Electronegativity and location of anionic ligands drive yttrium NMR for molecular, surface and solid-state structures

Yttrium is present in various forms in molecular compounds and solid-state structures; it typically provides specific mechanical and optical properties. Hence, yttrium containing compounds are used in a broad range of applications such as catalysis, lasers and optical devices. Obtaining descriptors that can provide access to a detailed structure–property relationship would therefore be a strong base for the rational design of such applications. Towards this goal, 89Y (100% abundant spin ½ nucleus), is associated with a broad range of NMR chemical shifts that greatly depend on the coordination environment of Y, rendering 89Y NMR an attractive method for the characterization of yttrium containing compounds. However, to date, it has been difficult to obtain a direct relationship between 89Y chemical shifts and its coordination environment. Here, we use computational chemistry to model the chemical shift of a broad range of Y(III) molecular compounds with the goal to reveal the underlying factors that determine the 89Y chemical shift. We show through natural chemical shift (NCS)-analysis that isotropic chemical shifts can easily help to distinguish between different types of ligands solely based on the electronegativity of the central atom of the anionic ligands directly bound to Y(III). NCS-analysis further demonstrates that the second most important parameter is the degree of pyramidalization of the three anionic ligands imposed by additional neutral ligands. While isotropic chemical shifts can be similar due to compensating effects, investigation of the chemical shift anisotropy (CSA) enables discriminating between the coordination environment of Y.ISSN:2041-6520ISSN:2041-653

VRA 3: What Are the Risks of Causing a New Outbreak of Foot and Mouth Disease (FMD) by Moving Stray Susceptible Animals from Roads within a Restricted Zone?

No abstract available

Identification of favourable silica surface sites for single‐molecule magnets

International audienceIndustrial data storage application based on single‐molecule magnets (SMMs) necessitates not only strong magnetic remanence at high temperatures but also requires the implementation of SMMs into a solid material to increase their durability and addressability. While understanding relationship between local structure metal resulting behavior is well understood in molecular systems, it remains challenging establish similar for materials, especially isolated lanthanide sites surfaces. For instance, dispersed Dy(III) ions silica prepared via surface organometallic chemistry exhibit slow relaxation low temperatures, origin these properties unclear. In this work, we modelled ten neutral complexes with coordination numbers (CN) three six ([Dy(OSiF 3 ) (O(SiF 2 CN‐3 ]) representing possible investigated SMM potential ab initio CASSCF/RASSI‐SO calculations. Detailed analysis shows influence spatial position anionic ligands while play minor role properties. particular, T‐shape like orientation predicted good making promising targeted environment surface‐based SMMs

Classifying and understanding the reactivities of Mo based alkyne metathesis catalysts from 95Mo NMR chemical shift descriptors

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X3MoCR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the con-text of alkyne metathesis that 95Mo chemical shift tensors encode detailed information on the electronic structure of potent catalysts. Analysis by first principles calculations of 95Mo chemical shift tensors ex-tracted from solid-state 95Mo NMR spectra show a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2+2]-cycloaddition step, thus linking 95Mo chemical shifts to reactivity. In particular, the 95Mo chemical shifts are driven by ligand electronega-tivity (σ-donation) and electron delocalization through Mo-O π-interactions, thus explaining the unique reactivity of the silanolate canopy catalysts. These results further motivate exploration of transition-metal NMR signatures and their relations to electronic structure and reactivity

Increasing Olefin Metathesis Activity of Silica-Supported Molybdenum Imido Adamantylidene Complexes through E Ligand sigma-Donation

Molybdenum imido adamantylidene complexes with different substituents on the imido ligand (dipp=2,6-diisopropylphenyl, Ar-F5=C6F5, and Bu-t) having distinct electron donating abilities were investigated for the metathesis of internal and terminal olefins, for both molecular and silica-supported species using standardized protocols. Here we show that surface immobilization of these compounds results in dramatically increased activity compared to their molecular counterparts. Additionally, we show that electron withdrawing imido groups increase the activity of the compound towards terminal olefins while they simultaneously decrease the ability to metathesize internal olefins. Furthermore, these systems also show high stability when used as initiators in olefin metathesis, although the species that display higher initial activity deactivate faster than those that show more a more moderate reaction rate at first. Our catalytic studies, augmented by DFT calculations, show that all investigated compounds have a remarkably small energy difference between the trigonal bipyramidal (TBP) and square planar (SP) configurations of the metallacyclobutane intermediates, which has previously been linked to high activity.ISSN:0018-019XISSN:1522-267

W-oxo Adamantylidenes: Stable Molecular Precursors for Efficient Silica-Supported Metathesis Catalysts

