On the use of 17O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes.

17O-labelled tungsten siloxide complexes [WOCl2(OSitBu3)2] (1-Cl) and [WOMe2(OSitBu3)2] (1-Me) were prepared and characterized by 17O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking 17O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO2-700 afforded material 2, with surface species [(SiO)WOMe2(OSitBu3)] as shown by elemental analysis, IR and 1H and 13C MAS NMR. The DFT calculations of the grafting mechanism are in line with the observed reactivity. They indicate the occurrence of several isomeric species of close energy for the grafted W centers, precluding efficient 17O MAS NMR studies. The lack of catalytic activity in olefin metathesis and ring-opening olefin metathesis polymerization indicates that initiation by α-H elimination is not operative in 2, contrary to related tungsten surface species, which illustrates the crucial influence of the nature of the metal coordination sphere.

Solid-state NMR of quadrupolar nuclei for investigations into supported organometallic catalysts: scope and frontiers.

The rational development of heterogeneous catalysts is a prerequisite for improving current catalytic technologies. One of the most successful approaches to achieve this goal is the grafting of organometallic complexes onto a given support, via surface organometallic chemistry (SOMC). In such cases, the immobilization of (transition) metal centres onto an inorganic support affords single-site catalysts with precise tuning of the coordination sphere and thus target-specific catalytic reactivity. In this context, the precise characterization of the surface sites by several spectroscopic techniques in parallel is of paramount importance. Solid-state NMR, in particular for half-integer quadrupolar nuclei, is a method that has increased its scope in the field of catalysis, especially due to recent methodological breakthroughs, occasionally combined with DFT calculations. In this tutorial review, we provide an overview of the most robust and efficient NMR experiments for half-integer quadrupolar nuclei, from simple one-dimensional acquisition under static and MAS conditions up to high-resolution and heteronuclear correlation two-dimensional spectra, as illustrated by selected examples.

Preparation of monopodal and bipodal aluminum surface species by selective protonolysis of highly reactive [AlH3(NMe2Et)] on silica.

The synthesis and characterization of silica-grafted monopodal and bipodal aluminum hydrides has been achieved starting from 200 °C- and 700 °C-annealed silica and [AlH3(NMe2Et)]. The mechanism by which aluminum trishydride reacts with isolated and vicinal silanols, assisted by the amine, has been investigated computationally at the ωB97XD-DFT level.

On the use of solid-state 45Sc NMR for structural investigations of molecular and silica-supported scandium amide catalysts.

Tris- and tetra-coordinated scandium amides were grafted onto silica and probed as catalysts for alkyne dimerization. 45Sc NMR was carried out, providing information about the metal coordination sphere. The increasing number of coordinated ligands was correlated with decreased catalytic activity.

17O MAS NMR studies of oxo-based olefin metathesis catalysts: a critical assessment of signal enhancement methods.

The DFS enhancement method as applied to 17O MAS NMR was critically assessed, first on NaPO3, a simple binary glass system, and in a second step, on a series of catalysis-related organometallic molecules and materials. The robustness of DFS was investigated for the wide range of anisotropic parameters (quadrupolar coupling and chemical shift anisotropy) encountered in these samples. Emphasis has been put on the variation of signal enhancements with respect to the DFS final sweep frequency, pulse amplitude and pulse duration, while line shape distortion issues were also addressed. Finally, the robustness of DFS enhancement of the 17O MAS NMR signal is shown through its successful application to silica-supported olefin metathesis catalysts.

Chiral phenoxyimino-amido aluminum complexes for the asymmetric cyanation of aldehydes.

The reactivity of triethylaluminum towards salicylaldimine sulfonamides was probed, affording well-defined complexes through consecutive protonolysis of two Al-C bonds by the proligand. These complexes, when combined with an achiral anilinic N-oxide, catalyze the asymmetric addition of trimethylsilylcyanide to a wide range of aldehydes, with good activity and enantioselectivity (up to 91% ee). Insertion of the benzaldehyde substrate into the Al-N amido bond was observed, bringing elements for discussion around the nature of the actual active species.