Weakly interacting solvation spheres surrounding a calixarene-protected tetrairidium carbonyl cluster: contrasting effects on reactivity of alkane solvent and silica support

Abstract

A substituted tetrairidium carbonyl cluster on a silica support undergoes hydrogen activation at a rate and with a mechanism different from the cluster in solution.

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

“…We previously argued this to be a consequence of the much larger size of the calixarene ligands compared with the tetrahedral metal frame of 3 and consistent with the structure of 3 by single-crystal X-ray diffraction . Previously, we observed selective bonding of H 2 on basal-plane Ir atoms of a silica-supported cluster derived from 3 , in a noncompetitive manner in the presence of ethylene as hydrocarbon, which only bonds to apical Ir atoms of the same cluster. − This observation of selective molecular recognition at the basal-plane Ir atoms is consistent with the discrete nature of these CO bonding sites (i.e., lack of CO scrambling and phosphine mobility) observed here for 3 in toluene solution. The similar steric environment enforced by the calixarene ligands at the apical position versus at substituted basal-plane Ir atoms in 3 ( vide supra ) supports the reason for this previously observed selectivity to be electronic rather than steric, which is consistent with our previous hypothesis .…”

“…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). , …”

Bulky Calixarene Ligands Stabilize Supported Iridium Pair-Site Catalysts

“…This result is contrasted with that characterizing the silicasupported molecular catalyst consisting of phosphine-substituted tetrairidium clusters Ir 4 L 1 3 /SiO 2 (L 1 = calixarenephosphine), which lost approximately three of its nine CO ligands at 311 K in flowing helium. 28 Moreover, the initial rate of CO dissociation from Ir 4 L 1 3 /SiO 2 was independent of whether H 2 was present, 32 demonstrating that the CO dissociation was not hydrogen-assisted, 28,39 contrary to what was observed for 1-SiO 2 .…”

“…During this reaction, we observed the appearance and growth of a new IR band, at 2130 cm −1 (Figures S19 and S25), assigned to Ir−H. 32,39 Because time scales corresponding to both the loss of CO ligands and the formation of the iridium hydride match the time scale observed for increases in catalytic activity during time on stream under ethylene hydrogenation conditions (Figure S21), we conclude that the activation of 1-SiO 2 for catalysis was accompanied by the replacement of a single CO ligand with a bound hydride, which subsequently reacted rapidly with ethylene. Subsequent CO treatment after the reaction of 1-SiO 2 with flowing H 2 led to a fast recarbonylation, to give a species again incorporating four CO ligands per Ir 2 moiety (Figure S18).…”

Continuous Hydrogenation of Ethyl Levulinate to 1,4‐Pentanediol over 2.8Cu‐3.5Fe/SBA‐15 Catalyst at Low Loading: The Effect of Fe Doping

“…Reducing the water content in the system could preserve the active sites maximally. In addition, the stronger adsorption of oxygen‐containing groups and the promotion of hydrogen activation on silica supports in alkane solvents, rather than water or alcohol reported in several works, were critical factors for obtaining the better GVL yields in n ‐hexane solvent.…”