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

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-d8. 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.