Cationic Group VI Metal Imido Alkylidene N‐Heterocyclic Carbene Nitrile Complexes: Bench‐Stable, Functional‐Group‐Tolerant Olefin Metathesis Catalysts

Abstract

Despite their excellent selectivities and activities,Moand W-based catalysts for olefin metathesis have not gained the same widespread use as Ru-based systems,mainly due to their inherent air sensitivity.Herein, we describe the synthesis of airstable cationic-at-metal molybdenum and tungsten imido alkylidene NHC nitrile complexes.T hey catalyze olefin metathesis reactions of substrates containing functional groups such as (thio-) esters,(thio-) ethers and alcohols without the need for prior activation, for example,byaLewis acid. The presence of an itrile ligand was found to be essential for their stability towards air,w hile no decrease in activity and productivity could be observed upon coordination of anitrile.V ariations of the imido and anionic ligand revealed that alkoxide complexes with electron-withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes.

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

“…This concept, that showed the importance of dissymmetry at the metal centre, was soon aer generalized to d 0 Mo and W catalysts, [M(E)(] CHR)(X)(Y)] with M(E) ¼ Mo/W(imido/oxo) (Scheme 1), 68,69 and shown to be a predictive guiding principle for the metathesis catalyst design not only in heterogeneous but also in homogeneous systems such as MAP, 25,[70][71][72][73][74][75][76][77][78] MAC, 79,80 or NHC-stabilized cationic catalysts. [81][82][83][84][85] Metallacyclobutane intermediates of W-based catalysts are usually more stable than those of their Mo-based congeners and are oen observed; the Mo analogues are rarely reported for either homogeneous or well-dened silica-supported systems. 25,86 Computational results indicate that weaker s-donating ligands X and Y lead to more stable metallacyclobutane intermediates, and the isomerization between the two metallacyclobutane isomers (TBP and SP) via a turnstile process tends to be more facile for W as compared to Mo and for weaker s-donor ligands.…”

“…This principle has been recently exploited in designing Mo and W catalysts containing N-heterocyclic carbenes (NHC)-very strong s-donor ligands (see Section 2.2). [81][82][83][84][85] The effect of the ancillary X ligand on the relative stability of metallacyclobutanes derived from the reaction of such silica-supported W imido alkylidenes with ethylene has been studied in detail by solid-state NMR spectroscopy. With X ¼ OtBu, a strong s-donor ligand, the SP metallacyclobutane was formed exclusively.…”

A Comprehensive Analysis of the Metal–Nitrile Bonding in an Organo-Diiron System

“…In the presence of significantly π-acidic substituents on the nitrile, the polarization may be inverted (N → C), leading to negative Δῦ values. Therefore, it has to be remarked that a positive value of Δῦ is not an index of absence of back-donation, in analogy to what previously demonstrated for carbonyl complexes, 41 and advising caution in the interpretation of the metal–nitrile bonding based on infrared data only [ 1 , 10 , 12 , 70 , 71 ].…”

“…Nitriles (N≡CR) are ubiquitous and versatile ligands in coordination chemistry [ 1 , 2 , 3 ]. They most commonly behave as monodentate ( end-on ) ligands, and have been widely employed as weakly coordinating agents in complexes of low- to middle-valent transition metals, since their substitution by more strongly coordinating ligands is a convenient strategy to access a multitude of derivatives, catalysts and materials [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ]. Furthermore, nitrile ligands are usually susceptible to nucleophilic attack [ 12 , 13 , 14 ] and may be engaged in a great variety of chemical transformations mediated by the adjacent metal centre, either catalytic [ 15 , 16 , 17 , 18 ] or stoichiometric [ 12 , 19 , 20 , 21 , 22 , 23 ].…”

Synthesis of d10 N-Heterocyclic Carbene Complexes with a Perimidine Scaffold

“…Remarkably, the case for six-membered-ring NHCs is reversed. The saturated variants have dominated the literature and conjugated examples based on a perimidine scaffold are rare (Figure A). − To date, the complexes that have been characterized are restricted to group 8–10 transition metals with d electron counts of 6 or 8. (Figure B).…”