Iridium(I)/N-Heterocyclic Carbene Hybrid Materials: Surface Stabilization of Low-Valent Iridium Species for High Catalytic Hydrogenation Performance

An Ir-I(NHC)-based hybrid material was prepared using a methodology which allowed the precise positioning and isolation of the Ir centers along the pore channels of a silica framework. The full characterization of the material by solid-state NMR spectroscopy showed that the supported Ir sites were stabilized by the silica surface, as low-coordinated single-site complexes. The material is extremely efficient for the hydrogenation of functional alkenes. The catalytic performance (TOF and TON) is one to two orders of magnitude higher than those of their molecular Ir analogues, and could be related to the prevention of the bimolecular deactivation of Ir complexes observed under homogeneous conditions

Hybrid polarizing solids for pure hyperpolarized liquids through dissolution dynamic nuclear polarization

Hyperpolarization of substrates for magnetic resonance spectroscopy (MRS) and imaging (MRI) by dissolution dynamic nuclear polarization (D-DNP) usually involves saturating the ESR transitions of polarizing agents (PAs; e.g., persistent radicals embedded in frozen glassy matrices). This approach has shown enormous potential to achieve greatly enhanced nuclear spin polarization, but the presence of PAs and/or glassing agents in the sample after dissolution can raise concerns for in vivo MRI applications, such as perturbing molecular interactions, and may induce the erosion of hyperpolarization in spectroscopy and MRI. We show that D-DNP can be performed efficiently with hybrid polarizing solids (HYPSOs) with 2,2,6,6-tetramethyl-piperidine-1-oxyl radicals incorporated in a mesostructured silica material and homogeneously distributed along its pore channels. The powder is wetted with a solution containing molecules of interest (for example, metabolites for MRS or MRI) to fill the pore channels (incipient wetness impregnation), and DNP is performed at low temperatures in a very efficient manner. This approach allows high polarization without the need for glass-forming agents and is applicable to a broad range of substrates, including peptides and metabolites. During dissolution, HYPSO is physically retained by simple filtration in the cryostat of the DNP polarizer, and a pure hyperpolarized solution is collected within a few seconds. The resulting solution contains the pure substrate, is free from any paramagnetic or other pollutants, and is ready for in vivo infusion

Preparation of Sn-doped 2-3 nm Ni nanoparticles supported on SiO2\ via surface organometallic chemistry for low temperature dry reforming catalyst:The effect of tin doping on activity, selectivity and stability

Silica supported nickel nanoparticles of 2.2 +/- 0.4 nm diameter were selectively doped with tin by surface organometallic chemistry while keeping the particle size nearly constant. The catalysts with various tin contents were doped and characterized by TEM, XRD and H-2 chemisorption. In contrast to what is found at high temperature (&gt;= 973 K), dry reforming tests performed at 773 K and successive TPO and TEM analysis showed that tin neither influences the catalyst deactivation rate nor prevents coke formation, present in the form of encapsulating carbon. The nickel dopant does not influence either the selectivity, ruled by reverse water gas shift Thermodynamics, but was shown to have a 3-4-fold decrease of intrinsic activity of the available surface nickel, thus indicating that Sn has a negative effect on adjacent Ni atoms. (C) 2014 Elsevier B.V. All rights reserved.</p

A highly ordered mesostructured material containing regularly distributed phenols: preparation and characterization at a molecular level through ultra-fast magic angle spinning proton NMR spectroscopy

Highly ordered organic-inorganic mesostructured material containing regularly distributed phenols is synthesized by combining a direct synthesis of the functional material and a protection-deprotection strategy and characterized at a molecular level through ultra-fast magic angle spinning proton NMR spectroscopy

Evidence for Metal-Surface Interactions and Their Role in Stabilizing Well-Defined Immobilized Ru-NHC Alkene Metathesis Catalysts

