The syntheses and single‐crystal X‐ray structures of a series of Mo–imido alkylidene N‐heterocyclic carbene (NHC) complexes (1–15) and of the first complexes containing bidentate NHC‐phenolate ligands (16–18) are reported. Mo(N‐2,6‐Me2‐C6H3)((1‐R‐phenethyl)‐3‐mesitylimidazolidin‐2‐ylidene)(CHR)(OTf)2 (R=CMe2Ph, 1) is the first enantiomerically pure Mo–imido alkylidene NHC catalyst. With [Mo(N‐2,6‐Me2‐C6H3)(IMes)(CHR)(CH3CN)(OTf)(CH3CN)+ B(ArF)4−] (7), turnover numbers up to 545 000 were achieved in the homometathesis (HM) of 1‐octene and 1‐nonene (≤95 % E). With 7 and 1‐nonene, a turnover frequency (TOF4 min) of 8860 min−1 was determined. Productivity and E/Z‐selectivity were correlated with catalyst structure. For 1, Mo(N‐3,5‐Me2‐C6H3)(IMesH2)(CHR)(OTf)2 (9) and Mo(N‐3,5‐Me2‐C6H3)(IMes)(CHR)(OTf)2 (10), productivity was correlated with the coalescence temperature of the two triflates, determined by variable‐temperature 19F NMR spectroscopy. The square‐planar conformer is postulated to be the most relevant for the catalyst activation.

The structures of the first insertion products (syn/anti, cis/trans) in the ring-opening metathesis polymerization (ROMP) of norbornene derivatives using both neutral and cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes based on an O-chelating NHC, i.e., [(3), have been identified. Also, syn/anti interconversion rates of catalysts 1−3 have been determined in acetonitrile. Correlation of these values with the rate constants of polymerization revealed the importance of a balanced ratio between these two values. Disrupting that balance by changing the solvent or the monomer or by switching to a similar, but more ROMP-active catalyst leads to significant changes in the cis/trans contents of the resulting polymers. Despite the chelating and bulky nature of the ligands, a mechanism that entails inversion at the metal center of the catalyst during polymerization is proposed. Thus, highly cis-syndiotactic ring-opening metathesis polymerization-derived polymers have been prepared with the aid of 3. On the basis of our results, we propose a comprehensive mechanism for the formation of cis-and trans-configured polymers with molybdenum imido complexes containing an O-chelating NHC.

The synthesis of titanium (IV) complexes bearing bidentate O‐chelating N‐heterocyclic carbenes (NHCs) is reported. Double deprotonation of the respective NHC precursor ligand and further reaction with one equivalent of TiCl4⋅THF2 leads to the formation of complexes Ti‐1–Ti‐3 of the general formula TiCl3(NHC‐R‐O)(THF), (NHC‐R‐O=1‐(2,6‐diisopropylphenyl)‐3‐(2‐O‐phenyl)‐4,5‐dihydroimidazol‐2‐ylidene, 1‐(mesityl)‐3‐(2‐O‐phenyl)‐4,5‐dihydroimidazol‐2‐ylidene, 1‐(2,6‐dimethylphenyl)‐3‐(2‐O‐phenyl)‐4,5‐dihydroimidazol‐2‐ylidene). Furthermore, these Ti‐(NHC‐R‐O) trichloride complexes can be reacted with the bulky aryloxo ligands 2,6‐ditBu‐4‐methylphenolate and 2,2’,4,4’,6,6’‐hexa‐iPr‐terphenoxide, respectively, to form the mixed titanium (aryloxo)(NHC‐R‐O) dichloride pre‐catalysts Ti‐4–Ti‐6. Single‐crystal X‐ray structures of complexes Ti‐1, Ti‐5 and Ti‐6 are presented. Depending on the aryloxo ligand used, the complexes possess either a trigonal bipyramidal or square pyramidal ligand sphere. The propensity of Ti‐1, Ti‐4–Ti‐6 to homopolymerize ethylene and to copolymerize ethylene with norborn‐2‐ene (NBE) and cyclopentene (CPE), respectively, was investigated and access to high‐molecular weight (co‐)polymers with such complexes is disclosed.

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