New tungsten and molybdenum alkylidyne complexes bearing mono-, bi-, and tridentate N-heterocyclic carbenes (NHCs) have been synthesized. Formation of unprecedented structures in complexes bearing N-tert-butyl substituents on the imidazol(in)-2-ylidene was observed, leading to molybdenum complexes containing an abnormal carbene (Mo-4) and a bridging O,C,C-pincer ligand (Mo-10) and to a tungsten complex containing a cationic imidazolinium-tagged alkoxide forming an inner salt with an anionic tungsten center (W-5). Both the abnormal carbene binding in Mo-4 and the O,C,C-pincer-type structure of Mo-10 were confirmed by singlecrystal X-ray analysis, and the proposed structure of W-5 is supported by the single-crystal X-ray structure of a minor byproduct (W-8) formed during the synthesis of W-4, displaying the aforementioned inner-salt-like structure. The novel alkylidyne complexes were also investigated for their capability to form a previously postulated quasi-cationic species with a weakly coordinating anion (WCA) during the alkyne homometathesis of 1phenyl-1-propyne. Overall, incorporation of bidentate and strongly σ donating NHCs as well as introduction of better leaving groups did not lead to the expected increase in catalytic activity. Despite identical ligand spheres, changing from molybdenum to tungsten led to complete loss of activity in the bidentate systems.

The first cationic and neutral tungsten alkylidyne N‐heterocyclic carbene (NHC) complexes bearing one triflate ligand were synthesized and tested for their reactivity in alkyne metathesis. Both types of tungsten alkylidyne complexes display a higher productivity in alkyne metathesis than the analogous neutral tungsten alkylidyne NHC trisalkoxide complexes. Reaction of W(≡CC6H4OMe)(1,3‐bis(1‐hydroxy‐1,1‐trifluoromethylethyl)‐imidazol‐2‐ylidene)Cl (W18) with AgB(ArF)4 (ArF = 3,5‐bis(trifluoromethyl)phenyl) resulted in the unexpected formation of, to the best of our knowledge, the first cationic ditungstatetrahedrane W2(1,3‐bis(1‐hydroxy‐1,1‐trifluoromethyl‐ethyl)‐imidazol‐2‐ylidene)2(MeCN)(µ‐((Ar)CC(Ar)))+ (B(ArF)4)– (W19, Ar = C6H4OMe), which suggests bimolecular decomposition as a possible decomposition pathway of cationic tungsten alkylidyne NHC complexes. Reaction of the cationic tungsten alkylidyne NHC complex W(≡CC6H4OMe)(1,3‐diisopropylimidazol‐2‐ylidene)(OC(CF3)2Me)2(NCtBu)+ (B(ArF)4)– (W7) with 1‐phenyl‐1‐propyne allowed for the isolation of a cationic tungstacyclobutadiene W(C3(Ph)(Me)(C6H4OMe))(1,3‐diisopropylimidazol‐2‐ylidene)(OC(CF3)2Me)2(NCtBu)+ (B(ArF)4)– (W20). Its formation strongly supports a cationic active species in the alkyne metathesis with tungsten alkylidyne NHC complexes.

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.

The reaction of pentamethylcyclopentadienyl titanium trichloride (Cp*TiCl3) with lithium N,N‐bis{6‐[2‐(diethylboryl)phenyl]pyridyl‐2‐yl}amide resulted in the formation of chlorido[η3‐N‐{6‐[2‐(diethylboryl)phenyl]pyridyl‐2‐yl}‐N‐(6‐C6H4)pyrid‐2‐yl)amido]pentamethylcyclopentadienyltitanium (Ti‐1). The structure of Ti‐1 was elucidated by using single‐crystal XRD. The same compound is formed upon the reaction of Cp*TiCl3 with lithium N‐{6‐[2‐(diethylboryl)phenyl]pyridyl‐2‐yl}‐N’‐[6‐(2‐bromophenyl)pyridyl‐2‐yl]amide. The reaction of N,N‐bis[6‐(2‐bromophen‐1‐yl)pyrid‐2‐yl]amine (4) with lithium hexamethyldisilazide followed by the addition of Cp*TiCl3 leads to the formation of [N,N‐bis{6‐(2‐bromophen‐1‐yl)pyrid‐2‐yl}amide]dichloridopentamethylcyclopentadienyltitanium (Ti‐2). The reaction of Ti‐1 with benzylmagnesium chloride yields the pentamethylcyclopentadienyl titanium vinyl complex pentamethylcyclopentadienyl{N‐(6‐phenylpyridyl‐2‐yl)‐N‐[6‐(2‐ethyl‐η2‐vinylboryl‐C6H4)pyrid‐2‐yl]amido}titanium (Ti‐3) by C−H activation; Ti‐3 is best described as a monosubstituted titanacyclopropane. Upon activation with methylaluminoxane (MAO), Ti‐1 produces linear, high‐density polyethylene with molecular weights >6×106 g mol−1, whereas Ti‐2 yielded ethyl‐branched polyethylene. Both Ti‐1 and Ti‐2 are able to copolymerize ethylene (E) with norborn‐2‐ene (NBE) and E with cyclopentene (CPE) and to terpolymerize E with NBE and CPE. Ti‐1 activated by MAO is also active in the ring‐opening metathesis polymerization of NBE to produce up to 97 % cis‐syndiotactic poly(NBE). The polymer structure of all resulting homo‐ and copolymers was elucidated by using 13C NMR spectroscopy.

No abstract