Secondary interactions are demonstrated to direct the stability of well-defined Ru-NHC-based heterogeneous alkene metathesis catalysts. By providing key stabilization of the active sites, higher catalytic performance is achieved. Specifically, they can be described as interactions between the metal center (active site) and the surface functionality of the support, and they have been detected by surface-enhanced H-1-Si-29 NMR spectroscopy of the ligand and P-31 solid-state NMR of the catalyst precursor. They are present only when the metal center is attached to the surface via a flexible linker (a propyl group), which allows the active site to either react with the substrate or relax, reversibly, to the surface, thus providing stability. In contrast, the use of a rigid linker (here mesitylphenyl) leads to a well-defined active site far away from the surface, stabilized only by a phosphine ligand which under reaction conditions leaves probably irreversibly, leading to faster decomposition and deactivation of the catalysts

Phenylazide Hybrid-Silica - Polarization Platform for Dynamic Nuclear Polarization at Cryogenic Temperatures

Hyperpolarization of NMR-active nuclei is key to gather high quality spectra of rare species and insensitive isotopes. We have recently established that silica-based materials containing regularly distributed nitroxyl radicals connected to the silica matrix by flexible linkers can serve as promising polarization matrices for dynamic nuclear polarization (DNP). Here we investigate the influence of the linker on the efficiency of the polarization. The materials were fully characterized and exhibit high surface areas and narrow pore size distributions with a tunable amount of phenyl azide groups over a broad range of concentrations. The phenyl azide groups can be easily functionalized via a two-step procedure with 4-carboxy-2,2,6,6-tetramethyl-1-oxylpiperidine (TEMPO) to give polarizing matrices with controllable radical content. The DNP efficiency was found to be similar as in materials with flexible linkers, both for magic angle spinning at 105 K and dissolution DNP at 4 K

Surface enhanced NMR spectroscopy by dynamic nuclear polarization.

International audienceIt is shown that surface NMR spectra can be greatly enhanced using dynamic nuclear polarization. Polarization is transferred from the protons of the solvent to the rare nuclei (here carbon-13 at natural isotopic abundance) at the surface, yielding at least a 50-fold signal enhancement for surface species covalently incorporated into a silica framework

A Slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials

A new nitroxide-based biradical having a long electron spin-lattice relaxation time (T-1e) has been developed as an exogenous polarization source for DNP solid-state NMR experiments. The performance of this new biradical is demonstrated on hybrid silica-based mesostructured materials impregnated with 1,1,2,2-tetrachloroethane radical containing solutions, as well as in frozen bulk solutions, yielding DNP enhancement factors (epsilon) of over 100 at a magnetic field of 9.4 T and sample temperatures of similar to 100 K. The effects of radical concentration on the DNP enhancement factors and on the overall sensitivity enhancements (Sigma(dagger)) are reported. The relatively high DNP efficiency of the biradical is attributed to an increased T-1e, which enables more effective saturation of the electron resonance. This new biradical is shown to outperform the polarizing agents used so far in DNP surface-enhanced NMR spectroscopy of materials, yielding a 113-fold increase in overall sensitivity for silicon-29 CPMAS spectra as compared to conventional NMR experiments at room temperature. This results in a reduction in experimental times by a factor >12 700, making the acquisition of C-13 and N-15 one- and two-dimensional NMR spectra at natural isotopic abundance rapid (hours). It has been used here to monitor a series of chemical reactions carried out on the surface functionalities of a hybrid organic-silica material

Solid-Phase Polarization Matrixes for Dynamic Nuclear Polarization from Homogeneously Distributed Radicals in Mesostructured Hybrid Silica Materials

Mesoporous hybrid silica-organic materials containing homogeneously distributed stable mono- or dinitroxide radicals covalently bound to the silica surface were developed as polarization matrixes for solid-state dynamic nuclear polarization (DNP) NMR experiments. For TEMPO-containing materials impregnated with water or 1,1,2,2-tetrachloroethane, enhancement factors of up to 36 were obtained at similar to 100 K and 9.4 T without the need for a glass-forming additive. We show that the homogeneous radical distribution and the subtle balance between the concentration of radical in the material and the fraction of radicals at a sufficient inter-radical distance to promote the cross-effect are the main determinants for the DNP enhancements we obtain. The material, as well as an analogue containing the poorly soluble biradical bTUrea, is used as a polarizing matrix for DNP NMR experiments of solutions containing alanine and pyruvic acid. The analyte is separated from the polarization matrix by simple filtration