NMR Signatures and Electronic Structure of Ti Sites in Titanosilicalite-1 from Solid-State 47/49Ti NMR Spectroscopy

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(OⁱPr)] 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.

Across the Board: J. Henrique Teles

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Dr. J. Henrique Teles, who highlights the importance of a holistic view of sustainability. This is particularly important when it comes to bringing results from the lab to industrial use.

The Role of Conjugative Interaction in Stable Carbenes of the 1,2,4-Triazol-5-ylidene Type and their Energy of Dimerization. An Ab Initio Study

The role of conjugative interaction in stable carbenes of the 1,2,4-triazol-5-ylidene type was studied at different levels of ab initio theory employing model compounds. In the case of parent 4,5-dihydro-1 H-1,2,4-triazol-5-ylidene this type of interaction was found to be of similar importance as in 1,2,4-triazole and pyridine.

Five stationary points were located on the hypersurface of the dimer (C2N3H3)2 . ΔE for all considered dimerization reactions was found to be negative, and the change of energy associated with the formation of the most stable isomer is about -9.5 kcal/mol. In spite of a high value for the singlet-triplet gap of about 90 kcal/mol for the carbene, the heavy atom skeleton of the most stable dimer is close to planarity. This finding is in perfect accordance with the predictions of other authors regarding the geometry of dimers from singlet carbenes stabilized by π-conjugation.

Linker-free, silica-bound olefin-metathesis catalysts: applications in heterogeneous catalysis

A set of heterogenized olefin-metathesis catalysts, which consisted of Ru complexes with the H(2)ITap ligand (1,3-bis(2',6'-dimethyl-4'dimethyl aminophenyl)-4,5-dihydroimidazol-2-ylidene) that had been adsorbed onto a silica support, has been prepared. These complexes showed strong binding to the solid support without the need for tethering groups on the complex or functionalized silica. The catalysts were tested in the ring-opening-ring-closing-metathesis (RO-RCM) of cyclooctene (COE) and the self-metathesis of methyl oleate under continuous-flow conditions. The best complexes showed a TON>4000, which surpasses the previously reported materials that were either based on the Grubbs-Hoveyda II complex on silica or on the classical heterogeneous Re(2)O(7)/B(2)O(3) catalyst.