enforcement of site isolation to study bonding of hydride, bridging peroxo, and ligands formed during hydrogenation catalysis in iridium clusters. − These data demonstrate the calix[4]­arene macrocycle as a steric barrier against metal cluster aggregation processes, much like rigid folds in a protein, which prevent the mutual annihilation of oppositely charged substituent groups on the backbone (e.g., acid and base groups). , …”

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst

“…The dioxygen ligands in 3 have been characterized by Raman spectroscopy in 18 O labeling experiments coupled with electronic-structure calculations (and a comparison with reference compounds that had been characterized previously by single-crystal X-ray diffraction crystallography (and synthesized by similar exposure to O 2 under ambient conditions)) . These bridging dioxygen ligands were shown not to react with H 2 and could not be displaced by CO, and now we show that the cluster incorporating them is catalytically competent for ethylene hydrogenation and H–D exchange in the reaction of H 2 with D 2 . By comparing the catalytic activities of the clusters with and without the bridging dioxygen ligands ( 2 and 3 ), we now have determined evidence of enhancement of the rate of ethylene hydrogenation catalysis caused by these ligands.…”

“…They also validate the premise that the protected but accessible tetrairidium template of 1 offers good opportunities for elucidating details of catalysis involving hydrogen transfer at isolated metal centers. The results link to a number of reported results characterizing tetrairidium clusters with various ligands, including results characterizing ethylene hydrogenation catalysis, ,,,− illustrating how to use tetrairidium clusters as templates to create isolated iridium sites for hydrogenation reactions. Incorporation of bridging dioxygen ligands at sites remote from the active site in the cluster increases the rate of ethylene hydrogenation catalysis 6-fold.…”

“…In the work reported here, we have built on our recently described ability to precisely modify the supported tetrairidium cluster 1s with the metal frame intact, now by incorporating bridging dioxygen ligands at Ir atoms adjacent to the catalytically active apical site, without altering the calixarene phosphine ligands bound to the basal plane . As reported (and shown in Figure ), cluster 3 formed by bonding of O 2 to it under ambient conditions, after the cluster had reacted with H 2 in a preceding step to form cluster 1s′ (Figure ). The reaction of O 2 with 1s′ resulted in the desorption of H 2 (but not H 2 O) and CO 2 , but the calixarene phosphine ligands remained bonded to the cluster .…”

“…Figure 2. Synthesis of 3 from 1 proceeds by sequential reactions involving H 2 (either in the presence or absence 47 of ethylene), which results in basal-plane decarbonylation, 39,41 metal complex catalysts containing organic ligands, 40,49 which have historically led to calls for the development of synthetic approaches to provide stable coordinatively unsaturated sites in metal clusters. 49 The robustness of 1 is manifested in the reversibility of the removal of CO ligands from it, either in solution or supported on silica.…”