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

Abstract

Rates of CO exchange in a comparative series of tetrairidium carbonyl clusters Ir4CO9L3 consisting of phosphine ligands of varying steric bulk (diphenyl­methyl­phosphine in 1, triphenyl­phosphine in 2, and calix[4]­arene phosphine in 3) have been investigated in toluene-d 8. The presence of bridging CO ligands and the same phosphine substitution pattern (axial, equatorial, and equatorial) as confirmed by 31P NMR spectroscopy enables the rigorous comparison of this series of isoelectronic clusters. Inverse gated decoupling 13C qNMR spectroscopy was applied for quantification and assignment of the entire spectrum, the carbonyl region of which was used to characterize CO exchange. A toluene solution of the calixarene-based cluster 3 exhibited no evidence of CO exchange up to 353 K. This included a lack of observed exchange involving apical CO ligands, which underwent scrambling by 323 K for 1 and 2. Activation energies for CO exchange in a toluene solution of 1 were <4.5 kcal/mol based on line-width analysis, whereas they could not be calculated for 2 because resonances were too broad to be analyzed by 353 K. Large differences in phosphine mobility between 1 and 2 relative to 3 were also reflected in the 31P NMR spectra, which for the latter remained unchanged up to 353 K, in contrast to significant broadening observed for the former two clusters. The observed trends here reinforce the crucial role of cumulative noncovalent interactions involving sterically bulky calixarene ligands in 3. These interactions are responsible for immobilizing phosphine ligands and encaging CO ligands, in a manner that limits their intramolecular exchange. These observations elucidate a previously observed mechanism of selective molecular recognition involving basal-plane bonding of hydrogen but not hydrocarbon (i.e., catalytic “S” sites) in a silica-supported cluster derived from 3, in particular its electronic rather than steric origin.

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

“…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.…”

“…Our approach to understanding these phenomena was to synthesize noble-metal catalysts incorporating isolated active sites in well-defined environments, at temperatures close to ambient, with controlled atomic-scale connectivity. − We used a molecular templatea tetrairidium carbonyl cluster, 1 in Figure , as a simple model comprising a tetrahedron of Ir atoms that neighbor each other; we chose this metal cluster because of its demonstrated robustness, which is enabled by a bulky calixarene phosphine ligand bonded to each basal-plane Ir atom in the tetrahedral iridium frame. ,,− The steric bulk of these ligands prevents bimolecular processes that have been invoked as the nucleation event for cluster agglomeration, as well as those that are responsible for CO and phosphine ligand fluxionality in trisubstituted tetrairidium clusters . The robustness of this catalyst is contrasted with the fragility of most metal clusters and supported mononuclear metal complex catalysts containing organic ligands, , which have historically led to calls for the development of synthetic approaches to provide stable coordinatively unsaturated sites in metal clusters …”

Nested Metal Catalysts: Metal Atoms and Clusters Stabilized by Confinement with Accessibility on Supports

“…Because the protection is associated with the bulkiness of the calixarene ligands, the strength of the interactions of the metal with the support is relatively unimportant, and tetrairidium clusters have been stabilized even on a weakly interacting support such as silica . The bulky calixarene ligands even freeze intramolecular CO exchange processes, which otherwise occur rampantly, on tetrairidium carbonyl clusters . This class of catalyst includes a silica-supported di-iridium cluster (Figure ), which could be reversibly decarbonylated in the presence of H 2 to synthesize a cluster hydride that was a stable catalyst for ethylene hydrogenation, in contrast to comparable catalysts lacking the calixarene ligands, which sintered …”

“… 10 The bulky calixarene ligands even freeze intramolecular CO exchange processes, which otherwise occur rampantly, on tetrairidium carbonyl clusters. 11 This class of catalyst includes a silica-supported di-iridium cluster ( Figure 1 ), which could be reversibly decarbonylated in the presence of H 2 to synthesize a cluster hydride that was a stable catalyst for ethylene hydrogenation, in contrast to comparable catalysts lacking the calixarene ligands, which sintered. 12 …”

The Journal of Physical Chemistry C Virtual Special Issue on Metal Clusters, Nanoparticles, and the Physical Chemistry of Catalysis

“…The second approach provides a novel opportunity to establish the correlation of the atomic structure and catalysis and to understand the effect of the ligand layer on the catalytic activity and the selectivity. This VSI contains 11 papers concerning MPCs, 5 of which are related to the chemical properties of MPCs: CO oxidation by a ferrocene-protected Au 8 cluster, 12 reductive elimination of 1,3-diynes from an alkynyl-protected alloy cluster, 13 the migration of a Pd atom within an Au MPC during CO oxidation, 14 CO exchange in a carbonyl Ir 4 cluster, 15 and the hydrogenation of nitroaromatics by supported Au n (n = 19, 20, and 22) MPCs. 16 Other topics reported include the synthesis of a new Au 5 Cu 6 alloy cluster, 17 Published: March 11, 2021 the controlled doping of a Cu atom to a Au-based alloy cluster, 18 the synthesis of ultrastable Au clusters protected by sulfonic thiolates, 19 the dynamics of the ligand layer playing a role in the MPC stability, 20 the photoluminescence properties of Ag MPCs, 21 and the optical properties of Au 9 MPC.…”