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.

The charge delocalization between the N-heterocyclic carbene (NHC) and the metal in cationic molybdenum imido alkylidene NHC mono(nonafluoro-tert-butoxide) complexes has been studied for different NHCs, i.e., 1,3-dimesitylimidazol-2ylidene (IMes), 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene (IM-esCl 2 ), 1,3-dimesityl-4,5-dimethylimidazol-2-ylidene (IMesMe 2 ), and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH 2 ). The binding situation in the corresponding cationic complexes Mo(N-2,6-Me 2 C 6 H 3 )(CHCMe 2 Ph)(NHC)(OC(CF 3 ) 3 ) + B(Ar F ) 4 − (NHC = IMes (1), IMesCl 2 (2), IMesMe 2 (3), and IMesH 2 (4) was compared to that of the analogous neutral Schrock catalyst Mo(N-2,6-Me 2 C 6 H 3 )(CHCMe 2 Ph)((OC(CF 3 ) 3 )) 2 (5). Singlecrystal X-ray data were used as a starting point for the optimization of the geometries of the catalysts at the PBE0-D3BJ/def2-SVP level of theory; the obtained data were compared to those obtained from X-ray absorption (XAS) and emission spectroscopy (XES). The very similar X-ray spectroscopic signatures of the XANES (X-ray absorption near-edge structure) and Kβ-XES of catalysts 1, 2, and 5 suggest that a similar oxidation state and charge are present at the Mo center in all three cases. However, charge delocalization is more pronounced in 1 and 2 compared to 5. This is supported by quantum chemical (QC) calculations, which reveal that all NHCs compensate to a very similar extent for the cationic charge at molybdenum, leading to charge model 5 (CM5) partial charges at Mo between +1.292 and +1.298. Accordingly, the partial charge in the NHCs was in the range of +0.486 to +0.515. This strong delocalization of the positive charge in cationic molybdenum imido alkylidene NHC (nonafluoro-tert-butoxide) complexes is also illustrated by the finding that the analogous neutral Schrock catalyst 5 has a more positive charge at molybdenum (+1.435) despite being a neutral 14-electron complex. Complementarily, charge analysis on complexes 1 and 2 and the acetonitrilecontaining derivatives 1•MeCN and 2•MeCN revealed that a small partial positive charge of about +0.1 was found on acetonitrile, accompanied by an increase in positive charge on Mo. Accordingly, the partial charges at the imido, the alkoxide, and NHC ligands decreased slightly. Finally, the catalytic activity of complexes 1−4 was determined for a number of purely hydrocarbon-based substrates in a set of olefin metathesis reactions. A correlation of the Tolman electronic parameter (TEP) with catalyst activity, expressed as the turnover frequency after 3 min, TOF 3min , was found for complexes 1−3 based on imidazol-2-ylidenes. 57 Fe-Mossbauer measurements on Mo(N-2,6-Me 2 C 6 H 3 )(CH-ferrocenyl)(NHC)(OTf) 2 and Mo(N-2,6-Me 2 C 6 H 3 )(CH-ferrocenyl)-(NHC)(OTf) + B(Ar F ) 4− (NHC = IMes (6, 8) and IMesH 2 (7, 9)) revealed significant changes in the quadrupole splitting of these complexes. These suggest a significantly more efficient charge distribution between the cationic molybdenum center and an imidazol-2-ylidene-based NHC compared t...

The stereospecific ring-opening metathesis polymerization (ROMP) of endo,exo-2,3-dimethoxymethylnorborn-5-ene (DMMNBE) was accomplished using cationic tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes as initiators. These even outperform a cationic molybdenum imido alkylidene...

A route to cationic tungsten alkylidyne N-heterocyclic carbene (NHC) complexes of the general formula [W­(CR)­X2(NHC)­(tBuCN) y +B­(ArF)4 –] (R = tBu, C6H4-4-OMe; X = Br, Cl, OTf; NHC = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene, IMes; 1,3-bis­(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, IDipp; 1,3-dimesityl-4,5-dichloro-1,3-dihydro-2H-imidazol-2-ylidene, IMesCl2; y = 1, 2; ArF = 3,5-bis­(trifluoro­methyl)­phenyl) is presented. Addition of 1 equiv of water to cationic tungsten benzylidyne and neopentylidyne NHC complexes results in the formation of the corresponding cationic tungsten oxo alkylidene complexes; however, only the resulting neopentylidene complexes were isolated as stable compounds and tested in selected olefin metathesis reactions. Interestingly, even though a strongly basic ligand in the form of the NHC is coordinated to the metal center, this bond is unaffected by the reaction with 1 equiv of water. The hydrolysis of cationic tungsten alkylidyne NHC complexes represents one pathway to cationic tungsten oxo alkylidene NHC complexes and enables the synthesis of complexes, which were so far not easily accessible by previously published procedures.

A series of molybdenum alkylidyne silyloxy N-heterocyclic carbene (NHC) complexes of the general formula [Mo(�C-(R))(OSiPh 3 ) 3 (NHC)] (R = tBu, 4-methoxyphenyl, 2,4,6-trimethylphenyl; NHC = 1,3-diisopropylimidazol-2-ylidene, 1,3-dicyclohexylimidazol-2-ylidene, 1,3-dicyclohexyl-4,5-dihydroimidazol-2ylidene, 1,3-dimethylimidazol-2-ylidene, 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene) was synthesized. Single crystal X-ray analyses revealed that with increasing steric demand of the alkylidyne group, enhanced air-stability of the complexes in the solid-state is achieved with the most stable complex (R = 2,4,6trimethylphenyl, NHC = 1,3-diisopropylimidazol-2-ylidene) being stable in air for 24 h without showing signs of decomposition in 1 H NMR. In contrast to previously reported air-stable molybdenum-based complexes, the novel catalysts proved to be highly active in alkyne metathesis, allowing for turnover numbers (TONs) of up to 6000 without further activation, and tolerant towards several functional groups such as tosyl, ether, ester, thioether and nitro moieties. Their air stability allows for facile handling of the catalysts in air and even after exposure to ambient atmosphere for one week, the most stable representative still displayed high productivity in alkyne metathesis.