[(Ph)₃PBr][Br₇], [(Bz)(Ph)₃P]₂[Br₈], [(<i>n</i>-Bu)₃MeN]₂[Br₂₀], [C₄MPyr]₂[Br₂₀], and [(Ph)₃PCl]₂[Cl₂I₁₄]: Extending the Horizon of Polyhalides via Synthesis in Ionic Liquids

The five polyhalides [(Ph)3PBr][Br7], [(Bz)(Ph)3P]2[Br8], [(n-Bu)3MeN]2[Br20], [C4MPyr]2[Br20] ([C4MPyr] = N-butyl-N-methylpyrrolidinium), and [(Ph)3PCl]2[Cl2I14] were prepared by the reaction of dibromine and iodine monochloride in ionic liquids. The compounds [(Ph)3PBr][Br7] and [(Bz)(Ph)3P]2[Br8] contain discrete pyramidal [Br7]− and Z-shaped [Br8]2– polybromide anions. [(n-Bu)3MeN]2[Br20] and [C4MPyr]2[Br20] exhibit new infinite two- and three-dimensional polybromide networks and contain the highest percentage of dibromine ever observed in a compound. [(Ph)3PCl]2[Cl2I14] also consists of a three-dimensional network and is the first example of an infinite polyiodine chloride. All compounds were obtained from ionic liquids as the solvent that, on the one hand, guarantees for a high stability against strongly oxidizing Br2 and ICl and that, on the other hand, reduces the high volatility of the molecular halogens

[(Ph)₃PBr][Br₇], [(Bz)(Ph)₃P]₂[Br₈], [(<i>n</i>-Bu)₃MeN]₂[Br₂₀], [C₄MPyr]₂[Br₂₀], and [(Ph)₃PCl]₂[Cl₂I₁₄]: Extending the Horizon of Polyhalides via Synthesis in Ionic Liquids

The five polyhalides [(Ph)₃PBr][Br₇], [(Bz)(Ph)₃P]₂[Br₈], [(<i>n</i>-Bu)₃MeN]₂[Br₂₀], [C₄MPyr]₂[Br₂₀] ([C₄MPyr] = <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium), and [(Ph)₃PCl]₂[Cl₂I₁₄] were prepared by the reaction of dibromine and iodine monochloride in ionic liquids. The compounds [(Ph)₃PBr][Br₇] and [(Bz)(Ph)₃P]₂[Br₈] contain discrete pyramidal [Br₇]⁻ and Z-shaped [Br₈]^2– polybromide anions. [(<i>n</i>-Bu)₃MeN]₂[Br₂₀] and [C₄MPyr]₂[Br₂₀] exhibit new infinite two- and three-dimensional polybromide networks and contain the highest percentage of dibromine ever observed in a compound. [(Ph)₃PCl]₂[Cl₂I₁₄] also consists of a three-dimensional network and is the first example of an infinite polyiodine chloride. All compounds were obtained from ionic liquids as the solvent that, on the one hand, guarantees for a high stability against strongly oxidizing Br₂ and ICl and that, on the other hand, reduces the high volatility of the molecular halogens

Al^III–Calix[4]arene Catalysts for Asymmetric Meerwein–Ponndorf–Verley Reduction

Chiral Al^III-calixarene complexes were investigated as catalysts for the asymmetric Meerwein–Ponndorf–Verley (MPV) reduction reaction when using chiral and achiral secondary alcohols as reductants. The most enantioselective catalyst consisted of a new axially chiral vaulted-hemispherical calix[4]­arene phosphite ligand, which attained an enantioselective excess of 99%. This ligand consists of two lower-rim hydroxyl groups, with the remaining two lower-rim oxygens directly connected to the phosphorus of the phosphite, which is derived from a chiral diol. The results emphasize the importance of the rigid calix[4]­arene lower-rim substituents and point to a possible role of a lower-rim chiral pocket and Lewis-basic phosphorus lone pairs in enhancing asymmetric hydride transfer

Control of Zeta Potential of Hierarchical Mesoporous Zeolite Y via Inorganic Surface Modification without Micropore Blockage and Phase Separation

We describe a selective postsynthetic surface modification of the interior surface of mesoporous Y zeolite (Meso-Y), which results in a uniform nanoscale silica, alumina, or aluminosilicate coating, without causing significant micropore blockage and without synthesizing a separate bulk alumina-containing phase. The approach relies on CTAB surfactant (required for Meso-Y-as synthesis) acting as a soft template. Our results demonstrate the importance of conducting surface modification under dry conditions at the stage of reacting alkoxide molecular precursors with the Meso-Y-as surface. If this is not performed, it leads to undesired micropore blockage by N2 physisorption at 77 K for thicker silicate overlayers as well as phase separation for alumina overlayers. This separate alumina phase is a consequence of the water sensitivity of the latter and the impenetrability of partially condensed alumina colloids into the Meso-Y particle, and it is visible on the external surface of the Meso-Y particle as a bulk micrometer-scale aggregate via SEM. In contrast, uniform layers with no evidence of either phase separation or zeolite micropore blockage could be achieved for oxide coating synthesized under dry conditions. In the case of alumina surface modification, these layers were imaged by TEM as uniform nanoscale (∼10 nm thick) overcoats, and their uniformity in modifying the internal surface of the Meso-Y particle is demonstrated by a combination of zeta-potential measurements and SEM/EDX data. We demonstrate that our approach for synthesizing uniform oxide coatings on Meso-Y particles can be used to control the surface charge, which is crucial for applications in adsorption and catalysis