Tungsten oxo adamantylidenes (=Ad) are a new class of readily accessible alkylidenes, that opens the possibility to evaluate a broader class of molecular and supported olefin metathesis catalysts. In this context, W(O)(=Ad)X2, with X=pyrrolide and perfluoro-tert-butoxide, were grafted on partially dehydroxylated silica, to generate the corresponding silica-supported tungsten oxo adamantylidenes. The high thermal stability of the molecular perfluoro-tert-butoxide complex, that exists as a dimer in the solid state, even enables its grafting through sublimation. Evaluation of their catalytic activities toward olefin metathesis using standardized protocols show that these supported species display high activity toward internal and terminal olefins. In particular, avoiding the use of solvent during grafting of the highly electrophilic perfluoro-tert-butoxide compounds, that can bind to the W center, enable to generate significantly more active catalysts.ISSN:0018-019XISSN:1522-267

Olefin Metathesis Catalysts Generated In Situ from Molybdenum(VI)-Oxo Complexes by Tuning Pendant Ligands

Tailored molybdenum(VI)-oxo complexes of the form MoOCl2(OR)2(OEt2) catalyse olefin metathesis upon reaction with an organosilicon reducing agent at 70 °C, in the presence of olefins. While this reactivity parallels what has recently been observed for the corresponding classical heterogeneous catalysts based on supported metal oxide under similar conditions, the well-defined nature of our starting molecular systems allows us to understand the influence of structural, spectroscopic and electronic characteristics of the catalytic precursor on the initiation and catalytic proficiency of the final species. The catalytic performances of the pre-catalysts are determined by the highly electron withdrawing (σ-donation) character of alkoxide ligands, OtBuF9 being the best. This activity correlates with both the 95Mo chemical shift and the reduction potential that follows the same trend: OtBuF9>OtBuF6>OtBuF3.ISSN:0947-6539ISSN:1521-376

Switching between classical/nonclassical crystallization pathways of TS-1 zeolite: implication on titanium distribution and catalysis

In the MFI zeolite crystallization process, the classical crystallization mechanism based upon the addition of silica species is often concomitant with the nonclassical route that is characteristic of the attachment of silica nanoparticle precursors. However, the factors that govern the preferences for each mechanism remain unclear. In this work, we present the impact of switching between these two crystallization pathways on the active sites and the resulting catalytic performance of the titanosilicate TS-1 zeolite. By controlling the self-assembled precursor structures in the early crystallization stage which are mediated by the Ti and H2O in the reaction system, we could achieve the preferred modes of crystal growth of the TS-1 zeolite. We indicate that by directing the predominant crystallization path from the classical to the nonclassical route, it is possible to generate more stable bridging peroxo species upon reaction with hydrogen peroxide, as confirmed by O-17 solid-state nuclear magnetic resonance spectroscopy, thus substantially increasing the catalytic performance of the resulting TS-1 for olefin epoxidation.ISSN:2041-6520ISSN:2041-653

Tracking Coordination Environment and Reaction Intermediates in Homo- and Heterogeneous Epoxidation Catalysts via Ti L2,3-edge NEXAFS

Ti-based molecules and materials are ubiquitous, and play a major role in both homogeneous and heterogeneous catalytic processes. Understanding the electronic structures of their active sites (oxidation state, local symmetry and ligand environment) is key to developing molecular-level structure-property relationships. In that context, X-ray absorption spectroscopy (XAS) offers a unique combination of element selectivity and sensitivity to local symmetry. Commonly, for early transition metals such as Ti, K-edge XAS is applied for in situ characterization and subsequent structural analysis with high sensitivity towards tetrahedral species. Ti L2,3-edge spectroscopy is in principle complementary and offers specific opportunities to interrogate the electronic structure of five-and six-coordinated species. It is, however, much more rarely implemented, because the use of soft X-rays implies ultra-high vacuum conditions. Furthermore, the interpretation of the data can be challenging. Here, we show how Ti L2,3-edge spectroscopy can help to obtain unique information about both homogenous and heterogeneous epoxidation catalysts and to develop a molecular-level relationship between spectroscopic signatures and electronic structures. Towards this goal, we first establish a spectral library of molecular Ti reference compounds, comprising various coordination environments with mono- and dimeric Ti species having O, N and Cl-ligands. We next implemented a computational methodology based on multiplet ligand field theory and maximally localized Wannier orbitals benchmarked on our library to understand Ti L2,3-edge spectroscopic signatures. We finally used this approach to track and predict spectra of catalytically relevant intermediates, focusing on Ti-based olefin epoxidation catalysts