The selectivity of several molybdenum and tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes in the ring opening metathesis polymerization (ROMP) of enantiomerically pure endo,exo-2,3-dicarbomethoxynorborn-5-ene (DCMNBE) was examined by 1H- and 13C NMR spectroscopy. With one exception, all complexes showed a strong bias toward the formation of trans-isotactic polymers, some yielding polymers based on >98% trans-isotactic repeat units. This high selectivity was successfully extended to the ROMP of other monomers such as endo and exo-N-(R)-(+)-α-methylbenzyl-5-norbornene-2,3-dicarboximide, 2,3-bis­[(menthyloxy)­carbonyl]­norbornadiene, and methyl-N-(S)-(−)-α-methylbenzyl-2-azabicyclo­[2.2.1]­hept-5-ene-3-carboxylate. The cationic initiators [Mo­(NAr)­(5 i Pr)­(CHCMe2Ph)­(X)]­[B­(ArF)4] (Ar = 2- t BuC6H4, 2,6-Me2C6H3, 2,6- i Pr2C6H3; 5 i Pr = 1,3-diisopropylimidazol-2-ylidene; X = pyrrolide, O-2,6-(2,4,6-Me3C6H2)2C6H3; B­(ArF)4 = B­(3,5-(CF3)2C6H3)4) were found to be the most reactive ones, while also maintaining very high isoselectivity. Finally, a large imido ligand improved stereospecificity, while the choice of the NHC ligand had only a minor influence. Polymerizations can be terminated with 2-methoxystyrene, as evidenced by matrix-assisted laser-desorption time-of flight mass spectrometry.

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.

For entropic reasons, the synthesis of macrocycles via olefin ring-closing metathesis (RCM) is impeded by competing acyclic diene metathesis (ADMET) oligomerization. With cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes confined in tailored ordered mesoporous silica, RCM can be run with macrocyclization selectivities up to 98% and high substrate concentrations up to 0.1 M. Molecular dynamics simulations show that the high conversions are a direct result of the proximity between the surface-bound catalyst, proven by extended X-ray absorption spectroscopy, and the surface-located substrates. Back-diffusion of the macrocycles decreases with decreasing pore diameter of the silica and is responsible for the high macrocyclization efficiency. Also, Z-selectivity increases with decreasing pore diameter and increasing Tolman electronic parameter of the NHC. Running reactions at different concentrations allows for identifying the optimum substrate concentration for each material and substrate combination.

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts.

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...