Many industrial catalysts contain isolated metal sites on the surface of oxide supports. Although such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This Review discusses how surface organometallic chemistry (SOMC) engineers surface sites with well-defined structures and provides insight into the nature of the active sites of industrial catalysts; the Review focuses in particular on olefin production and conversion processes.

Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have...

Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ-donating N-heterocyclic carbene ligands with weak σ-donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well-defined silica-supported catalysts, [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(+) ][B(Ar(F) )4 (-) ] [IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, B(Ar(F) )4 =B(3,5-(CF3 )2 C6 H3 )4 ] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.

Dendrimer-shielded polarizing agents for the application of DNP SENS to reactive surfaces.

Silica-supported cationic Mo-imido alkylidene N-heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ-donating property of the NHC ancillary ligand, which disfavors the formation of the parent square-planar metallacyclobutane, an off-cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified.

We report the synthesis of tungsten oxo alkylidene complexes bearing bulky thiophenoxide ligands [W(O)(CHtBu)-(SHMT) 2 (PMe 2 Ph)] and [W(O)(CHtBu)(SHMT) 2 ] (SHMT = 2,6-dimesitylthiophenoxide) and their grafting on partially dehydroxylated silica, affording the supported complexes [(SiO)W(O)(CHtBu)-(SHMT)(PMe 2 Ph)] and [(SiO)W(O)(CHtBu)(SHMT)]. While the molecular precursors are not significantly active in the metathesis of alkenes, the grafted analogue without bound phosphine ligands displays activity comparable to that of its aryloxide analogue [(SiO)W(O)( CHtBu)(OHMT)] (OHMT = 2,6-dimesitylphenoxide). It is worth noting that [(SiO)W(O)(CHtBu)(SHMT)] showed unprecedented activity in the metathesis of 1-alkenes, probably because of the lower stability of metallacyclobutane intermediates.A dvances in alkene metathesis over the last 30 years have transformed the way organic molecules are prepared. 1−3 Of d 0 alkene metathesis catalysts, 4−7 supported tungsten oxo alkylidene complexes have recently emerged and been shown to have unprecedented activities and stabilities. 8,9 In particular, silica-supported tungsten oxo alkylidene bearing phenolate ligands catalyze the metathesis of internal olefins at loading as low as 50 ppm. However, they are surprisingly sluggish for 1-alkenes, because of the formation of very stable unsubstituted metallacycles generated by the ethylene byproduct. This reversible deactivation process could be circumvented by using a reactor setup, allowing for the continuous removal of ethylene.DFT calculations have shown that dissymmetry at the metal center (X ≠ Y) for d 0 Schrock-type catalysts of general formula (X)(Y)M(E)(CHR) (M = Mo, W, Re; E = O, NR, CR) not only decreases the energy barrier for coordination of the olefin substrate (the key step in metathesis) but also raises the energy of the metallacyclobutane intermediates. 10−14 In silica-supported phenolate tungsten oxo alkylidene complexes such as [(SiO)W(O)(CHtBu)(OHMT)] and [(SiO)W( O)(CHtBu)(dAdPO)] (OHMT = 2,6-dimesitylphenoxide, dAdPO = 2,6-diadamantyl-4-methylphenoxide), the σ donations of the phenoxide and the surface siloxide ligands are very similar. We thus reasoned that thiolate analogues 15 would be ideal targets, since they are isoelectronic with the corresponding phenolates 16−19 with the advantage of restoring the dissymmetry at the metal center.Herein we report the synthesis, the grafting on partially dehydroxylated silica at 700°C (SiO 2-(700) ), and the catalytic performance of oxo alkylidene complexes with pendant thiolate ligands [W(O)(CHtBu)(SHMT) 2 (PMe 2 Ph)] (1) and [W(O)(CHtBu)(SHMT) 2 ] (2) (SHMT = 2,6-dimesitylthiophenoxide) 15,20 and the corresponding supported systems.The thiolate complex 1 was synthesized by salt metathesis of [W(O)(CHtBu)Cl 2 (PMe 2 Ph) 2 ] 21,22 with 2 equiv of potassium 2,6-dimesitylthiophenolate in 70% yield. A similar synthesis was independently described by Schrock and Hoveyda during the course of our studies. 16 The phosphinefree complex 2 was prepare...

Hemilabile tris(tert-butoxy)silanolate ligands allow stabilizing a mononuclear octahedral Mo(iii) complex without quenching its reactivity towards small molecules (N2, CO2, N2O).

Silica‐supported cationic Mo‐imido alkylidene N‐heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ‐donating property of the NHC ancillary ligand, which disfavors the formation of the parent square‐planar metallacyclobutane, an off‐cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified.