Bulky Calixarene Ligands Stabilize Supported Iridium Pair-Site Catalysts

Abstract

Although essentially molecular noble metal species provide active sites and highly tunable platforms for the design of supported catalysts, the susceptibility of the metals to reduction and aggregation and the consequent loss of catalytic activity and selectivity limit opportunities for their application. Here, we demonstrate a new construct to stabilize supported molecular noble-metal catalysts, taking advantage of sterically bulky ligands on the metal that serve as surrogate supports and isolate the active sites under conditions involving steady-state catalytic turnover in a reducing environment. The result is demonstrated with an iridium pair-site catalyst incorporating P-bridging calix[4]arene ligands dispersed on siliceous supports, chosen as prototypes because they offer weakly interacting surfaces on which metal aggregation is prone to occur. This catalyst was used for the hydrogenation of ethylene in a flow reactor. Atomic-resolution imaging of the Ir centers and spectra of the catalyst before and after use show that the metals resisted aggregation and deactivation, remaining atomically dispersed and accessible for catalysis. This strategy thus allows the stabilization of the catalysts even when they are weakly anchored to supports.

Resolving the structure of the E1state of Mo nitrogenase through Mo and Fe K-edge EXAFS and QM/MM calculations

“…These changes in DE 0 effectively change the phase of the individual t paths relative to one another, making the bond distances determined by said t effectively arbitrary (please see Section S4 of the ESI † for further discussion). This is an unfortunately common mistake in EXAFS tting; [59][60][61][62][63][64] to quote Scott Calvin -"There are few mistakes more common in published EXAFS work, and more unambiguously wrong, than publishing ts where every path has a unique E 0 .". 65 With the experimental bond distances of E 0 and E 1 in hand, we can compare the observed differences between these two states with those of model complexes and other FeS cluster proteins.…”

Supported Metal Pair-Site Catalysts

“…For example, iridium pair-site catalysts incorporating phosphorusbridging calix[4]arene ligands were found to be anchored (weakly) to silica when the precursor and support were slurried in an organic solvent. 28 The precursor remained essentially intact upon 23,29 In contrast, more stable rhodium and iridium pair-sites on MgO were synthesized by the reaction of M 2 (OCH 3 ) 2 (COD) 2 (M = Rh, Ir; COD = cyclooctadienyl) with partially dehydroxylated MgO (Figure 3b). 6a, 9a The nuclearity of the precursor was retained after chemisorption on MgO, even under reactive atmospheres containing CO, H 2 , or ethylene at temperatures up to 353 K. In the syntheses, one or two methoxy ligands on rhodium or iridium were replaced by bridging oxygen atoms that were part of the support surface, with the COD ligands remaining as protective ligands on the metal pairs.…”

“…A key question in these cases is whether the structure of the dimeric precursor, with the attached ligands, is preserved after adsorption on a support surface. Schöttle et al 28 used XAS to ascertain whether the structure of the [Ir(CO) 2 PPhL] 2 (L is the calixarene) was maintained on silica. In their XAS analysis, the authors used the known structure of the molecular complex to fix the coordination numbers of the numerous scattering paths in their XAS model and determined whether this model gave an acceptable fit of their data.…”

Single-Atom Catalysts Electrostatically Stabilized by Ionic Liquids

“…Specifically, the anions mainly function in stabilization and electronic modulation by direct interacting with Pt 1 species on the support, while the peripheral cations balance the charge and possibly offer additional steric inhibition. 10…”