Syntheses of “Phosphine-Free” Molybdenum Oxo Alkylidene Complexes through Addition of Water to Alkylidyne Complexes

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

Cationic d0 group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe3)(IMes)(OR)[X-]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-C6H3, C6F5; X- = B(3,5-(CF3)2C6H3)4 -, B(ArF)4, tetrakis(perfluoro-t-butoxy)aluminate (PFTA)). Immobilization of [Mo(O)(CHCMe3)(IMes)(O-1,3-dimesityl-C6H3)+B(ArF)4 -] on silica via surface organometallic chemistry yields an active alkene metathesis catalyst that shows the highest productivity towards terminal olefins amongst all existing molybdenum oxo alkylidene catalysts.

Organometallic Chemistry of Transition Metal Alkylidyne Complexes Centered at Metathesis Reactions

Transition metals form a variety of alkylidyne complexes with either a d⁰ metal center (high-valent) or a non-d⁰ metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as N≡CR, P≡CR, and N≡NR⁺. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo(VI) and W(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed.

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 [OR_F9, OTPP, OHMT - where OR_F9 = 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 = OR_F9) 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 OR_F9 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.

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.

183 W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. 1 H,183 W heteronuclear multiple bond correlation spectroscopy, exploiting a favorable 5 J-coupling between the 183 W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over-stabilized and the catalytic turnover brought to a halt. Guided by the 183 W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly π-donating all-alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentate tert-butoxy ligands, in terms of rate and functional-group compatibility.