Bulky Calixarene Ligands Stabilize Supported Iridium Pair-Site Catalysts

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

Cs-RHO Goes from Worst to Best as Water Enhances Equilibrium CO₂ Adsorption via Phase Change

The strong affinity of water to zeolite adsorbents has made adsorption of CO2 from humid gas mixtures such as flue gas nearly impossible under equilibrated conditions. Here, in this manuscript, we describe a unique cooperative adsorption mechanism between H2O and Cs+ cations on Cs-RHO zeolite, which actually facilitates the equilibrium adsorption of CO2 under humid conditions. Our data demonstrate that, at a relative humidity of 5%, Cs-RHO adsorbs 3-fold higher amounts of CO2 relative to dry conditions, at a temperature of 30 °C and CO2 pressure of 1 bar. A comparative investigation of univalent cation-exchanged RHO zeolites with H+, Li+, Na+, K+, Rb+, and Cs+ shows an increase of equilibrium CO2 adsorption under humid versus dry conditions to be unique to Cs-RHO. In situ powder X-ray diffraction indicates the appearance of a new phase with Im3̅m symmetry after H2O saturation of Cs-RHO. A mixed-cation exchanged NaCs-RHO exhibits similar phase transitions after humid CO2 adsorption; however, we found no evidence of cooperativity between Cs+ and Na+ cations in adsorption, in single-component H2O and CO2 adsorption. We hypothesize based on previous Rietveld refinements of CO2 adsorption in Cs-RHO zeolite that the observed phase change is related to solvation of extra-framework Cs+ cations by H2O. In the case of Cs-RHO, molecular modeling results suggest that hydration of these cations favors their migration from an original D8R position to S8R sites. We posit that this movement enables a trapdoor mechanism by which CO2 can interact with Cs+ at S8R sites to access the α-cage

Tandem Catalytic Antioxidant Nanoparticles Comprising Cerium Carbonate and Photoactive Metal Oxides

While photoactive metal oxides such as TiO2 find widespread use in paints and coatings as well as cosmetics and suncare products, they also generate reactive oxygen species (ROS), which degrade materials and are associated with human-health pathologies. Here, we demonstrate a robust and potent catalytic antioxidant consisting of earth-abundant cerium carbonate nanoparticles and micron-size Ce2(CO3)3.8H2O, which are characterized by powder X-ray diffraction and scanning nanobeam electron diffraction. When dispersed with photoactive metal oxides, these cerium carbonate catalysts decrease the photodecomposition rate of organic dyes and commercial pigment colorants in aqueous media by up to 820-fold, as well as in acrylic coatings. X-ray photoelectron spectroscopy and kinetic experiments support the same tandem catalysis mechanism of photoprotection when using both micron-size Ce2(CO3)3.8H2O and cerium carbonate nanoparticles. This mechanism involves ROS disproportionation (catalyzed by cerium carbonate) and H2O2 decomposition (partially catalyzed by TiO2) pathways, both of which cerium carbonate also catalyzes on its own, crudely mimicking the function of the cascade system of superoxide dismutase and catalase enzymes. When cerium carbonate nanoparticles were dispersed at 2 wt % in polymethylmethacrylate, the transparency of the polymer film was preserved and the photo-oxidative degradation of the polymer was prevented following UV irradiation at 254 nm, which otherwise resulted in the loss of optical properties and hydroxylation as characterized by ATR–FTIR spectroscopy, in the control polymer lacking cerium carbonate. Similar observations were made regarding color preservation in paint films comprising dye and insoluble commercial colorant pigments. The material chemistry associated with this photoprotection catalysis is subtle and emphasizes the importance of both Ce­(III) and carbonate together, as both CePO4 and Na2CO3 are inactive. This emphasis is also apparent in comparisons of photoprotection catalysis with previously reported cerium oxide nanoparticles, which are significantly less active compared with Ce2(CO3)3.8H2O under the same conditions

Outer-Sphere Control of Catalysis on Surfaces: A Comparative Study of Ti(IV) Single-Sites Grafted on Amorphous versus Crystalline Silicates for Alkene Epoxidation

The effect of outer-sphere environment on alkene epoxidation catalysis using an organic hydroperoxide oxidant is demonstrated for calix[4]­arene-Ti^IV single-sites grafted on amorphous vs crystalline delaminated zeotype (UCB-4) silicates as supports. A chelating calix[4]­arene macrocyclic ligand helps enforce a constant Ti^IV inner-sphere, as characterized by UV–visible and X-ray absorption spectroscopies, thus enabling the rigorous comparison of outer-sphere environments across different siliceous supports. These outer-sphere environments are characterized by solid-state ¹H NMR spectroscopy to comprise proximally organized silanols confined within 12 membered-ring cups in crystalline UCB-4, and are responsible for up to 5-fold enhancements in rates of epoxidation by Ti^IV centers

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

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals