Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

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 been used for decades in the petrochemical industry, detailed mechanistic studies and the development of molecular organometallic chemistry have led to the development of robust and widely used homogeneous catalysts based on well-defined alkylidenes that have found applications for the synthesis of fine and bulk chemicals and are also used in the polymer industry. The development of the chemistry of high-oxidation group 5-7 alkylidenes and the use of surface organometallic chemistry (SOMC) principles unlocked the preparation of so-called well-defined supported olefin metathesis catalysts. The high activity and stability (often superior to their molecular analogues) and molecular-level characterisation of these systems, that were first reported in 2001, opened the possibility for the first direct structure-activity relationships for supported metathesis catalysts. This review describes first the history of SOMC in the field of olefin metathesis, and then focuses on what has happened since 2007, the date of our last comprehensive reviews in this field.

Boosting the Metathesis Activity of Molybdenum Oxo Alkylidenes by Tuning the Anionic Ligand σ Donation.

The catalytic performances of molecular and silica-supported molybdenum oxo alkylidene species bearing anionic O ligands [ORF9, OTPP, OHMT - where ORF9 = OC(CF3)3, OTPP = 2,3,5,6-tetraphenylphenoxy, OHMT = hexamethylterphenoxy] with different σ-donation abilities and sizes are evaluated in the metathesis of both internal and terminal olefins. Here, we show that the presence of the anionic nonafluoro-tert-butoxy X ligand in Mo(O){═CH-4-(MeO)C6H4}(THF)2{X}2 (1; X = ORF9) significantly increases the catalytic performances in the metathesis of both terminal and internal olefins. Its silica-supported equivalent displays slightly lower activity, albeit with improved stability. In sharp contrast, the molecular complexes with large aryloxy anionic X ligands show little activity, whereas the activity of the corresponding silica-supported systems is greatly improved, illustrating that surface siloxy groups are significantly smaller anionic ligands. Of all of the systems, compound 1 stands out because of its unique high activity for both terminal and internal olefins. Density functional theory modeling indicates that the ORF9 ligand is ideal in this series because of its weak σ-donating ability, avoiding overstabilization of the metallacyclobutane intermediates while keeping low barriers for [2 + 2] cycloaddition and turnstile isomerization.

A reactive coordinatively saturated Mo(iii) complex: exploiting the hemi-lability of tris(tert-butoxy)silanolate ligands.

Coordinatively unsaturated Mo(iii) complexes have been identified as highly reactive species able to activate dinitrogen without the need for a sacrificial reducing agent. Here, we report a coordinatively saturated octahedral Mo(iii) complex stabilized by κ2-tris(tert-butoxy)silanolate ligands, which is yet highly reactive towards dinitrogen and small molecules. The combined high stability and activity are ascribed to the dual binding mode of the tris(tert-butoxy)silanolate ligands that allow unlocking a coordination site in the presence of reactive small molecules to promote their activation at low temperatures.

Silica-Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis.

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well-defined silica-supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo-based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off-cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica-supported Mo and W imido analogues.

Promoting Terminal Olefin Metathesis with a Supported Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalyst.

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.

Bridging the Gap between Industrial and Well-Defined Supported Catalysts.

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.

Dendritic polarizing agents for DNP SENS.

Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an effective method to significantly improve solid-state NMR investigation of solid surfaces. The presence of unpaired electrons (polarizing agents) is crucial for DNP, but it has drawbacks such as leading to faster nuclear spin relaxation, or even reaction with the substrate under investigation. The latter can be a particular problem for heterogeneous catalysts. Here, we present a series of carbosilane-based dendritic polarizing agents, in which the bulky dendrimer can reduce the interaction between the solid surface and the free radical. We thereby preserve long nuclear T'2 of the surface species, and even successfully enhance a reactive heterogeneous metathesis catalyst.

Cationic Silica-Supported N-Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis.

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.

Coordination complexes of NbX5 (X = F, Cl) with (N,O)- and (O,O)-donor ligands and the first X-ray characterization of a neutral NbF5 adduct.

A variety of fairly air-stable complexes were prepared by straightforward reactions of NbX5 (X = F, Cl) with a series of potential bidentate (N,O)- and (O,O)-donor ligands in CH2Cl2. NbF5 reacted with equimolar amounts of 2-[(2,6-diisopropylphenyl)iminomethyl]phenol (dpimpH) and 2-benzoyl-6-hydroxy-6-phenylpentafulvene (bhpfH) to afford the mononuclear complexes NbF5[κ(1)(O)-OC6H4CH=NHC6H3(CHMe2)2], 2, and NbF5[κ(1)(O)-bhpfH], 7, respectively. The 1:1 reactions of NbF5 with salicylaldehyde oxime (saoH2) and 2-phenylaminoethanol gave the hexafluoroniobato salts [NbF4{κ(1)(O)-ON(H)=CHC6H4OH}2][NbF6], 3, and [NbF4{κ(1)(O)-OCH2CH2NH2Ph}2][NbF6], 6, respectively. The syntheses of 2, 3 and 6 are accompanied by oxygen to nitrogen intramolecular H migration, leading to the formation of metal-alkoxide moieties. The parallel chemistry of NbCl5 is characterised by HCl release. The 1:1 reactions with dpimpH, saoH2 and bhpfH yielded, respectively, the ionic complex [NbCl2{κ(1)(N):κ(1)(O)-dpimp}2][NbCl6], 4a, the neutral-dinuclear Nb2Cl7[κ(1)(O)-saoH][κ(2)(O)-sao], 5, and the mononuclear NbCl4[κ(2)(O)-bhpf], 8. The tantalum species [TaCl2{κ(1)(N):κ(1)(O)-dpimp}2][TaCl6], 4b, was prepared from TaCl5/dpimpH. The new products 2-8 were fully characterized by analytical and spectroscopic techniques. Moreover the solid state structures of 2 and 8 were ascertained by X-ray diffraction studies; the structure of 2 exhibits an intramolecular bifurcated N-H···(O,F) hydrogen bond. DFT calculations were carried out in order to predict the lowest energy structures in the distinct cases, showing generally good agreement with the experimental data.