Studies on Molybdena- and Tungstenacyclobutadiene Complexes Supported by Fluoroalkoxy Ligands as Intermediates of Alkyne Metathesis

Abstract

The molybdenum and tungsten 2,4,6-trimethylbenzylidynes [MesCM{OC(CF 3 ) 2 Ph} 3 ] (12a, M = Mo; 12b, M = W) were prepared and structurally characterized as related complexes to already known [MesC M{OC(CF 3 ) 2 Me} 3 ] (MoF6, M = Mo; WF6, M = W). While treatment of 12a with 3-hexyne yielded the propylidyne complex [EtCMo{OC(CF 3 ) 2 Ph} 3 ] (13), the tungsten congener 12b formed isolable metallacyclobutadiene (MCBD) species 14−16 by reaction with 3-hexyne, 1-phenyl-1-propyne, and 2,4-hexadiyne, which can be correlated with the higher electrophilicity of the tungsten complex. Furthermore, the labile MCBD [(C 3 Et 3 )Mo{OC(CF 3 ) 2 Me} 3 ] ( 17) was isolated at low temperature from the reaction of the highly active MoF6 catalyst with 3-hexyne and could be characterized by X-ray diffraction analysis. At room temperature, the same reaction afforded [EtCMo{OC(CF 3 ) 2 Me} 3 ] ( 18), and the equilibrium reaction with 3-hexyne to form 17 was additionally studied by variable temperature NMR spectroscopy, which allowed determining ΔH°and ΔS°for the formation of MCBD 17. The experimental thermodynamic data were used to set the benchmark for DFT calculations. Moreover, the deprotiometallacyclobutadiene complex (DPMCBD) [{C 3 (Mes)(Ph)}Mo-{OC(CF 3 ) 3 } 2 ] (19), prepared from [MesCMo{OC(CF 3 ) 3 } 3 ] (MoF9) and phenylacetylene, was isolated and structurally characterized as a decomposition product of terminal alkyne metathesis and employed in the polymerization of phenylacetylenes.

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

“…The cationic tungsten alkylidyne NHC bis(hexafluoro‐ tert ‐butoxide) complex W7 was inactive in alkyne metathesis at 35 °C, whereas cationic W8 bearing two trifluoro‐ tert ‐butoxides showed reactivity at the same temperature. A similar trend was found in Tamm's tungsten trialkoxide complexes[4e] among which the tungsten alkylidyne tris(hexafluoro‐ tert ‐butoxide) complex was inactive, while the tungsten alkylidyne tris(trifluoro‐ tert ‐butoxide) complex showed the highest catalytic activity,[4c], [4e] The finding that W7 was inactive at 35 °C but active at 80 °C led us to believe that similar to the aforementioned trialkoxide complexes the tungstacyclobutadiene intermediate is overly stabilized and requires higher temperatures to undergo cycloreversion . Even though increased productivities compared to the previously published tungsten alkylidyne NHC complexes were observed,[14b] the cationic tungsten alkylidyne NHC complexes do not rival already established systems for alkyne metathesis.…”

“…Metallacyclobutadienes were early identified as intermediates in alkyne metathesis and isolated from reaction mixtures of catalyst and substrate. [2e], [4a], , Recently, a molybdatetrahedrane complex was also detected as a reaction intermediate in a Mo alkylidyne complex bearing a siloxide ligand . Similar to some previously published metallacyclobutadienes, W7 does not perform a full metathesis cycle at room temperature but stops at the stage of the metallacyclobutadiene.…”

A Ghost Trapped: Realization of the 1‐Titanacyclobuta‐2,3‐diene as the First Four‐Membered Group 4 Metallacycloallene

“…Evidently, changing from group 6 (Mo, W) to group 4 (Ti) metals leads to a different bonding situation in these metallacycles. Whereas the title compound has a very large angle of 150(1)° at the allenic C‐atoms, this is with 130–135° significant smaller for the examples from group 6 metals (Mo, W) for which a M‐β‐carbon atom bond is drawn in the papers . This is in contrast to the findings of of Reiß and Beweries for the new Ti complex (Scheme and Scheme ) .…”

“…Also, in four‐membered group 4 metallacycloallenes additional stabilization was found to be possible by interaction of the central carbon atoms with the metal center, resulting in an in‐plane aromatic the MC 3 ring. The mesomeric description for all these interactions in the unsaturated ring strained four membered MC 3 ring follow the presentations A – K (Scheme ) from which F was used in and E in …”

“…During the decomposition of an alkylidene catalyst with a terminal alkyne substrate Fürstner et al observed the formation of another 1‐molybdacyclobuta‐2,3‐diene . Very recently Tamm et al reported [(F 3 C) 3 CO)] 2 Mo[‐C(Mes)=C=C(Ph)‐], one further complex of this type . These compounds were described as more or less unusual products connected with alkyne metathesis experiments and not as products of a targeted synthesis.…”