Structural studies of AgPO3–MoO3 glasses using solid state NMR and vibrational spectroscopies

Vitreous samples (1-x) AgPO₃–x MoO₃ (0 ≤ x ≤ 0.5) were prepared by conventional melt-quenching and characterized by Differential Scanning Calorimetry (DSC). The structural evolution of the vitreous network was monitored by ³¹P solid state nuclear magnetic resonance and Raman scattering, and assignments were aided by corresponding studies on the model compound AgMoO₂PO₄. The ³¹P MAS-NMR data differentiate between species having two, one, and zero P―O―P linkages (Q⁽²⁾ Q⁽¹⁾, and Q⁽⁰⁾ species), respectively. Interatomic connectivities involving these units are revealed by two-dimensional INADEQUATE data, utilizing the formation of double quantum coherences mediated by indirect ³¹P–³¹P spin–spin interactions via P―O―P linkages. As this method discriminates against isolated P atoms, it also serves as an important spectral editing tool for constraining lineshape fits. ⁹⁵Mo NMR data and Raman spectra suggest that the Mo species are most likely six-coordinate, forming four P―O―Mo linkages and are otherwise invariant with composition, except at MoO₃ contents ≥ 40 mole %, where some Mo―O―Mo bonding and/or clustering is observed.     

Preparation of Bifunctional Mesoporous Silica Nanoparticles by Orthogonal Click Reactions and Their Application in Cooperative Catalysis

The synthesis of bifunctional mesoporous silica nanoparticles is described. Two chemically orthogonal functionalities are incorporated into mesoporous silica by co‐condensation of tetraethoxysilane with two orthogonally functionalized triethoxyalkylsilanes. Post‐functionalization is achieved by orthogonal surface chemistry. A thiol–ene reaction, Cu‐catalyzed 1,3‐dipolar alkyne/azide cycloaddition, and a radical nitroxide exchange reaction are used as orthogonal processes to install two functionalities at the surface that differ in reactivity. Preparation of mesoporous silica nanoparticles bearing acidic and basic sites by this approach is discussed. Particles are analyzed by solid state NMR spectroscopy, elemental analysis, infrared‐spectroscopy, and scanning electron microscopy. As a first application, these particles are successfully used as cooperative catalysts in the Henry reaction.     

A Phosphorus(III)‐Mediated (4+1)‐Cycloaddition of 1,2‐Dicarbonyls and Aza‐o‐Quinone Methides to Access 2,3‐Dihydroindoles

A (4+1)‐cycloaddition is reported between 1,2‐dicarbonyls and aza‐o‐quinone methide precursors to access 2,3‐dihydroindoles bearing a tetra‐substituted carbon center. The utilization of dioxyphospholenes as carbene surrogates provided dihydroindoles in 20–90 % yield, wherein the electronic nature of the dioxyphospholene impacts its role in the reaction.     

Metallic Glass Films with Nanostructured Periodic Density Fluctuations Supported on Si/SiO2 as an Efficient Hydrogen Sorber

Nanostructured metallic glass films (NMGF) can exhibit surface and intrinsic effects that give rise to unique physical and chemical properties. Here, a facile synthesis and electrochemical, structural, and morphologic characterization of Pd-Au-Si based MGs of approximately 50 nm thickness supported on Si/SiO₂ is reported. Impressively, the maximum total hydrogen charge stored in the Pd-Au-Si nanofilm is equal to that in polycrystalline Pd films with 1 μm thickness in 0.1 m H₂SO₄ electrolyte. The same NMGF has a volumetric desorption charge that is more than eight times and 25 % higher than that of polycrystalline PdNF and Pd-Cu-Si NMGF with the same thickness supported on Si/SiO₂, respectively. A significant number of nanovoids originating from PdH_x crystals, and an increase in the average interatomic spacing is detected in Pd-Au-Si NMGF by high-resolution TEM. Such a high amount of hydrogen sorption is linked to the unique density fluctuations without any chemical segregation exclusively observed for this NMGF.     

A Path to More Sustainable Catalysis: The Critical Role of LiBr in Avoiding Catalyst Death and its Impact on Cross-Coupling

The role that LiBr plays in the lifetime of Pd-NHC complexes has been investigated. A bromide ion is proposed to coordinate to Pd thereby preventing beta hydride elimination (BHE) (to form NHC-H⁺) of the reductive elimination (RE) intermediate that normally completes with the desired cross-coupling catalytic cycle. Coordinating groups, such as anilines, are able to bind suitably well to Pd to prevent this pathway from occurring, thus reducing the need for the added salt. The metal hydride formed from BHE is very unstable and RE of the hydride to the NHC ligand occurs very rapidly giving rise to the corresponding hydrido-NHC (i.e., NHC-H⁺). The use of the per deuterated dibutylzinc shows a significant deuterium isotope effect, shutting down catalyst death almost completely. The use of bis-neopentylzinc, now possessing no hydrides, eliminates catalyst death all together leading to a very long-lived catalytic cycle and confirming the untoward role of BHE.     

P to Se cross polarization in β-P4Se3

Cross polarization from ³¹P to ⁷⁷Se is demonstrated in β-P₄Se₃. This material, an inorganic glass, is readily synthesized from the elements and serves as a convenient sample for setting the Hartmann-Hahn condition.     

Spectral editing in MAS NMR of aprotic solids P---Cd cross-polarization and heteronuclear double-quantum filtering studies in II-IV-V2 semiconductor alloys

Magic-angle-spinning NMR spectra of aprotic solids, ceramics and glasses frequently suffer from poor site resolution due to wide chemical shift distribution effects. In such cases, cross-polarization and heteronuclear double-quantum filtering experiments involving nuclei other than ¹H offer unique spectral editing capabilities. The utility of such assignment techniques for examining site populations in semiconductor alloys is demonstrated for the chalcopyrite systems CdGeAs_2−xP_x, CdSiAs_2−xP_x and Zn_xCd_1−xGeP₂. The results permit a distinction between local and non-local effects on experimental chemical shift trends and reveal that compositional dependences observed in these alloys are dominated by non-local chemical shift contributions.     

Processing dependence of Young's modulus of Ti-base nanostructured alloys

The cooling rate and heat treatment dependence of Young's modulus for Ti-base multicomponent nanostructure-dendrite composites exhibits a very different response compared with the monolithic nanostructured or normal grain-sized Ti alloys. With increasing cooling rate, the decrease in the volume fraction of the micrometer-sized β-phase dendrites induces a significant increase in Young's modulus for most of these composites. This increase can cover the decrease in Young's modulus induced by the reduction of grain size of the nanostructured matrix. Heat treatment induces a significant increase in Young's modulus due to the precipitation of intermetallics and/or α-phase in the nanostructured matrix.