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

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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 Δ H ° and Δ S ° 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.