Cationic Tungsten Alkylidyne N‐Heterocyclic Carbene Complexes: Synthesis and Reactivity in Alkyne Metathesis

Abstract

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.

Cationic Tungsten Oxo Alkylidene N-Heterocyclic Carbene Complexes via Hydrolysis of Cationic Alkylidyne Progenitors

“…In view of these findings and in continuation of our research on neutral and cationic molybdenum and tungsten alkylidyne NHC complexes, we synthesized cationic alkylidyne NHC complexes bearing halides or triflates. Reaction of [W­(CC 6 H 4 -4-OMe)­Br 3 (DME)] with the free carbenes 1,3-dimesityl-1,3-dihydro-2 H -imidazol-2-ylidene (IMes) and 1,3-bis­(2,6-diisopropyl­phenyl)-1,3-dihydro-2 H -imidazole-2-ylidene (IDipp) in THF yielded complexes W01 and W02 (Scheme ).…”

183W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

“…The coordination geometry about the tungsten center of 17 (Figure 3) is much closer to square pyramidal ( τ 5 ≈0.14) than to the trigonal bipyramidal arrangement, which basically all other known tungstenacyclobutadiene complexes adopt; [14b, 20, 23, 24b, 37–40] this difference is likely a consequence of the chelate ligand framework and may contribute to the low reactivity of 17 [41] . The equatorial siloxide serves as a notably better π‐electron donor to tungsten, since the W1‐O2 bond (1.903(1) Å), which is essentially coplanar with the metallacycle, is significantly shorter than the orthogonal W1‐O1 (1.988(1) Å) and W1‐O3 (1.952(1) Å) bonds.…”

Catalytic Alkyne and Diyne Metathesis with Mixed Fluoroalkoxy-Siloxy Molybdenum Alkylidyne Complexes

“…16,19 Convenient access to these complexes was provided by the development of reliable protocols for their preparation from Mo(CO)6 or W(CO)6, respectively, 20,21 which also furnished MoF6-type initiators for ring-opening alkyne metathesis polymerization (ROAMP) [22][23][24] as well as N-heterocyclic carbene (NHC) adducts of related molybdenum and tungsten benzylidyne complexes. [25][26][27][28] Recently, related molybdenum benzylidyne complexes were even used for olefin metathesis reactions. 29 Siloxide-supported alkylidyne complexes are another important class of alkyne metathesis catalysts, 30 and the triphenylsiloxy molybdenum complex I is probably the most frequently used species, 31,32 in particular for natural product synthesis through ring-closing alkyne metathesis (RCAM).…”

“…In the case of tungsten, a lower degree of fluorination is sufficient, and [MesCW­{OC­(CF 3 )­Me 2 } 3 ] ( WF3 ) is arguably the most active tungsten-based alkyne metathesis catalyst reported. , Convenient access to these complexes was provided by the development of reliable protocols for their preparation from Mo­(CO) 6 or W­(CO) 6 , respectively, , which also furnished MoF6 -type initiators for ring-opening alkyne metathesis polymerization (ROAMP) − as well as N-heterocyclic carbene (NHC) adducts of related molybdenum and tungsten benzylidyne complexes. − Recently, related molybdenum benzylidyne complexes were even used for olefin metathesis reactions …”