Although titanosilicalite-1 (TS-1) is among the most successful oxidation catalysts used in industry, its active site structure is still debated. Recent efforts have mostly focused on understanding the role of defect sites and extraframework Ti. Here, we report the 47/49Ti signature of TS-1 and molecular analogues [Ti(OTBOS)4] and [Ti(OTBOS)3(O i Pr)] using novel MAS CryoProbe to enhance the sensitivity. While the dehydrated TS-1 displays chemical shifts similar to those of molecular homologues, confirming the tetrahedral environment of Ti consistent with X-ray absorption spectroscopy, it is associated with a distribution of larger quadrupolar coupling constants, indicating an asymmetric environment. Detailed computational studies on cluster models highlights the high sensitivity of the NMR signatures (chemical shift and quadrupolar coupling constant) to small local structural changes. These calculations show that, while it will be difficult to distinguish mono- vs dinuclear sites, the sensitivity of the 47/49Ti NMR signature should enable distinguishing the Ti location among specific T site positions.
Ziegler-Natta (ZN) catalysts – typically formulated as TiCl4/MgCl2/AlR3 and possibly containing additional organic ligands – are essential to the production of polyethylene and polypropylene. Despite their industrial relevance and years of research on these materials, the role of each constituent (support, organic ligands, post-treatment with organic or inorganic modifiers…) on the structure of Ti surface sites responsible for polymerization remains poorly understood, partly because of the high complexity of such materials. Herein, we show how high-field 47/49Ti NMR can bring about new lights on the structures of the Ti surface sites in ZN pre-catalysts (prior to activation with alkyl aluminium) resulting from adsorption of TiCl4 on MgCl2 followed by a post-treatment with BCl3, an additive used to improve catalytic activity by increasing the amounts of active sites. The implementation of high-field NMR (900 MHz), low temperature (~100 K), magic angle spinning (10 kHz), CPMG echo train acquisition and DFT modelling, to study this material (TiCl4/MgCl2/BCl3) and molecular analogs, allows the detection of a 47/49Ti NMR signature and the development of a molecular level understanding of the NMR signature of Ti surface sites. The extracted 49Ti NMR parameters (δiso, exp = –170 ppm and CQ, exp = 9.3 MHz) from this signature analyzed by DFT modeling indicate the presence of one specific coordination sphere for Ti, namely a fully chlorinated hexacoordinated Ti site with a symmetric charge distribution, due to the post-treatment with BCl3 (that removes the alkoxide ligands) and the coordination environment provided by surface of an amorphous MgCl2.
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.
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